fy1995 study of ultra high resolution laser and microwave
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Summary of New Energy and Industrial Technology Development Organization Entrusted R&D Report for FY 1996
Date of preparation March 31, 1997
Field/Project number
Field: Human Society Technology No. F — 145
Research organ!zation
National Research Laboratory of Metrology,Agency of Industrial Science and Technology
Post of the researchcoordinator
Chief, Quantum Measurement Section,Quantum Metrology Department
Name of theresearch coordinator
Jun YODA (Signature)
Title of the project A Study of Ultra High Resolution Laser and Microwave Spectroscopy and its Application to Future Standards
Duration of theNovember 1, 1995 ~ March 31, 1997
projec t
Purpose of the projec t
Ultra high resolution spectroscopy is carried out in a range from microwave to ultraviolet using lasers with laser cooling, saturated absorption, 2 photon absorption and ion trap, and optical parametric osci- lation is studied for optical frequency measurement.
Summary of the resu 11s
A temperature of 10 u K and that of 150 mK were obtained for Cs atoms and a single Yb ion, respectively.A stability of 3. 3x10"12 was obtained for iodine stabilized YAG lasers. A saturation signal of C2H2 was observed during more than 9 months. An optical parametric oscillator, which was fabricated to measure an optical frequency, was probed to work continuously more than 3 hours.
Publication, patens, etc.
1 998 Conference of Precision Electromagnetic Measurement, Annual Meeting of Japanese Physics Society and that of Japanese Applied Physics.
Future plans Studies of future standards will be carried out, because it takes long time to fabricate the future s t andards.
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1) S. Ohshima, T. Kurosu, T. Ikegami, Y. Nakadan; "Cesium Atomic Fountain with
Two-Dimentional Moving Molasses”, Jpn. J.App 1.Phys., 34, LI 1 70, (1 995).
2) S. Ohshima, T. Kurosu, T. Ikegami, Y. Nakadan; ’’Multipulse Operation of Cesium
Atomic Fountain”, Proc. of the 5th Symposium on Frequency Standard and
Metrology, World Scientific, p. 60, (1 995).
3) G. A. Cos tanzo, S. Ohshima, K. Hagimoto, Y. Nakadan, Y. Koga; " A New Laser System for
the Cs Fountain Frequency Standard at NRLM", Proc. 11th European Frequency
and Time Forum, (1 997)
4) *<§, n« m±, ; {±'>0#565# #59#, p. 959, (1 996)
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Saturation Spectroscopy of Acetylene Molecule towards Frequency Standard at 1550 nm Region
CPEM96 Braunschweig Germany 18 June 1996.
• A. Onae, K. Okumura, Y. Miki, T. Kurosawa, E. Sakuma,J. Yoda, K. Nakagawa
Saturation spectroscopy of acetylene molecule at 1550 nm region using erubium doped fiber amplifier
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-35-
####
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[1] T. J. Quinn: Metrologia 30, 1 994, 523.
[2] D. Touahri, 0. Acef, A. Clairon and J.-j. Zondy, " Frequency measurement of the
5S1/2 (F = 3)-5D5/2 (F = 5) two-photon transition in rubidium.": Optics Com.
(to be pub 1ished)
[3] F. Nez, F. Biraben, R. Felder, Y. Millerioux: Optics Com. 1 02, 1 993, 432.
[ 4 ] R. Felder, D. Touahri, 0. Acef. L. Hilico, J. J. Zondy, A. Clairon, B. de Beauvoir,
F. Biraben, F. Nez, L. julien, Y. Millerioux: SP IE 2 378, 1 995, 52.
(5] M. de Labache 1 er i e and G.Passedat: Applied Optics 32, 1 993, 2 69.
[ 6 ] Wf'tfaS> 4 1 , B8fa 5 1 ¥ 2 £ ,
1 .
[7] Tim Day, Michael Brownell, and I-Fan Wu: SP IE 2378, 1 99 5, 35.
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—76—
[ 1 ] A. Arie, S. Schiller, E.K. Gustafson, R.L. Byer: Opt. Lett. 17, 1204 (1992)
[ 2 ] A. Arie, R.L. Byer: J. Opt. Soc. Am. B 10, 1990 (1993)
[ 3 ] A. Arie, R.L. Byer: Appl. Opt. 32, 7382 (1993)
C 4 ] M.L. Eickhoff, J.L. Hall: IEEE Trans. Instrum. Meas. IM-44, 155 (1995)
[ 5 ] P.A. Jungner, S. Swartz, M. Eickhoff, J. Ye, J.L. Hall, S. Waltman: IEEE Trans. Instrum. Meas.
IM-44, 151 (1995)
[ 6 ] J. H. Shirley: Opt. Lett. 7, 537 (1982)
-77
TEMPERATURECONTROL SERVO ■ - -
ANALOG *
MULTIPLIER*
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532 nm OUTPUT 5 OLP
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■> IDENTICAL SYSTEM
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[1] M X. ^, D. J. Wineland, Science, 226, 395 (1 984) : Proceedings of the 4 th
Symposium on Frequency Standards and Metrology, Ancona, 1988, edited by
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4f13(2F70/2)5d6s
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—117—
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Neuchatel, Switzerland.
-118-
fo=Frequency of 1064nm Nd:YAG laser (282THz) to be measured
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1) S. Ohshima, T. Kurosu, T. Ikegami, Y. Nakadan: Multipulse Operation of
Cesium Atomic Fountain. Proc. of the 5th Symposium on Frequency
Standard and Metrology. World Scientific p. 60 (1995)
2) mm, mm,6 5 #, # 9 ^ p 9 5 9 (1996).
3) G. A. Costanzo, S. Ohshima, K. Hagimoto, K. Nakadan and Y. Koga:A New Laser
System for the Cs Fountain Frequency Standard at NRLM.
Proc. 11th European Frequency and Time Forum. (1997)
1) Y. Nakadan, K. Hagimoto, C. H. Oh, T. Ikegami, Y. Koga and S. Ohshima:
Reconstruction of Optically Pumped Cs Frequency Standard at NRLM.
Proc. of 96 CPEM. (1 996)
2) E*, mm,
S4 4 28pZC—5 1 997^32.
3) ±#, m;')u mm, am, ^
1997^.
4) mm, am,
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#5 10##^##^ 1 996^3^.
2) A. Onae, K. Okumura and J. Yoda: Saturation Spectroscopy of Acetylene
Molecule towards Frequency Standard at 1550 nm Region.
Proc. of 96 CPEM. (1 99 6)
3) m#, mm,
m s 2 28pya-4 1997^3^.
4) 4wn, *b : 1. 5
S5iHji/-if- • u^^m%s<Dm$LWtum 1997^.
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1) F-L Hong and J. Ishikawa: Saturation Spectroscopy of Molecular Iodine at
532 nm by Nd:YAG Lasers and their Application for Frequency
Stabilization.
Proc. of 96 CPEM. (1996)
2) 5 HI, Yoon :j;3#2c^fb 5 3 2nm Nd : Y A G lx-1f
^4 4^^^#)##^;#^^ 29&NC-10 1 997^32.3) 5)11 : 3 2 nm Nd : YAG
ms imy-if- - 1997^.
1) K. Sugiyama and J.Yoda: Investigation for High-resolution Spectroscopy of
the 2S1/2 - 2D5/2 Transition of Laser-cooled Trappd Yb+.
Proc. of 96 CPEM. (1 996)
2) #lLl, Mm, teffl : h? y 7°$n/t^-Yb+# 1/ — if — 7^7 h □ ^ — 7 — (D M % o
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6 ) O P O
1) T. Ikegami, S. Slyusarev, S. Ohshima and E. Sakuma : Long Term Operation of
a CW Doubly Resonant Optical Parametric Oscillator.
Jpn. J. Appl. Phys. 35 (1 996) p2690.
-129-
2) S. Slyusarev, T.Ikegami, S. Ohshiraa and E. Sakuma : Influence of Electro-
chromic Damage of a KTiOPOi Crystal on a Phase-Locked CW Optical
Parametrie Oscillator.
Jpn. J. App 1. Phys. 35 ( 1 9 9 6) p3459.
3) T. Ikegami, S. Slyusarev, S. Ohshima and E. Sakuma : Accuracy of an Optical
Parametric Oscillator as an Optical Frequency Divider.
Optics Commun. 127 (1996) p69.
4) S. Slyusarev, T.Ikegami, S. Ohshima and E. Sakuma : Frequency Measurement
of Accurate Sideband of an Optical Frequency Comb Generator.
Optics Commun. 1 35 (1 997) p2 23.
1) S. Slyusarev and T. Ikegami : Development of a stable 8 W YAG laser and
its SHG system for optical frequency measurement
sb 5 @ i/-if- - tfom 1997#.
3) X'Jifk7> #.± : Development of 10 W YAG laser and its SHG system for
optical frequency measurement.
% 5 01/-if- - 1997^.
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Jpn. J. Appl. Phys. Vol. 34 (1995)Part 2, No. 9A, 1 September 1995
Cesium Atomic Fountain with Two-Dimensional Moving MolassesShin-ichi Ohshima, Takayuki Kurosu, Takeshi IKEGAMI and Yasuhiro NakadanNational Research Laboratory of Metrology, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
(Received May 17, 1995; accepted for publication July 14, 1995)
In this paper we describe a cesium atomic fountain with two-dimensional moving molasses. This method is applicable for multipulse operation of the cesium atomic fountain frequency standard, which will improve the frequency stability and reduce the frequency shift due to spin exchange collisions of atomic fountain frequency standards. Multipulse launching is demonstrated experimentally, and the frequency shift due to the fluorescent light from the loaded atoms is estimated.
KEYWORDS: atomic fountain, cesium, frequency standard, laser cooling, light shift
1. IntroductionA cesium atomic fountain frequency standard is ex
pected to be a high-performance primary frequency standard, and the achievable accuracy is expected to be on the order of 10_16.1'3) This new type of standard requires the latest techniques of laser cooling and manipulation of cooled atoms, and such standards are under study in many laboratories throughout the world. The atomic fountain will overcome problems in the conventional standards, such as cavity phase shift, second-order Doppler shift, and cavity pulling. The sequence of the cesium fountain operated up to now has been the pulsed mode, and only one pulsed signal is obtained in one cycle from the loading phase to the detection phase. This degrades the stability because the frequency of the local oscillator is only periodically sampled.^ In order to obtain the best stability using the fountain, we must use a local oscillator that has extremely high short-term stability, such as a cryogenic oscillator. If we can realize many pulses in one cycle of the fountain, which we call multipulse operation here, the stability will approach the potential performance of the fountain even if we use a quartz oscillator or a hydrogen maser as the local oscillator. Another problem of atomic fountain frequency standards up to now is the frequency shift due to spin exchange collision,3,5) for which we must reduce the number density of cesium atoms to reduce its influence. Multipulse operation will reduce this influence because we can reduce the number density without reducing the total number of atoms in a unit time.
In this work, we demonstrate the launching of laser- cooled cesium atoms by two-dimensional moving molasses, which will be a useful technique for realizing multipulse operation of the cesium fountain frequency standard. The problem of multipulse operation is the frequency shift due to the fluorescent light from the loaded atoms, which is called light shift. Its effect is estimated and a method of removing it is discussed.
2. Launching by Two-Dimensional Moving MolassesFigure 1 shows the setup for experiments on the cesi
um atomic fountain, and Fig. 2 shows energy levels of the Cs D2 line relevant to this experiment. This setup is constructed with a loading area employing a magnetooptical trap (MOT), an optical pumping area from F = 4
to F = 3, a microwave cavity and drift region for obtaining Ramsey resonance, and a detection area of cesium atoms returned after the ballistic motion. Moving molasses is a very efficient method to launch cooled atoms from the loading area, and one-dimensional moving molasses is usually used.2,3) When we employ one- dimensional moving molasses, we cannot load the next atoms until the previous atoms return and are detected. We employed two-dimensional moving molasses, as shown in Fig. 3, to prevent the laser light from illuminating the launched atoms. The same configuration of MOT was proposed in ref. 6. With this configuration, we can obtain the velocity
v= -X-8u (l)
where A is the wavelength of the D2 line and 8u is the frequency difference between the two pairs of laser light, as shown in Fig. 3.
Two-dimensional moving molasses is slightly more complicated than the one-dimensional one, because we must take care as to the alignment and balance of the power of two pairs of laser beams to launch the atoms into the exactly vertical direction. The power of each
Microwave Cavity
Pumping Lasers
Loading/Launching Lasers and Repumping Lasers
Loaded Atoms
Anti-Helmholtz Coils
Detection Lasers
Fig. 1. Experimental setup of cesium atomic fountain.
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Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 2, No. 9A S. Ohshima et al.
8 v=10~70MHz<—
c
9192MHz
A A
R D
F"
-5
F
•4
Fig. 2. Energy levels of Cs D2 line. (C: cooling/launching, R: repumping, P: pumping, D: detection.)
v-dv v-Sv
Moving Molasses —v=V2->v 8v
45° Launching Lasers
v+dv v+8vFig. 3. Two-dimensional moving molasses.
cooling laser beam introduced to MOT is about 10 mWand its beam diameter is 15-20 mm. We loaded about1 x io7 atoms using this MOT in the loading time of 640 ms. The cooling laser frequency is detuned to about 16 MHz below the resonance frequency of F=4 —F'=5. After the loading, the current to the anti- Helmholtz coil is switched off, and the atoms are further cooled by decreasing the laser power for 6 ms. With the frequency difference of 6u = 3.2 MHz, we can obtain the vertical velocity of 3.9 m/s which gives the maximum height of 77 cm above the loading point. This launching is mostly completed in 1 ms with maximum laser power, and the 2 ms following the launching
(c) Multipulse launch
(b) Single-pulse launch
(a) Free fall from loading point (xlOO)
Time after Launching (ms)
Fig. 4. Examples of time-of-flight (TOF) signal, (a): Free fall from the loading point. The gain of this is 100 times smaller than in (b) or (c). (b): Single-pulse launch with loading time of 640 ms. (c): Multipulse launch with loading time of 130 ms.
is spent for postcooling by reducing laser power and increasing the frequency detuning. The detection area is 33 cm under the loading point, and Fig. 4 shows examples of time-of-flight (TOF) signals. Curve (a) shows free fall from the loading point, whose delay time corresponds to the distance between the loading point and detection area; curve (b) shows the atoms returned after being launched over a height of 77 cm with the loading time of 640 ms; and curve (c) shows TOF of multipulse launching with the loading time of 130 ms. We can see successive pulses every 170 ms. This duration is limited only by the launching and postcooling, and we will be able to reduce it to less than 100 ms. We cannot obtain further successive pulses because the detection area is placed under the loading area in the present setup and the returned atoms are recaptured by the next loading cycle. The detection area must be placed between the loading area and the microwave cavity to operate the standard in a practical multipulse mode. The signal-to-noise ratio of curve (b) is about 500 and the signal amplitude is not decreased so much in curve (c) because the loading of atoms in curve (b) is nearly saturated. If we increase the vapor pressure of cesium, we will be able to load the atoms in a short time. We can also expect to obtain a larger signal amplitude by moving the detection area to between the loading area and the microwave cavity. The background vacuum pressure was around 1 x 10-6 Pa at the ion pumps ofthe vacuum chamber, and there is a possibility of increasing the signal amplitude by reducing the background vacuum pressure. The present signal-to-noise ratio of curve (b) is, however, sufficient to observe the Ramsey resonance signal, as shown in Fig. 5. In order to achieve this, the cesium atoms are optically pumped to F = 3 by introducing a laser beam at 1.5 cm above the loading point. Since the microwave cavity is placed 52 cm above the loading point, we can obtain only the height of 25 cm for the drift of cesium atoms. This still gives about a 1-Hz-wide Ramsey resonance, as shown in Fig. 5. The variation of the signal amplitude is due to
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Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 2, No. 9A S. Oh SHIM A et al.
measured
Detuning of Microwave Frequency (Hz)
Fig. 5. Ramsey resonance signal observed using single-pulse operation (Fig. 4(b)). The width is about 1 Hz.
F=5 (xlO-6)
F=4 (xIO-7)
F=3 (xlO-7)
Fluorescence from Loaded Atoms
Drift Time (ms)
Fig. 6. Example of the estimated fractional frequency shift due to the fluorescent light from the loaded atoms. The maximum intensity is assumed to be 1 W/m2 at the distance of 1 m from the loading point.
the change of the vapor pressure of cesium during the measurement period.
If we assume operation of the standard in the multipulse mode whose period of launching is 100 ms, we can obtain 17 pulses in 1.7 s which is the cycle time of single-pulse operation in this experiment, including a short dead time. This means we can expect to improve the stability about fourfold when we operate the standard under the condition of the same number density of atoms per pulse, namely, with the same collisional frequency shift.
3. Consideration of Light ShiftWe must give serious consideration to the light shift.
We discuss the shift due to the fluorescent light from the loaded atoms, but we do not discuss the scattered light because it depends on the structure of the apparatus. The strongest fluorescence is due to F=4—>F' =5 transition of cesium atoms in the loading process. We consider only the shift of the state, g:F = 4, mF = 0, because the resonance frequency of F = 3 is far from that of fluorescent light, and is expressed as7^
E2Awg-—Ef\(fx-e)fg
UJ — Ufg—k-V{to — tOfg — krv)2 + {l/2rf) ’
(2)
where E and e are the amplitude and the unit vector of the electric field of light, k is the wave vector of light, ujjg is the angular frequency of the transition from the excited state / to the ground state g, v is the velocity of atoms, fx is the dipole moment, and 77 is the lifetime of state/, which is 32.7 ns.8)
We assume that the distance between the loaded atoms and the microwave cavity is 50 cm, and the drift height from the cavity is 30 cm. For simplicity, the number of loaded atoms is assumed to increase linearly with time, the loading time is 100 ms, and the launching time is ignored. The spectrum of the fluorescence is Lorentzian, and e vector and magnetic field, C-field, are perpendicular to each other. Figure 6 shows fractional frequency shift calculated at every moment in the drift duration under the condition that the intensity of light from the loading area at the distance of 1 m is 1
W/m2 at the end of the every loading cycle. The figure shows the contributions of the coupling with the components F,=3, 4 and 5. The resulting frequency shift is obtained by averaging the shift over the whole drift duration. The influence of F,=5 is roughly compensated by the change of the sign of velocity between the upward and downward moving of launched atoms, but the change of the intensity due to the loading of atoms makes the compensation imperfect. We can reduce the light shift by making the loading cycle fast, and reduction to less than 10% of the value in this example is possible in principle. The influence of F' = 3, 4 does not depend on the loading cycle, and it can be reduced only by increasing the distance between the loading area and the cavity. When we assume that the maximum number of loaded atoms is 1 x 106, which is 10% of that in the experimental condition in the previous section, and the atomic transition is saturated by the laser light, the intensity of the fluorescent light at the distance of 1 m is 2.8 x 10-7 W/m2 and the fractional shift is 4.8 x 10-14. To achieve accuracy on the order of 10-16, we must estimate the shift to the accuracy of 1%. According to our experience in the accuracy evaluation of cesium frequency standards, this appears difficult to achieve, and we may have to reduce the number of atoms, for example, to less than 1 x 105 per pulse to obtain an accuracy on the order of 10-16.
The best way to reduce the influence of the light shift is to prevent the penetration of the fluorescent light into the region above the microwave cavity, which can be accomplished by bending the trajectory of cesium atoms slightly. The use of an inhomogeneous magnetic field as in a conventional cesium frequency standard is one technique, because the atomic velocity is very slow and we can take a long time of interaction to bend the trajectory.
4. ConclusionsWe demonstrated the launching of cesium atoms by
using two-dimensional moving molasses. This will be applicable for realizing multipulse operation of cesium
—141 —
fountain frequency standards. We estimated the light shift due to the fluorescent light from the loading area, and it will be a serious problem to obtain the accuracy of 10-16. However, it should be overcome by using a mechanism to bend the atomic trajectory, such as inhomogeneous magnetic field employed in conventional frequency standards, which will prevent the fluorescent light from penetrating into the region above the microwave cavity.
AcknowledgmentWe acknowledge the helptul advice on this project
from Dr. C. Salomon and Dr. Y. Koga.
Jpn. J. Appl. Phys. Vol. 34 (1995) Pt. 2, No. 9A
1) M. Kasevich, E. Riis, S. Chu and R. DeVoe: Phys. Rev. Lett. 63 (1989) 612.
2) A. Clairon, C. Salomon, S. Guellati and W. D. Phillips: Europhys. Lett. 16 (1991) 165.
3) K. Gibble and S. Chu: Phys. Rev. Lett. 70 (1993) 1771.4) G. J. Dick: Proc. 19th Annual Precise Time and Time Interval
Applications and Planning Meeting (1987) p. 133.5) E. Tiesinga, B. J. Verhaar, H. T. C. Stool and D. van Bragt:
Phys. Rev. A 45 (1992) R2671.6) R. J. Douglas and J.-S. Boulanger: Proc. 1992 IEEE Frequency
Control Symp. (IEEE Catalog No. 92CH3083-3) p. 6.7) N. Bloembergen and M. D. Levenson: High Resolution Laser
Spectroscopy, ed. K. Shimoda (Springer-Verlag, Berlin, Heidelberg, New York, 1976) p. 330.
8) R. W. Schmieder, A. Lurio, W. Happer and A. Khadjavi: Phys. Rev. A 2 (1970) 1216.
S. Ohshima et al.
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MULTIPULSE OPERATION OF CESIUM ATOMIC FOUNTAIN
SHIN-ICHI OHSHIMA, TAKAYUKI KUROSU, TAKESHI IKEGAMI and YASUHIRO NAKADAN
National Research Laboratory of Metrology,1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
E-mail: [email protected]
ABSTRACT
The frequency of two pairs of counter propagating laser beams in three dimensional molasses was shifted by equal and opposite amounts, and hence successive launching of cold cesium atoms has been demonstrated. This method is applicable for multipulse operation of the cesium fountain atomic frequency standard, which will be useful for the reduction of the frequency shift due to the collisions of slow atoms and for the improvement of the overall frequency stability. A serious disadvantage of this method is the light shift. Experiments to reduce this light shift are described.
1. Introduction
Research on the cesium atomic fountain frequency standard has progressed to the point where the apparatus for a primary standard is now operating.1,2,3’4 This type of standard is expected to have excellent performance and will overcome problems associated with conventional standards, such as cavity phase shift, second-order Doppler shift, cavity pulling and so on. However, the atomic fountain standard requires the latest techniques of laser cooling and manipulation of cooled atoms, an ultra-high vacuum system and a well designed magnetic field. We have constructed an experimental apparatus to carry out basic research into the design and development of the cesium atomic fountain frequency standard in NRLM. In this report, we present recent experiments performed using this apparatus.
So far the atomic fountain has been operated in the pulsed mode. Although excellent performance is expected, some problems has been pointed out. One of them is the frequency shift due to the collision of slow atoms.3,4,5 In order to realize the accurate standard, we cannot increase the number density of atoms for one pulse to reduce the collisional frequency shift. Another problem is the degradation of frequency stability due to the down-conversion of the local oscillator's noise, which is caused by the dead time of pulsed mode operation.6 We launched cesium atoms by causing a frequency difference in two pairs of counter propagating laser beams in the three-dimensional molasses.3,7 This will make it possible to launch the atoms successively and thus we can operate the standard without dead time. If this is realized, we can reduce the number density of atoms per one pulse without changing the number of atoms in a unit time. The experiments to confirm this method is not completed yet, but some positive results have been obtained.
Proc. the 5th Symp. on Freq. Standards and Metrology, pp60-64, (1995)
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2. Experimental Apparatus for the Atomic Fountain
2.1. Configuration of the Apparatus
Figure 1 shows the experimental apparatus developed to perform basic experiments for the development of a cesium atomic fountain frequency standard. This setup is constructed with three parts; a microwave resonance part, a loading part and a detection part. Ion pumps are attached to the microwave resonance part and detection part. The vacuum chambers are connected to each other by flexible tubes to obtain adjustment flexibility. It is possible to exchange the location of each of the parts and to insert other components between them to cope with the different requirements of various kinds of experiments.A solenoidal coil to generate a C-field and a three-layer Fig. 1 Experimental apparatus for the development ofmagnetic shield case are mounted cesium atomic fountain frequency standard.
outside the vacuum chamber. A TE011 mode cylindrical microwave resonator is mounted at a height of 20 cm above the base of the outer magnetic shield case. The maximum height of the drift region for Ramsey resonance is about 85 cm. The loading part employs a magneto-optical trap (MOT), which has anti-Helmholtz coils mounted outside the vacuum chamber and six windows to introduce laser light. The whole of this part is surrounded by a single magnetic shield case. A reservoir of cesium is connected to this chamber through a valve. The detection part consists of light collection mirrors and a photodetector. The collection efficiency is estimated to be several tens of percent. This part is also surrounded by a single magnetic shield case.
2.2. Loading and Launching of Cesium Atoms
The present set up for loading and launching is as follows. The six laser beams are
Ion Pump
VacuumChamber
Micrawave Resonance Part :tic Shield
MicrowaveResonator C-field Coil
— 9 GHz Loaded Atoms
9 GHz
Loading PartCoolingLasers
Anti-HelmholtzCoils
>. Ion PumpDetectionLasers Detection Part
PhotodetectorMirrors
— 144—
introduced by the method shown in Fig.l. Two beams are counter-propagating in the horizontal plane and the other two pairs of beams are in the same vertical plane.7 In this configuration, we can choose the laser beam diameter freely without the restriction of the diameter of the microwave cavity hole. At present, the power of each laser beam is about 10 mW and the beam diameter is 15-20 mm. The frequency is detuned to about 20 MHz below the resonance frequency of F=4-F=5 transition in the D2 line in the loading period. We can load about Ixl07~lxl08 atoms using this setup in the loading time of about 1 s. Before the launch, the current to the anti-Helmholtz coils is switched off, and the atoms are further cooled by decreasing the laser power for 6 ms to reach a temperature of several micro-Kelvin.
We caused a frequency difference Sv between the two pairs of counter propagating laser beams in the vertical plane and used this to launch the cooled atoms. We obtained a velocity of,
v = \f2-X-Sv,
where X is the wavelength of the D2 line. For example, with a frequency difference of 6v=3.0 MHz, we can obtain a vertical velocity of 3.6 m/s which gives a maximum height of 77 cm above the loading point. This launching is mostly completed in 1 ms with maximum laser power, and the 2 ms following the launching is spent for post cooling by reducing the laser power and increasing the frequency detuning. We introduced another laser beam for optical pumping from F=4 to F=3, which makes it easier to observe the Ramsey resonance signal. We started the experiment with a loading time of 640 ms. The distance between the microwave cavity and the loading point was 52 cm and the detection point was placed at 33 cm below the loading point. Under these conditions, only 1~2 percent of the launched atoms returned to the detection point. i.oThis percentage is very sensitive to the background vacuum pressure which is typically around lxlO’6 Pa at the ion pumps. We intend to improve the vacuum to obtain a larger and more stable signal amplitude. The amplitude fluctuation of individual pulse was around 1% in RMS, and it is close to the reported value in ref.4. This fluctuation may be minimized by reducing the fluctuation of the laser intensity and beam direction, which are mostly generated in
§E
-6 -4 -2 0 2 4 6Microwave Frequency (Hz)
Fig.2 Example of observed 1-Hz-wide Ramsey resonance.
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acousto-optic modulators (AOM) to control the laser's frequency and amplitude. Figure 2 is an example of the observed Ramsey resonance signal. Each data point is an average of 16 measurements. Although we can only obtain a height of 25 cm for the drift of cesium atoms in this configuration, this still gives about a 1-Hz-wide Ramsey resonance, as shown in Fig.2. There is a long term variation of the signal amplitude which is caused by the change of the vapor pressure of cesium, frequency drift of lasers and so on.
3. Multipulse Launch and Light Shift
When the atoms are launched successively as in the configuration of Fig.l, they are recaptured by the MOT following their ballistic motion. Then we exchanged the location of the loading part and the detection part. Figure 3 shows an example of observed successive pulses launched every 100 ms. These were observed during the rise of atoms through the detection part. The loading time of every pulse is 70 ms and the launching process
Fig.3 Cesium atoms launched successively with a duration of 100 ms.
including initial velocity is the same as in the previous section. The peak height is about 20% of that of the 640 ms loading time, which is larger than the ratio of the loading times because of the saturation of the loading. If we assume operation of the standard in the multipulse mode whose period of launching is 100 ms, we can obtain 17 pulses in 1.7 s which is the cycle time of single-pulse operation in the above experiment, including an additional short dead time. This means we can expect to improve the stability about fourfold when we operate the standard under the condition of the same number density of atoms per pulse, namely, with the same collisional frequency shift. Furthermore, there is no dead time which will generate the down-conversion of the local oscillator's noise.
We must give serious consideration to the light shift, so we estimated the frequency shift due to the fluorescent light from the loaded atoms. Even if we assume that the maximum number of loaded atoms is as small as 1X106, the fractional frequency shift is estimated to be 5 X10'14.8 This looks too large to achieve accuracy of the order of 10"16. The best way to reduce the influence of the light shift is to prevent the penetration of the fluorescent light into the region above the microwave cavity.
—146—
4. Magnetic Deflection of Slow Atoms
We can remove the fluorescent light by deflecting the trajectories of the launched atoms. One approach is to use an inhomogeneous magnetic field as in a conventional cesium beam frequency standard. The atomic velocity is very slow and we can use the long interaction time to deflect the atoms. The scheme of the operation will be as shown in Fig.4.Atoms are launched at an angle to the vertical line and optically pumped to the F=3 state on the way to the deflection magnet. Deflected atoms will be split in space according to the Zeeman sublevels.Only the atoms of m^O will return to the detection point after the ballistic motion, and only atoms which made a transition to the F=4 state will be detected.
Initially, we tried the deflection of F=4 atoms to confirm the effect of the magnetic field. We employed a solenoidal coil with an iron core to generate the magnetic field and we utilized the magnetic field diffusing from the end of the coil as the inhomogeneous magnetic field. The distance between the loading point and deflection magnet is 18 cm and the detection point is 13 cm above the magnet. The atoms are detected during the rising. Figure 5 shows an example of the deflection of atoms which were launched with the initial velocity of 2.88~3.84m/s. The maximum signal was obtained at 3.60 m/s because the launching condition is optimized at that velocity. The deflection angle is estimated to be 2.6 degrees. The deflection efficiency is not estimated accurately yet, but it will not be smaller than a couple of tens of percent. The peaks in Fig.5 move to the right in the figure as expected when the initial velocity is increased. We can see the structures due to the Zeeman sublevels. The maximum peaks correspond to mp=0 and the components of negative mF levels lie to the right of the peaks in the figure. The components of positive mF levels lie to the left of the peaks, but are very close to each other and are not resolved. The separation in space is not enough to observe the Zeeman components independently, but it will be improved when we observe the atoms after the
MicrowaveCavity
DetectionLasers
Inhomogeneous Magnetic Field
PumpingLasers
MovingMolasses
LaunchingLasers
Fig.4 Cesium atomic fountain with magnetic deflection.
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ballistic motion. As thesignals obtained in this 0.9 experiment are reasonable, we intend to carry out a further investigation of the magnetic deflection of slow atoms and its use in the cesium fountain atomic
3.60m/s
frequency standard. It will be very important to develop a well designed magnetic field.
0.0 1,,-------- .-------- 1---------,---------1---------,---------,---------.-------- 10.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Current to Solenoidal Coil (A)
4. ConclusionsFig.5 Atoms deflected by an inhomogeneous magnetic fieldgenerated by a solenoidal coil.
We launched cesium atoms by causing a frequency difference in two pairs of counter propagating laser beams in three-dimensional molasses and we have observed a Ramsey resonance signal. Successive launching of atoms every 100 ms has also been demonstrated. Multipulse operation will be useful to reduce the effect of collisional frequency shift and remove the dead time of operation. In order to reduce the light shift due to the light from the loading part, we tried deflecting launched atoms in an inhomogeneous magnetic field. Reasonable results have been observed, but further investigation is required.
References
1. M. Kasevich, E. Riis, S. Chu and R. DeVoe, Phys. Rev. Lett. 63 (1989) 612.2. A. Clairon, C. Salomon, S. Guellati and W. D. Phillips, Europhys. Lett. 16 (1991) 165.3. K. Gibble and S. Chu, Phys. Rev. Lett. 70 (1993) 1771.4. A. Clairon, P. Laurent, G. Santarelli, S. Ghezali, S. N. Lea and M. Bahoura, IEEE
Trans. Instrum. Meas., 44 (1995) 128.5 E. Tiesinga, B. J. Verhaar, H. T. C. Stoof and D. van Bragt, Phys. Rev. A 45 (1992)
R2671.6. G. J. Dick, in Proc. 19th Annual Precise Time and Time Interval Applications and
Planning Meeting (1987) p.133.7. R. J. Douglas and J. -S. Boulanger, in Proc. 1992 IEEE Frequency Control Symp.,
(1992) p.6.8. S. Ohshima, T. Kurosu, T. Dcegami and Y. Nakadan, Jpn. J. Appl. Phys. 34 (1995)
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1) M. H. Anderson, J. R. Ensher, M. R. Matthews, C. W. Wieman and E. A. Cornell: Science 269, 189 (1995).
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Frequency Control Symp., p. 6 (1992).7) C. Klonroe, H. Robinson and C. Wieman: Opt. Lett. 16, 761
(1991).8) Y. Wang, Y. Li, J. Gan, X. Chen and D. Yang: Frequency
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(World Scientific Publishing Co., 1996), p 60.(1996 ^ 6 R 3 B SSL)
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Proc. 11th European Frequency and Time Forum (1997)
A NEW LASER SYSTEM FOR THE CS FOUNTAIN FREQUENCY STANDARD AT NRLM
G.A. Costanzo*, S. Ohshima, K. Hagimoto, Y.Nakadan, Y. Koga
National Research Laboratory of Metrology 1-1-4, Umezono, Tsukuba, Ibaraki 305, Japan
e-mail: [email protected]
1. ABSTRACT
The National Research Laboratory of Metrology is developing an atomic Cs fountain frequency standard. An experimental apparatus, useful to perform basic experiments on cooling and launching of Cs atoms, was realized.In the past years we proposed a multipulse scheme and showed the feasibility of the reduction of some frequency inaccuracies (Ref 1) due to collisional frequency shift and the down conversion of the local oscillator noise.To improve the laser beam quality, the MOT characteristics and the long term stability we realized a new optical set-up, which will be an important part of the standard under development The technical solutions adopted in the- Cs atomic fountain standard and the related experiments on the cold atomic source are reported in this paper.
Keywords: laser system, cold atoms, Cs fountain.
2. INTRODUCTION
In the recent years the laser cooling and trapping of neutral atoms allowed a set of new apparatus for basic experiments in physics and spectroscopic fields (Ref.2). Particularly the frequency and time community took advantage of these methods which seem necessary to overcome some inaccuracies in the traditional atomic Cs beam standard.With the aim of a reduction of some causes of inaccuracies new technical solutions were adopted in many primary frequency standards (Ref. 3,4).At this moment the Zacharias idea of the atomic fountain seems the best solution to achieve an accuracy budget in the low 10*15 because of the characteristics which distinguishes this type of standard. Among these the long interaction time, the monokinetic velocity of the atoms and the intrinsic reduction of the cavity related shift arc relevant benchmarks. On the contrary the complexity of the Cs standards using the fountains scheme is increased of almost an order of magnitude because of the large number of lasers necessary to trap, pump and probe the Cs atoms. Consequently theperformances of the laser system is crucial because the cold atomic source is strongly correlated to many parameters of the laser set-up.Actually many laboratories are involved in the project of Cs fountain apparatus and a set of new solutions were proposed to overcome some sources of inaccuracies. Finally the LPTF laboratory already carried out many remarkable results on the fountain frequency standard and the preliminary evaluation of the operating standard reported an accuracy of the order of few parts in 10*15 (Ref. 5).
G.A.Co«anzo U i member of Llt.c.i. - Dip. Electronic* , Potitecnieo di Torino (luly)
3. THE CS FOUNTAIN AT NRLM
NRLM planned to build a fountain frequency standard operating beside the optically pumped Cs clock which was realized in the late 80’s (Ref. 6, 7). Many papers have been already published (Ref. 1, 8, 9) and some basic experiments have been carried out to evaluate the feasibility and the performances of this new standard.In fig.l a rough set-up for the experimental apparatus is sketched our. Six independent laser beams, tuned to the red of the (F=4-F'=5) cycling transition, are introduced in the loading part of the vacuum chamber for trapping and cooling of the Cs atoms.
Pressure ̂O'4 Pa
Interactionzone
Detectionzone
LoadingzoneTnppmg beam*
meMagnetic
Fig. 1 : Scheme of the Cs fountain at NRLM (horizontal beams orthogonal to the MOT coils are not shown).
The quadruple magnetic field, useful for magneto-optical trapping, is generated by two anti-Helmoltz coils wound around the vacuum conflat flanges in the loading zone which is surrounded by a magnetic shield.
-154-
The horizontal beams arc orthogonal to the Helmoltz coils plane whereas the vertical ones have an inclination of 45°. in this manner it is possible to introduce the vertical beams without the spatial reduction due to the passing holes in the end caps of the microwave cavity.An additional laser beam, which is frequency locked on the (F=3-F’=4) transition, is then used to repump the atoms from the F=3 level.The 3D cr+ - a" MOT method is used to trap the atoms in the bottom part of the vacuum chamber connected through a valve to a Cs reservoir at the laboratory temperature.After the launching sequence, which is performed with the moving molasses technique with a few millisecond post cooling period, the Cs atoms interact with the microwave frequency in the cylindrical copper cavity which resonates in the TEon mode.Thanks to their parabolic flight the atoms pass twice in the microwave cavity performing the two separate oscillatory field Ramsey resonance method. After that the atoms are detected by fluorescence using a probe laser, tuned on the (F=4-F’=5) cycling transition, and a light collection mirrors with an efficiency of several tens percent.During the past years a couple of experiments have been realized using this apparatus with the purpose to increase the background knowledge towards this new standards.During the first experiments the amplitude fluctuations of the lasers intensity and the beams positions were the main source of instabilities in the cold atomic source for the apparatus.To overcome these problems we projected and realized a new optical set-up also with the aim to obtain a handling and modular structure which will permit an easier and faster maintenance of the final standard as delineated in the following paragraph.
4. THE OPTICAL SETUP
In the Cs fountains the laser system is an important part which has a remarkable complexity compared to the scheme used in the optically pumped Cs beam frequency standards.To develop the optical set-up it is necessary to mind many parameters to achieve the goal of a highly stable trapped cold atoms. Frequency stability, beam intensity, beam quality, good flexibility in the tuning of the AOM devices are essential.Nowadays semiconductor lasers in the near infrared region is a useful tool in the Cs spectroscopy. Extended cavity laser diodes are commonly used and commercial products are also available. Nevertheless the provided power is not exceeding several milliwatt.An efficient solution (Ref. 10,11) at this problem is achieved with the master-laser configuration which transfers the spectral purity of a low power stabilized master laser (ML) tothe higher power of the slave injected laser (SL). For this purpose only several microwatt of the ML are necessary and, moreover, no servo electronics.In Fig. 2 the new optical set-up for the Cs fountain is roughly depicted. In this scheme a commercially available diode laser system mounted in Littman-Metcalf configuration is locked on the crossover resonance between the (F=4-F*=5) and (F=4-F’=4) lines using the saturated absorption spectrum of a Cs cell which is surrounded by a magnetic shield to prevent the frequency instabilities due to the variations of the magnetic field in the laboratory environment. Few percent of
the total power of the laser beam is inserted n an_electro- optical modulator driven by a 30 MHz synth etizer.In this manner the common FM sideband method is applied without the modulation of the diode laser current which introduce detrimental sidebands in the main beam. The saturated absorption signal is then detected by a PIN photodiode and subsequently demodulated in order to get the error signal which is used to control the laser current and the PZT voltage by the loop filter.From the analysis of the Fourier Transform of the error signal in closed loop conditions it is possible to assume a value of several kHz for the laser line width which is dominated by the low frequency jitter due to the sensitivity of the extended cavity to the mechanical vibrations.
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^ pumpedFreq.
dividerCs beam standard
pp m
i/4 E 3Pol mint.
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O
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AOM
LASER
AMP. (HA)
INJECTEDLASER
LASER (OIL)LOCKEDOFFSET
Fig.2: The optical set-up for the Cs fountain (the dashed areashows one of the three injected slave lasers).
Afterwards the main power of the master laser is coupled into a single mode fiber and then splitted into two parts: one is used for the NRLM optically pumped Cs beam frequency standard and the other one for the fountain apparatus.Each fiber provides about 1.5 mW useful to lock another laser (OLL).The offset frequency is about 125 MHz and it is accurately tuned by referring to the maximum fluorescence signal from the detection of the atomic Cs beam in the optically pumped Cs standard. The beat note between the ML
MASTERLASER
-155-
and the OLL is detected by an avalanche photodiode and then is fed in an AC coupled amplifier.After a prescaling of the IF by a digital divider by 10 the signal is inserted in a digital phase and frequency discriminator (PFD) (Ref. 11,12) which ensures a reliable lock for long time but showing a high sensitivity to the acoustic noise picked-up from the optical table. The output of the PFD is splitted in a fast component which control the laser current and a slow component for long term stabilization by the PZT voltage input of the extended cavity laser.Fig. 3 shows the beat note between the ML and the offset locked laser: the signal to noise ratio is of the order of 30 dB in a bandwidth of the spectrum analyzer of 10 kHz. Thus the main power of the OLL is firstly up-shifted of (2x75)MHz using an AOM in double pass configuration and cat's eye combination and finally is injected in a single mode laser amplifier (ILA) with a CW power of 150 mW.
CENTER FREQUENCY - 115MHz SFAN - 1 MHz. RBW - 10 KHz
Fig. 3 : Beat note between the master laser and the offset locked laser in closed loop condition.
To monitor the laser spectrum we used a scanning Fabry- Perot interferometer with a full spectral range of 2 GHz. The injection beam (P=2mW) is aligned in the laser amplifier and, with the purpose to optimize the locking bandwidth, we measured the induced photocurrent when the ILA was switched off.In this simple manner the maximum induced photocurrent is about 400 pA and it is possible coarsely evaluate the amount of the total injected power which is of the order of 400p\V in the hypothesis of an unitary quantum efficiency.A nearly optimum mode matching is performed by the temperature setting of the Peltier cooling element in the laser package. Because during the launching sequence the OLL frequency undergoes a large frequency shift, the efficiency of the AOM and, consequently, the power useful to inject the ILA is decreased. Following that we drove the ILA at lower power with the aim to obtain the highest locking bandwidth for this laser.When the ILA power is about 20 mW and the injected power is several hundreds of microwatt, we obtained about 10 GHz of locking bandwidth which ensures a good and robust stabilization for long term operation and still when we produce a frequency shift of the order of 10-15 MHz from the operating frequency (75 MHz) of the AOM.Finally we coupled the ILA in a singlemode fiber which distributes the power to three independent slave lasers. Each beam coming up from the fiber is collimated by a grin lens and then injected in the SL. The three SLs emit about 150mW with a locking bandwidth of more than 2 GHz. The main laser power is splitted into two parts and each one
undergoes a negative frequency shift using AOM in double pass configuration. Finally the laser beam is fed in an optical fiber. An half wave plate is used before the fiber aligner to accurately match the laser beam polarization with the polarization axis of the fiber.After the beam expander the beam intensity has a full gaussian shape with a diameter of 30 mm (1/e2 points). A quarter wave plate is placed close to the fiber output to achieve the desired <y* or a'polarization.All the AOMs are driven by independent direct digital synthetizers (DOS) which frequencies are set by remote control with a computer. The output of each DDS is fed in a RF power amplifier to achieve the 1W power level necessary to properly drive the AOM.For the fluorescence detection we used a DBR laser which is stabilized by injection locking using few percent of the OLL power. The DBR laser beam is then coupled in a optical fiber and used as probe in the detection zone of the fountain apparatus.In this scheme problems are the long term instabilities of the laser power and the poor fiber coupling efficiency (about 30%). The former is mainly ascribed to the beam drift when the temperature variations slightly change the relative positions of the optical components.Furthermore the coupling efficiency to the fiber is strongly correlated to the beam positions. Even if we used a double pass configuration for the AOM devices the laser beam intensities still showed a sensitivity to the operating RF frequencies driving the AOM.Fig. 4 shows the laser intensities variations at the fibers output when the frequencies of the AOM devices are swept with a span of 10MHz: only the laser beam #4 shows a symmetric and bandpass-like characteristic unlike the beams #3 and #6.
Effective operating
•laser beam #3 •laser beam #6 j laser beam 84 >
AOM tieiag fmteeac? (MHz|
Fig. 4 : Laser beam intensity when AOM frequency is swept around the 85 MHz working point (for an easier caption only 3 of the 6 laser beams characteristics are shown).
In order to get more stable laser beam conditions we stabilized the power at the exit of the fibers controlling the RF power driving the AOM devices.For this purpose we used optical beam samplers placed in front of the end of each fiber. A relatively low percentage of the laser power is detected by a photodiode and then fed in a control loop filter useful for the long term stabilization of the laser power.This solution ensure a constant level of the laser power even if the AOM devices are tuned at different frequencies during the launching sequence. With the active control it is also possible to reduce the amplitude noise which is induced
—156—
when the optical fibers undergoes some unwanted micro bend.
5.EXPER1MENTAL RESULTS ON THE COLD ATOMIC SOURCE
With the optical set-up described in the previous chapter we performed some basic experiments with the aim to investigate on the performances of this new laser system which will be used for the fountain standard apparatus.Firstly by a CCD camera we observed the characteristics of the 3D cY - cT MOT: about 10*-10* atoms are trapped when the six laser beams intensities are of the order of 2mW/cm2 and the laser frequency detuning is -20MHz. A stable shape of the atomic cloud is kept for long time.Then the time of flight (TOP) method was used to measure the molasses temperature: after a 200ms loading time the MOT coils are switched off in few milliseconds and the molasses are further cooled by decreasing the intensity of the trapping beams to a value of about 0.3mW/cm2 and increasing the detuning. This cooling period can be adjusted by computer in order to get the best TOP signal. In this manner a final temperature of several microkelvin can be achieved.After the ballistic flight the cold Cs atoms are detected in the lower part of the vacuum chamber (not shown in Fig.l) where a wide area photodiode collects the fluorescence signal obtained when the probe laser excites the (F=4-F’=5) cycling transition. A diaphragm is used to reduce the laser beam size to 2mm.When the laser detuning is about -20MHz the detected TOF signal has a S/N of 65dB (1Hz of bandwidth) limited by the electronic noise and the scattered light From integration of the TOF signal still the number of detected atoms is consistent with the number roughly evaluated from the MOT observation by CCD camera.Moreover we can detect pure Cs molasses directly trapped with MOT coils switched off. In this case the number of loaded atoms is about ten times lower.With this simple experiment it is possible to measure the molasses temperature by comparisons of the spread of the TOF signal with the related temperature of a maxwellian velocity distribution which best fits the TOF signal.Fig. 5 shows the results obtained from these measurements at different laser frequencies detuning and laser beam intensities used during the cooling phase occurring after the MOT coils arc switched off.
P=0.3 mW/cm2 P=0.15 mW/cm2
D«nimag (-MHz)
Fig. 5 : Temperature of the molasses at different laser detuning.
In Fig. 5 the TOF broadening due to the geometrical size of the probe laser and molasses is not removed and limits the resolution of the measurement. As expected the molasses temperatures decrease with the laser detuning and the intensities of the lasers beams during the cooling phase. A saturation level of a couple of microkelvin is achieved for detuning larger than -20MHz.Finally the TOF signal can be used to check the stability of the cold atomic source which is strongly related to the laser beams parameters (intensity, frequency stability, frequency detuning).When laser power at the output of the fibers is not stabilized the fluctuations of the TOF signal are of the order of 10% peak to peak. Moreover we noticed an evident jitter of the arrival time of the molasses clouds.These instabilities arc notably reduced when the laser power control loop is operating: the fluctuations of the TOF signal become not exceeding 1% peak to peak even for long measurement time.At this moment this value is probably limited by the fluctuations of background stray atoms and the Cs partial vapour pressure which change the trapping efficiency and affect the ballistic flight of the cold Cs atomic source.
6. SUMMARY
In this paper we presented the new optical set-up which will be used in the Cs fountain frequency standard at the National Research Laboratory of Metrology (Japan).The high complexity of the lasers system was analyzed and we described some technical solutions as the offset locking and the injection locking of lasers. Moreover we used optical fibers for laser beam distribution in order to reduce the position fluctuations of the lasers beams. The main goal was to obtain a better lasers configuration which is necessary to increase the efficiency and the stability of the cold Cs atomic source.With the aim to investigate and confirm the performances of this new optical set-up we also showed basic experimental results concerning the temperature measurements of the molasses by time of flight method. Finally the improved stability of the cold atomic source was here reported.
7. ACKNOWLEDGEMENTS
G.A. Costanzo wish to express his gratitude for the funding support from the Japanese Science and Technology Agency (STA) through the grant which permit his fruitful and agreeable stay in the NRLM Quantum Standard Section.
8. REFERENCES
[1] S. Ohshima, T. Kurosu, T. Ikegami, Y. Nakadan: “Multi pulse operation of Cesium atomic fountain”, Proceeding of the 5th Symp. on Freq. Standard and Metrology, pp.60-65, Woods Hole (1995)[2] see Journal of the Optical Society of America B, special issue on laser cooling and trapping of atoms, vol.6, n.ll (1989)[3] W.D. Lee, J.H. Shirley, J.L. Lowe, R.E. Drullinger: The evaluation of NIST-7”, IEEE Trans, on I.&M., vol.44, n.2, pp. 120-123 (1995)[4] A. De March!: "The hi»h C-field concept for an accurate Cs beam resonator, Proc. 7“ EFTF, Neuchatel (1993)
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[5] A. Clairon, S. Ghezali, G. Santarelli, Ph. Laurent, S.N. Lea, M. Bahoura, E. Simon, S. Weyers, K. Szymaniec: ’’Preliminary accuracy evaluation of a Cesium fountain frequency standard”, Proceeding of the 5th Symposium on Frequency Standard and Metrology, Woods Hole (1995)[6] S. Ohshima, Y. Nakadan, Y. Koga: “Development of an optically pumped Cs frequency standard at the NRJLM”, IEEE Trans, on I.&M., vol.37, n.3, pp. 409-413 (1988)[7] S. Ohshima, Y. Nakadan, T. Ikegami, Y. Koga, R. Drullinger, L. HoIIberg: “Characteristics of an optically pumped Cs frequency standard at the NRLM”, IEEE Trans, on I.&M., vol.38, n.2, pp. 533-536 (1989)[8] S. Ohshima, T. Kurosu, T. Ikegami, Y. Nakadan: ”Cs atomic fountain with two dimensional moving molasses”, Jpn. Journal of Applied Physics, vol.34, pp. LI 170-L1173 (1995)[9] T. Ikegami, S. Ohshima, Y. Nakadan: “Laser cooling of Cs atomic beam at the NRLM”, Proc. 5th EF 1'F, Besancon (1991)[10] J. Bouyer, C. Breant, P. Schanne: “Injection-locking mechanism in semiconductor lasers”, SPIE, vol. 1837, pp.324-335 (1992)[11] S. Kobayashi, T. Kimura: “Injection locking in AJGaAs semiconductor laser”, IEEE J. of Q.E., vol QE-17, n.5, 1981, pp. 681-689(1981)[12] M. Prevedelli, T. Freegarde, T.W. Hansch: “Phase locking of grating-tuned diode lasers”, Appl.Phys. B, vol.60, pp.S241-5248 (1995)[13] G. Santarelli, A. Clairon, S.N. Lea, G.M. Tino: ’’Heterodyne optical phase-locking of extended-cavity semiconductor lasers at 9GHz”, Optics communications vol. 104, pp.339-344 (1994)
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SATURATION SPECTROSCOPY OF ACETYLENE MOLECULE TOWARDS FREQUENCY STANDARD AT 1550 nm REGION
A. Onae, K.Okumura*, and J.Yoda National Research Laboratory of Metrology
1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan
K.NakagawaTokyo Institute of Polytechnics
1583 Iiyama, Atsugi, Kanagawa 243-02, Japan
I
(
Abstracts
We have observed saturated absorption of the vi+v3 band of l3C2H2 using an intracavity cell. These narrow lines can be expected to use as a frequency standard at 1550 nm region. We also demonstrate it is possible to saturate a weak molecular transition using a fiber amplifier.
Introduction
Frequency Standard at 1550 nm region is currently of great interest in optical communication, atomic and molecular spectroscopy and metrology. Although vibrational-rotational transitions of molecule such as C2H2 and HCN provide almost equally-spaced nice frequency markers in this near infrared region, their quite small transition dipole moment ( - few x 10*3 Debye) due to overtone band has prevented us from applying well-established and powerful saturation spectroscopy techniques to these transitions.
Recently K. Nakagawa and M. de Labachelerie in Ohtsu group have developed a very elegant method to obtain narrow saturated absorption spectrum of molecules using a extended cavity laser diode (ECLD) with a narrow linewidth and a Fabry-
Perot (FP) cavity cell, that builds up the laser field and increase sensitivity of the system owing to the effectively long interaction region. [1,2] Furthermore, they have demonstrated the frequency stabilization of an ECLD to these molecular transitions and have measured the absolute frequencies referring to the Rb two-photon transition at 778 nm [3] as a secondary frequency standard with an accuracy of 10"9 (-100 kHz) [4,5] e
Preliminary Result with FP Cavity Cell
Encouraged by their nice results that shows an ECLD frequency-locked to molecular lines becomes a very promising frequency standard, we have started a plan to make a working standard at 1550 nm region. We have made two identical ECLDs, have measured characteristics of the lasers such as linewidth, tuning range, and output power and have observed saturated absorption spectrum with a preliminary setup. Figure 1 shows the experimental apparatus. The reflective finesse and the free spectrum range of FP cavity are 5000 and 6.25 GHz, respectively. Inside this FP cavity, 12 mtorr of acetylene gas is filled and about 2 mW of laser power is introduced to the cavity cell. The laser frequency is locked to a cavity mode using the FM side band technique. The signal from a detector (Dl) is
A/ 2
ECLD
1 SO
PBSP Z T
EOM
PBS
F Pcavity
t
Fig. 1 Experimental setup of intracavity saturated absorption spectroscopy.
ISO, isolator; EOM, electro-optical modulator; PBS, polarization beam splitter ; PZT, piezoelectric transducer
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demodulated at the EOM driving frequency (7.59 MHz) and provides error signal for servo-locking. Sweeping and dithering voltages applied to a PZT of the FP cavity, i.e. the laser frequency, the first derivative of the saturated absorption signal has been observed by lock-in detection of a signal from a detector (D2). Figure 2 shows the recorded Doppler-free spectrum of the P(10) transition of the vi+v3 band of isotope acetylene l3C2H2. The width of spectrum is mainly determined by transit time and laser line width.
From the point of view of long term operation, the present cell has a serious problem. We can not bake the cell because we use dielectric mirrors on both ends. The PZT element attached by glue cases vacuum leak. Consequently, the life time of the cell is few days. To make it longer, we have designed a new scheme. In the next stage, we are going to use a FP cavity spaced by ULE holding a baked and completely sealed-off cell with Brewster windows. In this case, we have to replace a V4 plate (Fig.l) with a Faraday rotator. As a result of the improvement as well as reduction of the laser frequency noise mainly due to acoustic, we hopefully expect realization of a frequency standard with an accuracy of 10"10 .
Experiment with ED FA
Recent progress in optical communication system provides us nice devices such as Erbium Doped Fiber Amplifier (EDFA) which amplify - mW of ECLD’s power up to 50 - 100 mW. We have demonstrated that we can use this EDFA instead of FP cavity to enhance the laser intensity and to saturate the weak C2H2 transitions. The output from the PBS in Fig. 1 is fed to the EDFA. The frequency is locked to the FP cavity to improve the short term frequency stability and is modulated at fin = 332 Hz. After collimated, the output beam of the EDFA enters a 1-m long cell containing l3C2H2 and is reflected by a mirror. The retro beam through the cell is introduced to a detector and the signal is lock-in detected at the frequency of fin. We have observed a saturated absorption signal with the pressure of 50 mtorr, the laser power at the cell entrance of 40 mW and the beam diameter of around 1 mm. We can also apply FM side band technique to improve sensitivity of this spectrometer. The signal from a DBM can be used as a error signal for servo-locking of the laser frequency stabilization.
Now it is turned out that we can saturate these weak molecular transitions using EDFA without the restriction of longitudinal mode FP cavity, it is also interesting to apply double resonance schemes and to study broadening of saturated absorption spectrum and relaxation process of molecule. *
8 MHz
Fig. 2 A typical derivative signal around the saturation
dip of the P(10) of the vx+v3 of isotope acetylene
We are sincerely grateful to Dr. Y. Awaji inOhtsu group for his substantial help to construct aECLD.
References
[1] M. de Labachelerie, K. Nakagawa, and M.Ohtsu, “Ultranarrow saturated-absorption lines at 1.5 pm", Opt. Lett.. Vol. 19 No. 11, pp. 840-842, 1994.
[2] M. de Labachelerie, K. Nakagawa, Y. Awaji, and M. Ohtsu, “High-frequency-stability laser at 1.5 pm using Doppler-free molecular lines”, Opt. Lett.. Vol. 20, No. 6, pp. 572-574, 1995.
[3] F.Nez, F. Biiaben, R Felder, and Y. Millerioux/’Optical frequency determination of the hypeifine components of the SSi^-SDa^ two-photon transitions in rubidium", Opt. Cornmun. Vol. 102, pp. 432-438, 1993.
[4] K. Nakagawa, M. de Labachelerie, Y. Awaji, M. Kourogi, T. Enami, and M. Ohtsu, “Highly precise 1-THz optical frequency-difference measurement of 1.5 pm molecular absorption lines”, Opt. Lett.. Vol. 20, No.4, pp. 410-412, 1995.
[5] Y. Awaji, K. Nakagawa, M. de Labachelerie, and M. Ohtsu, “Optical frequency measurement of H12CMN Lamb-dip-stabilized 1.5 pm diode laser", Opt. Lett.. Vol. 20, No. 19, pp. 2024-2026, 1995.
* researcher of New Energy and Industrial Technology Development Organization (NEDO)
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TUP7-5.
INVESTIGATION FOR HIGH-RESOLUTION SPECTROSCOPY OF THE ZSL<2- ZD^ TRANSITION OFLASER-COOLED TRAPPED Yb*
Kazuhiko Sugiyama and Jun Yoda National Research Laboratory of Metrology
1-4 Umezono 1-chome, Tsukuba, Ibaraki 305, JAPAN
Abstract
We drove the * 2S^, - 2D3y2 transition of laser-cooled trapped Yb+by frequency-doubled diode-laser radiation. The 2F7/2 state, pumped by driving this transition, was deexcited by irradiation at 3.43 £tm, 638 nm, and 828 nm.
Introduction
Yb* is one of the candidates which provide a frequency reference in future frequency standards [1]. Yb* has several clock transitions from the microwave to the optical region. We are interested in three clock transitions in the violet region, i.e., the 411 nm 2SW - 2D5/2 [2], the 435 nm 2SW - 2D3f7 [3] and the 467 nm 2SW-2F7/2 transitions. The wavelengths of these transitions are accessible by frequency-doubled all-solid-state lasers including diode lasers .
We are currently investigating the possibility of high-resolution spectroscopy of the 2Sl/2 - 2D&7 transition. An advantage of the 2S^ - 2D%% transition is that its excitation can be easily detected, because electron shelving in the 2F7/2 state with a lifetime of weeks [4] occurs due to a decay from the 2D5/2 state. The 6S-8S two-photon transition of Cs atoms, which is 0.5 THz away from the 2SW - 2D%% transition of Yb*, has the potential for use as a reference or transfer standard. However, pumping to the 2F7/2state requires some method for deexciting the 2F7/2 state quickly. Researchers at NPL drove the 2S1/2 - 2DW transition of laser-cooled Yb*by a frequency-doubled Ti:Al203 laser and found that driving the 2F7Z2 - 'D[5/2]transition at 638 nm rejuvenated fluorescence by the 2Pl/2 - 2S1/2 spontaneous emission[2]. 2
In this paper, we report driving of the 2S1/2 - D3/2 transition of laser-cooled Yb* by a frequency-doubled diode laser, and investigation of deexcitation of the 2F7/2 state by several methods realized by all-solid-state lasers.
Experimental setup
Laser cooling of Yb* was achieved by driving the2S^ - 2P1/2 transition at 370 nm and depleting the 2DJ/2 state with radiation at 935 nm which drives the 2D3/2 -3D[3/2] °1/2 transition [5]. UV radiation at 370 nm wasproduced by second-harmonic generation of dye-laserradiation with an external enhancement cavity technique.DR radiation at 935 nm was generated by a ring-Ti:Al203 laser.
For other radiations, we used diode lasers in an extended cavity and a commercial diode-laser-pumped Nd:YAG laser. The frequency conversions required are described later. The wavelengths of these lasers were measured by a commercial evacuated wavemeter with a resolution and accuracy of ±1 x 10"6 * 150.
We confined Yb*in a rf trap of which inner diameter was 5 mm. RF driving frequency was 2.2 MHz. The background pressure was below 3 x 10"* Pa. We added two compensation electrodes and applied DC voltages to them and to one of the endcap electrodes to minimize micromotion-induced sidebands in spectra.
Results
Laser cooling of Yb+. Unlike from otherlaboratories, we prepared laser cooled Yb+ from a natural isotope mixture source [6]. We found that it was possible to laser cool single 174Yb* even from the isotope mixture [7]. The number of Yb* was measured by observing electron shelving to the 2F7/2 state by driving the - 1D[3/2]°3/2 transition at 861 nm [5]. A temperature of150 mK was obtained. The question arises whether other isotopes would be simultaneously trapped. We observed the excitation of a stretch-mode of ion crystals from a decrease in fluorescence intensity with rf voltage applied between endcap electrodes.This can beused to investigate the existence of other isotopes. We will make detail investigations of our laser-cooling.
Deexcitation of the 2F7/2 state. We originally intended to deexcite the 2F7/2 state by irradiation at 369.48 nm [4,8]. However, we found that this wavelength is that for photodissociation of YbH* [7], and need true deexcitation lines. Deexcitation of the 2F7/2 state was investigated with a large number of laser-cooled Yb* at a temperature of several tens kelvin. We irradiated Yb* with test radiation focused with a lens of focal length 500 mm.
A straightforward method of deexcitation of the 2F7/2 state is to drive the 2F7/2 - 2D5/2 transition at 3.43 fum. Fluorescence recovery by driving this transition was observed in the presence of buffer gas [9]. It should be possible to generate radiation at 3.43 /mu by difference- frequency mixing of diode-laser-pumped Nd:YAG-laser radiation at 1064 nm and diode-laser radiation at 812 nm or 1543 nm. AgGaS, and KTA (or KTP) crystals should be phase-matched in the case of 812 nm and 1543 nm, respectively. We generated about 100 nW of 3.43 /mi radiation from 65 mW of 812 nm diode-laser
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2000including construction of a highly stable diode-laser- based spectrometer.
10coh-z3oo
o
T i-----1 l----------------------------1 U861nm 3.43pm 861nm
on on ^| on
- stray light level t min.
Fig. 1 UV fluorescence recovery by driving 2F7/2 - 2Dm transition with 3.43 pm radiation. (qz= 0.56, without DC field compensation)
and 150 mW of Nd:YAG-laser radiation with a AgGaS2 crystal of length 5 mm. We observed UV fluorescence recovery of 172 *Yb+ at 3.4317 pm as shown in Fig. 1.
If we find efficient deexcitation lines which can be driven by one laser, those are a better choice. We confirmed fluorescence recovery by the 638 nm 2F7/2 - 3D[5/2] transition for 174Yb* as reported in ref.2 where 172Yb* was used (see Fig.2). Recovery time of fluorescence intensity to the level before pumping to the 2F7/2 state was sometimes within 5 s with 360 pW of radiation at 638.619 nm.
. We also found that fluorescence can be recovered by driving the 2F7/2 - 3D[9/2]7/2 transition at 828 nm. A recovery time of 10 s was observed with 13 mW of radiation at 828.098 nm for174Yb+.
, Efficiency of fluorescence recovery depends on the lifetime and branching of upper states. However, those of the ‘D[5/2] ^ and 3D[9/2]7/2 states are not known. Further investigation is needed to estimate the efficiency of these transitions. We will irradiate Yb*in the presence of buffer gas with these deexcitation radiations, because there is room for the question that these wavelengths would be photodissociation lines of molecular ions.
Driving the 2S1/2 - 2D5/2 transition. Radiation at 411 nm was produced by second-harmonic generation using an external enhancement cavity technique. We obtained IpW of 411 nm radiation from 50 mW of diode-laser radiation with a LiI03 crystal of length 3 mm, and focused it with a lens of focal length 500 mm. We observed a decay in 370 nm fluorescence and recovery by irradiation at 638 nm for 174Yb+ as shown in Fig. 2. This shows that Yb* was pumped to the 2F7/2 state by excitation to the 2Dm state. The decay occurred at a diode laser wavelength of 821.939 nm and we needed a fine frequency tuning below the resolution of the wavemeter.
Conclusion
We observed excitation of the 411 nm2Sw - 2D5/2transition by frequency-doubled diode-laser radiation. We found several methods for deexciting the 2F7/2 state.
We intend to perform single-ion experiments of driving the 2SV2- 2DW and the 2F7/2 deexcitation transitions
3000
Fig.2 UV fluorescence decay by 411 nm radiation and its recovery by 638 nm radiation. (qz = 0.27, with DC field compensation)
References
[1] See articles in Frequency Standards and Metrology. Ed. A DeMarchi, Berlin: Springer, 1989.
[2] P. Gill, H. A. Klein, A. P. Levick, M. Roberts,W. R. C. Rowley, and P. Taylor, "Measurement of the 2S^ - 2D%2 411-nm interval in laser-cooled trapped 172Yb*ions", Phys. Rev. Vol.A52, pp.R909-R912, August 1995.
[3] Chr. Tamm and D. Engelke, "Frequency standard investigations on trapped ytterbium ions using semiconductor laser sources", Proc. European Frequency Time Forum 1994, March 1994.
[4] H. Lemitz, J. Hattendrf-Ledwoch, R. Blatt, and H. Harde, "Population trapping in excited Yb ions", Phys. Rev. Lett., Vol.62, pp.2108-2111, May 1989.
[5] A S. Bell, P. Gill, H. A. Klein, A P. Levick,Chr. Tamm and D. Schnier, "Laser cooling of trapped ytterbium ions using a four-level optical excitation scheme", Phys. Rev., Vol.A44, pp.R20-R22,July 1991.
[6] K Sugiyama and J. Yoda, "Laser cooling of a natural isotope mixture of Yb+ stored in an RF trap", IEEE Trans. Instrum. Meas. Vol.44, pp.140- 143, April 1995.
[7] K Sugiyama and J. Yoda, "Characteristics of buffer- gas-cooled and laser-cooled Yb* in rf Trap", 5th Symposium on Frequency Standards and Metrology, October, 1995.
[8] A Bauch, D. Schnier, and Chr. Tamm, "Collisional population trapping and optical de-excitation of ytterbium ions in a radiofrequency trap", J. Mod. Opt. Vol.39, pp.389-401, February 1992.
[9] A S. Bell, P. Gill, H. A. Klein, A P. Levick, and W. R. C. Rowley, "Precision measurement of the 2F7/2- 2D^3.43 pm interval in trapped 172Yb*",J. Mod. Opt. Vol.39, pp.381-387, February 1992.
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Development of a diode-laser spectrometer for high resolution spectroscopy of trapped single Yb+
Ntnl Res. Lab. of Metrology, Toho U.A, Science U. of Tokyo8
Kazuhiko Sugiyama, Kaoru SasakiA, Akinori Wakita , and Jun Yoda
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[3] Drever et. al., Appl. Phys., B31. 97 (1983).[4] Sugiyama and Morinaga, JJAP, 30, L1811 (1991).
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m 1 mf#@m#®fii 0 2 2s1/2 - ^5/2 m&ommmm[1] Sugiyama and Yoda, IEEE Trans. Instrum. Meas., IM 44, 140 (1995).[2] e.g., Madej and Sankey, Opt. Lett., 15, 634 (1990).[3] Sugiyama and Yoda, Phys. Rev., A55, RIO (1997).[4] Gill et al., Phys. Rev., A52, R909 (1995).[5] Drever et al., Appl. Phys., B 31, 97 (1991).[6] Sugiyama and Morinaga, JJAP, 30, L1811 (1991).
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Jpn. J. Appl. Phys. Vol. 35 (1996)Part 1, No. 5A, May 1996
Short Note
Long Term Operation of a CW Doubly Resonant Optical Parametric OscillatorTakeshi Ikegami*, Sergey SLYUSAREV, Shin-ichi OHSHIMA and Eiichi SAKUMA National Research Laboratory of Metrology, 1-1~4, Umezono, Tsukuba, Ibaraki 305, Japan
(Received January 8, 1996; accepted for publication February 19, 1996)
Long-term operation of a cw doubly resonant optical parametric oscillator was demonstrated. The minimum threshold was 9mW and the slope efficiency' was 25%. Using a rigid structure cavity and a stable pump laser, it operated in a single longitudinal signal-idler mode pair for over 3 h without any mode hopping.
KEYWORDS: optical parametric oscillator, stable operation, low threshold
An optical parametric oscillator (0P0) is a highly coherent source of optical radiation tunable over a wide frequency range. For cw operation, a doubly resonant OPO (DRO) is used exclusively because an extremely high-power laser is necessary to pump a singly resonant OPO. However, the cw DRO has not been recognized as a practical light source because of the instability of the single signal-idler mode pair oscillation and the difficulty of frequency tuning. These problems were recently overcome.1”5^ The revival of the DRO led to interesting and unique proposals such as an optical frequency chain.6) For precise and practical applications such as high-resolution spectroscopy, coherent optical communication and frequency chains, the DRO must be stably operated in a single signal-idler mode pair. However, long-term operation of the cw DRO for periods more than 1 h has not been reported.
We demonstrate that the DRO can be operated stably over 3 h without any mode hopping, by adopting a type-II phase-matching configuration,2) a rigid cavity structure and a good pumping laser, even if a monolithic cavity is not used. We believe that this is the longest continuous operation of a cw DRO. In addition, the frequency is tunable over a wide range owing to the fact that a monolithic structure is not used.
The DRO setup is shown in Fig. 1. We used similar physical parameters to those described in ref. 2. A cw Nd:YAG laser (MISER, Lightwave Inc., A = 532 nm, P = 100 mW, spectral linewidth=lOkHz) was used as the pump laser. A 8mm long potassium titanyl phosphate (KTP) nonlinear crystal (HOYA Corp.) with a 3 mm x 3 mm cross section was cut with the same angle as that of the type-II SHG for 1064 nm light. Both sides of the crystal were dual AR-coated for 1064 nm and 532 nm. The input mirror had a 20 mm radius of curvature and was coated for maximum reflection (99.95%) for 1064 nm light. The transmission of the input mirror for 532 nm light was about 94%. The output coupler had a 25 mm radius of curvature and its transmission was 0.7% for 1064 nm light. It was coated for maximum reflection (99.97%) for 532 nm light. To obtain high mechanical stability, the use of mounts including springs was avoided and the cavity spacer was made from a single aluminum block. The spacer was a box 70 mm wide x 70 mm long x 70 mm high with a wall thickness of 8 mm and two holes in two sides for the light path. The KTP crystal was mounted on a Peltier thermoelectric (TE)
"E-mail address: [email protected]
cooler, and a temperature sensor was attached to it to form a single assembly. This crystal assembly was then placed on a rotation stage which was glued to the spacer, which allowed us to change the angle of the crystal relative to the incident pump beam. The input mirror and a PZT for cavity tuning, to which the output coupler was attached, were glued to aluminum plates and the plates were tightly bolted to the cavity spacer. After all of the components were placed inside the box, the upper part of the box was closed to eliminate the influence of air turbulence and acoustic noise.
The dependence of the pumping threshold on the incident angle of the pump beam to the crystal is shown in Fig. 2. The crystal was rotated around the z-axis. Thus, the output wavelength could be tuned by about 5 THz (I9nm) around 1064nm because of the change in the phase-matching condition. The minimum threshold was 9mW for a near concentric configulation (the effective cavity length Lcff7) between the two mirrors ~ 44
DRO cavity spacer box
Temp. LoopController Filter
Fig. 1. Experimental setup. L1,L2: Lenses for mode matching, A/2: half-waveplate, Ml: input mirror, M2: output mirror, TE:thermoelectric cooler, PD: photodiode, A: amplifier.
0 2 4 6 8 10Incident Angle (deg)
Fig. 2. OPO pumping threshold dependence on the incident angle of the pump beam to the crystal.
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We turned off the DRO here.
D. = 1.0
0 5000 10000 15000Time (s)
Fig. 3. OPO output intensity under the intensity-locking condition.
mm, where Leff = lxfn + l2, h is the crystal length, l2 is the length of the air space between the two mirrors and n is the refractive index of the crystal). We found that the DRO threshold had a slight dependence on Lcff. Also, more stable outputs were obtained with shorter cavity configuration and the Lcff was set to 39mm. In this configuration, the minimum threshold was slightly increased to 15 mW. The slope efficiency was about 25%. The spectral linewidth of the beat signal between the signal and the idler was about 5 kHz which shows that it is not influenced much by cavity vibration even if a monolithic cavity is not used.
When the cavity length was scanned using the PZT, OPO oscillation occured at discrete Le{{ at intervals of ALeff = 5.6 nm because the refractive indices for the signal and the idler are different for type-II phase matching, and they must resonate simultaneously.2^ Because stable single-mode operation was obtained for only several minutes under the free-running condition, we used a PZT intensity servo to lock the DRO to a side of the
resonance mode and obtained a stable cw output with a residual short-term intensity noise of 0.5% rms. Under the intensity-locked condition, stable outputs without any mode hopping were obtained for longer Than 3 h, as shown in Fig. 3.
We demonstrated that a cw DRO can be operated stably for 3 h by adopting a semi-monolithic cavity and a good pumping laser and that the frequency of the DRO was tunable over a range of 4 THz. Because the visible optical power 20-30 mW and the spectral linewidth less than 100 kHz necessary to pump the cw DRO are easily supplied by recent external cavity diode lasers, the DRO can be widely used as a practical and tunable light source in the near IR region, for which there are no widely tunable lasers such as Thsapphire lasers.
The authors thank K. Sato and H. Toratani of HOYA Corporation for providing us with excellent-quality KTP crystals.
1) J. Opt. Soc. Am. B 10 (1993) No.9, the special issue on OPO and OPA.
2) D. Lee and N. C. Wong: J. Opt. Soc. Am. B 10 (1993) 1659.3) T. R. Stevenson, F. G. Colville, M. H. Dunn and M. J. Padgett:
Opt. Lett. 20 (1995) 722.4) G. Breitenbach, S. Schiller and J. Mlynek: J. Opt. Soc. Am. B
12 (1995) 2095.5) M. Scheudt, B. Beier, R. Knappe, K. J. Boiler and R. Wallen
stein: J. Opt. Soc. Am. B 12 (1995) 2087.6) N. C. Wong: Opt. Lett. 17 (1992) 1155.7) A. Ashkin, G. D. Boyd and J. M. Dziedzic: IEEE J. Quantum.
Electron. 2 (1966) 109.8) L. Ricci, M. Weidmxiller, T. Esslinger, A. Hemmerich, C. Zim-
mermann, V. Vuletic, W. Konig and T. W. Hansch: Opt. Com- mun. 117 (1995) 541.
T. IKECAMI et al.
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Short Note
Influence of Electrochromic Damage of a KT1OPO4 Crystal on a Phase-Locked CW Optical Parametric OscillatorSergey SLYUSAREV*, Takeshi Ikegami, Shin-ichi OHSHIMA and Eiichi SAKUMA National Research Laboratory of Metrology, l-l-f, Umezono, Tsukuba, Ibaraki 305, Japan
(Received January 19, 1996; accepted for publication March 25, 1996)
The influence of electrochromic damage on a phase-locked cw doubly resonant optical parametric oscillator (OPO) was examined for KTi0P04(KTP) crystals grown by three different methods. The damage occurred at a relatively low E-field of ~3 V/mm. Additional thermal feedback control was effective to reduce the E-field necessary for the phase locking for long-term operation.
KEYWORDS: cw optical parametric oscillator, optical phase locking, electrochromic damage, thermal feedback control
An optical parametric oscillator (OPO) is a highly coherent and widely tunable source of optical radiation. For cw operation, a doubly resonant OPO (DRO) is exclusively used because a laser of extremely high power is necessary for the pumping of a singly resonant OPO. However, the cw DRO has not been considered as a practical light source because of the instability of oscillation and the difficulty of frequency tuning. These problems were recently overcome.1) For precise uses such as in high-resolution spectroscopy, coherent optical communication and the frequency chain,^ the DRO must operate stably in a single signal-idler mode pair and the oscillation frequency of the signal and idler must be precisely known. For this purpose, the difference frequency between the signal and the idler was successfully phase locked.7)
As a nonlinear optical crystal, potassium titanyl phosphate (KTi0P04=KTP) is very popular due to its large nonlinear optical constant, nonhygroscopicity, transparency, nearly noncritical phase matchability for SHG with a Nd:YAG laser, and ease of treatment.3) However, electrochromic damage has been observed in KTP crystals.4) This is a phenomenon in which black lines run along the E-field in a crystal when an external E-field is applied to the crystal. This is considered to be due to the high ionic conductivity of the KTP crystal.
For practical applications including the frequency chain, long-term operation of the cw DRO up to days is necessary. Therefore, avoidance of the electrochromic damage is essential. However, the influence of electrochromic damage on practical systems such as the DRO has not been examined yet.
We examined the effect of electrochromic damage on the operation of a cw phase-locked DRO. Based on these results, an additional thermal feedback system was
adopted and we found that it could effectively reduce thecontrol E-field applied to the crystal.
The experimental setup is shown in Fig. 1. The details of our DRO are given in ref. 5 and we describe it briefly. A cw Nd:YAG laser (MISER, LightWave Inc., A = 532 nm, P = 100 mW, spectral linewidth=10 kHz) was adopted as a pump laser. As a nonlinear crystal, a 3 mm x 3 mm x 8 mm KTP crystal was cut at the same angle as that of the type-II SHG of 1064 nm. We examined three types of KTP crystal, a flux-grown one (FLUX), a hydrothermal one (HT), and a uniformly flux-grown one
'E-mail address: [email protected]
(U-FLUX).6) Both sides of each crystal were AR-coated for 1064nm and 532 nm. To tune the beat frequency between a signal and an idler making use of the EO effect of the crystal, the crystal’s z-faces were coated with gold.
Because the stable single mode oscillation was obtained only for several minutes in the free running condition, we controlled the cavity length with PZT to lock the DRO power to one side of the resonance mode and obtained a stable cw output with a residual intensity noise of 0.5% rms.
Because direct application of a high dc E-field to the crystal might damage the crystal irreversibly, we first measured the small signal frequency response of the E- field tuning coefficient of the signal-idler beat frequency for each crystal. As shown in Fig. 2, the E-field tuning coefficient for FLUX was significantly lower than that of HT and U-FLUX. The E-field tuning coefficient decreased as the frequency decreased and the EO effect was completely damped for the dc E-field. Thus, phase locking of the DRO utilizing the EO effect is impossible for FLUX.
Next, we applied a dc E-field to the crystal to observe the effect of the damage directly. We observed suppression of the DRO oscillation due to dichroic absorption increases for the pump and the generated light and the black line formation in the crystal for the dc E-fields of 3.7 V/mm and 5.0 V/mm for FLUX and HT, respectively. For U-FLUX, no degradation was observed up to the E-field of ~66V/mm, which was the maximum E- field applied. Because the other two crystals were damaged, we used U-FLUX for the following experiments.
ri SpectrumSynthesizer Analyzer
Light
R-LI A/2L2
OpticalIsolator
Temp. Loop PhaseController -Eill£L Detector
M2 IjPZTYDf
MicrowaveSynthesizer
Signal&Idler
TE
DRO Cavity
miA/2
PBS°)--
PD
LoopFilter
DBM
Fig. 1. Experimental setup. LI, L2: Lens for mode matching, A/2: half-wave plate, Ml: input mirror, M2: output mirror, TE:thermoelectric cooler, A/4: quarter-wave plate, PBS: polarization beam splitter, PD: photodiode, A: amplifier, DBM: double balanced mixer, ATT: attenuator, +: sum amplifier.
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Jpn. J. Appl. Phys. Vol. 35 (1996) Pt. 1, No. 6A
U-FLUX KTP
FLUX KTP
3 500
100000Frequency (Hz)
Fig. 2. Small signal frequency response of the beat frequency EO tuning coefficient. The tuning coefficient is defined as the amplitude of the FM modulation divided by the amplitude of the modulation E-field.
(a) RBW: 1kHz, Sweep: 500ms (b) RBW: 30Hz, Sweep: 100s
5kHz— h 30 Hz
1
A/ yA A aa Ml.20kHz/div 200Hz/div
Fig. 3. Beat spectrum between the signal and the idler, (a) Freerunning condition, (b) Phase-locked condition.
For the phase locking, the beat signal between the signal and the idler (typically in the microwave range) was detected by a high-speed photodetector and downcon- verted to the lower rf frequency range by mixing with a signal from a microwave synthesizer. A typical rf beat- note spectrum of the free running DRO is shown in Fig. 3(a). The spectral linewidth (FWHM) of the beat signal was about 5 kHz. The rf signal was again mixed with a signal from an rf synthesizer with a double balanced mixer (DBM) used as a phase detector. The output from the DBM was integrated and fed back to the E-field fast tuning circuit to control the signal-idler beat frequency and phase. Figure 3(b) shows the beatnote spectrum phase locked at the frequency of 2.6 GHz. Under the phase-locked condition, the observed linewidth was 30 Hz, which was limited by the resolution of the spectrum analyzer used. The following equations hold
us — -(z/p + ^pu), — g(z/p — ^pu), ^pl = z/s — z/j, (1)
where up, us, wx are the frequencies of the pump, the signal and the idler, respectively. Therefore, the frequency fluctuation of the signal and the idler is considered to be about one half of that of the pump because APL is kept constant as shown in Fig. 3(b).
Figure 4, before 550 s, shows the control E-field applied to the crystal for the phase locking. The El-field amplitude of ~1 V/mm had to be applied to the crystal to maintain the phase-locking condition. On the other hand, the control E-field applied to the crystal had to be kept below 3-5 V/mm to avoid the damage for FLUX and HT. Because these are comparable orders of magnitude, the control E-field must be decreased to avoid the damage and to obtain long-term operation. Operation with small control El-field is considered to be favorable even for U-FLUX to avoid possible damage under longer- term operation over several hours. The low-frequency
S. Slyusarev et al.
Temp.Control ON
Time(s)
Fig. 4. Control E-field applied to the crystal for EO tuning for phase locking.
phase error between the demodulated beat note and the rf reference reflects the temperature fluctuation of the crystal, and the temperature fluctuation must be further controlled to decrease the necessary El-field. However, the better temperature control is difficult because the temperature fluctuation of our system is already of the order of lOmK. Furthermore, we cannot know the precise temperature of the crystal because the temperature sensor cannot be inserted into the crystal and the output from the sensor is not a direct measure of the crystal temperature. However, the slow component of the error signal of the phase detector is a measure of the crystal temperature fluctuation as mentioned earlier, and this can be used to control the temperature of the crystal. Actually, by applying the attenuated phase detector output signal to the thermal control circuit, we could decrease the control E-field fluctuation to ~ 1/3 of that originally observed as shown in Fig. 4 beyond 550 s.
Thus, the required dynamic range could be obtained using thermal feedback control and the appropriate speed for the phase locking could be obtained using E- field feedback control. This method should be applicable to HT because its frequency response is similar to that of U-FLUX. It should also be applicable to the phase locking with FLUX although the gain for the temperature control must be higher to compensate for the lack of the dc EO response.
We obtained stable phase locking of the beat signal between signal and idler of the DRO. For long-term operation of the DRO, we investigated the influence of elec- trochromic damage. To avoid the damage under El-field application, we utilized additional thermal feedback. Additional thermal feedback was effective to reduce the E- field.
Acknowledgements
The authors thank K. Sato and H. Toratani of HOYA Corporation for providing us with KTP crystals of excellent quality.
1) J. Opt. Soc. Am. B 10 (1993) No. 9, the special issue on OPO and OPA.
2) N. C. Wong: Opt. Lett. 17 (1992) 1155.3) J. D. Bierlein and H. Vanherzeele: J. Opt. Soc. Am. B 6 (1989)
622.4) P. A. Morris, M. K. Craroterd, M. G. Roelofs, J. D. Bierlein
and T. M. Boer: Proc. SPIE 1541 (1991) 104.5) S. Slyusarev, T. Ikegami, S. Ohshima and E. Sakuma: Proc.
SPIE 2379 (1995) 192.6) K. Sato, K. Okada and H. Toratani: presented at Symp. of the
Japanese Spectroscopy Society, 1994.
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Optics Communications 127 (1996)
OmcsCommunications
ELSEVIER
Accuracy of an optical parametric oscillator as an optical frequency divider
Takeshi Ikegami*, Sergey Slyusarev, Shin-ichi Ohshima, Eiichi SakumaNational Research Laboratory of Metrology, 1-1-4, Umezono, Tsukuba-Shi, Ibaraki-Ken 305, Japan
Received 21 December 1995; revised 29 January 1996; accepted 22 February 1996
Abstract
The coherence and the accuracy of a cw optical parametric oscillator (OPO) as an optical frequency divider was measured. The phase coherence between the pump and the signal or the idler was confirmed. The accuracy of the OPO as an optical frequency divider was found to be better than 5 x 10-18.
PACS: 42.65.Cq; 06.90.fvKeywords: Optical parametric oscillator; Optical frequency division; Optical frequency chain; Coherence
1. Introduction
An OPO is a highly coherent and widely tunable source of optical radiation. For the cw operation, a doubly resonant OPO (DRO) is exclusively used because an extremely high power laser is necessary for the pumping of a singly resonant OPO. However, the cw DRO had not been recognized as a practical light source because of the instability of single signal-idler mode pair oscillation and the difficulty of frequency tuning. These problems were recently overcome [ 1]. On the other hand, a new type EO modulator (an optical frequency comb generator; OFCG) makes hundreds of sidebands to light frequency with a frequency interval of a few GHz and a total span of a few THz [2,3]. Based on these advanced techniques, new types of frequency chains, which use the lasers and the nonlinear frequency converters only in the visible and the near IR, were proposed [4-6].
* Corresponding author. E-mail: [email protected]; fax: +81 298 54 4135.
The energy conservation in OPO requires
vp=vs + Vi, (1)
where vp,vs, vt are the frequency of the pump, the signal, and the idler, respectively. On the other hand, the following relation holds:
vs - vi = Av, (2)
where Av is the frequency difference between the signal and the idler. From these equations,
- {(vp + Av), Vi = \{vp - Av). (3)
Although these relations should hold, no direct measurement was carried out. In addition, the coherence property in a DRO was measured once only for a special case of a type-I monolithic DRO configuration [7]. The confirmation of these facts is important for the realization of a frequency chain because the accuracy of most of the optical frequency measurement methods utilizing an optical cavity was found to
0030-4018/96/$l2.00 © 1996 Elsevier Science B.V. All rights reserved PII 50030-4018(96)00141-1
T. lkegami et al./Optics Communications 127 (1996)
Green Phase lockedMISER (pump) OPO
IR'(reference)
PBS
Idler;
DBM
ADConverter
FrequencyCounter
rf Synthesizer
Fig. 1. Experimental setup. MISER: dual wavelength Nd:YAG laser, PBS:polarization beam splitter, A/2: quarter waveplate, M: mirror, HM: half mirror, PD: photodiode, DBM: double balanced mixer, PC: computer.
be limited due to the dispersion effect of the cavity. The experimental confirmation is necessary because a DRO is also a complex system including dispersive media (a crystal and a cavity) and they might possibly give rise to frequency shifts [8].
We examined the performance of a DRO as an element of a phase coherent frequency chain. At first, we confirmed the phase coherence between the pump and the signal (or the idler). The accuracy of the OPO as an optical frequency divider was also investigated.
Time (s)Fig. 2. Beat signal between the signal and the reference down- converted to 1 Hz.
tion was 9 mW at the frequency degeneracy condition (Asignai — Ajdier — 1064 nm). The free running spectral linewidth of the beat note between the signal and the idler was 5 kHz. The beat frequency between the signal and the idler (the Av in Eq. (2)) was phase locked to a signal from a synthesizer [9], which is not shown in the figure. We denote the frequency of this synthesizer as /synthesizer, which was typically 500 MHz. Because type-II phase matching was used, the signal and the idler were orthogonally polarized and could be separated by a polarization beam splitter. The separated signal (idler) was superimposed with the reference beam on a photodetector.
3. Results
2. Experimental setup
The experimental setup is shown in Fig. 1. The details of our DRO are described in Ref. [7] and we give here a brief description. A Nd: YAG laser (a dual wavelength MISER, LightWave Inc.) produced IR light (A = 1064 nm, P = 5 mW, spectral linewidth = 5 kHz) and its second harmonics (532 nm, 100 mW, 10 kHz), which we hereafter call the reference beam and the pump beam, respectively. The pump beam was used to excite the DRO. A KTP was used as a nonlinear crystal. Although conventionally the higher frequency light from the OPO is considered a signal and the other is considered an idler, we call the horizontally polarized light a signal and the vertically polarized one an idler, for definiteness. The pump beam was horizontally polarized for type-II phase matching because the z-axis of the crystal is the vertical direction in our setup. The minimum threshold for cw oscilla-
First of all, we measured the phase coherence between the reference and the signal (or the idler). For this purpose, we downconverted the beat signal between the reference and the signal (or the idler) to 1 Hz by mixing it with a signal from an rf synthesizer shown in the figure. This synthesizer and the first one used to phase-lock the OPO, which was described in the preceding section, were locked to the same frequency standard. The frequency of this synthesizer was set to \/synthesizer + 1 Hz. The acquisition of the 1 Hz beat signal data was made by a computerthrough a AD converter and the power spectral density was calculated. Figs. 2 and 3 show the measured 1 Hz beat signal and its power spectral density, respectively. Because the spectral linewidth was well below 1 Hz, we could confirm that the phase coherence was maintained during the second harmonic and the optical parametric processes and the OPO can be considered a phase coherent frequency divider. The characteristic
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T. Ike garni et al./Optics Communications 127 (1996)
.d 3000
Frequency (Hz)Fig. 3. Power spectral density of the downconverted 1 Hz beat signal between the signal and the reference.
Time(s)
Fig. 5. Beat signal between the signal and the reference down- converted 0 Hz. Initially, the beat frequency was set to 1 Hz for reference.
H 15
£ -10
-IS
Fig. 4. Measured beat frequencies between the signal and the reference (V) and between the idler and the reference (H). Vertical axis is the measured beat frequency minus 10 MHz. Each interval correspond to about 500 s of integration time.
of the beat signal between the reference and the idler was similar to the one between the reference and the signal although it is not shown.
Because the phase coherence was confirmed, we next measured the frequency between vs (p,) and Reference directly with a frequency counter to estimate the accuracy of the OPO as a frequency divider, where freference is the frequency of the reference beam. The frequency of the synthesizer was set to \ /synthesizer+10 MHz so as to be measured easily with the frequency counter. Fig. 4 shows the measured beat frequency minus 10 MHz, and this value should be zero if the OPO is a perfect optical frequency divider. The signal and the idler was switched alternately, and H and V mean the beat frequency measurement intervals for Fv ^reference and V[ ^reference* respectively. Each interval corresponds to about 500 s of integration time. We calculated the total average for all H-intervals and V-intervals, respectively, and found it to be
10 11 12 13 14 IS 16
^reference Fr 2 /synthesizer
= —0.1 ± 1.5 (mHz),
^reference — vi ~ ^-/synthesizer
= —0.3 ± 1.6 (mHz).
(4)
(5)
Simulated data
Normalized voltage
Fig. 6. The distribution of the voltage of the 0 Hz beat signal between the signal and the reference. The measured data are shown in the bold solid line with square symbols and the simulated data are shown in the thin solid line.
We can conclude that the result was consistent with zero within the experimental error of 1.5 mHz. The accuracy of the OPO as a frequency divider, which means the accuracy of Eq. (3), was confirmed to be better than 1.5 mHz/pio64nm = 5 x 10-18, if we assume 2^reference = vp in the second harmonic process. This result can also be interpreted as the accuracy of the combined processes of the second harmonic generation and the optical parametric oscillator.
4. Summary and discussion
Our results are summarized as follows.(i) The phase coherence of the type-II KTP OPO
was confirmed.(ii) The accuracy of the OPO as an optical fre
quency divider was estimated to be better than 5 x 10-18.
Finally, we discuss the origin of the noise in the beat signal in Fig. 2, which limited the accuracy of our measurement. To see the origin of the noise, we
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T. Ikegami et al./Optics Communications 127 (1996)
observed the phase fluctuation directly by setting the frequency of the synthesizer to 5/synthesizer and the beat frequency to 0 Hz, which is shown in Fig. 5. In this case, A shup = A sin(preference -^signal) was observed, where A, preference and psignai are the amplitude of the beat signal, the phase of the reference light and the phase of the signal light, respectively. The beat frequency was switched from 1 Hz to 0 Hz around time of 15 s for reference. To know the characteristics of this phase fluctuation, we plotted the distribution of the voltage of the 0 Hz beat signal, as shown in Fig. 6. In the horizontal axis, the maximum observed voltage is normalized to unity. Simulated data is also plotted assuming random phase fluctuation and 5% intensity fluctuation, corresponding to the measured data. The simulation results agree with the measured data, although there is some variance in the data due to insufficient measurement time.
Because the path length difference between the reference beam and the signal beam was about 2 m in our setup, a 277 phase difference is caused by the change of the refractive index of 5 x 10-7, which is easily caused by air turbulence in a room [11]. The observed random phase fluctuation is considered to be due to the refractive index change caused by the air turbulence. This can be eliminated by evacuating the system or letting the two light paths coincide. Further improvement of the accuracy is possible because there is no reason to limit the reduction of the phase fluctuation physically.
Although our experiment was done close to the frequency degeneracy, the behavior of the OPO may be different for the region far from the frequency degeneracy and the confirmation in this region is also necessary. For this purpose, the frequency difference between the signal and the idler can be expanded by using a higher speed photodetector or an OFCG. In addition, the accuracy of the OFCG, which is another important component for a frequency chain, has been checked with a similar technique [ 10].
References
[1] J. Opt. Soc. Am. B 10, No. 9 (1993), special issue on OPO and OPA.
[2] M. Kourogi, K. Nakagawaand M. Ohtsu, IEEE. J. Quantum Electron. 29 (1993) 2693.
[3] L.R. Brothers, D. Lee and N.C. Wong, Optics Lett. 19 (1994) 245.
[4] H.R. Telle, D. Meschede and T.W. Hansch, Optics Lett. 15 (1990) 532.
[5] K. Nakagawa, K. Kourogi and M. Ohtsu, Appl. Phys. B 57 (1993) 425.
[6] N.C. Wong, Optics Lett. 17 (1992) 1155.[7] C D. Nabors, S.T. Yang, T. Day and R.L. Byer, J. Opt. Soc.
Am. B 7 (1990) 815.[8] R. Wynands, O. Coste, C. Rempe and D. Meschede, Optics
Lett. 20 (1995) 1095.[9] S. Slyusarev, T. Ikegami, S. Ohshima and E. Sakuma, Proc.
SPIE., SPIE., USA, 2379 (1995) 192.[10] T. Ikegami, S. Slyusarev, S. Ohshima and E. Sakuma, in:
Proc. 5th Symp. on Frequency Standards and Metrology (Woods Hole, October 1995), ed. J.C. Beigquist (World Scientific, Singapore), to be published.
[11] K.P. Birch and M.J. Downs, Metrologia 30 (1993) 155.
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15 February 1997
j
ELSEVIER Optics Communications 135(1997)
OpticsCommunications
Frequency measurement of accurate sidebands of an optical frequency comb generator
Sergey Slyusarev \ Takeshi Ikegami, Shin-ichi Ohshima, Eiichi SakumaNational Research Laboratory of Metrology, 1-1-4, Umezono, Tsukuba-shi, lbaraki-ken 305, Japan
Received 19 March 1996; revised version received 25 September 1996; accepted 16 October 1996
Abstract
The accuracy of the optical sideband generation of an optical frequency comb generator (OFCG) was examined. The fundamental light and its second harmonics from a YAG laser were used as input to the OFCG and as pump light to an optical parametric oscillator (OPO), respectively. The frequency between the + 15th order OFCG sideband and the OPO signal light was compared with that between the — 15th order sideband and the idler. The frequency shift of the generated sideband was estimated to be smaller than 6 X 10“13 at a frequency span of 276 GHz.
PACS: 42.60.Fc; 06.90. + vKeywords: Optical frequency comb generator, Optical frequency chain; Coherence
1. Introduction
An OFCG is known as a modulator that can produce a wideband optical frequency comb of equally spaced modes from a cw coherent light source [1-3]. Its frequency span approaches 10 THz with a modulation frequency of a few GHz. In addition, a cw OPO has become a practical light source owing to the recent improvement of nonlinear optical crystals [4-6]. Based on these new techniques, a number of novel schemes for building a completely solid state and compact optical frequency chain have been proposed [7-9]. It is assumed that the following equations hold precisely for the OPO and the OFCG^pump ” ^signal ~P ^ idler » CO
^carrier mvrnoi * (2)
where and are the frequencies of thepump light, the signal, the idler of the OPO; vm, vcarrier and vmod are the frequencies of the m-th sideband, the carrier light and the microwave modulation of the OFCG, respec-
1 Fax: +81 298 54 4135; E-mail: [email protected].
lively; m is the order of the sideband of the OFCG. Recently, the frequency ratio between the fundamental and its second harmonic optical frequency was checked by measuring the frequency difference between the second harmonics of two diode lasers and comparing it with the frequency difference of the fundamentals [10]. For the OPO, the phase coherence between the pump and the signal and the accuracy of Eq. (1) were confirmed close to the frequency degeneracy [11]. However, no experiments have treated the accuracy of the OFCG sideband generation yet. Experimental confirmation is necessary because an OFCG is also a complex system including dispersive media (a crystal and a cavity). The frequency-dependent phase-matching condition on sideband generation and a drift of the cavity length can lead to frequency shifts. Each OFCG sideband frequency must match the resonant frequency of the cavity to produce a number of sidebands effectively. However, due to the large dispersion in the LNb03 crystal used in the OFCG, the frequency mismatch between z/carTier + mumod and the corresponding cavity resonance can be about 5 MHz at a frequency span of 1 THz. This might cause deformation of the spectrum of the modulated light and apparent frequency shift. If such a
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S. Slyusarev et al./ Optics Communications 135 (1997)
OFCG
V-mJr' A V-n
<r
OPOVo
Vidler
1/2
Vcarner (DW MISER) Vpump
) ^ Vmod
V'+m "F ^ V-+//I
V ->
Vstgnal
1 1/2 Vj>k ^
k-
-<£
Fig. 1. Diagram showing the use of the OPO for checking of accuracy of OFCG side band generation.
dispersion dependent frequency shift exists, the frequency of the sideband will be
v . _ = (3)where v+m and v_m are the frequencies of +m-th and — m-th order sidebands, respectively, and Ai/±m is the term representing the dispersion dependent frequency shift as shown in Fig. 1. This can be checked by using the OPO assuming that it is a perfect optical frequency divider [11]. If we use the second harmonic of ncarTkr as the pump light for the OPO, ppump = 2 pcaiTicr. Defining the adjustable frequency difference between the signal and the idler as
^signal ^idler Using Eq. (l), Signal, idler ^carrier
± If we fix the difference frequency between vidler and the closest OFCG sideband (v_m) by phase locking to a reference frequency (v0), we obtain:^0 ^idler (^carrier ^ ^mod ^ Hn) ’
V = ( ^carrier ~^* m ^mod ^ m) ^signal > (^)
6-lock
GreenPump
; IR carrierIdler
+m-th comb
m-th comb
DBM
DBM
MISER
OFCG
rf Synthesizer
rf Synthesizer
Filter
OPO
FrequencyCounter
Fig. 2. Experimental setup. MISER: dual wavelength Nd:YAG laser, PBS: polarization beam splitter, 1/2: quarter waveplate, M: mirror, HM: half mirror, PD: photodiode, DBM: double balanced mixer.
where v is the beat frequency between u+m and i/si By measuring v and comparing with v0, we can evaluate the sum of Av_m and Ai/+m :A^= U0-U= Ridler + ^signal-2vcarTicr + Al/_m+Al/+m
= Ai/_m + Ai/+m. (5)
2. Experimental setup
The experimental setup is shown in Fig. 2. For the OFCG, we adopted a microstripline structure [2]. The substrate was z-cut LiNb03 crystal with a length of 25 mm along the propagation direction of the light, width of 8 mm and thickness of 0.5 mm. The matched modulation frequency (vmod) was 9.2 GHz for a microstripline width of 2.5 mm. The substrate was placed inside an optical cavity, whose radius of curvature and reflectivity was 250 mm and 99.2%, respectively. The optical cavity length was set to 67 mm and the FSR was equal to |i/mod. About 4.5 W of the microwave power was coupled into the OFCG. The IR light from a YAG laser (DW MISER, Light-Wave Inc., P532nm = 100 mW, Pl064nm = 5 mW, Av532nm= 10 kHz) was used as carrier light for the OFCG. The cavity was servo locked to the YAG laser with the lock point at the trough of the FM line shape.
We used a three-element type doubly resonant OPO (DRO). Potassium titanyl phosphate (KTP) was used as a nonlinear optical crystal with type-II phase matching configuration [6], The green light from the YAG laser was used to pump the DRO. The orthogonally polarized signal and idler from the DRO were separated with a polarization beam splitter. The DRO was initially angle tuned so that the signal and the idler were near the + 15th and the — 15th order sidebands of the modulated IR respectively, giving a signal-to-noise ratio of at least 40 dB, so that the counter will not miss zero crossings of the electric signal. The lower-frequency idler beam and — 15th order comb sideband were recombined and detected with a high-speed photodetector. The typical beat frequency was ~ 1.5 GHz. The heterodyne beat was amplified and down-converted to a low frequency by a double-balanced mixer and a synthesizer. The resultant beat signal was amplified, bandpass filtered, and phase locked to a reference frequency (v0) by means of the electro-optic effect of the DRO. Under phase-locked conditions, the beat signal between the signal and the + 15th order comb sideband was detected by another high speed photodetector.
3. Results and discussions
Fig. 3 shows the downconverted beat-note spectrum with a signal-to-noise ratio greater than 40 dB for a resolution bandwidth of 100 Hz. The two sidebands at ±2 cHz are due to dithering of the cavity mirror for servo
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S. Slyusareu et al./ Optics Communications 135 (1997)
locking the modulator cavity. All reference frequencies used in the experiment are derived from a Cs atomic clock. The downconverted beat signal (u) between the DRO signal and the + 15th comb sideband was measured directly by a frequency counter (HP 53131 A), while the frequency between the idler and the — 15th comb sideband (vq) was fixed by the phase locking loop. Thus, the difference between these frequencies provides us with the hypothetical frequency shift (A y) in Eq. (5), if it exists. We chose frequencies of all radio-frequency synthesizers so that the measurement beat signal was in the operational range of the bandpass amplifier. The gate time of the frequency counter was chosen to be two seconds in order to obtain the maximum measurement resolution, which determines the number of digits displayed. The integration time for each data set was about 500 s. Fig. 4 shows the measured beat frequency centered at v0. We also undertook statistical analysis of each data set. Our data appeared to be normally distributed and the resultant average values were calculated to be Av = 0.15 mHz with standard deviation of 0.9 mHz. In other words we obtain an upper limit for frequency shifts:
Lv/v< 0.9 mHz/1.5 GHz = 6X 10"13. (6)
The experimental uncertainty is assumed to be caused by the measurement error of the counter, which is composed of random factors including short-term timebase stability, trigger noise and signal-to-noise ratio of the measurement signal. The reference frequency of the counter was 10 MHz output from the Cs atomic clock and thus systematic uncertainty, which depends on time base error, was substantially reduced. Under this condition the counter displays results within an error of + 0.4 mHz to within 1 — sigma confidence with 2 s gate time. Another source
lOdB/div
lkHz/divFig. 3. Downconverted signal and the + m-th comb side band beat-note spectrum (vertical scale, 10 dB/division; horizontal scale, 1 kHz/division; center frequency, 50 MHz; resolution bandwidth, 100 Hz; sweep time, 3 s).
N
E><
43210
-1•2-3-4
1 1 -r.
___ i___ i__—i___ i____i___ i___ i-----1 2 3 4 5 6 7 8
IntervalFig. 4. Measured difference frequency between (v+15 - vsignal) and v0. Each interval corresponds to an integration time of about 500 s.
of experimental error is considered to come from the random phase fluctuation caused by the change of the refractive index by air turbulence [11].
4. Conclusion
The frequency shift of the generated sidebands was measured to be smaller than 0.9 mHz at a frequency span of 276 GHz assuming the applicability of Eq. (l) to the OPO. However, if we assume Eq. (2) instead of Eq. (1), this result is inversely interpreted to represent the accuracy of the OPO at a large frequency span. Although the accuracy of the OPO frequency division was evaluated close to the frequency degeneracy condition (/signai -/d!er ~ 500 MHz) [11], we can consider that it is accurate to 0.9 mHz / /1064nm = 3 X 10™18 up to a frequency of 276 GHz.
Because we have mainly focused on the dispersion dependent frequency shift, our experiment is sensitive to A%/+m and Ay_m of the same sign. Especially, for equal magnitude of Az/+#n and Av_m with opposite signs, our experiment is completely insensitive. We can check this possible symmetric shift by measuring fsignal — Av+m, /idler - A v_ m, /idlcr - A v_ m _, simultaneously. Although the frequency span was limited to 276 GHz due to the low performance of our OFCG, it can be increased up to ~ 5 THz in principle by improving and optimizing our OFCG [1,2].
References
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S. Slyusarev et al./ Optics Communications 135 (1997)
[7] K. Nakagawa, M. Kourogi and M. Ohtsu, Appl. Phys. B 57 [10] R. Wynands, O. Coste, C. Rembe and D. Meschede, Optics(1993) 425. Lett. 20 (1995) 1095.
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