Frequency and intensity stabilization of a high outputpower, internal mirror He-Ne laser usinginterferometry

Tsutomu Araki, Yu Nakajima, and Norihito Suzuki

The output amplitude and frequency of a conventional internal mirror He-Ne laser have been stabilized usinga new approach. For this scheme, the polarization properties of three axial-mode spectra are examined.Differences in intensities between the side mode spectra are detected using a newly designed Michelsoninterferometer. The resultant differential signal is used to maintain the central mode spectrum at maximumgain. The cavity length of the laser is thermally controlled with a drift-compensated interferometer usingdifferential photodetection feedback. The result is an intense and stabilized single-mode output laser.Excellent amplitude (0.1%) and frequency (2 parts in 109) stability are achieved.

1. Introduction

There is considerable need for frequency- and am-plitude-stabilized single-mode lasers. The polariza-tion properties of a multimode internal mirror(MMIM) laser can be used to control such stabiliza-tion. The longitudinal oscillation modes of MMIMlasers are linearly polarized, while adjacent modes arepolarized at right angles to each other.' This is due toreflection anisotropy of the laser mirrors.2 Generallysuch lasers are stabilized by operating at two axialmodes; the cavity length is then controlled to maintaina constant difference (or ratio) between the two ampli-tudes. Several stabilized laser systems have been de-signed3-6 using this concept. Unfortunately, the out-put spectrum is weak since operation is at the tail of thelaser gain curve.

Our goal was to stabilize both the laser frequencyand its amplitude without power loss, by forcing thelaser to operate at the center of the gain curve. Signalmodulation techniques can be used to tune the laserfrequency with maximum output.7 However, themodulated output is generally inconvenient for practi-cal use. To avoid these problems, a new feedbacksystem incorporating a Michelson interferometer,

Yu Nakajima is with Nakajima Spectroscopy Company, Ltd.,Fushimi, Kyoto 612, Japan; the other authors are with University ofTokushima, Department of Mechanical Engineering, Tokushima770, Japan.

Received 20 July 1988.0003-6935/89/081525-04$02.00/0.© 1989 Optical Society of America.

photodetection circuits, and a cavity length controllerwas designed. A conventional MMIM He-Ne laser,operated at three axial modes, was used. The designand performance of the new laser system are described.

II. Principle

The oscillation gain bandwidth (FWHM) of a He-Ne laser is -1 GHz. Therefore, a 30-cm cavity laserwith a mode-to-mode frequency difference of 0.5 GHzis adequate. As shown in Fig. 1, the gain curve of thislaser can contain three modes (a central mode spec-trum M2 , and two side mode spectra M1 and M3 ) whichare orthogonally polarized with respect to each other.M2 provides the main laser output, while M1 and M3are used to control the position of M2 on the laser gaincurve. Because the frequency difference between theadjacent modes is constant, the frequency of M2 can bemaintained at the center of the gain curve if the spec-tral intensities of M1 and M3 are kept equal. Frequen-cy stabilization of M2, with maximum amplitude, isachieved when the gain curve is symmetric (asymme-try results in a reduction of output power).

A Michelson interferometer (MIF) is employed hereto determine the differences in spectral intensities be-tween M1 and M3. When the three-mode laser beam isapplied to the MIF, interferometric output fringes,IF1 (X), IF2(X) and IF3(X), are formed as:

IF,(X) = I 1 + cos2ir f-) X],

IF2(X) = I2 1 + cos27r f X

IF3(X) = I3 [1 + cos27r + 2) ]

(1)

15 April 1989 / Vol. 28, No. 8 / APPLIED OPTICS 1525

* Polarization

I

IFrequency

Fig. 1. Schematic diagram of the output power profile of a three-axial mode laser. Three spectra, Ml, M2 , and M3, separated by 0.5GHz, are illustrated. The polarization direction of the adjacent

modes are at right angles with respect to each other.

where I, I2, and I3 are the spectral intensities of M1 ,M2, and M3; f is the oscillation frequency of M2; L is thecavity length of the laser; c is the speed of light; and Xis the interferometric path length. The polarization ofMl and M3 is perpendicular to that of M2. ThereforeIF2(X) can be isolated from IF1(X) and IF3(X) using apolarizer. After separation of IF2(X), F1(X) andIF 3 (X) interfere with each other to form IF(X) suchthat

IF(X) = IF1(X) + IF3(X)

=I +I) [1 + Cos2fX) Cos(7 X)

I

LL

II> 13

I = I3

11< 13

-+ XFig. 2. Profiles of the interferogram IF(X) for I, = I3 and I, F I3.

The interferometer system is shown in Fig. 3.

P

PZT StageLaser Tube

+ (I, - I3) sin(2f X) sin(L ). (2)

If the interferometric path length is set to be close toone-half of the cavity length (X - 2/L), one obtains

IF(X) = (I, + I3) - (I -I3) sin(-X) * (3)c

Equation (3) indicates that the interferometric fringeis observed when I, 5, 13; the fringe disappears when I,= I3. This is shown in Fig. 2.

When the fringe disappears (e.g., I, = I3), the spec-tral intensity of Ml must be equal to that of M3.Therefore spectrum M 2 is forced to operate at maxi-mum gain. Thus, a frequency- and amplitude-stabi-lized laser can be realized via control of its cavitylength by continually adjusting the fringe of IF(X) = 0.

111. Experimental

A schematic diagram of the resultant laser system isshown in Fig. 3. The back beam of the laser (contain-ing modes Ml, M2 , and M3) is incident on the MIFwhich is fabricated from two roof mirrors, RI and R2.The path length of the MIF is set to 15 cm (one-half ofthe laser cavity length (=30 cm)). This length isprecisely controllable with a piezoelectric stage (PZTstage) on which R2 is mounted. The corner angles ofR1 and R2 are slightly different (,0.1°) from the rightangle, so that there is a tilt between the interferometricbeams, resulting in the formation of 2-D interferomet-ric fringes. As previously noted, IF2(X) and IF(X) areseparated using a polarization beam splitter (PBS).The image of IF(X) is focused on photodetectors PD1and PD 2 , while that of I1l2(X) is focused on a slit (S).

Cavity- LengthControl

Circuit

PD2

Fig. 3. Schematic diagram of the frequency and amplitude stabili-zation system for an internal mirror He-Ne laser: P, polarizer; H,heater coiled around the laser cavity; RI and R2, roof mirrors; HM,half-mirror; PBS, polarization beam splitter; PD1-PD3, photodi-odes; LI and L 2, lenses (f = 3 cm); S, slit; PZT, piezoelectric movingstage. The polarization direction of the light beam is indicated by e

and .

I1= 1I3

I1 I3

Fig. 4. Representation of the interferometric fringes focused onphotodiodes PD, and PD2. When I, = I3, the fringe pattern disap-

pears and a flat halftone image is observed.

PD3 is used to monitor the brightness of the fringe line.The pitch of the interferometric fringe is determinedby the tilt between the interferometric beams and bythe lens magnification (lOX). They are fixed for these

1526 APPLIED OPTICS / Vol. 28, No. 8 / 15 April 1989

Ikk 10k +15v

Fig. 5. Cavity length control circuit: PD1 andPD2 , photodiodes; Al-A 5 , low drift operational am-

plifiers; H, heater for the laser cavity.

illklll, #, ,. I ,,, i i, i ; ! , ! i I

1 . I I I . I. j . ., I . I. ., . , ,, j i

i I . . '

10

. i I . II I :

i !

I I .

20 30

i !

40 50 6 0 min

-- ___- ___ !.1..............Jir1 l 2

Fig. 6. Amplitude fluctuations of the stabilized

4 6

Timelaser output during the first hour

operation.

8 10 hrs

(upper trace), and the next 10 h (lower trace) of laser

II

Hi II

10 minFig. 7. Amplitude fluctuations of the stabilized laser with a magnified scale (20X). The laser was first preheated for 1 h.

experiments, while the fringe phase is established bythe instantaneous interferometer length X. To detectthe disappearance of fringe IF(X), PD1 and PD9 areaccurately positioned on the bright and dark lines,respectively, as shown in Fig. 4. With the disappear-ance of fringe formation, PD1 and PD2 generate identi-cal photocurrents. For long-term stabilization, anydrift in X requires compensation. For this, the inten-sity of the bright line of IF2(X) is monitored with PD3;the PZT stage is then controlled to keep the outputsignal of PD3 constant.

Figure 5 shows the electric circuit designed to con-trol the cavity length. Thermal control is achieved viathe heater coil located around the laser tube. Theoutput difference between PD1 and PD9 is detectedwith a differential amplifier (composed of operationalamplifiers A-A 5). The differential output signal isused as a bias supply for the heater coil driver. Thecavity length is adjusted thermally until PD and PD,supply the same photocurrent.

15 April 1989 / Vol. 28, No. 8 / APPLIED OPTICS 1527

100

50 F

O LiC

4>

4

100 r

50 -

0 L

6-. - -[-J _1 -- ,1 H t 1 1 -

6 E .1 - ]---V I:4- _ '

i l :::: ; : l l . . l.

l O0k

I I!i; : :!

I .

. p : I ; iI ! :

I I I

II ; ,i: I , : I.;; :!i1

i.II ;

! .I..

i; Ii Ii i

!I.. II. .III�:,I

I: .!.1I,: ,i:I .

i

I

� i

�JIiI

iI

I; iiii i;

i

I Ii

I

PP

PD

LSoectrumAnalyzer

Fig. 8. Measuring system for determining laser frequency stabil-ity: P polarizer; PD, pin photodiode; HM, half-mirror; f, outputfrequency of the test laser; fir output frequency of the standard laser

(Zeeman stabilized laser); fb, beat frequency due to f and fs.

22

t

. 21

- 20U-

C 19

0

-

0g * 0****

000 0 0.00 0000

0 0 0 0 00 000 0 000. * 00

0 00

*000 0 0 0

10 20 30 40Time (min)

50 60

Fig. 9. Frequency fluctuations of the stabilized laser.

IV. Results

A. Amplitude Stability

Amplitude stability was checked by measuring theintensity of the front beam, M2 , as shown in Fig. 6.Periodic amplitude fluctuations are observed for thefirst 25 min of operation due to mode hopping. Fol-lowing the initial warm-up, the laser output intensityremains stabilized for many hours. This stable opera-tion is shown in expanded form (20X) in Fig. 7. Arelative amplitude fluctuation of ±0.1% was calculatedbased on Fig. 7.

B. Frequency Stability

Frequency stability was examined by measuring abeat frequency with respect to a standard laser, asshown in Fig. 8. A frequency-stabilized Zeeman la-ser,8 designed in our laboratory (473612328.0 ± 0.5MHz), was used as the standard laser. Figure 9 shows

changes in beat frequency with time. The resultantbeats were measured every 30 s for 1 h. From this wefound that the scattered range of the beat was 2.1 MHz,so the relative frequency stability was calculated as ±2X 10-9 (2 parts in 109). Note, stabilization of aMMIM He-Ne laser output using a conventional two-mode approach results in amplitude and frequencystabilization of ±0.1% and ±2 X 10-8, respectively. 1 3 5

These compare with ±10% (intensity) and ±10-6 (fre-quency) for a typical unstabilized MMIM He-Ne la-ser.

V. Conclusion

An amplitude and frequency-stabilized single-modelaser has been made using a three-axial mode laser andan intensity interferometer. An internal-mirror He-Ne laser with 30-cm cavity length is suitable for thisuse. By keeping the central mode spectrum at maxi-mum gain using a feedback system, intense emissionwith relative amplitude and frequency stabilities of±0.1% and ±2 X 10-9, respectively, are attained. Al-though the instrumentation is somewhat complicated,the resultant degree of stabilization warrants the ef-fort. The resultant laser is applicable to many preci-sion measurements and experiments.

We express our thanks to K. Domoto of Fujikura,Ltd. and the staff of Nakajima Spectroscopy Co., Ltd.for their help with this research.

References1. S. J. Bennett, R. E. Ward, and D. C. Wilson, "Comments on:

Frequency Stabilization of Internal Mirror He-Ne Lasers," Appl.Opt. 12, 1406 (1973).

2. T. Yoshino, "Reflection Anisotropy of 6328A Laser Mirrors,"Jpn. J. Appl. Phys. 18, 1503 (1979).

3. R. Balhorn, H. Kunzmann, and F. Lebowsky, "Frequency Stabili-zation of Internal-Mirror Helium-Neon Lasers," Appl. Opt. 11,742 (1972).

4. S. K. Gordon and S. F. Jacobs, "Modification of InexpensiveMultimode Lasers to Produce a Stabilized Single FrequencyBeam," Appl. Opt. 13, 231 (1974).

5. T. Yoshino, "Frequency Stabilization of Internal-Mirror He-Ne(X = 633 nm) Lasers Using the Polarization Properties," Jpn. J.Appl. Phys. 19, 2181 (1980).

6. A. Sasaki and T. Hayashi, "Amplitude and Frequency Stabiliza-tion of an Internal-Mirror He-Ne Laser," Jpn. J. Appl. Phys. 21,1455 (1982).

7. W. R. C. Rowley and D. C. Wilson, "Wavelength Stabilization ofan Optical Maser," Nature London 200, 745 (1963).

8. T. Araki, Y. Nakajima, and N. Suzuki, "Measurement of theRelationship Between Output Frequency and Zeeman Beat Fre-quency of Transverse Zeeman He-Ne Lasers," IEEE-QE (1989),submitted for publication.

0

1528 APPLIED OPTICS / Vol. 28, No. 8 / 15 April 1989

t s