Baseline noise reduction in laser pulse trainsJames M. Thorne, Thomas R. Loree, and Gene H. McCall Citation: Applied Physics Letters 22, 259 (1973); doi: 10.1063/1.1654631 View online: http://dx.doi.org/10.1063/1.1654631 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/22/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Laser noise reduction in air Appl. Phys. Lett. 88, 251112 (2006); 10.1063/1.2216402 Development of linebased noise pollution reduction programs J. Acoust. Soc. Am. 108, 2524 (2000); 10.1121/1.4743336 Gated baseline restorer without ‘‘droop’’ Rev. Sci. Instrum. 58, 1104 (1987); 10.1063/1.1139613 TimeSharing Modulation at 200 kc Applied to Broad and Narrow Line NMR for BaseLine Stability Rev. Sci. Instrum. 36, 1495 (1965); 10.1063/1.1719366 Reduction of Baseline Shift in PulseAmplitude Measurements Rev. Sci. Instrum. 32, 1057 (1961); 10.1063/1.1717615

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Base-line noise reduction in laser pulse trains*

James M. Thornet , Thomas R. Loree, and Gene H. McCall University of California. Los Alamos Scientific Laboratory. Los Alamos. New Mexico 87544 (Received 6 November 1972)

The intensity-dependent rotation of elliptically polarized light by carbon disulfide is used to discriminate against low-intensity laser light and pass high-intensity pulses through crossed polarizers. The transmitted intensity is shown to be proportional to the third power of the incident intensity. This technique has been used to reduce base-line noise and low-intensity pulses in the output pulse train of a mode-locked Nd: YAG oscillator.

The short pulses required for some laser fusion experi ~ ments must have a peak-to-base~line intenSity ratio at the oscillator output of at least 106 to avoid premature ionization of the target. It is extremely difficult to pro~ duce such a pulse with a mode~locked oscillator, and it would be useful to have a well ~characterized device with a picosecond response time which could be used to im ~ prove the peak-to-base~line intensity ratio. We have constructed such a device, utilizing the phenomenon of intensity~dependent rotation of the plane of polarization in an optically nonlinear material.

The early experiments of Maker, Terhune, and Savage l

have shown that the major axis of elliptically polarized light is rotated as the light passes through a material in which it induces an intensity-dependent nonlinear polarization. The induced rotation 13 is given by

13= (rrw/cn)B(E_E! -E.E!)z, (1)

where w is the frequency of the light, n is the refractive index, B is the nonlinear susceptibility of the material, and z is the path length along the light beam. E. and K are the electric field magnitudes of circularly polarized light and are given by

E.= (Ex + iEy)/..f2,

E_= (Ex -iEy)/..f2.

More recent publications2,3 have presented more re­

fined measurements of B, but have left the basiC theory essentially unchanged.

If the intensity-dependent rotation occurs between crossed polarizers, high -intensity light will not be at­tenuated as severely as lOW-intensity light. Figure 1 shows such an optical system. Note that elliptical light is converted to plane-polarized light before striking the second polarizer for maximum attenuation of unrotated light. A similar but intentionally less efficient optical system has been used by Dahlstrom as a paSSive mode~ locking device within a laser cavity. 4 In this letter, we describe the results of placing the optical system of Fig. 1 outside the cavity to suppress base-line noise and to compress pulses in both time and space.

Consider a beam of plane -polarized light incident on a plate with retardance CP. Let the retarder axes be coin­cident with the x and y axes, and the electric vector of the light make an angle 0 measured clockwise from the y axis. The emergent elliptically polarized light can be described in terms of its two Circularly polarized components:

1"'" r'" to. ...... E= ("2V 2)EI1 (E. + ~J sinO -i(E. - E.) cosO exp( -iCP)]. (2)

259 Appl. Phys. Lett., Vol. 22, No.5, 1 March 1973

After this light has passed through the carbon disulfide cell (or other nonlinear optical material), there will be a phase difference 13 between the Circularly polarized components, resulting in

E= tEo{[SR. -i exp( - icp)CK]x+ [iSR~ + exp( - icp)CR.]Y} ,

where S = SinO, C == cosO, and Rz = 1 ± exp( - i(3).

As shown in Fig. 1, this elliptically polarized and ro­tated light next passes through another linear retarder with phase shift cp and orientation - O. Its effect is to

CFj

@

CS2 D ~--¢

CP2

'YL LP2 LSJ

" B FIG. 1. Optical system of the intensity-dependent filter. LPt and LP2 are Glan prisms (crossed), CPt and CP2 are i-wave retardation p\ates, and CS2 is a carbon disulfide cell. Optical element orientations are given in the right-hand column, and the state of light polarization is shown between the elements.

Copyright © 1973 American Institute of Physics 259 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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260 Thorne, Loree, and McCall: Base·line noise reduction in laser pulse trains 260

FIG. 2. Tracing of oscillo­graph of superimposed pulse trains. In each pulse pair the leading pulse is the C~ de­vice output, and the lagging pulse is a sample of the in­put beam which has been optically delayed and attenu­ated. Time scale, 10 nsec/ div (indicated on figure).

shift the phase of E" by exp( - i</». The portion of this light passing the second polarizer is

Eoot = E"C - EyS= tEo{iK[SZ + C2 exp( -i2</>)]}. (3)

For rotation (3 small enough that i{3 '" 1 - exp( - i(3) = R_, this reduces to

(4)

From Eqs. (1) and (2) we find {3 = - B~ sin26 sin</>. Sub­stituting this into Eq. (4) and squaring to obtain the out­put intensity, we have

lout =113n(B' z)Z~ sinz</> (S4 + C4 + 2S2CZ cos2</», (5)

whereSz=sin26, Iln=~ andB'=(1Twlcn)B. Thel13n

dependence shows the extremely strong relative dis­crimination against low-level light-far stronger than that obtained with saturable dyes. 5,6 A Signal-to -noise ratio of 10 will be increased to 1000, provided the noise level is well above the device light leakage from im­perfections in alignment and optical elements.

The low -intensity portions of large pulses will also be strongly attenuated. For Gaussian pulses, this amounts to reduction by a factor of ..f3 in both the temporal and spatial half -widths.

The maximum contrast ratio is achieved for 6 = ± 45° (t -wave retardation plates). Equation (5) predicts that the maximum contrast ratio can also be obtained for </> > 45° with appropriate adjustment of 6, and that the contrast is not very sensitive to the exact value of 6. As long as one retarder is at + 6 and the other at - 6 , values of 6 from 35° to 55° should give excellent con­trast ratios. Although the peak intensity transmission is unity ({3= 90°), intensity and energy transmission integrated over Gaussian pulses are lower because of the compression mentioned earlier. This is also evi­dent from the fact that {3 cannot be 90° over the entire Gaussian profile.

The 113n dependence of the device has been verified by displaying the photodiode signal from both the treated beam and a portion of the input beam on the same oscil­lograph (Fig. 2). Enhanced attenuation of the low -inten­sity fulses is clearly evident. Figure 3 is a plot of loot vs lin (data taken from the photograph used in Fig. 2) and is linear where the approximation for {3 [Eq. (4)] holds, or about 30° rotation. A pulse shortening of ap­prOximately 10% was measured by two-photon fluores­cence. The narrowing of {3 expected for a Gaussian pulse was not achieved, probably because the pulse shape was non-Gaussian.

Appl. Phys. Lett., Vol. 22, No.5, 1 March 1973

30

10 15

FIG. 3. Plot of oscillograph peak heights from Fig. 2. (Rela­tive scales corrected for attenuation.)

It appears possible to cascade amplifiers and optical systems similar to the one described in order to achieve even higher contrast ratios and pulse compres­sions. Care should be taken to adjust the carbon disul­fide path lengths so the peak rotation does not exceed about 30°, or distortion of the pulse shape will result.

Filament formation by self-trapping of the beam in the carbon disulfide does not destroy the effectiveness of the device. It does, however, modify the relationship of the input and output intensities in a complicated man­ner which depends on power density and optical system geometry. Typical power in these experiments was ap­prOximately 109 W Icm2

• Self-trapped filaments were not observed in 1 cm of CSz at this power level.

*Work performed under the auspices of the U. S. Atomic Energy Commission.

tpermanent address: Brigham Young University, Provo, Utah 84601.

ip.D. Maker, R. W. Terhune, and C.M. Savage, Phys. Rev. Lett. 12, 507 (1964).

2R. Y. Chiao and J. Godine, Phys. Rev. 186, 430 (1969). 3P.D. McWane and D.A. Sealer, Appl. Phys. Lett. 8, 278 (1966).

4L. DahlstrBm, opt. Commun. 6, 157 (1972). 5R.J. Harrach, T.D"MacVicar, G.I. Kachen, and L.L. Steinmetz, Lawrence Livermore Laboratory Report No. UCRL-51008, 1971 (unpublished).

sp.G. Kryukov and V.S. Letokhov, IEEE J. Quantum Electron. QE-8, 766 (1972).

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