Coherent optical image amplification by an injectionlocked dye amplifier at 632.8 nmRobert Akins and Sing Lee Citation: Applied Physics Letters 35, 660 (1979); doi: 10.1063/1.91246 View online: http://dx.doi.org/10.1063/1.91246 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/35/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Selection and amplification of modes of an optical frequency comb using a femtosecond laser injection-lockingtechnique Appl. Phys. Lett. 89, 181110 (2006); 10.1063/1.2374680 Injectionlocked flashlamppumped dye lasers of very narrow linewidth in the 570–720 nm range J. Appl. Phys. 62, 23 (1987); 10.1063/1.339188 Mode locking of laser oscillators by injectionlocking Appl. Phys. Lett. 28, 258 (1976); 10.1063/1.88730 Lifetimes of 127I2 Molecules Excited by the 632.8 nm He/Ne Laser J. Chem. Phys. 56, 1012 (1972); 10.1063/1.1677205 Fluorescence of Na2 Induced by a Helium–Neon Laser at 632.8 and 640.1 nm J. Chem. Phys. 52, 6441 (1970); 10.1063/1.1672973

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absorptive theory,·8 is not strictly applicable here because we utilize a Gaussian incident profile, and detailed explanation of the form of Fig. 2 must await a more complete theory, but the qualitative conclusion that bistability is easier to observe at higher order is borne out by this experiment. It is worth noting too that, both theoretically and from our experi­ments, at higher orders it is no longer necessary to tune the resonator to obtain bistability.

It is also apparent from these results (Fig. 2) that the transitions become further apart in power with increasing order. This is qualitatively consistent with a self-defocusing nonlinearity since we would expect the beam to be spread out inside the crystal to some extent and hence to be less effective in producing refractive index changes due to its reduced in­tensity. This behavior certainly contrasts strongly with the calculations of Marburger and Felber8 on a self-focusing nonlinearity where the transitions are expected to get closer with increasing order, eventually becoming indistinguish­able. It may be that very-high-order operation can only be achieved successfully with a defocusing nonlinearity.

Using a modification of this simple system we have also been able to observe "optical transistor" action, where a powerful beam is modulated by a weak beam, with an overall small signal power gain greater than six. Clear peaks in the gain can be seen which correspond to the adjacent orders of interference in the bistability observation. This experiment, which is therefore a demonstration not just of differential gain capable of amplifying changes in a single beam but of true optical transistor signal gain action where changes in one beam result in amplified changes in another, will be dis­cussed elsewhere. 17

We have therefore demonstrated optical "circuit ele­ments" in a simple solid state device where the active volume is only 580 X 200 pm diam. Operation of similar devices at higher temperatures is a possibility since this refractive effect has also been observed at 77 K (Ref. 9). This simplicity should make it possible to manufacture similar devices for use in integrated optical systems for switching and signal amplification.

'H.M. Gibbs, S.L. McCall, and T.N.C. Venkatesan, Phys. Rev. Lett. 36, 1135 (1976).

'T.N.C. Venkatesan and S.L. McCall, App!. Phys. Lett. 30, 282 (1977). 'S.L. McCall and H.M. Gibbs, J. Opt. Soc. Am. 68,1378 (1978). 'H.M. Gibbs, T.N.C. Venkatesan, S.L. McCall, A. Passner, A.C. Gossard, and W. Wiegmann, App!. Phys. Lett. 35, 451 (1979).

'T. Bischofberger and YR. Shen, App!. Phys. Lett. 32,156 (1978). 'T. Bischofberger and YR. Shen, Opt. Lett. 4, 40 (1979). 'F.S. Felber and J.H. Marburger, App!. Phys. Lett. 28, 731 (1976). "J.H. Marburger and F.S. Felber, Phys. Rev. A 17, 335 (1978). 'D.A.B. Miller, M.H. Mozolowski, A. Miller, and S.D. Smith, Opt. Com­mun.27, 133 (1978).

,0D. Weaire, B.S. Wherrett, D.A.B. Miller, and S.D. Smith, Opt. Lett. (to be published).

"M. Neuberger, in Handbook of Electronic Materials, (Plenum, New York, 1971), Vo!. 2.

"D.A.B. Miller, Ph.D. thesis (Heriot-Watt University, Edinburgh, 1979) lJD.A.B. Miller, B.S. Wherrett, and S.D. Smith (unpublished); see also

G.D. Holah, J. Dempsey, D.A.B. Miller, B.S. Wherrett, and A. Miller, Inst. Phys. Conf. SeT. 43, 505 (1979).

"H.M. Gibbs, T.N.C. Venkatesan, S.L. McCall, A. Passner, A.C. Gossard, and W. Wiegmann, App!. Phys. Lett. 34, 511 (1979).

"H.M. Gibbs, A.C. Gossard, S.L. McCall, A. Passner, W. Wiegmann, and T.N.C. Venkatesan, Solid State Commun. 30, 271 (1979).

"J.E. Bjorkholm and A. Ashkin, Phys. Rev. Lett. 32,129 (1974). "D.A.B. Miller and S.D. Smith, Optics Commun. (to be published). 18D.A.B. Miller and S.D. Smith, App!. Opt. 17, 3804 (1978).

Coherent optical image amplification by an injection-locked dye amplifier at 632.8 nma)

Robert Akins and Sing Lee Department of Applied Physics and Information Science, University of California, San Diego, California 92100

(Received 23 July 1979; accepted for publication 23 August 1979)

Coherent amplification of images illuminated by the 632.8-nm radiation of He-Ne has been successfully demonstrated using a two-stage injection-locked pulsed dye amplifier technique. A gain of three times has been measured using Cresyl Violet 620 and Rhodamine B in ethanol, while maintaining signal bandwidth, relative phase, and direction, over a one-quarter ILs pulse duration. A space bandwidth product of 500000 was obtained by incorporation of the image amplifier into an optical image processing system.

PACS numbers: 42.30.Va, 42.55.Mv

Coherent amplification of weak monochromatic light beams by injection-locked laser techniques has been success­fully demonstrated over the past decade. H Dye lasers are

particularly well suited for this purpose due to their high gains, broad tunability, and homogeneous broadening on a nanosecond time scale. Effective amplifications of more than 106 have been reported using dye amplifiers with bandwidths equal to or less than that of the injected radiation. I

')Work supported by Air Force Department of Scientific Research under grant AF 77-3208 and the National Science Foundation under grant ENG 75-234-22.

Considerable recent interest has been expressed in the development of a coherent two-dimensional (image) amplifi-

660 Appl. Phys. Lett. 35(9), 1 November 1979 0003-6951/79/210660-04$00.50 © 1979 American Institute of Physics 660

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r-----------------------------..,

AMPLIFIED IMAGE

FLASH~AMP DYE LASER ( R-6GI

SHUT Tl~ ,

~ /~ .

AMP lIFIER SPATIAL FILTER

H.

Ne

FIG. I. Schematic diagram of the experimental system showing the ftash­lamp driven injection-locked dye laser (in dotted box), and dye amplifier in single-pass processor, below.

er. Such interest was first launched by Hansch et al. with the construction of a broadband (100 A.), single-pass dye image amplifier capable of high gain and resolution over a few nanoseconds pulse duration.6 As 'the light source and ampli­fier utilized identical dyes, matching of the signal frequency profile to gain profile was insured, minimizing Sf N problems.

In many optical processing systems, however, high gain is not as vital to an amplifier as is preservation of signal frequency bandwidth. Furthermore, active times of suffi­cient duration to allow passage of light through macroscopic systems (order of tens or hundreds of nanoseconds) are need­ed. 7 In this letter we report on a two-stage injection-locked dye image amplifiers of a-its duration, capable of maintain­ing frequency, phase, and directional information of the im­age. Because of the inefficiencies of using a broadband dye amplifier for monochromatic amplification, the input signal intensity is first boosted by an injection-locked dye laser to a level well above competition with spontaneous emission in the single-pass dye amplifier. This, together with increased active time duration, necessarily means that operation is nearer saturation than with a broadband system, and smaller gains are to be expected.

A schematic diagram of the apparatus is shown in Fig. I. Narrow-band injected radiation at 632. 8 nm was obtained from a He-Ne laser (Spectra Physics model 125) tuned by an intercavity etalon to produce a TEMoo single longitudinal mode, with a coherence length of approximately 200 m and a cw power of up to 6.5 m W. Injection into the dye laser cavity is accomplished by means of the input! output coupler with a reflectivity of75%. A polarizing beam splitter and quarter wave plate isolate the two cavities to prevent mode beating effects. A grating (75 I fmm) lowers the locking threshold and helps to reduce noise by placing a physical bandpass in the cavity. The dye solution, a mixture of 5.5 X 10-4_ M Cresyl Violet 620 and 0.4 X 1O-4-M Rhodamine B in ethanol, is placed in a I-cm spectroscopic cell tilted slightly to discour­age multiple reflections. Cavity length is 10 cm. As has been recently demonstrated, longitudinal mode matching is not always necessary for strong injection locking. ).4 The dye la-

661 Appl. Phys. Lett., Vol. 35, No.9, 1 November 1979

ser is transversely pumped to about 20% over threshold by a flashlamp pumped dye laser (Phase-R model 2100c) using Rhodamine 6G lasing broadband, focused by a cylindrical lens to a point just short of the dye cell. This was observed to produce a very uniformly pumped volume approximately 3 mm into the cell in which the injected radiation could be well centered by means of an imaging lens and screen. Two I-mm apertures limit the lasing volume to approximately 8 X 10-) cm). With the grating replaced by a flat broadband reflector, free-running dye laser output was tuned to center at 632.8 nm to within 0.05 nm by analyzing the output with a Spex double monochrometer while varying the dye mixture. Good emission and absorption matching between the dyes in use insured efficient energy coupling from pump to laser and minimized dye degradation.

Injection locking could be observed with 3 m W of in­jected power for a period of about 250 nsec, which is slightly shorter than the 500-nsec FWHM pulse duration of the flashlamp pumped dye laser. Locking efficiency was deter­mined by analyzing the output with a plane parallel Fabry­Perot and measuring the ratio of energy deposited on a fringe to between fringes with a UDT detector. With 6.5 mW of injected power, a locking efficiency of approximately 80% was obtained. Interferometer mirror spacing was 20 cm, in­dicating a coherence length of at least 40 cm. Locking was observed to drive the dye laser cavity from its free-running linear polarization induced by the grating, to a polarization circular to within 1 % as dictated by the optical isolator. Peak output power was 5 kW, yielding an effective gain of approximately 106

•

Stage two of the systems consists of a single-pass stain­less steel dye cell obliquely pumped by the same flashlamp pumped dye laser (see Fig. 1) to provide synchronous oper­ation. The dye mixture used is identical to that employed in the injection-locked dye laser, tuned to 632.8 nm for maxi­mum efficiency and to minimize any frequency pulling ef­fects. The active volume measures 3 mm diameter X 3 mm deep. The ends are sealed by parallel antireflection coated quartz windows.

The dye amplifier has been incorporated into three opti­cal processing configurations as shown in Fig. 2. Configura-

c cTER DYE

AMPLIFIER FROM INJEC TION

LOCKED DYE LASER

AJG~I,· §~ .. ACTIVE ~ T3mm

VOLUME ~ J.­OB JECT "' mr~~~8·~I-~

" ill]- ~---E-FOURIER OBJECT PLANE

FIG. 2. Dye amplifier cell in test configurations demonstrating (a) beam amplifier, (b) image amplification, and (c) amplification of Fourier trans­form plane.

R. Akins and S. Lee 661

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(a)

(b)

(c)

FIG. 3. (a) Time history of beam amplifier output without gain (smaller trace) and with gain (larger trace). Horizontal time scale is 200 nsec per division. (b) Output of image amplifier showing simultaneously unampli­fied (upper) and amplified (lower) halves of the image. (c) Output of Fourier transform amplifier.

tion (a) is used for simple beam amplification, (b) for image amplification, and (c) for amplification of the Fourier trans­form of an image. A narrow bandpass filter at 632.8 nm helps reject broadband noise.

The time history of the beam amplification experiment is shown in Fig. 3(a). The smaller of the two traces shows the output of the injection-locked dye laser without amplifica­tion (horizontal time scale is 200 nsec per division). The large trace is the same output pulse amplified single pass by the dye amplifier, demonstrating an enhancement of 3 in peak power and approximately 3.6 in pulse energy.

A three-bar resolution chart was then placed in the ob­ject plane of the simple processors shown in Figs. 2(b) and

662 Appl. Phys. Lett., Vol. 35, No.9, 1 November 1979

2(c). Figure 3(b) shows the output from the image amplifier configuration of Fig. 2(b), recorded on high contrast film with the active volume of the dye amplifier shifted slightly off axis to provide a simultaneous comparison of the unam­plified (upper) and amplified (lower) halves of the image. Distortion along the boundary of the active volume may be caused by the propagation of a thermal shock wave induced by the pump pulse. 9 This is not a problem under normal operating conditions when the edge of the active area is well outside the image boundary.

From the configuration of Fig. 2(c), the Fourier trans­form amplifier, the result of Fig. 3(c) is obtained, showing the output amplified at its Fourier plane. The maximum re­solvable frequency corresponds to approximately 14 lines/mm, which is the theoretical limit also imposed by the gain cell aperture and lenses used. The active area of the dye amplifier together with the lenses used yields a space band­width product of approximately 5 X 105

•

As shown by Ganiel, Hardy, and Treves,1O the equiv­alent noise input signal 10 (A ) can be defined by

10 (A ) = 81Tn2cg/ A 4 ,

whereg is the geometrical fraction of molecules whose spon­taneous emission is added to the photon flux. The onset of locking will occur when the stimulated emission rate at the signal wavelength Ai is comparable to the integrated stimu­lated emission rate of the gain profile ..1,1. This requires the flux at Ai to be greater than Imin where

CT e (,1;)1 min ~ ( CT e (A ')l 0 (A ') dA , )<1,<

and CTe is the stimulated emission cross section. Evaluating this integral for TEMoo (dye laser) and highly multimode operation (dye amplifier) yields required locking intensities of approxiately 25 uW and 60 W, respectively. Note that these denote the onset oflocking. For strong locking, several times this power must be supplied. In our experiment, a fac­tor of approximately lOO times these requirements is injected.

As has been observed previously,6 thermal distortion of the dye media maximizes at a time delay of 2-10 msec after absorption of the pump radiation, limiting repetition rate to approximately lOO pps. With approximately 5 X 105 resolv­able spatial directions, coherent amplifiers with channel through-put rate of 5 X 107 could be obtained for use in co­herent optical processing systems. Many immediate applica­tions, especially in optical feedback systems where repeated passage through a filter is required, already exist. 7 Work to increase coherence length and S / N of the amplifier is in progress.

'L.E. Erickson and A. Szabo, Appl. Phys. Lett. 18,433 (1971). '1.1. Turner, E.1. Moses, and C.L. Tang, Appl. Phys. Lett. 27, 441 (1971). 'Q.H.F. Vrehen and AJ. Breimer, Opt. Commun. 4, 416 (1972). 'I. Itzkan and F. Cunningham, IEEE 1. Quantum Electron. QE-8, 101 (1972).

R. Akins and S. Lee 662

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'A.l. Gibson and L. Thomas, 1. Phys. D 11, L 59 (1978). 'T.W. Hansch, F. Varsanyi, and A.L. Schawlow, Appl. Phys. Lett. IS, 108 (1971).

'I. Cederquist and S.H. Lee, Proc. of Electro-Optics/Laser Conference, Anaheim, California, pp. 221-225 (unpublished).

"R.P. Akins and S.H. Lee, 1. Opt. Soc. Am. Oct. 68, 1361 (1978). 'Irving Itzkan (private communication). H'V. Ganiel, A. Hardy, D. Treves, IEEE 1. Quantum Electron. QE-12, 704

(1976).

Backward Raman compression of XeCllaser pulse in Pb vapor N.Ojeu Naval Research Laboratory, Washington, D.C. 20375

(Received 21 May 1979; accepted for publication 23 August 1979)

Pulse compression has been observed for the spontaneously generated backward Raman wave in Pb vapor pumped by the XeCllaser. A compression factor in excess of 6.7 was observed with a photon efficiency of 42%.

PACS numbers: 42.65.Cq, 42.60.Kg

Recently, there has been a resurgence of interest in stimulated backward Raman scattering as a means for com­pressing in time high-power optical pulses. l First discovered a decade ago, the effect has been shown to be capable of producing large peak power magnifications. 2 Current re­search on the backward Raman compressor with its intend­ed application to laser fusion has been stimulated by the emergence in the past several years of the rare-gas halide excimer lasers. While several of the rare-gas halide lasers show very respectable efficiency, their pulse lengths are gen­erally far too long to be suitable for the fusion target envi­sioned at present. Backward Raman compression appears to be an attractive way to reshape the optical pulse to shorter durations, and the combined laser-compressor system could be a viable fusion driver candidate.

As discussed in Ref. 1, one of the limitations on the compression of the backward Raman wave that can be achieved is the spontaneous generationof the backward sec­ond Stokes wave. The severity of this constraint is reduced if a medium is chosen which exhibits appreciable Raman gain only for the first Stokes. Such a medium may be an atomic vapor posessing an optical transition which is in near reso­nance with the pump photon, and it was to exploit this po­tential advantage that the present work was undertaken. The pump and Raman transitions involved in the present scheme are shown in Fig. 1 in relation to the lowest-lying energy levels in Pb. It can be seen that the virtual level established by the XeCI pump photon interacting with the Pb atom is only, approximately, 2800 cm - J from the 7s 3Pstate, which has large oscillator strengths to both the ground 6p2 3 Po state and the final 6p2'P2 state involved in the Raman transition. By contrast, the first Stokes is as much as 13 400 cm - J from the 7s 3p state, which remains the dominant intermediate state for the generation of the second Stokes wave. As a re­sult, the Raman gain coefficient for first Stokes can be ex­pected to be much larger than for second Stokes. While the frequency shift of XeCllaser output in Pb is sufficient to discriminate against the build-up of the second Stokes, it is still modest compared to the pump frequency. Therefore, the combined laser (XeCl) compressor (Pb) system can be ex-

pected to have an overall efficiency quite comparable to that of the XeCllaser itself.l Initial work on forward Raman scat­tering ofXeCI pump radiation in Pb vapor has already been described elsewhere. 4

.5 Here, we report the observation of the

compression of a spontaneously generated backward Raman wave in the same system.

To minimize as much as possible the difference between forward and backward Raman gains caused by the band­width of the pump laser, a narrow-band XeCI oscillator­amplifier system was used as the pump source in the present experiment. Both the oscillator and the amplifier were of the uv-preionized discharge variety. The oscillator had an active length of 30 cm, and its cavity consisted of a 50%-reflecting lO-m-radius mirror and a 98.76 grooves/mm (630 26') echelle grating separated by 70 cm. Intracavity solid quartz etalons of 1 and 10 mm thicknesses and 75% reflectivity per surface were used to further reduce the output bandwidth. While oscillation within a single mode of the 10-mm etalon ought to have given an output bandwidth of no greater than 0.03 cm - J, the actual spectrum of the output remains to be

7s 3pO---

6p2 IS ---

6p21 0 ---

6p23P2 ---

6p23PI __ _

t; v '" 2,800 em·1 --------T

PUMP (308nm)

I t;v" J 3,400 em-I

_________ J \ 2nd

\TOKES

FIG. 1. Lowest lying energy levels in Pband their relation to pump and first Stokes photons.

663 Appl. Phys. Lett. 35(9), 1 November 1979 0003-6951/79/210663-03$00.50 663

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