Ultrafast pulse shaping with a traveling wave Kerr cell andpicosecond rise time electrical pulses

Michael Stavola, John A. Agostinelli, and Mark G. Sceats

A traveling wave CS 2 Kerr cell has been used to shape optical pulses at 1.06 ,um. The rise time of '20 psecreported here is possiile because the group velocities for electrical and optical pulses are equal in CS 2 at 1.06tim. An electrical pulse generated by a light activated semiconductor switch is used to drive the device.High contrast jitter-free operation appears to be possible.

1. IntroductionUntil recently a fast enough, jitter-free, electrical

driver has not been available to make possible the activeshaping of the subnanosecond optical pulses producedby mode-locked lasers.1 For this reason much attentionhas been paid to passive shaping techniques. Passivepulse shaping typically involves pulse stacking meth-ods2 or dispersive elements 3 and suffers from theproblems of low throughput, degradation of beamquality, high sensitivity to variations in the input pulse,and inflexibility.

Recently, a technique for generating high voltageelectrical pulses with fast rise times has been reported.Auston 4 5 first showed that 1-kV pulses of variable du-ration and rise times of -25 psec could be generated. Agapped microstrip transmission line was biased with ahigh voltage. The doubled light from a mode-lockedNd:glass laser was used to generate a conducting elec-tron-hole plasma in the gap resulting in the transmis-sion of a voltage pulse with a rise time about equal to thewidth of the optical pulse. This technique of using ahigh-power pulsed laser to generate carriers in a semi-conductor and thereby switching high voltages has beenfurther developed,6-8 so that now the switching of up to10 kV is possible. In addition to the fast rise time, be-cause of the way the electrical switch is optically acti-vated, it is essentially jitter-free, that is, well synchro-nized with the optical pulse.

All authors are with University of Rochester, Rochester, New York14627; they are in the Chemistry Department and the Institute ofOptics.

Received 6 June 1979.0003-6935/79/244101-05$00.50/0.© 1979 Optical Society of America.

Now that the required fast rise time jitter-free elec-trical driver does exist and will be improved in its fur-ther stages of development, the problem of devisingactive optical modulators that can realize the 25-50-psec rise times of the driver becomes attractive. SinceAuston's work on fast electrical switching the followingreports of fast rise time optical modulators have ap-peared. LeFur and Auston5 have made a LiTaO3stripline Pockels cell, which had a measured rise timeof 25 psec. Mourou9 has performed a similar experi-ment and has informed us that this type of device iscapable of only low contrast operation (about 20:1).Antonetti et al. 6 have reported measurements of opticalrise times of 100 psec for a traveling wave nitrobenzeneKerr cell and a Lasermetrics 1080 Pockels cell. Agos-tinelli et al. 10 have performed a pulse-shaping experi-ment of the same type we will describe here using aLasermetrics 2038 Pockels cell and demonstrated a risetime of 40 psec.

In this paper we describe a traveling wave CS 2 Kerrcell, which is limited in rise time here by the charac-teristics of the electrical driving pulse. Because laserfusion experiments have provided the primary moti-vation for the development of fast pulse-shaping tech-niques, we include discussion of the applicability of ourKerr cell to laser fusion studies.

il. Kerr Cell DesignWith optically manipulated electronic devices that

can produce up to 10-kV pulses with rise times of 50psec or less and essentially no jitter, the problem ofdesigning an active light modulator with comparablespeed becomes tractable. In media of interest the in-herent response time (for example, the rotational re-orientation time of nitrobenzene or CS 2) is less than 50psec so that the effects that limit the speed of a lightmodulating device will be degradation of the fast elec-trical pulse and transit time effects. We discuss how

15 December 1979 / Vol. 18, No. 24 / APPLIED OPTICS 4101

.1-06uH *- L -p1

Fig. 1. A traveling wave CS2 Kerr cell driven by a GaAs switch.

these effects relate to our CS2 Kerr cell, schematicallydepicted in Fig. 1. (Although not shown, the device isbetween crossed polarizers.)

We consider a rectangular electrical pulse of widthT propagating down a microstrip transmission line oflength L and an optical pulse traveling between themicrostrip and ground plane. The velocities of theelectrical and optical pulses will be C/(Er)'/ 2 and c/ng(w),respectively, where Er is the low frequency dielectricconstant, and ng( () is the group index. Generally,these velocities will not be equal and give rise to a finiteinteraction length

d =I (E-)/2 ng M

and a finite rise timell

1 1 TR 12 cL.

In light modulating devices driven by Auston-typeswitches reported previously,5 6 the length of the lightmodulator was typically a millimeter or two, to keep therise time small. As pointed out by Gaddy et al. 1 2 theelectrical and optical group velocities in CS2 are verynearly equal so that the interaction length can be largeand rise time small. The operation of high frequencycw Kerr effect devices using CS2 has been reported.13

The relative permittivity of CS2 has been reportedl2to be 2.62 from 2 GHz to 3 GHz [so (r) 1 /2 = 1.62]. Thisis to be compared to the group velocity of optical pulsesin CS2. We note that group dispersion effects are im-portant. From the work of Topp and Orner'4 we haveng (0.53 ,um) = 1.769 and fortuitously ng (1.06 ,um) =1.624. We have chosen a length of L = 5 cm for ourKerr cell giving it a rise time of 25 psec at 0.53 ,um and,because the velocity mismatch is less than 1% at 1.06,um, a rise time less than 1.7 psec at 1.06 ,um (that is, atransit time effect rise time). (The rotational reorien-tation time'5 is about 1.8 psec in CS2.)

Gaddy et al. 12 have measured the loss tangent of re-agent grade CS2 and found it to be an excellent micro-wave dielectric with lower loss than Teflon. Also theconductivity is less than 10-9 mho-m-1, and thebreakdown field exceeds 107 V/m. We observe no at-tenuation of the electrical pulse after it travels throughour Kerr cell and expect the losses in our cables to ex-ceed those in the cell. (We note that nitrobenzene isexcessively lossy at microwave frequencies,' 2 and thatthe addition of 3% nitrobenzene to CS2 increases the losstangent by more than a factor of 10.) In a properlydesigned cell there is no problem with degradation ofthe electrical pulse.

To minimize the halfwave voltage of the cell we chosean electrode spacing that is nearly as small as is con-

sistent with the lossless propagation of a Gaussian laserbeam along the 5-cm length. The minimum value ofh2/L is given by16

h2/L = (S24X)/nir,

where the confocal parameter of the beam is made equalto L. Here h is the electrode spacing, and S is a safetyfactor. S = 3 corresponds to difficult alignment butonly a small loss, and S = 6 corresponds to easy align-ment. With S = 6, L = 5 cm, and X = 1.06 Atm, we findh = 400 Aim. In our cell we have h = 660 Aim.

In order to minimize electrical reflections, our Kerrcell was designed to have a 50-Q impedance. From themicrostrip line design formulas set down by Schneid-er,17 we have calculated a width-to-height ratio of w/h= 2.7. The microstrip was made of brass shim stock-1.5 mm wide and soldered under tension to type HNhigh voltage connectors. The gap height was adjustedto minimize the reflection coefficient as determinedfrom TDR measurements. The cell had a reflectioncoefficient of about 0.1 from which we estimate z = 50L 10 Q. Some ringing characteristic of the cell length

was observed both in reflection and transmission mea-surements. No special precautions were taken to en-sure a smooth transition from the electrical connectorsto the strip line.

The double-refraction phase shift of an optical pulseis given by 6 = 27rBE 2L for a Kerr cell with length L thatis short compared with the interaction length as is thecase here. B is the Kerr constant of the cell liquid andis proportional to the inverse wavelength. At 0.589 jim,B = 3.39 X 10-4 M/V 2 for CS2.18 The transmission ofthe Kerr cell placed between crossed polarizers is givenby sin2(5/2). The halfwave voltage V,1 is defined to bethat voltage that gives a phase shift of r. We esti-mate that V, = 15.2 kV at 1.06 Aim for h 660 jim andL = 5 cm.

111. Experimental SetupFigure 2 is a schematic diagram of our experimental

setup. This system is described here.

Fig. 2. A schematic diagram of the pulse-shaping and diagnosticsetup.

4102 APPLIED OPTICS / Vol. 18, No. 24 / 15 December 1979

A passively mode-locked Nd:YAG oscillator gener-ates a train of pulses at 1.06 m separated by about 10nsec. A single pulse switchout system consisting ofKrytron drive electronics and a fast Pockels cell is usedto select a pulse out of the mode-locked train. A typicalpulse has a 40-50-psec full width at half-maximum(FWHM) and an energy of about 100 jiJ. The pulseenergy is then increased to about 2 mJ by a single-pass.Nd:YAG amplifier.

The amplified pulse is then split into two beams.One beam, after a variable optical delay, is focused ontoa coaxial GaAs switch.6-8 In such a switch, a high-re-sistivity chromium doped GaAs bar interrupts thecenter pin of a coaxial connector. In our experiment an

1-cm charge line was biased to 8 kV by a high-voltagepower supply through a 300-kQ resistor. Upon thepulsed irradiation at 1.06 jim of the GaAs bar, photo-generated carriers restore the continuity of the coaxialconnector, discharging the 1-cm line to produce a fastrise time pulse. The short charge line and the fast re-combination time7 19 for free carriers in the GaAs act toproduce a fast electrical fall time. In our experiment,about 100 J of optical energy was used to activate theswitch. A 10-90% rise time of 40-50 psec is typical forthese electrical switches.6 A similar fall time would beconsistent with our pulse shaping results. These fastelectrical pulses are used to drive the Kerr cell describedin Sec. II.

The other part of the amplified YAG pulse isstretched in time to give a pulse with a 70-90-psecFWHM. This is accomplished by a grating pair thatfrequency modulates the short (large bandwidth) inputpulse to give a broadened output pulse.20 Thestretched pulse is split into two beams. About 10 J isfocused with a 40-cm f.l. lens through our Kerr cell,which is between crossed polarizers. The arrival of aproperly timed electrical pulse will cause a part of thestretched optical pulse to be transmitted through thepolarizers. This actively shaped optical pulse and theother part of the stretched optical pulse are focused ontothe entrance slit of a Hadland macon 675 optical streakcamera at slightly different times. Detection of theoutput of the streak camera is by a Princeton AppliedResearch 500-bin optical multichannel analyzer. Ourdata consist of photographs of the OMA display of in-tensity vs time, uncorrected for streak camera or OMAdistortion.

IV. Experimental ResultsIn this section we present the transmission charac-

teristics of our err cell and our pulse-shaping re-sults.

The transmission characteristics of our cell weremeasured using a P-I-N photodiode and neutral densityfilters. In the absence of an applied voltage, the ex-tinction ratio for the crossed polarizers and Kerr cell wasabout 105. Better extinction is possible than for aPockels cell (typically 103) because the Kerr cell liquidhas no natural birefringence. The line delivers 4-kVvoltage pulses (half of the bias) to the Kerr cell. About1.3% transmission of optical pulses at 1.06 m was ob-

Fig. 3. A bounce etalon used to produce a pulse train to calibrate the- streak camera OMA setup.

served. This measurement gives a half-wave voltageof V 7 - 15 kV for our Kerr cell at 1.06 jm. This agreeswell with our estimate of 15.2 kV based on cell dimen-sions.

We now describe our OMA photographs (Figs. 3-6).Each figure is a plot of intensity vs time with time in-creasing from left to right. Figure 3 shows a typicaloscillator pulse with a FWHM of about 50 psec. Abounce etalon with faces coated for 70% reflectivity at1.06 m has been used to produce pulse reflections with125-psec separation such that each successive pulse hasone half of the peak intensity of the previous pulse. Inthis way the streak camera, OMA setup has been cali-brated for both intensity response and sweep speed;there is about 100 psec/div in the photographs. In Figs.4-6 are displayed our pulse shaping results, obtainedas described in Sec. III. The pulse to the left of thephotograph is the stretched oscillator pulse, brought tothe streak camera without shaping. Its FWHM istypically 70-90 psec. To the right of the photographis the same pulse as transmitted by the Kerr cell andcrossed polarizers.

Figure 4(a) shows a stretched optical pulse to the leftand the transmission of the crossed polarizers, defeatedslightly with a halfwave plate, in the absence of an ap-plied voltage at the Kerr cell to the right. Figure 4(b)shows that the Kerr cell, driven by a short electricalpulse, has opened and closed within the duration of the70-psec pulse. Figure 4(c) is an enlargement of theshaped pulse. The 10-90% rise time of this pulse isabout 25 psec and corresponds to the rise time of theKerr cell. The fast fall time of the shaped optical pulsecorresponds to the fast fall time of the electrical pulse,this being due to the short charge line preceding theGaAs switch and the fast recombination time of theGaAs.

In Fig. 5(a) a shaped optical pulse is shown where avery long charge line has been used. The entire backend of the optical pulse has been transmitted. This isto be contrasted with Fig. 5(b) where an -1-cm longcharge line has been used giving a somewhat faster falltime. The front end of each shaped pulse has about a25-psec rise time.

15 December 1979 / Vol. 18, No. 24 / APPLIED OPTICS 4103

Figure 6 shows the effect of varying the relative ar-rival times of the electrical and optical pulses at the Kerrcell. In Fig. 6(a) the long rise time is due to the earlyarrival of the electrical signal. In each succeedingphotograph, Figs. 6(b)-6(d), the arrival of the electricalpulse has been delayed in 20-psec increments. Risetimes are again 10-90% in about 25 psec.

V. ConclusionIn this paper we have described a high contrast light

modulator with an optical rise time of about 25 psec.That the Kerr cell should have a rise time faster thanthat of the electrical driving pulse (estimated to beabout 50 psec) is not surprising because for low trans-mission one has a driving voltage to the fourth powerdependence.

At 1.06 jm, the transit time rise time is estimated tobe only 1.7 psec, and also the rotational reorientationtime of CS2 is only 1.8 psec, so we expect that the opticalrise time we have observed is limited by the rise time ofthe electrical pulse. The electrical rise time mighteventually be as small as a few picoseconds 45 with fur-ther development so that the full potential for fast op-eration of the CS2 Kerr cell might be realized. If such

tC)

Fig. 4. (a) A stretched optical pulse to the left and the transmissionof the crossed polarizers in the absence of a voltage at the Kerr cellto the right; (b) a short electrical pulse at the Kerr cell opens and closes

the cell; (c) an enlargement of the shaped pulse.

Fig. 5. (a) An optical pulse shaped by a long electrical pulse; (b) anoptical pulse shaped by a short electrical pulse.

Fig. 6. The electrical signal is delayed in 20-psec increments fromthe top to the bottom (a)-(d).

4104 APPLIED OPTICS / Vol. 18, No. 24 / 15 December 1979

(a)

(D)

(a)

(a)

(b)

(c)

(d)

(D)

fast rise time electrical signals are generated, an inte-grated design for the semiconductor switch and Kerr cellmight be necessary to prevent degradation of the drivingpulse through connectors and cables.

The appealing features of our CS 2 Kerr cell are thevelocity match of electrical and optical signals at 1.06jim making possible high-speed operation, the high-contrast, and that CS2 is an excellent microwave di-electric. There are also difficulties to be pointed out.Our CS2 Kerr cell, even with a 5-cm path length, has alarge halfwave voltage, V - 15 kV. This has made itnecessary for us to work with low transmission. Also,to keep the halfwave voltage reasonable, it has beennecessary to keep the aperture of the device small.Another serious problem is that of self-focusing. CS 2has a large nonlinear index, n2 = 1.5 X 10-20 m2/V at1.06 jim. We estimate that for our cell, with a 660-gimaperture and 5-cm length, the phase shift due to self-focusing, the B integral, is 0.7 rad with 10 jJ of energyin 50-psec pulses.

Laser fusion studies require pulses with rise times<200 psec and timing accuracies of 100 psec. 21 Theserequirements are easily met by our CS2 Kerr cell drivenby an Auston switch. Our Kerr cell cannot provide thenecessary beam quality due to the small aperture andself-focusing. Pulses with 100 J of energy, which aretypical of simultaneously mode-locked and Q-switchedNd:YAG oscillators, will surely undergo beam breakupdue to self-focusing in a 5-cm length of CS2 for the beamwaists required here.

While there are these problems, with the further de-velopment of electrical switching capability and lasersystems, our Kerr cell could be a useful pulse-shapingdevice for laser fusion studies. A factor of 2 in theamplitude of the electrical driving pulses would increasethe transmission of our Kerr cell to about 20%. Further,an approach other than the simultaneously mode-locked and Q-switched Nd:YAG oscillator has beenproposed for reliable short pulse generation.22 23 Asingle pulse from a cw mode-locked oscillator is ampli-fied in a separate regenerative amplifier. Studies haveshown that pulse shape was unaffected in the amplifierby an energy gain of 106. One possible scheme forproviding shaped pulses would be to use a CS 2 Kerr cellto shape the pulse before amplification. At this pointthere would be negligible self-focusing because of thesmall pulse power. One should note that this schemerequires the synchronization of a high-power fast pulseto activate the electrical switch with a long low-powerpulse. This capability could perhaps be realized by adual-pulse regenerative amplifier system.23

In summary, we have described an ultrafast light

modulator, which, with further development of both theelectrical driver and laser systems, may provide themeans for actively shaping optical pulses with rise timesand timing accuracies that exceed the requirements oflaser fusion studies.

We wish to acknowledge the encouragement andadvice given by Gerard Mourou and Professor CongerGabel. This research was supported, in part, by theNSF (DMR77-04515) and the Office of Naval Research.Much of the equipment used was provided by theLaboratory for Laser Energetics.

References1. B. C. Johnson, K. R. Guinn, W. E. Martin, and W. D. Fountain,

J. Appl. Phys. 49, 75 (1978).2. C. E. Thomas and L. D. Siebert, Appl. Opt. 15,462 (1976); W. E.

Martin and D. Milam, Appl. Opt. 15, 3054 (1976).3. J. Desbois, F. Gires, and P. Tournois, IEEE J. Quantum Electron.

QE-9, 213 (1973); C. Froehly, A. Lacourt, and J. Vi6not, Nouv.Rev. Opt. 4, 183 (1973).

4. D. H. Auston, Appl. Phys. Lett. 26, 101 (1975).5. P. LeFur and D. H. Auston, Appl. Phys. Lett. 28, 21 (1976).6. A. Antonetti, M. M. Malley, G. Mourou, and A. Orszag, Opt.

Commun. 23, 435 (1977).7. G. Mourou and W. Knox, paper presented at CLEA, Washington,

D.C., May-June 1979.8. C. H. Lee, Appl. Phys. Lett. 30, 84 (1977).9. G. Mourou, U. Rochester; private communication.

10. J. Agostinelli, C. Gabel, and G. Mourou, to be published in Appl.'Phys. Lett., Nov. 1979.

11. G. White and G. M. Chin, Opt. Commun. 5, 374 (1972).12. O. L. Gaddy, D. F. Holshouser, and R. E. Stanfield, in Quantum

Electronics III, P. Privet and N. Bloembergen, Eds. (ColumbiaU.P., New York, 1964), p. 1679.

13. A. J. Chenoweth, 0. L. Gaddy, and D. F. Holshouser, Appl. Opt.5, 1652 (1966).

14. M. R. Topp and G. C. Orner, Opt. Commun. 13, 276 (1975).15. H. P. Broida and S. L. Shapiro, Phys. Rev. 154,129 (1967); E. P.

Ippen and C. V. Shank, Appl. Phys. Lett. 26, 92 (1975).16. I. P. Kaminow and E. H. Turner, Appl. Opt. 5,1612 (1966).17. M. V. Schneider, Bell Syst. Tech. J. 48, 1421 (1969).18. C. G. LeFeure and R. J. W. LeFeure, in Physical Methods of

Chemistry, A. Weissberger and B. W. Rossiter, Eds. (Wiley-Interscience, New York, 1972), Part 3C, p. 399.

19. C. H. Lee, A. Antonetti, and G. Mourou, Opt. Commun. 21, 158(1977).

20. J. Agostinelli, G. Harvey, T. Stone, and C. Gabel, Appl Opt. 18,2500 (1979).

21. J. D. Lindl and W. C. Mead, Phys. Rev. Lett. 34, 1273 (1975).22. W. H. Lowdermilk, in Laser Program Annual Report-1976, E.

Coyle, Ed., Lawrence Livermore Laboratory, Rept. UCRL-50021-76 (1976), p. 2-318.

23. J. E. Murray and W. H. Lowdermilk, in Picosecond Phenomena,C. V. Shank, E. P. Ippen, and S. L. Shapiro, Eds. (Springer, NewYork, 1978), pp. 281-284.

S

15 December 1979 / Vol. 18, No. 24 / APPLIED OPTICS 4105