Frequency-switchable CO2 laser: design and performance

R. L. Shoemaker, R. E. Scotti, B. Comaskey, and Jose Soto M.

A CO2 laser with an intracavity CdTe modulator is described whose output frequency can be rapidlyswitched or chirped, while the output intensity remains constant to better than 2%. Currently, frequencyswitches of up to 20 MHz in 50 nsec can be performed, with the limitation being only the performance of thepulse generator which drives the crystal. The problems which must be addressed in designing such a laserare discussed along with the approaches we have taken to solve them. The laser performance is analyzedand compared with theory, and a novel laser frequency locking scheme is presented which eliminates switch-ing transients in the CO2 gain medium.

1. Introduction

There are a substantial number of applications whereone wants to rapidly change the frequency of a laser, e.g.,in FM optical communications systems, in rapid scanspectroscopy, and in frequency chirped laser radarsystems. In general, an electrooptic crystal placed in-side the laser resonator provides the best method ofproducing very rapid frequency changes. The crystalis cut and placed in the cavity so that light passingthrough it does not have its state of polarization altered.Instead, voltages applied to the crystal simply changeindex of refraction of the crystal, thereby changing theoptical path length of the laser resonator and shiftinglaser mode frequency. In this paper we discuss thedesign and performance of a C02 laser which containsa CdTe electrooptic modulator inside the laser cavityand allows the laser frequency to be rapidly switchedor chirped from one output frequency to another fre-quency 1-20 MHz away in times as short as 50 nsec.Furthermore, the system is designed in such a way thatthe laser amplitude remains practically constant (<2%amplitude variation) during the frequency switch.Much larger frequency switches should be possible if ashorter-cavity high-pressure CO2 laser is used. Wehave not pursued this possibility because our applica-tion of the laser is to the study of coherent transienteffects in time-resolved spectroscopy, and this workdoes not require larger frequency switches.

The authors are with University of Arizona, Optical SciencesCenter, Tucson, Arizona 85721.

Received 21 November 1981.0003-6935/82/050961-06$01.00/0.© 1982 Optical Society of America.

There have been a number of previous papers de-scribing CO2 lasers with intracavity CdTe modula-tions.1 -6 However, these papers have been almost ex-clusively concerned with the behavior of the FM side-bands produced when the crystal is driven by a high-frequency sine wave. We are concerned here with theresponse of the laser output frequency to step functionor square wave pulses applied to the CdTe crystal. Thisbehavior in CO2 lasers has not been specifically dis-cussed except for the case where the square wave fre-quency is coincident with a cavity resonance, a situationwe want to avoid.6 The behavior of a frequency-switched dye laser has been examined previously,7 andwe compare the behavior of our laser to the theory givenin that work in Sec. III. First, however, we present adiscussion of the design considerations that go intobuilding a CO2 laser with an intracavity modulator.Practical details of how to construct such a laser and theproblems which must be addressed are lacking in mostof the previously mentioned work.

II. Design Considerations

In applications requiring IR electrooptic modulators,it is preferable to have the modulator placed outside thelaser cavity. Unfortunately, this is not possible if onewants to shift the laser frequency to a new value andhold it there for any length of time. To frequency shiftoutside the laser cavity one must apply a voltage rampV(t) to the crystal so that dV/dt is a constant for as longas the frequency is shifted, and the peak voltages re-quired for this ramp rapidly become impractical.8 Withthe modulator inside the laser cavity, a dc voltage on thecrystal will hold the frequency at a shifted value indef-initely. However, several potential problems are in-troduced by placing the crystal inside the laser reso-nator. These include heating and thermal lensing inthe crystal, excitation of mechanical resonances in thecrystal by the piezoelectric effect, alignment difficulties,

1 March 1982 / Vol. 21, No. 5 / APPLIED OPTICS 961

and the excitation of transient effects in the laser me-dium itself. Each of these problems are discussedbelow.

CdTe, the current material of choice for 10-gmmodulators, has a residual bulk absorption coefficientof a 0.0015 cm-' in commercially available pieces.9The absorption arising from this can be significant be-cause of the very high light intensities inside the lasercavity. Two effects are of concern. First, thetransverse intensity profile of the laser mode producesnonuniform heating of the crystal, and this will causea thermally induced lens to appear in the crystal. Wecan estimate the size of this effect by assuming thecrystal to be cylindrical in shape with its edges held atconstant temperature and having a plane wave frontGaussian beam passing down the axis. Using standardheat flow equations, one finds

T(r) - T(O) w- K-g - exp(-2x2/w2) dx, (1)

where T(r) is the temperature of the crystal at radiusr from the center (r = 0), Io is the intensity in the centerof the laser beam, w is the beam spot radius, and KT =0.06 W/cm C is the thermal conductivity of CdTe.The integral in Eq. (1) can be expressed in terms oftabulated functions and the induced wave front cur-vature calculated numerically. However, a crude es-timate (which overestimates the size of the effect) maybe obtained by expanding the exponential exp(-2r 2 /w2 )and keeping only the first two terms. We then find

T(r)-T(O) = Ka r2, (2)27VKTW2

(an aP r12,n(r) = nlo (TJ 27rKT W 3

where P = 1/2 ww210 is the total power in the beam, andno is the refractive index in the center of the crystal.We can then use Yariv's treatment of Gaussian beampropagation in a medium with a quadratic index profileto find that a plane wave entering the crystal will leavewith a wave front radius of curvature, 0

R = 1rKTW2

aPl(an/T) (4)where is the length of the crystal, and an/aT = 1.07 X10-4/C is the temperature dependence of the refractiveindex.9

For a 5- X 5- X 50-mm crystal, a spot size w such that3w = 5 mm and 10 W of circulating power (corre-sponding to a 90% output mirror reflectance and 1-Wlaser output power), we find R = 650 cm. Such a weaklens should not cause problems, but if the laser outputpower were raised to 10 W, there would be cause forconcern.

In addition to thermal lensing, crystal heating due tothe absorption causes a change in the overall refractiveindex of the crystal. This changes the optical pathlength of the laser cavity and hence shifts the cavitymode frequencies. Using the known value of On/OT, wefind that a temperature rise of just 0.2 K in the crystalshifts the laser frequency by 20 MHz [Av v(l/L)(On/OT)AT, where L is the overall cavity length].

This is significant compared with the CO2 laser gainbandwidth of -80 MHz. As a result, some kind oftemperature control system is required to compensatefor crystal heating effects due to absorption as well asambient temperature changes.

Another troublesome property of CdTe is that it ispiezoelectric. This effect is large when the voltagewaveform applied to the crystal has frequency compo-nents at the mechanical resonance frequencies of thecrystal. These lie in the 100-500-kHz range for typicalcrystal sizes used in modulators. If one stays far fromthis frequency region, piezoelectric effects are not asevere problem. However, the step function or squarewave pulses we use to drive the crystal have Fouriercomponents over a very broad frequency band, and itis impossible to avoid exciting the mechanical reso-nances. The result is that the crystal vibrates in itsmount, causing birefringence and disturbing thealignment of the laser resonator. This produces outputpower fluctuations. Here one can only try to minimizethe effects by designing a CdTe mount which damps themechanical resonances of the crystal. This basicallyinvolves finding a mount material which has a goodacoustic impedance match to CdTe, so that any inducedmechanical vibrations are immediately transmitted outof the crystal and into the mount where they can beabsorbed.

Yet another difficulty which arises when CdTe is usedinside the laser cavity is that of aligning the laser.Typically the two ends of the crystal are cut so thatthere is a wedge angle of 10 min of arc or more betweenthe ends. Although the ends are AR coated, some re-sidual reflectivity remains, and the wedge is necessaryto prevent the crystal from acting like a Fabry-Perotetalon inside laser cavity. The wedge introduces a de-viation in the CO2 beam passing through it, however, sothat one cannot just place the crystal in an aligned lasercavity and expect it to lase. In addition, CdTe isopaque in the visible so that one cannot use the usualHe-Ne laser alignment methods.

Finally, one must consider the effects of switching thelaser frequency on the CO2 gain medium itself. Thephenomena that occur here depend upon the speed atwhich one switches from one frequency to another. Thevarious effects that can be seen and comparisons be-tween theory and experiment are given in the nextsection.

We now consider how the problems discussed abovehave been eliminated or controlled in our frequency-switched laser. The crystal we used is a 5- X 5- X50-mm piece of CdTe supplied by II-VI, Inc.9 It wascut to provide maximum phase modulation with the 5-X 5-mm ends being 1101 planes, the top and bottombeing {211} planes, and the sides being 111 planes. Thetwo ends were wedged by 25 min of arc with respect toeach other and given a broadband AR coating centeredat 10 Aim. The sides have evaporated metal electrodesso that electric fields can be applied in the [111] direc-tion.

The design of the crystal mount is the place where theheating and piezoelectric problems must be addressed.

962 APPLIED OPTICS / Vol. 21, No. 5 / 1 March 1982

'-WATER COOLING

Fig. 1. Cross section of CdTe modulator mount. The CdTe-BeO-Kovar assembly is contained within a Lucite box and held togetherby plastic screws pressing against soft rubber pads. The Kovarelectrodes are connected to the pulse generator by brass strap

conductors.

Figure 1 shows a cross section of the mount we use. Itwas designed in collaboration with engineers at II-VI,Inc., and is currently available as a commercial productfrom them.9 The best one can do for heating problemsis to ensure that all four sides of the crystal are ther-mally contacted to a constant temperature reservoir.To do this, beryllium oxide pieces are pressed againstthe top and bottom of the crystal and against Kovarmetal electrodes which also contact the sides of thecrystal. The BeO and Kovar are maintained at con-stant temperature by water cooling from a constanttemperature chilled water system with the water pres-sure regulated to maintain a constant flow rate. Theassembly is held together by two screws pressing againstsoft rubber pads. In constructing the mount, care mustbe taken to provide gentle even pressure on all sides ofthe crystal. Nonuniformities can strain the crystal andintroduce birefringence or can even fracture it sinceCdTe only has a mechanical strength comparable withthat of chalk.

An acoustical impedance match into the mount isprovided by using Kovar for the metal electrodes. Inaddition, a very thin layer of Apiezon T vacuum greaseis placed between the BeO and the CdTe. This seemsto improve both thermal and acoustic energy transferto the mount.

The BeO-Kovar assembly is contained in a Lucitehousing, and strap electrodes lead from the Kovarelectrodes to a pulse generator. The housing is held inan optical mount which provides the 5 degrees of free-dom necessary to properly orient the crystal in the lasercavity.

The CO2 laser is an ultrastable design of the typedeveloped by Freed." 1 A ZnSe 95% reflector with 2-mradius of curvature and a gold coated 80-lines/mmmaster diffraction grating are mounted in granite blocksand separated by Invar rods to form the laser cavity.The cavity length is 149 cm, giving a TEMoo mode spotradius of 1.8 mm at the grating end of the resonatorwhere the crystal is located. A sealed-off CO2 dischargetube having a 90-cm active length with NaCl Brewster

windows and an 11.6-mm diam bore is positioned in-side the resonator along with the modulator. A TEMOOmode output is ensured by two apertures in the cavity:a 9-mm diam aperture at the ZnSe output mirror anda 4-mm aperture on the CdTe crystal housing. Themaximum CdTe aperture is limited to just over 4 mmby small tabs which hold the crystal in place in itsmount. The 4-mm aperture causes significant powerloss, and it would have been preferable to use a 6- X6-mm CdTe modulator. With the crystal in place, anoutput beam of -15 mW is available from zero-orderreflection off the diffraction grating. This is used tostabilize the operating frequency of the laser as de-scribed in the next section. The main output beamfrom the ZnSe mirror gives TEMoo mode output powersof 300-500 mW.

To align the crystal in the laser cavity, we found thatthe following procedure works well: First, the wedgeangles of the entrance and exit faces with respect to thecrystal axis are accurately measured using a pair ofDavidson alignment telescopes facing each other withthe crystal between. This setup is also used to calibratethe angular motion of the crystal mount adjustments.The laser is then aligned so that it lases without themodulator. The modulator is inserted into the lasercavity, and a He-Ne beam injected via a pellicle beamsplitter is used to center the crystal and make the frontface perpendicular to the laser axis. The modulator isthen rotated by the calculated amount required to makethe CO2 beam pass down the crystal axis. This usuallyaligns the crystal accurately enough so that the laserlases.

The 145-cm cavity length results in a mode spacingof 100 MHz. The frequency shift due to a voltage Vapplied to the crystal is given by

cn3r4 l V12XLd

(5)

where n3r4 = 10-8 cm/V is the electrooptic figure ofmerit for CdTe, L is the cavity length, and I and d arethe length and cross section of the CdTe modulator.This gives Av = 0.0095 MHz/V. The crystal is drivenby a Velonex high-power pulse generator which canproduce 0-2000-V pulses with rise and fall times of -50nsec. Thus frequency shifts of up to 20 MHz are pos-sible. A 200-4 load resistor in parallel with the crystalis required to obtain the minimum rise and fall times.

Ill. Performance and Comparison with Theory

The most important characteristic of the fre-quency-switched laser's performance is that it switchescleanly and smoothly to a new frequency without pro-ducing other optical frequencies. In addition we wantthe frequency switch to produce little or no change inthe laser beam amplitude. In this section we describethe laser's performance and compare it with two theo-retical models.

A theoretical treatment of the output frequency ofa laser whose cavity length is rapidly changed by a rampvoltage applied to an intracavity phase modulator hasbeen given by Genack and Brewer. 7 The main con-

1 March 1982 / Vol. 21, No. 5 / APPLIED OPTICS 963

Fig. 2. Frequency-switched CO2 laser response to a 0.8-psec 650-Vpulse. The 0-MHz frequency switch is made visible by heterodyningthe laser output with a second stable CO2 laser. The apparent am-plitude change in the beat signal is due to the frequency response of

the detector used.

clusion of this treatment is that a voltage pulse appliedto the modulator should have a rise time which is longcompared with the round-trip time for the light in thelaser cavity. If this condition is not satisfied, other lasercavity modes will be excited, resulting in a mixture ofoutput frequencies. A simple way to understand therise time restriction is to observe that a pulse whose risetime is of the order of the cavity round-trip time willintroduce large Fourier components into the light at newfrequencies that are c/2L away from the current oscil-lation frequency. Such frequencies overlap adjacentnonoscillating cavity mode frequencies that can be ex-cited by this injected signal.

In our laser, the cavity round-trrp time is 9.9 nsec,whereas the fastest pulse rise time we could obtain is-50 nsec. Hence any excitation of other cavity modesshould be minimal. Figure 2 shows the switching be-havior of the laser. Here the frequency-switched laseroutput was heterodyned with a second stable CO2 laser.Thus the frequency switch appears as a change in thebeat frequency. The large change in the beat amplitudeis due to roll-off in the frequency response of theHgCdTe detector and video preamp being used. Whenthe detector-preamp frequency response is taken intoaccount, one finds no appreciable amplitude variation(on this time scale at least) when the frequency switchis made. Note that the new frequency appears within1 cycle of the 6.7-MHz beat frequency and that no in-terruptions in phase or other transients are evident.Our detector bandwidth did not allow us to observedirectly whether adjacent cavity modes (100 MHzaway) were being excited, but any excitation of this sortwould have to be quite small, or we would at least seeamplitude changes in the beat signal.

On a longer time scale, amplitude changes do occurdue to the piezoelectric effect discussed earlier. Figure3 shows an example of the laser amplitude variationsproduced by this effect. The damped oscillations seen

in the figure are due to birefringence and cavity mis-alignments caused by the piezoelectrically induced vi-brations of the crystal in its mount. The size of theseoscillations is critically dependent on the way the crystalis mounted and its alignment in the laser cavity. Usingthe crystal mount described earlier and with properalignment, the peak-to-peak amplitude changes can bereduced to <2%.

Finally, substantial amplitude changes on a micro-second time scale are also observed under some condi-tions. Here a large transient lasting about microsec-onds is observed when a frequency switch is performed.Depending on where the laser is operating within theCO2 gain curve, however, the sign of the transient canbe made positive or negative, and at one position underthe gain curve the transient disappears. These effectsare shown in Fig. 4.

The effect clearly arises in the CO2 gain medium, andthere are several possibilities one might consider. Onepossibility is that the frequency switch produces co-herent transient effects in the CO2 gain medium.

-2

39

*0a .

0

0

_J

+20 5 10 15 20

Time (usec)

Fig. 3. Laser output power transients following a step functionfrequency switch. Note that the laser power remains within 2% of

its steady state value.

(a)

(bI

(c)X,

0 5 10 15

TIME (sec)

Fig. 4. Transients observed when the frequency switched laser isoperating at different positions under the gain curve. A 200-V pulseis turned on at t = 5 ,usec and turned off at t = 10 /Isec. (a) The laseris switched from one side of the gain curve toward line center by thepulse and back to the side when the pulse is turned off. (b) The laseris switched between two positions symmetrically located with respectto the center of the gain curve. (c) The laser is switched from thecenter of the gain curve toward one side and then back to line centerwhen the pulse is turned off. (Note that the output power changeshere are of the order of 20%. The vertical scale in Fig. 3 is expanded

by over a factor of 10 compared with the traces here.)

964 APPLIED OPTICS / Vol. 21, No. 5 / 1 March 1982

However, the dephasing time for rotational relaxationis -10 nsec at the 10-15-Torr pressures we use, and ourfrequency switch occurs slowly (rise time ' 50 nsec)with respect to this relaxation time. The more likelypossibility is that this effect has the same origin as thelarge amplitude perturbations near 250 kHz found byKiefer et al. 1 in their work on intracavity frequencymodulation of CO2 lasers. In this time regime a rateequation treatment can be used to describe the gainmedium behavior. We have done this using a set of rateequations similar to those given by Christensen et al. 1 2

in their treatment of gain saturation and diffusion inCO2 lasers.

One finds that the solutions are damped sine func-tions whose period and damping rate give qualitativeagreement with the experimental result shown in Fig.4. The solutions agree fully with experiment in regardto their dependence on position under the gain curve.Switching toward line center produces a transient in-crease in laser intensity, and switching away producesa transient decrease.

The basic physical mechanism at work here is some-what similar to what happens in laser relaxation oscil-lations, except that here the damping is too large toallow more than 1 or 2 cycles of oscillation. Suppose thelaser is initially on one side of the gain curve and issuddenly switched to line center at t = 0. Up until t =

0, the saturated gain equals the losses, as required forsteady state operation. When the switch occurs, how-ever, the cavity mode frequency shifts to line centerwhere the gain is higher, and we now have gain > losses.Thus there will be a sudden increase in output powerbefore the gain adjusts to a new steady state value.Similarly, if the laser is switched from line center to oneside of the gain curve, the laser will have gain < lossesjust after the switch, producing a transient drop in thelaser power.

To avoid the transient just discussed, one must fre-quency switch the CO2 laser between two points whichare symmetrically placed with respect to line center onthe CO2 gain curve. To do this requires some kind offrequency locking scheme in which the laser frequencycan be offset from line center. Furthermore, the lockmust not interfere with the frequency switching. Wehave developed a very simple and effective way of doingthis. Instead of trying to directly frequency lock thelaser, we monitor the switching transient itself and feeda signal proportional to the transient's amplitude backto the laser in such a way that the transient's amplitudeis driven to zero.

Figure 5 shows how the transient suppression is ac-complished. The laser output intensity is monitoredwith a Ge:Au detector using the zero-order reflectionfrom the laser's diffraction grating to obtain a monitorbeam. The detector output is fed into a spare channelin a PAR 162 boxcar integrator. The boxcar channelis triggered by a sync pulse from the pulse generator thatcauses the laser frequency switch, and the boxcar sam-pling gate is set to coincide with the peak of the laser'sswitching transient. The boxcar's output is cleared andreset 100 times/sec by an asynchronous clock, while wetypically frequency switch the laser 10,000 times/sec.Hence the boxcar integrates -100 transients, and at theend of the integration, just before the clear signal, theboxcar output is latched into a sample and hold and fedto an operational amplifier set up as an integrator witha time constant of -50 msec. The integrator output isamplified and fed to a PZT on the laser output mirrorwhich adjusts the laser's cavity length until the inte-grator output is zero. This system is quite stable andworks very reliably. From observations of the errorvoltage fed to the PZT, we estimate the frequency sta-bility of the laser to be better than 1 MHz with this lockscheme.

Pressure RegulaledChilled Waler

Fig. 5. Block diagram of the frequency-switched CO2 laser along with the electronics used to drive the crystal and to control the laser frequencyso that switching transients are suppressed.

1 March 1982 / Vol. 21, No. 5 / APPLIED OPTICS 965

IV. Conclusions

Although further improvements are certainly possi-ble, the frequency switched laser described here worksvery well for its intended function. We have been usingthe laser successfully in coherent transient experimentson molecular gases for some time now. The primaryimprovement which could be made would be to use aCdTe crystal with a larger cross section. The current5- X 5-mm size causes a rather large insertion loss andmakes the laser unnecessarily difficult to align. In allother respects, the laser performance is quite satisfac-tory. The <2% amplitude stability is particularlygratifying.

Larger frequency switches than 20 MHz are feasible,particularly if one uses a high-pressure waveguide CO2laser instead of the conventional type used here. Thewaveguide laser has a much broader, fairly flat gaincurve, thus reducing or perhaps eliminating the switchtransient discussed in the previous section. Further-more, the overall laser length can be much shorter,producing a larger frequency switch for a given appliedvoltage. A requirement for high laser output power willcause problems, however, as thermal lensing rapidlybecomes quite severe. For such cases the use of a CO2amplifier to obtain higher power is probably the bestapproach. Finally, if switching times much more rapidthan the 50 nsec reported here are needed, problems willbe encountered with excitation of cold cavity modes andcoherent transients in the CO2 gain medium. The useof an electrooptic modulator outside the laser cavity isthen likely to be a better choice.

The support of the National Science Foundationunder grant CHE-7824085 is gratefully acknowledged,and J.S.M. acknowledges scholarship support by CO-NACYT (Mexico). We also wish to thank M. Hen-derson of Hughes Aircraft Co., El Segundo, Calif. forsuggesting the use of Kovar and Apiezon T in the CdTemount.

References1. J. E. Kiefer, T. A. Nussmeier, and F. E. Goodwin, IEEE J.

Quantum Electron. QE-8, 173 (1972).2. C. Huang, Y. Pao, P. C. Claspy, and F. W. Phelps Jr., IEEE J.

Quantum Electron. QE-10, 186 (1974).3. T. A. Nussmeier, F. E. Goodwin, and J. E. Zavin, IEEE J. Quan-

tum Electron. QE-10, 230 (1974).4. W. R. Leeb and C. J. Peruso, IEEE J. Quantum Electron. QE-13,

61 (1977).5. W. R. Leeb and A. L. Scholtz, IEEE J. Quantum Electron. QE-13,

925 (1977).6. A. L. Scholtz, W. R. Leeb, H. Kastl, and F. Sommer, IEEE J.

Quantum Electron. QE-15, 1079 (1979).7. A. Z. Genack and R. G. Brewer, Phys. Rev. A 17, 1463 (1978).8. D. Anafi, R. Goldstein, and J. Machewirth, Laser Focus 13, 72

(1977).9. II-VI, Inc., Saxonburg Blvd., Saxonburg, Pa. 16056.

10. A Yariv, Introduction to Optical Electronics (Holt, Rinehart andWinston, New York, 1971).

11. C. Freed, IEEE J. Quantum Electron. QE-4, 404 (1968); QE-3,203 (1967).

12. C. P. Christensen, C. Freed, and H. A. Haus, IEEE J. QuantumElectron. QE-5, 276 (1969).

Meetings Calendar continued from page 850

1982October

17-22 First Electrochemical Soc. Int. Symp. on Very Large ScaleIntegration Science & Technology, Detroit C.Dell'Oca, LSI Logic Corp., 1601 McCarthy Blvd.,Milpitas, Calif. 95035

18-22 OSA Natl. Mtg., Tucson OSA, Mtgs. Dept., 1816Jefferson P1. N. W., Wash., D.C. 20036

November

1-5 APS Div. of Plasma Physics, New Orleans W. W.Havens, Jr., 335 E. 45 St., N. Y., N.Y. 10017

1983

January

10-12 Excimer Lasers, OSA Top. Mtg., Lake Tahoe OSA,Mtgs. Dept., 1816 Jefferson P., N.W., Wash., D.C.20036

10-12 Optical Techniques for Remote Probing of the At-mosphere, OSA Top. Mtg., Lake Tahoe OSA, Mtgs.Dept., 1816 Jefferson P., N. W., Wash., D.C. 20036

12-14 Meteorological Optics, OSA Top. Mtg., Lake TahoeOSA, Mtgs. Dept., 1816 Jefferson P., N. W., Wash.,D.C. 20036

12-14 Signal Recovery & Synthesis with Incomplete Dataor Partial Constraints, OSA Top. Mtg., Lake TahoeOSA, Mtgs. Dept., 1816 Jefferson P., N. W., Wash.,D.C. 0036

17-19 Optical Storage of Digital Data, OSA Top. Mtg., LakeTahoe OSA, Mtgs. Dept., 1816 Jefferson P., N. W.,Wash., D.C. 20036

February

28-2 Mar. Optical Fiber Communication, OSA Top. Mtg., NewOrleans OSA, Mtgs. Dept., 1816 Jefferson P., Wash.,D.C. 20036

March

20-25 185th ACS Natl. Mtg., Seattle A. T. Winstead, 1155 16thSt. N. W., Wash., D.C. 20036

May

17-19 OSA Conf. on Lasers & Electro-Optics, BaltimoreOSA, Mtgs. Dept., 1816 Jefferson P., Wash., D.C.20036

continued on page 970

966 APPLIED OPTICS / Vol. 21, No. 5 / 1 March 1982