May 15,1991 / Vol. 16, No. 10 / OPTICS LETTERS 773

Eye-safe coherent laser radar system at 2.1 ,m using Tm,Ho:YAGlasers

Sammy W. Henderson, Charley P. Hale, James R. Magee, Michael J. Kavaya, and A. Vincent Huffaker

Coherent Technologies, Inc., 3300 Mitchell Lane, Suite 330, Boulder, Colorado 80301-2272

Received January 8,1991

An eye-safe pulsed coherent laser radar has been developed by using single-frequency Tm,Ho:YAG lasers and

heterodyne detection. Returns from a mountainside located 145 km from the laser radar system and the measure-ment of wind velocity to ranges exceeding 20 km have been demonstrated with transmitted pulse energies of 22 mJ.

Compared with incoherent (direct) detection laser ra-dar remote sensing, coherent (heterodyne) detectionlaser radar has the advantages of greater sensitivityand the straightforward measurement of the velocityof the target. The majority of coherent laser radar(CLR) remote sensing performed to date has utilizedCO2 gas laser technology at eye-safe wavelengths of 9-11 Aim (e.g., see Ref. 1). However, the use of rapidlydeveloping solid-state laser technologies and theirshorter wavelengths (versus those of C02) offers manypotential advantages. We recently reported the firstdemonstration of Nd:YAG solid-state CLR remoteprofiling of wind velocity at the wavelength of 1.064,um.2 In this Letter we describe the design and dem-onstration of what is to our knowledge the first eye-safe solid-state CLR system, utilizing Tm,Ho:YAG la-ser technology at 2.1,4m.

A diagram of the Tm,Ho:YAG CLR system is shownin Fig. 1. The master oscillator (MO) is a continuous-wave diode-laser-pumped single-longitudinal-modeTm,Ho:YAG laser that exhibits a short-term (t < 1 s)frequency stability of <0.5 MHz and produces -100mW of power at wavelengths near 2090 or near 2096nm.3'4 The laser utilizes a linear cavity with an intra-cavity Brewster plate for polarization control and twosolid fused-silica intracavity 6talons for frequencycontrol. The Tm,Ho:YAG crystal in the MO is cooledto approximately -40°C with a thermoelectric cooler.By tilt tuning the intracavity 6talons, one can continu-ously tune the wavelength between 2086 and 2100 nm.By a change in the reflectivity of the output coupler,tuning near 2121 nm has also been demonstrated.

The output of the MO is first passed through apermanent-magnet yttrium iron garnet Faraday isola-tor, which provides -40 dB of isolation from the rest ofthe system. Additional isolation from the Q-switchedslave oscillator (SO) is provided by a 98% reflector.Portions of the reflected MO power serve as the localoscillator (LO) beams for both the frequency monitorand signal detectors. The MO power passing throughthe 98% reflector is frequency upshifted by 27.12 MHzwith a chalcogenide glass acousto-optic modulator(AOM). The frequency-shifted beam is then modematched and injected into the SO by using the intra-cavity thin-film Brewster polarizer. The flash-lamp-

pumped, Q-switched, Cr,Tm,Ho:YAG SO utilizes thepolarizer and a X/4 plate for output coupling. The flathighly reflecting (R > 99.9%) end mirrors for the SOare separated by -1.2 m and are supported by a three-bar Super Invar resonator structure. The laser crystalis pumped by two xenon flash lamps in a close-cou-pled, diffuse pump chamber and is maintained at ap-proximately +181C with a closed-loop water circula-tor. The oscillator is dynamically stabilized for large-volume TEMoo operation with the thermal lenscreated by the pumped laser rod.5 Q-switched opera-tion is accomplished by using a lithium niobate Pock-els cell in conjunction with the intracavity polarizer.The spectral transmission peak of this polarizer varieswith the tilt angle, allowing us to tune to different laserlines by making small changes in the polarizer angle.Using this technique, we have demonstrated SO oper-ation in four different wavelength regions near 2090,2096, 2121, and 2128 nm. The frequency match be-tween the SO and the MO required for successful in-jection seeding is maintained by an automatic servosystem6 that controls the SO cavity length with an endmirror mounted onto a piezoelectric translator (PZT).We have demonstrated that the injection-seeded Q-switched SO can reliably produce 50-mJ single-longi-tudinal-mode TEMoo pulses at a pulse repetition fre-quency of -3 Hz. The FWHM duration of the 50-mJ

AOM R-98% FARADAYISOLATOR

F; VARIABLE ATTENUATOR f MIRROR, R>99.5% !J 1/2 WAVE PLATE

| 15% REFLECTOR 0 LENS | 1/4WAVEPLATE

Fig. 1. Schematic of the Tm,Ho:YAG CLR system.

0146-9592/91/100773-03$5.00/0 © 1991 Optical Society of America

774 OPTICS LETTERS / Vol. 16, No. 10 / May 15,1991

E

E00ns/DIV 15 20 25 30 35 40200ns/DIV FREQUENCY (MHz)

Fig. 2. (a) Heterodyne beat between the transmitted pulseand the LO. (b) Sample spectrum of the heterodyne pulse.

single-longitudinal-mode pulses is -150 ns. For allthe measurements described below, the wavelength,the pulse energy, and the pulse repetition frequencywere set to 2090 nm, -22 mJ, and 3.2 Hz, respectively,which resulted in a FWHM pulse duration of -220 ns.More detail concerning the SO and our injection-seed-ing studies is available. 4

The majority of the SO output is incident upon a40X eccentric-pupil Dall-Kirkham telescope, whichexpands the TEMoo beam to -16 cm (1/e2 intensitydiameter) and transmits the pulse to the target.Backscattered radiation from the target or atmo-sphere is collected by the 20-cm-diameter telescopeand is then mixed with -1.4 mW of LO power with a15% reflector. The signal and LO beams are thenfocused onto the signal detector-a 100-,um-diameterroom-temperature extended-wavelength InGaAs p-i-nphotodiode.7 Care is taken to match the size and thewave front of the backpropagated LO beam to that ofthe transmitted beam. The quantum efficiency of thesignal detector was measured to be 0.69 electron/pho-ton at the 2.09-,4m lidar wavelength. The heterodynesignal from the detector in the bandwidth between 9and 50 MHz (target radial velocities of +19 to -24 m/s) is amplified with a low-noise 47-dB-gain preamplifi-er before being digitized. With 1.4 mW of LO powerincident upon the signal detector, the LO-generatedshot noise dominates all other noise by -13 dB.

A small fraction of the SO output, which transmitsthrough the 99% reflector, is mixed with a fraction ofthe LO power with another 15% reflector. Thesebeams are then incident upon the frequency monitordetector-a second InGaAs detector that is essentiallyidentical to that used as the signal detector. Theoutput of this detector passes through a 15-MHzFWHM bandpass filter centered at -27 MHz beforebeing digitized.

The outputs of both detector circuits are digitized atspeeds up to 100 megasamples/s using separate chan-nels of a LeCroy 9400 dual-channel digital storageoscilloscope (DSO). The DSO data (8 bits) are trans-ferred to a DEC MicroVax II digital computer andthen postprocessed to extract backscatter, Dopplervelocity, and velocity width information. Data fromthe frequency monitor detector are utilized to charac-terize the frequency content of each outgoing pulse, inparticular, to obtain an estimate of the zero-velocityoffset frequency /0, defined as the difference between

the MO and SO frequencies. This information is nec-essary for accurate measurements of target velocity,since the shot-to-shot variation in f0 can be severalmegahertz owing to imperfect locking of the SO to theMO seed radiation. Figure 2(a) shows an example ofthe output of the frequency monitor detector versustime as captured by the DSO. Note the expected -37-.ns period (frequency -27 MHz) of this signal owing tobeating between the frequency of the 220-ns FWHMSO pulse and the LO frequency. The spectral contentof this pulse is shown with a linear ordinate in Fig.2(b). This sample spectrum was calculated using a256-point fast Fourier transform (FFT) of the tempo-ral data shown in Fig. 2(a). Note that the FWHM ofthe spectrum is -1.6 MHz, which may be broadenedsomewhat owing to the FFT frequency resolution of fr= 0.39 MHz. This is computed from fr = (NFFTTs)- 1 ,where NFFT = 256 points and the DSO sample periodT, = 10ns.

In order to assess the velocity measurement accura-cy of the entire CLR system, a number of single-shotvelocity estimates of a nonmoving hard target located-1 km from the CLR system were made. Figure 3shows the histogram resulting from 96 high-signal-to-noise ratio (SNR) single-shot measurements made atour remote field test site; 100 sequential shots werefired at the target, but 4 shots observed to have signalstrengths of less than -10% of the mean signalstrength (due to speckle) were deleted with no knowl-

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Fig. 3. Velocity estimate histogram of a nonmoving hardtarget located -1 km from the laser radar.

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Fig. 4. Twenty-seven-shot average A-scope display of laserradar return from Pikes Peak, located 145 km downrange.

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May 15, 1991 / Vol. 16, No. 10 / OPTICS LETTERS 775

atmospheric aerosol re-

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Fig. 6. Estimated radial wind velocity versus range.

edge of the computed velocity estimates. A samplespectrum (i.e., a periodogram) is calculated for eachCLR shot using a 512-point FFT. The first momentof the periodogram, used for velocity estimation, wascalculated after we applied the zero-offset frequencycorrection to the periodogram and after we set to zeroall periodogram values farther than 5 MHz from thefrequency of the peak value and below 20% of thespectral density of the peak value (i.e., frequency andspectral density thresholding). The mean value of the96 high-SNR velocity estimates was -3.3 cm/s, andthe standard deviation was 11 cm/s. Accurate veloci-ty measurements can be made, even though theFWHM pulse duration is only 220 ns.

Figure 4 shows a return obtained on November 1,1990, from a mountainside (Pikes Peak) located -145km from the CLR system. When this measurementwas made, the telescope was focused at infinity, theDSO sample period was 80 ns, and the atmosphericrefractive turbulence was visually estimated to be low.The figure shows the 27-shot average of the square ofthe intermediate-frequency signal as displayed by theDSO (i.e., an A-scope display). The background fluc-tuation seen across the figure is due to the LO-generat-ed shot noise. The large signal at the far left of thefigure is due to backscatter from atmospheric aerosolparticles, and the return from Pikes Peak is seen nearthe center of the figure. This long-distance returnindicates high atmospheric transmission at X = 2090nm.

Figures 5 and 6 show an example of atmospheric

wind measurement. For this measurement, the tele-scope was focused at infinity, the beam was aimedapproximately north and approximately +0.4° fromhorizontal, and the DSO sample period was 10 ns.Periodograms at 20 evenly spaced range gates between1 and 30 km were calculated from each of the single-shot intermediate-frequency records using 128-pointFFT's and corrected by using the measured LO/SO-offset frequency. Twenty single-shot periodogramswere averaged for each range gate before the first mo-ment (estimated velocity) calculation was performed.Figure 5 shows representative averaged periodogramscentered at ranges of 7, 16, and 28 km with logarithmicordinate. Figure 6 shows the estimated radial windvelocity versus range. The moment calculations ateach range employed only those periodogram valueswithin 5 MHz and above 20% of the peak value. Al-though the SNR drops substantially at the fartherranges, this 22-mJ 20-shot measurement producedcontinuous signals throughout the entire 30-km rec-ord. We believe that the feature near 16 km is due to ashear in the radial wind velocity near a hilltop located-18 km downrange. (The CLR beam passed within afew meters of the hilltop.)

In conclusion, a pulsed eye-safe Tm,Ho:YAG coher-ent laser radar system has been developed and tested.Range-resolved atmospheric wind velocity and long-range hard target measurements were demonstrated.Future efforts using this system may include calibrat-ed backscatter measurements and performance com-parisons with a CO2 CLR system.

We acknowledge many helpful discussions with R.M. Huffaker. The 2.1-,gm CLR system was developedwith a Small Business Innovative Research Phase IIgrant from the Defense Meteorological Systems Ad-vanced Technology Office at the U.S. Air Force SpaceDivision (L. Scacca, S. E. D. Ferry, and A. T. Lee,technical monitors). Improvement of the MO designwas funded by NASA/Marshall Space Flight Center(W. D. Jones, technical monitor). Initial field demon-stration of the system has been sponsored by theWright Research and Development Center, Wright-Patterson Air Force Base (R. D. Richmond, technicalmonitor).

References

1. R. T. Menzies and R. M. Hardesty, Proc. IEEE 77, 449(1989); M. J. Post and R. E. Cupp, Appl. Opt. 29, 4145(1990).

2. M. J. Kavaya, S. W. Henderson, J. R. Magee, C. P. Hale,and R. M. Huffaker, Opt. Lett. 14, 776 (1989).

3. S. W. Henderson and C. P. Hale, Appl. Opt. 29, 1716(1990).

4. S. W. Henderson, C. P. Hale, and J. R. Magee, in Proceed-ings of Meeting on Advanced Solid-State Lasers (OpticalSociety of America, Washington, D.C., 1990), paper WAL.

5. V. Magni, Appl. Opt. 25, 107 (1986).6. L. Rahn, Appl. Opt. 24,940 (1985).7. R. U. Martinelli, T. J. Zamerowski, and P. A. Longeway,

Appl. Phys. Lett. 53, 989 (1988).

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Fig. 5. Averaged periodograms ofturn at ranges of 7, 16, and 28 km.

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