Coherent laser radar at 3.6 �m

Frank Hanson and Mark Lasher

Coherent laser radar systems in the mid-IR wavelength region can have advantages in low-altitudeenvironments because they are less sensitive to scattering, turbulence, and humidity, which can affectshorter- or longer-wavelength systems. We describe a coherent laser radar at 3.6 �m based on asingle-frequency optical parametric oscillator and demonstrate the system over short ranges outdoors.The system was used to make micro-Doppler measurements from idling trucks that were processed togive surface vibration spectra. © 2002 Optical Society of America

OCIS codes: 280.0280, 280.3340, 280.3640, 190.4970.

1. Introduction

Coherent laser radar has been used for a wide rangeof remote sensing applications for many years. It isan especially useful tool to measure small surfacevibrations from remote hard targets that can aide intheir identification. A number of solid-state lasersources based on neodymium at 1 �m, semiconduc-tors or erbium fiber at 1.5 �m, and holmium at 2 �mas well as CO2 lasers that operate over the 9–11-�mregion have been used successfully for theseapplications.1–5 However, there are often environ-mental conditions associated with poor visibility, tur-bulence, or high humidity where it might be desirableto operate in the mid-IR band to improve systemperformance.6 Such conditions are not uncommonat low altitudes and in marine environments. Al-though this seems intuitive, it is useful to quan-tify this assertion with calculations based on thewell-developed atmospheric transmission model inFASCOD3P.7 We calculated monochromatic trans-missions as a function of meteorological range for a20-km low-altitude path as a function of meteorolog-ical range in a tropical atmosphere using the Navymaritime aerosol model �ICSTL equal to 3�. The cal-culations were performed at wavelengths associatedwith erbium lasers �1.555 �m�, holmium lasers�2.096 �m�, and 12C18O2 lasers �9.115 �m� and at

The authors are with the Space and Naval Warfare SystemsCenter, Code 2853, 53490 Dow Street, San Diego, California92152. The e-mail address for F. Hanson is hansonfe@spawar.navy.mil.

Received 28 May 2002; revised manuscript received 11 Septem-ber 2002.

0003-6935�02�367689-05$15.00�0© 2002 Optical Society of America

2

3.823 �m, which is a preferred wavelength in themid-IR band. The results in Fig. 1 clearly indicatethat longer wavelengths are less sensitive to aerosolscattering. Transmission at 9.115 �m on the R�20�line is the highest of all wavelengths in the 9–11-�mband readily accessible with isotopic CO2 composi-tions; however, severe attenuation that is due to thewater continuum absorption affects all of them.

Unfortunately, there are inherent difficulties thatare generally associated with demonstrating practi-cal solid-state laser sources with direct output in themid-IR. However, a common technique that has be-come widely used to generate tunable output in themid-IR from 3 to 5 �m is based on the optical para-metric oscillator �OPO�. Single-frequency OPO de-vices have been described,8–10 and we have recentlycharacterized the frequency stability of a cw OPOsource based on periodically poled lithium niobate�PPLN�, similar to the one reported in Ref. 9. In Ref.11 we demonstrated that this source could be used tomake micro-Doppler heterodyne measurements ofpiezoelectrically driven vibrating targets in the labo-ratory. In this paper we describe further character-ization of the OPO source and some outdoormeasurements of surface vibration spectra from op-erating trucks.

2. Optical Parametric Oscillator Mid-IR Source

The mid-IR source used in these heterodyne experi-ments was built at Lightwave Electronics Corpora-tion and is based on a ring OPO resonantly pumpedwith a single-frequency Nd:YAG monolithic ring la-ser. The essential feature of this device is that theexcellent frequency stability of the 1-�m pump laseris transferred to the idler output. The idler fre-quency fluctuations follow the pump frequency with areduced scaling of � �� . The linear spectral density

i p

0 December 2002 � Vol. 41, No. 36 � APPLIED OPTICS 7689

of frequency noise, S �Hz�Hz1�2�, of both the pumpand the idler was found to have 1�f behavior between1 and 1000 Hz.

In preparation for a possible field experiment atextended range it was necessary to precisely tune andaccurately determine the idler wavelength becausethere are many strong molecular absorption linesthat are primarily due to water vapor throughout themid-IR. A transmission spectrum calculated withFASCOD3P over the tuning region of the OPO is shownin Fig. 2 for a low-altitude 4-km path in a tropicalatmosphere. The absorption lines are less than 1cm�1 wide with a typical peak attenuation of �7.5dB�km. Clearly one needs to operate the OPO inone of the relatively wide transmission windows be-tween the sharp lines. We found the coarse wave-length tuning of the idler with temperature of thePPLN crystal indirectly by measuring the wave-length of the weak leakage of red output that is dueto the sum frequency of the resonated pump andsignal waves in the OPO cavity. The result for theidler at �3.6 �m was �2.4 nm�deg; however, theabsolute uncertainty in idler wavelength obtained

from this method scales as 2��i��p�2 or �23 times theuncertainty in pump wavelength. We wanted toknow the idler wavelength to within an accuracy of 1nm and therefore decided to make a direct measure-ment with a scanning monochromator. A coarsegrating, 128 lines�mm, was used in first order at 3.6�m and was calibrated with neon spectral lines inhigher order. We found that, with the PPLN crystalat 204° C, the idler wavelength was 3.605 �m andnearly centered in a 6-nm-wide transmission window�Fig. 2�. In our early research we incorrectly re-ported an idler wavelength of 3.57 �m at 200° C. Itshould be emphasized that the coarse tuning of �2.4nm�deg is due to the phase-matching properties ofthe PPLN crystal used in the OPO. The OPO cavitylength is actively controlled to maintain resonancewith the single-frequency pump wave. Under theseconditions and for this particular OPO configuration,the temperature dependence of the single-mode fre-quency for the signal and idler wave was calculated tobe ���57 MHz�deg.11 For small temperature fluc-tuations of the PPLN crystal, the output remains in aspecific longitudinal mode because the wavelengthdependence of the nonlinear gain that is due to phasematching is relatively broad. Larger temperaturechanges of course will cause mode hops. In fact, ourprevious measurements of the frequency stability ofthe OPO indicate that, for frequencies above �1 Hz,the output frequency fluctuations are due to the 1-�mpump frequency fluctuations and not temperaturefluctuations.

3. Experiment

We previously demonstrated that this source could beused in the laboratory to make sensitive micro-Doppler measurements of a piezoelectrically driventarget using heterodyne detection. Here we de-scribe a monostatic laser radar experiment using thissource and report measurements of vibration spectrafrom trucks idling in a parking lot adjacent to thelaboratory. The range to target was �15 m, andtherefore atmospheric turbulence and scatteringwere not significant factors. In fact, the strong ab-sorption from the various water lines �Fig. 2� in thisspectral region are not significant at ranges shorterthan 100 m.

All the optical components were arranged on a 2 ft. 4 ft. �0.61 m 1.22 m� optical table as shownschematically in Fig. 3. The lenses were all un-coated CaF2. A germanium acousto-optic Bragg cell�Brimrose� that operated at 40 MHz was able to shiftapproximately 80% or 15 mW of the output into thefirst order. This frequency-shifted output was usedas the transmit beam. It was expanded �13 andcollimated with a telescope consisting of a 7.5-cmfocusing lens and a 1-m off-axis parabolic reflectorwith a 10-cm aperture. We obtained a near-fieldtwo-dimensional intensity profile of the output beamby stepping a point detector in a raster pattern in aplane near the output aperture. The profile shownin Fig. 4 is smooth, roughly Gaussian, and nearlysymmetric with a full width at half-maximum

Fig. 1. Fixed-path transmission over 20 km at low altitude cal-culated for monochromatic laser wavelengths in a tropical mari-time atmosphere as a function of meteorological range.

Fig. 2. Low-altitude atmospheric transmission spectrum calcu-lated with FASCOD3P for a 4-km path in tropical atmosphere, Navymaritime aerosol with the ICSTL parameter equal to 3.

7690 APPLIED OPTICS � Vol. 41, No. 36 � 20 December 2002

�FWHM� of �2 cm. In the gray-scale image of thebeam one can see the diffraction from a small un-coated spot on the center of the primary mirror usedfor alignment. In fact, our beam was not quite cen-tered on the primary mirror.

The remaining �20% of the unshifted light in zero

order from the acousto-optic cell was used as the localoscillator �LO� beam. A 50–50 beam splitter be-tween the primary mirror and the secondary lensesreflected the output beam and transmitted the returnlight. An adjustable iris was placed at the telescopefocus for alignment purposes and to set the field ofview for a visible camera that could be used with apickoff mirror in the return path after the telescope.The telescope was pointed by fine adjustments of theprimary mirror, and the target position was easilytracked with the visible camera.

To simplify this initial demonstration of the laserradar outside of the laboratory and avoid speckle fluc-tuations, we used a hollow retroreflector target with a4-in. �10-cm� aperture. Measurements were madewith this target bolted onto either a 1992 Ford Ex-plorer or a 1998 Toyota 4Runner. In both cases, theretroreflector was mounted on the top-front part of theengine compartment with the hood raised. The lineof sight was nearly perpendicular to the side plane ofthe car, that is, we were sensitive to horizontal-sideways vibrational motion. As noted above, thesecars were positioned �15 m from the telescope aper-ture so that the beam remained nominally collimatedover the round-trip path. The reflected light wastransmitted through the first beam splitter and com-bined with the LO by use of a second beam splitter andfocused onto a high-speed room-temperature HgC-dZnTe photovoltaic detector �Vigo Systems Model PDI-4�. Although the quantum efficiency of this detectorwas only 1%, it was easy to see the dc signal fromeither the LO or reflected return and make coarseinitial alignment of the beams for overlap on the de-tector. The 40-MHz heterodyne signal was amplifiedwith a 2000-� fast ac coupled transimpedance ampli-fier. The output was bandpass filtered and mixedwith a stable rf signal at 40.05 MHz to give a down-shifted signal centered at 50 kHz. All the Dopplershifts were well within 50 kHz, which corresponds to aradial target velocity of 9 cm�s at 3.6 �m. The down-shifted signal was low-pass filtered and digitized at 106

samples�s for up to 10 s.We processed the data files of �107 voltage points

using a spectrogram technique in which successiveblocks of 1024 points were windowed with a Hanningfunction and Fourier transformed. The frequencycentroid of the peak near 50 kHz was obtained for eachsegment every 1.024 ms. This data set was againFourier transformed to give a power spectrum of Dopp-ler frequency shift SD

2 �Hz2�Hz� from which the powerspectrum of displacement �Sd

2�, velocity �Sv2�, or accel-

eration �Sa2� could be obtained according to

Sd2� f � � ���4 �f �2SD

2� f � , (1a)

S�2� f � � �2SD

2� f ��4 , (1b)

Sa2� f � � �2�f ��2SD

2� f ��4 . (1c)

With both trucks running at idle, we obtained adistinctive vibration structure as evident in Fig. 5where average velocity power spectra for two inde-pendent 10-s measurements are shown. Both the

Fig. 3. Coherent heterodyne laser radar optical setup. BS1 andBS2, 50–50 beam splitters; P, 10-cm telescope primary; S1 and S2,telescope secondary lenses for transmit and receive; FOV, an iristhat sets the field of view for a visible camera placed after S2; AO,acousto-optic frequency shifter.

Fig. 4. Top, transmit beam intensity gray-scale map showingdefect in telescope primary. Bottom, horizontal profile throughdashed line.

20 December 2002 � Vol. 41, No. 36 � APPLIED OPTICS 7691

Explorer and 4Runner have 6-cylinder engines andwere idling at �800 and �700 rpm, respectively. Avibration feature at the cylinder firing frequency,which is three times the crankshaft rate, can be seenat �40 Hz for the Explorer but more clearly at 36 Hzfor the 4Runner. Generally, the 4Runner had muchnarrower characteristic vibration features over the10-s measurement time. In fact, the Explorer wouldtypically shift between slightly different idling ratesevery few seconds, and this led to the broader inho-mogeneous nature of the spectrum. Several vibra-tion features at lower frequency were due tocomponents such as the alternator or water pump inboth trucks. At idle, the total power in the velocityspectra was almost entirely concentrated below 50 Hzfor both trucks. The measured rms velocity was1.09 and 1.48 mm�s for the Explorer and 4Runner,respectively. The corresponding rms amplitudeswere 45.5 and 18.8 �m.

We were able to compare the laser radar results forthe Toyota with measurements from a commercialmultiaxis accelerometer �Crossbow Model DMU-VGX�, which we mounted on the 4Runner frame closeto the position of the laser radar target. The acceler-ometer was oriented so that the measurementdirection was within �10 deg of the laser radar line-of-sight direction. This was a microelectromechani-cal system-based sensor that had an analog bandwidthof 100 Hz ��3 dB� and provided an output voltageproportional to acceleration. Measurements were notmade with both sensors simultaneously; however, con-ditions were nominally the same with the 4Runnerwarmed up and idling steady at �700 rpm. The laserradar data were processed to give a frequency spec-trum of acceleration Eq. �1c��, and this is comparedwith the spectrum from the accelerometer in Fig. 6.Agreement between the two sensors is generally good,especially for the three dominant vibration featuresbelow 40 Hz. Small differences might be attributed tomisalignment of the measurement axes, placementand mounting of the sensors, and operating conditionsof the truck. The measured rms acceleration from 0

to 100 Hz was 210 and 162 mm�s2 with the laser radarand accelerometer, respectively.

4. Discussion

For a laser radar system to be practical for longer-range applications, at least two issues must be ad-dressed. First, a certain average power, dependingon many factors, would be required from the trans-mitting source. In this study, the OPO idler outputwas �90 mW. After transmission losses from vari-ous elements, the actual transmitted beam powerfrom the telescope was estimated to be �10 mW.This could be scaled to some degree, but to realizemuch more than an order-of-magnitude increase, anew approach probably would be required. For ex-ample, either the signal wave or idler wave from thecw OPO could be used to seed a higher-power pulsedslave OPO while still providing the idler for the LOreference.

A second concern might be the frequency fluctua-tions or noise of the OPO idler wave as during theround-trip time to the target, any frequency shift inthe LO will be indistinguishable from a Doppler shiftbecause of target motion. In general, the powerspectral density of Doppler shift SD

2 �Hz2�Hz� is anincoherent combination of spectra because of the tar-get ST

2, atmospheric path-length fluctuations Satm2,

LO frequency drift SLO2, and instrumental noise Sn

2

according to

SD2� f � � ST

2 � Satm2 � 4SLO

2 sin2��f�� � Sn2 , (2)

Fig. 5. Power spectrum of vibration velocity for Ford Explorer at800-rpm idle �scaled by 4 and offset� and Toyota 4Runner at700-rpm idle.

Fig. 6. Log power spectrum of acceleration for the Toyota 4Run-ner at 700-rpm idle measured with laser radar and accelerometer.

7692 APPLIED OPTICS � Vol. 41, No. 36 � 20 December 2002

where � is the round-trip time to the target. In fact,over sufficiently turbulent paths, the atmosphericpath-length fluctuations or piston noise can be thedominate noise source, but it is nevertheless useful tounderstand how the LO frequency noise relates to thedesired target spectrum. For example, we considera measurement made over a time tm that is less thanthe coherence time of a particular target vibration atfrequency fv. We can then calculate a limiting rangeRLO for this source where the LO noise at the vibra-tion frequency fv within a bandwidth �f � 1�tm isequal to the power PT from the target vibration in thisbandwidth.

RLO � sin�1 �PT�SLO2 �f �1�2�

c4�f�

. (3)

Because PT � 8��Avfv���2, we can derive a relationbetween RLO and vibration amplitude Av. This isshown in Fig. 7 for the OPO source used in this studyand when we take SLO�f� � 4.8 104�f from the pre-viously measured idler frequency spectrum. Thesecurves for RLO represent a boundary such that, athigher frequency or shorter range, the power that isdue to LO frequency noise is less than the target powerin the vibration bandwidth. When fv� � 2fvRLO�c��1 in Eq. �2�, RLO increases linearly with vibrationamplitude according to RLO � c Av�� SLO�2�f. Also,at low vibration frequencies, the limiting range for thissource would increase linearly with frequency becauseof the 1�f behavior of SLO�f�. We can expect that formany situations the LO noise would not be a concerneven without further efforts to reduce it. For exam-ple, with a 1-s measurement time and �f � 1 Hz, weobtain RLO � 120 km for a 1-�m vibration at 100 Hz.However, the OPO frequency stability could probablybe further improved if we incorporated a feedback con-trol loop from a stable reference, such as a high-finesseetalon.

5. Summary

In summary, we have described a monostatic coherentlaser radar using a single-frequency OPO source at 3.6�m and a demonstration of heterodyne laser radarmeasurements of two different operating trucks. De-tailed surface vibration measurements were obtainedfrom spectrogram processing of the micro-Doppler-shifted return signal. The wavelength of the OPOsource was tuned to 3.605 �m, which lies within anatmospheric window between sharp water lines thatwould be necessary for a longer-range experiment.Also, the frequency stability of the cw source wasshown to be adequate for low-amplitude vibrationmeasurements at long range.

This research was supported by the Internal Lab-oratory Independent Research Program at the Spaceand Naval Warfare Systems Center, and we aregrateful to Ayax Ramirez for use of the Crossbowaccelerometer.

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Fig. 7. Calculated range RLO where band-integrated LO fre-quency noise power from the OPO source is equal to signal powerPT from a vibrating target for given vibration amplitudes.

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