Development of the - PDF document

1 Development of the Lidar Atmospheric Sen
Development of the Lidar Atmospheric Sensing Experiment (LASE) -An Advanced Airborne DIAL Instrument Alvah S. Moore, Jr., Kevin E. Brown, William M. Hall, James C. Barnes, William C. Abstract. The Lidar Atmospheric Sensing Experiment (LASE) Instrument is the ° C tC. After power is applied, the operation of LASE is totally autonomous. LASE caavontinuous operation with PILOT SWITCH ER - 2 XMIT PCM DOWN LINK PCM OPTICS & l l FILTERS APD DET.88% SIGNAL RETURN LASER DIODE Nd:YAG PUMPLASER Ti:SAPPHIRE POWER OSCILLATOR TUNABLE LASER SUBSYSTEM (TLS) ll ONll OFFll ONll OFFFIBER OPTICCOMMANDS & DATA ON/OFF ll SEEDDBL PULSE TIME BASE LASER OUTPUTTROPOSPHERE APD DET. 12% ABCDHIMEDLOW DIGIT. DIGIT. DIGIT. GAIN CONT.SIGNAL PROCESSOR RECORDERMICRO COMPUTER TUNABLE LASER RECEIVERSIGNAL PROCESSORhown in Figure 1, LASE is divided into four main parts: Tunable LaserSubsystem (TLS), Receiver Subsystem (RCS), Signal Processor Subsystem (SPS) andthe Control and Data Subsystem (CDS). Each of these subsystems are discussedThe TLS was designed to operate in a double-pulse mode (separated by 400 sec) at5 Hz, with energy outputs of up to 150 mJ per pulse in the 813 to 819 nm wavelengthregion and with 99% of the output energy within a spectranterval of 1.Ti:Sapphire (Ti:Al) power oscillator was constructed using a frequency-doubledNd:YAG laser as the pump sournd a single mode diode laser as an injectionseeder for the Ti:Allaser [5,6,7].In the laser schematic of Figure 2, a flashlamp pumped Nd:YAG laser delivers 1.4 J at5Hz. Is doubleulsey a Q-switch into a highly deuterated CD*A secondharmonirystal whicenerates 530 mJ of 532 nm laser energy for pumping of the Ti:Al power oscillator. The Ti:Al power oscillator produces output energies of Nd:YAG Oscillator Isolation Optics Amp 1 Amp 2 CD*A GRM HR HRR BRF BS Diode Seed Faraday Isolators H 2 O Reference Cell Etalon Detector M M M M M BS M Ti:Al 2 O 3 Output Fig. 2. Sch

2 ematic representation of the LASE TLS op
ematic representation of the LASE TLS optical layout. M The Ti:Al power oscillator cavity is an unstable resonator design [8] having acavity length of 1.5 metersEach of the twrewster cut Ti:Al rodhas anactive length of 18 mmOther oscillator components include a graded reflectivemirroGRM) output coupler, a 97% reflectivnd mirror (HR), a four platebirefringent filteBRF) trovide coursontrol and narrowing of the outputwavelength, and a hollow retro-reflector (HRR)which provides alignmentinsensitivity in the horizontal planeThe unseeded Ti:Al laser has a spectrallinewidth of 1 nm and a tuning range which includes the required 813 nm tnmwavelength region. The laser pulsewidth is 30-40 ns FWHM and is dependent on thepump energy fluence. Its beam diameter is 3.2 (at 1/e) with a beam quality of 1.3times diffraction limited. Fine linewidth and wavelength control of the Ti:Al laseris achievey using the output of a single mode diode laser as an injection seedsource [9]. The diode seeeam is ontinuous wave of 100 mW and is injectedthrough the 97% HR end mirror. Typically 1 mW of seed power is transmitted into thepower oscillator cavity. Injection seeding allows control of the spectranewidth toless than the required 1.0 pm anrovides wavelength stability tetter than +/-0.25pm as measured using a high finesse Fabry-Perot Interferometer. Injection seedingalso achieves the LASE spectral-purity requirement of maintainireater than 99percent of the laser output energy within a 1.06-pm interval. Spectral purity wasmeasured using thbsorption-to-transmission ratif laser pulse through a 200mpath length water vapor filled cell. To achieve this required spectral fidelity, the diodelaser wavelength is locked onto a preselected water absorption line feature by passinga fraction of its frequency modulated lighhrough a multipass reference cell filledwith water vapor and detecting the cell transmission of the light. By detecting the nullof the transmitted

3 light, the diode wavelength can be locke
light, the diode wavelength can be lockento thbsorptionfeature. Electronic feedback control of the diode temperaturnd current serve to maintain the diode’s wavelength line-locked to the feature. Typically, 3 mA/pm is thecontroaw rate for the feedback in controlling the wavelength. Thurrent can beadjusted in 1 uA increments. The tunable diode laser seeds the pulsed laser alternatelybetween "on-line" wavelength, the first pulse of the pulse pair, located at the center ofthe water vapor linnd "off-line" wavelength, the seconulse of the pulse pair,typically located 20 tm away from the "on-line" wavelength. The "on-line" and"off-line" wavelengths are measured to within +/-1pm using a wavemeter. Theaccuracy of the "on-line" wavelength is verifiey comparisoto the line-lockedwavelength of the diodnd is further validatey spectral purity measurements.Ninety (90) db of isolation between the diode and the Ti:Al HR end mirror is usedto prevent pulsed Ti:Al energy feedback to the diode laser. Laser energy feedbackto the diodould cause diode damagnd mode-hopping. A 150 m thick etalon isused to suppress any side bands of the diode laser output before it enters the Ti:Alresonator where it would decrease spectral purity.A new diode seeding technique was developed to enhance the capability of the LASEinstrument [10]. The stroertical absorption gradient of atmospheric water vaporhad required the LASE measuremeno typically usstrong water vapor line todetect low concentrations of water vapor at high altitudes and weak water vapor linesto detect mucigher concentrations aower altitudes. Operationally this has meantrecording high altitude water vapor profiles over a predetermined aircraft groundtrack in one leg of a flighanthen retracing this ground trackin nother leg ofthe flight to record low altitude water vapor dataThe new multi-wavelengthsequential diode seeding approach uses a wavelength thas accurately positioned (towithin 0.m) on

4 the slope of a strong water vapor absorp
the slope of a strong water vapor absorption feature, hence,enabling thurate selection of the size of absorption cross-section te probed.Thilope position is accomplishey a precisurrent pulse to the diode that hasbeen characterized. The new approach allows a single strong water vapor line tused trobe both the higher and lower altitudes along a single ground trackIn arepeating sequence, a pulse pair of "on-line" and "off-line" (which probes highaltitude water vapor) alternates with a pulse pair o"side-line" and "off-line" (whichprobes the lower altitude water vapor). In this way nearly simultaneous measurementsof thtmosphere from sea leveo about 14 km aromplished along a singleThe LASE RCS telescope is a mechanically and thermally stable F/21 Dall-Kirkhamdesign. The barrel of the telescope is graphite-epoxy anesigned to maintain theseparation of the primary and secondary mirrors, over the LASE operatingenvironment, to within 25 m for longitudinal stability. The telescope optics are light-weighted Zerodur for thermal stability with an 800-cm focal length, a 0.1 mcollectingThe RCS aft optics are polarization insensitive and include an interference filter that isactively tilt-tuned to the desired water absorption line wavelength. The received light plit for three output channels: an engineering channel and two scienhannels.Thngineering channel uses a 20 diameter silicon quaetector for measuringthe laser-to-telescoplignment. A 1.5-diameter silicon avalanche photodiodedetector (APD) is placed at the focus of each of the two optical science channels. Theaft optics layout is shown in Figure 3. A microprocessor based Receiver Control Unitcontrols and monitors all of thlectro-mechanical functions including the shutter, The light enters the aft optics through the primary hole and is folded into the plane ofthft optics using a turn mirror. A shutter and adjustable field stop (not shown) areplaced ahe telescope focus. The shutter is periodical

5 ly closed for backgroundelectronic noise
ly closed for backgroundelectronic noise measurement and calibration. The field-of-view is adjusted using aniris mounted in a rotary stepper motor. Approximately 1 percent of the light is directedto the quaetector channel. The field stop is imagento the quaetector forground alignment of the laser to the telescope. The remaining ercent of the lightpasses through one of two interference filters designed to reduce background lightlevels. The light is then split at the APD beamsplitter in order to increase the dynamicrange of the system; ercens reflected to the low-gain APD channel anpercens transmitted to the high-gain APD channel. The beamsplitter is polarizationinsensitive and slightly wedged. A uniform image at the APDs is obtained by imagingThe Receiver contains two interference filters mounten a rotation stage. One filteris used for day and the other for night missions. The typical day filter has a peaktransmission of 0.48 and a full widtaft maximum (FWHM) bandwidth of 350 pm.The peak transmission of a typical night filter is 0.65 with a FWHM of m. Theselected filter is tilt-tuned to the desired laser wavelength and is adjusted tocompensate for ambienemperaturhanges. The filters are used icollimated Triplet for High GainChannel Triplet for Low GainChannelAPDBeamsplitterBeamsplitter Triplet for Quad Detector Interference FilterField LensSecond TurnMirror light, with a half conngle of 1.egrees and a 0.egree obscuration, thauniform with incident angle. The filter throughpus a function of the filter centerwavelength, thffective index of refraction, the wavelength of incidenght, theThe light signals from the RCS are converted to electrical signals ahe APDs, whichare followey transimpedanmplifiers. Each APD and amplifier is housed in adetector preamplifier unit (DPU) in the SPS (Fig. 1). The low gain DPU maintainsconstant responsivity over the operating temperature range by actively adjusting theAPD bias voltage. Because the hiain DPU

6 is more noise sensitive, low noisndcons
is more noise sensitive, low noisndconstant responsivity are maintainey cooling its APD to ontrolled settemperature of egrees Celsius whilpplying a fixeias voltageTheresponsivity of each of the DPUs was nominally seamps per watt aheoperating wavelengthsThe responsivity was determined as ompromise betweenthe effects of a high responsivity, resulting in an undesirable high excess noise factorat high signaevels, and a low responsivity, resulting in adesirable hioiseequivalent power aow signaevelsThe bandwidth of each of the DPUs wasThlectrical outputs of both DPUs enter the differentianputs of the SignalProcessing Module (SPM) to receive further amplification and filteringThe twoSPM inputs arlectrically spn order to further increase the dynamic rangeThehigh gain DPU output into channels A and B, and the low gain DPU outpusplit into a switch selectable channel C or D. Before leaving the SPM, all three outputchannels pass through a 1.5 MHz Bessel filter, to sehe system analog bandwidth,beforntering theiespective it digitizerThe digitizer conversion speed is 5MHz which exceeds the Nyquist sampling criteriaThe LASE Coherent Timebaseprovides the digitizer clock and trigger pulses which synchronize the digital output ofThe operation and data acquisition of LASE is controlled by the CDS subsystem. Thebasic functions aroordinatey internal commands through thontrol states of 2modes ansubmodes. The modes are STANDBY and OPERATE. The submodesare WAIT, TUNEand DATA. After the LASE Instrument receives power duringpreflight, the initial command is STANDBY:WAIT. The instrumens held in thisstate while thircraft climbs to altitude, during which time the laser cavitytemperature itabilized. Safety inter-locks and a pilot switcave te satisfiedbefore LASE can advance out of this state. When the ER-2 is at operational altitude, apilot switch will activate the OPERATE state. The TUNE submode begins thescanning of a pre-selected water-line with the seedin

7 g diode laser. The scan consist ofoarsnd
g diode laser. The scan consist ofoarsnd fine scan to ensurn acceptable lock on the water-line. When the scan analyses aromplete, the submode switches to DATA. Ahis time the Nd:YAGpump laser flashlamps begin flashing with a delayed Q-switch for pre-conditioning ofthe optics. The timing of the Q-switch is gradually shifted to maximize laser beamenergy. The SPS iynchronized to the return signals by the time base module whenthe laser pulse leaves the instrument. After storing the return signals from the firstlaser pulse, the second laser pulse is emitted (400 sec after the first). The digitizationof this second set of return signals completes the cycle for one DIAL data sample. TheLASE Instrument repeats this operation at 5 Hz throughouhe mission. LASE willreturn to STANDBY on deactivation of the pilot switch or thltitude interlockswitch. The CDS monitors engineering parameters from the instrument, comparingeach to a set of limit conditions. When an irrecoverable lims exceeded, the CDSwill return the instrument to STANDBY and, through a FAIL light, inform the pilot toreturn to base. For more critical limits, the CDS may remove power from one or moreThe CDS controller is bunto a CAMAC crate using off-the-shelf microprocessoranata-acquisition modules. The unique hardware functions were bun-house tobe CAMAC-compatible. The CDS data recorder is a single-board-computer PC-basedsystem using two 1-Gbyte off-the-shelf hard disk drives. The disk drives are containedin a pressurnister to allow high-altitude operations. Total recording time is 9.5hours. When flights are within a 230 mile range, a down-link of the LASE data streamto the ground station is made possible using the RF transmitter aboard the ER-2aircraft. This allows the processing of LASE water vapor and aerosol data foealLASE is an advanced lidar instrumenhas fully engineered to meeherequirements of making precise DIAL measurements of water vapor for the totalcolumn in the troposphe

8 re. The LASE Instrument hauccessfully co
re. The LASE Instrument hauccessfully completedextensive characterizatioin the lab and validation testing in the field. LASE ielf-calibrating anes not need external reference data ttain accurate water vaporIn addition to collecting science data for NASA's Mission To Planet Earth Program,LASE will be used as a testbed temonstratdvanced airbornnd spaceborneDIAL technologies. Thesdvancements wnclude new lasers, detectors and The authors wish to thank the technical staff at Langley for their efforts in designing,developing and testing to make LASE a world class instrument to support the Mission1.Browell, E. V., Wilkerson, T. D., McIlrath, T. J.: Water vapor differential absorption lidar2.Browell, E.: Remote sensing of tropospheric gases and aerosols with an airborne DIALsystem. In Optical Laser Remote Sensing, editey D. K. Killinger and A. Mooradian,3.Ismail, S., Browell, E. V.Airbornnd spaceborne lidar measurements of water vapor4.Browell, E. V.: LASE Validation Experiment. In 18th ILRC Proceedings, Springer Verlag5.Barnes, J. C., Edwards, W. C., Petway, L. B., Wang, L. G.: NASA Lidar AtmosphericSensing Experiment's Titanium-doped Sapphire Tunable Laser System, Optical Remote , Salt Lake City, Utah, March 1993.Barnes, N. P., Barnes, J. C.: Injection Seeding I: Theory, IEEE Journal of QuantumBarnes, J. C., Barnes, N. P., Wang, L. G., Edwards, W. C.: Injection Seeding II: Ti:AlExperiments, IEEE Journal of Quantum Electronics, Vol. 29, No. 10, October, 1993, pp.Rines, G. A., Moulton, P. F., Harrison, J.: Narrowband, High Energy Ti:Al LidarTransmitter for Spacecraft Sensing, Proceedings of the OSA Topical Meeting on TunableWang, L. G., Barnes, J. C., Edwards, W. C., Hess, R. V., Ponsardin, P., Sasche, G.: DiodeLasenjection Seeding of a Pulsed Ti:Al Laser for Remote Sensing, OSA 1991Sachse, G. W., Wang, L. G., Ismail, S., Browell, E. V., Banziger, C.: Multi-WavelengthSequential Seeding Method for Water Vapor DIAL Measurements, Optical R