NASA LANGLEY AIRBORNE HIGH SPECTRAL RESOLUTION LIDAR INSTRUMENT DESCRIPTION David .B. Harper(1), Anthony Cook(1), Chris Hostetler(1), John W. Hair(1), Terry L. Mack(2) (1) NASA Langley Research Center, MS468 Hampton, Virginia, 23681-2199, USA, [email protected] (2) Lockheed Martin, MS473 Hampton, Virginia, 23681-2199, USA, [email protected] ABSTRACT iodine absorption line, and an injection-seeded pulsed Nd:YAG laser slaved to the seed [3] and an output NASA Langley Research Center (LaRC) recently conditioning module that provides control over beam developed the LaRC Airborne High Spectral steering, output power, and polarization orientation. Resolution Lidar (HSRL) to make measurements of The seed laser and pulsed slave laser were developed aerosol and cloud distribution and optical properties. by Innolight GmbH and Fibertek, Inc., respectively. The Airborne HSRL has undergone as series of test The electroptic control loop and the output flights and was successfully deployed on the Megacity conditioning modules were designed and implemented Initiative: Local and Global Research Observations by LaRC. (MILAGRO) field mission in March 2006 (see Hair et al. in these proceedings). This paper provides an The receiver employs a 16-inch Newtonian telescope to overview of the design of the Airborne HSRL and which an aft-optics module is kinematically mounted. descriptions of some key subsystems unique to this The aft optics module separates wavelengths, filters the instrument. return to reject out-of-band background light, and separates polarization components. The 532 nm parallel-polarized backscatter is further split in the aft 1 INSTRUMENT OVERVIEW optics module: approximately 4% of the 532 parallel return is sent to the boresight subsystem which A functional block diagram of the HSRL instrument is automatically maintains alignment between the shown in Fig.1. The Airborne HSRL operates as a transmit and receive optical axes; approximately 10% high spectral resolution lidar at 532 nm using the of the parallel backscatter is transferred directly to the iodine vapor filter technique [1-2] and a backscatter detector subsystem; and the remaining 86% of the lidar at 1064 nm. Depolarization is measured at both parallel 532 nm backscatter is directed to the iodine wavelengths. vapor filter module which provides rejection of the Mie component of the backscatter. The 10% channel The laser transmitter consists of four basic measures total (molecular plus aerosol) parallel components: an Nd:YAG CW seed laser, an electro- backscatter and the 86% channel is sensitive to only the optic feedback loop that locks the seed laser to an 11006644nnmm PPaarraalllleell 553322nnmm PPoollaarriizzaattiioonn AAPPDD PPaarraalllleell PPMMTT CChhaannnneell PPoollaarriizzaattiioonn CChhaannnneell 11006644nnmm 553322nnmm PPPPeeoorrppllaaeerrnniizzddaaiiccttiiuuoollnnaa rr AAPPDD PPPPeeoorrppllaaeerrnniizzddaaiiccttiiuuoollnnaa rr PPMMTT CChhaannnneell CChhaannnneell 11006644nnmm TT ransmitteransmitte LaserLaser ((00..TT44ee mmllee ddssiiccaammooppeeeetteerr)) IInn((00ttee..44rrff eennrrmmee nnFFccWWee HHFFiiMMllttee))rr 553322((nn44mm00 ppEEmmttaa))lloonn II22FFVViillaatteepprroorr BBMMaaCC55ccoohh33kkllaaee22ssnnccccnnuunnaammlleettaatt lleerr rr rr PPPMMMTTT TTrraannssmmiitttteedd PPuullssee EEnneerrggiieess BBoorreessiigghhtt 11006644nnmm == 11..00 mmJJ DDeetteeccttoorr 553322nnmm ==22..55 mmJJ Fig. 1. HSRL Functional Diagram molecular backscatter. The resulting optical channels output of the detector is amplified, passed through a are fiber-optically coupled to the various detector phase shifter, and mixed with the original 240 MHz subsystems. signal used to drive the phase modulator. The output of the mixer produces an error signal that is proportional to the difference of the doubled seed laser 2 LASER FREQUENCY LOCKING frequency and the center frequency of the iodine absorption line. The HSRL technique as employed in this system relies on stable locking of the transmitted laser frequency to The error signal from the frequency mixer is fed into a the center of an iodine absorption line. Frequency dual proportional-integral (PI) controller circuit to vary locking of the pulsed laser output is accomplished by the frequency of the seed laser. The first PI controller controlling the frequency of the CW seed laser. A sets the output signal to the piezo element providing piezo element on the seed laser crystal provides fast fast tuning of the seed laser output. However, the frequency tuning and a thermo-electric cooler (TEC) dynamic range of the piezo element is limited, and provides slow frequency tuning. The signals that drive temperature drifts in the seed laser can put the optimal the piezo element and the TEC which control the seed control position beyond the range of the piezo element. laser wavelength are generated in an electro-optic Therefore, a second PI loop is used to monitor the feedback loop involving the frequency doubled seed piezo control output, adjusting the seed laser crystal output at 532 nm. temperature as needed to keep the piezo control output within limits. A block diagram of the laser frequency locking subsystem is shown in Fig. 2. The 532 nm output from the seed laser is phase modulated to produce 3 SPECTRAL PURITY sidebands that are at ± 240MHz about 532-nm output frequency [4]. The phase modulated output is passed In addition to precise output frequency control, the through an iodine vapor cell where the intensity of the HSRL technique relies upon high spectral purity of the center and sideband frequencies are attenuated by output: i.e., the power in the main laser mode centered varying degrees depending upon the difference on the iodine absorption line must exceed the power of between the doubled seed laser frequency and the modes off line center by orders of magnitude. center of the iodine absorption line. When the doubled Otherwise, the signal measured in the “molecular seed frequency is centered on the iodine line, the power channel” downstream of the iodine cell in the receiver in the two sidebands is equal; when the seed is not (different from the iodine cell in the frequency locking centered on the iodine line, a mismatch in sideband subsystem) would include excessive Mie backscatter, power is produced. The optical signal from the phase invalidating the assumptions inherent in the retrieval of modulator (i.e., the doubled laser frequency and the extinction. (We note that this Mie cross-talk could be two sidebands) is incident onto a silicon detector. The characterized and incorporated in the retrieval; however, a system that provides nearly complete 11006644 nnmm sseeeedd bbeeaamm fff +++///---222444000 MMMHHHzzz ((ttoo ppuullsseedd llaasseerr)) fffooo+++///---222444000 MMMHHHzzz ooo yyy yyyyyy nsitnsitnsit sitysitysity nsitnsitnsitnsitnsitnsit eee nnn eeeeee intintint ntentente intintintintintint fff iii fff ooo fff ooo fffrrreeeqqquuueeennncccyyy ooo ffffffrrrrrreeeeeeqqqqqquuuuuueeeeeennnnnnccccccyyyyyy fffrrreeeqqquuueeennncccyyy CCWW sseeeedd llaasseerr PPhhaassee IIooddiinnee PPhhoottoo-- ((ddoouubblleedd NNdd::YYAAGG)) 553322 nnmm MMoodduullaattoorr cceellll ddiiooddee m.m.m. ee sss Crystal TemperaturCrystal Temperatur Crystal PressureCrystal Pressure ttIIuunnnnppiiuunnttggss oo2244sscc00ii llMMllaattHHoozzrr mmiixxeerrI2 tranI2 tranI2 tran fffrrreeeqqqfffuuuccceeennncccyyy pphhaassee aammpp sshhiifftteerr DDuuaall PPII LLoooopp eerrrroorr ssiiggnnaall Fig. 2. HSRL Laser Frequency Locking System CCoommppaarraattoorr PPuullsseedd LLaasseerr PPuullssee IIooddiinnee PPhhoottoo-- LLaasseerr FFiibbeerr OOppttiicc cceellll ddiiooddee AAmmpp -- OOnnee CCaabbllee ++ SShhoott TThhrreesshhoolldd AANNDD DDiiggiittiizzeerr GGaattee TTrriiggggeerr QQ--SSwwiittcchh SSyynncc PPuullssee DDeellaayy Fig. 3. Spectral Purity Detector System rejection of the Mie backscatter can be more easily vapor filter in the receiver is measured periodically. calibrated and provide a measurement with lower Figure 4 shows the receiver iodine filter and calibration systematic error.) subsystem. The filter and calibration system are housed in a light-tight enclosure which includes three Although the laser frequency locking system is very separate optical paths and detectors. The PMT is the robust and performs well, a decision was made during science channel detector and the iodine cell is the design phase of the instrument to add a subsystem positioned upstream of this detector during science data for measuring the spectral purity of the pulsed laser acquisition. During calibration operations, the iodine output to insure that the system was performing as cell is moved to the position in front of the PIN diode designed on a shot-to-shot basis. When the system labeled PIN 1, and 532 nm CW light from the seed detects a laser shot that is not spectrally pure, it over- laser is directed through the cell to PIN 1 and through rides the digitizer trigger and prevents the offending an open path to PIN 2. The seed laser frequency is profile from being recorded. tuned to scan through the iodine line chosen for the measurement. PIN 1 measures the relative shape of the The block diagram of the spectral purity detection line and PIN 2 provides a reference for calculating the subsystem is shown if Fig. 3. A fraction of the transmission through the cell. A confocal transmitted pulsed 532 nm laser beam (~0.5%) is interferometer (not shown) is used to provide picked off and delivered to the spectral purity frequency markers and determine the frequency scan subsystem via optical fiber. The light exiting the fiber range of the seed laser during the scan. is collimated and passed through an iodine vapor cell (separate from the cells in the laser locking and LLiigghhtt TTiigghhtt EEnncclloossuurree receiver subsystems) and then focused onto a detector. 553322nnmm When the laser is operating on a single longitudinal MMoolleeccuullaarr IIooddiinnee mode centered on an iodine absorption line (line center CChhaannnneell cceellll PPMMTT transmission value of 10-6) the light in the spectral IInnppuutt purity subsystem is effectively extinguished in the CCaalliibbrraattiioonn iodine cell. When the laser is operating off line center and/or other longitudinal modes are propagating, light passes through the cell, producing a large pulse at the PPIINN 11 output of the detector. The output pulse from the CCaalliibbrraattiioonn detector is amplified and sent to a comparator where an SSccaann IInnppuutt ffrroomm SSeeeedd PPIINN 22 experimentally determined discrimination threshold is LLaasseerr implemented. Detected pulses with amplitudes above the threshold, hence not spectrally pure, will trip the comparator and gating logic, thereby effectively Fig. 4. Iodine Filter Calibration System defeating the digitizer trigger as shown if Fig 3. Under typical conditions, the measured spectral purity of the system (ratio of offline to online transmission through The receiver iodine cell was built and implemented as a the spectral purity iodine cell) is greater than 5000:1, “starved cell”: a fixed amount of iodine was introduced and the spectral purity system screens zero shots. into the cell before sealing and the cell is heated such that all the iodine molecules are in the gas phase (56º in this system). Implemented as such, the line shape and transmission of the cell are determined solely by 4 IODINE FILTER CALIBRATION Doppler broadening and pressure broadening linewidth To provide a near real-time, internally calibrated changes that are a function of the cell temperature and measurement of backscatter at 532 nm, the absorption pressure, and not variations in iodine density [5]. line shape and absolute transmission of the iodine Measurements of the iodine filter spectrum have shown that the transmission is extremely stable: better than boresight control signals and reduce the achieved 1% over the range of the molecular scattering spectrum pointing accuracy/stability. To avoid these (+/- 5 GHz). Table 1 shows the molecular scattering complications, the Airborne HSRL boresight detection transmission through the filter (~29%) based on the subsystem was implemented using a novel quad fiber iodine transmission values measured for the specified bundle rather than a quad detector. Each quadrant is a dates. bundle of hundreds of fibers with a 25 um diameter core. The output end of each quadrant bundle is Table 1: Molecular scattering (275K, 0.75Pa) spatially randomized and split out to a separate SMA transmission through cell based on the measured iodine fiber connector. The quad fiber bundle can be filter spectrum for the given dates. fabricated to be up to many millimeters in diameter; the diameter of the quad fiber used in the Airborne HSRL Transmission Date is 4.7 mm. The 10 mm dead space between fiber 0.290 9/15/2004 quadrants is much smaller than available in commercial 0.283 10/14/2005 quad detectors and the spatial randomization of the 0.281 4/24/2006 individual fibers within each quad bundle eliminates spatial variability of sensitivity at the field stop image. The light from each quad bundle is allowed to diverge onto a separate single-element large-area PMT, where the signal is amplified, filtered, and digitized. A 5 ACTIVE BORESIGHT SYSTEM software loop running on the data acquisition computer The HSRL instrument utilizes a unique quadrant calculates the pointing error according to (1) and (2) detector to measure the pointing error of the and feeds that error into a PI control loop to determine transmitted laser with respect to the receiver field of the corrective laser beam steering output. view. Typical lidar boresight systems image the field stop on a quadrant PMT or APD to determine boresight error signals as shown in Fig. 5. 6 REFERENCES 1. Piironen, P. and E. W. Eloranta, "Demonstration of a High-Spectral-Resolution Lidar based on a AAA BBB Iodine Absorption Filter". Optics Letters, 19, 3, 234-236, 1994. YY 2. Z. Liu, I. Matsui, and N. Sugimoto, “High- spectral-resolution lidar using an iodine absorption filter for atmospheric measurements,” Opt. Eng. CCC DDD 38, 1661–1670, 1999. XX 3. Floyd E. Hovis, Michael Rhoades, Ralph L. Fig. 5. Typcial Quadrant Detector Burnham, Jason D. Force, T. Schum, Bruce M. Gentry, Huailin Chen, Steven X. Li, Johnathan W. Hair, Anthony L. Cook, Chris A. Hostetler, “Single-frequency lasers for remote (B+D)-(A+C) sensing”, Proc. SPIE Vol. 5332, p. 263-270, Solid x = (1) o A+B+C+D State Lasers XIII: Technology and Devices; Richard Scheps, Hanna J. Hoffman; Eds., July (A+B)-(C+D) 2004. yo = A+B+C+D (2) 4. A. Arie and R. L. Byer, "Frequency stabilization of the 1064 nm Nd:YAG lasers to Doppler- broadened lines of iodine", Applied Optics , 32, 7382-7386, 1993. There are issues which complicate the use of quadrant 5. Crafton, Jim, Campbell D Carter, and Gregory S or multi-element PMT and APD detectors [6], Elliott, “Three-component phase-averaged however. The size of commercially available quadrant velocity measurements of an optically perturbed detectors is very limited, imposing undesirable supersonic jet using multi-component planar constraints on the optical design of the system. Also, Doppler velocimetry”, Meas. Sci. Technol. 12 the dead zone between quadrants is much larger than 409–419, 2001. desired (at least for our optical configuration) and there 6. Sharmin, Paul, ‘Position sensing with can often be a large spatial variation in responsivity photodiodes’ Laser Focus World February, 2002. over the detector active area. It was believed that these issues could lead to large systematic errors in the