Prototype holographic atmospheric scanner for environmental remote sensing

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D18, PAGES 22,287-22,292, SEPTEMBER 27, 1999 Prooype holographic amospheric scanner for environmenal remoe sensing David V. Guerra, Geary K. Schwemmer 2, Alber D. Wooen Jr. 3, Sandipan S. Chaudhuri 3, and Thomas D. Wilkerson 4 Absrac. A ground-based amospheric lidar sysem ha uilizes a holographic opical elescope and scanner has been developed and successfully operaed o obain amospheric backscaer profiles. The Prooype Holographic Amospheric Scanner for Environmenal Remoe Sensing is buil around a volume phase reflecion holographic opical elemen (HOE). This single opical elemen boh direcs and collimaes he ougoing laser beam as well as collecs, focuses, and filers he amospheric laser backscaer while offering significan weigh savings over exising elescope mirror echnology. Conical scanning is accomplished as he HOE roaes on a urnable sweeping he 1.2 mrad field of view around a 42 ø cone. During his echnology demonsraion, amospheric aerosol and cloud reurn signals have been received in boh saionary and scanning modes. The success of his program has led o he furher developmen of his echnology for inegraion ino airborne and evenually saellie Earh-observing scanning lidar elescopes. 1. Inroducion HOTS echnology, we have reesed he opical properies of he The ground-based es faciliy for he holographic opical HOE ha has been operaed in he PHASERS sysem for almos 5 elescope and scanner (HOTS) echnology [Schwemmer, 1993] is years of field use. This is he firs assessmen of opical qualiies buil around a volume phase reflecion Holographic Opical of a HOE afer long-erm use in a lidar sysem. Elemen (HOE). This Prooype Holographic Amospheric Scanner for Environmenal Remoe Sensing (PHASERS) 2. Holographic Opical Elemen employs he HOE o collimae and direc he ougoing laser beam, as well as o collec, focus, and filer he amospheric laser A HOE is a hologram ha exhibis opical power, i.e., he backscaer. As he HOE roaes abou he opical axis of he abiliy o focus and/or direc ligh [Magar#ios and Coleman, sysem, is 1.2 mrad field of view sweeps ou a 42 ø conical scan. 1985]. I derives his opical power from a diffracion paern The laser beam is bore-sighed wih he field of view (FOV) of which is manifes as an index modulaion hroughouhe he HOE as deermined by he field sop. The field sop, which is hickness of a hin film [Kogelnick, 1969]. A reflecion HOE, as locaed on he HOE roaion axis, hen racks he ougoing laser used in PHASERS, is produced by exposing a glass plae coaed beam hroughouhe scan. wih a film of dichromaed gelain emulsion o wo muually In his echnology demonsraion projec, he unique properies coheren laser beams. To produce a focusing HOE, he objec of he HOTS have been uilized o perform boh unidirecional beam emanaes from a pinhole producing spherical wave frons, and conical scans during daa acquisiion. In boh operaional while a second plane wave beam serves as he reference beam, modes, he HOTS echnology has operaed successfully in inerfering wih he objec beam in he gelain. The angle making measuremens of he aerosol backscaer profiles [Guerra beween each beam and he plae deermines he diffracion angle e. al. 1998]. The developmen of his echnology will allow during reconsrucion. Molecular cross links are formed in he larger opical and infrared planeary and Earh-observing phoosensiized gelain wih exposure o ligh, so he inerference scanning lidar elescopes o be deployed, while offering fringes are regisered in he film as variaions in hardness and significan weigh savings over exising elescope mirror index of refracion [Magariios and Coleman, 1985]. The echnology. In his paper, we will give a brief overview of he phoosensiive dye is removed from he gelain during sysem and presen daa aken wih he sysem operaing in boh posexposure chemical processing, and he resuling hologram is saionary and scan modes. Along wih he field es of he relaively free of absorpion. The HOE is hen dried and hermeically sealed wih a cover glass cemened o he film and sealed around he edges. When he compleed HOE is Deparmen of Physics, Sain Anselm College, Mancheser, illuminaed wih a plane wave monochromaic source conjugae New Hampshire. o he consrucion reference wave, a conjugae of he original 2Laboraory for Amospheres, NASA Goddard Space Fligh Cener, objec beam forms a focus or image of he original poin source. Greenbel, Maryland. The opical properies of he HOE in PHASERS can be 3Deparmen of Physics, Wesern Maryland College, Wesminser, undersood as he combinaion of an on-axis inerferomeric zone Maryland. 4Cener for Amospheric and Space Sciences, Uah Sae Universiy, plae [Horman and Chau, 1967] and a slaned opical graing Logan. [Kogelnik, 1969]. To achieve a specific focal lengh f a a given wavelengh l for a reflecion HOE, he radii *i of he circular Copyrigh 1999 by he American Geophysical Union. aperures of he zone plae mus be spaced such ha Paper number 1999JD /99/1999JD ,287

2 22,288 GUERRA ET AL.' PHASERS where i is he number of he circular aperures, wih i = 0, 1, 2,..., couning from he cener of he opic ou o he radial edge, and m is he diffracive order of he ligh ha is focused [Kamiya, 1963]. Assuming an inpu ray normal o he surface of he HOE, he angle 4, a which he HOE deflecs he beam, is given by he graing equaion k=dll + sin (90ø + 40 In (2) where n is he index of refracion, and d is he graing period. The addiional 90 ø is added because he fringe planes of he reflecion HOE are oriened parallel, insead of perpendicular, o he surface. Unlike he radii of he circular aperures, which vary d, across he plane of he HOE, he graing period varies hrough he deph of he HOE. Togeher, he graing perio d and he radii, % Do define he "bowl-shaped" fringe planes ha give he HOE is opical power. I is imporan o reierae ha physically, he HOE is fla and ha he curvaure of he fringe planes is independen of he subsrae. In he lidar applicaion, backscaered laser ligh acs as he Figure 1. The Prooype Holographic Amospheric Scanner for Environmenal Remoe Sensing sysem consiss of a (a) HOE, a (b) laser ransmier, a (c) phoomuliplier deecor, and (d) reconsrucion beam and is focused by he HOE. The wavelengh focusing and collimaing opics. of reconsrucion does no have o mach ha of he consrucion ligh, he HOE can be ailored o achieve maximum efficiency and remove wavelengh-induced aberraions by a number of echniques described by Rallison and Schicker [1995], Jannson possible and plainly observable. The wides FOV of he HOE is and Jannson [1985], and Assenheimer e. al. [1988]. The HOE limied by hose off-axis rays which no longer saisfy he Bragg can also serve as he scan mirror, by roaing i in is own plane. condiion for diffracion. This FOV varies wih HOE design and This will direc he ougoing laser beam such ha i will sweep is narrower for hicker films. However, hinner films wih wider ou a cone of ligh. By placing he field sop on he axis of accepance angles and bandwidhs end o have lower diffracion roaion, he focal plane opics and deecor can remain saic. efficiency. In 1991 he measuremens of diffracion efficiency were The PHASERS HOE, designed a a wavelengh of 532 nm, is performed using a echnique ha included sampling he power of a circular volume phase reflecion hologram of a poin source. he inciden radiaion across he collimaed beam, and wih Mouned on a fla glass subsrae, he HOE has a oal acive knowledge of he deecor head area, he power densiy of he diameer of 40 cm. The HOE was independenly esed in 1991 prior o deploymen in he PHASERS sysem and again by he radiaion inciden on he HOE was calculaed. This power auhors in The ess consised of measuremens of focal densiy was averaged across he enire beam, wih consideraion of he il angle of he HOE and he obscuraion of he collimaor lengh, focal spo size and shape, diffracion angle, and pinhole assembly. The power of he radiaion focuses off he diffracion efficiency. A collimaed ligh source a 532 nm ha HOE was hen measured wih he same deecor. The values of filled he HOE was used o simulae he lidar-backscaered signal averaged inpu power and measured oupu power were divided for he es measuremens. The 1/e 2 diameer of he focal spo o ge he refleciviy percenage. In 1998 he refleciviy was was measured using a beam scan sysem equipped wih a CCD measured by collecing he radiaion from a similar collimaed camera and sofware ha produced a real-ime display along wih spo wih a large Fresnel lens before and afer reflecion. The frame-grabbing capabiliy. This provided he abiliy o adjus he measured values of inpu and oupu power were divided o find locaion of he camera o find he poin of he smalles spo. The he reflecion percenage. Anoher echnique was used o confirm focal lengh was found by measuring he disance from he HOE he resuls from he measuremen mehod employed in In o he locaion of he smalles spo produced by he opic. his echnique, we uilized a mask wih a verical and a horizonal Ineresingly, hese exercises revealed ha he focal spo of he line of holes ha passed hrough he cener of he circular mask. HOE did no vary significanly over a variaion of nearly +2 ø of The 13 holes in he mask made a cross, which had hree holes diffracion angle from he one ha produced he minimum spo. above, below, o he lef, and o he righ of he cener hole. Wih The focal spo size subends 1.2 mrad, herefore limiing he his mask placed in he pah of he collimaed radiaion, before smalles useful FOV and consraining measuremens made wih he HOE, a power meer was used o measure he inciden and our 1.0 mj laser o nigh-ime. The aberraions of he HOE are a refleced power of each poin. Care was aken o be cerain ha combinaion of spherical, asigmaism, and coma. Also, he spo we were measuring he same poin of ligh before and afer he produced by he HOE had a deph of focus comparable o a HOE. Also, i is imporan o noe ha he holes in he grid were mirror wih similar opical parameers. smaller han he size of he deecion surface of he power meer. Since he HOE acs as a lens or spherical mirror, images of Alhough hese wo echniques are significanly differen, he exended sources oher han a infinie objec disance are resuls of reflecion efficiency varied by +2% for he HOEs measured. This agreemen has given us confidence in our efficiency measuremens aken in A summary of he Table 1. Opical Tes Resuls of he PHASERS HOE Propery April 1991 January 1998 resuls from all he opical ess performed on he PHASERS Focal lengh m m HOE are lised in Table 1. Focal spo diameer mrad mrad The echnique employed o measure he refleciviy of he Diffracion angle 43.2 ø + 0.5" 42" +0.5" HOE in 1991 was repeaed in 1998, bu he variaion in he Diffracion efficiency 73% + 8% 59% + 2% measuremens was exreme, + 10%. Thus we abandoned his

3 GUERRA ET AL.' PHASERS 22, " A d (m) I I Us Backscaering Raio Figure 2. A single 1 min. average daa file. The daa are ploed as signal srengh (phoon coun) as a funcion of aliude (meers). mehod and used he mehods explained he above paragraph. and scanning. A ripod suppors he conical baffles, a spider The difference in refleiviies measured in 1991 and 1998, were assembly for he laser-seering mirror, and he phoon-couning due, in par, o he unavailabiliy of he same equipmen and deecor package locaed a he op. The FOV of he hologram faciliies in makes a 42 ø angle wih he normal o he plane of he disk. As All differences in he opical measuremens lised in Table 1 can be visualized in Figure 1, he FOV sweeps ou a conical scan are nearly wihin one sandard deviaion. The small shifs in as i spins abou a perpendicular line hrough is cener, which is diffracion angle may be due o gradual shrinkage and aligned wih he opic axis of he sysem. The hologram is he densificaion of he holographic film over ime, bu we believe only moving componen of his elescope, allowing a ha mos of he differences in he measuremens are due o significanly ligher srucure for supporing he deecor package unavoidable differences in he measuremen echniques. Overall, and baffling, boh of which remain fixed. The ransmier laser hese resuls demonsrae ha he opical characerisics of he beam is direced o a urning mirror posiioned above he HOE, HOE remain fairly consan even under he unconrolled which direcs he beam along he roaion axis o he HOE environmen of he PHASERS roof op faciliy. surface. The beam is hen direced off he HOE coaxially wih he insananeous FOV of he elescope. Since he HOE is employed as he final reflecion surface of he ougoing beam, a 3. Experimenal Apparaus lens is used wih he HOE o form a ransmier-collimaing elescope, which expands he beam. The beam is enclosed by The PHASERS, depiced in Figure 1, is a complee ubes as i propagaes hrough he elescope sysem o help ground-based lidar sysem operaing wih he HOTS echnology. eliminae scaered ligh from enering he deecor and The HOE is placed on a compuer-conrolled, moorized roaing overloading he deecor, which could corrup he weak able ha allows for PC-based remoe operaion for boh poining amospheric backscaer signals.

4 22,290 GUERRA ET AL.' PHASERS 3026 A i d e (m) 744.z bs o Time (Min.) Figure 3. Surface plo of he relaive backscaer signal srengh as a funcion of aliude (meers) and ime (minues). The frequency-doubled, diode-pumped, Q-swiched, Nd:YAG ' damage hreshold esing has no been done, bu i is believed ha laser ransmier for he sysem is hermeically sealed, which he epoxy used o cemenhe cover glass o he HOE film and helps reduce complicaions due o humidiy and emperaure subsae will be he limiing facor. flucuaions. The oupu of he laser oscillaor is exernally frequency doubled o mach he wavelengh of he HOE. To achieve he maximum doubling efficiency, he fundamenal radiaion is focused ino a 3 x 3x 10 mm poassium ianyl phosphae (KTP) crysal. The laser is usually operaed a a repeiion rae of 2 khz. In his operaional mode, he oupu from he Nd:YAG laser has a pulse widh of 20 ns and a power of 4.2 W in he fundamenal, giving an energy per pulse of 4. Resuls PHASERS has been coninually operaed and upgrade during is approximaely 5 years of operaion. In he following secion, hree daa are presened o clearly demonsrae differen abiliies of he sysem. The firs daa se, given in Figure 2, is ha of a 2.1 mj. The frequency-doubled radiaion has a slighly shorer single 1 min averaged daa file aken a 2000 LT on Ocober average pulse widh of 19 ns and a power of 1.9 W, which gives The daa in his figure are represened as backscaering an energy per pulse of- 1.0 mj a 532 nm. This repeiion rae raio as a funcion of aliude in meers. In he earlier version of was chosen because i is he one a which he highes energy per pulse in he green is achieved. The PHASERS HOE showed no degradaion wih exposure o he 1.0 mj, 2 khz laser, which provides a power densiy of 0.5 PHASERS, wih which hese daa were aken, a lower repeiion rae laser was operaed a 1.0 mj a 20 Hz and 1 min averaged daa files were aken. The daa have been background subraced and r 2 correced. The background subracion enailed fiing a mj/cm 2 a he HOE. In a relaed experimen, a similar HOE was polynomial o he daa such ha he base-line of he phoon coun observed o bum in response o illuminaion by a 600 mj, 30 Hz, 532 nm, Q-swiched Nd:YAG laser wih an approximae power was sraigh and se o zero a all values of aliude. From hese daa i is clear ha he sysem can deec reurns from muliple densiy of 190 mj/cm 2 a he HOE. Alhough quaniaive opical layers in he amosphere.

5 GUERRA ET AL.: PHASERS 22, O g i L d e (m) v oo Scan Angle (degrees) Figure 4. Surface plo of he relaive backscaer signal srengh as a funcion of aliude (meers) and scan angle (degrees). The areas of maximum conour have a backscaering raio of In is saionary mode of operaion he HOE is oriened o poin in one direcion and daa are aken over a given period of raio value of 1.870, and he addiional conour lines are a 0.15 inervals. The surface plo in Figure 4 is he projecion of he ime. The 100 consecuive, 1 min average daa files presened cones of daa, swep ou during he scan, ono a fla surface. in he surface plo in Figure 3 were aken beween 2000 and 2140 LT on Ocober 1996 wih he sysem poining norh. The lines of maximum conour are a he backscaering raio value of 1.203, Thus he daa a each aliude on he surface 'plo can be undersood as a series of unraveled circles of daa each saring and ending a Norh. For each scan, he circles have a common and he addiional conour lines are a 0.15 inervals. Wih his axis, wih a circumference ha increases wih aliude. In his ime sequence of daa files i is evidenha he sysem can deec he change in muliple aerosolayers as a funcion of ime. In is scan mode of operaion he HOE is roaed a a fixed rae, and daa are aken coninuously and averaged in predeermined inervals. This essenially divides he sky ino equal segmens of a hree-dimensional hollow cone. A daa se of forma, he srucure ha firs appears disincly in he second scan a -600 ø shifs farher o he souheas in he hird scan. This capabiliy of he sysem will be sudied furher, and a wind measuremen echnique ha uilizes he cross correlaion of aerosol densiies deviaion, as described by Elorana e al. [1975] and Sroga e al. [1980] will be explored. hree coninuous conical scans using he 2 khz laser was aken beween 1900 and 1930 LT on November 3, During his daa acquisiion he HOE was roaed a a consan rae of one revoluion every 10 min, and daa files were sored in 1 min averages. Thus each file represens one enh of he sky, and each 5. Summary This sudy represens he firs successful use of he HOTS echnology o make amospheric backscaering measuremens. scan begins and ends in he norh. I is imporano recognize The HOE was used in boh saionary and scanning modes. ha he cones of daa produced in his process represen he ime evoluion of he sky in he FOV swep ou by he HOE. By Fuure plans include he addiion of a narrow-band filer and he necessary opics for dayime measuremens, along wih addiional analyzing hese daa wih his in mind, amospheric srucures, auomaion of he sysem and daa handling. Because of he such as clouds, can be seen advecing across he sky. A composie diagram of hese daa are presened in a surface plo in success of his demonsraion of he HOTS echnology a new class of ligh weigh scanning elescopes for lidar remoe sensing Figure 4. The lines of maximum conour are a he backscaering from aircraf and spacecraf plaforms are under developmen and

6 22,292 GUERRA ET AL.: PHASERS being esed for addiional applicaions [Schwemmer, 1998; Kamiya, K., Theory of Fresnel zone plae, Sci. Ligh, 2(2), 35-44, Wilkerson e al. 1998]. Kogelnick, H., Coupled wave heory for hick hologram graings, Bell HOEs similar o he one described here are available from a Sys. Tech. J., 48, , Magarifios, J.R., and D.J Coleman, Holographic mirrors, Proc. SPIE, variey of manufacures around he world a coss compeiive 523, , wih convenional reflecive opics of similar aperure size. Two Rallison R.D., and M.A. Schicker, Wavelengh compensaion by ime U.S. lidar sysem vendors have licensed he HOTS echnology for reverse ray racing, Proc. SPIE, 2404, , use in commercialidar sysems. More informaion is available Schwemmer, G., Conically Scanned Holographic Lidar Telescope, U.S. from he offices of echnology ransfer and paens a NASA Pa. 5,255,065, Washingon, D.C., Goddard Space Fligh Cener (GSFC). Schwemmer, G., Holographic Airborne Roaing Lidar Insrumen Experimen (HARLIE), in Proceedings of he 19h Inernaional Laser Radar Conference, NASA Conf. Publ., 623, Acknowledgmens. This research was funded by he Laboraory for Sroga, J.T, E.W Elorana, and T. Barber, Lidar measuremen of wind Amospheres a NASA Goddard Space Fligh Cener. Thanks o Dave velociy profiles in he boundary layer, J. Appl. Meeorol., 19, , Kubalak of Orbial Science Corp. for his help in esing he opical properies of he HOE. Wilkerson, T. D., M. Hammond, and V. B Wickwar, Increased capabiliies for convenional lidars using holographic opics, in References Proceedings of he 19h Inernaional Laser Radar Conference, NASA Conf. Publ., 923, Assenheimer, M., Y. Amiai, and A. Friesem,, Recursive design for an efficien HOE wih differen recording and readou wavelenghs, Appl. Op., 27, , Elorana, E.W., J.M.King, and J.A. Weinman, The deerminaion of D. V. Guerra, Deparmen of Physics, Sain Anselm College, 100 wind speeds in he boundary layer by monosaic lidar, J. Appl. Sain Anselm Drive, Mancheser, NH (dguerra anselm. edu) Meeorol., 14, , G. K. Schwemmer, Laboraory for Amospheres, NASA Goddard Guerra, D. V., A.D. Wooen Jr., S. Chaudhuri, and G. K. Schwemmer, Space Fligh Cener, Greenbel, MD (geary virl.gsfc.nasa.gov) Operaion of he Prooype Holographic Amospheric Scanner for Sandipan S. Chaudhuri and A.D. Wooen Jr., Deparmen of Physics, Environmenal Remoe Sensing (PHASERS), in Proceedings of he Wesern Maryland College, 2 College Hill, Wesminser, MD h Inernaional Laser Radar Conference, NASA Conf. Publ., 879, T. D. Wilkerson, Cener for Amospheric and Space Sciences, Uah Sae Universiy, Logan, UT (dw sdl.usu.edu) Horman, M. and H. Chau, Zone plae heory based on holography, Appl. Op., 6(2), , Jannson, T., and J. Jannson, Bragg holograms and concenraor opics, (Recieved December 18, 1998; revised April 26, 1999; Proc. $PIE, 523, , acceped May 10, 1999.)