Adaptive Beam Director for a Tiled Fiber Array

Transcription

1 Adaptive Beam Director for a Tiled Fiber Array Mikhail A. Vorontov Intelligent Optic Laboratory, Computational and Information Science Directorate, U.S. Army Reearch Laboratory, 2800 Powder Mill Road, Adelphi, Maryland and Intelligent Optic Laboratory, Intitute for Sytem Reearch, Univerity of Maryland, 2107 Technology Venture Building, College Park, Maryland 20742; mvorontov@arl.army.mil Jim F. Riker AFRL/DESM, Air Force Reearch Laboratory, 535 Lipoa Parkway, uite 200, Kiehei, HI 96753; Jim.Riker@maui.afmc.af.mil Ernt Polnau Intelligent Optic Laboratory, Intitute for Sytem Reearch, Univerity of Maryland, 2107 Technology Venture Building, College Park, Maryland 20742; epolnau@mail.umd.edu Svetlana L. Lachinova Intelligent Optic Laboratory, Intitute for Sytem Reearch, Univerity of Maryland, 2107 Technology Venture Building, College Park, Maryland 20742; llachin@mail.umd.edu V. S. Rao Gudimetla AFRL/DESM, Air Force Reearch Laboratory, 535 Lipoa Parkway, uite 200, Kiehei, HI 96753; Rao.Gudimetla@maui.afmc.af.mil Abtract We preent the concept development of a novel atmopheric compenation ytem baed on adaptive tiled fiber array architecture operating with target-in-the-loop cenario for directed beam application. The adaptive tiled fiber array ytem i integrated with adaptive beam director (ABD). Wavefront control and ening function are performed directly on the beam director telecope primary mirror. The beam control of the adaptive tiled fiber array aim to compenate atmopheric turbulence-induced dynamic phae aberration and reult in a correponding brightne increae on the illuated extended object. The ytem i pecifically deigned for tiled fiber ytem architecture operating in trong intenity cintillation and peckle-modulation condition typical for laerilluated extended object and include both local (on-tile) wavefront ditortion compenation and phae locking of ub-ytem. The compenation algorithm are baed on adaptive optimization of performance metric. Local wavefront ditortion compenation i performed uing on-tile tochatic parallel gradient decent (SPGD) optimization of local peckle metric directly meaured on each fiber-tile. Phae locking i performed uing SPGD optimization of a compoed metric, that i, the metric combined from the local metric. An experimental etup i developed to evaluate the feaibility of controlling beam quality by uing peckle metric baed on the temporal analyi of the peckle pattern of light which i backcattered from a laer-illuated extended object and recorded by a ingle photo-detector. The experimental etup i ued to invetigate beam quality improvement, adaptive proce convergence, and the influence of the illuated object hape.

2 1. Introduction It i now well undertood that further development of exiting large-aperture beam forg telecope will lead to a heavy and bulky expenive aembly compoed of monolithic primary and econdary mirror. The hortcog of large-aperture monolithic ytem timulated the recent development of beam director ytem compoed of a phae-locked (coherent) array of mall denely packed laer tranmitter telecope (ub-ytem), referred to a conformal optical ytem. Indeed, the conformal beam forg aembly can be more compact, ignificantly le expenive, and lighter. Moreover, the conformal architecture i calable and robut to element failure. The increaing interet in phae-locked conformal optical ytem ha recently timulated ignificant growth of phaelocking technique. Neverthele, the development of optical conformal beam forg ytem perforg in the preence of atmopheric turbulence repreent a very challenging problem. In fact, it can be hown that tiled fiber array under atmopheric condition with relatively mall Fried parameter r0 require everal thouand highly accurate aligned fiber-tile for full compenation of atmopheric effect if only a piton control (phae-locking) i ued. One of the major problem reulting from the complicated architecture i that the entire ytem i highly vulnerable to environmental influence. For example, vibration, high acceleration, and alo high-thermal gradient are problematic for uch ytem. Thi lead to a trong interet in new robut adaptive beam control capabilitie. In thi paper we introduce a new concept, referred to a Adaptive Beam Director (ABD). The ABD ytem conit of a beam forg telecope with wavefront compenation integrated olely on it primary mirror. Thi new ytem i capable of operating in trong-cintillation condition with both unreolved (point-ource) and reolved (extended) object toward which the beam i directed. The ABD ytem can either be combined with phae-locked tiled fiber array ytem to improve their tability againt environmental influence or it can be implemented in conventional beam director with the monolithic aperture to achieve a robut beam forg capability. The new ABD control ytem introduce additional wavefront compenation degree of freedom beide the piton control into a phae-locked tiled fiber array ytem. Thi introduction ha everal advantage. Firtly, it allow u to achieve good ytem performance with maller number (100 or le) of intalled and mutually aligned fiber-tile. Secondly, thee additional degree of freedom make poible the compenation of fiber-tile mialignment caued by dynamically changing thermal expanion and vibration within the ytem. Thi make the whole ytem more robut to environmental influence. In thi new concept the control of the on-mirror adaptive optic can be performed uing the tochatic parallel gradient decent (SPGD) algorithm. The SPGD algorithm i deigned for laer beam control ytem in which reflected from an illuated object light i ued for SPGD performance metric meaurement. Thee metric act a quality parameter, providing information about the ize of the laer hot pot on the object. Depending on the application cenario and/or the object type, different type of metric can be conidered in the ABD ytem. In thi work we motly concentrate on the time-varying peckle metric which are baed on temporal fluctuation of the peckle field reflected from the extended laer-illuated object. In particular, we invetigate a enitivity of thee metric to the laer beam intenity concentration on the extended object urface and further compare it to the enitivity of the o-called power-in-the-bucket (PIB) metric, typically aociated with unreolved object cenario.

3 2. ABD control ytem 2.1. Deign of ABD ytem Fig.1 illutrate the new on-mirror adaptive beam director concept. The ABD work baically a a telecope, coniting of a primary mirror and a econdary mirror. Thi primary mirror i a key component of the ABD telecope. It contain an array of pocket machined on it backide and i therefore alo called a pocket-mirror. Within thee pocket enor lenlet with photo-detector array, the SPGD controller, on-chip amplifier, and piezo-electrical component are placed (ee Fig.2). The piezo-electrical component conit of electrically ectioned piezo-ceramic annular ring made from thin (~0.3mm) bimorph dic glued to the pocket bottom. By applying a voltage to the electrode of the element, the curvature of the local front (mirror) urface oppoite to the pocket bottom can be changed. Fig.1. Adaptive beam director concept uing either phae-locked tiled fiber array or a olid-tate high energy laer a input (left). On-pocket mirror with integrated wavefront ening and control ub-ytem (right). On the front urface of the pocket-mirror a pecial dielectric layer i depoited which i highly reflective for one wavelength λ il and emi-tranparent for a lightly different wavelength λ b. Thi enable the ytem to work with two different laer beam at thee wavelength. The ource of radiation with wavelength λ il can either be a olid tate laer ource or the output from a tiled fiber array. Thi beam i combined with a econd beam λ b by a beam plitter. The laer ource, the beam plitter and the ABD telecope arranged geometrically in uch a way that after paing the beam plitter, the combined beam are directed to the econdary mirror where they are reflected to the primary mirror and then further reflected toward the targeted object. In the cae of a tiled fiber array a a ource of the beam λ il, the pocket array geometry matche the fiber-collimator array, o that each beamlet of the tiled fiber array ub-aperture enter the correponding pocket region of the pocket-mirror and i then reflected from the pocket window. We note that there exit a modified verion of the ABD ytem working with a ingle laer beam λ il. In

4 thi cae the wavefront controlling element are located within the primary mirror pocket, wherea the photo-element needed to detere the value of the metric are ituated eparately Function of the ABD ytem A mentioned earlier, the ABD control ytem operate in a target-in-the-loop configuration. In thi configuration the laer beam i ent through a diturbing media (atmopheric turbulence) toward a targeted object where it i cattered back and Fig.2. Schematic for wavefront control and ening pocket of the ABD telecope primary mirror. received by a detector. The detector ignal i then ued to cloe the control loop, erving a the input for a beam forg adaptive optic. The goal i to reduce the influence of the diturbing media and optimize the beam quality (hot-pot ize) on the illuated object. In the decribed ABD ytem two laer beam with wavelength λ b and λ il are leaving the ABD telecope and then are directed toward the targeted object through the turbulent air, which ditort the wavefront of both beam in the ame way (ee Fig.1). The laer beam with wavelength λ b produce a beacon on the laer-illuated object. Radiation from both beam i reflected back from the object toward the pocket mirror of ABD telecope but only the reflected radiation from the beacon beam λ b can enter through the dielectric layer into the pocket wherea the cattered radiation from econd beam i completely reflected. The back cattered light from the beacon beam λ b i therefore detected by the photo-enitive element within the pocket and ued for computing an input for the control algorithm which teer the wavefront correcting element. Thi mean that the adaptive element primarily are correcting the turbulence induced wavefront aberration of the beacon beam. Since the difference in the wavelength between the two ued laer beam i only mall which avoid wavelength anioplanatim, the optimization of the pot ize on the illuated object for the laer beam with wavelength λ b automatically alo reduced the influence of the turbulent air and thu alo optimize the pot ize for the laer beam with wavelength λ il. By uing two eparate beam and a dielectric layer which block the tranmiion of illuator beam into the pocket, it i poible to ue on-mirror ening of back cattered light within the beam path without the danger of too powerful illuator beam diturbing the ening of the back cattered light from the illuated object or influence the functional element within the pocket in an other unwanted way, e.g. by cauing trong temperature gradient. Sening of the wave returned from the illuated object i performed inide each pocket either by a ingle photodetector or by a photo-detector array. The ignal from thee detector are ued to detere the metric value J which act a a quality parameter containing information about laer beam intenity concentration on the illuated object. Thi metric i ued a the input for the control algorithm. Wavefront control in thi ytem can be performed uing the tochatic parallel gradient decent (SPGD) technique [1,2]. Uing metric value a the input, the SPGD algorithm compute iteratively new etting for the control voltage applied to the electrode of the piezo-element. Since the voltage of the piezo-element control the curvature of the pocket-window front urface, thi provide a compenation of low-order aberration at each ABD pocket-window. If a phae-locked tiled fiber array i ued a a ource of the input for the illuating beam of the ABD ytem, the pocket mirror provide higher order wavefront correction beyond piton-control for each beamlet.

5 Beide trongly reducing the number of fiber-tile neceary to correct the turbulence-induced wavefront aberration, thi alo offer the opportunity to correct a poible mialignment of the fiber-tile in the phae-locked tiled fiber array, thu making the whole ytem more robut. If otherwie a olid tate laer i ued a a ource, the pocket mirror of the ABD ytem i the only adaptive beam forg element within the whole ytem which provide the capability to optimize the beam on the illuated object. 3. Temporal peckle field analyi 3.1. Time-varying peckle metric and time averaged power-in-the-bucket metric In thi ection we dicu in more detail different type of metric required for adaptive laer beam control. The type of the ued metric depend on the application. Power-in-the-bucket (PIB) metric, averaging the total cattered radiation from the illuated object received by photo-detector, are ueful in the cae of point-ource object or a long a the beam ize i larger than the illuated object. A reduction of the beam ize reult in thi cae in a higher concentration of the laer energy on the object. Thi again lead to a higher intenity of the cattered radiation received by a detector. However, for the cae of extended object, that i when the intenity ditribution of the laer beam completely fall within the outline of the object, thee metric are problematical. In thi cae the power-in-the-bucket metric perform poorly or not at all, depending on the curvature of the urface. Therefore an alternative metric i needed for extended object. Poible candidate for extended object metric are time-varying peckle metric. Thee are baed on the obervation that the ignal of a ingle photo-detector how fluctuation if the detector i placed in the peckle field reflected from the object and if the incident laer beam and object are moving relative to each other. Important i that the trength of thee fluctuation and hence the power pectrum of the ignal are dependent on the hot-pot ize on the object. The maller the hot-pot ize the bigger the fluctuation and alo the higher the frequency component which are preent in the power pectrum. In the ABD ytem, the following cenario can be conidered: (a) unreolved object; (b) extended fat pinning peckle-object; and (c) extended moving peckle-object with low pin or none at all. Depending on the cenario the following method for detering local and global metric can be ued: Ditributed power-in-the-bucket metric for enor unreolved object. Photo-detector, placed in lenlet N p PIB focal plane, allow meaurement of the power-in-the-bucket (PIB) metric JPIB = J j= 1 j, where Np i PIB the number of pocket and J j i the contribution to the PIB metric from the j-th pocket. Maximization of J PIB lead to a compenation of phae aberration and correpondingly increae the object hot-pot brightne. Ditributed parallel time-varying peckle metric for extended object. Thi type of metric i baed on meaurement of temporal and/or patial correlation characteritic of the peckle field reulting from cattering off an extended object urface (peckle metric J ) [3,4]. Adaptive optic optimization of the peckle-metric allow mitigation of atmopheric effect for extended peckle-object [3,5]. Thi proceing of the returned wave require a ingle photo-detector located in the pocket-lenlet focal plane. To eparate different frequency component of the ignal, ignal proceing include pectral analyi of photo-current fluctuation. Since the above decribed ABD ytem ha the capability to work with reolved object it i neceary to invetigate the behavior of time-varying peckle metric more cloely. The relative to the object movement of the incident laer

6 beam needed for time-varying peckle metric can be achieved in two way. Either the object itelf i moving or the laer beam i canning over the urface of a tationary object. Therefore two type of experimental etup one with a pinning object and another with a canning beam are ued to invetigate the time-varying peckle metric [6] Spinning object Reflected peckle pattern Rotating cylinder Optical etup Laer beam Photo detector Fig.3. A chematic (left) and a photo (right) of the experimental etup for peckle experiment with a pinning object. Fig.3 repreent the experimental etup ued for the time-varying peckle metric behavior evaluation for the cae of moving (pinning) object. In thi etup a laer beam with a wavelength of 532nm i cleaned with a micro-optic pinhole combination and collimated by a collimator len with a focal length of 1m. A diaphragm i ued to limit the beam ize after collimation to 50mm. A econd len with a focal length of 300mm i mounted on a moveable tage and focue the laer beam. After the len a rotation ymmetric object with a diameter of 14mm and a rough urface i placed within the laer beam in uch a way that the axi of rotation of the object i perpendicular to the optical axi. The object i mounted on a drill with a rotation peed of 12rpm. By moving the poition of the focuing len, the poition of the focal point relative to the object urface can be moved within a range of ±180mm. By increaing the ditance of the focal point from the urface, the hot-pot ize on the object can be controlled up to a diameter of 30mm. The imum hot-pot ize i 15μm. A photo-detector i placed in the reflected peckle pattern at a 50mmditance from the object urface. The detector ignal i digitized and read into a computer by a digital acquiition board. Fig.4. Dependence of the normalized amplitude of everal frequency component on the pot ize on the object. To ue the peckle metric a a quality parameter for laer beam intenity concentration, it i important to know the

7 dependence of the frequency component amplitude on the laer hot-pot ize. Fig.4. how thi dependence for four elected frequency component (1kHz, 5kHz, 10kHz, 15kHz), each with a bandwidth of 1kHz on the poition of the focal point, and hence on the pot ize on the object a meaured with the decribed experimental etup. A een, all frequency component have their maximum at the ame poition for the mallet hot-pot ize. In addition, the width of the peak for higher frequencie i maller than the width for maller frequencie. Fig.5 how the behavior of the timeaveraging metric <J PIB >, the time-varying metric J, and the combined metric J Σ in dependence on the hot-pot ize normalized to a pinning cylindrically haped object. Here J i a weighted um of the frequency component hown in Fig.4. The weight are choen in uch a way that for a mallet hot-pot ize every frequency contribute equal to J. A een from the Fig5, in the vicinity of mall hot-pot ize, the time-averaging metric <J PIB > how only weak enitivity, wherea the time-varying metric J demontrate rather high enitivity. Thi lead to the concluion that the time-varying peckle metric are in particular ueful for the optimization of the hot-pot ize toward the poible limit. Fig.5 alo how that the combined metric J Σ i advantageou before the ingle metric J and <J PIB >. Although the time-varying metric J and Fig.5. Dependence of the time-averaging metric <J PIB >, the timevarying metric J, and the combined metric J Σ on the hot-pot ize normalized to a pinning cylindrically haped object. The inet are the zoomed maximum region of the metric <J PIB > and J Σ together with the tandard error. The haded area how the accuracy with which the poition of the maximum value, and therefore the hot-pot ize, can be detered with each metric. alo the combined metric J Σ have a larger etimation error for mall hot-pot ize compared to the time averaging metric <J PIB > a i hown in the inet in Fig.5 the range within which the hot-pot ize can be confined due to etimation error i much maller for the time-varying metric and the combined metric. Wherea the combined metric J Σ confine the hot-pot ize within a diameter of 60μm, the time-averaging metric only detere the hotpot ize within a 1.3mm-range. The energy denity within the hot pot can in thi cae be enlarged by a factor of (1.3mm/ 60μm 470) uing the combined metric. Note that for other hape of the illuated object the time averaging metric might perform even wore for mall hot-pot ize. E.g. for object with a uniform flat urface the time averaging metric i not enitive to the pot ize at all if the ize of the hot pot i already maller than the object ize and if the intenity ditribution fall completely within the outline of the object. According to the Lambert coine law which per definition hold for a diffue reflecting Lambertian urface, the time-averaged power-in-the-bucket metric how a dependency on the hot-pot ize only for a curved urface. In addition to the poibility for a better final energy denity the teeper gradient hould alo reult in a fater convergence peed toward final optimization for the control algorithm of an adaptive optic. Thi i important if gradient baed control algorithm like SPGD.

8 3.3. Scanning laer beam Fig.6. A chematic of the experimental etup for peckle experiment with a canning laer beam. To invetigate the ue of peckle metric for non-moving (tationary) object by mean of a canning laer beam an experimental etup i ued containing a tip-tilt mirror to can the laer beam over the urface of the object. The baic etup i hown Fig.6. A laer beam i cleaned with a micro-optic pinhole combination and collimated to a ize of 13mm. The beam i then reflected by a tip-tilt mirror with a diameter of 14mm toward a relay-len combination to increae the beam ize to 50mm and focu it on the object at a ditance of 8.25m. A beam plitter i placed between the tip-tilt mirror and the firt relay len, making it poible to image the illuated object on the focal plane array of a CCD camera. One pixel of the camera cover a quare of about mm on the object. The tiptilt mirror i connected to two ignal generator which are et to 3kHz (horizontal) and 2.5kHz (vertical). Since the relative peed between the object and the laer beam i much higher in thi etup than in the non-canning experimental etup decribed above higher frequencie (5kHz, 50kHz, 100kHz, 150kHz) with a bandwidth of 10kHz are choen to detere the time-varying peckle metric J. One important quetion for the canning cae i how the canning amplitude influence the behavior of the peckle metric. The meaured dependence of peckle metric on the hot-pot ize b for three different canning amplitude i hown in Fig.7. The inet how the image of the laer on the object (mallet hot-pot ize b ) for a) no canning, b) canning with an amplitude of one time the imum hot-pot ize b, c) canning with an amplitude of two time the imum pot ize b, and d) canning with an amplitude of three time the imum pot ize b. A een from the figure, the maximum of all curve in Fig.7 are one the ame poition correponding to the imum hot-pot ize b. The width and hape of the curve are the ame for all canning amplitude. Only the magnitude of the metric J i changing for different canning amplitude.

9 4. Concluion The ABD ytem ha everal advantage compared to conventional tiled fiber array ued for laer beam control. Firtly, there i a dramatic reduction of ytem complexity and improvement of robutne againt environmental influence which i offered by the direct integration of variou beam control ytem directly on the primary mirror. Secondly, the ytem ha a wide range of potential application ince thi concept of beam control work with unreolved and extended pinning and/or fat moving object in the condition of trong peckle modulation and intenity cintillation. Finally, the beam control approach dicued in thi paper allow fat and high reolution return-wave ening achieved with multiple photoenor and control hardware operating in parallel. Our experiment how that time-varying peckle metric can be ued a metric Fig.7. The dependence of peckle metric on hot-pot ize for three different canning amplitude i hown. The inet how the image of the laer on the illuated object (mallet hot-pot ize b ) for a) no canning, b) canning with an amplitude of one time the pot ize b, c) canning with an amplitude of two time the pot ize b, and d) canning with an amplitude of three time the pot ize b. containing the information about the pot ize on the illuated extended object. Due to trong enitivity of thee metric for mall laer pot ize, they can increae the convergence peed of gradient baed control algorithm, uch a SPGD algorithm, and are in particular ueful for the optimization of the laer pot toward the poible optimum. Moreover, a combination of the time-varying peckle metric and the power-in-the-bucket metric reult in a univeral metric which can be ued for both the unreolved a well a for reolved object. 5. REFERENCES 1. Weyrauch T., Vorontov M., Berenev L., and Liu L., Atmopheric compenation over a 2.3 km propagation path with a multi-conjugate (piton-mems/modal DM) adaptive ytem, Proc. SPIE, Vol. 5552, 73-84, Vorontov, M. A., Carhart G. W., Cohen M. and Cauwenbergh G., Adaptive optic baed on analog parallel tochatic optimization: analyi and experimental demontration, JOSA A, Vol. 17, , Vorontov M.A. and Carhart G., Adaptive phae ditortion correction in trong peckle-modulation condition, Optic Letter, Vol. 27, , Polejaev V.I. and Vorontov M.A., Adaptive active imaging ytem baed on radiation focuing for extended target, SPIE, Vol. 3126, , Vorontov M.A. and Koloov V., Target-in-the-loop beam control: baic conideration for analyi and wavefront ening, JOSA A, Vol. 22, , Polnau E., Vorontov M.A., Beam Quality Metric Baed on Temporal Speckle Field Analyi in Directed Energy Application for Extended Target. JOSA A to be publihed, 2006