Innovative technologies are needed to support and augment the development of various types of deformable mirrors (DM), such as Micro Electro Mechanical Systems (MEMS), segmented, bimorph and membrane types that are currently used in adaptive-optic (AO) systems. The paper discusses the results of initial studies that, could, potentially, be employed for full characterization of the dynamic behavior of adaptive optics mirrors. The experimental data were obtained from a typical bimorph mirror using both, a Shack-Hartman wavefront sensor (SHWFS) and an Imaging Laser Doppler Vibrometer (ILDV) developed exclusively by AS and T Inc. These two sensors were employed for quantitative measurement of both the spatial and temporal dynamics of the DM under broadband excitation via the piezo electric drive elements. The need to characterize the spatial and temporal dynamic response of current and future DM mirror designs is essential for optimizing their performance to a level adequate for high bandwidth AO systems, such as those employed for real-time compensation of wavefront perturbations.
Atmospheric turbulences can significantly deteriorate the performance of long-range conventional imaging systems and
create difficulties for target identification and recognition. Our in-house developed adaptive optics (AO) system, which
contains high-performance deformable mirrors (DMs) and the fast stochastic parallel gradient decent (SPGD) control
mechanism, allows effective compensation of such turbulence-induced wavefront aberrations and result in significant
improvement on the image quality. In addition, we developed advanced digital synthetic imaging and processing
technique, “lucky-region” fusion (LRF), to mitigate the image degradation over large field-of-view (FOV). The LRF
algorithm extracts sharp regions from each image obtained from a series of short exposure frames and fuses them into a
final improved image. We further implemented such algorithm into a VIRTEX-7 field programmable gate array (FPGA)
and achieved real-time video processing. Experiments were performed by combining both AO and hardware
implemented LRF processing technique over a near-horizontal 2.3km atmospheric propagation path. Our approach can
also generate a universal real-time imaging and processing system with a general camera link input, a user controller
interface, and a DVI video output.
Experimental study of internal phase locking of a seven fiber collimator array is presented. As a
metric for the feedback loop the periphery areas (tails) of beams outgoing from three adjacent fiber tips
are used before the beams are clipped by the lens apertures. The "intercepted" tails of beams are
redirected back into the collimator array forming an interference pattern located between adjacent
collimators. Optical energy from one region of the pattern is selected by a pinhole, detected with a
photo-diode and used as a metric signal for an SPGD controller to lock the phase of the three adjacent
beam tails. The non-common phase difference of the outgoing wavefronts from these three collimators
can be manipulated by altering the position of the pinhole in focal plane of the interference pattern and
is removed (set to "zero" or 2π increments) by a displacement selected to produce the expected far field
interference pattern. To phase lock the beams from seven collimators arranged in a hexagonal array,
three pinholes, each of which receives some light from the center collimator are used. A sum of the
signals from the three photo-diodes placed behind these pinholes is used to lock the phase of the six
periphery beams to the central uncontrolled reference without beam splitters or a remote target-in-theloop
metric.
We describe bimorph piezoelectric fiber actuators, which enable tip and tilt control in fiber collimators, as well as their
integration into a fiber-array based beam-projection system. A mechanism for alignment of individual fiber actuators
within the array was developed. It provides six degrees of freedom and allows for high subaperture density. The
alignment procedures for a seven-subaperture prototype system as well as first results from evaluation experiments are
presented.
We present the design and evaluation of compact adaptive optical antennas with apertures diameters of 16 mm and
100 mm for 5Gbit/s-class free-space optical communication systems. The antennas provide a bi-directional optically
transparent link between fiber-optical wavelength-division multiplex systems and allow for mitigation of atmospheric-turbulence
induced wavefront phase distortions with adaptive optics components. Beam steering is implemented in the
antennas either with mirrors on novel tip/tilt platforms or a fiber-tip positioning system, both enabling operation
bandwidths of more than 1 kHz. Bimorph piezoelectric actuated deformable mirrors are used for low-order phase-distortion
compensation. An imaging system is integrated in the antennas for coarse pointing and tracking. Beam
steering and wavefront control is based on blind maximization of the received signal level using a stochastic parallel
gradient descent algorithm. The adaptive optics control architecture allowed the use of feedback signals provided locally
within each transceiver system and remotely by the opposite transceiver system via an RF link. First atmospheric
compensation results from communication experiments over a 250 m near-ground propagation path are presented.
Fast piezoelectric controlled adaptive optics elements are developed allowing one to design the adaptive optics
system with compensation of optical phase aberrations in frequency range exceeding 10 kHz. Adaptive optic elements
include: 13-channel deformable mirrors DM-13 with aperture 25 and 30mm, combine device formed with DM-13
attached to the tip/tilt platform, 49-channel deformable mirror PM-49 based on in-pockets piezoelectric multi-pixel
actuators. The designs, fabrication and performances of the developed devices are described. The emphasis is made on
the experimental investigation of resonance behavior of the devices and influence of the passive damping on the
frequency bandwidth.
In this paper, we present the recent development of a conformal optical system with three adaptive phase-locked fiber elements. The coherent beam combining based on stochastic parallel gradient descent (SPGD) algorithm is investigated. We implement both phase-locking control and wavefront phase tip-tilt control in our conformal optical system. The phase-locking control is performed with fiber-coupled lithium niobate phase shifters which are modulated by an AVR micro-processor based SPGD controller. The perturbation rate of this SPGD controller is ~95,000 iterations per second. Phase-locking compensation bandwidth for phase distortion amplitude of 2π-radian phase shift is >100Hz. The tip-tilt control is realized with piezoelectric fiber positioners which are modulated by a computer-based software SPGD controller. The perturbation rate of the tip-tilt SPGD controller is up to ~950 iterations per second. The tip-tilt compensation bandwidth using fiber positioners is ~10Hz at 60-μrad. jitter swing angle.
A newly developed adaptive optical transceiver telescope is used to investigate the possibility of correcting wavefront
aberrations under strong atmospheric turbulence conditions over a distance of several kilometers. A fiber laser is
connected to a fiber positioner within the telescope, which acts as a transmitter by sending laser light at wavelength 1550
nm through the turbulent air to a retro reflector mounted on the top of a water tower at a distance of 2.33 km. The
reflected laser light is received by the telescope -(acting this time as a receiver) and focused onto the fiber tip. The light
picked up by the fiber is - guided to a photo detector by means of a fiber splitter. The signal from the photo detector is
recorded by a PC and used as feedback signal for the adaptive optics controller, which controls the fiber-tip positioner
as well as an six-channel adaptive mirror using a stochastic parallel gradient descent optimization algorithm.
Experimental results are reported in this paper.
An adaptive laser communication system capable of minimizing the impact of atmospherically induced tip-tilt and defocus distortions and tracking an optical antenna mounted on a mobile platform is presented. The laser communication system consists of two independently operating adaptive optical transceivers mounted on wide angle pan-tilt gimbals that each transmit and receive optical data propagating on a single bidirectional laser beam. Each optical antenna incorporates a 3-channel tip-tilt and defocus adaptive mirror constructed at the Army Research Laboratory. The adaptive mirror control system of each optical transceiver is based on phase control of the outgoing wavefront by stochastic parallel gradient descent optimization of the optical power received by the other optical transceiver. A tracking capability of an optical transceiver mounted on a mobile platform is implemnted by using the pan-tilt control signals of each adaptive mirror to define signals that drive the pan-tilt axes of the gimbal on which the adaptive mirror is mounted. System performance in the presence of laboratory generated turbulence is characterized and reported. Results demonstrate that the adaptive laser communication system can reduce signal fading induced by atmospheric tip-tilt distortions and improve the concentration of laser beam energy delivered to each optical transceiver and therefore reduce system bit error rates.
The proper collimating of the laser beam between the fiber optics transmitter and receiver is among the most important requirements to free space telecommunications. The targeting of the receiver over near-horizontal propagation in kilometers long distance requires fast control of the tip/tilt deviation of the laser beam emitting from the fiber optic transmitter because of beam direction fluctuations due to atmospheric turbulences. This deviation can be accomplished with fast deviation of the fiber optics emitting tip placed into focus of the collimating lens. Similar control could allow one to keep the input fiber optics tip in focus of the receiver's input lens.
In this report we describe the design of the compact adaptive fiber optics collimator based on the X-Y positioner of the fiber optics tip utilizing the piezoelectric bimorph actuators for X-Y displacement of the fiber tip placed in the focal planes of the transceiver.
The compact adaptive optics mirror is described, which allows compensation of wave front distortions related to low-order aberrations such as tip/tilt, focus/defocus, astigmatism, etc. of the laser beam distorted, for example, because of propagation through atmospheric turbulences.
We discuss the expansion of wavefront distortion compensation based on stochastic parallel gradient descent (SPGD) optimization to the control of several wavefront correctors. We describe then a SPGD adaptive optics system that uses a low-order deformable mirror with modal control and a high-resolution (either 132 or 320 control channels) piston-type MEMS mirror. The system was installed at a 2.3km near-horizontal propagation and used for atmospheric compensation experiments. Results obtained for different system configurations are presented.
A method of fabrication of twisted nematic device is described utilizing the monomolecular chemisorbed layer on at least one internal transparent electrode of capillary cell. The layer is capable of aligning and re-aligning the liquid crystal molecules upon UV irradiation. The preliminary uniform LC orientation with high contrast of the order of 1000 occurs after filling the LC material into the cell, which is irradiated before filling with small UV dosage of polarized light of the order of 1 mJ/cm2. The obtained orientation can be further re-aligned and desirable local twist configurations can be obtained using the second irradiation with UV light having polarization different from that of the first irradiation.
A high-resolution phase contrast technique is applied for the measurement of an atmospheric-like artificial turbulence distribution along laser beam propagation path. This technique is based on a nonlinear Zernike wave front sensor that employs an optically addressed ferroelectric liquid crystal spatial light modulator. The measurements of dynamically changing phase distortions located at different distances from the wave front sensor were obtained. The spatial resolution, dynamic range, and time response of this technique are examined and compared with a theoretical analysis.
The experimental search of the shape of electrical voltage is presented for controlling the optical phase modulator based on dual frequency nematic liquid crystal. Two basic approaches were tested. One is based on universal relation between two driving frequencies, below and above inversion point, respectively. Another approach uses the combination of pulses of both frequencies, which allow us, at first, to provide the fast transition between chosen retardations and , at second, to hold final retardation for the necessary duration.
A sub-millisecond wavefront sensor based on a non-linear Zernike filter is described. The sensor's key element is an optically addressed spatial light modulator (OASLM). The OASLM is composed of ferroelectric liquid crystal and an amorphous silicon carbide photo-conducting film. The OASLM is placed into the focal plane of a Fourier filtering system. The sensor operates with circularly polarized input light. For the high intensity (zero order) spectral component the deviation of the ferroelectric liquid crystal director by (pi) /4 (45 degree(s)) results in a (pi) /2 phase shift. This provides wavefront phase distortion visualization similar to a conventional Zernike filter. The registered characteristic response time of the sensor is near 0.2 ms for light intensity on the order of 100 nW.
The reflective type optically addressed spatial light modulators (OASLMs) with an amorphous hydrogenated carbon (a- C:H) light-blocking layer (LBL) sandwiched between an intrinsic hydrogenated silicon carbide (a-Si:C:H) photoconductor and broad-band and narrow-band dielectric mirrors have been developed. FLC was used as a light modulating medium. The DHF effect in an FLC with tilt angle (Theta) equals 39 degrees and SS (Clark-Lagerwall) effect with angle 22,5 degree(s) were employed. The study showed that a flexible design of the OASLMs are possible. As a result of optimal design of the reflective type OASLMs with the dielectric mirrors of two types, the following performance characteristics have been obtained. Diffraction efficiency (DE) was about 30%, net diffraction efficiency (NE) was about 20% (spatial frequency equal 30 lp/mm and frame refreshment rate equal 200 Hz). This net diffraction efficiency practically does not depend on the direction of the reading- out light polarization for the OASLMs operating on the DHF mode.
A conventional Zernike filter measures wavefront phase by superimposing the aberrated input beam with a phase-shifted version of its zero-order spectral component. The Fourier- domain phase-shifting is performed by a fixed phase-shifting dot on a glass slide in the focal plane of a Fourier- transforming lens. Using an optically-controlled phase spatial light modulator (SLM) instead of the fixed phase-shifting dot, we have simulated and experimentally demonstrated a nonlinear Zernike filter robust to wavefront tilt misalignments. In the experiments, a liquid-crystal light valve (LCLV) was used as the phase SLM. The terminology 'nonlinear' Zernike filter refers to the nonlinear filtering that takes place in the Fourier domain due to the phase change for field spectral components being proportional to the spectral component intensities. Because the Zernike filer output intensity is directly related to input wavefront phase, a parallel, distributed feedback system can replace the wavefront reconstruction calculations normally required in adaptive- optic phase correction systems. Applications include high- resolution phase distortion suppression for atmospheric turbulence, optical phase microscopy, and compensation of aberrations in optical system components. A factor of eight improvement in Strehl ratio was obtained experimentally, and simulation results suggest that even better performance could be obtained by replacing the LCLV with a more sophisticated optically-controlled phase SLM.
High-resolution phase-contrast wavefront sensors based on optically addressed phase spatial light modulators and micro- mirror/LC arrays are introduced. Wavefront sensor efficiency is analyzed for atmospheric turbulence-induced phase distortions described by the Kolmogorov and Andrews models. A nonlinear Zernike filter wavefront sensor based on an optically addressed liquid crystal phase spatial light modulator is experimentally demonstrated. The results demonstrate high-resolution visualization of dynamically changing phase distortions within the sensor time response of about 10 msec.
A novel liquid crystalline material for optical phase modulation is proposed. The material consists of banana shaped dimer molecules, which form a lamellar phase, e.g. a smectic phase. A helical superstructure is induced by means of chiral fragments included in the molecular structure. Due to polyphilic segregation of hydrogenated and fluorinated molecule segments a cancellation of transversal dipole moments can be avoided and a net polarization of single layers will be obtained. The polarized layers may form ferroelectric or antiferroelectric structures. Application of an electric field in light propagation direction will deform the helix until total unwinding without reorienting the optical axes of the material, but changing the refractive index for one polarization direction. An estimation of the refractive indices of the material, its index modulation capability and the behavior under electric field is presented in this paper. The possible application of the material in phase modulators is discussed.
The reflective type optically addressed spatial light modulators (OASLMs) with an amorphous hydrogenated carbon (a-C:H) light-blocking layer sandwiched between an intrinsic hydrogenated silicon carbide (A-Si:C:H) photoconductor and broad-band and narrow-band dielectric mirrors have been developed. FLC was used as a light modulating medium. The DHF effect in an FLC with tilt angle (Theta) equals 39 degree(s) and SS (Clark-Lagerwall) effect with angle 22.5 degree(s) were employed. The study showed that a flexible design of the OASLMs are possible. As a result of optimal design of the reflective type OASLMs with the dielectric mirrors of two types, the following performance characteristics have been obtained. Diffraction efficiency was about 30%, net diffraction efficiency (NE) was about 20% (spatial frequency equal 30 lp/mm and frame refreshment rate equal 200 Hz). Sensitivity was in the interval from 50 to 100 (mu) W/cm2, depending on the mirror type. NE equals 12.5% for spatial frequency 100 lp/mm. The results of our work show promise for the development of novel FLC OASLMs with a significant decrease in the loss of light, operating in reflective mode. This net diffraction efficiency practically does not depend on the direction of the reading-out light polarization for the OASLMs operating on the DHF mode.
We demonstrate real time severe large and small scale phase distortion correction of arbitrary coherent and incoherent images, using ferroelectric liquid crystal spatial light modulators to record the dynamic hologram of the phase distortions.
Calculation analysis and experimental measurements are described of the operation of optical phase modulator composed from two DHF (deformed helix ferroelectric) liquid crystal cells, mounted in series. The limit dependence of effective refractive index on molecular tilt angle in helical smectic C* phase is calculated as well as the influence of electric field on effective refractive index for different values of molecular refractive index components and tilt angle. The presence of gray scale in DHF regime is shown for crossed DHF cells and jump in change of effective refractive index was found, though the smooth deviation of average refraction index ellipsoid of single DHF layer was found at change of applied voltage from zero to untwisting value. Ferroelectric liquid crystal mixtures are developed with response times less than 0.5 ms. Developed FLC materials have very large tilt angle about 40 degrees and very short pitch of helix less than 0.2 micrometers . The model phase modulator was composed from two DHF cells mounted in series. The behavior of optical phase shift was investigated in Fizeau interferometer. The gray scale phase modulation only is obtained in accordance with calculations. The limit change of phase retardation was found 0.75 (lambda) in transmission mode between undisturbed and totally untwisted helix for cell thickness 16.5 micrometers . The strong nonlinear behavior of the phase shift versus d.c. electric field was found. The quantitative results agree with the theoretical calculations.
Polarization mode dispersion (PMD) hampers development of greater than or equal to 10 Gb/s trunk lines but can be compensated by endless polarization controllers and birefringent fiber. We demonstrate automatic 10 Gb/s PMD compensation using three deformed-helix ferroelectric liquid crystal cells.
Given are the results of experimental study on the quasi real time holographic correction for the lens distortions in the passive observational telescope in the visible range of spectrum, using the liquid crystal optically addressed spatial light modulator.
The basic characteristics of the optically addressed spatial light modulators are presented based on ferroelectric liquid crystal (FLC) as a light modulating media and amorphous hydrogenated silicon carbide a-SiC:H and dye-doped polyimide films as photoconducting layers. The parameters of the constituent parts are described, among them: photoconductivity of photosensitive layers, data about newly developed light blocking layers, characteristics of the FLC materials, utilizing the deformed helix ferroelectric effect. The dynamics of the response in hundreds Hz region, the diffraction efficiency of 20% at spatial resolution better than 501 p/mm, and sensitivity in range of microWatt/cm2 are obtained.
Given are the results of experimental study on the quasi real time holographic correction for the lens distortions in the passive observational telescope in the visible range of spectrum, using the liquid crystal optically addressed spatial light modulator.
Arkady Onokhov, Vladimir Berenberg, Aleksander Chaika, Nataliya Ivanova, Mikhail Isaev, Nikolai Feoktistov, Leonid Beresnev, Wolfgang Dultz, Wolfgang Haase
The subjects of the study were spatial light modulators (SLM), comprised by polymer photoconductor (PC) or a-Si1-x:Cx:H photoconductor, and ferroelectric liquid crystal. The polymer PC provides the highest available diffraction efficiency (DE) among all layers when the spatial frequency of the hologram exceeds approximately 100 lp/mm. At the same time the parameters of the a-Si1-x:Cx:H PC layer can be varied across the wide range and thus the proper tradeoff between the spatial resolution and reversibly can be chosen.
Given are the results of experimental study on the quasi real time holographic correction for the lens distortions in the passive observational telescope in the visible range of spectrum, using the liquid crystal optically addressed spatial light modulator.
Binary mixtures of achiral liquid-crystalline copper complexes with a chiral nonmesomorphic dopant were prepared. The achiral copper complexes incorporating Schiff's base ligands show enantiotropic smectic C and smectic A mesophases. A study of the ferroelectric properties of these materials was performed. The spontaneous polarization PS and the switching time were determined by means of the polarization reversal method. The dielectric behavior of one copper compound was investigated by dielectric relaxation spectroscopy in the frequency range from 10 Hz to 13 MHz. It exhibits the Goldstone mode, the soft mode, and a molecular mode.
The observation of the growth with time of chiral C* nuclei (domains) in the surrounding chiral nematic melt is presented for ferroelectric substances possessing the first order phase transition chiral nematic N* - chiral smectic C*, which is characterized by a strong jump of the tilt angle at the phase transition temperature. For the first time we report the observations of ferroelectric domains, having a 'screw' shape in the cell bulk; the domains grow much faster along the screw axis than across it. The screw pitch is appreciably larger than the helix pitch in the C* phase: the axis of the intrinsic helix turned out to be parallel to the screw axis. Both the screw- and the helix pitches were observed by optical microscopy. We provide a model, giving our interpretation of the phenomenon, based on the fundamental properties of the ferroelectric smectic C* phase at interfaces with various media. It was shown earlier that, due to the specific features of the surface tension of the ferroelectric smectic material, the smectic layers must be inclined with respect to the surface normal, and such an inclination is a function of the material parameters (for instance, the piezoelectric modulus), of the interface parameters (for instance, the anchoring strength) and of the temperature. In fact, the inclination of the smectic layers is related to the appearance of dislocation nets and depends on the interface anisotropy. We describe the growth of the smectic C*/N* interface, which is stabilized by free ions, decreasing the surface tension due to the neutralization of the polarization charges. The estimates of the inclination angle and of the ratio between the screw- and the intrinsic helix pitch are given.
We have developed laboratory prototypes of nematic and ferroelectric liquid crystal optically addressed spatial light modulators with photosensitive layers based on ZnSe, alpha-SiH, pin alpha-SiH, alpha-SiCH, and pin alpha-SiCH. The SLM performance characteristics measured using standard projection and holographic techniques are presented. The advantages and shortcoming of different SLM design versions are analyzed for the use in the wavefront correction and dynamic hologram recording systems. The examples are given for specific implementation of the SLMs in these systems.
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