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Synthetic aperture imaging ladar (SAIL) is one of the most possible optical active imaging methods to break the diffraction limit and achieve super-resolution in a long distance. Nevertheless, two-dimensional reconstructed images of the natural targets have not been achieved. Polarization state change of the backscattered light, which is always determined by the interaction of the light and the materials on the target plane, will affect the imaging of SAIL. The Mueller matrices can describe the complex polarization features of the target reflection and treat this interaction. In this paper, a measurement of the Mueller matrices for different target materials will be designed, and the influences of polarization characteristic of targets on resolution element imaging in side-looking and down-looking SAILs will be theoretically analyzed.

Down-looking synthetic aperture imaging ladar have bidirectional (positive and negative direction) modulation in the orthogonal direction of travel during phase modulation. The return signal can be also collected in the two directions. The imaging processing with bidirectional modulation is used and demonstrated. The signal-to-noise ratio can be enhanced in this mode synthetic aperture imaging ladar. Meanwhile, the velocity of carrying-platform can be faster. In the experiment, the return signals with bidirectional modulation are stacked and rebuilt. Compared to the unidirectional modulation imaging, the faster and clearer imaging is realized with bidirectional modulation. This technique has a great potential for applications in extensive synthetic aperture imaging ladar fields.

Performance of satellite laser communications between GEO and LEO satellites can be influenced by background light noise appeared in the field of view due to sunlight or planets and some comets. Such influences should be studied on the ground testing platform before the space application. In this paper, we introduce a simulator that can simulate the real case of background light noise in space environment during the data talking via laser beam between two lonely satellites. This simulator can not only simulate the effect of multi-wavelength spectrum, but also the effects of adjustable angles of field-of-view, large range of adjustable optical power and adjustable deflection speeds of light noise in space environment. We integrate these functions into a device with small and compact size for easily mobile use. Software control function is also achieved via personal computer to adjust these functions arbitrarily. Keywords

Active polarization imaging technology has recently become the hot research field all over the world, which has great potential application value in the military and civil area. By introducing active light source, the Mueller matrix of the target can be calculated according to the incident light and the emitted or reflected light. Compared with conventional direct detection technology, optical heterodyne detection technology have higher receiving sensitivities, which can obtain the whole amplitude, frequency and phase information of the signal light. In this paper, an active polarization imaging system will be designed. Based on optical heterodyne balanced receiver, the system can acquire the horizontal and vertical polarization of reflected optical field simultaneously, which contain the polarization characteristic of the target. Besides, signal to noise ratio and imaging distance can be greatly improved.

To perform the in-orbit test, verification and surveillance task, the laser simulation and test station must be constructed. Cooperated with the fixed laser communication ground station, we can not only test the main specifications of the laser communication terminals, but also test the performances of the laser backbone link. In this paper, we first give the basic theory of the in-orbit test. Then designed the laser simulation and test station, which consists of laser transmitter module, laser receiver module, and general test module. In the GEO-to-LEO laser communication terminal test progress, the laser simulation and test station responsible for the simulation of the LEO laser communication terminal. In the LEO-GEO-Ground laser link performance test progress, the laser simulation and test station simulate the LEO satellite, which transfer high date rate data flow to GEO satellite, then the GEO satellite route the data flow to the GEO-to-Ground laser communication terminal through optical router, finally GEO satellite transfer the LEO data down to fixed laser receiver on ground.

A structure similar to the direct detection of the DPSK signals using polarization dependent free space Mach-Zehnder interferometer is set up to validate its ability for coherent ranging. M sequence is adopted for its superior performance in code compression. The resultant signal voltage is sampled instead of being zero crossing detected and cross-correlated with the modulation signal sampled at the same rate. The ranging peak appears in the one target setup after proper calibration of the interferometric bias point but doesn’t imply any correct range information. For the two range resolved targets, the output image doesn’t depict two independent peaks. The further study is being conducted.

In Synthetic aperture imaging ladar (SAIL), the polarization state change of the backscattered light will affect the imaging. Polarization state of the reflected field is always determined by the interaction of the light and the materials on the target plane. The Stokes parameters, which can provide the information on both light intensity and polarization state, are the ideal quantities for characterizing the above features. In this paper, a measurement system of the polarization characteristic for the SAIL target materials is designed. The measurement results are expected to be useful in target identification and recognition.

On the basis of the Poisson photocurrent statistics in the photon-limited heterodyne detection, in this paper, the signal-to-noise ratios in the receiver in the time domain and on the focused 1-D image and 2-D image in the space domain are derived for both the down-looking and side-looking synthetic aperture imaging ladars using PIN or APD photodiodes. The major shot noises in the down-looking SAIL and the side-looking SAIL are, respectively, from the dark current of photodiode and the local beam current. It is found that the ratio of 1-D image SNR to receiver SNR is proportional to the number of resolution elements in the cross direction of travel and the ratio of 2-D image SNR to 1-D image SNR is proportional to the number of resolution elements in the travel direction. And the sensitivity, the effect of Fourier transform of sampled signal, and the influence of time response of detection circuit are discussed, too. The study will help to correctly design a SAIL system.

The design and laboratory experiment of a demonstrator of all-optronic down-looking synthetic aperture imaging ladar (SAL) is presented in this paper, in which the sensing-to-processing chain is carried out with light. The ultra-fast processing capability from image acquisition to real-time reconstruction is shown. The demonstrator consists of a down-looking SAL unit with a beam scanner and an optical processor. The down-looking SAL unit has a transmitter of two coaxial orthogonally polarized beams and a receiver of polarization-interference self-heterodyne balanced detection. The linear phase modulation and the quadratic phase history are produced by the projection of movable cylindrical lenses. Three functions of strip-map mode, spotlight mode and static mode are available. The optical processor is an astigmatic optical system, which reduces to a Fourier transform system and a free-space of the Fresnel diffraction to realize the matched filtering. A spatial light modulator is used as the input interface. The experiment is performed with an optical collimator. The system design is given, too. The down-looking SAL has the features such as a big coverage with an enhanced receiving aperture and little influence from atmospheric turbulence and the optical processor is simple.

The static-mode down-looking synthetic aperture imaging ladar (SAIL) can keep the target and carrying-platform still during the collection process. Improvement of the signal-to-noise ratio in static-mode down-looking SAIL is investigated. The signal-to-noise ratio is improved by increasing scanning time and sampling rate in static-mode down-looking SAIL. In the experiment, the targets are reconstructed in different scanning time and different sampling rate. As the increasing of the scanning time and sampling rate, the reconstructed images become clearer. These techniques have a great potential for applications in extensive synthetic aperture imaging ladar fields.

The technique of differential phase shift keying(DPSK) modulation is applied into demodulating phase information in the coherent optical receiver. The dual rate free-space receiving structure on the base of Mach-Zehnder delay interferometer with the lens is used suitably for differential delay which is equal to the one bit corresponding to a certain data rate. Delay distance at the interference receiver is varied with transmission rata from satellite to ground. Differential information is obtained by the subtraction of the two successive wave-front phases when made to interfere. The phase demodulation is extremely sensitive to phase fluctuation. Because of the incident light through atmospheric turbulence, the wave-front of optical signal became jittered in the temporal and spatial domain rapidly. In the paper, the dual rate free-space laser communication receiver for phase lock to stable signal light phase is proposed, increasing the homodyne efficiency and decreasing the bit error rate.

Large field of view of the coherent receiver requires large detector photosurface area, but with the increase of detector photosurface area, the SNR (signal-to-noise ratio) of the coherent receiver will decline. A balanced photodetection with large photosurface APD can not only increase the angular field of view, and can guarantee the detection sensitivity, which satisfy the coherent receiver’s requirements. An experimental measurement of a balanced APD photodetection is reported.

The implementation of down-looking Synthetic Aperture Imaging Ladar(SAIL) uses quadratic phase history reconstruction in the travel direction and linear phase modulation reconstruction in the orthogonal direction. And the linear phase modulation in the orthogonal direction is generated by the shift of two cylindrical lenses in the two polarization-orthogonal beams. Therefore, the fast-moving of two cylindrical lenses is necessary for airborne down-looking SAIL to match the aircraft flight speed and to realize the compression of the orthogonal direction, but the quick start and the quick stop of the cylindrical lenses must greatly damage the motor and make the motion trail non-uniform. To reduce the damage and get relatively well trajectory, we make the motor move like a sinusoidal curve to make it more realistic movement, and through a resampling interpolation imaging algorithm, we can transform the nonlinear phase to linear phase, and get good reconstruction results of point target and area target in laboratory. The influences on imaging quality in different sampling positions when the motor make a sinusoidal motion and the necessity of the algorithm are analyzed. At last, we perform a comparison of the results of two cases in resolution.

A 3 × 3 free-space optical router, which comprises optical switches and polarizing beam splitter (PBS) and based on crossbar network, is proposed in this paper. A control algorithm for the 3 × 3 free-space optical router is also developed to achieve rapid control without rearrangement. In order to test the performance of the network based on 3 × 3 free-space optical router and that of the algorithm developed for the optical router, experiments are designed. The experiment results show that the interconnection network based on the 3 × 3 free-space optical router has low cross talk, fast connection speed. Under the control of the algorithm developed, a non-block and real free interconnection network is obtained based on the 3 × 3 free-space optical router we proposed.

Down-looking synthetic aperture imaging ladar(SAIL) has overcome many difficulties in side-looking SAIL. However, it is inevitably impacted by the speckle effect. There is temporally varying speckle effect due to the angular deflecting of two coaxial polarization-orthogonal beams transmitted in the orthogonal direction of travel, and a spatial varying speckle effect in the travel direction. Under the coaxial heterodyne, phase variations caused by speckle effect are compensated, leaving the amplitude variations of speckle field. In this paper, the speckle effect in the down-looking SAIL is analyzed, expressions for two-dimensional data collection contained speckle effect are obtained and the two-dimensional image influenced by speckle effect is simulated.

Synthetic aperture radar interferometry (InSAR) can gain three-dimensional topography with high spatial resolution and height accuracy using across track interferometry[1]. Conventional InSAR produce three-dimensional images from SAR data. But when the working wavelength transit from microwave to optical wave, the transmission antenna and receive antenna become very sensitive to platform vibration and beam quality[2]. Through differential receive antenna formation, we can relax the requirement of platform and laser using synthetic aperture imaging ladar (SAIL) concept[3]. Line-of-sight motion constraints are reduced by several orders of magnitude. We introduce two distinctive forms of antenna formation according to the position of interferogram. The first architecture can simplify the interferogram processing and phase extraction algorithm under time-division multiplex operation. The second architecture can process the 2D coordinate and height coordinate at the same time. Using optical diffraction theory, a systematic theory of side-looking SAIL is mathematically formulated and the necessary conditions for assuring a correct phase history are established[4]. Based on optical transformation and regulation of wavefront, a side-looking SAIL of two distinctive architectures is invented and the basic principle, systematic theory, design equations and necessary conditions are presented. It is shown that high height accuracy can be reached and the influences from atmospheric turbulence and unmodeled line-of-sight motion can be automatically compensated.

Compared to synthetic aperture radar (SAR), synthetic aperture imaging ladar (SAIL) is more sensitive to the phase errors induced by atmospheric turbulence, undesirable line-of-sight translation-vibration and waveform phase error, because the light wavelength is about 3-6 orders of magnitude less than that of the radio frequency. This phase errors will deteriorate the imaging results. In this paper, an algorithm based on low-pass filtering to suppress the phase error is proposed. In this algorithm, the azimuth quadratic phase history with phase error is compensated, then the fast Fourier transform (FFT) is performed in azimuth direction, after the low-pass filtering, the inverse FFT is performed, then the image is reconstructed simultaneously in the range and azimuth direction by the two-dimensional (2D) FFT. The highfrequency phase error can be effectively eliminated hence the imaging results can be optimized by this algorithm. The mathematical analysis by virtue of data-collection equation of side-looking SAIL is presented. The theoretical modeling results are also given. In addition, based on this algorithm, a principle scheme of optical processor is proposed. The verified experiment is performed employing the data obtained from a SAIL demonstrator.

The down-looking synthetic aperture imaging ladar (SAIL) with electro-optic modulation was proposed. The measurement uses electrically controlled scanner to produce beams with spatial parabolic phase difference, which consists of electro-optic crystal and cylindrical lens. Due to the high modulation rate without mechanical scanning, this technique has a great potential for applications in extensive synthetic aperture imaging ladar fields. The phase mapping of electrically controlled scanner under the different applied voltage is achieved and measured by the polarized digital holographic interferometry. The phase mappings of the scanner in the down-looking SAIL with the o-polarized light and e-polarized light are obtained. The linear phase distribution and the parabolic phase distribution are observed after applying the external electric field. The corresponding analyses and discussions are proposed to explain the phenomena.

We propose a new step imaging method based on astigmatism Fourier transform for synthetic-aperture ladar imaging processor, which is mainly used in optical imaging processing system of synthetic-aperture imaging ladar. The time-domain data is translated into spatial coordinate expression suitable for space optical conversion. The Fourier transform is realized by astigmatism principle. It can simultaneously achieve radar goals focusing both on distance and azimuth. Processor scale is effectively reduced. The process of target echo confocal imaging data is simplified. The requirements of ladar imaging processing system are reduced. It has a great advantage in the synthetic-aperture imaging ladar target echo confocal imaging data processing.

Due to the restriction of the aperture effect and the aperture transverse delay, it is very difficult to get large instantaneous bandwidth under wide scan range for conventional phased array radar. Optical controlled beam forming network (OCBFN) technology can solve this problem. The key of the OCBFN is the optical true time delay line network. In this paper, we propose a new differential time delay line network based on the free-space integrated prism array technology. The new method can realize any linear time delay for the space multi-beam radar. Furthermore, the time delay can be adjusted very easily. In order to validate the new method, we design a 4×4 differential time delay line network, and the time delay ranges from 0 ps to 100 ps.

Atmospheric turbulence influences the wave-front, and reduces homodyne detection efficiency and bit error rate in the Satellite-to-Ground Laser Communication. Free-space differential interference structure based on differential phase shift keying (DPSK) is applied in the optical signal receiver. The free-space Mach-Zehnder delay interferometer without lens is suited for differential delay which is equal to the one bit period. Differential information is obtained by the subtraction of the two successive wave-front phases when made to interfere. Differential distance at the interference receiver is varied with transmission rate from satellite to ground. And through the receiving telescope, the spot size of incident signal light within the interference became small than before, which influences the interference efficiency of the two unequal branches. So that, it is significant for increasing homodyne efficiency to determine the optical signal rate and choose the magnification of receiving telescope. In this paper, the effect of the spot size of incident light and transmission data rate on homodyne detection efficiency is analysed. By the simulation result of efficiency in different spot size and transmission date of incident light, the homodyne efficiency will be predicted in the given data rate and light spot size on the basis of experiment setup. And application condition of free-space differential structure at DPSK differential receiver is proposed.

We investigate the random phase fluctuations of coherent laser propagate through the turbulent atmosphere, and introduce a model of its impact on optical heterodyne reception free space coherent laser optical communication (FSO) system. A polarization based shearing interferometer is used to detect the distorted laser wave-front and reconstruct the wave-front after propagate through a 1Km near-ground atmospheric channel. Further, the heterodyne efficiency of the heterodyne reception system would be given under special consideration of the mismatch between the signal field and the local oscillator. By analyzing the heterodyne efficiency data and the real-time atmospheric coherence length data, a mathematical model of the effects of atmospheric turbulence on FSO system performance is given.

Optimization of high fixed diffraction efficiency in LiNbO 3 :Ce:Cu is investigated. Holographic gratings are recorded using typical recording wavelengths including 488, 514 and 633 nm in LiNbO 3 :Ce:Cu for different doped concentration. Optimal switching from recording step to fixing step is taken into consideration. The experiment results can confirm the optimal recording wavelength and doped concentration for high fixed diffraction efficiency in LiNbO 3 :Ce:Cu crystal by optimal switching from holographic recording to fixing.

Phase-space optics has its advantage over the research of laser beam propagating through atmospheric turbulence channel due to its special joint space-spatial frequency representation, which can be used to describe the property of optical signals between variables that form a Fourier transform pair. In this paper, we introduce the effect of atmospheric turbulence in terms of matrix into the phase-space analysis of laser beam propagation through turbulence. The far field optical intensity in the channel will be given in terms of phase-space representation and the phase-space distribution of received optical intensity can be plotted. It may be a new method analyzing optical scintillation under the turbulent condition.

A new design and fabrication of flat optical lens by use of 90-degree volume holographic grating recording geometry is presented. It is recorded by the interference of a plane wave and a spherical wave in a Fe-doped LiNbO3. The flat optical lens can laterally transfer and focus the plane wave which perpendicularly incidents on the planar crystal surface. The optimal switching from recording to thermal fixing is taken into consideration in order to obtain the nonvolatile hologram with maximum fixing efficiency. The flat optical lens with different recording and reconstructing wavelength has also been discussed. The flat optical lens has the advantages of light weight, laterally transferring and focusing, small duty ratio, and easy for the microstructure integration. The testing results measured verify that the flat optical lens can successfully be used for the free space optical communication.

A system of angle-Doppler resolved reflective tomography imaging Lidar and its algorithm are given. The issue between transverse range resolution and the sampling time of single angle is solved. The condition of far-field diffraction transmission in the laboratory is designed, and the angle-Doppler reflective projections of the target are collected. Filtered back-projection algorithm is used to reconstruct the cross section of the target image. Because of the utilization of coherent detection of coaxial beams, both the imaging signal to noise ratio and the receiving sensitivity are improved. Due to the simplification in configuration and operations without involving signal phase processing, this technique has a great potential for applications in extensive imaging Lidar fields.

Reflective tomography is one of the most promising high-resolution imaging methods for the remote objects. But in practical application, because of the sampling angle error and limited view of projections in signal collecting process, anisotropic resolution is inevitable and the reconstruction image quality of reflective tomography system degrades. In this paper, the theoretical imaging resolution derived from Fourier-Slice theorem is presented, computer simulation and experimental verification are also given. Imaging analysis in this paper will make a complement and perfection of the theory in reflective tomography imaging ladar.

Synthetic aperture laser imaging ladar’s optical receiver is a heterodyne detect system, so the atmosphere turbulence will affect the phase history of the ladar seriously. Unlike in conventional passive optical imaging system, both the wavefront distortion and piston term will decrease the imaging resolution. The distortion of the wavefrom decreases the heterodyne mixing efficient; the piston will cause the pulse-to-pulse phase errors. Multiaperture receiver technology can improve the heterodyne mixing efficient efficiently, but the pattern of multiaperture should be optimized according to different turbulence intensity. In this paper, we optimize a four-aperture receiver pattern in different turbulence environment. The simulation and experiment results are obtain in our ladar system.

The synthetic aperture imaging ladar (SAIL) systems typically generate large amounts of data difficult to compress with digital method. This paper presents an optical SAIL processor based on compensation of quadratic phase of echo in azimuth direction and two dimensional Fourier transform. The optical processor mainly consists of one phase-only liquid crystal spatial modulator(LCSLM) to load the phase data of target echo and one cylindrical lens to compensate the quadratic phase and one spherical lens to fulfill the task of two dimensional Fourier transform. We show the imaging processing result of practical target echo obtained by a synthetic aperture imaging ladar demonstrator. The optical processor is compact and lightweight and could provide inherent parallel and the speed-of-light computing capability, it has a promising application future especially in onboard and satellite borne SAIL systems.

As synthetic aperture imaging ladar employs the linear chirp laser signal, it is inevitably impacted by the space-time varying speckle effect. In many SAIL two-dimensional reconstructed images, the laser speckle effect severely reduces the image quality. In this paper, we analyze and simulate the influence of space-time speckle effect to the resolution element imaging both in range direction and in azimuth direction. Expressions for two-dimensional data collection contained space-time speckle effect are obtained, and computer simulation results of resolution degradation both in range direction and in cross-range direction are presented.

Laser communication links between satellite and ground remains a bottleneck problem that has not been solved in free space communication network now. Atmospheric disturbances have badly influenced the wave-front of signal light and destroyed the integrality of optical phase, so the bit error rate (BER) is increased at the receiving terminal in the space-to-ground laser communication. With conventional coherent reception, the contrast of coherent light increased dramatically, and transmission efficiency of Space to ground laser communication decreased. Receiving technology based on differential phase shift keying (DPSK) is proposed here to overcome the effects of atmosphere here. Differential phase shift keying without the integrality and compensation of the optical phases, is suited for high rate space to ground communication links due to its immunity of the wavefront of a beam passing atmosphere. A Mach-Zehnder delay interferometer is used for differential delay which is equal to the one bit period. The differential data information can be obtained from the optical phase changes. Differential phase modulation technique can be a promising optical receiving technology.

The high data-rate satellite-to-ground coherent optical communication link is limited because the phase integrity of a beam is impaired when passing through the atmospheric turbulence. Based on the interference of two successive data bits in an unequal arm-length Mach-Zehnder delay interferometer, the differential phase shift keying receiver is suited for high data-rate satellite-to-ground coherent optical communication links due to its immunity of the wave front impairment when passing through the atmospheric turbulence. In the time-delay self-homodyne interferometric detection used in 2×4 90 degree optical hybrid, the optical path difference corresponds to the duration of one bit. The optical path difference is stabilized to below one thousandth of the wavelength by moving a finely motorized platform with the close-loop control using the phase feedback from the outputs of the 90 degree hybrid. The 2.5 Gbps optical communication link has already been verified between two buildings over a distance of 2.4km in the worst-case atmospheric conditions. The design and experimental results are given in this paper.

In the inter-satellite laser communication, the laser beam transmitted from the optical terminals is required to be highly collimated and its divergence approaches diffraction-limit. For testing the diffraction-limit wavefront, a polarization phase-shifting cyclic Jamin shearing interferometer is proposed. It is composed of a Jamin plate with a PBS film coated on its front surface, a right-angle prism reflecting beams two times, a shearing plate shearing beams by its rotation and a polarization phase shifter. The laser beam to be test is incident on the Jamin plate and gives rise to two interference beams with mutually perpendicular polarization directions by the PBS film. The two beams falls on the right-angle prism before or after passing through the shearing plate. With reflection of the right-angle prism, a cyclic Jamin shearing interferometric light path is formed. Two emitted beams go into the polarization phase shifter to obtain phase-shifting interferograms. In this interferometer, the cyclic interferometric light path can eliminate error of the surface profile of the optical element and the effect of environment. The interferometer has polarization phase shifting function and its fringe visibility is high. Therefore the interferometer can obtain high accuracy with variable shearing amount. In experiments, phase-shifting interferograms are obtained and the usefulness of the interferometer is verified.

At present inter-satellite laser communications have made great success, such as SILEX, TerraSAR-X LCT etc. But satellite to ground laser communications still at the experimental stages because of the atmosphere turbulence channel and the clouds. Once the satellite to ground laser communication technology obtains a breakthrough, the all laser spacebased communication era is coming. In this paper, we suggest a DPSK modulation/self-coherent homodyne reception scheme to overcome the atmosphere turbulence. The key in the scheme lies in the phase error compensation with the external environment change. In our experiment, we use two parallel plates rotating to compensate the phase error. The communication data rate reaches 2.5Gbps in the field experiment. The real time bit error rate was obtained with the variation of the communication channel’s turbulence.

The phase-space description of signals is a joint space-spatial frequency representation in local domain. It employ the phase-space representation functions to describe the property of optical signals between two variables that form a Fourier transform pair and provide a valuable analysis tool for signals when this new mathematical analysis tool applied to an optical system. This method can also be applied to the analysis of propagation of laser beam within free space or atmosphere condition. In this paper, we give the analytical formula of optical field distribution of laser beam after propagation within turbulence in the domain of phase-space. The results show an instructing aspect of optical signal propagation and are helpful to the future application of phase-space method to the laser communications.

The paper overviews various supporting ways for large-size movement mirrors, and the advantages and disadvantages of the support methods are summarized. Some valuable optimization methods to improve support effects are also introduced. As a case study, a radial segment-face contact support method is proposed to solve the support problem for a large-aperture rotating prism, and a two-step optimization method is implemented to improve the support effects. The surface deformations under different support separated angles are evaluated. The overview can be as good references for large-size mirror support design in similar opto-mechanical systems especially under movement conditions.

The cross-orbit scanning is very important for Fresnel telescope synthetic aperture imaging ladar system. This paper presents a design of large-angle high speed scanner based on electro-optic crystal for the cross-orbit scanning in Fresnel telescope synthetic aperture imaging ladar system. The designed scanner is based on the space-charge-controlled EO effect in KTN. In the experiment the crystal temperature should be kept a little higher above Tc to obtain a large EO effect and the polarization of the laser beam should be parallel to the direction of the driving electric field. Compared with other conventional EO crystal scanner, the new scanner can greatly improve the scanner angle by several times when maintains high speed and accuracy, which will have a great potential for cross-orbit scanning applications in Fresnel telescope synthetic aperture imaging ladar system.

A theoretical explanation of fixed holograms with high diffraction efficiency is given based on jointly solving the material equations and the coupled-wave equations. The dynamics of the recording and fixing diffraction efficiency can be effectively described and analyzed by using this method. The optimal recording wavelength for high fixed diffraction efficiency is discussed in detail. There is an optimal switching time for the maximum diffraction efficiency up to an ideal of 100%. Theoretical results can confirm the optimal recording wavelength for maximum diffraction efficiency of thermal fixing in Fe doped LiNbO 3 crystal.

The effect of dopants in doubly-doped LiNbO 3 crystals on persistence of two-center recording is investigated experimentally and theoretically in this paper. Six kinds of doubly-doped LiNbO 3 crystals are used in experiments with 633nm recording and 404nm sensitizing, the results show that LiNbO 3 :Ce:Cu crystal has the largest persistence. The dependence of persistence on bulk photo-voltaic coefficient and photo-excitation coefficient of deep center for recording light (K 12 and S 12 ) is discussed based on jointly solving the two-center material equations and the coupled-wave equations. The results show that both the increase of S 12 and the decrease of K 12 will result in the decrease of the persistence. When a holographic storage system is designed, the appropriate dopant will help to obtain the high persistence and dynamic range simultaneously.

During the signal acquisition for synthetic aperture imaging ladar, any phase error will deteriorate the phase-matched filtering results. The phase differential algorithm (PDA) is presented to correct the phase error. The quadratic spatial phase history can be reconstructed from the phase information submerged by the phase error from platform line-of-sight translation–vibration and nonlinear chirp. The theoretical modeling results and experimental results are presented.

Synthetic aperture laser ladar is one of the most possible long-distance super-resolution active imaging methods. Many institutes have obtained two-dimensional synthetic aperture laser ladar’s reconstructed images. However, in these images, the laser speckle effect severely reduces the image quality. In the paper, we use mutli-receiver and multinoncoherent image superposition methods to reduce the impact of the laser speckle. According to the scalar diffraction theory, we deduced reconstructed image with the multi-receiver and the multi-noncoherent image superposition methods.

Acquisition strategy is very important during the inter-satellite laser communications systems. Spatial acquisition of the companion terminals using very narrow beacon laser beams is a very difficult task especially under the laser terminal scanning errors situation. Acquisition is a statistical process. In this paper, we detailed the optimized scanning overlap factor of the beacon laser beam which depends on the scanning accuracy of the laser terminals.

In the inter-satellite laser communication, the diffraction-limit wavefront is required. To test the wavefront, we have developed a Jamin double-shearing interferometer. The interferometer is consists of two Jamin plates to form lateral shearing and four wedge plates to divide the aperture. The laser beam to be test is incident on the first Jamin plate and gives rise to two beams. One is reflected from the front surface of the first Jamin plate, then passes two wedge plates and is reflected from the rear surface of the second Jamin plate. The other is reflected from the rear surface of the first Jamin plate, then passes other two wedge plates and is reflected from the front surface of the second Jamin plate. The two beams are recombined to form the interferogram. For the interferometer, simple phase-shifting method to improve the measurement accuracy of the wavefront by moving four wedge plates is proposed in this paper. When the wedge plates are moved along its surface or in the incidence direction of the interferometer, the optical path difference of two interferometric beams is changed to form phase shift. The added optical path difference introduced by the movement of wedge plates is liner function of displacement, thus the phase-shifting amount is easy to be control. Wedge plates can be moved in four steps with interval of quarter wavelength and the phase can be unwrapped using the four-step phase-shifting algorithm. In experiments, phase-shifting interferograms are obtained. The usefulness of the phase-shifting methods is verified.

In this paper, we present a laboratory demonstration of Fresnel telescope imaging ladar system for imaging the faraway objects with high resolution. Two concentric and coaxial quadratic wavefront with orthogonal polarization are used as scanning beams to illuminate the target. The scattered light from the target is heterodyne detected by a 90 degree 2×4 optical hybrid with two balanced receivers. The target image can be reconstructed by digital processing of the output signals of the balanced receivers. Point targets 4.3m away are reconstructed with high resolution in experiments.

Volume holographic gratings (VHG) are of wide interest in many applications because of their properties of high diffraction efficiency, excellent wavelength selectivity and angular selectivity. Recently, because of more free parameters, multi-layer volume holographic gratings (MVHG) have become an ideal candidate for various promising technological applications such as optical interconnects, pulse shaping and optical filters. Therefore, some knowledge of the diffraction behaviors of such system would be very valuable for characterizing and optimizing such volume diffractive optical elements. In this paper, we extend the coupled wave theory of multi-layer gratings to study the Bragg diffraction properties of continuous wave, ultrashort laser pulse and chirped ultrashort pulse, and present a systematically theoretical analysis on the spectrum distribution of the diffracted intensities, the diffraction bandwidth of a system of MRVHG. The system of MRVHG is composed of multiple layers of reflection VHG separated by intermediate layers. The comparisons of the diffraction characteristics for these beams are investigated.

Range-resolved reflective tomography is one of the most effective high-resolution imaging methods for laser sensing and imaging technologies. In experiments reported earlier by MIT Lab, only the outline of the target was recovered using reflective tomography algorithm. In our experiment, we adopt a novel imaging method which can get an imaging result of the whole region covering the target. A target of letter "E" is placed on a plane with a tilt angle to the horizontal plane and rotated about the axis perpendicular to it, the target is illuminated by parallel light pulses, the range-dependent return signal is collected by a non-imaging optical system. Filtered back-projection and algebraic reconstruction techniques algorithm are used to reconstruct the target, then we get an image result which has clear description of the target. After that, the imaging quality and resolution of this new approach are discussed. Our experiment system reported in this paper can achieve high imaging quality in real two-dimension image construction using reflective tomography algorithm, thus it has a great practical significance for applications in extensive imaging fields.

An efficient technique of utilizing Dammann grating and phase plate to get high power and high brightness laser beam from phase-locked laser array is presented. The conjugate Dammann grating and the phase plate are placed in the back and front focal plane of a Fourier lens respectively. In order to improve the beam combining efficiency, Continuous grating of high diffraction efficiency is used to replace the Dammann grating. Analysis shows that the Continuous grating which has a higher diffraction of efficiency is also suitable for beam combining of the presented system.

The phase perturbations due to propagation effects can destroy the high resolution imagery of Synthetic Aperture Imaging Ladar (SAIL). Some autofocus algorithms for Synthetic Aperture Radar (SAR) were developed and implemented. Phase Gradient Algorithm (PGA) is a well-known one for its robustness and wide application, and Phase Curvature Algorithm (PCA) as a similar algorithm expands its applied field to strip map mode. In this paper the autofocus algorithms utilized in optical frequency domain are proposed, including optical PGA and PCA respectively implemented in spotlight and strip map mode. Firstly, the mathematical flows of optical PGA and PCA in SAIL are derived. The simulations model of the airborne SAIL is established, and the compensation simulations of the synthetic aperture laser images corrupted by the random errors, linear phase errors and quadratic phase errors are executed. The compensation effect and the cycle index of the simulation are discussed. The simulation results show that both the two optical autofocus algorithms are effective while the optical PGA outperforms the optical PCA, which keeps consistency with the theory.