Laser beam projection with adaptive array of fiber collimators. I. Basic considerations for analysis

We present a mathematical model and provide an analysis of optical beam director systems composed of adaptive arrays of fiber collimators (subapertures), referred to here as conformal optical systems. Performances of the following two system architectures are compared: A conformal-beam director with mutually incoherent output laser beams transmitted through fiber collimators (beamlets), and a corresponding coherent system whose beamlets can be coherently combined (phase locked) at a remote target plane. The effect of the major characteristics of the conformal systems on the efficiency of laser beam projection is evaluated both analytically and through numerical simulations. The characteristics considered here are the number of fiber collimators and the subaperture and conformal aperture fill factors, as well as the accuracy of beamlet pointing.     

Light scattering with single-mode fiber collimators

The recent development and availability of fiber-optic components including graded-index (GRIN) microlenses and the unique optical properties of single-mode optical fibers make it possible to build ideal detector systems for light-scattering measurements. We show that the simple coupling of a 0.25-pitch GRIN lens and a single-mode optical fiber to form a collimator makes a nonimaging detector system with properties that are superior to conventional setups based on pinholes and that approaches the theoretical limit of a perfectly coherent detector.     

Optimization of the arrangement of fiber laser array in beam coherent combining

Based on the expanding of the fundamental mode of a step-index fiber in terms of Laguerre–Gaussian modes, the accurate expression for the beam propagation of a fiber laser in free space is obtained. Thereby, the coherent combining beam of three fiber laser arrays including circular arrangement, square arrangement, and diamond arrangement are numerically analyzed. The study shows that all the beams gradually concentrate centrally on the propagation axis and the highest far-field peak intensity can be obtained by using the circular arrangement. Meanwhile, the far-field intensity of the circular arrangement by using the Laguerre–Gaussian approximation is also compared with that by using the pure Gaussian approximation, which indicates that the pure Gaussian approximation will induce much error in the far-field intensity. Finally, the influence of the radius of a circular fiber laser array on the far-field intensity is studied, of which the result shows that the far-field intensity decreases with increasing radius. Therefore, the fiber laser elements are suggested to be placed as close as possible.     

Laser beam projection with adaptive array of fiber collimators. II. Analysis of atmospheric compensation efficiency

We analyze the potential efficiency of laser beam projection onto a remote object in atmosphere with incoherent and coherent phase-locked conformal-beam director systems composed of an adaptive array of fiber collimators. Adaptive optics compensation of turbulence-induced phase aberrations in these systems is performed at each fiber collimator. Our analysis is based on a derived expression for the atmospheric-averaged value of the mean square residual phase error as well as direct numerical simulations. Operation of both conformal-beam projection systems is compared for various adaptive system configurations characterized by the number of fiber collimators, the adaptive compensation resolution, and atmospheric turbulence conditions.     

Active phase locking of fiber amplifiers using sine-cosine single-frequency dithering technique

The sine-cosine single-frequency dithering technique for active phase locking of fiber amplifiers is presented for the first time to our knowledge. It has twice the phase control speed as the single-frequency dithering technique. Detailed theoretical development has been presented and the relevant experiment has been done. In the experiment, nine 10 W level fiber amplifiers are tiled into 3×3 array and the total output power is about 100 W. The sine-cosine single-frequency dithering algorithm is run on a signal processor based on a field-programmable gate array for phase control on the fiber amplifiers. When the phase control system is in a closed loop, the fringe contrast of far-field intensity pattern is improved by more than 90% from 21% in an open loop, and the residual phase error is less than λ/20.     

High-strength fiber Bragg gratings for a temperature-sensing array

We have successfully demonstrated a one-step laser process of fabricating fiber Bragg grating arrays directly through fiber buffer. A new polysiloxane-based buffer provides high 244-nm transmission and showed no degradation due to UV irradiation or thermal annealing as verified by Weibull analysis of tension tests. The FBG array can withstand over 400-kpsi tension tests and 220/spl deg/C temperature. The spectral quality of the FBG is compatible with the FBGs currently used by the sensing industry and, therefore, can be interrogated with existing instruments. The laser fabrication process is robust with no coating, stripping, or recoating required.     

光纤激光相干合成研究进展

近年来,以激光大气传输为应用背景的光束相干合成技术被广泛研究,而关于该项技术在空间光通信中的应用研究却不多。事实上,基于光束相干合成的多孔径接收天线结构可有效缓解大气湍流影响,提高空间激光通信系统的性能。本论文简要介绍了中国科学院自适应光学重点实验室面向激光大气传输应用的激光组束传输与湍流校正技术研究近况;重点介绍关于多孔径接收空间光通信系统中的光纤相干合成研究进展,主要包括基于3 dB光纤耦合器的相干合成和基于光纤偏振合束器的相干偏振合成两种方法,在空间光通信系统中具有极大的潜在应用。

     

Reprogrammable optical phase array

A novel reprogrammable optical phase array (ROPA) device is presented as a reconfigurable electro-optic element. One specific application of the ROPA, a 1 x 6 electro-optic space switch, is fully described. Switching angles are within 2 degrees , and switching is achieved through a complementary metal-oxide semiconductor (CMOS) controlled, diffraction based, optical phase array in a bulk BaTiO3 crystal. The crystal is flip-chipped to the CMOS chip, creating a compact fully integrated device. The design, optical simulation, and fabrication of the device are described, and preliminary experimental results are presented.     

Number of phase levels of a talbot array illuminator

The number of phase levels of a Talbot array illuminator is an important factor in the estimation of practical fabrication complexity and cost. We show that the number (L) of phase levels of a Talbot array illuminator has a simple relationship to the prime number. When there is an alternative pi-phase modulation in the output array, the relations are similar.     

Effect of fiber array on damping behaviors of fiber composites

This study aims to investigate the fiber array effect on modal damping behaviors of fiber composites. Three different fiber arrays, i.e., square edge packing (SEP), square diagonal packing (SDP), and hexagonal packing (HP), were considered to represent the microstructures of the unidirectional composites. Repeating unit cells (RUCs) suitable for describing the characteristics of the microstructure were adopted in the generalized method of cell (GMC) micromechanical analysis. The energy dissipation concept was then employed to calculate the specific damping capacities of composites in the material principal directions. The specific damping capacities obtained from micromechanical analysis were regarded as the equivalent damping properties homogenizing in the composites. In conjunction with the modal shapes of the composite structures determined from the finite element analysis, the specific damping capacity was extended to characterize the corresponding modal damping of the composite rods and plates. Both free–free and clamped-free boundary conditions were taken into account in the composite structures. Results indicated that the structures constructed from the composites with SDP fibers exhibit better damping behaviors than the other two cases.     

Enhanced phase locking in a Josephson junction driven by current pulses

The constant voltage steps generated by a slightly damped Josephson junction driven by short rectangular current pulses are studied within the RSJ model for a broad range of dc and rf bias. The height of the rf-induced steps strongly depends on model parameters but always largely exceeds that of a sinusoidally driven junction. The dynamical response of the junction is also briefly analyzed.     

Theoretical analysis of tuning coherent laser array for several applications

Theoretical analysis for the far-field diffraction of the coherent laser array (CLA) is presented. Based on the theoretical model of the propagation of CLA, the conditions and restrictions for coherent beam combination, two-dimensional steering, and dark-hollow beam generation are theoretically described in detail. With properly organized phase distributions and tilt control, the peak location of the far-field pattern of the CLA could shift two-dimensionally in a large scale of steering angle. With additional amplitude modulation, the far-field pattern could have special shapes. The simulated results agree with the theoretical analysis. It is a feasible way to realize all these applications by a CLA with well-arranged phase distributions and/or additional amplitude modulation.     

Interconnection of a two-dimensional array of vertical-cavity surface-emitting lasers to a receiver array by means of a fiber image guide

The implementation of a 10-channel parallel optical interconnect consisting of a two-dimensional array of vertical-cavity surface-emitting lasers, a 1.35-m fiber image guide, and a metal-semiconductor-metal receiver array is described. Transmission rates of 250 Mbits/s per channel are demonstrated with an optical cross talk of less than -27 dB and a loss of -3 dB. Coupling issues associated with image guides are analyzed and discussed.     

声矢量阵相干源方位估计算法

基于声矢量阵的质点速度场平滑（ParticleVelocityFieldSmoothing,PVFS）算法是一种有效的解相干算法,但是当存在大量相干源时,该算法性能急剧下降甚至失效。在PVFS算法的基础上,提出了矩阵平方平滑（MatrixSquareSmoothing,MSS）-PVFS算法,该算法是对PVFS算法的改进,通过对PVFS算法构造的数据协方差矩阵进行平方、矩阵分块以及矩阵块间交叉相乘等数学运算,增强了PVFS算法解相干的能力,并大大增加了其分辨相干源的数目。计算机仿真结果表明,MSS—PVFS算法的效果与空间平滑（SpatialSmoothing,SS）-PVFS算法大致相同,但在低信噪比条件下,该方法具有更高的DOA估计精度。     

Multimodal coherent anti-Stokes Raman spectroscopic imaging with a fiber optical parametric oscillator

We report on multimodal coherent anti-Stokes Raman scattering (CARS) imaging with a source composed of a femtosecond fiber laser and a photonic crystal fiber (PCF)-based optical parametric oscillator (FOPO). By switching between two PCFs with different zero dispersion wavelengths, a tunable signal beam from the FOPO covering the range from 840 to 930 nm was produced. By combining the femtosecond fiber laser and the FOPO output, simultaneous CARS imaging of a myelin sheath and two-photon excitation fluorescence imaging of a labeled axons in rat spinal cord have been demonstrated at the speed of 20 μs per pixel.     

Application of a plastic scintillating fiber array for low-energy x-ray imaging

Interest in digital imaging has led to the development of new detectors in the form of large-area displays. Most of the recent improvements are based on charge-coupled devices, a:Si photodiodes arrays, and so on. Some of these photodetectors must be coupled to scintillating screens to convert the ionizing radiation into light. Fiber-optic screens offer an advantage for achieving this interface because the length (thickness) of the interaction medium does not contribute too much to the degradation of the spatial resolution. We discuss the possibility of using a plastic scintillating fiber (PSF) array for x-ray detection and imaging in the 10-keV range. Modulation-transfer-function (MTF) measurements of the PSF array are compared with the optics MTF of the imaging system (without the sample); cross talk in the fiber array is negligible, even though the fiber array thickness is 20 mm. The optimal thickness of the array is estimated experimentally.     

Multilevel phase-only array generators with a trapezoidal phase topology

The implementation of a two-stage design process for the design of diffractive optical elements for array illumination is described. Results are presented for on-axis two-dimensional array illuminators for which this method is used. The final designs are theoretically within 5% of the calculated diffraction efficiency upper bound, and theoretical signal reconstruction error is below 1%. Experimental verification of the design theory is given, with experimental diffraction efficiencies within 4% of design values and signal reconstruction error below 6%.     

Approximate phase conjugation with a retroreflecting array in microscopy

Some experiments on image recovery in microscopy are described. The aperture stop of a microscope objective is partially obscured, and a retroreflecting array is used as an approximate phase conjugator to return the beam through the unobstructed part of the aperture. An application is made to the measurement of the absolute reflectance of small samples by use of a variant of Strong's method.     

基于光纤耦合的光纤激光阵列像差探测

提出了一种基于光纤耦合的光纤激光阵列像差探测方法,介绍了其结构和波前复原过程,采用数值仿真模拟其复原湍流像差的过程,并进行了7单元自适应光纤准直器(AFOC)阵列复原静态像差的实验。仿真结果表明,本文提出的波前传感方法能够有效复原出湍流畸变波前,且对于不同单元数的六边形排布阵列,存在不同的最优复原阶数。阵列填充因子的降低会增大复原残差,填充因子大于0.8可保证复原残差RMS相较于填充因子为1时的增幅不超过10%。实验结果表明,利用填充因子为0.875的7单元AFOC阵列,复原以离焦为主的低阶像差时,初始畸变波前RMS为0.433 μm,复原残差小于0.075 μm。仿真和实验结果验证了本文提出的基于光纤耦合的光纤激光阵列像差探测方法的有效性。该技术有望在激光阵列大气传输及湍流校正等系统中得到进一步应用。