Adaptive optical transmitter and receiver for space communication through thin clouds

Optical space communication from satellite to ground or air to air consists of clouds as part of communication channels. Propagation of optical pulses through clouds causes widening and deformation in the time domain and attenuation of the pulse radiant power. These effects decrease the received signal and limit the information bandwidth of the communication system. Having dealt with the other effects previously, here we concentrate on pulse broadening in the time domain. We derive a mathematical model of an adaptive optical communication system with a multiscattering channel (atmospheric cloud). We use knowledge about the impulse response function of the cloud to adapt the communication parameters to the transfer function of the cloud. The communication system includes a receiver and a transmitter. We adapted the transmitter to atmospheric conditions by changing the bit error rate. One can adapt the receiver to the atmospheric condition by changing the parameters of the detector and the filter. An example for a practical communication system between a low Earth orbit satellite and a ground station cover by cloud is given. Comparison and analysis of an adaptive and semiadaptive system with cloud channels are presented. Our conclusion is that in some cases only by such adaptive methods is optical communication possible.     

Polymeric thermo-optic space switches for optical communications

Solid state optical space switches based on the thermo-optic (t.o.) effect in polymeric optical waveguides have now reached the commercial stage. The application of these switches is in network protection and network reconfiguration functions for fiber optic communications systems. The requirements for these applications include polarization and wavelength independence, low insertion loss, low cross talk, low drive power with step-like (digital) response, millisecond switching times and small size. In addition the reliability of the component must meet the demanding requirements of telecom applications. It will be shown that polymeric t.o. space switches can meet all functional requirements due to the exceptional thermal and t.o. effects of polymers combined with their tunability and processing versatility. Furthermore, it will be shown that polymer optical chips components can withstand extreme lifetime tests with success.     

Experimental demonstration of an optical phased array antenna for laser space communications

The feasibility of an optical phased array antenna applicable for spaceborne laser communications was experimentally demonstrated. Heterodyne optical phase-locked loops provide for a defined phase relationship between the collimated output beams of three single-mode fibers. In the far field the beams interfere with a measured efficiency of 99%. The main lobe of the interference pattern can be moved by phase shifting the subaperture output beams. The setup permitted agile beam steering within an angular range of 1 mr and a response time of 0.7 ms. We propose an operational optical phased array antenna fed by seven lasers, featuring high transmit power and redundance.     

Experimental implementation of fiber optic bundle array wide FOV free space optical communications receiver

A gimbal-free wide field-of-regard (FOR) optical receiver has been built in a laboratory setting for proof-of-concept testing. Multiple datasets are presented that examine the overall FOR of the system and the receiver's ability to track and collect a signal from a moving source. The design is not intended to compete with traditional free space optical communication systems, but rather offer an alternative design that minimizes the number and complexity of mechanical components required at the surface of a small mobile platform. The receiver is composed of a micro-lens array and hexagonal bundles of large core optical fibers that route the optical signal to remote detectors and electronics. Each fiber in the bundle collects power from a distinct solid angle of space and a piezo-electric transducer is used to translate the micro-lens array and optimize coupling into a given fiber core in the bundle. The micro-lens to fiber bundle design is scalable, modular, and can be replicated in an array to increase aperture size.     

Slant-propagation characteristics in tapered optical waveguides: analysis

Although several analysis methods for tapered waveguides have been proposed, they dealt with the problem of a light wave traveling parallel to the tapered direction of the waveguide. Light waves that propagate at a slant in the tapered direction are discussed as mode-coupling problems, including radiation modes that are made discrete by the hypothetical boundary method. We propose a method of determining the three-dimensional wave vectors of the discrete radiation modes in the slant propagation; the slant-propagation characteristics, such as the transmission efficiency of the tapered waveguide, are analyzed.     

光扭矩传感器信号转换误差分析

光扭矩传感器的输出信号在进行光电转换过程中，由于温度影响，造成了直流电平漂移误差，影响系统的测量精度。对直流电平漂移误差进行了分析，针对产生误差的原因提出了采用正负温度补偿、热敏电阻和差动电路三种补偿方案，并进行了比较。实验结果表明，采用差动补偿电路的方法，效果良好，能有效地消除直流电平漂移误差。     

光纤传输的激光振动自混合信号的分析

基于自混合效应的传感技术由于其装置的简单和紧凑而获得广泛应用。提出了光纤作为传输介质条件下观察自混合信号的方法,并观察了振动物体振幅为0.74μm,频率为998Hz时的自混合信号,与自由空间中自混合信号进行了比较。在此基础上,分别基于不同的反馈参数对两种自混合信号进行了模拟,理论模拟和实验结果相符合。分析了反馈参数C对自混合信号的不对称度和幅度的影响,研究表明,自混合信号的不对称度和幅度随着C的增大而增加。光纤作为传输介质时的自混合测振信号幅度和不对称度劣于自由空间系统中的自混合信号,但光纤自混合系统由于其特有的优点仍将广泛应用于二维速度和振动的测量。     

子母弹抛撒加速度信号特征小波分析

针对子母弹爆炸抛撒过程中加速度信号为典型非平稳随机信号,应用Fourier分析无法同时获取信号时域和频域特性的问题,提出应用小波分析法处理子母弹爆炸抛撒过程加速度信号的方法.通过对信号进行分层重构,获得不同频带上的加速度时间曲线,其重构信号与实测信号相对误差较小,且加速度信号频率成分丰富;证明小波分析是一种处理爆炸抛撒加速度信号的有效途径,其对爆炸抛撒加速度信号的时频分析可以给出丰富的细节特征,对弹药的研制和生产有一定的指导作用.     

Image transfer through cirrus clouds. I. Ray trace analysis and wave-front reconstruction

A new technique for modeling image transfer through cirrus clouds is presented. The technique uses a ray trace to model beam propagation through a three-dimensional volume of polydisperse, hexagonal ice crystals. Beyond the cloud, the technique makes use of standard Huygens-Fresnel propagation methods. At the air-cloud interface, each wave front is resolved into a ray distribution for input to the ray trace software. Similarly, a wave front is reconstructed from the output ray distribution at the cloud-air interface. Simulation output from the ray trace program is presented and the modulation transfer function for stars imaged through cirrus clouds of varying depths is discussed.     

Compact matrix formalism for phase space analysis of complex optical systems

Phase space analysis is a powerful approximate scheme for the analysis of complex optical systems. The matrix formalism introduced makes it possible to determine all important beam characteristics as a function of the distance from the source. An algebraic approach comprising matrix transformations and determinants was consistently employed rather than numerical phase space integrations. While the primary application is x-ray and neutron optics, the approach can be used more generally for characterizing finite-size beams near the optical axis.     

Performance of differential pulse-position modulation (DPPM) with concatenated coding over optical wireless communications [optical wireless communications]

The concatenation of marker and Reed-Solomon codes in order to correct insertion/ deletion errors in differential pulse-position modulation (DPPM) over optical wireless communications is presented. The concatenated code decoding algorithms with hard-decision and soft-decision detection are presented. The performance of the hard-decision coded DPPM system is evaluated over both nondispersive and dispersive channels via analysis and simulation. It is shown that the coding gain provided by the concatenated code is approximately 4 dB when the code rate is about 0.7 and the channel is nondispersive. Over a dispersive channel, the coded system performs better than the uncoded system when the ratio of delay spread to bit duration is not high. A soft-decision detector is employed to combat intersymbol interference. The soft- decision decoding algorithm, which has low complexity and can be practically implemented, is described. The performance over nondispersive and dispersive channels is evaluated by analysis and simulation. It is shown that the soft-decision system requires approximately 2 dB less transmit power than the hard-decision system for additive white Gaussian noise and low-dispersive channels. Soft decoding also provides a performance improvement in high-dispersive channels.     

A new experimental approach to second-order polarization mode dispersion analysis for optical communications systems

This article analyzes the polarization mode dispersion (PMD) in optical fibers and presents a new approach for its measurement. This approach permits the combined analysis of the first-order PMD and the second-order polarization mode dispersion (SOPMD) in optical systems for different values of temperature. The relation between SOPMD and the differential group delay (DGD) in a single-mode fiber is discussed. The analysis is based on time/wavelength and temperature variations changing/impacting PMD and DGD measured values. We present long-term statistical characteristics of SOPMD acquired by a PMD emulator constructed with some pieces of high birefringence fiber, and the correlation between the SOPMD, depolarization (Depol) and polarization-dependent chromatic dispersion (PCD) was investigated. A new interpretation of SOPMD is presented to analyze the problem of the temperature spectral stability. The actual techniques for SOPMD determination, consequently Depol and PCD, use the variation with time and/or wavelength. In some studies, the results obtained for SOPMD are correlated to first-order PMD. We demonstrate experimentally that the temperature is an important factor for the correlation between SOPMD and PMD and cannot be neglected in the optimization of high-speed optical communications links.     

Experimental prediction of the wavelength-dependent path-length factor for optical intrinsic signal analysis

Analysis of the optical intrinsic signal of an exposed cortex has been applied to measurement of functional brain activation. It is important for accurate measurement of concentration changes in oxygenated hemoglobin and deoxygenated hemoglobin to consider the wavelength dependence of the mean optical path lengths for the reflectance of cortical tissue. A method is proposed to experimentally estimate the wavelength dependence of the mean optical path length in cortical tissue from the multispectral reflectance of the exposed cortex without any additional instruments. The trend in the wavelength dependence of the mean optical path length estimated by the proposed method agrees with that estimated by the model-based prediction, whereas the magnitude of the wavelength dependence predicted by the proposed method is greater than that of the model-based prediction. The experimentally predicted mean optical path length minimizes the difference in the measured changes in the concentrations of the oxygenated hemoglobin and deoxygenated hemoglobin calculated from different wavelength pairs.     

Two-dimensional optical wavelet transform in space domain and its performance analysis

A new architecture of an optical wavelet transform system with a lenslet array is proposed, and its optical performance and optical limits are analyzed.     

Effects of unwanted feedback on synchronized chaotic optical communications

The effects of unwanted external optical feedback on synchronized chaotic optical communication systems are studied numerically. We consider an open-loop configuration consisting of a transmitter laser with double external optical feedbacks and a receiver laser with optical injection from the transmitter laser. First, including the effects of unwanted optical feedback, the synchronization performances of both the complete synchronization and the generalized synchronization are examined. Then the encoding and decoding performances of the generalized synchronization and the effects of the introduced feedback are investigated, respectively. Finally, we study the control of the unwanted feedback on the dynamics of the transmitter laser and briefly discuss the system security when the transmitter laser is driven to operate in a steady state or periodic oscillation state by the additional feedback.     

小波分析在光纤陀螺信号处理中的工程应用

光纤陀螺作为敏感角速度信息的元件对其精度要求较高,仅在硬件方面提高它的精度会极大的增加成本,工程应用中不经济,而从软件设计对光纤陀螺信号进行处理以提高精度便显得尤为重要.小波分析作为一种新型时频共域滤波器,处理二维信号取得了很大的进展,而在实时处理时间信号领域成果相对较少,为此,介绍了小波分析理论并提出了小波分析对光纤陀螺信号实时处理的工程应用方法.     

Free electron model for optical polarizabilities: Role of dimensionality

Abstract

The free electron model proposed by Rustagi and Ducuing to explain the enhancement of the second-order hyperpolarizabilities of π-con-jugated linear molecules with increasing conjugation length has been extended to two and three dimensions. The sign and magnitude for the linear (α) and third order (γ) polarizabilities as a function of the size and electron density of the systems have been obtained. Some simple general conclusions on the role of dimensionality in the nonlinear response have been obtained.