自由空间相干光通信技术及发展

对比了相干探测和直接探测，以及自由空间相干光通信和光纤相干通信，对相干自由空间激光通信系统的特点和关键技术进行了讨论，回顾了近年来国外在自由空间相干光通信领域内开展的有关研究计划。最后对相干光通信技术的应用前景进行了展望。

自由空间激光远距离传输的地面模拟研究

根据透镜的傅里叶变换性质，提出了采用光学傅里叶变换加级联光学成像放大并结合有限口径接收的方法来实现自由空间激光光束远距离传输的实验室模拟。由此原理设计了自由空间激光远距离传输模拟装置，该装置主要由大口径、长焦距的傅里叶变换平行光管和三级成像放大镜所组成，最大等效传输距离达2.4×10^5km，可用于星间激光通信终端综合通信性能的评估，在设定的误码率下测量终端可能的通信距离，或者在设定的作用通信距离下检测通信的误码率。

卫星激光通信终端光跟踪检测的数理基础

卫星激光通信光束的传播在空间属于远场衍射，而在地面光学跟踪检验中属于近场衍射。采用菲涅耳衍射理论研究了远场衍射和近场衍射对于产生光学跟踪位置误差信号的本质上的差别并且分析了等效条件，证明了近场检验中采用卫星相对角度运动轨迹的光束扫描可以准确模拟远场相对运动，同时也得到了光斑特性和卫星抖动模拟的分析。澄清了卫星激光通信终端光学跟踪性能检验中的基本概念，具有实际指导意义。

Performance analysis of bit error rate for free space optical communication with tip-tilt compensation based on gamma-gamma distribution

In this paper, the gamma-gamma probability distribution is used to model turbulent channels. The bit error rate (BER) performance of free space optical (FSO) communication systems employing on-off keying (OOK) or subcarrier binary phase-shift keying (BPSK) modulation format is derived. A tip-tilt adaptive optics system is also incorporated with a FSO system using the above modulation formats. The tip-tilt compensation can alleviate effects of atmospheric turbulence and thereby improve the BER performance. The improvement is different for different turbulence strengths and modulation formats. In addition, the BER performance of communication systems employing subcarrier BPSK modulation is much better than that of compatible systems employing OOK modulation with or without tip-tilt compensation.

卫星激光通信Ⅱ地面检测和验证技术

星间激光通信终端的主要技术指标和运行性能必须事先在地面实验室条件下进行模拟检验, 因此在研制卫星激光通信终端的同时必须发展相应的系统性检测和验证平台, 主要包括激光通信性能检验、光跟瞄性能检验和光束质量检验。本文综述了卫星激光通信终端检验技术的国外进展, 介绍了我们全物理模拟的地面检测验证思路和方法。

Measurement of the wavefront of a laser diode system for intersatellite communication by a Jamin double-shearing interferometer

The frame of a laser diode transmitter for intersatellite communication is concisely introduced. A simple, novel and visual method for measuring the diffraction-limited wavefront of the transmitter by a Jamin double-shearing interferometer is proposed. To verify the validity of the measurement, the far-field divergence of beam is additionally rigorously analysed in terms of the Fraunhofer diffraction. The measurement, the necessary analyses and discussion are given in detail. By directly measuring the fringe widths and quantitatively interpreting the interference fringes, the minimum detectable wavefront height (DWH) of the wavefront is only 0.2 gimel (the distance between the perfect plane wavefront and the actual wavefront at the transmitting aperture) and the corresponding divergence is only 65.84 mu rad. This indicates that the wavefront approaches the diffraction-limited condition. The results show that this interferometer is a powerful tool for testing the semiconductor laser beam's wavefront, especially the diffraction-limited wavefront.

Wireless optical communication-based spatial pattern

We propose a novel communication technique which utilizes a set of mutually distinguishable optical patterns instead of convergent facula to transmit information. The communication capacity is increased by exploiting the optical spatial bandwidth resources. An optimum detector for this communication is proposed based on maximum-likelihood decision. The fundamental rule of designing signal spatial pattern is formulated from analysis of the probability of error decision. Finally, we present a typical electro-optical system scheme of the proposed communication. (c) 2006 Elsevier GmbH. All rights reserved.

Far-field spot compression without energy loss in main lob in wireless laser communication

Far-field spot compression without energy loss in main lob is of great significance to wireless laser communication. In this letter, we propose two schemes to obtain far-field spot compression without energy loss in main lob. One scheme is based on the simulated annealing (SA) algorithm. Using SA algorithm, we design the phase profile of the diffractive phase element (DPE). Using the designed DPE, far-field spot compression without energy loss in main lob is achieved. The other scheme is based on YG algorithm. By means of YG algorithm, we appropriately designed the DPE in the emitting plane. Using the DPE, far-field spot compression without energy loss in main lob is obtained. (c) 2007 Elsevier GmbH. All rights reserved.

Designing a coupler for the intersatellite optical communication system

Coupling a single-mode laser diode with 200 mW to a single-mode fiber (SMF) through an orthonormal aspherical cylindrical lens and a GRIN lens for the intersatellite optical communication system is proposed and demonstrated. We experimentally studied how the coupling efficiency changes with the SMF's position displacement and axial angle variation, and obtained 80 mW output power at the end of the SMF, which shows that the coupling units have satisfied the designed request. (c) 2007 Elsevier GmbH. All rights reserved.

Effects of atmosphere visibility on performances of non-line-of-sight ultraviolet communication systems

Dependence of performances of non-line-of-sight (NLOS) solar-blind ultraviolet (UV) communication systems on atmosphere visibility is investigated numerically by correlating the propagation of UV radiation with the visibility. A simplified solar-blind UV atmospheric propagation model is introduced, and the NLOS UV communication system model is constituted based on the single scattering assumption. Using the model, numerical simulation is conducted for two typical geometry configurations and different modulation formats. The results indicate that the performance of the NLOS UV communication system is insensitive to variation of visibility in quite a large range, and deteriorates significantly only in very low-visibility weather, and is also dependent on the geometry configuration of the system. The results also show that the pulse position modulation (PPM) is preferable due to its high-power efficiency to improve the system performance. (c) 2007 Elsevier GmbH. All rights reserved.

Multi-channel 2-D pattern transfer model based on imaging optics for submarine laser uplink communication

An optical communication scheme of 2-D pattern transfer based on imaging optics for submarine laser uplink communication (SLUC) is suggested. Unlike the methods aiming at avoiding neighboring crosstalk used in traditional multi-channel optical beam transferring, we make full use of the overlapping of each spreading beam other than controlling divergence effect of each beam to avoid interference noise. The apparent parameters have been introduced to simplify theoretical analysis of optical pattern transfer problem involving underwater condition, with the help of which the complex beam propagation inside two kinds of mediums can be easily reduced to brief beam transfer only inside air medium. In this paper, optical transmission path and receiver terminal optics geometry have been described in detail. The link range equation and system uplink performance analysis have also been given. At last, results of a proof-of-concept experiment indicate good feasibility of the proposed SLUC model. © 2007 Elsevier GmbH. All rights reserved.

Double-cladding Yb3+ fiber for free-space optical communication

The experiment result of Nd:YVO4 laser pumped by laser diode that was amplified by double-cladding Yb3+ fiber is reported. Stable mode-locking pulses are obtained at repetition rate of 320 MHz and the output power is 15 mW. When laser power is amplified by Yb3+- doped double-cladding fiber amplifier, its power can get to 600 mW. Based on these, experiment of double-frequency is carried out, and green laser with power of 4 mW is obtained. (c) 2007 Wiley Periodicals, Inc.

Acoustic signals of Chinese alligators (Alligator sinensis): Social communication

This paper reports the first systematic study of acoustic signals during social interactions of the Chinese alligator (Alligator sinensis). Sound pressure level (SPL) measurements revealed that Chinese alligators have an elaborate acoustic communication system with both long-distance signal-bellowing-and short-distance signals that include tooting, bubble blowing, hissing, mooing, head slapping and whining. Bellows have high SPL and appear to play an important role in the alligator's long range intercommunion. Sounds characterized by low SPL are short-distance signals used when alligators are in close spatial proximity to one another. The signal spectrographic analysis showed that the acoustic signals of Chinese alligators have a very low dominant frequency, less than 500 Hz. These frequencies are consistent with adaptation to a habitat with high density vegetation. Low dominant frequency sound attenuates less and could therefore cover a larger spatial range by diffraction in a densely vegetated environment relative to a higher dominant frequency sound. (C) 2007 Acoustical Society of America.

Detecting quantum entanglement by means of local operations and classical communication

Based on the positive maps separability criterion, we present a method for the detection of quantum entanglement of a shared bipartite quantum state, within the "distant labs" paradigm, using only local operations and classical communication.