Noise power spectra of optical two-beam interferometers induced by the laser phase noise

Dynamic range of many optical signal processing and sensing devices incorporating two-beam interferometers (such as Mach-Zehnder and Michelson interferometers) can be limited by random phase fluctuations of the optical source emission field. This paper is concerned with the intensity fluctuations, originating from the laser source phase noise, at the output of a two-beam interferometer. Closed-form theoretical expressions are presented for the autocovariance function, variance, and noise power spectral density of the instantaneous output intensity which are valid for any optical biasing phase and any source coherence time. Application of the results in the noise performance evaluation of optical devices incorporating two-beam interferometers will also be shown.     

Source-induced noise in optical systems driven by low-coherencesources

High-power quasimonochromatic light sources may deliver enough power into optical and fiber-optic systems to make the source noise dominant at the system output. With an RMS value proportional to light intensity, this noise limits the system sensitivity and dynamic range to levels which cannot be improved by injecting more light into the system. Source-related noise may originate not only from the source intensity fluctuations, but also from a phase-to-intensity conversion process which is characteristic of many single-spatial-mode multiple-path optical systems. The shape of the power spectral density of the source-induced noise, being critically dependent on the physical structure of the system, is analyzed for a self-homodyne Mach-Zehnder structure and for a recirculating delay line. For single-path communication systems, it is shown that source-originated noise exceeds both shot and thermal noise for a received optical power of only a few tens of microwatts     

Source phase-induced noise in unbalanced time domain multiplexedsensor networks

The dynamic range of many optical signal processing and sensing devices incorporating two-beam interferometers can be limited by random phase fluctuations of the optical source emission field. We report the theoretical expressions of the autocovariance function of the output intensity and the noise power spectral density due to the source phase-induced noise in unbalanced time domain multiplexed (TDM) sensor networks. Application of the results in the performance evaluation of all-fiber TDM sensor networks illustrating the important trade-offs between high duty cycle, sensitivity, and noise power are also presented     

基于稀土掺杂光纤荧光强度比的温度传感

相比于普通电传感器,光纤温度传感器具有精度高、传感范围宽、不受电磁干扰等优点。基于荧光强度、荧光寿命的稀土掺杂光纤温度传感器容易受到外部环境的干扰,测量精度存在不确定性。利用稀土离子两个能级的荧光强度的比值随温度变化的特性进行测温,可消除这种干扰（如泵浦源的噪声和波动）,而且荧光强度比对应力的不敏感性,使得它可以在很多应力与温度交叉敏感的传感系统中作为温度补偿。总结了国内外相关稀土掺杂光纤的荧光强度比测温的发展状况,分析了掺铒、掺钕、掺镱等石英光纤的荧光强度比温度测量的特点。     

The effect of semiconductor laser intensity fluctuations on the performance of lightwave transmission systems

The performance of a high-bit-rate lightwave transmission system which uses a dc-operated semiconductor laser with external modulation may be affected by intrinsic intensity fluctuations in the laser output. The effect of such random variations in the transmitted optical pulse stream on the average probability of error is determined. The results indicate that while the effect of the intensity fluctuations is strongly influenced by the average gain of the photodetector, these random variations are not a serious source of transmission errors at a bit rate of 8 Gbit/s.     

Soft Optomechanical Systems for Sensing,Modulation, and Actuation

Soft optomechanical systems have the ability to reversibly respond to optical and mechanical external stimuli by changing their own properties (e.g., shape, size, viscosity, stiffness, color or transmittance). These systems typically combine the optical properties of plasmonic, dielectric or carbon-based nanomaterials with the high elasticity and deformability of soft polymers, thus opening the path for the development of new mechanically tunable optical systems, sensors, and actuators for a wide range of applications. This review focuses on the recent progresses in soft optomechanical systems, which are here classified according to their applications and mechanisms of optomechanical response. The first part summarizes the soft optomechanical systems for mechanical sensing and optical modulation based on the variation of their optical response under external mechanical stimuli, thereby inducing mechanochromic or intensity modulation effects. The second part describes the soft optomechanical systems for the development of light induced mechanical actuators based on different actuation mechanisms, such as photothermal effects and phase transitions, among others. The final section provides a critical analysis of the main limitations of current soft optomechanical systems and the progress that is required for future devices.     

光纤光栅在全光通信网中的应用

随着社会对通信业务需求的不断增加,基于DWDM的全光网络已成为未来光通信的发展趋势,光纤光栅具有附加损耗小,体积小,能与光纤很好地耦合,可与其他光纤器件融成一体等特性,这将使光纤光栅成为未来全光网中的基石,从目前的研究来看,光纤光栅已经能够为全光通信系统不光源,光放大,色散补偿,OTM（光终端复接器）,OTDM（光时分复用）,OXC（光交叉连接）等等关键部件提供优秀的解决方案。     

Spectral tuning and linewidth narrowing of shallow-junction surface emitting GaAs LED́s through γ-ray irradiation

Alterations of device characteristics as a result of γ-ray irradiation of shallow-junction surface emitting devices are different from those of deep-junction devices with respect to LED emission intensity reduction, I-V curve, line shape, and spectral shift. In particular, much larger spectral shifts in the opposite direction-toward longer wavelengths-are reported here than those found in the literature for deep-junction devices. A qualitative model based upon photochemical doping and changes in surface band bending is proposed to explain these phenomena. Changes in surface emitting shallow-junction optical radiation source device characteristics brought about by γ-ray irradiation are desirable ones for utilization in most optical fiber communication systems. These changes include linewidth narrowing, decreased time response, and decreased material dispersion because of the emission wavelength change.     

强度调制高速光通信系统传输性能研究

码型调制格式是提高通信传输质量的一项关键技术,针对高速光传输系统中码型调制格式及其传输性能进行理论分析,重点研究了强度调制格式在高速光传输系统中的性能;通过光通信仿真软件搭建强度调制格式的产生装置,得到各种调制格式的光谱,对强度调制格式在系统中抗色散能力和抗非线性能力进行了研究和分析。     

Coherent optical processing

The principles of communication theory were applied in the 1950's to optical imaging systems and to the analysis of images. Optical systems were analyzed in much the same way as linear systems (modulation transfer functions and channel capacities) and images were characterized in ways analogous to time signals (space-bandwidth products, spatial frequency content, etc.). Both coherently and incoherently illuminated optical systems can be treated using these concepts. Coherently illuminated systems are most useful for performing operation such as convolution, cross correlation, and spectral analysis because the Fourier transform of an optical signal physically exists and can, therefore, be measured or modified. The basic Fourier transform relationship for coherently illuminated systems is developed in this paper. It can be detected directly and used to estimate the distribution of spatial frequencies contained in the signal. Methods for constructing complex-valued spatial filters are described; these filters can be used to realize such operations as convolution or cross correlation, addition or subtraction, and differentiation or integration. Experimental results are given to illustrate the concepts and to susgest potential applications. To extend the range of applications, interface devices are needed to allow optical processing of two-dimensional raster-scanned time signals, wide bandwidth electrical signals, and incoherent optical signals. Interface devices are often needed to convert the output optical signal to an electrical signal for post-processing by a digital computer. For some applications, interface devices are needed to construct spatial filters in real time, so different operations can be performed on a given signal. The desired characteristics of these three interface devices and the current state of their development are briefly reviewed.     

Realization of broad-band phase devices by using W-typeeccentric-core optical fibers

Axially nonsymmetric W-type optical fibers are proposed as broadband phase devices for optical-fiber systems to provide the Fresnel's rhomb retarder in classical optical systems. Propagation characteristics of the W-type eccentric-core optical fibers are investigated theoretically using the boundary-integral method. The results of the numerical analysis indicate that such broadband phase devices can be realized by using the wavelength-dependent birefringence of axially nonsymmetric W-type optical fibers. It is theoretically found in the case of eccentric cores that a relative bandwidth of 18.2% with a phase retardation error of 2% is realizable without the propagation of higher-order mode waves     

大气信道下窄线宽激光器对相干通信系统性能影响研究

在空间下行相干激光通信系统中,激光器的线宽大小会影响通信系统的性能,而窄线宽激光器能有效降低激光器线宽引起的激光器相位噪声,目前已经成为相干激光通信系统的首选。光信号在大气信道传输时,大气湍流会引起光信号强度和相位的波动,从而进一步影响系统的通信性能。针对上述问题,基于四相移键控(Quadri Phase Shift Keying, QPSK)通信系统的工作原理,进一步考虑窄线宽激光器线宽和大气湍流引起的光信号强度和相位的波动,给出了空间下行QPSK通信系统误码率模型。并基于该模型,数值仿真分析了窄线宽激光器对空间下行相干激光通信系统性能的影响。结果表明：大气湍流不仅严重影响系统性能,也会弱化激光器线宽对系统性能的影响。而对于大气湍流影响而言,其引起的相位波动要大于光强波动的影响。此外,随着通信速率增大,激光器线宽对于系统性能影响也会随之降低。文中对于空间下行相干激光通信系统的优化设计和调试具有一定的实际参考意义。     

Architecture of an all-optical circuit-switched multistageinterconnection network

An architecture of an all-optical multistage interconnection network is proposed. The network supports a circuit-switching model of communication and can provide parallel optical paths among input and output ports. It uses an address-based routing algorithm for path setup which, due to its decentralized nature, makes this network suitable for designing high-speed switching systems. These switches are commonly used in telephony and multiprocessor systems. The proposed architecture uses bistable optical devices, such as interference filters, as essential components of its switching modules. Since these devices can be easily fabricated, the implementation of this architecture is feasible. Various design issues related to optical clock generation, its distribution, data synchronization, and intensity restoration are also discussed     

Influence of facet reflection on the performance of a DFB laserintegrated with an optical amplifier/modulator

The influence of output facet reflection on characteristics of a monolithically integrated optical device consisting of a distributed feedback (DFB) laser and an optical amplifier/modulator is analyzed. Basic equations used for the analysis are the characteristic equation for a DFB laser and the rate equations. As a result, it is known that static and dynamic properties of such optical integrated devices are seriously degraded by the optical feedback from the output facet. It is concluded that the power reflectivity of the output facet should be reduced to at least less than 0.1% for the integrated region to work as an optical amplifier and less than 0.01% as an intensity modulator     

Fused conically tapered fibre-optical power divider

We describe a novel, conically tapered fibre-optical power divider which substantially increases the usable power of an LED source and provides highly homogeneous power division among the output fibres. Such devices are of use in optical signal distribution systems such as fibre-optical local computer networks. The performance of a 25-fibre power divider is described.     

倒锥透镜型光纤端的出射光场性质

倒锥透镜型光纤端系经过特殊技术加工而成。本文用二阶统计相关函数,采用单色相干光源,计算了倒锥透镜型光纤端的光场分布及其统计相关性质,并与平端的情况作了对比。在实验上,用He-Ne激光作为单色相干光源,对理论进行了验证。     

Quantum-well perfection requirements for large-scale applicationsof exciton-base devices

Exciton-based devices have been shown to display remarkable properties that can find applications in optical information processing. However, if these devices are to be used in systems where many devices and chips involving the excitonic effect are to be used, there has to be stringent control over the quantum-well perfection. In this paper we examine the effect of exciton linewidth and well size fluctuations on a number of device-related effects based on the quantum-confined Stark effect. We find that for many applications a one-monolayer variation in structures fabricated in different runs will drastically diminish the system performance     

Fiber-optic lattice signal processing

We discuss the implementation of fiber-optic lattice structures incorporating single-mode fibers and directional couplers. These fiber structures can be used to perform various high-speed time-domain and frequency-domain functions such as matrix operations and frequency filtering. In this paper we mainly consider systems in which the signals (optical intensities) and coupling coefficients are nonnegative quantities; these systems fit well in the theory of positive systems. We use this theory to conclude, for example, that for such systems the pole of the system transfer function with the largest magnitude is simple and positive-valued (in the Z-plane), and that the magnitude of the frequency response can nowhere exceed its value at the origin. We also discuss the effects of various noise phenomena on the performance of fiber-optic signal processors, particularly considering the effects of laser source phase fluctuations. Experimental results are presented showing that the dynamic range of the fiber systems, discussed in this paper, is limited, not by the laser source intensity noise or shot noise, but by the laser phase-induced intensity noise. Mathematical analyses of lattice structures as well as additional applications are also presented.     

Optical wavelength converters

Devices and techniques for shifting the frequency/wavelength of an optical carrier wave are reviewed, with particular reference to applications in wavelength/frequency division multiplexed optical communication systems. Considered first are devices which make use of semiconductor laser amplifiers or oscillators. Some of these techniques come close to satisfying the requirements for a suitable wavelength shifter, though none appear to offer the potential for a transparent network independent of data modulation format. A second main category, using nonlinear optical materials, promises to satisfy the requirements for frequency translation in a network which is fully transparent to the modulation format. However, the conversion efficiencies currently available appear too low for cost-effective communication systems engineering so that further advances in nonlinear optical materials/techniques are still required. It seems likely that guided wave techniques will provide the best means of improving the efficiency by increasing the interaction length and the optical intensity for a given power. Schematic examples of such a converter are discussed. All of the existing techniques have some deficiencies compared to an ideal system so that wavelength conversion for optical communication systems is still a problem in search of a good solution     

Intensity-induced stopband bending in periodic nonlinearmultilayers

A simple and physically appealing theory is developed to describe the intensity-induced “bending” of the stopband region of nonlinear multilayered devices. The theory is subsequently used to graphically map the general response of devices as a function of input intensity and phase detuning. Deeper insight into conventional (known) phenomena is obtained, while new phenomena are revealed, such as “hard” optical limiting, intensity discrimination, and a new arrangement for bistability