The reliability of laser diodes and laser transmitter modules

This paper reviews the reliability of laser transmitter modules for use in optical fibre transmission systems. Methods for reliability testing and lifetime prediction are discussed and the dominant failure mechanisms affecting laser modules are described. The current status of laser module reliability is discussed, based on both published results, and on the findings of a study at BT Laboratories of the reliability of commercial laser modules from ten manufacturers. It is concluded that significant progress has been made in the reliability of laser diodes, through the understanding of basic failure mechanisms, leading to long predicted lifetimes for a number of different laser structures. However, module packaging is less reliable and further work is required to identify and eliminate those materials and processing technologies which lead to the risk of early failure.     

Bragg grating-based laser sensors systems with interferometricinterrogation and wavelength division multiplexing

We describe the use of fiber Bragg grating based lasers as sensors. An interferometric detection technique is presented for interrogating laser wavelength shifts due to measurand induced laser cavity strain with high resolution from both single-mode and multimode lasers. The principle of integrating multiple sensors to form an array is demonstrated using a wavelength division multiplexing approach     

PIN diodes in meander-line filters for multiplexing applications

A new application of PIN diodes is described in terms of a meander-line switchable filter. With the diodes forward-biased, bandpass properties result, but under reverse bias the passband is converted into a stopband. These filters are intended for use in multiplexing systems.     

Properties of loss-coupled distributed feedback laser arrays forwavelength division multiplexing systems

The characteristics of loss-coupled distributed feedback (DFB) semiconductor laser arrays are investigated both theoretically and experimentally. Using simulations based on a transfer matrix method, the strong influence of the residual facet reflectivity on the singlemode yield and the statistical fluctuation of the emission wavelength for as-cleaved and AR/HR coated loss-coupled DFB lasers is pointed out and compared to purely index-coupled λ/4 phase-shifted devices. Experimental results and the fabrication techniques are given for loss-coupled 1.55 μm InGaAs/InGaAlAs/InP DFB laser arrays with four channels and integrated striped thin-film heaters, which were successfully used for fine tuning the channel spacings     

Elimination of phase noise of pulsed laser diodes in electroopticsampling systems

The multimode operation of pulsed laser diode sources is converted into excess noise in electrooptic probing systems for electrical waveforms and prevents measurements with shot-noise-limited sensitivity. The transfer function of the sampling system is determined and analyzed in terms of the wavelength and electric field dependence. It is shown that by proper orientation of the polarizing components within the optical beam path, points of operation can by found which have both a vanishing wavelength dependence of the transfer function and maximum sensitivity to the electric fields to be measured. Excess noise sources due to wavelength fluctuations of the laser are thus eliminated, and shot-noise-limited voltage resolution is obtained. The system has been optimized to yield a voltage sensitivity of 2.3 mV/√Hz for a longitudinal LiNbO₃ probe crystal     

Analysis of controlled multiplexing systems via numericalstochastic control methods

The paper demonstrates the usefulness of numerical methods of stochastic control theory for the design, analysis and control of multiplexing type systems and networks, as well as ATM type systems. The sources are of the Markov-modulated type, although the final results hold for other types (e.g. low-order AR schemes). Control problems arise when we wish to control cell loss due to buffer overflows by regulating the sources. The basic control mechanism is the deletion of selected low-priority cells, according to an appropriate state dependent rule. But the same results hold for many other schemes (e.g. purchasing incremental bandwidth). By exploiting the large size of the system (large number of users), the systems can be efficiently approximated by diffusion type processes, whether there is a control term or not, and for many types of control mechanisms. The basic controls are of the “low-priority cell deletion” type, and various extensions. They might be state dependent, and we can obtain optimal controls for cost functions that weigh buffer overflow, controller cell deletion loss as well as queue length. The limit equations are an effective aggregation of the original system. It is shown that there are substantial savings in losses with the use of optimal control techniques. The numerical methods can be used to balance the losses at the control with those due to buffer overflow, to minimize losses at the controller subject to constraints on buffer overflow, and to explore various approximations, systems aggregations, the interaction of multiple source classes of different priorities     

Uncooled DBR laser directly Modulated at 3.125 gb/s as athermal transmitter for low-cost WDM systems

An uncooled three-section tunable distributed Bragg reflector laser is demonstrated as an athermal transmitter for low-cost uncooled wavelength-division-multiplexing (WDM) systems with tight channel spacing. A /spl plusmn/0.02-nm thermal wavelength drift is achieved under continuous-wave operation up to 70/spl deg/C. Dynamic sidemode suppression ratio of greater than 35 dB is consistently obtained under 3.125-Gb/s direct modulation over a 20/spl deg/C-70/spl deg/C temperature range, with wavelength variation of as low as /spl plusmn/0.2 nm. This indicates that more than an order of magnitude reduction in coarse WDM channel spacing is possible using this source.     

Noise characteristics of high-frequency superposed laser diodes for optical disc systems

Excess noise peaks in an index-guided single-mode laser can be suppressed by superposing high-frequency current. By this method, index-guided lasers have stable noise characteristics and the noise level is about 7?12 dB lower than that in gain-guided multimode lasers at output power of about 3 to 5 mW.     

半导体激光器列阵器件光束复合研究

文中针对半导体激光器列阵的波长复合和偏振复合技术开展了设计和实验研究。首先利用半导体材料波长易调节的特点,设计了AlGaInAs/GaAs/AlGaAs压应变量子阱结构,得到了760nm、800nm、860nm、930nm、976nm五个波长激射的半导体列阵激光器,同时设计了四个短波通滤波片参数,开展了半导体列阵激光器的多波长光束复合技术的实验研究;其次利用1/2波片和偏振复合棱镜将两束不同偏振状态的光束进行了复合,并设计了光束聚焦系统。最终实现了5个波长,10条半导体激光器列阵的光束复合,得到了196W的激光功率输出,总体效率为76%,其中,波长复合效率可以达到92.4%,输出聚焦光斑尺寸为144μm×1 330μm,聚焦光功率密度达到1.02×105W/cm2,与单条半导体激光器列阵相比,合束光的光功率密度提高了4.3倍。     

Phase-locking of laser diodes

Phase-locking of two optically prestabilised laser diodes is realised by controlling the injection current and the feedback cavity resonance frequency of one of the lasers.<>     

Transmit selection in spatial multiplexing systems

In this letter we solve the transmit antenna selection problem for a zero forcing spatial multiplexing system with knowledge of the channel statistics at the transmitter. We show through Wishart matrix analysis that the signal-to-noise ratio on the kth stream is a weighted Chi-squared variable with the weight equal to-the kth diagonal entry of the inverted transmit correlation matrix. We use this result to develop selection algorithms for two cases-maximizing ergodic capacity and minimizing the average probability of error. Monte Carlo simulations illustrate potential performance improvements.     

Electrooptic modulation of multilongitudinal mode laser diodes: Demonstration at 850 nm with simultaneous data transmission by coherence multiplexing

An electrooptic modulation method based on signal coding through optical delays larger than the source coherence length is described. The method is illustrated with the simultaneous transmission of several signals by path-difference multiplexing. In this method, the signals are coded as a sequence of optical delays introduced by a series of electrooptic modulators working as delay lines. Decoding is carried out by temporal correlation performed by another sequence of optical delays introduced by birefringent plates, and matched to those used at the emission process. The permissible delays are shown to be ruled by the source coherence length and by the number of laser modes.     

基于半导体激光器高精度恒温系统设计

为了在宽温环境中保证半导体激光器能够稳定地输出功率和波长,针对半导体激光器体积小、重量轻及对温度稳定性要求高等特点,在采用负温度系数热敏电阻作为温度传感器并对其输出信号进行处理的基础上,设计了基于微控制单元的半导体激光器温度控制系统,并在软件上采用了直接比例积分微分算法等方法。通过微控制单元调整输出调制信号脉宽和幅值,从而改变半导体致冷器的驱动电流的大小和方向,并进行了相应理论分析和实验验证,可知半导体激光器工作温度稳定在25℃左右,且温度稳定精度为±0.1℃。结果表明,该高精度温度控制系统在宽温环境中控制精度高、响应速率快,优于其它同类产品。     

Switching between diversity and multiplexing in MIMO systems

Multiple-input multiple-output (MIMO) wireless communication systems can offer high data rates through spatial multiplexing or substantial diversity using transmit diversity. In this letter, switching between spatial multiplexing and transmit diversity is proposed as a simple way to improve the diversity performance of spatial multiplexing. In the proposed approach, for a fixed rate, either multiplexing or diversity is chosen based on the instantaneous channel state and the decision is conveyed to the transmitter via a low-rate feedback channel. The minimum Euclidean distance at the receiver is computed for spatial multiplexing and transmit diversity and is used to derive the selection criterion. Additionally, the Demmel condition number of the matrix channel is shown to provide a sufficient condition for multiplexing to outperform diversity. Monte Carlo simulations demonstrate improvement over either multiplexing or diversity individually in terms of bit error rate.     

Influence of extinction ratio on performance of optical receivers incorporating laser preamplifiers

Reports the performance degradation of optical receivers incorporating semiconductor laser amplifiers (SLAs) caused by the nonzero extinction ratio of the input optical signal. The resulting sensitivity penalty dependencies on bit rate, SLA gain and facet reflectivity are also investigated. The results clearly show that the new receiver can be more affected by a finite extinction ratio compared to a conventional pin or APD receiver. Using a bandpass optical filter to reduce amplifier spontaneous emission noise will increase the extinction ratio penalty.<>     

Analysis of the spectral linewidth of distributed feedback laser diodes

The spectral linewidth of DFB laser diodes is analysed. DFB lasers with long cavities are shown to have very narrow spectral linewidth, and it is further narrowed if a phase shift is introduced at the centre of the cavities.     

Analysis of the spectral linewidth of distributed feedback laser diodes

The spectral linewidth of distributed feedback (DFB) laser diodes is theoretically studied. Numerical calculation shows that DFB lasers with long cavity lengths and large coupling coefficients have very narrow spectral linewidth less than 1 MHz, The effects of the phase shift and mirror facets on the spectral characteristics of DFB lasers are also analyzed, It is shown that the phase-shifter further narrows the spectral linewidth of DFB lasers. Its numerical result and physical meaning are also shown.     

Temporal wavelength multiplexing of quantum cascade laser

A quantum cascade (QC) laser is presented that is both temporally and wavelength multiplexed, i.e. it emits two widely different wavelengths at two alternate time slots. A bidirectional and multi-wavelength QC laser source that emits at 10.8 m wavelength for a positive polarity current and 8.6 m for a negative polarity current was used. A diode based adder circuit was designed to drive the QC laser such that it added two opposite polarity current pulses with a variable time delay. Application of two polarity 100 ns pulse width current pulses at 80 kHz with time delays of 0.5 to 2.5 s resulted in a versatile time and wavelength multiplexed QC laser.     

Capacity of QAM SCM systems utilising optically linearisedMach-Zehnder modulator as transmitter

The limitation imposed by nonlinear distortion and Gaussian noise on the capacity of a subcarrier multiplexed system with quadrature amplitude modulated channels and an optically linearised Mach-Zehnder externally modulated laser transmitter is calculated. A typical system can support 140 6 MHz quadrature phaseshift keying channels (an aggregate bit rate of 1.4 Gbit/s) at 10^-9 bit error rate with a received power of -25 dBm or 300 64-QAM channels (8.9 Gbit/s) with a received power of -15 dBm