Realization of ultrahigh coherence in semiconductor lasers bynegative electrical feedback

To be useful, an ultrahigh-coherence semiconductor laser source requires high frequency stability, narrow linewidth, the capability of frequency tracking to a master laser, and stable frequency tuning. Negative electrical feedback is proposed to meet these four requirements simultaneously. Although the degree of frequency fluctuation that can be reduced by negative electrical feedback is limited by the noise contained in the feedback signal, theoretical calculations show that the fluctuations can be lower than the quantum noise limit. Experimental results obtained recently by the author are reviewed     

Efficient frequency noise reduction of GaAlAs semiconductor lasersby optical feedback from an external high-finesse resonator

The reduction of frequency noise power level of non-antireflection-coated GaAlAs laser diodes by several orders of magnitude using optical feedback from an external high-finesse Fabry-Perot resonator is discussed. Optimum oscillation parameters were derived from a steady-state model. Long-term performance was obtained by electronic feedback phase control     

Stabilised PSK transmitter with negative electrical feedback to a semiconductor laser

A method for PSK signal generation using a semiconductor laser in an optical phase-locked loop is presented. The error signal caused by imperfect PM response and phase noise is obtained by optical homodyne detection and fed back to the semiconductor laser. Flat PM response up to approximately 8 MHz is achieved and PSK signal generation is demonstrated.     

电反馈半导体激光器的稳定性

本文分析了电反馈(NFFB)半导体激光器(ILD)的稳定性,并研究了其调制和噪声特性的改善。     

Linewidth reduction of a semiconductor laser by electrical feedback

An electrical feedback technique was proposed to stably reduce the linewidth of a semiconductor laser without changing its cavity structure. Calculations and experiments were carried out to reduce the linewidth of a 1.5 μm InGaAsP laser (DFB type) according to the following procedure. A compact Fabry-Perot interferometer was used as a freqeuncy discriminator. The minimum attainable linewidth, limited by the detector noise, was estimated as being narrower than 1 kHz when the reflectance of the interferometer used was higher than 0.9. The minimum linewidth obtained in the experiment was 330 kHz, which was 15 times as narrow as in the case of a free-running laser. The improvements of this experimental result can be expected by simultaneously reducing the AM noise of the laser.     

Simple method for feedback parameter determination in optical feedback controlled semiconductor lasers

A simple method is proposed to measure the X parameter which determines the optical feedback influence on the semiconductor laser spectral characteristics. Experimental results are in good agreement with high spectral resolution measurements.     

提高半导体激光二极管功率密度的光束整形方法

针对半导体激光二极管由束散角大（14~46）导致的激光功率密度在传播过程中不断衰减的问题,提出了一种提高激光功率密度的光束整形方法。首先以X型柱面平凸透镜和Y型柱面平凸透镜对激光二极管输出光束慢轴和快轴方向进行准直,然后通过一对平凸透镜组合进行扩束,进一步提高光束平行度,最后由单片平凸透镜将光束聚焦为高功率密度的光点。采用Light Tools软件仿真光路、优化光学元件参数,对光学元件进行实际选型后安装并调试光束整形系统。测试结果表明:半导体激光二极管输出光束的67%激光能量汇聚于直径1 mm圆内,激光功率密度优于30 W/cm2。     

半导体激光器混沌电路延时反馈控制方法

由于当前方法未能建立物理模型,导致混沌电路延时反馈控制复杂度和控制耗时增加,容错率下降,为此提出一种半导体激光器混沌电路延时反馈控制方法。组建半导体激光器混沌电路延时反馈条件下激光器电流激发混沌的物理模型,获取激光器非线性增益和线宽增强因子特性,并获取控制延迟可控条件。分析延迟量和反馈增益的解析关系,获取目标周期轨道控制参数的分岔图,以此为依据进行半导体激光器混沌电路延时反馈控制。实验结果表明,所提方法能够有效降低混沌电路延时反馈控制复杂度和控制耗时,大幅度提升容错率。     

Thermal feedback in power semiconductor devices

Thermal feedback (TF) is an important aspect for the thermal management of semiconductor devices and high-power density integrated circuits. Different features of positive and negative TF in transistors are reviewed and summarized for the macroscopic domain. The thermal feedback mechanism is applied to the microscopic domain of noise fluctuations in semiconductor devices. It is argued that TF may be responsible for a major part of 1/fflicker or excess noise. Some experimental evidence is presented which supports this thermal feedback 1/f-noise theory for bipolar and MOS field effect transistors. Device and circuit design rules for the minimization of transmitter noise are given.     

Dynamic regimes in semiconductor lasers subject to incoherent optical feedback

The dynamics of a semiconductor laser subject to polarization-rotated (incoherent) optical feedback in the long cavity limit has been numerically and experimentally investigated. The results show that the induced dynamics can be grouped into four regimes (stable, chaotic, pulsed, and two state) and that the transverse-magnetic mode of the laser never lases in the ring cavity configuration studied. In addition, unlike the earlier short-cavity work, the boundaries between the regimes in the long-cavity case were found to be independent of the external-cavity delay time.     

The optimal loop gain design for the spectral linewidth reductionin an electrical feedback semiconductor laser

The design of the optimal feedback loop gain for the spectral linewidth reduction in a semiconductor laser with electrical feedback is presented using a model of self-heterodyne type electrical feedback. The results show that the feedback loop gain must be suitably chosen to satisfy the requirement of narrowing linewidth. Due to the influence of AM noise in the feedback process, there is an optimal value for the feedback loop gain to reduce the linewidth for a given feedback condition. The influence of other feedback parameters on the linewidth reduction is also presented     

A method to dramatically improve subcarrier tracking

A method is presented for achieving a considerable improvement in phase tracking of square-wave subcarriers or other square waves. The amplitude of the phase quadrature reference signal is set to zero, except near the zero crossings of the input signal. Without changing the loop bandwidth, the variance of the phase error can be reduced by approximately Wσ₀² where σ ₀² is the phase error variance without windowing and W is the fraction of cycle in which the reference signal has a nonzero value. Simulation results confirm the results of the analysis and establish minimum W versus SNR (signal-to-noise ratio). Typically, the window can be made so narrow as to achieve a phase error variance of 1.5 σ₀⁴     

Noise properties of semiconductor lasers due to optical feedback

It has been recognized for some time that reflections back into the cavity of an injection laser from a mechanically unstable external mirror cause noise in the output of the laser (reflection noise). In this paper, noise measurements are presented for index-guided lasers as functions of the output power, the degree of optical isolation and external cavity length (5-15 cm), while current modulation (using 2-4 mA at 50-200 MHz) has been used to reduce the noise levels by as much as 20 dB. It is demonstrated that the mechanism of noise reduction by modulation is the frequency modulation of the laser's spectrum and its interaction with the modes of the external cavity. It is shown that using an optical isolator can give a relative intensity noise of -110 dB while using modulation in addition can reduce this to -125 dB; the bandwidth performance of optical storage systems is discussed in terms of these two schemes.     

Sensitivity to external feedback for gain-coupled DFB semiconductor lasers

Performances of DFB semiconductor lasers with both index and gain spatial modulation are theoretically analysed in terms of threshold gain, emission wavelength, model discrimination and sensitivity to external optical feedback. It is found that gain coupling provides low threshold gain, high spectral selection in favour of the mode oscillating at the Bragg wavelength with a relative immunity from facet reflection, and the sensitivity to external feedback is not significantly reduced in comparison with the case of pure index coupling.<>     

半导体多纵模激光器相干特性分析

本文运用傅立叶干涉分光计的原理和matlab工具 ,从半导体多纵模激光器的光谱图来分析其相干特性 ,指出其作为短相干光源的局限性 ,理论分析与实验结果基本一致。     

Community based feedback techniques to improve video search

In this paper, we present a novel approach to aid users in the difficult task of video search. We use a graph based model based on implicit feedback mined from the interactions of previous users of our video search system to provide recommendations to aid users in their search tasks. This approach means that users are not burdened with providing explicit feedback, while still getting the benefits of recommendations. The goal of this approach is to improve the quality of the results that users find, and in doing so also help users to explore a large and difficult information space. In particular we wish to make the challenging task of video search much easier for users. The results of our evaluation indicate that we achieved our goals, the performance of the users in retrieving relevant videos improved, and users were able to explore the collection to a greater extent.     

Chaos in the coherence collapse of semiconductor lasers

The observation of a quasiperiodic route to chaos in the coherence collapse of a single-mode semiconductor laser subjected to back-reflections from an external cavity is discussed. Also, a simple deterministic model that correlates well with the data is presented     

Detailed analysis of coherence collapse in semiconductor lasers

Experimental and theoretical studies of coherence collapse in GaAs/AlGaAs laser diodes with weak optical feedback show two distinct routes to chaos. In each case we observe undamped relaxation oscillations, then external cavity mode beating, and finally coherence collapse. When there is frequency locking between the relaxation oscillations and external cavity modes, a period doubling sequence is followed, otherwise the route to chaos is via quasiperiodicity     

A dynamic detection method to improve SLAM performance

Simultaneous localization and mapping (SLAM) technology is a research hotspot in the field of intelligent mobile robot, and many researchers have developed many classic systems in the past few decades. However, most of the existing SLAM methods assume that the environment of the robot is static, which results in the performance of the system being greatly reduced in the dynamic environment. To solve this problem, a new dynamic object detection method based on point cloud motion analysis is proposed and incorporated into ORB-SLAM2. First, the method is regarded as a preprocessing stage, detecting moving objects in the scene, and then removing the moving objects to enhance the performance of the SLAM system. Experiments performed on a public RGB-D dataset show that the motion cancellation method proposed in this paper can effectively improve the performance of ORB-SLAM2 in a highly dynamic environment.     

Characteristics of chaos synchronization in semiconductor lasers subject to polarization-rotated optical feedback

We numerically investigate the detailed characteristics of chaos synchronization in semiconductor lasers subject to polarization-rotated optical feedback. The emission of the dominant TE mode of a drive laser is rotated 90 deg and fed back to the laser with time delay. The polarization-rotated TE mode is also injected with time delay into the TM mode of a second laser. Two types of synchronization with different time-lags are found, as in the case for synchronization in semiconductor lasers with nonrotated optical feedback. However, a significant difference to the nonrotated optical feedback case is that neither of the two types of synchronization requires matching of optical carrier frequency between the two lasers.