Theory of digital modulation of semiconductor laser diode remotely

The authors present a new theory that can be applied to modulate the semiconductor laser diode (LD) remotely. The proposed scheme utilizes the effect of incoherent external optical feedback (EOF) on the LD output optical signal. In particular, owing to the high increase in threshold carrier density, the LD will be impelled to turn off when the value of external reflectivity is equal to the laser back reflectivity. Thus, by exposing the LD, which is injected with a dc-current higher than threshold, to incoherent EOF digital signal (which conveys the transmitted information), the output optical signal of LD can be modulated accordingly.     

Effect of carrier heating on chirp and modulation response of directly modulated semiconductor laser diodes

Abstract

This paper examines the effect of carrier heating due to hot carrier injection in directly modulated semiconductor laser diodes. Using a single-mode rate equation model including an equation for the energy stored in the conduction band carriers, the changes in carrier temperature due to hot carrier injection from the barrier layer in quantum-well laser diodes are seen to cause a degradation in the device's sinusoidal modulation response. This degradation is coupled with an increase in the wavelength chirp of the light output. It is concluded that in order to minimize the detrimental effect of carrier heating on directly modulated quantum well laser diodes the barrier height should be kept as small as practical. Additionally, the appreciable change in device performance brought about by carrier heating should be accounted for when modelling such devices.     

Behaviour of lasing wavelength change during degradation in a buried-heterostructure (BH)-type DFB laser

The behaviour of lasing wavelength change during degradation is experimentally clarified for a buried heterostructure (BH) InGaAsP/InP DFB laser, and the mechanism of the wavelength change is theoretically analysed.     

Random speckle modulation technique for laser interferometry

In spite of its obvious advantages over conventional contact and immersion techniques, laser interferometry has not yet become a practical tool in ultrasonic nondestructive evaluation since its sensitivity is insufficient in most practical applications. Part of the problem is that the maximum signal-to-noise ratio often cited in scientific publications and manufacturers' specifications cannot be maintained on ordinary diffusely reflecting surfaces. Although these surfaces reflect a fair amount (5–50%) of the incident laser light, this energy is randomly distributed among a large number of bright speckles. Unless the detector happens to see one of these bright speckles, the interferometer's signal-to-noise ratio will be much lower than the optimum. This adverse effect is almost completely eliminated by the suggested random speckle modulation technique. The conventional interferometric technique was modified to assure random occurrence of a few very bright speckles and to move the whole speckle pattern around at an appropriate speed. Random but frequent bright flashes detected from the surface of the specimen resulted. The bright periods are 0.1 ms or longer, sufficient to trigger the ultrasonic pulser and detect the transmitted signals before the flash subsides. As much as 5–10 times improvement of the optical sensitivity was achieved by this novel approach and close to maximum signal-to-noise ratio was maintained everywhere on the surface of a diffuse object.     

Electrothermal dynamics of semiconductor nanowires under local carrier modulation

Charge transfer, surface/interface, defect states, and internal fields strongly influence carrier statics and dynamics in semiconductor nanowires. These effects are usually probed using spatially resolved scanning current techniques, where charge carriers are driven to move by diffusion force due to a density gradient, drift force due to internal fields, and thermoelectric force due to a temperature gradient. However, in the analysis of experimental data, analytical formulas are usually used which are based on the assumption that a single component of these forces dominates the carrier dynamics. In this work we show that this simplification is generally not justified even in the simplest configurations, and the scanning microscopy data need to be analyzed with caution. We performed a comprehensive numerical modeling of the electrothermal dynamics of free charge carriers in the scanning photocurrent microscopy configuration. The simulation allows us to reveal and predict important, surprising effects that are previously not recognized, and assess the limitation as well as potential of these scanning current techniques in nanowire characterization.     

All-plastic organic dye laser with distributed feedback resonator structure

We demonstrated an all-plastic waveguide organic dye laser with distributed feedback (DFB) resonator. We fabricated DFB structure on a surface of SU-8 2002 photoresist polymer using the interference of two beams of a frequency-tripled Nd:YAG pulse laser at 355 nm. The typical grating pitch of fabricated DFB structure was 190, 380 and 570 nm, corresponding to the number of mode m, and each corresponding grating amplitude was ca. 1, 4-5, and 25 nm, respectively. Shallow amplitude of 1 or 4-5 nm is ascribed to the wide incidence angle of the interference beams. Threshold of lasing for m = 3 is lower than that for m = 1 or m = 2 depending on the emission wavelength. DFB structure with the smaller amplitude of gratings at m = 1 and 2 is required for the higher threshold of laser emission. By controlling the grating pitch with nanometer scale, we can tune the wavelength of laser emission with 40 to 60 nm tunability. Effective energy transfer via nonradiative Förster transfer mechanism assists in lowering the threshold of laser emission.     

Characterization of laser hardened steels by laser induced ultrasonic surface waves

Ultrasonic surface waves are suitable for the characterization of surface hardened materials. This is shown on laser hardened turbine blades. The martensitic microstructure within the surface layer of surface hardened steels has a lower surface wave propagation velocity than the annealed or normalized substrate material. Because the propagation velocity depends on the ratio of layer thickness to wavelengthd/, its measurement allows the determination of the hardening depth. If the surface wave frequency is high enough, the surface wave propagates mainly within the hardened layer. A correlation of the surface wave velocity to the surface hardness has been found. Because the variation of the surface velocity in hardened steels is small, a high measurement accuracy is necessary to obtain the interesting hardening parameters with sufficient certainty. Therefore, a measuring arrangement has been developed where laser pulses, guided by optical fibers to the surface hardened structure, generate simultaneously surface wave pulses at two different positions. The two ultrasonic pulses are received by a piezoelectric transducer. The surface wave velocity is obtained from the time delay between these pulses which is determined by the cross-correlation method. To evaluate simultaneously surface waves with different penetration depths from the same signal acquisition, digital filtering has been used in connection with the cross-correlation.     

一种时域解调激光干涉仪调制信号的方法

针对某低频水平振动校准设备的激光干涉仪,利用NI公司的多功能采集卡,采集激光干涉仪的调制信号及与其同相位的电容式位移传感器的输出信号.根据激光干涉仪调制原理,提出一种时域解调调制信号的方法.根据调制信号条纹的时域特性和它被限幅的实际特点,利用labview软件实现了信号的解调,并将电容位移传感器信号作为参考信号,还原解调后信号的相位,从而得到振动台高精度的振动波形.实验证明,此方法对连续振动信号的解调效果良好,具有较好的重复性和稳定性,精度可达到激光干涉仪发射激光波长的四分之一.     

隧道再生大功率半导体激光器模式特性分析

利用有效折射率法计算了隧道再生双有源区大功率半导体激光器的工作模式,并对计算得到的激射模式的近场、远场与实验结果进行比较,得到激射模式为一阶模,二者十分吻合,这有助于进一步优化设计波导结构。     

大功率半导体激光器全固态风冷散热系统

设计并制作了一种全固态大功率半导体激光阵列恒温散热系统。它利用半导体制冷器对大功率半导体激光阵列吸热,然后经由风冷散热。经测试,单bar激光阵列连续输出功率达到15.28W,双 bar 阵列输出达 27.8W 时,全部达到风冷散热控温精度±0.1 ;当环境温度达到 45 时,仍然能够保证激光阵列的正常使用。     

Crystallization of amorphous Ge films induced by semiconductor diode laser annealing

Crystallization of amorphous Ge (a-Ge) films induced by semiconductor diode laser (SDL) irradiation has been investigated. 500-nm-thick a-Ge films are crystallized by scanning 807 nm CW SDL light focused to 4.0 × 0.1 mm² with the scanning speed (v) ranging from 100 to 350 mm/s. From X-ray diffraction and Raman scattering spectra, it is confirmed that the films crystallized at v = 100 mm/s show [220] preferential orientation with no residual amorphous component. On the basis of in-situ monitoring, it has been confirmed that a-Ge films annealed at a v higher than 200 mm/s are transformed to crystalline phase via solid phase crystallization, while films annealed with v lower than 170 mm/s are melted and recrystallized.     

大功率半导体激光器热沉结构的数值模拟

为了提高激光器的冷却效果,设计了3种具有不同扩展结构的热沉,包括矩形平行肋、菱形肋片和30°矩形斜肋,模拟计算3种结构对应的热沉芯片的最高温度、器件热阻和水泵功耗。结果表明30°矩形斜肋布置方式可以增强不同流道内流体的扰动,破坏热边界层,提高对流传热系数,换热性能最好;然而,由于倾斜角的存在增加了流动阻力,需要以更高的功耗为代价。综合考虑器件热阻与水泵功耗,矩形平行肋片综合性能最好。     

Using the optical feedback regime for evaluating the slope of the modulation characteristic of a semiconductor laser

A new simple method based on the optical feedback phenomena is proposed for evaluating the slope of the modulation characteristic (generation frequency versus injection current) of a semiconductor laser. The slope is determined using the frequency of a signal formed as a result of the interference between the initial and scattered radiation, which is measured at the output of a photodiode built into the laser. The proposed method is theoretically justified, the scheme of realization is described, and the results of experimental verification using a particular laser diode are presented.     

Influence of the optical feedback on the large signal modulation characteristics of the external cavity semiconductor laser

After taking into account the multiple reflections of light in external cavity, the influence of the optical feedback on the large signal modulation characteristics of the external cavity semiconductor laser (ECSL) has been theoretically investigated. The numerical simulations show that, with the increase of the modulation index, the peak photon number of the ECSL tends to higher level on the whole. When the optical feedback is strong or weak, the peak photon number of the ECSL shows single period. However, the peak photon number exhibits bifurcation or chaos for different modulation index when the optical feedback is intermediate.     

椭圆柱透镜组准直半导体激光束的优化设计

采用光线矢量法设计了两个相互垂直的椭圆截面柱透镜组来准直半导体激光束。利用多目标优化中的加权和法进行优化计算,并提出自适应加权系数优化法。此方法能根据计算结果自动调整加权系数,从而达到最佳的优化结果。计算结果表明,该准直系统可达到45μrad左右的发散角,远远优于其它截面柱透镜组（毫弧度数量级）的准直效果。同时两柱透镜间距取适当值,还可在远场形成圆形光宽。     

半导体激光器后腔面高反射涂层的研究

采用金属银(Ag)做为高反射镀膜材料,ZrO2介质膜作为与激光器端面的绝缘层和Ag保护膜,在激光器后腔面实现了高反射涂层。经激光器光电特性测试表明,该高反射膜系能使激光器输出功率提高60％,阈值电流减小20％～50％,并且具有良好的化学稳定性、热稳定性和机械强度,能有效保护半导体激光器后腔面。     

Correlation between 1/f noise and semiconductor laser degradation

The origin of the increase in residual spectral linewidth during device degradation is experimentally and theoretically clarified in a multiple quantum well (MQW) distributed feedback (DFB) laser. Non-radiative recombination current increases during device degradation and causes 1/f noise to increase. This current 1/f noise is the origin of the increase in the residual spectral linewidth. Through these degradation behaviours, a model showing a correlation between 1/f noise and the semiconductor laser degradation is proposed.     

Ion implantation and laser treatment of III–V compound semiconductor: A brief report

The progress in the area of applications of ion implantation and laser treatment to III–V semiconductors is reviewed. The achievements till todate are discussed alongwith the yet unresolved problems.     

941nm连续波高功率半导体激光器线阵列

用金属有机化合物气相淀积(MOCVD)技术生长了InGaAs／GaAs／AlGaAs分别限制应变单量子阱激光器材料。利用该材料制成了半导体激光器线阵列,连续波工作条件下的中心激射波长为940．5nm,输出功率高达37．7W(45A、2．0V),斜率效率可达0．99W／A(外微分量子效率为75％),最高转换效率超过45％,阈值电流密度为117A／cm^2,该波长的半导体激光器是Yb：YAG固体激光器的理想泵浦源。     

Experimental method based on wavelength-modulation spectroscopy for the characterization of semiconductor lasers under direct modulation

An experimental method is presented for characterization of the combined intensity and frequency modulation produced when the injection current of a laser diode is modulated. The reported technique is based on the analysis of the harmonic signals produced when a modulated laser is used to probe a gas absorption line by the so-called wavelength-modulation spectroscopy method. Based on a theoretical model of this technique, we present two methods that facilitate the determination of (i) the deviation in laser frequency and (ii) the phase shift between intensity and frequency modulation. These methods are illustrated experimentally by measurement of the modulation parameters of a 2-microm distributed-feedback laser by use of a CO2 absorption line. The experimental results have been compared with those obtained with another traditional method and have shown full agreement in the frequency range (400 Hz-30 kHz) considered.