Near threshold operation of semiconductor lasers and resonant-typelaser amplifiers

The characteristics of a semiconductor laser operating in the threshold region are investigated systematically. In this transition region from below to above threshold, the linewidth versus injection current characteristic is found to be nonmonotonic: a local minimum of linewidth just below threshold and a local maximum just above threshold are confirmed experimentally. If a semiconductor laser works below threshold as a resonant optical amplifier or optical filter, the small-signal frequency bandwidth is found to be equivalent to the spontaneous emission linewidth. When the laser amplifier is used simultaneously as a photodetector, the maximum value of photodetection sensitivity is achieved with the laser amplifier biased between 98% and 99% of the threshold current. The Fokker-Planck equation method is employed in the linewidth calculation. A numerical computer simulation is also performed using the rate-equation model. A reasonable agreement between theory and experiment is obtained     

Mode selection and stability of a semiconductor laser with weakoptical feedback

The authors present measurements that show that a semiconductor laser with weak optical feedback oscillates in the external-cavity mode with the narrowest linewidth rather than the mode with minimum threshold gain. A frequency fluctuation model that explains this narrowest linewidth operation is proposed. This model also correctly predicts mode-hopping frequencies. It is shown that the frequency span of the external-cavity modes determines the critical feedback level that causes coherence collapse     

千赫兹量级窄线宽半导体激光器研究进展

窄线宽半导体激光器由于其高单色性、低频率噪声、高可调谐性等优点,广泛应用于高速相干光通信、分布式传感、激光雷达等领域.随着高品质因子(Q)光学谐振腔、硅光异构集成芯片等技术的发展,窄线宽半导体激光器近十年经历了革命式发展,线宽压缩至千赫兹(kHz)量级,甚至到亚千赫兹量级.文章阐述了千赫兹量级窄线宽半导体激光器的最新进展,针对不同压缩线宽机制的窄线宽激光器进行了分类介绍,深入讨论了优化耦合系数、减少外腔损耗等对窄线宽激光器性能的影响,并针对未来应用需求展望了千赫兹量级窄线宽激光器在进一步压缩线宽、提升输出光功率方面的发展方向.     

Optical feedback phase stabilization of a semiconductor laser

The phase of optical feedback into a semiconductor laser was stabilized to maintain a minimum linewidth of 2 MHz. The scheme makes use of the linewidth information provided by a 1-MHz width external resonator. A further reduction in the laser linewidth to 300 kHz can be achieved and stabilized by using an external resonator with a narrower width.     

A Vertical-Cavity Surface-Emitting Laser at Threshold

Linewidth is measured over the threshold transition of a vertical-cavity surface-emitting laser and results are compared to a FokkerPlanck semiconductor laser model. The linewidth shows nonmonotonous behavior at threshold similar to that previously observed in distributed feedback lasers. The behavior agrees with the FokkerPlanck model if the linewidth enhancement factor $alpha approx 5.0$. Measurement of the relaxation oscillation frequency and the damping rate using an injected optical probe allows, together with the FokkerPlanck model, the extraction of major laser parameters.      

Spectral properties of a semiconductor laser coupled to a single mode fiber resonator

The effect of optical feedback from a single mode fiber resonator on the instantaneous frequency fluctuation spectrum and on the field fluctuation spectrum of an AlGaAs semiconductor laser is studied theoretically and experimentally. Single mode fiber resonators of different lengths and finesses are used for the experiments. The minimum linewidth conditions are given and it is shown that the linewidth reduction ratio increases with the fiber resonator selectivity. The limit for multistability operation depends on the resonator finesse. Theory and experiments are found to be in good agreement. Finally, the effect of resonator loaded semiconductor laser phase noise on BER performance in optical DPSK heterodyne systems is theoretically analyzed.     

Spectral linewidth rebroadening with short wavelength detuning in external cavity lasers

The first experimental observation of spectral linewidth rebroadening when an external cavity semiconductor laser is detuned well away from the laser gain peak on the shorter wavelength side has been made. This confirms a recent theoretical prediction of the variation of the effective linewidth enhancement factor with shortwave detuning and has significant implications for the design of narrow linewidth optical sources.<>     

Coherent optical interference effects in external-cavity semiconductor lasers

A broadening of the apparent linewidth of the semiconductor laser modes with external optical feedback is observed. This is shown to be due to the coherent nature of the feedback and multiple reflections in the external cavity. A theory for the steady-state behavior of the external-cavity semiconductor laser taking into account such coherent optical effects is developed. The inclusion of these effects is also important in the interpretation of the threshold data of such lasers.     

Spectral linewidth reduction in semiconductor lasers by an external cavity with weak optical feedback

The linewidth narrowing of a semiconductor laser due to weak optical feedback is analysed, taking into account both phase condition and threshold change for the feedback-induced modes. The achievable linewidth reduction lies in between two limiting cases, 1/(1 + X ?(1 + ?2))2 and 1/(1 + X)2, where ? and X are the linewidth enhancement factor and the feedback parameter, respectively.     

Emission frequency stability in single-mode-fibre optical feedback controlled semiconductor lasers

A newly designed semiconductor laser emitter with adjustable linewidth by controlling the coupling to a single-mode fibre is described with emphasis on thermal requirements to obtain a high-frequency stability. A stable operation has been achieved during more than 1 h using an AlGaAs laser diode with optical feedback from a 50 cm-long single-mode fibre resulting in a 50-fold reduction of the spectral linewidth.     

A tool to calculate the linewidth of complicated semiconductor lasers

A theory of the low-frequency phase fluctuations in the output of a semiconductor laser due to spontaneous emission is developed. The theory can be used as a tool to numerically calculate the linewidth of complicated laser structures, e.g., by use of the transfer matrix formulation. The results for the single Fabry-Perot laser are shown to be in exact agreement with the most accurate treatments published so far. Results are then presented for both DFB and F-P lasers with external optical feedback showing how the linewidth varies with the threshold gain, with the coupling coefficient, and with the external feedback conditions.     

Measurement of carrier lifetime and linewidth enhancement factor for 1.5- mu m ridge-waveguide laser amplifier

Semiconductor optical amplifiers are used for investigation of the effective carrier lifetime and the linewidth enhancement factor. Contrary to semiconductor lasers, semiconductor optical amplifiers allow measurement at high levels of injected carrier density. The carrier lifetime and the linewidth enhancement factor are measured with a simple dynamic self-heterodyne method. Carrier lifetimes of 750 ps at the threshold current for the SOA without antireflection coating and 200 ps at high injection have been found. The linewidth enhancement factor is measured to be between 4 and 17 which fits with a simple empirical expression.<>     

基于半导体激光器窄线宽光子微波信号获取

为了研究光注入半导体激光器(SL)产生的光子微波信号的性能, 基于SL的速率方程和光纤布喇格光栅(FBG)滤波理论, 采用数值仿真的方法进行了理论分析, 得到了各种注入参量下的光谱、功率谱和线宽, 并讨论了反馈参量对微波线宽的影响, 考虑到光注入下产生的微波线宽较宽, 进一步引入FBG光反馈窄化了微波信号的线宽。结果表明, 当SL仅在光注入作用下时, 通过改变注入参量, 可实现微波频率连续可调谐和微波强度最大化; 微波线宽随着反馈强度的增加逐渐变窄, 通过适当调节反馈参量可将微波线宽压缩到10kHz以下。该研究结果可为半导体激光器在光生微波中的应用提供一定的理论参考。     

窄线宽半导体激光器件

分布反馈半导体激光器的线宽一般较大,难以满足光纤传感等领域的要求。根据C.H. Henry于1982年提出的半导体激光器的线宽理论,通过适当设计DFB半导体激光器的腔长、耦合系数、微分增益、光限制因子,能有效地减小激光器的线宽。同时,空间烧孔现象也可限制DFB半导体激光器的线宽,为此需要合理设计光栅结构。在此基础上,DFB激光器的线宽能达到几十千赫兹的量级。此外,采用DBR结构或者外腔结构,也可以获得相当窄的线宽。     

半导体激光器的电噪声

通过测量激光器的电噪声,可以用来在线监测器件的诸多特性,如阈值电流的大小,是否有模式跳变发生,以及谱线宽窄等.另外,根据电噪声的大小,还可以对器件的质量和可靠性作出评价,具有快速、方便、无损等优点.文章概述了半导体激光器的电噪声,对其主要应用进行了综合和讨论,概括性评述了该领域目前的研究进展.     

Dual-Wavelength InAlGaAs–InP Laterally Coupled Distributed Feedback Laser

The design and operation of a novel dual-wavelength laterally coupled (LC) distributed feedback (DFB) semiconductor laser are reported. The laser has two different grating periods, one on each sidewall. Stable dual-mode emission, with low threshold currents and good extinction ratio approaching 30 dB, has been demonstrated. Generation of millimeter-wave signals by photomixing has been performed, and the linewidths of each of the individual modes and of the beat mode have been measured as a function of the injection current. It is observed that while the linewidth of the separate modes has a modest dependence on the injection level, the linewidth of the beat mode experiences a strong decrease as the injected current increases     

Gain saturation and the linewidth enhancement factor insemiconductor lasers

There is an asymmetry in the optical spectrum of the semiconductor laser under weak current modulation. It arises because the linewidth enhancement factor that describes the proportionality between the real and imaginary parts of the optical susceptibility due to the differential gain is not appropriate for the gain saturation. This asymmetry can be used to determine the proportionality for the gain saturation term, and to measure the value of the linewidth enhancement factor, a parameter that has been difficult to accurately measure. Data for an oxide-confined vertical-cavity surface-emitting laser is presented that shows a gain saturation term with a different, but nonnegligible, proportionality     

Bistability in grating-tuned external-cavity semiconductor lasers

Bistability has been observed in the tuning characteristic and power versus current relation of a 1.3 μm grating-tuned external-cavity semiconductor laser. Tuning-direction reversal, current variations, and feedback interruption can change the output power and threshold current at a given wavelength. These effects are shown theoretically to be due to the coupling of the semiconductor gain and index of refraction. From measurements of the semiconductor chip facet reflectivity, solitary laser diode mode spectrum, and tuning curve in the presence of external feedback, the analysis yields values for the external feedback strength, semiconductor modal loss, and linewidth enhancement factor. Using an InGaAsP double-channel-planar-buried-heterostructure laser diode with a 4 percent reflectivity antireflection-coated facet inside an external cavity consisting of a 0.60 numerical aperture lens and a 1200 line/mm diffraction grating, we found 22 percent external feedback, 60 cm^-1modal loss, and a linewidth enhancement factoralpha = -7.1.