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     

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     

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。     

Intensity dependence of the linewidth enhancement factor and itsimplications for semiconductor lasers

The linewidth enhancement factor is shown to become intensity dependent when the intraband relaxation effects responsible for nonlinear gain and index changes are incorporated in the theory of semiconductor lasers. The intensity dependence of the linewidth enhancement factor influences many laser characteristics such as the frequency chirp, the modulation response, the injection-locking range, and the phase noise. In particular, it leads to a power-independent contribution to the laser linewidth. Furthermore, for semiconductor lasers detuned to operate away from the gain peak, the nonlinear index changes can even lead to a rebroadening of the laser linewidth at high-output powers     

Spontaneous emission model of surface-emitting DFB semiconductorlasers

A general spontaneous emission model is developed for surface-emitting (SE) distributed feedback (DFB) semiconductor lasers. The frequency distribution of spontaneous emission noise below lasing threshold and the spontaneous emission rate in lasing operation are formulated by using a transfer matrix method combined with the Green's function method. The effective linewidth enhancement factor is obtained from this model in terms of the elements of the transfer matrix. By way of example, the author applies the formulation to a standard SE DFB laser, and a SE λ/4-shifted DFB laser with a distributed Bragg reflector (DBR) mirror. In particular, the author analyzes the below-threshold spectrum, the threshold current density, the differential quantum efficiency, and the spectral linewidth of these lasers     

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.     

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。     

Dependence of the linewidth of a semiconductor laser on the modedistribution

A theory and measurements that show the dependence of the frequency noise spectrum and, therefore, of the semiconductor laser linewidth on the mode distribution are presented. The theory is based on rate equations where the linewidth is strongly influenced by the gain saturation coefficients (nonlinear gain). Analytical results are restricted to two modes. Measurements show that the linewidth change of the main mode depends on the wavelength side of the side mode (related to the wavelength mode of the main mode) and the effect decreases with increasing mode separation. Furthermore, the theoretical and measured frequency noise spectra of a three-mode laser are depicted, showing a characteristic resonance peak for certain mode distributions. This property of the frequency noise spectrum is also described by the theory     

外腔半导体激光器的场功率谱和线宽

本文采用低频近似的方法,用半经典理论,给出了外腔半导体激光器的场功率谱和线宽公式,理论分析与实验结果是吻合的。     

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.     

Linewidth of a semiconductor laser operating near threshold

The linewidth of a single-mode semiconductor laser operating in the threshold region has been studied both theoretically and experimentally. Due to strong phase-amplitude coupling in a semiconductor laser, its linewidth versus current characteristics exhibit a local minimum below threshold and a local maximum just about threshold. This implies a limitation in the minimum optical bandwidth achievable in a resonant-type semiconductor laser optical amplifier. The theoretical prediction has been verified experimentally on a conventional DFB semiconductor laser     

Controling the linewidth of a semiconductor laser withphotorefractive phase conjugate feedback

A novel technique for narrowing the linewidth of a single laser diode using photorefractive phase conjugate feedback is presented. The behavior of this system can be modeled by adapting the previously developed theory to include the phase constraints of the phase conjugate external cavity. The experiments show excellent agreement with the model. This technique produces a stable, narrow linewidth in a semiconductor laser. The comparison to the conventional case is discussed     

高稳定性单纵模布里渊光纤激光器的实验研究

本文提出了一种用可调谐半导体激光器（TLS）泵浦的高稳定性单纵模布里渊光纤激光器（SLMBFL）。该激光器利用Brillouin泵浦光（TLS激光）只在环形器内运行一次,这样Brillouin泵浦光就不会在谐振腔内共振,确保了激光器谐振腔的高稳定性从而使Brillouin激光在该谐振腔内形成共振而输出。实验发现当Brillouin增益介质光纤的长度缩短到13．4m时谐振腔的自由光谱范围变为15．2MHz,正好与光纤的Bfillouin增益带宽接近,此时SLMBFL开始表现为单纵模振荡。实验结果表明,该光纤激光器的功率转化效率为42．5％,最大输出功率为267．6roW。单纵模激光输出的信噪比与其泵浦激光的信噪比提高近10dB。SLMBFL对TLS的高频噪声有很好的抑制作用,其输出线宽小于5KHz。     

Semiconductor laser lineshape and parameter determination from fringe visibility measurements

The field autocorrelation function of single-mode 1.3 ?m InGaAsP lasers has been measured using a Michelson interferometer. Comparison with theory for semiconductor laser line broadening yields the relaxation oscillation frequency, its damping rate and the linewidth broadening factor. A comparison with other techniques for lineshape measurement is presented.     

准直透镜失调对Littman-Metcalf光栅外腔激光器线宽的影响

以光栅外腔半导体激光器的理论知识为基础,对Littman-Metcalf型外腔半导体激光器的工作原理进行了说明,并详细地讨论了外腔半导体激光器的线宽压窄以及模式选择机制,采用严格的耦合理论和光线变换矩阵推导了系统结构参数对光场耦合效率影响的计算公式。同时,对影响Littman-Metcalf外腔激光器输出激光线宽的几个重要因素进行了分析,重点讨论了系统中准直透镜位置失调导致的线宽变化规律。计算结果表明:合理地控制Littman-Metcalf光栅外腔半导体激光器的外腔参数可以将中心波长为785 nm半导体激光器的本征线宽压窄四个数量级,该外腔系统中准直透镜位置失调会影响系统出射光场与经外腔反馈光场之间的耦合效率,进而影响光栅外腔半导体激光器的输出线宽。     

Simultaneous measurement of the linewidth, linewidth enhancementfactor α, and FM and AM response of a semiconductor laser

Using a commercially available computer controlled spectrum analyser with tracking generator, optical input section, and optical delay line it is possible to measure the linewidth, linewidth enhancement factor α, and FM and AM response of a semiconductor laser in one process. The determination of the linewidth yields also information about the frequency noise density and the determination of a delivers information about the nonlinear gain. Assuming an optical input power of 0 dBm, a laser linewidth <50 MHz and a modulation response of the laser without cut-off, AM indices m>0.01% and FM deviations of about >10 MHz up to 20 GHz can be detected     

Laser induced line narrowing and saturation broadening effects in OPFIRL

The variation of the laser linewidth within an optically pumped NH3 FIR laser has been studied theoretically on the basis of three-level quantum system approach. It has been shown that the activated medium within the sample tube of an optically pumped ASE FIR laser can be divided into three regions: A). Laser induced line narrowing region, in which the FIR laser linewidth is less than Lorentz linewidth; B). Saturation broadening region, in which the power density of the FIR. signal becomes large enough to make saturation effect important and the linewidth broadens; C), Self-absorption narrowing region, in which the pumping energy density is below the threshold, so self-absorption dominates and the linewidth decreases. The necessary linewidth of the OPFIRL can be obtained by appropriate selection of the length of the sample tube.     

Excess phase noise in self-heterodyne detection

The interpretation of self-heterodyne spectra is difficult if, apart from spontaneous emission, additional noise sources are presented. Measurements on an external-cavity semiconductor laser show how, for a relatively long delay, the high-frequency (Lorentzian) wings of the self-heterodyne spectrum are a sensitive measure for the quantum-limited (Schawlow-Townes) laser linewidth. The quantum-limited laser linewidth is shown to be inversely proportional to the output power. Values below 5 kHz are routinely measured. The full width at half maximum (FWHM) laser linewidth is larger than this due to excess low-frequency fluctuations, which are shown to result from the presence of side modes     

Transition from the lowest linewidth mode operation to coherencecollapse in a semiconductor laser with feedback from a distant reflector

The influences of optical feedback from a distant reflector on single-mode semiconductor laser operation are analyzed theoretically. The stable operation in the lowest linewidth mode and the feedback parameter for the onset of coherence collapse are systematically derived. In the feedback regime for the lowest linewidth mode operation, the laser principally operates at the emission frequency of the solitary laser at the beginning of the operation with the feedback. It is shown exactly by asymptotic analysis of the equation for the field phase in the semiconductor cavity considering the time-lag of the feedback that the laser comes to operate in the lowest linewidth mode. In the lowest linewidth mode operation, the laser is stably phase-locked to the feedback. In the semiconductor cavity, the field amplitude and the field phase oscillate in almost the same phase in the fluctuation modes related to the relaxation oscillations, In the lowest linewidth mode operation, the phase oscillations enhance the amplitude oscillations through the feedback-action's dependence upon the phase difference between the field in the semiconductor cavity and the feedback. Thus, the damping of the relaxation oscillations decreases with further increased feedback and the transition to the coherence collapse occurs