Polarization-dependent nonlinear gain in semiconductor lasers

We have numerically studied the nonlinear gain coefficients in terms of spectral hole burning for the optical fields in parallel and orthogonal polarizations in semiconductor lasers by solving the equation of motion for the density matrix in perturbation series. The electronic band structures and the transition matrix elements used in the calculations are obtained by diagonalizing Luttinger's Hamiltonian. In the present analysis for InGaAsP lasers, the cross-saturation coefficient for the parallel polarizations is twice as large as the self-saturation. Also, the cross-saturation coefficient for the orthogonal polarizations, which affects the polarization switching and polarization bistable operations of the laser, rests between the two. The relative magnitude of self-saturation coefficients and cross-saturation coefficients for orthogonal polarizations satisfies the condition for polarization bistable operations. We also discuss the effect of carrier heating on gain saturation coefficients     

Effective nonlinear gain in semiconductor lasers

An effective nonlinear gain is introduced for semiconductor lasers by taking into account the effect of laser structure and the associated distribution of the mode intensity along the cavity length. It should be used in the analysis of laser dynamics and noise in place of the material nonlinear gain parameter. A general expression for the effective nonlinear gain is given by using the Green's function method. The results obtained for Fabry-Perot and distributed feedback lasers show that the effective nonlinear gain could be considerably enhanced. The exact value of the enhancement factor depends on cavity parameters. Affected by the laser structure, the nonlinear gain has a different power dependence than expected from material considerations alone     

外腔半导体激光器单频输出的长期稳定

一、引言 由于光纤通讯、光纤传感、检测二光谱分析、频标、光陀螺等领域的需要；单频半导体激光器的研究近年来受到广泛重视并得到迅速发展，由于采用了长外腔选模技术，已实现了线宽小于100 kHz的单频输出~[1-3]。而频率长期稳定工作则刚刚开始~[4]。通常认为，造成半导体激光器输出频率漂移及跳模的原因是管芯温度和注入电流的波动，因此稳频工作     

Effect of nonlinear gain on single-frequency behaviour of semiconductor lasers

The effect of nonlinear gain on the single-frequency behaviour of semiconductor lasers is analysed using a two-mode rate-equation model. We find that the asymmetric nature of the nonlinear gain is responsible for bringing the sidemode above threshold when the power in the main mode exceeds a critical level. We obtain an analytic expression for the critical current density at which the sidemode reaches threshold and apply it to discuss the single-frequency range of distributed-feedback semiconductor lasers.     

Effect of nonlinear gain on the bandwidth of semiconductor lasers

The effect of nonlinear gain on the frequency response of semiconductor lasers at high bias powers is analysed using the multimode rate equations. In spite of the strong damping of the relaxation oscillations due to nonlinear gain, bandwidths of over 40 GHz appear to be attainable in semiconductor lasers.     

单模半导体激光器的相位噪声

一、前言 对半导体激光器的内在噪音如散粒噪音、模式噪音等已经作过很多研究。近年来随着采用大接收灵敏度检测器、使用光放大器和具有紧凑空间通道的多路复用技术以及自差、外差式光纤通讯系统等的进展,光纤通讯系统中使用的半导体激光器的内在相位噪音已成为上述技术的基本限制,因而对单模半导体激光器的相位噪音和光谱线型的理论研究和实验观测已为众争学者注目。 通常的激光理论并没有正确地讨论在室温下工作的单模半导体激光器的特性,首先对注入式半导体激光器线宽与光功率关系进行仔细测量后,发现在300K时,带宽⊿f比用通常激光理论计算的半导体激光器的带宽大50倍,但他们没有解释这种异常增宽的原因。     

Transverse and longitudinal mode control in semiconductor injection lasers

Mechanisms which determine the oscillating transverse and longitudinal modes in semiconductor injection lasers are discussed in this paper. The analysis is based on the semiclassical method in which the optical field is represented by Maxwell equations and the lasing phenomenon is analyzed quantum mechanically using the density matrix formalism. Guided modes are classified by the relation between refractive index and gain-loss differences at the boundaries of the active region as normal guided mode (index guiding), active-guided mode (gain guiding), and leaky mode (anti-index guiding). The guiding loss and cutoff conditions are given for these modes. The optimum range to obtain stable fundamental transverse mode operation is discussed with respect to several guiding factors, such as width of active region, the refractive index difference, and gain-loss differences at the boundaries of the active region. Longitudinal mode behavior is discussed in terms of electron transition mechanism in semiconductor crystals. The relaxation effect of the electron wave is introduced in this model. Profiles of the saturated gain and the spatial diffusion of the electron are related to this relaxation effect. Mode competition phenomena are analyzed, and a strong gain suppression among the longitudinal modes is shown to be as an intrinsic property of semiconductor lasers. The possibility of obtaining single longitudinal mode operation is postulated. Physical influences for stable single longitudinal mode operation are discussed in terms of transverse mode control (or stripe structure), spontaneous emission, threshold current level, impurity concentration in the active region, and direct modulation. Some experimental results are also given to support these analyses.     

Asymmetric gain induced by longitudinal spatial carrier burning in semiconductor lasers

Nonlinear gain caused by dielectric corrugation resulting from the cavity standing wave of a lasing mode in semiconductor lasers is investigated using the perturbation approach. The results show that the nonlinear gain spectrum is asymmetric when the linewidth enhancement factor alpha not=0, and the possibility of single mode operation is greater at alpha =0.<>     

Lateral and longitudinal mode discrimination in index-guidedcircular ring semiconductor lasers

We present the first analysis of the mode structure and thresholds of index-guided ring lasers, and on longitudinal mode discrimination of X-junction-coupled ring lasers. Submilliampere thresholds can be achieved with single-lateral-mode ring lasers having small refractive index steps. In addition, strong longitudinal mode discrimination can be obtained with ring lasers of different cavity lengths coupled via an X junction     

Effects of nonlinear gain on mode-hopping in semiconductor laserdiodes

A systematic and comprehensive analysis of longitudinal mode-hopping, due to nonlinear gain, and its influence on the design criteria of transverse-mode-controlled semiconductor laser diodes are presented. An existing nonlinear model, which was derived using a density matrix formalism, has been extended in this paper to generate the nonlinear gain coefficient matrix. Properties of the nonlinear gain coefficient matrix, which describes the interaction among cavity modes, are discussed. Using the new nonlinear gain in the steady-state multimode rate equations, conventional Fabry-Perot (FP) and short cavity Fabry-Perot (SFP) semiconductor laser diodes have been numerically simulated. Design issues such as cavity length, cavity volume, facet reflectivity, spontaneous emission factor, mode wavelength, intraband relaxation time, linewidth enhancement factor, and laser structure are also discussed. It is shown that increasing the injection current causes the lasing mode to jump to longer wavelengths. Furthermore, increasing the spontaneous emission factor reduces the dynamic range of laser operation without mode-hopping, and vice versa for short cavity. It has been also shown that the carrier density in the active region shifts to higher values (i.e., experiences a kink) at the onset of mode-hopping. Finally, the total modal gain (linear and nonlinear) competes as the injection current increases     

Theoretical analysis of longitudinal mode coupling in external cavity semiconductor lasers

The single longitudinal mode behavior in long external cavity semiconductor lasers is discussed. Experimentally, the laser exhibits a single frequency oscillation even for an external cavity length of 100 cm. The mode selectivity of a composite cavity is shown to be insufficient to explain the experiments. Longitudinal mode coupling in semiconductor lasers is found to arise from an interference effect between the modes on the interband transition probability of electrons. Mode coupling equations are derived, which indicates that the single mode oscillation in long external cavity semiconductor lasers is brought about when the coupling strength goes beyond a critical value. It is shown that the effect of the hole burning in the external cavity semiconductor lasers is similar to that in the solitary laser.     

Effect of the nonlinear gain in semiconductor lasers on theperformance of multigigabit-per-second lightwave systems

The implications on performance of the functional form of the nonlinear gain in single-mode semiconductor lasers are studied for multigigabit-per-second, intensity-modulation, direct-detection lightwave systems. Compared to a previously used functional dependence on the photon and carrier densities, a new result due to G. Agrawal (1988) can, depending on the fiber dispersion and gain compression factor, yield quite different receiver sensitivities     

The influence of mode partition fluctuations in nearly single longitudinal mode lasers

A simple relationship between the mode ratio of a two-mode laser and an empirical mode partition coefficient is derived. The effects of the mode ratio of a two-mode laser on the bit-error-rate floor of an optical fiber system are calculated, and the power penalties due to mode partitioning in the presence of Gaussian receiver noise are evaluated. The analysis provides a simple way to calculate the effects of mode ratio on the sensitivity of optical receivers.     

Noise reduction in single longitudinal mode lasers by high-reflectivity coatings

Feedback-induced noise in single longitudinal mode semiconductor lasers was suppressed over a wide range of the amount of the optical feedback light by high-reflectivity facet coatings. A relative intensity noise value was as low as -143 dB/Hz at 3 mW output power under 1 percent of optical feedback in the GaAlAs lasers with high-reflectivity (0.75) facets.     

基于无源腔多波长单模布里渊光纤激光器

为了实现一种单模多波长布里渊光纤激光器,采用了将掺铒光纤放大器放置于1个10m左右的无源光纤振荡腔外,各阶布里渊激光通过耦合器可反馈回送到掺铒光纤放大器中再次放大,作为下一阶布里渊激光的抽运光来产生多阶布里渊激光的方法,并进行了理论分析和实验验证。结果表明,在现有掺铒光纤放大器输出功率的水平,在这种多波长单模布里渊光纤激光器中观察到了7个布里渊激光波长。预计其在光生微波、密集波分复用系统、光学惯性旋转传感和超高精度光谱分析等领域将会有着极其广泛的应用。     

Comparison of the effects of nonlinear gain and weak opticalfeedback on the dynamics of semiconductor lasers

The influence of nonlinear gain and optical feedback on the dynamics of single-mode semiconductor lasers are numerically investigated based on the Lang and Kobayashi model. It is well known that the nonlinear gain tends to stabilize the dynamics, while the optical feedback tends to increase the instabilities. In this paper, we study the behavior of the attractors when the feedback level k and the gain saturation coefficient ε vary and show that the effects of these parameters are surprisingly opposite. For example, we find that the route to chaos that the external cavity modes follow for increasing k is reversed for increasing ε in an almost identical manner. When the feedback increases the modes follow the usual quasi-periodic route and turn into torus. If k continues to increase, the torus become chaotic attractors as the result of several period-doubling bifurcations or a third Hopf bifurcation. Further increase of k causes the chaotic attractors to lose stability, Contrarily, if the value of the parameter ε is increased, the attractors recover their stability and reverse the route becoming simple torus again. If ε is increased further, the torus reverse the quasi-periodic route and turn into stable modes again. We also find that on the contrary to k, the parameter ε enhances the stability region of an attractor. We show that the feedback level above which a limit cycle emerges from a stable mode, the feedback level above which a torus emerges from a limit cycle, and the feedback level above which a chaotic attractor loses stability are all increasing functions of ε     

Lateral mode dynamics of semiconductor lasers

The interaction between two lateral modes in a waveguide semiconductor laser is studied. The effects of carrier diffusion are included into the governing rate equations. A linear stability analysis is used to identify regions of frequency locking between the lateral modes. Numerical integration shows the effects of the diffusion length and other system parameters on the lateral mode dynamics. It is shown that variations in the carrier diffusion length give rise to regimes of single-mode dominance, oscillation, chaos, and frequency locking. Direct experimental observations of higher order lateral modes in lasers exhibiting beam instabilities are presented. Observations of bistable beam steering with hysteresis are presented and explained     

半导体激光器的输出模谱

利用半导体激光器多模速率方程组的隐式解析解，考虑到实际激光器的峰值增益波长与峰值谐振波长往往不一致的情况，从理论上对不同电流下半导体激光器的输出模谱进行了研究。