Analysis of polarization bistability of phase-shifted DFB laser dueto TM light injection

As a method of high-bit-rate optical processing, the TM light injection-induced optical polarization bistability using Fabry-Perot type laser diodes has been experimentally demonstrated and theoretically analyzed. In these studies, although switching times less than 1 ns both for switch-up and switch-down have been reported, both of the TE and TM outputs exhibit longitudinal multimode spectra. In practical processing systems, a dynamically stable operation of the polarization switching is considered to be essential. It is the purpose of this paper to present an analytical study of the TM light injection-induced polarization bistability using a λ/4-shifted distributed feedback (DFB) laser model and to describe the detailed picture of the mechanism     

Dynamic properties of transverse-magnetic wave injectedsemiconductor lasers

Dynamic responses of optical polarization bistability and self-sustained pulsation in transverse-magnetic (TM) wave injected semiconductor lasers are analyzed. Due to the TM wave injection, laser amplification of TM wave and laser oscillation in the transverse-electric (TE) mode simultaneously occurs in the laser. The calculated dynamic responses of optical polarization bistability are compared with measured responses. The upper limit of the switching time and the repetition frequency are clarified from calculations. The self-sustained pulsation triggered by a change of TM input power is found in the course of the calculation. The conditions needed for the pulsation are discussed     

Pitchfork bifurcation polarization bistability in laser diodes withexternal cavities

A new pitchfork bifurcation polarization bistability in laser diodes with a two-armed polarization-selective external cavity is examined theoretically and experimentally. The rate-equation analysis taking into account gain saturation has been carried out using the model for a laser diode whose laser thresholds of the TE and the TM modes can be changed independently through the changing of their photon lifetimes. It is shown from the analysis that pitchfork bifurcation polarization bistability appears when the laser thresholds for both modes are very close to each other and the switching between two stable states is obtained by means of a photon lifetime changing or a coherent or incoherent light injection. In the experiment, a 1.3-μm InGaAsP laser diodes with a T-shaped polarization-selective external cavity mas used. A new type of polarization bistability was observed when one arm was blocked and then recoupled. The two stable states mere also switched by means of an optical injection. All-optical flip-flop operation has been successfully demonstrated by using both TE and TM trigger inputs     

The optical gain coupling saturation effect on the linewidthproperty of complex coupled DFB lasers

The cavity length dependence of the linewidth property of complex coupled (CC) DFB lasers is investigated. In CC-DFB lasers with a fixed geometrical structure of the active region, the cavity length strongly affects the ratio of index coupling strength to the gain coupling one, which results in the variation of the effective linewidth enhancement factor α_eff. It is also found that pure gain coupled (GC) DFB lasers have the saturation characteristic in the linewidth power product, with the extension of the cavity length, These results indicate the importance of the selection of the cavity length in CC-DFB lasers     

Tunneling injection quantum-dot lasers with polarization-dependent photon-mediated carrier redistribution and gain narrowing

A theoretical and experimental study of a particular transverse-electric (TE) mode lasing mechanism of a tunneling injection InP quantum-dot (QD) laser is reported. In the experiment, the TE mode lasing action takes place at the first excited state of InP biaxially compressively strained QDs. This QD state is coupled to the ground state of two tensile-strained InGaP quantum wells (QWs) although the tensile-strained QW structure favors the transverse-magnetic (TM) polarization light emission. The measured TE and TM modal gain spectra show a typical QW gain evolution behavior at low injection currents, which can be theoretically modeled by the quasi-equilibrium of carrier distribution. When the injection current is increased near threshold, a TE gain narrowing and a simultaneous TM gain pinning are observed in the measured modal gain spectra, which cannot be explained via the quasi-equilibrium model. We propose a polarization-dependent photon-mediated carrier redistribution in the QD-coupled-QW structure to explain this TE and TM gain evolution behavior. When the injection current is just below threshold, the strong carrier depletion via stimulated emission due to coupling between the InP QD and InGaP QW states plays an important role in carrier redistribution, which depends on the optical transition energy and polarization. This concept of the polarization-dependent photon-mediated carrier redistribution explains the TE gain narrowing and TM gain pinning behavior. In addition, a coupled rate equation model is established, and the calculated polarization power ratio based on the coupled rate equations explains the experimental observation.     

Relaxation oscillation frequency of DFB lasers with gain coupling

Using the spatially-dependent rate equations based on the Green's function analysis, we investigate the dependency of the relaxation oscillation frequency on the complex coupling coefficient and other parameters of gain-coupled DFB lasers by simultaneously considering spatial-hole-burning, gain saturation and gain compression. An explicit expression for the relaxation oscillation frequency for DFB lasers including the longitudinal spatial effects has been obtained. It is found that antiphase gain-coupling significantly enhances the local effective differential gain in the gain-coupled DFB laser and hence increases the relaxation oscillation frequency. We have also shown for the first time that the modal linewidth enhancement factor α_M plays an important role in determining the relaxation oscillation frequency of gain-coupled DFB lasers, especially when the built-in index coupling is weak     

Optical polarization bistability in TM wave injected semiconductorlasers

Experimental results of optical polarization bistability in InGaAsP lasers are reported and the causes of polarization switching and nonlinear response in this bistability are discussed. The input light signal consisted of the transverse magnetic (TM) wave, while the semiconductor lasers operated in a fundamental transverse electric (TE) mode when the light input was not injected. The light output versus light input characteristics were dependent on the input wavelength. Switching times of up to a few hundred picoseconds were achieved for both switch-up and switch-down. It was found that the TE and TM outputs originate from the TE oscillation and the TM amplification, respectively. Calculations using the rate equations showed that the TM input induces the nonlinearity of the carrier density. These results indicate that this bistability is a type of dispersive bistability     

Fabrication approach for antiphase narrow linewidth complex coupled 1.55 /spl mu/m DFB lasers

A new method for fabricating narrow linewidth antiphase complex coupled MQW DFB lasers by periodically etching the active layer and quarternary InGaAsP overgrowth is reported. The minimum linewidth for a 375 /spl mu/m long ridge waveguide laser is only 250 kHz at an optical output power of 4 mW.     

分布反馈光纤激光器模式特性分析

根据耦合波理论,在分析分布反馈(DFB)光纤激光器纵模特性的基础上,着重阐述其偏振特性。另外给出了偏振态同耦合系数和双偏振态相移量差的关系。理论分析结果表明,当分布反馈光纤激光器输出为0阶模时,输出激光的偏振状态由耦合系数和双偏振态的相移量差共同决定,即在耦合系数一定的情况下,通过增加双偏振态相移量的差,或在双偏振态相移量差一定的情况下,通过减小耦合系数,可以实现单偏振输出。实验中在经载氢处理的掺铒光纤上制作分布反馈光纤激光器,由于耦合系数较大和双折射效应过小,输出为双偏振态。     

Uncooled 1.3-/spl mu/m complex-coupled DFB BH laser diodes with the Fe-doped InGaAsP-InP hybrid current-blocking grating

In this letter, we proposed an alternate method by using the Fe-doped InGaAsP-InP hybrid grating layers to fabricate the 1.3-/spl mu/m current-blocking-grating complex-coupled distributed-feedback (CBG CC-DFB) laser diodes (LDs) grown by metal-organic chemical vapor deposition (MOCVD). By combining the Fe-doped InGaAsP-InP grating layers, the CBG CC-DFB LDs can provide high optical DFB coupling coefficient and high current confining ability. Moreover, the current aperture in the lateral direction can be easily controlled by the self-aligned MOCVD regrowth process. Therefore, the manufacture of CBG CC-DFB buried heterostructure LDs is easy as the ridge-waveguide LDs. The LDs exhibit a low threshold current of 5.3 mA, a high slope efficiency of 0.42 mW/mA, and a stable single mode with a high sidemode suppression ratio of /spl sim/42 dB at two times the threshold (10.5 mA). Even at high temperatures, these LDs still have an extremely low threshold current of 15.8 mA at 90/spl deg/ and a small variation in slope efficient of only -1 dB at the temperatures between 20/spl deg/ and 80/spl deg/. Furthermore, these LDs show a high-speed characteristic of more than 11.8 GHz at 20/spl deg/, which are suitable for 10-Gb/s Ethernet and OC-192 applications.     

Bistability and optical switching in a polarization-bistable laserdiode

Dependences of the polarization-bistable condition on the coefficients of linear and nonlinear gains of a laser diode are analyzed, including S-shaped and pitchfork types of bistability, in the optical output versus injection current characteristics. Moreover, the steady-state and dynamic characteristics of the optical output for optical input are investigated, taking account of a detuning of incident optical beam from the cavity resonant frequency of a polarization-bistable laser diode, TE-TM polarization switching is shown to be attained in the optical output of the polarization-bistable laser diode in a very fast switching time less than a nanosecond by the application of TE and TM optical input pulses     

A ridge waveguide DFB laser model including transverse carrier andoptical effects

In this paper, a steady-state model for ridge waveguide DFB lasers is presented. The complex two-dimensional semivectorial optical mode in the transverse direction is solved using a finite difference scheme without introducing any approximations. The electron and hole diffusion in the lateral direction is also considered, Along the longitudinal direction, a novel “Superposition of Spectral Power Method” is used, based on the coupled mode formulation. This model enables one to examine the interaction between the optical and carrier profiles for different injection levels and arbitrary transverse index profiles. As such, it is useful for studying CW characteristics such as lasing wavelength and threshold current. Finally, the results from this model are compared with experimental data from a varying ridge width laser array fabricated from a 1.55 μm InGaAsP-InP compressively strained multiquantum-well loss-coupled DFB structure     

Influence of standing waves on DFB lasers including saturableabsorptive gratings

The influence of standing waves on the performance of distributed feedback (DFB) lasers is investigated, whereby local changes of the wavenumber as induced by gain compression or inhomogeneous saturation of an absorptive grating are incorporated into the coupled wave equations. With the help of this model, it is shown that gain compression ultimatively limits the maximum output power for monomode operation. As compared to in-phase complex-coupled lasers, it is shown that the worse monomode capability of antiphase complex-coupled lasers is related to the better stability properties of these lasers. Additionally, a characteristic interaction between the saturable absorptive grating and the optical field is found, which furthermore reduces the maximum monomode output power of antiphase complex-coupled lasers with saturable absorptive gratings     

Digital optical signal processing with polarization-bistable semiconductor lasers

The operations of a complete set of optical AND, NAND, OR, and NOR gates and clocked opticalS-R, D, J-K,andTflip-flops are demonstrated, based on direct polarization switching and polarization bistability, which we have recently observed in InGaAsP/InP semiconductor lasers. By operating the laser in the direct-polarization-switchable mode, the output of the laser can be directly switched between the TM00and TE00modes with high extinction ratios by changing the injection-current level, and optical logic gates are constructed with two optoelectronic switches or photodetectors. In the polarization-bistable mode, the laser exhibits controllable hysteresis loops in the polarization-resolved power versus current characteristics. When the laser is biased in the middle of hysteresis loop, the light output can be switched between the two polarization states by injection of short electrical or optical pulses, and clocked optical flip-flops are constructed with a few optoelectronic switches and/or photodetectors. The 1 and 0 states of these devices are defined through polarization changes of the laser and direct complement functions are obtainable from the TE and TM output signals from the same laser. Switching of the polarization-bistable lasers with fast-rising current pulses has an instrument-limited mode-switching time on the order of 1 ns. With fast optoelectronic switches and/or fast photodetectors, the overall switching speed of the logic gates and flip-flops is limited by the polarization-bistable laser to < 1 ns. We have demonstrated the operations of these devices using optical signals generated by semiconductor lasers. The proposed schemes of our devices are compatible with monolithic integration based on current fabrication technology and are applicable to other types of bistable semiconductor lasers.     

基于两次衰减全反射结构的激光偏振控制器

为了实现激光偏振态的控制和提高两偏振态消光比,用由各向异性材料加工而成的耦合棱镜并采用两次衰减全反射方法,使具有相同入射角的TE和TM偏振态的共振激发条件不同,即当TE偏振态满足共振激发条件能量耦合进波导时,TM偏振态却不满足共振激发条件而被反射来实现其在空间上的分离.同时将电光系数较大、光学性能和热稳定性好的聚合物材料作为波导层,利用其电光效应来改变共振条件来实现输出光束偏振态的调谐.结果表明,反射型结构插入损耗小于0.2dB,器件的消光比高达27dB,驱动电压为19V.该方案预计在光信息存储、光路由、光开光以及图像处理等光学系统里有着极其广泛的应用.     

Self-Pulsating Amplified Feedback Laser Based on a Loss-Coupled DFB Laser

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.     

Enhanced AM and FM modulation response of complex coupled DFBlasers

Significant improvements in the small AM bandwidth and FM efficiency of a DFB laser are obtained by introducing a carrier dependent complex coupling coefficient. A simulation using a large-signal dynamic model shows that the 3-dB cutoff frequencies for AM can be increased by factors of three by having the index and gain coefficients in antiphase. In contrast, the efficiency of FM, at low frequencies, can be similarly improved if the index and gain coupling are in phase     

Enhanced wavelength tuning range in two-section complex-coupled DFBlasers by alternating gain and loss coupling

A novel tunable two-section complex-coupled distributed feedback (DFB) lasers were demonstrated which exhibited an enhanced wavelength tuning range beyond 7 nm by current injection only. By a simple master-and-slave type of injection current control, either in-phase gain-coupling or antiphase loss-coupling mechanism can be activated, in such a way that one of the two Bragg modes around DFB stopband will become the lasing mode, and its wavelength can be simply and independently tuned by corresponding injection current. More then 45 dB side-mode suppression ratio (SMSR) over entire tuning range were obtained. Good system performance for 375 km transmission with butt-coupled III-V Mach-Zehnder (MZ) modulator at 2.5 Gb/s over nondispersion-shifted fiber was also demonstrated     

Effect of in-phase and antiphase gain coupling on high-speedproperties of MQW DFB lasers

Effect of in-phase and antiphase gain-coupling on high-speed properties is studied for MQW DFB lasers with periodically truncated quantum-wells. The enhancement of modulation bandwidth due to antiphase gain-coupling is found significantly suppressed, and gain-coupled DFB lasers with high κL are preferred for large modulation bandwidth due to the presence of linear gain saturation in MQW lasers     

Effect of oblique incidence on bistability of nonlinear microcavity

Based on the nonlinear medium transfer theory, optical bistability of nonlinear microcavity is investigated at oblique incidence. It is found that the critical frequency of incident light inducing bistability phenomenon is related to the polarization and incident angle. The critical frequency increases with the incident angle. Increasing the incident angle can make bistability happen easier for focusing Kerr medium, but will hinder bistability for defocusing Kerr medium. The bistability switch threshold of TE mode is significantly lower than that of TM mode at a large incident angle.