Optical gain and saturation characteristics of quantum-dot semiconductor optical amplifiers

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs.     

Optical bistability in semiconductor periodic structures

A theoretical demonstration of optical bistability in periodic layered media, particularly in long-period GaAs/AlAs superlattices is presented. The proposed structure consists of a periodic multilayer system, as opposed to previously demonstrated nonlinear bistable devices which employed Fabry-Perot etalons. The optical resonance effect which is essential for bistable devices is, in this case, induced by a refractive index modulation. The nonlinear active medium is distributed in the whole structure rather than placed between the two mirrors of a Fabry-Perot cavity. It is shown by a complete calculation of wave propagation in the periodic nonlinear medium that a multiple valued feature appears in the structure's nonlinear reflectivity spectrum. The input/output characteristics of the structure exhibit bistable hysteresis similar to that of a nonlinear Fabry-Perot etalon     

Optical bistability in reflective fiber gratings

Optical bistability in a nonlinearly reflective fiber grating through the mode coupling between the LP₀₁ and counter propagation LP₀₂ modes (i,e., the reflective LP₀₁&rlhar2;LP₀₂ mode converter) is investigated by using the coupled-mode theory. Both the transmissive and the reflective properties of this nonlinear device are analyzed, which show that the output-versus-input relation exhibits the optical bistability. The switching power and the bistable hysteresis performances are also discussed. In addition, the comparison between the nonlinearly reflective mode converter (LP₀₁&rlhar2;LP₀₂) and the nonlinear fiber Bragg grating reflector (i,e., the LP₀₁ and counter-propagation LP₀₁ modes) is also presented. It is shown that the former has much lower switching power than the latter     

Optical bistability in semiconductor lasers under intermodal lightinjection

Optical bistability in semiconductor lasers under intermodal light injection is predicted using small-signal analysis. The optical bistability is a special case of sidemode injection locking and originates from the strong gain nonlinearity introduced by external light injection. The theory can be applied to the optical bistability under both intramodal and intermodal light injection. And the difference of the optical bistabilities between intermodal and intramodal light injection is discussed. Expression for the bistable loop width is presented. Optical frequency-bistability and power-bistability in semiconductor lasers can be realized by intermodal light injection, which imply that a small (a few GHz) change of the injected light frequency or a small (several μW) change of the injected light power will induce a large (up to THz) change of the laser emitting frequency. Besides, hybrid optical bistability can be achieved by varying the bias current (only a few mA) of the laser. Dynamic properties, such as turn-on and turn-off delay and carrier overshoot during switching are discussed and some consideration of the practical aspects of the optical switching is presented     

Optical bistability and nonlinear resonance in a resonant-type semiconductor laser amplifier

We report here on the first observation of hysteresis in the asymmetrical tuning curve of a semiconductor laser operating as a resonant-type amplifier. This bistable amplification resulting from a nonlinear resonance phenomenon is caused by the dependence of the refractive index on the injected carrier density.     

Theory of short-pulse gain saturation in semiconductor laseramplifiers

Carrier heating and spectral hole burning are shown to have a strong influence on the amplification of ultrashort pulses using semiconductor laser amplifiers. Approximate analytical expressions for the effective saturation energy in different pulsewidth regimes are derived     

Optical bistability in an L-band dual-wavelength erbium-doped fiber laser with overlapping cavities

The authors investigate the optical bistability in an L-band dual-wavelength erbium-doped fiber (EDF) laser with overlapping cavities. The bistable input-output hysteresis behavior of the two lasing wavelengths evolves in antiphase. The bistable region is tens of milliwatts wide. The bistability is essentially due to the homogeneous saturable absorption of EDF. The cross-gain saturation causes the bistabilities of the two lasing lines to evolve in antiphase with respect to each other. The gain clamping effect helps to widen the bistable region, which may be controlled by the cavity loss and by the EDF length.     

谐波锁模SOA光纤环形激光器的改进方案及其数值研究

对谐波锁模半导体光放大器(SOA)光纤环形激光器的工作条件提出改进,并采用自再现理论对其进行了数值研究.数值结果表明:激光器在新的工作条件下,即调制SOA被高直流偏置,对自发辐射信号进行调制并提供锁模脉冲克服腔损耗所需的增益,而此时的增益SOA被低直流偏置充当增益补偿器,输出的锁模脉冲质量显著提高.     

Effects of gain saturation in semiconductor laser amplifier links

The effects of gain saturation due to amplified spontaneous emission in systems comprising cascaded semiconductor laser amplifiers and attenuators are investigated. The influence of the relative spatial distribution in the link of gain and loss is analyzed numerically, and conclusions regarding noise optimal structures are drawn. It is shown that the performance of links with gain saturation can be quite different from the ideal, unsaturated case     

Electrooptic tuning of a fiber ring semiconductor laser

This letter reports recent results obtained in the area of wavelength tuning of semiconductor lasers. The laser is constructed from an InGaAsP 1300-nm semiconductor optical travelling-wave amplifier. The cavity consists of a fiber ring resonator using a low fiber coupler as the output mirror. The intra-cavity electrooptic tuner works as a one-stage Lyot filter and allows wavelength tuning over a spectral range of 20 nm with a tuning rate of 0.05 nm V^-1. This is one of the best results reported using electrooptic birefringent tuning systems     

Strong optical bistability in a simple L-band tunable erbium-doped fiber ring laser

We demonstrate strong optical bistability in a widely tunable L-band erbium-doped fiber ring laser pumped by a 980-nm laser diode. The bistable region is as much as 150-mW wide and can be controlled by the lasing wavelength or the length of erbium-doped fiber. The mechanism for the bistability is not typical cross gain saturation in a two-mode operation, but rather saturable absorption strongly enhanced in the proposed scheme.     

Optical bistability in a traveling-wave SOA connected to a DFB laser diode: theory and experiment

We investigate the bistable response of a traveling-wave semiconductor optical amplifier (SOA) connected bidirectionally to a distributed feedback (DFB) laser diode. By contrast to dispersive DFB amplifiers, bistability is possible for a broad wavelength range of the injected optical signal. A theoretical analysis clarifies the origin of the phenomenon and the conditions under which it occurs. Numerical simulations and experimental results confirm the theoretical analysis. Experimentally obtained step heights up to 25 dB and widths of the bistability domain of 1-4 dB have been found.     

半导体光放大器增益饱和及其开关特性的研究

从理论上研究了行波半导体光放大器的增益饱和特性。由半导体的单模速率方程出发，应用半导体光放大器的传输波动方程，推导了放大器的增益特性表达式。根据半导体光放大器增益饱和特性，研究了开关特性。     

Switching-wavelength pulse source constructed from a dispersion-managed SOA fiber ring laser

A new approach to generate switching-wavelength picosecond pulses is developed based on a dispersion-managed semiconductor optical amplifier (SOA) fiber laser cavity. Gain modulation on a 1.55-/spl mu/m SOA is used together with subharmonic gating of pulses in the cavity. An 8-GHz, eight-wavelength output is obtained with a constant spacing of 0.91 nm between neighboring wavelengths. The wavelength channels have nearly identical peak powers. The total amplitude and time jitter of the output is measured to be smaller than 0.4 ps. The spacing and the number of output wavelengths can be easily tuned by changing the operating frequency and the subharmonic ratio without modifying the optical cavity.     

Effects of multiple wave mixing and gain saturation in an idealtraveling wave semiconductor laser amplifier

Nonlinear distortions induced by four-wave mixing and gain saturation in a semiconductor laser amplifier can affect the overall performance of lightwave systems using such devices. A model which includes both types of distortion in a three-channel optical transmission system is presented. Large-signal analysis is used to calculate distortion in amplifiers operating near saturation. The technique provides more realistic predictions than the small-signal rate equation analysis used in previous theoretical studies of four-wave mixing in optical amplifiers     

Optical pulse shaping by cross-phase modulation in a harmonic mode-locked semiconductor fiber ring laser under large cavity detuning

The authors experimentally investigate the phenomena of the pulse-shape distortion due to the drift of cavity length or modulation frequency in a harmonically mode-locked semiconductor fiber ring laser (SFRL). Also, it is experimentally demonstrated that the optical pulse shaping by cross-phase modulation (XPM) can be applied to enhance the robustness of a harmonic mode-locked semiconductor ring laser to the environmental factors, e.g., the modulation-frequency drift and/or the cavity-length drift. The mechanism of pulse-shape distortion due to the modulation-frequency drift or cavity-length drift is explained and experimentally verified. As the locking condition of the SFRL goes to the rational harmonic mode-locking (RHML) regime, the pulse-shape distortion becomes dominant. To validate the explanation, a simple equation for estimating and comparing the amount of cavity-length detuning in terms of frequency is proposed for comparison, which is called the equivalent frequency detuning (EFD). The distortion mostly appeared in the trailing edge of the pulse. By selectively filtering the XPM-induced red-chirped wavelength components in the rising edge of the pulse, the mode-locked pulse train with large pulse-shape distortion can be reshaped to regenerate a mode-locked pulse train with less pulse-shape distortion than the pulse train without the proposed scheme. This method can be used to reduce the pulse-shape distortion effectively, maintaining the pulse repetition rate under the large modulation-frequency detuning, or equivalently, large cavity-length detuning. The proposed scheme is able to generate the pulse-trains robust to the modulation-frequency drift and/or cavity-length drift of the mode-locked SFRL.     

External cavity controlled operation of a semiconductor diode gain element in series with an optical fiber

The series combination of a semiconductor diode gain element (a diode laser whose end facets have been antireflection coated) and an optical fiber has been placed inside an external cavity and the combined system has lased in a single spectral line whose width was less than the1.7 times 10^{-5}-nm (7.5-MHz) resolution of the scanning Fabry-Perot interferometer used in the measurement. This result has been achieved with either a single-mode or a multimode optical fiber in series with the diode gain element and for experiments in which a polarizer, oriented so its polarization is parallel to the diode gain element polarization, was placed at the other end of either fiber. The output of the external cavity is temporally stable as measured both by a 225-MHz-bandwidth detector system and a spectrum analyzer. Over a one minute interval the maximum fluctuation in the output frequency of the external cavity was found to be 2 MHz. Analysis of the threshold behavior of the external cavity for the cases where the multimode or single-mode fiber are in the cavity indicates that the coupling coefficients for either fiber are nearly the same. Without an external cavity, the coupling coefficient from the diode gain element (or from a similar, not anti-reflection-coated, laser) to the multimode fiber is over five times larger than that to the single-mode fiber. These coupling results are explained by postulating that for the multimode fiber, only one or a selected number of the many modes of the fiber can participate in the laser action of the external cavity. It is believed that only these mode(s) are reflected back into the fiber by the spherical mirror in appropriate phase and angle to participate in the laser emission.     

半导体光纤环形腔激光器调谐特性的实验研究

采用国产半导体器件,组建了半导体光纤环形腔激光器(FRSLs)的实验装置及相应的测试系统,系统地研究了FRSLs的输出和调谐特性.结果表明,已实现高速调制情况下的动态单纵模调谐,并获得波长调谐范围大于36nm的稳定光脉冲输出.确立了FRSLs的阈值特性、输出功率等物理参量与振荡波长之间的关系,提出通过控制分光比来优化和兼顾FRSLs的调谐范围及输出功率.实验结果较好地与理论模拟结果相吻合,为进一步改善FRSLs的性能指标提供了实验依据.同时,这些分析与结论对于相关结构类型的激光器设计也具有一定的参考价值.     

Time-dependent simulation of a semiconductor laser amplifier: pulsecompression in a ring configuration and dynamic optical bistability

A dynamic theory for a semiconductor laser amplifier with time-dependent optical input signal and driving injection current is presented. Previous treatments are extended by including a carrier density-dependent refractive index, a frequency-dependent gain, and multimode operation. The simulation yields optical bi- and tristability strongly depending on the speed at which the optical input-output characteristic is scanned. Tristability can be found in case of high pumping and large linewidth enhancement factor only. Applying the theory to a ring laser configuration, asymptotic pulse compression, which can lead to the emission of a stable sequence of short pulses with widths down to about 6 ps is found     

Pulse energy gain saturation in subpico- and picosecond pulseamplification by a traveling-wave semiconductor laser amplifier

Optical pulses with durations ranging from 0.49 to 21 ps are amplified by a traveling-wave semiconductor laser amplifier. The pulse energy gain is determined by pulse energy only. The dependence of pulse energy gain on output pulse energy does not change in the pulse duration range. The saturation characteristics are successfully explained by a four-level system model