Nonlinear gain coefficients in semiconductor lasers: effects ofcarrier heating

Nonlinear gain coefficients due to the effects of carrier heating are derived from the rate equations of carrier energy transfer in semiconductor lasers. We find that, in the modulation responses of semiconductor lasers, stimulated recombination heating will affect the resonant frequency and damping rate in a same form as the effects of spectral hole burning, while free carrier absorption heating will only affect the damping rate. The effects of injection heating and nonstimulated recombination heating are also discussed. The carrier energy relaxation time is calculated from first principles by considering the interactions between carriers and polar optical phonons, deformation potential optical phonons, deformation potential acoustic phonons, piezoelectric acoustic phonons. At the same time, the hot phonon effects associated with the optical phonons are evaluated because their negligible group velocity and finite decay time. We show that the carrier-polar longitudinal optical phonon interaction is the major channel of carrier energy relaxation processes for both electron and holes. We also point out the importance of the longitudinal optical phonon lifetime in evaluating the carrier energy relaxation time. Neglecting the finite decay time of longitudinal optical phonons will significantly underestimate the carrier energy relaxation time, this not only contradicts the experimental results but also severely underestimates the nonlinear gain coefficients due to carrier heating. The effects of spectral hole burning, stimulated recombination heating, and free carrier absorption heating on limiting the modulation bandwidth in semiconductor lasers are also discussed     

掺铒光纤中浓度效应对光脉冲群速的影响

利用相干布居振荡技术在介质吸收光谱上产生烧孔,孔宽大约为基态粒子数恢复时间的倒数.由增益理论分析得出不同抽运光强度对介质吸收状态的影响.在介质的吸收区域,振荡导致光脉冲经历饱和吸收,脉冲传输延迟;在介质的增益区域,振荡又导致光脉冲经历增益饱和、脉冲传输超前.将此技术应用在掺铒光纤(EDOF)中实现了光速人为可控.根据布居振荡效应及增益理论,由速率方程出发,得到了探测光的群折射率的理论解析表达式.考虑掺铒浓度对光脉冲群速度的影响,分别对四种浓度的光纤进行实验研究,观测到的最慢群速分别为3.45×102m/s,相应感生群折射率8.7×105.     

A normal amorphous silicon-based separate absorption andmultiplication avalanche photodiode (SAMAPD) with very high optical gain

A normal amorphous silicon-based separate absorption and multiplication avalanche photodiode (SAMAPD) with very high optical gain of the avalanche photodiode has been developed successfully by plasma-enhanced chemical vapor deposition (PECVD). Based on experimental results, using undoped α-Si:H as avalanche layer material and α-Si_1-xGe_x:H as absorption layer material, the hole-injection (HI) type SAMAPD yields a very high optical gain of 686 at a reverse bias of 16 V under an incident light power of P_in =1 μW and has a rise time of 145 μs at a load resistance R=10 kΩ. Thus the amorphous silicon-based SAMAPD is a good candidate for the long-range optical communication applications     

Amplification and excited state absorption in longitudinally pumped laser dyes

Direct measurements of optical gain using a probe laser beam for the laser dye cresyl violet perchlorate (CVP) in methanol solution using longitudinal pumping at two different wavelengths are described. An analysis of these longitudinal pumping experiments is developed and analytical expressions relating measured gain to the measured pump laser transmission are presented. This analysis is developed in terms of relevant optical cross sections including the influence of excited state absorption. The results demonstrate the influence of excited state absorption at the pump wavelength on pumping efficiency and on the gain per pass at the probe laser wavelength.     

Analysis of basic four-wave mixing characteristics in asemiconductor optical amplifier by the finite-difference beampropagation method

We have numerically analyzed nondegenerate four-wave mixing (FWM) among short optical pulses in a semiconductor optical amplifier (SOA) by the finite-difference beam propagation method (FD-BPM). We used the nonlinear propagation equation taking into account gain spectrum dynamic gain saturation which depends on carrier depression, carrier heating, and spectral hole-burning, group velocity dispersion, self-phase modulation, and two-photon absorption. To analyze FWM in an SOA, the evolution in time and spectral domain of two input optical pulses with different frequencies during propagation was calculated. From this simulation, it has become clear that the method me used here is a very useful technique for simulating FWM characteristics in SOA's. We also found that the wavelength dependence of the gain is crucial if the detuning is larger than 1 THz     

Photocurrent in self-organized InAs quantum dots in 1.3 μm InAs/InGaAs/GaAs semiconductor laser heterostructures

The photocurrent spectra of InAs/InGaAs/GaAs laser heterostructures with self-organized InAs quantum dots (QDs) are studied. The study has been performed with a sample illuminated perpendicularly or in parallel to the QD plane. The optical density of QDs, maximum gain in the laser structure, radiative recombination time, and polarization characteristics of absorption have been determined from the experiment. The photocurrent spectra were correlated with specific features of lasing. The absorption spectrum is interpreted as a superposition of optical transitions between states of the discrete energy spectrum, those between the states of the continuous spectrum, and “mixed” transitions.     

On the dimensionality of optical absorption, gain, andrecombination in quantum-confined structures

The purpose of this paper is to explore the dimensionality of the optoelectronic properties of quantum-well and dot systems by expressing carrier distributions in the confinement directions in terms of envelope functions rather than assuming that carriers are localized to the geometrical extent of the confining potential. The conclusions apply to an ideal two-dimensional (2-D) system or a structure where only the n=1 electron and hole subbands are populated. We show that optical absorption normal to the plane of a QW cannot be expressed as an absorption coefficient but should be specified as a fraction of light transmitted or absorbed per well. The modal gain for light propagating along the plane of a QW does not scale with well width and the variation of the material gain inversely proportional to the well width is a consequence of the definition of the confinement factor and has no independent physical significance. Coupling to the optical mode can be specified as a mode width without the need to assume the gain medium is localized in the well. Optical absorption and gain by quantum dots should be expressed as a cross section per dot. The radiative recombination current should be expressed in terms of a two-dimensional recombination coefficient and use of an equivalent three-dimensional coefficient introduces an artificial dependence on well width which can lead to errors in the comparison of QW systems. We provide an analysis of experimental data for optical absorption in GaAs wells and show that, using the correct dimensional forms, it is straightforward to use this to estimate modal gain and the recombination coefficient     

Temperature dependence of electrical and optical modulationresponses of quantum-well lasers

We present theory and experiment for high-speed optical injection in the absorption region of a quantum-well laser and compare the results with those of electrical injection including the carrier transport effect. We show that the main difference between the two responses is the low-frequency roll-off. By using both injection methods, we obtain more accurate and consistent measurements of many important dynamic laser parameters, including the differential gain, carrier lifetime, K factor, and gain compression factor. Temperature-dependent data of the test laser are presented which show that the most dominant effect is the linear degradation of differential gain and injection efficiency with increasing temperature. While the K-factor is insensitive to temperature variation for multiple-quantum-well lasers, we find that the carrier capture time and nonlinear gain suppression coefficient decreases as temperature increases     

Optical gain and absorption of quantum dots measured using an alternative segmented contact method

An alternative segmented-contact method for accurate measurement of the optical gain and absorption of quantum-dot and quantum-dash active materials with small optical gain is reported. The usual error from unguided spontaneous emission is reduced by subtracting signals acquired from three independently controlled sections as opposed to just two found in the conventional technique. The quantum-dot gain spectra are measured to a precision of less than 0.2 cm/sup -1/ at nominal gain values below 2 cm/sup -1/, and gain spectrum of quantum-dash sample is calculated with an error less than 0.3 cm/sup -1/ at a gain less than 1 cm/sup -1/. These accuracies are checked with a self-calibrating method. The internal optical mode loss measurement is also described.     

Optimum output efficiency of homogeneously broadened lasers with constant loss

The exact solution of a laser power extraction model is analyzed that relates the extraction efficiency with the laser parameters such as output coupling, small-signal gain, absorption loss, and laser cavity length. The model assumes a stable optical resonator and a homogeneously broadened gain medium. Optimum output coupling and resulting maximum extraction efficiency are determined for a range of values of the small-signal gain and absorption loss per pass. A relation is derived that allows determination of the intrinsic laser parameters from output power measurements when output mirrors are utilized with three different reflectances.     

New mechanism of excitonic enhanced optical gain for wide-gap quantum wells

New mechanism of optical gain in quantum wells are proposed using excitonic effects. Exciton in wide-gap semiconductors plays an important role in optical phenomena since it has a large binding energy and could be stable at room temperature. However, its bound state is constructed by the electron-hole Coulomb interaction and should be related to the electron and hole distributions when the ground state has many electron and holes. We have evaluated the current-current correlation function, i.e. conductivity, treating the mechanism of optical gain and exciton on equal footing. It is shown that the recombination of the exciton does not yield optical gain directly but that excitonic effects enhance an oscillator strength of the coupled transition. Taking into account a localized level in the energy gap, the optical gain in terms of the population inversion between the localized level and one of the band edge subband states is produced with the very small carrier concentration. Simultaneously, the excitonic absorption occurs due to the band edge electron-hole interaction. It is found that the former optical gain is enhanced extremely by the latter excitonic effect through the coupling between the two transitions. This enhanced optical gain might show a possibility of very low threshold current density for wide-gap laser diodes.     

铒镱共掺光波导放大器的优化

本文介绍了光波导放大器的理论,分析了由于铒离子吸收截面小和铒离子浓度过高带来的聚集效应,并认为由该聚集效应引起的协同上转换和激发态吸收会限制掺铒磷酸盐光波导放大器的增益.将镱离子作为敏化剂掺入到光波导放大器中,不仅可以降低饵离子高浓度引起的聚集,而且可以有效地解决光波导的增益问题.在考虑协同上转换效应的基础上,用龙库算法研究了饵镱共掺的光波导放大器,讨论了镱离子浓度和波导长度两个主要影响因素,并且计算出了最佳的波导长度和最佳的掺镱浓度.     

CW periodically poled LiNbO3 optical parametricoscillator model with strong idler absorption

An analytical model is described for predicting the gain and performance of CW singly resonant optical parametric oscillators with strong idler absorption, strong pump depletion, and thermally induced dephasing. Pump and signal beam focusing effects are included by using the Guha et al. h_sm parameters. It is shown that significant parametric gain can be generated with an idler absorption-length product exceeding 10, and in some cases idler absorption is seen to enhance the pump depletion by impeding back conversion. The model accurately predicts current experimental results with weak idler absorption. Validation with strong idler absorption awaits experimental measurements to be completed     

The role of high growth temperature GaAs spacer layers in 1.3-/spl mu/m In(Ga)As quantum-dot lasers

We investigate the mechanisms by which high growth temperature spacer layers (HGTSLs) reduce the threshold current of 1.3-/spl mu/m emitting multilayer quantum-dot lasers. Measured optical loss and gain spectra are used to characterize samples that are nominally identical except for the HGTSL. We find that the use of the HGTSL leads to the internal optical mode loss being reduced from 15 /spl plusmn/ 2 to 3.5 /spl plusmn/ 2 cm/sup -1/, better defined absorption features, and more absorption at the ground state resulting from reduced inhomogenous broadening and a greater dot density. These characteristics, together with a reduced defect density, lead to greater modal gain at a given current density.     

Theory of optical-gain measurements

The widely used technique for determining optical gain and absorption from single-pass, amplified spontaneous emission (ASE) is analyzed with particular attention given to active-volume saturation and three-dimensional effects. The results for homogeneously broadened media show that the population inversion and hence the gain in the active region is diminished by high levels of spontaneous and ASE. The externally measured gain is further reduced for large output collection apertures that admit incompletely amplified off-axis rays. Criteria are given for the excitation and detection geometries required to ensure accurate (∼10-percent) gain measurement. The simultaneous measurement of the equilibrium absorption coefficient can be made under less restrictive conditions.     

Two-dimensional simulation of constricted-mesa InGaAsP/InPburied-heterostructure lasers

A fully two-dimensional self-consistent numerical model of the steady-state behavior of 1.3 μm constricted-mesa InGaAsP/InP buried-heterostructure lasers is presented. Devices operating at this wavelength are very temperature sensitive and therefore the model for the first time includes coupled solutions to the thermal as well as the electrical and optical equation sets. The temperature dependence is included in the Fermi-Dirac statistics, bandgaps, mobilities, densities of states, Auger recombination coefficients, intervalence band absorption, optical gain, and thermal conductivities. The lasing mode profiles, carrier distributions, threshold currents, and temperature characteristics are analyzed and good agreement is found with experimental results, including the temperature dependence of the threshold current and the prediction of a break-point temperature. The optimum design parameters are investigated for reduced threshold currents, and the effect of optical loss in the blocking regions on lateral-mode control is analyzed     

Optical gain spectra of InGaAsP/InP double heterostructures

We report the first optical gain measurements on InGaAsP/ InP double heterostructures with composition corresponding to an emission wavelength of about 1.3 μm at 300 K. At optical pumping levels of about 1 MW/cm²the maximum gain values of the best samples available are 800 cm^-1at 2 K, 500 cm^-1at 77 K, and 200 cm^-1at 300 K. We conclude from low-intensity luminescence and absorption spectra, that the laser transition corresponds to free-carrier recombination between band-tail states, which are present even in not intentionally doped material. Although these tail states result in rather broad low-intensity luminescence, narrow gain spectra comparable to those of GaAs/GaAlAs double heterostructures are obtained.     

线性损耗和双光子吸收下硅基波导调制不稳定性

为了分析线性损耗和双光子吸收对硅基波导调制不稳定性的影响,采用理论论证和数值模拟相结合的研究方法,推导了调制不稳定性增益谱、峰值增益、峰值增益频率和调制带宽的表达式。论证了波导的线性损耗、双光子吸收系数和脉冲光功率等参量对调制不稳定性的影响,并对给定结构参量的脊波导进行了仿真分析。结果表明,即使在微弱的光功率(几十毫瓦)下,在反常色散区仍然存在强烈的调制不稳定性现象,其增益是相同功率下光纤介质的102～102倍;峰值增益频率和增益带宽随波导的线性损耗指数衰减;峰值增益也随双光子吸收系数指数减少。这为硅基波导调制不稳定性实验研究和超连续谱产生提供了理论参考。     

All-optical bistable switching dynamics in 1.55-μm two-segmentstrained multiquantum-well distributed-feedback lasers

All-optical bistable switching dynamics of 1.55-μm two-segment strained multiquantum-well (MQW) distributed-feedback (DFB) lasers were systematically studied both experimentally and theoretically. Some fundamental optical functionalities, including all-optical set-reset (flip-flop) operations, were demonstrated. The switching responses of these bistable lasers were studied, for the first time, with optical injection from a single-mode DFB laser, indicating that the switching dynamics based on gain quenching and absorption saturation are inherently different. A theoretical model including optical injection was developed to study these all-optical bistable switching characteristics in segmented bistable lasers. It was found that the nonuniform distribution of the photon density in the bistable laser cavity induced by optical injection was essential to perform the time-domain switching operations. Simulations showed a good agreement with experimental observations and indicated design improvements. Although the switching responses in the range of tens of picoseconds can be obtained with these bistable lasers, the maximum repetition frequency of the bistable systems would still be limited to the hundreds of megahertz due to the slow carrier recovery time (5 ns) of the lasers