Numerical study on gain and optical properties of AlGaInAs, InGaNAs, and InGaAsP material systems for 1.3-μm semiconductor lasers

In reference to real devices fabricated in laboratories, the optical properties of AlGaInAs, InGaNAs, and InGaAsP semiconductor material systems for 1.3-μm semiconductor lasers are systematically studied. Simulation results show that both the AlGaInAs/InP and InGaNAs/GaAs material systems have better gain performance and smaller transparency carrier density than the InGaAsP/InP material system. For the AlGaInAs/InP material system, the characteristic temperature is improved by using compensating tensile strain in barrier. Specifically, for a 250-μm-long short-cavity AlGaInAs/InP laser, when the barrier is with a compensating tensile strain of 0.39%, the characteristic temperatures in 290-330 K and 330-350 K can be enhanced to 121.7 K and 58.9 K, respectively. For the InGaNAs/GaAs material system, simulation results suggest that the laser performance can be significantly improved when the laser is with strain-compensated GaNAs barriers.     

Reduced linewidth enhancement factor due to excited state transition of quantum dot lasers

The carrier induced refractive index change and linewidth enhancement factor α due to ground-state (GS) and excited-state (ES) transitions have been compared by measuring the optical gain spectra from an InAs/GaAs quantum dot (QD) laser structure. It is shown that the ES transition exhibits a reduced α-factor compared to the value due to the GS transition. This result can be explained by the α-factor due to the ES transition having a smaller increase from the non-resonant carriers in the combined state of the wetting layer and InGaAs strain reducing layer than the α-factor increase due to the GS transition, since the relaxation time for carriers from the combined state of the wetting layer and InGaAs strain reducing layer to the ES is shorter than to the GS. The result reported here shows another advantage of using ES QD lasers for optical communication, in addition to their higher modulation speed.     

光纤光栅的温度增敏实验

用热膨胀系数较大的聚合物材料对光纤光栅进行封装处理,极大地提高了光纤光栅的温度灵敏度。我们将光纤光栅封装于两种不同的聚合物材料中,其温度灵敏度分别提高6倍和23倍之多。这是迄今有文献报道的最大的光纤光栅温度灵敏度。     

(001)和(111)取向的张应变量子阱光学特性的比较

以(001)和(111)价带Lutinger-Kohn哈密顿量为基础,讨论了这两种取向的量子阱的能带结构、态密度(DOS)、跃迁矩阵元和增益特性。由于材料结构的各向异性,(111)取向的量子阱在平行生长面内有较小的重空穴有效质量,无应变和压应变激光器可以很好地利用这一点。而(001)取向的量子阱虽有较小的平面内轻空穴有效质量,但并不比(111)量子阱的小多少;加上(001)量子阱的价带耦合效应强,使其DOS比(111)量子阱的大,在相同载流子注入下,张应变(111)量子阱的增益系数比相应(001)量子阱的要高。所以张应变(111)量子阱激光器仍然比相应的(001)量子阱激光器性能要好。可以认为匹配和压应变的(111)激光器优异的性能不仅来自小的面内有效质量,也来自于弱的价带耦合效应。     

Designing an all epitaxial 1,550 nm intra-cavity VCSEL using GaInAsN/AlGaInAs in the active region and AlGaAsSb/AlAsSb in top and bottom DBRs

In this paper, a 1,550 nm Intracavity structure vertical cavity surface emitting laser (VCSEL) has been designed using quaternary compound QW/barrier materials of GaInAsN/AlGaInAs matched with InP substrate. This choice has been made instead of choosing widely used GaInAsP/InP and AlGaInAs/InP to gain some advantages. In addition to the introduction of new combination in the active region, a different compound semiconductor combination AlGaAsSb/AlAsSb has been used as the DBR material for achieving lattice matching and also for achieving higher refractive index contrast. Compared to widely used GaAs/AlGaAs DBR mirror system, which needed wafer fusion with the top and bottom sides of the active region at 1,550 nm, the chosen DBR of this design is advantageous. The active material compositions have been chosen to obtain a peak gain at 1,550 nm and all other compositions have been chosen to obtain close lattice match at the same time to obtain the desired bandgap at the desired layers. The end result of this design is a VCSEL based on InP substrate which is capable of producing 1,550 nm light output and which can be constructed using widely used epitaxial techniques because all of the layers are lattice matched.     

Numerical study on strained InGaAsP/InGaP quantum wells for 850-nm vertical-cavity surface-emitting lasers

The physical and optical properties of compressively strained InGaAsP/InGaP quantum wells for 850-nm vertical-cavity surface-emitting lasers are numerically studied. The simulation results show that the maximum optical gain, transparency carrier densities, transparency radiative current densities, and differential gain of InGaAsP quantum wells can be efficiently improved by employing a compressive strain of approximately 1.24% in the InGaAsP quantum wells. The simulation results suggest that the 850-nm InGaAsP/InGaP vertical-cavity surface-emitting lasers have the best laser performance when the number of quantum wells is one, which is mainly attributed to the non-uniform hole distribution in multiple quantum wells due to high valence band offset. PACS 42.55.Px; 78.20.-e; 78.20.Bh; 78.30.Fs     

Investigation of Q-switched InP-based 1550 nm semiconductor lasers

High energy picosecond pulse generation from a two contact tapered 5 quantum well (QW) InGaAlAs/InP diode laser (1550 nm) is investigated using a passive Q-switching technique. Single peak pulses with pulse energies as high as 500 pJ and durations of typically hundreds of picoseconds are obtained from the device by applying reverse bias voltages in the range of 0 V to ?18 V to the absorber section of the device. It is also demonstrated that more symmetrical Q-switched pulses are obtained by reducing the duration of electrical pulses applied to the gain section of the laser. Such an improvement is attributed to the reduced time of the population inversion in the gain section due to shorter electrical pulse. We also show comparatively the dependence of optical spectra on the reverse bias voltage for diode lasers emitting at 1550 nm and 1350 nm, and demonstrate that better spectral output is obtained from AlGaInAs lasers emitting at a wavelength of 1550 nm.     

Effect of Quantum Well Width Reduction for GaInNAs/GaAs Lasers

We investigated the function of the quantum well (QW) width for laser characteristics especially for reduction of the well width. We pointed out that such reduction has almost no influence on the optical gain or the carrier overflow for a large conduction band offset system, such as GaInNAs QWs. A thin QW is advantageous for suppression of the carrier overflow to the higher quantized energy levels which results in good temperature and gain characteristics. Thin GaInNAs QWs is a good candidate for an active layer structure of the lasers utilized in the next optical communication systems.     

Analytical and visual modeling of InGaN/GaN single quantum well laser based on rate equations

An analytical, visual and open source model based on solving the rate equations for InGaN/GaN single quantum well (QW) lasers has been carried out. In the numerical computations, the fourth-order Runge–Kutta method has been used for solving the differential rate equations. The rate equations which have been considered in this simulation include the two level rate equations for the well and separate confinement heterostructure (SCH) layers. We present a new and inexpensive modeling method with analytical, visual and open source capabilities to investigate and comprehend the QW laser characteristics such as time behavior of carriers in SCHs and QW, photon density, output power and gain, and also the output power versus current which presents the threshold current of the laser. The characteristics of the QW lasers, which include laser time response (P–t), turn-on delay time of lasing and output power–current (P–I) characteristic and related features such as threshold current and slope efficiency have been investigated. Our model accurately computes the P–t and P–I characteristics such as turn-on delay time, threshold current and slope efficiency, and also illustrates the effects of parameters such as the injection current and geometry.     

Structuring of material parameters in lithium niobate crystals with low-mass, high-energy ion radiation

Ferroelectric lithium niobate crystals offer a great potential for applications in modern optics. To provide powerful optical components, tailoring of key material parameters, especially of the refractive index n and the ferroelectric domain landscape, is required. Irradiation of lithium niobate crystals with accelerated ions causes strong structured modifications in the material. The effects induced by low-mass, high-energy ions (such as ³He with 41?MeV, which are not implanted, but transmit through the entire crystal volume) are reviewed. Irradiation yields large changes of the refractive index ??n, improved domain engineering capability within the material along the ion track, and waveguiding structures. The periodic modification of ??n as well as the formation of periodically poled lithium niobate (PPLN) (supported by radiation damage) is described. Two-step knock-on displacement processes, ³He??Nb and ³He??O causing thermal spikes, are identified as origin for the material modifications.     

Lateral Current Injection (LCI) Multiple Quantum-Well 1.55 μm Laser with Improved Gain Uniformity Across the Active Region

A simulation study of lateral current injection 1.55 m laser with strain-compensated multiple quantum-well (MQW) active region (InGaAsP well, InGaAlAs barrier) is presented using self-consistent 2D numerical simulations. The effects of different mesa width and p-doping in the QWs on the carrier and gain uniformity across the active region are explored. A high p-doping in the quantum wells is found to increases the carrier and gain non-uniformity across the active region. The QW region close to the n-contact side does not provide much gain at high optical powers. An asymmetric optical waveguide design is proposed to help reduce the gain non-uniformity across the active region. By shifting the optical modal peak toward the p-side, the modal overlap between the gain region and the optical mode is improved and a more even carrier and gain distribution is obtained. However, due to reduced bandgap of the quaternary InGaAsP p-cladding, an enhanced electron leakage out of the QWs into the p-cladding degrades the laser efficiency and increases the threshold current. Transient time–domain simulations are also performed to determine the small-signal modulation response of the laser promising a simulated high modulation bandwidth suitable for direct-modulation applications.     

The effects of carrier dependant nonlinear gain on quantum well VCSEL characteristics

In this paper, we present a numerical opto-electro-thermal model for studying vertical cavity surface emitting lasers operation. The model is applied to an index-guided structure with an oxide aperture and multiple quantum-wells in active layer. The interdependent process of carrier transport, heat generation and optical field are solved self-consistently using finite difference time domain in cylindrical system. The gain of quantum wells (QWs) is calculated based on the solution of Schrödinger equation considering heavy hole-light hole band-mixing effect. The calculated maximum gain versus injected carriers is fitted by a 3th order polynomial function and used in opto-electro-thermal model. The inclusion of QW maximum gain calculation for constant wavelength in the model allows us to study threshold current value and higher order transverse modes as well as their dependencies on variation of gain and refractive index induced by carrier and heat more accurately than linear gain approximation. The results show a lower threshold current compared with linear gain approximation. For injection current above the threshold, we consider the spatial hole burning, thermal lensing and self focusing effects.     

Optical gain analysis of strain-compensated InGaN–AlGaN quantum well active regions for lasers emitting at 420–500 nm

Strain-compensated InGaN quantum wells with tensile AlGaN barriers are analyzed as improved gain media for laser diodes emitting at 420–500 nm. The band structure is calculated using the 6-band k ·p formalism, taking into account valence band mixing, strain effect, and spontaneous and piezoelectric polarizations. The optical gain analysis exhibits significant improvement in the peak optical gain and differential gain for the strain-compensated structures. The calculation also shows a significant reduction of threshold carrier density and current density for diode lasers employing the strain-compensated InGaN–AlGaN QW active regions.     

高功率808nm InGaAsP—GaAs分别限制结构的半导体激光器

介绍了研究分别限制结构ＩｎＧａＡｓＰ－ＧａＡｓ半导体激光器所得到的最新成果。利用引进的俄国技术，基于量子阱结构的ＩｎＧａＡｓＰ－ＧａＡｓ激光器，可用短时间液相外延技术制造。在ＧａＡｓ衬底上制成的ＩｎＧａＡｓＰ－ＧａＡｓ分别限制结构的激光器，主要参数如下：发射波入λ＝８０８ｎｍ，阈值电流密度Ｊ＝３００Ａ／ｃｍ＾２，对于条宽ω＝１００μｍ的激光器，连续功率为１－２Ｗ。     

Injection level dependence of the gain, refractive index variation, and alpha (α) parameter in broad-area InGaAs deep quantum-well lasers

In this study, a single, simple and an accurate computer-aided design model is developed in order to obtain the injection level dependence of the critical quantities of broad-area (with a width of 50 μm or more) InGaAs deep quantum-well (QW) lasers. Each of these quantities (gain, refractive index variation, and alpha (α) parameter) requires lengthy mathematical calculations with the use of different theories, assumptions, approximations, and estimations of some parameter values. The model is based on artificial neural network (ANN) approach that the total computational time is in the order of microseconds for the whole quantities in order to get their accurate values. The results are in very good agreement with the previously obtained results from an InGaAs deep QW laser sample.     

Gain and threshold properties of InGaAsN/GaAsN material system for 1.3-μm semiconductor lasers

The gain properties and valence subbands of InGaAsN/GaAsN quantum-well structures are numerically investigated with a self-consistent LASTIP simulation program. The simulation results show that the InGaAsN/GaAsN has lower transparency carrier density than the conventional InGaAsP/InP material system for 1.3-μm semiconductor lasers. The material gain and radiative current density of InGaAsN/GaAsN with different compressive strains in quantum well and tensile strains in barrier are also studied. The material gain and radiative current density as functions of strain in quantum well and barrier are determined. The simulation results suggest that the laser performance and Auger recombination rate of the 1.3-μm InGaAsN semiconductor laser may be markedly improved when the traditional GaAs barriers are replaced with the AlGaAs graded barriers.     

InGaAsP分别限制量子阱激光器

长波长InGaAsP量子阱激光器,以其低阈值、窄光谱线宽和高的调制频带宽等优良特性而成为大容量通信的基础。为此,我们利用低压MOVCD技术生长了1.62μm和1.3μm的InGaAsP材料,测得其77K光荧光(PL)谱线半峰高宽分别为18.7meV和28meV.利用X射线双晶衍射测得两种材料的晶格失配度不大于1×10^-3.并生长了四个不同阱宽的InGaAsP/InP量子阱结构,测得77K温度下的PL谱,分析了阱宽对发光波长及半峰宽的影响,并提出在量子阱激光器中减小界面层影响的方法。在此基础上,生长了分别限制量子阱激光器结构,并利用质子轰击制备出条形结构激光器,测得其最低阈值电流为100mA.直流工作光谱峰值波长为1.52μm左右,单面输出外微分量子效率约为36%.     

Fuzzy logic based device to implement a single CAD model for a laser diode based on characteristic quantities

In this study, a single modelling approach based on fuzzy logic (FL) device is designed for a quantum-well (QW) laser diode to obtain the characteristic quantities and their dependences on wavelength and injection currents. The fuzzy rules are compiled using the input variables which are namely the wavelength and the injection currents, while the output parameters are the modal gain difference, the effective refractive index difference and alpha (α) parameter (linewidth enhancement factor). The suggested approach provides fast and reliable predictions which can prevent the engineers from rigorous experiments/calculations. The FL device provides the predictions of modal gain difference, effective refractive index difference and alpha parameter against the wavelength and the injection currents within the accuracy level of 90–99.9%.     

Exciton effects on the refractive index and optical absorption in wurtzite quantum wells via a fractional-dimensional space approach

The optical refractive index changes and absorption coefficients of quantum wells (QWs) are theoretically investigated with considering exciton effects within the framework of the fractional-dimensional space approach. The exciton wave functions and bound energies are obtained as a function of spatial dimensionality, and the dimension increases with the well width increasing. Then optical properties are obtained by using the compact-density matrix approach and an iterative method. Numerical results are presented for wurtzite ZnO/Mg_xZn_1−xO QWs. The calculated results show that the changes of refractive index and absorption coefficients are greatly enhanced due to the quantum confinement of exciton. And the smaller the QW width (dimension) is, the larger influence of exciton on the optical properties will be. Furthermore, the exciton effects make the resonant peaks move to a lower energy. In addition, the optical properties are related to the QW width, the incident optical intensity and carrier density.     

增益非线性对超短腔激光器速率方程的影响

用量子阱激光器增益与载流子浓度的对数关系（增益饱和效应）取代了体材料的线性关系，得到了适用于量子阱结构的速率方程。详细分析和计算了这一修正的影响。计算表明，对长腔（低损耗）器件线性关系是较好的近似；对短腔（高损耗）、线性关系则有较大的误差，必须考虑增益饱和的影响，否则将过高估算自发辐射因子的测量值，过高估计弛豫振荡频率和最大调制频率。这一结果对微腔激光器的研究具有重要的意义。