Estimating the parameters of semiconductor lasers based on weak optical feedback self-mixing interferometry

The paper presents a practical approach for measuring the linewidth enhancement factor /spl alpha/ of semiconductor lasers and the optical feedback level factor C in a semiconductor laser with an external cavity. The proposed approach is based on the analysis of the signals observed in an optical feedback self-mixing interferometric system. The parameters /spl alpha/ and C are estimated using a gradient-based optimization algorithm that achieves best data-to-theoretical model match. The effectiveness and accuracy of the method has been confirmed and tested by computer simulations and experiments, which show that the proposed approach is able to estimate /spl alpha/ and C with an accuracy of 6.7% and 4.63%, respectively.     

Comparison of linewidth enhancement factor between p-doped and undoped quantum-dot lasers

The optical linewidth enhancement factor (LEF) of a p-doped quantum-dot (QD) laser is measured below threshold and compared with a theoretical calculation. The optical gain, refractive index, and LEF are well matched with our theoretical model when the thermal effect is isolated by an additional pulse current measurement of the LEF. We also theoretically calculate the LEF of an undoped QD Fabry-Pe/spl acute/rot (FP) laser assuming that the structure of the undoped FP QD laser is the same as that of the p-doped QD FP laser except the p-type doping. The changes in modal gain and refractive index due to the respective QD ground and excited states are calculated. Based on the theoretical results, we show that the LEF of the p-doped QD laser is smaller than that of the undoped QD laser due to the reduced transparency carrier density.     

CW technique for measurement of linewidth enhancement factor: application to 735-nm tensile-strained GaAsP quantum-well lasers

We present a procedure for determining the linewidth enhancement factor (/spl alpha/ parameter) in semiconductor lasers under continuous-wave (CW) operation. It is based on the measurement of the amplified spontaneous emission spectra, with a proper correction of thermal effects. The method is applied to 735-nm tensile strained GaAsP-AlGaAs quantum-well lasers and it is validated by comparing CW results, after correcting thermal effects, with pulsed measurements. The results show a low value of the /spl alpha/ parameter attributed to the tensile strain.     

Chirp and linewidth enhancement factor of 1.55 /spl mu/m VCSEL with buried tunnel junction

Chirp characteristics of a 1.55 /spl mu/m vertical-cavity surface-emitting laser (VCSEL) employing a buried tunnel junction are reported for the first time. From the measurements the linewidth enhancement factor /spl alpha//sub H/ is derived and presented.     

RF-modulation measurement of linewidth enhancement factor and nonlinear gain of vertical-cavity surface-emitting lasers

Using RF-modulation technique of pump current, the linewidth enhancement factor /spl alpha/ and the nonlinear gain of a vertical-cavity surface-emitting laser (VCSEL) were obtained from the measured normalized phase-modulation index versus modulation frequency, is found as 2.7/spl plusmn/0.5 and the measured nonlinear gain of the VCSEL device is found to behave differently from that of edge-emitting lasers with an increase in pump level.     

Chirp and linewidth enhancement factor of tunable, optically-pumped long wavelength VCSEL

Small-signal frequency modulation response and chirp characteristics of a tunable, optically-pumped 1.6 /spl mu/m vertical cavity surface emitting laser (VCSEL) based on microelectromechanical at wavelength tuning are presented for the first time. From the measurements the linewidth enhancement factor /spl alpha//sub H/ has been derived and is discussed.     

Design of quantum-dot lasers with an indirect bandgap short-period Superlattice for reducing the linewidth enhancement factor

A novel gain medium consisting of direct bandgap semiconductor quantum dots (QDs) embedded in a short-period superlattice of indirect bandgap as barrier material is proposed to produce QD lasers with extremely small linewidth enhancement factor /spl alpha/. Our analysis has shown that at least one order-of-magnitude reduction in /spl alpha/ can be achieved at room temperature compared to that of similar QDs embedded in direct-bandgap barriers, making low chirp, narrow linewidth semiconductor lasers feasible.     

半导体激光器参数估计的改进算法

为了精确估计带外腔半导体激光器的线宽展宽因数,采用了光反馈自混合技术和梯度最优算法,可以达到数据—理论的最佳拟合,同时提出了改进的梯度迭代公式,并在参数迭代时引入可变步长,取得了仿真和实验数据,该算法不仅测量精度较高,而且抗噪声性能较好,仿真结果表明当信噪比大于20分贝时,线宽展宽因数的相对标准差小于2.55% ,实验数据处理结果表明线宽展宽因数的相对标准差小于5.1%,这对半导体激光器的应用是有帮助的。     

Linewidth, tunability, and VHF-millimeter wave frequency synthesis of vertical-cavity GaAs quantum-well surface-emitting laser diode arrays

The authors report the measurement of the laser linewidth, wavelength tunability, and generation of microwave frequencies between individually addressable elements of a vertical-cavity GaAs quantum-well surface-emitting laser diode array (lasing in the wavelength range 850-865 nm). Using heterodyne techniques, the authors obtain a deconvolved 65 MHz laser linewidth from the 109 MHz beat signal. The laser linewidth corresponds to a semiconductor laser linewidth enhancement factor alpha =5.7, which is in excellent agreement with that obtained independently from optical gain measurements and corresponding calculated refractive index changes. The authors measured heterodyne beat frequencies of 2-20 GHz. The bandwidth was limited by the microwave amplifiers. A simple calculation shows that a tuning range of 65 MHz to 3 THz can be achieved.<>     

适度光反馈机制下线宽展宽因数的自动测量

介绍了光反馈自混合干涉(OFSMI)法测量半导体激光器线宽展宽因数(LEF)的基本原理。针对不同参数条件,分析了该测量方法的理论误差,找出了最佳测量条件和最佳测量条纹位置。并根据光反馈自混合干涉信号特点,设计了一个自动测量算法。该算法可以自动识别提取一个振动周期内的光反馈自混合干涉信号所有特征值,采用最佳测量条纹位置所对应的特征值,就可以精确得到线宽展宽因数。仿真数据验证了自动测量算法的正确性。实验结果表明,自动测量算法在实际数据处理时测量参数的精确度较好,当自混合信号的峰-峰值为1.75 V时,线宽展宽因数的相对标准差只有3.26%。     

Noise Correlation, Regenerative Amplification, and the Linewidth Enhancement Factor of a Vertical-Cavity Surface-Emitting Laser

This letter reports on amplitude and frequency noise correlation measurements as well as on optical injection experiments related to the linewidth enhancement factor a of a vertical- cavity surface-emitting laser (VCSEL). It is shown that the correlation in VCSELs is not dominated by alpha; consequently, such experiments yield very little information on its value. Measuring the regenerative amplification of a weak injected optical probe turned out to be a robust approach that can not only give accurately the relaxation oscillation frequency and the damping rate but a good estimate of alpha as well. Finally, these experiments are combined with previously published data to increase the number of techniques applied to the measurement of alpha of a single semiconductor laser up to 11.     

Linewidth enhancement in distributed-feedback semiconductor lasers

It is demonstrated that the linewidth enhancement in distributed-feedback (DFB) laser diodes can differ significantly from (1+ alpha /sup 2/), where alpha is Henry's linewidth enhancement factor. The influence of the relevant laser parameters, the detuning of the gain peak wavelength, a pi /2 phase shift and a reflecting end facet on the linewidth enhancement is discussed with respect to an improved DFB laser design.<>     

Measurement of gain, group index, group velocity dispersion, and linewidth enhancement factor of an InGaN multiple quantum-well laser diode

Gain, group index, group velocity dispersion (GVD), temperature variation of refractive index, and linewidth enhancement factor of an In/sub 0.15/Ga/sub 0.85/N/In/sub 0.02/Ga/sub 0.98/N multiple quantum-well blue laser diode was measured using the Fourier transform method as a function of wavelength from 400 to 410 nm. At the lasing wavelength (403.5 nm), the group index is 3.4, the GVD (dn/sub g//d/spl lambda/) is -37 /spl mu/m/sup -1/, the temperature variation of refractive index dn/dT is 1.3/spl times/10/sup -4/ K/sup -1/, and the linewidth enhancement factor is 5.6.     

Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers

A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QC) laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schro/spl uml/dinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.     

Self-phase modulation coefficient of multiple-quantum-well optical amplifiers

The self-phase modulation coefficient /spl gamma/ of 1310 nm multiple-quantum-well (MQW) semiconductor optical amplifiers has been investigated. It is found to vary from 16/spl times/10/sup 4/ W/sup -1/ m/sup -1/ for low driving conditions to 3/spl times/10/sup 4/ W/sup -1/ m/sup -1/ for high-driving conditions. This implies that the amount of self-phase modulation occurring in the amplifier is between 1.5-10/spl times/ more than that occurring in the optical fiber following the amplifier. The additional self-phase modulation caused by the semiconductor optical amplifier may be used to achieve compensation for fiber dispersion in optical communication systems at significantly lower average power levels. The linewidth enhancement factor /spl alpha//sub H/ was found to increase from a value of 2 at low driving conditions, in agreement with results reported for MQW lasers, to a value of 3 at high-driving conditions.     

Gain, refractive index, and /spl alpha/-parameter in InGaAs-GaAs SQW broad-area lasers

Gain, refractive index, and the linewidth enhancement factor, or /spl alpha/-parameter, are measured in broad-area InGaAs-GaAs single-quantum-well semiconductor lasers using below-threshold amplified spontaneous emission spectra and a far-field filtering technique. The /spl alpha/ parameter is shown to increase dramatically with increasing carrier density and wavelength. Modes propagating in the transparent substrate of the lasers are shown to have a significant influence on the measured value of /spl alpha/.     

Dependence of the linewidth enhancement factor on the number of compressively strained quantum well in lasers

We have systematically studied the well number dependence of the linewidth enhancement factor in strained quantum-well (QW) lasers and have demonstrated experimentally that the linewidth enhancement factor can be reduced from /spl sim/9.4 to /spl sim/2.0 by increasing the number of compressively strained QW's from 2 to 8. This behavior is primarily due to an increase in the differential gain with the number of QW's.     

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.     

Optical and electrical properties of nanostructured metal-silicon-metal photodetectors

We report an experimental evaluation of the performance of silicon (Si) photodetectors incorporating one-dimensional (1-D) arrays of rectangular and triangular-shaped nanoscale structures within their active regions. A significant (/spl sim/2/spl times/) enhancement in photoresponse is achieved in these devices across the 400- to 900-nm spectral region due to the modification of optical absorption properties that results from structuring the Si surface on physical optics scales smaller than the wavelength, which both reduces the reflectivity and concentrates the optical field closer to the surface. Both patterned (triangular and rectangular lineshape) and planar Ni-Si back-to-back Schottky barrier metal-semiconductor-metal photodetectors on n-type (/spl sim/5/spl times/10/sup 14/ cm/sup -3/) bulk Si were studied. 1-D /spl sim/50-250-nm linewidth, /spl sim/1000-nm depth, grating structures were fabricated by a combination of interferometric lithography and dry etching. The nanoscale grating structures significantly modify the absorption, reflectance, and transmission characteristics of the semiconductor: air interface. These changes result in improved electrical response leading to increased external quantum efficiency (from /spl sim/44% for planar to /spl sim/81% for structured devices at /spl lambda/=700 nm). In addition, a faster time constant (/spl sim/1700 ps for planar to /spl sim/600 ps for structured at /spl lambda/=900 nm) is achieved by increasing the absorption near the surface where the carriers can be rapidly collected. Experimental quantum efficiency and photocurrents results are compared with a theoretical photocurrent model based on rigorous coupled-wave analysis of nanostructured gratings.     

Linewidth of InP-based 1.55 /spl mu/m VCSELs with buried tunnel junction

Spectral linewidth measurements of 1.55 /spl mu/m InGaAlAs/InP vertical-cavity surface-emitting lasers (VCSELs) employing a buried tunnel junction are reported. A narrow linewidth around 28 MHz was obtained at a power level of 0.5 mW using the self-heterodyne method, and an estimation for the linewidth enhancement factor is given.