Transfer-matrix analysis of the intensity and phase noise ofmultisection DFB semiconductor lasers

A general small-signal model for the intensity and phase noise spectra of multisection distributed feedback (DFB) semiconductor lasers is developed by using the transfer-matrix approach based on the Green's function method. The spontaneous emission enhancements due to nonuniform longitudinal field distribution and the effective amplitude-phase coupling effect (the effective linewidth enhancement factor) are taken into account in the formulation. Analytical expressions for the spectra of the relative intensity noise and the FM noise of the main mode in the multimode operation are presented by using the transfer functions in a flow-graph representation. Facet reflectivities and external optical feedback are included in the model. The effects of the grating coupling coefficient, the random grating-phase at the facets, the phase-shift position, the external optical feedback, and the side mode on the noise spectra are analyzed systematically for a λ/4-shifted DFB laser     

Evaluation of an automatic method to extract the grating couplingcoefficient in different types of fabricated DFB lasers

Distributed feedback (DFB) laser parameters such as grating coupling coefficient, effective indices, facet reflectances, and the phases of facet reflectances have been determined using a method based on least-square fitting of theoretical spectra to measured, subthreshold DFB laser emission spectra. The only inputs needed are geometrical parameters such as length, grating period, and internal grating phase shifts. A larger number of devices have been successfully characterized, and consistent results have been obtained in both 1.3-μm multi-quantum-well (MQW) DFB lasers with both facets as-cleaved, and in 1.55-μm MQW DFB lasers with no, one, or two facets as-cleaved     

Coupled phase-shift distributed-feedback semiconductor lasers fornarrow linewidth operation

Mode properties of a coupled phase-shift distributed-feedback (DFB) structure are analyzed and the feasibility of narrow-linewidth emission is shown theoretically. The structure consists of a multiple number of phase-shift DFB laser units which are arranged in tandem and coupled with each other through additional phase shift of corrugation. Optimum structure parameters were determined for two coupled laser units so that the frequency detuning is zero and the mode pattern is the flattest. The mode analysis shows that the intensity distribution is flatter and the normalized coupling constant can be larger than those for the conventional DFB laser with a single phase shift. This indicates that the cavity length can be extended with less influence from the longitudinal spatial hole burning effect     

基于单边带调制的前馈技术压缩半导体激光器线宽

提出了一种基于电光单边带调制的前馈线宽压缩系统,该系统主要包括相位噪声检测和单边带调制两大部分。相位噪声检测部分输出一个与相位误差成比例的误差电压信号,电压信号通过线性调频作用于单边带强度调制器,实现对激光信号的调制。单边带调制器采用马赫增德尔强度调制器的结构,相当于内嵌四个相位调制器,通过相位调制达到强度调制的目的,最终得到线宽压缩的激光信号。实验针对波长为1552.52nm、功率10mW 的分布反馈（DFB）半导体激光器,通过对闭环系统的搭建与仿真,激光器的线宽从原来的0.5nm减小到0.016 nm,实现了对半导体激光器线宽的压窄。     

Characterization of the modulation and noise properties of athree-electrode DFB laser

The modulation and noise properties of a three-electrode DFB laser have been measured. At proper biasing, the frequency modulation response could be chosen to be in phase or 180° out of phase with respect to the intensity modulation by selecting the modulated electrode. Applying a first order equalization/filter, a flat frequency modulation response was measured up to 5 GHz. A 20-dB broadband suppression of the intensity modulation was accomplished when modulating two sections in push-pull. The spectral densities of the FM and intermodulation noise were measured showing remarkably flat intermodulation and FM noise spectra     

Traveling wave analysis of semiconductor lasers: modulationresponses, mode stability and quantum mechanical treatment of noisespectra

We present a traveling wave analysis of a general class of semiconductor lasers, which includes multisection DFB/DBR lasers and gain-coupled DFB lasers. The analysis leads to new semianalytic expressions for the small-signal IM and FM modulation responses, the intensity and FM noise spectra, and the linewidth. The expressions are given in terms of solutions to four coupled linear homogeneous differential equations and can easily be evaluated numerically. We also derive a stability parameter σ, for which σ<0 indicates that the mode is unstable with respect to small-scale fluctuations. The noise spectra are derived from semiclassical calculations as well as from calculations based on quantized fields, and we discuss the limitations of the semiclassical approach. The formalism of the quantum mechanical treatment has a built-in relationship between the relative intensity noise and the noise of the injection current. This relationship is discussed and illustrated by numerical examples     

Numerical modeling of intensity and phase noise in semiconductorlasers

A self-consistent numerical approach is demonstrated to analyze intensity and phase noise in semiconductor lasers. The approach takes into account the intrinsic fluctuations of the photon number, carrier number, and phase. A new systematic technique is proposed to generate the Langevin noise sources that derive the laser rate equations keeping their cross-correlations satisfied. The simulation is applied to AlGaAs lasers operating in a single mode. The time-varying profiles of the fluctuating photon and carrier numbers and the instantaneous shift of the oscillating frequency are presented. Statistical analysis of the intensity and phase fluctuations is given. The frequency spectra of intensity and phase noise are calculated with help of the fast Fourier transform. The importance of taking into account the carrier number noise source and its cross-correlation with the noise source on the phase is examined by comparing our results with those by conventional methods     

Study of wavelength shift in InGaAs/InAlGaAs QW DFB lasers based onlaser parameters from a comparison of experiment and theory

Experimental data of InGaAs/InAlGaAs quantum well distributed feedback (DFB) lasers such as spectra, under continuous and pulsed biasing, relative intensity noise and linewidth, are compared with the results of model calculations based on a transfer matrix method. Using experimental data of different lasers, a set of physical DFB laser parameters was determined. We succeeded in describing all the experimental data of different lasers by the same set. The determined parameter set was further applied to study the influence of facet properties on the wavelength shift of DFB lasers. We found a very strong dependence of the wavelength tunability on the end facet phases. The wavelength shift varies by a factor up to three between different end facet phases and coatings. This is crucial for the yield of, for example, tunable multisection DFB lasers with an envisaged large tuning range     

Criterion of external feedback sensitivity in index-coupled andgain-coupled DFB semiconductor lasers to be free from excess intensitynoise

Sensitivities of index-coupled as well as gain-coupled DFB semiconductor lasers to external optical feedback are theoretically investigated in this work. The conditions of getting free operation from excess intensity noise in these two types of DFB lasers are obtained. It is found that the sensitivity of external feedback for these two types of DFB laser depends significantly on the structure such as the internal grating, the facet reflection, and the phase relation between the grating and the facet. The condition of making a DFB laser less sensitive to external optical feedback is to adjust the lasing wavelength to the Bragg wavelength in both index-coupled and gain-coupled structures. A phase-adjusting mechanism such as the λ/4 shift region in the index-coupled structure is effective to avoid the external feedback     

Intensity noise characteristics of erbium-doped distributed-feedback fiber lasers

We present an experimental and theoretical investigation into the low-frequency intensity noise characteristics of erbium-doped distributed feedback (DFB) fiber lasers. The intensity noise characteristics of six nonidentical erbium-doped DFB fiber lasers are presented along with the characteristics of the grating and doped fibers. An analytical model has been used to predict the intensity noise generated in a linear fiber laser and explain the observed noise characteristics. Overall we find good agreement between our analytical model and observations. In particular, we find the intensity noise at frequencies close to the relaxation oscillation frequency significantly elevated due to excess noise from either spontaneous emission or cavity loss modulation. These results can be used to optimize the fiber laser design for sensor applications.     

Transfer-matrix formulation of spontaneous emission noise of DFBsemiconductor lasers

A unified formulation of the spontaneous emission noise in semiconductor DFB (distributed feedback) lasers is presented by using a transfer-matrix approach. Analytical expressions for the noise power per unit frequency bandwidth below threshold and the spontaneous emission rate into the lasing mode are obtained based on the Green's function method. Three DFB laser structures are analyzed: (1) a standard DFB structure with facet reflectivities, (2) a multisection DFB structure composed of n sections which models a phase-shifted DFB laser and a multielectrode (tunable) DFB laser, and (3) a periodic layered DFB structure which models a surface-emitting DFB laser. It is shown that the spontaneous emission noise of a complicated DFB laser structure can be calculated easily by the transfer matrix of each section of the structure and its derivative to frequency     

A complete small-signal equivalent circuit model of cooled butterfly-type 2.5 Gbps DFB laser modules and its application to improve high frequency characteristics

A small-signal equivalent circuit model of 2.5 Gbps DFB laser modules with butterfly-type dual-in-line packages has been proposed and verified using extracted parameters. Parameters related to the equivalent circuit have been extracted from measured S parameters using the modified two-port black box model. This model includes small-signal equivalent circuits of components used for 2.5 Gbps DFB laser modules such as DFB laser, coplanar waveguides, matching resistor, bonding wires, and thermoelectric cooler (TEC). From this equivalent circuit modeling, we show that calculated frequency characteristics of DFB lasers on submount and complete DFB laser modules are similar to their measured frequency characteristics, respectively. Based on this equivalent circuit model, we propose and demonstrate a method that can improve frequency characteristics of 2.5 Gbps DFB laser modules through both experiments and simulations.     

Amplitude-phase decorrelation: a method for reducing intensitynoise in semiconductor lasers

It has been shown that the method of amplitude-phase decorrelation can reduce the fundamental intensity noise floor of semiconductor laser light over a wide bandwidth by the ratio 1/(1+α²), where α is the linewidth enhancement factor. The method uses a dispersive element to convert phase noise into intensity noise. This technique was recently demonstrated by reducing intensity noise from a DFB (distributed feedback) laser as much as 7 dB below its intrinsic level. In the present work, the authors extend these results by characterizing the frequency dependence of the noise reduction. Optimum reduction is achieved in the flat region of the spectrum and diminishes at higher frequencies approaching the relaxation resonance. The correlation properties of the fluctuations are also investigated     

Effects of phase-to-intensity noise conversion by multiplereflections on gigabit-per-second DFB laser transmission systems

Large power penalties and bit-error-rate floors have been observed in some Gb/s systems using distributed feedback (DFB) lasers, which could be attributed to interferometric conversion of laser phase noise to intensity noise by multiple reflections at connectors and splices. The authors calculated the power spectral density of the interferometric noise and its impact on system performance as a function of both the magnitude and number of reflections, and they compare the theoretical predictions with experimental results. Their studies indicate that connectors and splices with return losses of more than about 25 dB are required for the reliable operation of Gb/s fiber transmission systems, even if optical isolators are used     

Impact of laser intensity noise on ASK two-port optical homodyne receivers

The letter describes initial experimental results obtained with a multiport optical homodyne receiver employing a DFB laser. The receiver performance is found to be limited by the intensity noise of the local oscillator rather than by the phase noise, even when the product of the IF linewidth and the bit duration is as large as 0.56. A relative intensity noise level of at least ? 140dB/Hz will be required for a satisfactory receiver performance with ? 15dBm local oscillator power.     

Etude des performances ďun récepteur homodyne à laser local synchronisé pour la détection de signaux optiques modulés en phase

An injection-locked DFB semiconductor laser can be used as a local oscillator in a receiver for phase modulated optical signals. The receiver performances are affected by phase noise of the emitter and local lasers, shot noise and the noise resulting from the local laser locking by a modulated carrier. The performances are shown to be slightly better than those of the phase-locked loop for laser linewidths below one megahertz. A design procedure for the receiver is given.     

Facet phases and sub-threshold spectra of DFB lasers: spectralextraction, features, explanations and verification

The sub-threshold spectra of distributed feedback (DFB) lasers are heavily influenced by the phase of the internal grating with respect to the end facets. In this paper, we document features commonly observed in sub-threshold spectra and explain these features as manifestations of the facet phases. We extract estimates of facet phases by fitting a probability-amplitude transfer-matrix model to spectra from six truncated-well DFB lasers, and use the probability-amplitude model to document, isolate, and explain the sub-threshold spectral dependence on facet phase. To verify the accuracy of the approach that we have taken, we compare estimates of the facet phases from the fits to independent measurements of the facet phases using a scanning photoluminescence method. The results from the two methods are compared and are found to be in agreement. The agreement validates our use of the probability-amplitude model in this paper to explain laser facet phase phenomena     

Static and dynamic properties of InGaAsP-InP distributed feedbacklasers-a detailed comparison between experiment and theory

We have investigated the static and dynamic characteristics of phase shifted InGaAsP-InP DFB lasers mainly focusing on a comprehensive comparison between experimental results and numerical simulations. Experimental data of InGaAsP-InP mushroom type DFB lasers have been recorded, such as optical spectra, variations of the mode wavelengths with continuous and pulsed injection current, side mode suppression ratio, relative intensity noise, small signal amplitude modulation, and the transient response to 10 and 15 Gb/s large signal modulation. The theoretical model calculations in this paper are based on the transfer matrix method in combination with a rate equation analysis and take into account longitudinal mode spatial hole burning which is modified by the inhomogeneous current injection resulting from the axially varying Fermi voltage in both the static and the dynamic case. A good agreement between the experimental data and the theoretical simulations has been obtained extracting a set of parameters which consistently describes the measurements of our devices     

选择区域生长高质量InGaAsP多量子阱材料

采用 L P- MOVPE在 Si O2 掩膜的 In P衬底上实现了高质量的 In Ga As P多量子阱 (MQW)的选择区域生长(SAG) .通过改变生长温度和生长压力 ,MQW的适用范围由 C波段扩展至 L 波段 ,即 MQW的光致发光波长从15 46 nm延展至 16 2 1nm.光致发光 (PL)测试表明 :在宽达 75 nm的波长范围内 ,MQW的质量与非选择生长的MQW质量相当 ,并成功制作出电吸收调制 DFB激光器 (EML) .     

选择区域生长高质量InGaAsP多量子阱材料

采用LP-MOVPE在SiO2掩膜的InP衬底上实现了高质量的InGaAsP多量子阱(MQW)的选择区域生长(SAG).通过改变生长温度和生长压力,MQW的适用范围由C波段扩展至L波段,即MQW的光致发光波长从1546nm延展至1621nm.光致发光（PL）测试表明：在宽达75nm的波长范围内,MQW的质量与非选择生长的MQW质量相当,并成功制作出电吸收调制DFB激光器（EML）.