Axial profile of grating coupled radiation from second-order DFBlasers with phase shifts

Axial profiles of the grating-coupled radiation field emitted in a direction normal to the surface of distributed-feedback (DFB) lasers with phase-shifted second-order corrugation are theoretically analyzed. The profiles are calculated for nonreflecting DFB lasers with the following phase shifts: λ/4 shift, two λ/8 shifts, and two 3λ/8 shifts. It is demonstrated that the radiation field can be controlled by changing the phase shifts. This result suggests a new method for modulating the surface emission of a DFB laser     

Above-Threshold Spectrum of the Radiation Field in Surface-Emitting DFB Lasers

This paper addresses the above-threshold analysis of the amplified spontaneous emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers with a second-order grating. To the best of the authors' knowledge, this paper is the first report in this regard. The analysis takes advantage of the time- and frequency-domain approaches. The essence of this method is obtaining the spectrum of the laser power using the fields in the frequency domain, whereas the above-threshold carrier distribution inside the cavity is obtained by the finite-difference time-domain approach. The use of this approach can be justified by the fact that the wave equations in the time and frequency domains are related by the Fourier transform. Applying this method to the surface-emitting DFB lasers with a second-order grating, the authors will demonstrate that excitation of the radiation field by the interference between the counter-propagating waves inside the cavity provides additional filtering mechanism, which makes the side-mode suppression ratio of the power emitted from the surface different than that from the edge. More importantly, it is shown that the interesting features of a properly designed quarter-wave phase-shifted surface-emitting DFB lasers with a second-order grating can be exploited in the design of transmitters for optical communications     

Analysis of radiation mode effects on oscillating properties of DFBlasers

The effects of radiation mode on the oscillating properties of distributed feedback (DFB) lasers with second-order corrugations are analyzed for designing a new type of DFB laser. A formulation based on the transfer matrix technique is applied to calculating Streifer's ζ-terms added to the coupled-wave equations. These terms represent the effects of radiation and evanescent modes. This formulation greatly simplifies the analysis of distributed resonance along multilayered waveguide structures with arbitrary-shaped second-order corrugations. The effects of vertical resonance are also incorporated into the formulation. Various types of DFB lasers with phase-shifted second-order corrugations are analyzed using this method. It is found that the phase shift and the blaze of the corrugations greatly affect the longitudinal mode selectivity. A new phase-shift DFB laser structure with two complementary blazing regions connected at the shift is proposed. It is demonstrated that this structure has small radiation loss resulting in low-threshold performance despite employing second-order corrugations     

Large-signal dynamic model of the DFB laser

A computer model is proposed to analyze the characteristics of distributed feedback (DFB) lasers. The model is based on time-dependent coupled wave equations, with spontaneous emission taken into account. In order to avoid uncertain phase factors in spontaneous emission, a method of converting field equations to power equations in a matrix format before computation is introduced. The steady-state LI curve and transient response to the pulse excitation are calculated in the λ/4 phase-shifted DFB lasers. The longitudinal variations of the carrier and photon densities as well as of the refractive index are considered in the model     

Extraction of gain parameters for truncated-well gain-coupled DFBlasers

An amplified spontaneous emission transfer matrix model for prediction of the subthreshold spectral output of distributed-feedback (DFB) lasers was developed and fitted to the spectra of truncated-well gain-coupled DFB lasers using a least-squares-fitting algorithm. Modal gains for the high- and low-gain segments of the truncated-well DFB lasers were extracted, and their evolution as a function of injection current was examined. Results explain the tendency for the truncated-well gain coupled DFB lasers to have higher yields of single-frequency lasers and larger sidemode suppression ratios than are expected from simple considerations     

半导体相移分布反馈激光器模式及调制相应特性的研究--牛顿-矢量方法的应用

给出了适于分析DFB激光器稳态特性的数值模型和分析振幅及频率调制响应特性的解析模型.研究了3相移DFB激光器的调制响应特性,并提出了一种能够快速精确得到DFB激光器多个模式解的新方法--矢量牛顿法.该方法将稳定的矢量法与精确的牛顿法结合,保证了求解质量.实践表明该方法非常适合于求解高度非线性方程的多解问题.用此方法,研究了3相移及简单DFB激光器的纵向光子浓度分布,纵模及调制响应特性.结果表明,3相移DFB具有与简单的DFB激光器同样好的调制响应特性,相移的引入在一定程度上抑制了纵向空间烧孔效应,并且有利于DFB激光器的单模输出.     

Static and Dynamic Modeling of Circular Grating-Coupled Distributed Feedback Lasers

A computer model is developed to analyze the steady-state and transient responses of circular grating-coupled distributed feedback (CGC DFB) lasers. The model solved the time-dependent coupled-mode equations and carrier rate equation self-consistently by using a modified time-domain finite-difference method. The nonuniform distributions of carrier concentration and refractive index are taken into calculation. Using the model, the temporal and spatial variations of electric field and carrier concentration of the CGC DFB lasers are examined. It is found that the laser performance would be underestimated if the carrier-induced index change is ignored in the calculation. On the other hand, it is noted that the output power and resonant oscillation frequency as well as damping rate of the lasers could be maximized simultaneously by an optimal design for the grating.     

Amplified-spontaneous-emission spectrum of the radiation field in surface-emitting DFB lasers

In this paper, the authors calculate the amplified-spontaneous-emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers. The response of the laser cavity to the Langevin noise source in the frequency domain is obtained using the newly developed Green's functions for the slowly varying amplitudes of the guided waves. The authors show that the power spectra from the surface and the edge are different, and this discrepancy is due to excitation of the radiation field by the interference between the counter-propagating waves inside the cavity. This feature can be properly exploited in the design of surface-emitting DFB lasers for optical communications.     

Improvement of single-mode gain margin in gain-coupled DFB lasers

Using the Bloch-wave analysis, this paper investigates the effect of the gain grating on the single-mode condition in DFB lasers. Various factors affecting the threshold gain of gain-coupled DFB lasers are analyzed in some detail. It is shown for the first time that unequal section lengths in the gain grating can have a significant effect on the single-mode gain margin of gain-coupled DFB lasers, especially when the linewidth enhancement factor α_M is large, because the long and shortwavelength Bloch waves are in phase and in antiphase with the index grating of DFB lasers, respectively     

半导体相移分布反馈激光器模式及调制相应特性的研究--牛顿-矢量方法的应用

给出了适于分析DFB激光器稳态特性的数值模型和分析振幅及频率调制响应特性的解析模型.研究了3相移DFB激光器的调制响应特性,并提出了一种能够快速精确得到DFB激光器多个模式解的新方法--矢量牛顿法.该方法将稳定的矢量法与精确的牛顿法结合,保证了求解质量.实践表明该方法非常适合于求解高度非线性方程的多解问题.用此方法,研究了3相移及简单DFB激光器的纵向光子浓度分布,纵模及调制响应特性.结果表明,3相移DFB具有与简单的DFB激光器同样好的调制响应特性,相移的引入在一定程度上抑制了纵向空间烧孔效应,并且有利于DFB激光器的单模输出.     

Mode analysis of DFB SE lasers

This paper presents theoretical results on mode characteristics of surface-emitting (SE) lasers utilizing an active second-order grating section. Based on a coupled-mode approach, longitudinal modes and the associated space-harmonic transverse modes are calculated via a numerical technique. From these, the lasing-mode spectrum, near- and far-field patterns of the radiation mode, and the surface-emission power efficiency are obtained. Effects of the substrate reflector and the grating parameters are also investigated. Finally, comparisons are made with conventional, edge-emitting DFB lasers. The results indicate that with a suitable choice of structural parameter values, DFB SE lasers can be made to possess both the spectral discrimination of the conventional DFB lasers and the advantages of SE lasers at the same time and also that the second lowest longitudinal mode may be preferred over the fundamental longitudinal mode for many applications due to its symmetric field distribution     

Static, dynamic, and noise analysis of multisection DFB lasersusing frequency-domain transmission line model

A novel frequency-domain transmission line model for multisection distributed feedback (DFB) lasers is developed. The characteristic impedances of active periodic structures are derived. A multisection DFB laser is described as a transmission line network, with each section represented by a transmission lint segment with a corresponding characteristic impedance. Static, dynamic, and noise analysis of multisection DFB lasers is demonstrated. The reflections at the junctions between sections are evaluated more accurately. The resonant condition of the equivalent transmission line network, instead of cumbersome Wronskian, is used to reformulate the rate equations. The diffusion coefficient of Langevin noise terms of any two different positions is solved for the first time, resulting in a more accurate noise analysis of multisection semiconductor lasers. Analytical expressions of the dynamic responses and noise properties of multisection DFB semiconductor lasers are derived     

Continuous-wave operation of 1.5 ?m distributed-feedback ridge-waveguide lasers

CW operation of 1.5 ?m ridge-waveguide DFB lasers is reported for the first time. The ridge-waveguide DFB laser structure offers the prospect of high modulation speeds due to the absence of parasitic capacitances associated with reverse-biased current-blocking layers. These devices also represent the first report of CW operation of DFB lasers fabricated using the hybrid LPE/MOCVD crystal-growth technique and also of DFB lasers with gratings produced by electron-beam lithography.     

Improved coupled mode analysis of corrugated waveguides and lasers

Corrugated waveguides and lasers in resonant and non-resonant situations are analyzed by an improved coupled mode theory based on a set of the coupled mode equations for guided modes and radiation continuum. The distributed feedback (DFB) coefficient and the radiation loss coefficient are given in closed forms. The formulation can be applicable to arbitrarily shaped gratings and multilayer waveguide structures. The accuracy of the theory is examined by comparing it with Tamir's exact calculation for a nonresonant situation and also with Streifer's one for a DFB structure. Reasonable accuracy is obtained by the proper choice of the unperturbed waveguide parameter. The dependence of the two coefficients on the grating depth, the grating period, the guide layer thickness, and the refractive index difference between core and cladding layers is obtained for all Bragg orders up to the fourth, and for four typical grating shapes, namely, for rectangular, sinusoidal, symmetric triangular, and sawtooth gratings. Both the threshold gain of DFB lasers utilizing higher order Bragg reflection and the output coupling efficiency of grating beam couplers are also calculated for these parameters. A new multilayer structure for controlling the radiation loss is proposed and analyzed. This structure is suitable for the suppression of the radiation loss in DBR reflectors as well as for the improvement of the output coupling efficiency in grating beam couplers.     

Effects of radiation loss on the performance of second-order DFBsemiconductor lasers

The effects of radiation loss on the performance of second-order distributed-feedback (DFB) semiconductor lasers with a symmetrical grating are analyzed systematically. The threshold gains of the two lowest modes, the differential quantum efficiency, and the spectral linewidth are calculated as a function of the complex coupling coefficient. The comparison is also made between a conventional DFB and a quarter-wave phase-shifted DFB laser. It is shown that the effects of radiation loss are different in these two laser types. In addition, a simple approximate expression for threshold gain is derived. By way of example, a DFB laser with a rectangular grating is analyzed. It is shown that the performance of a second-order DFB laser is not sensitive to the grating pitch over a fairly wide range, provided that it is close to the pitch which gives the maximum coupling coefficient     

Selective lasing of InGaAs quantum-wire array on a V-grooved GaAssubstrate by distributed optical feedback

Distributed-feedback (DFB) lasers were fabricated by using strained InGaAs quantum-wire (QWR) arrays on V-grooved GaAs substrates as an active grating. After characterizing the luminescence from the QWRs and parasitic quantum wells (QWLs), a DFB laser cavity incorporating such a QWR array with its emission wavelength matched to the Bragg wavelength was designed and fabricated. The wavelength selectivity of the DFB cavity was found to strongly support the QWR emission, and DFB lasing from QWR gain up to 145 K has been achieved under pulsed current. The emission from the parasitic QWLs was suppressed by the DFB filtering and the loss induced by coupling to radiation modes. The DFB cavity was shown to be essential for obtaining lasing from QWRs on V-grooved substrates     

FM response of InGaAsP buried heterostructure distributed feedbacklasers and their applications in incoherent FSK systems

Measurements are reported of the frequency modulation (FM) response of InGaAsP buried heterostructure distributed feedback (DFB) lasers and the system performance of a 1.7-Gb/s and 622-Mb/s incoherent frequency-shift-keyingn (FSK) system using these lasers. The measured lasers include 1.55-μm conventional DFB lasers and quarter-wave shifted DFB lasers. The thermal dips in the FM response of quarter-wave shifted DFB lasers usually occur at the lower frequencies, compared with that of conventional (250-μm-long) DFB lasers. A receiver sensitivity of -32.5 dBm (-39.5 dBm) for a 1.7-GB/s (622-Mb/s) incoherent frequency-shift-keying (FSK) system was achieved using a quarter-wave shifted DFB laser     

基于DSP的DFB激光器温度控制系统设计

为了提高红外分布反馈式激光器的工作稳定性,减少其受工作温度波动的影响,采用PID控制技术,设计并研制了一种基于DSP的DFB激光器智能温度控制系统,并利用该系统对DFB激光器进行温度测试仿真实验。实验表明,该系统的温度控制精度为±0.1℃,上升时间为19s,达到稳定的时间为70s,超调量为11.3%,温度控制范围为10℃~60℃,优越于基于单片机的激光器温度控制系统,具有一定的应用价值。     

Relaxation oscillation frequency of DFB lasers with gain coupling

Using the spatially-dependent rate equations based on the Green's function analysis, we investigate the dependency of the relaxation oscillation frequency on the complex coupling coefficient and other parameters of gain-coupled DFB lasers by simultaneously considering spatial-hole-burning, gain saturation and gain compression. An explicit expression for the relaxation oscillation frequency for DFB lasers including the longitudinal spatial effects has been obtained. It is found that antiphase gain-coupling significantly enhances the local effective differential gain in the gain-coupled DFB laser and hence increases the relaxation oscillation frequency. We have also shown for the first time that the modal linewidth enhancement factor α_M plays an important role in determining the relaxation oscillation frequency of gain-coupled DFB lasers, especially when the built-in index coupling is weak     

Second-order DFB lasers fabricated by deep UV contact lithography and GSMBE

DFB ridge waveguide lasers at 1.55 mu m with uniform second-order gratings defined by deep UV lithography have been realised for the first time. The lasers have been fabricated using gas source molecular beam epitaxial (GSMBE) heterostructures grown in a two-step process. The characteristics of the DFB lasers (28 mA minimum threshold current, single-mode behaviour at output power in excess of 5 mW for more than 80% of the lasers and very low dispersion (+or-0.6 nm) of the lasing wavelength) demonstrate that deep UV lithography can be successfully used for the fabrication of DFB lasers.<>