4-shifted Distributed Feedback Laser Research Articles

1.3 μm InGaAlAs asymmetric corrugation‐pitch‐modulated DFB lasers with high mask margin at 28 Gbit/s

Direct modulation of a 1.3 μm InGaAlAs asymmetric corrugation-pitch-modulated (ACPM) distributed feedback (DFB) laser operated at 28 Gbit/s was experimentally demonstrated. The laser exhibits linear light-current characteristics and a high-frequency response with a small roll-off at low frequency compared with a conventional λ /4-shifted DFB laser. Moreover, it attains a high mask margin (20%) under 28 Gbit/s operation at 55°C. These superior properties are due to a superior ACPM grating that suppresses longitudinal spatial hole-burning.

Enhancement of third-order nonlinear optical susceptibilities in compressively strained quantum wells under the population inversion condition

Third-order optical nonlinear susceptibilities /spl chi//sup (3)/ in compressively strained and nonstrained InGaAs-InGaAsP quantum wells (QW's) under the population inversion condition are discussed. The small effective mass of compressively strained QW's increases the contribution of the carrier density pulsation effect and the carrier heating effect of /spl chi//sup (3)/. The hole burning effect is also increased due to the decrease of the carrier-carrier scattering rate. The calculation including these effects shows an enhancement of factor 3 due to 0.8% compressive strain. The values of /spl chi//sup (3)/ are experimentally estimated from the data of nondegenerate four-wave mixing in /spl lambda//4-shifted distributed feedback lasers. /spl chi//sup (3)/ in 0.8% compressively strained QW's is three times larger than that in nonstrained QW's with the same linear gain.

An accurate rate-equation description for DFB lasers and some interesting solutions

Starting from the coupled-wave equations, we have derived an alternative set of rate equations which are valid for most single-section distributed feedback (DFB) lasers. These rate equations are in many respects more useful than the conventional rate equations and have also been used to derive the influence of spatial hole burning on characteristics such as the chirp, the linewidth, or the harmonic distortion. Numerical results are presented for a DFB laser with both facets cleaved, AR-coated, and for a /spl lambda//4-shifted DFB laser.

Effect of linear gain saturation on small-signal dynamics of MQW DFB lasers

The linear gain saturation effect is shown to be important in determining the dynamics of multiple-quantum-well (MQW) distributed-feedback (DFB) lasers. A more realistic logarithmic dependence of material gain on carrier density is assumed in a comprehensive MQW DFB laser model. It is found through simulation that because of the linear gain saturation, the interplay between modal gain and differential gain leads to an optimal kappa L for maximum small-signal modulation bandwidth in lambda /4-shifted MQW DFB lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Spontaneous emission model of surface-emitting DFB semiconductor lasers

A general spontaneous emission model is developed for surface-emitting (SE) distributed feedback (DFB) semiconductor lasers. The frequency distribution of spontaneous emission noise below lasing threshold and the spontaneous emission rate in lasing operation are formulated by using a transfer matrix method combined with the Green's function method. The effective linewidth enhancement factor is obtained from this model in terms of the elements of the transfer matrix. By way of example, the author applies the formulation to a standard SE DFB laser, and a SE lambda /4-shifted DFB laser with a distributed Bragg reflector (DBR) mirror. In particular, the author analyzes the below-threshold spectrum, the threshold current density, the differential quantum efficiency, and the spectral linewidth of these lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Investigation on the spectral characteristics of DFB lasers with different grating configurations made by electron-beam lithography

The single-mode stability for distributed-feedback (DFB) lasers with various electron-beam-written grating configurations has been investigated theoretically and experimentally, for both as-cleaved and AR-coated lasers. Other laser properties interesting for coherent and multichannel communications systems, such as linewidth and tunability, have also briefly been investigated. Lasers with more sophisticated grating structures, such as an optimized multiple phase-shifted or a corrugation-pitch-modulated grating, did not exhibit performance significantly superior to that of lambda /4-shifted DFB lasers with an appropriate coupling coefficient. Antireflection (AR)-coating of the end facets proved indispensable for obtaining a high yield for lasers with single-mode operation at high output power and for reducing the large chip-to-chip variation seen for the as-cleaved lasers. A theoretical investigation of the effect of end reflections on the stopband and of the problem of determining the coupling coefficient was also made. >

Dependence of high-speed properties on the number of quantum wells in 1.55 mu m InGaAs-InGaAsP MQW lambda /4-shifted DFB lasers

The dependence of intrinsic dynamic properties, such as relaxation oscillation frequency, damping K-factor, and spectral chirping under 10-Gb/s direct modulation, on the number of quantum wells is systematically investigated in 1.55- mu m multiquantum-well lambda /4-shifted distributed-feedback lasers. The theoretical and experimental results indicate that the dependence of the above three factors on the number of quantum wells is clearly explained by the linear gain saturation of the quantum-well lasers. >

Gain-coupled DFB lasers versus index-coupled and phase shifted DFB lasers: a comparison based on spatial hole burning corrected yield

A statistical yield analysis is presented for gain- and index-coupled distributed feedback (DFB) laser structures, allowing a comparison of their single longitudinal mode (SLM) yield capabilities. For the yield calculations, the threshold gain difference and the longitudinal spatial hole burning (SHB) are taken into account. By comparing the experimental and theoretical yield of index-coupled DFB lasers, the significance of SHB for correct yield predictions is illustrated. For the purpose of comparison, yield calculations for various lambda /4-shifted DFB lasers (with low facet reflectivities) are presented. The most emphasis, however, is on partly gain-coupled DFB lasers. Estimations of practical gain coupling coefficient values for gain and for loss gratings are discussed. >

Transfer-matrix analysis of the intensity and phase noise of multisection 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 lambda /4-shifted DFB laser. >

Stabilization of the longitudinal mode against spatial hole burning in lambda /4-shifted DFB lasers by quantum size effect

A novel method for stabilizing the spectral properties of lambda /4-shifted distributed-feedback (DFB) lasers against the longitudinal spatial hole burning by using the quantum size effect in MQW structures is proposed and demonstrated experimentally and theoretically. The effect of the longitudinal spatial hole burning on the spectral behavior is shown to be determined by the alpha parameter, not by the differential index or by the optical confinement in the active layer. The reduction of the alpha parameter is found to be very effective in suppressing the spectral instabilities induced by the longitudinal spatial hole burning. >

Spectral linewidth of DFB lasers including the effects of spatial holeburning and nonuniform current injection

An analytic expression for the linewidth of distributed-feedback (DFB) lasers is derived. Local fluctuations in carrier density are shown to provide a significant contribution to laser linewidth in the presence of spatial holeburning. In a single-electrode DFB laser the effect can be avoided by careful optimization of the kl-product, but for a multielectrode DFB laser the linewidth can also be reduced by adjusting the injected current profile to compensate for spatial holeburning. Numerical results for a three-electrode lambda /4-shifted DFB laser are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

5-Gb/s performance of integrated light source consisting of lambda /4-shifted DFB laser and EA modulator with SI InP BH structure

The fabrication process and characteristics, including 5-Gb/s transmission, of an integrated light source consisting of a lambda /4-shifted distributed feedback laser and an electroabsorption (EA) modulator are discussed. By introducing a semi-insulating (SI) InP on the butt-joint region, both the large electrical isolation resistance (>10 M Omega ) between the laser and the modulator and a high optical coupling efficiency (>80%) between them are achieved. A typical threshold current was 50-70 mA, and single-mode operation at 1.576 mu m was maintained up to 5.5-mW output power. The modulation voltage to swing between 90% transmission and 10% transmission was 6.2-12 V, depending on the modulator length in the range 1000-400 mu m. The 3-dB bandwidth is 7.7 GHz and a linewidth enhancement factor ( alpha ) of 0.9 is estimated from the sideband-to-carrier ratio of the spectra.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Suppression of longitudinal spatial hole-burning effect in lambda /4-shifted DFB lasers by nonuniform current distribution

The mode and the output properties of an asymmetric lambda /4-shifted DFB (distributed-feedback) laser with a distributed injection current density along the cavity are investigated theoretically and experimentally. Coupled wave equations that describe the longitudinal spatial hole-burning phenomenon due to both distributions of a stimulated recombination carrier density and an injection current density are developed. The calculations show that an appropriate distribution of the injection current similar to the intensity profile can be effectively suppress the longitudinal spatial hole-burning effect from the aspect of threshold gai difference Delta alpha /sub th/. A 1.55- mu m InGaAsP/InP asymmetric lambda /4-shifted DFB laser was fabricated with three divided electrodes. Improvement of the linearity of the output characteristics, decrease of threshold current, stability of the single-mode property, and narrowing of the spectral linewidth by injection current and distribution along the cavity were observed. >

Theoretical analysis of tuning properties for a phase-tunable DFB laser

The device consists of a conventional or a lambda /4-shifted DFB (distributed-feedback) laser monolithically integrated with an external waveguide section with separate electrodes for phase control. Tuning curves are presented for threshold current, frequency, linewidth, and output power. The analysis demonstrates a generally higher side mode suppression and larger frequency tuning range but also a larger linewidth for the lambda /4-shifted device compared to the device with a conventional DFB section. >