An invertible Volterra model of second-order distortion in DFBlasers

A closed-form nonlinear model of the distributed feedback (DFB) laser accounting for all of its distortion mechanisms is presented. The laser's nonlinearity is analyzed by combining Volterra models of the laser's driving circuit and pn heterojunction nonlinearity with rate equation based models of the active region nonlinearities. The prediction of the model is then compared to distortion measurements     

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     

Electrically pumped circular-grating distributed-Bragg-reflectorlasers

The design, fabrication, and lasing characteristics of electrically pumped circular-grating distributed-Bragg-reflector (CG-DBR) surface-emitting lasers, were studied. Surface-emitted lasing action was achieved at room temperature under pulsed condition. Although the measured threshold current is still higher than theoretical calculations, CW operation should be possible by improving the fabrication and the device structure     

Above-threshold analysis of loss-coupled DFB lasers: threshold current and power efficiency

The threshold current and power efficiency of loss-coupled DFB lasers have been calculated numerically using an above-threshold model. Compared with perfectly AR-coated devices, lasers with asymmetric facet coatings have substantially lower threshold current. However, significantly higher front-facet power efficiency is only possible at small grating-coupling coefficients. In particular, the range of loss-coupling coefficients for higher efficiency becomes narrower with larger grating duty cycle and larger index-coupling coefficient. These results can be explained by the interdependence of the reflectivity and extra loss associated with loss-coupled gratings.     

Nonlinear operation of a planar circular-grating DBR laser

An approximate method for the analysis of the nonlinear operation of a planar circular-grating distributed Bragg reflector laser is presented. The analysis is based upon vector-wave self-consistent coupled-mode equations modified to take into account gain saturation effects. With the help of an energy theorem and threshold field approximation, an approximate formula relating small-signal gain to the output power and laser parameters is derived. The laser characteristics obtained reveal behavior of the optimal coupling strength of the Bragg reflector, which provides maximal power efficiency as a function of the laser parameters. It is also shown that the gain saturation effect provides mode selectivity in the laser structure     

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     

Analytical formulas for modulation responses of semiconductor DFBlasers

Analytical expressions for the modulation responses of semiconductor DFB lasers are derived from a rigorous standing-wave rate equation formulations. With proper approximations, these expressions are reduced to simple and insightful formulas, similar to those obtained from the conventional rate equations. Salient features of the DFB lasers, such as complex-couplings, phase-shifts, facet conditions, as well as variations of carrier and photon densities along the cavity caused by the longitudinal hole-burning, are considered in these approximate formulas. It is demonstrated that the modulation responses, predicted by the simple formulas are in excellent agreement with the exact solutions     

Global and local effects in gain-coupled multiple-quantum-well DFBlasers

Global and local effects in gain-coupled multiple-quantum-well distributed feedback lasers are analyzed using a new approach that combines a submicrometer two-dimensional analysis together with a longitudinal transfer-matrix calculation. Issues such as complex coupling, the standing wave effect, hole burning, and barrier effects are examined in some detail. It is also shown that, due to the local interactions, the grating contrast is nonuniform along the cavity, leading to the new phenomenon of differential longitudinal spatial hole burning     

Selection of transverse oscillation modes in tilted ridge DFBlasers

In this paper, the effects of tilt angle on multi-transverse moded ridge-waveguide DFB lasers is presented. A small tilt angle is introduced between the waveguide orientation and the gratings to control the lasing wavelength. The authors find that although each transverse mode experiences a wavelength shift that approximately varies as 1/cos &thetas;, the accompanying variation in κ dictates which transverse mode will oscillate. At small tilt angles the fundamental E _x,0 mode has the lowest threshold gain; at larger angles the higher order transverse modes will be sequentially selected while all other modes are attenuated. As a result, the wavelength versus tilt angle exhibits sudden jumps due to this transverse mode selection in addition to the 1/cos &thetas; dependence. For the analysis, the authors develop a two-dimensional finite-difference method for modeling the multilayered ridge waveguide modes. The effect of tilt angle on the intramode coupling coefficient is derived and the behavior of lasing wavelength, threshold gain and stop-band width are studied     

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     

General nonlinear analysis of second-order oscillators

A general nonlinear analysis of second-order oscillators is presented. The oscillator equation is given in perturbation form, involving a small perturbation parameter, and its periodic solution derived, if any exists, which is calculated using an asymptotic method. Analytical relations between design constraints and circuit parameters are deduced and applied     

Electrically pumped circular-grating surface-emitting DBR laser onInGaAs strained single-quantum-well structure

The authors demonstrate the fabrication and room temperature operation of an electrically pumped circular-grating surface-emitting distributed-Bragg-reflector laser. An InGaAs/GaAs single quantum well (SQW) graded-index separate confinement heterostructure (GRINSCH) structure was grown by one-step molecular beam epitaxy (MBE). Circular gratings were defined by focused ion beam lithography. The lasing wavelength was 942 nm, and the threshold current was 280 mA. This is the first demonstration of these lasers with no epitaxial regrowth     

Above-threshold mode discrimination in external reflector andphase-shifted DFB lasers

The longitudinal mode discrimination above threshold in both distributed feedback lasers with one external reflector and those with phase-shifted grating structures is studied. By numerically solving a pair of coupled mode equations, it is shown that when these types of lasers have nonsaturable index coupling, they lose their mode discrimination at powers only a few times the saturation power of the medium due to the effects of spatial hole burning     

Enhanced AM and FM modulation response of complex coupled DFBlasers

Significant improvements in the small AM bandwidth and FM efficiency of a DFB laser are obtained by introducing a carrier dependent complex coupling coefficient. A simulation using a large-signal dynamic model shows that the 3-dB cutoff frequencies for AM can be increased by factors of three by having the index and gain coefficients in antiphase. In contrast, the efficiency of FM, at low frequencies, can be similarly improved if the index and gain coupling are in phase     

Gigahertz self-pulsation in 1.5 μm wavelength multisection DFBlasers

Self-pulsation in InGaAsP/InP multisection distributed feedback (DFB) lasers was generated reproducibly by adjusting appropriate injection conditions. Frequencies of up to some gigahertz were achieved. It was demonstrated that-in contrast to Fabry-Perot (FP) elements-no selective treatment of one section is required for creating the self-pulsation. It is concluded that the self-pulsation in DFB elements is of a different type than in FP elements     

Time-resolved performance analysis of a second-order PMD compensator

Design, test, and performance requirement and analysis for a polarization-mode-dispersion compensator (PMDC) with four degrees of freedom is presented. The performance is analyzed on the basis of time-integrated and time-resolved bit-error ratio (BER) measurements. Signal impairments are generated by both, first- and higher-order emulators. The probability distributions of bit errors measured over many one second intervals exhibit very long tails. Therefore even a PMDC with a good average BER performance may result in a significant total outage time for a given system.     

Above-threshold parameter extraction and modeling for amorphous silicon thin-film transistors

This paper presents modeling and parameter extraction of the above-threshold characteristics of hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) in both linear and saturation regions of operation. A bias- and geometry-independent definition for field effect mobility considering the ratio of free-to-trapped carriers is introduced, which conveys the properties of the active semiconducting layer. A method for extraction of model parameters such as threshold voltage, effective mobility, band-tail slope, and contact resistance from the measurement results is presented. This not only provides insight to the device properties, which are highly fabrication-dependent, but also enables accurate and reliable TFT circuit simulation. The techniques presented here form the basis for extraction of physical parameters for other TFTs with similar gap properties, such as organic and polymer TFTs.     

Above-threshold leakage in semiconductor lasers: An analyticalphysical model

We present an analytical physical model for above-threshold leakage in semiconductor lasers. The model can be applied to estimate whether heterobarrier lowering and accompanying overbarrier leakage are within reach of having serious deleterious effects on laser performance. The model uses two-dimensional fully self-consistent numerical equations that arise from comprehensive systems of partial coupled differential equations. The effect of temperature and doping on laser efficiency is analyzed for two lasers, one designed for operation at 1.3 μm and the other at 1.55 μm. Both devices are assumed to be built in the InGaAsP-InP material system. We show that, even in a 1.55-μm laser, overbarrier leakage can cause severe performance degradation at typical operating temperatures and doping levels, and we argue that overbarrier leakage deserves to be treated as a potential threat to laser performance at telecommunication wavelengths     

Spectral aspect of degradation in 1.55 μm long-cavity MQW DFBlasers

Degradation behaviors, particularly for the increase in spectral linewidth and FM efficiency, in 1.55 μm long-cavity MQW (multiple quantum well) DFB (distributed feedback) lasers, with a spectral linewidth of around 1 MHz, are presented from the viewpoint of coherent system application. Through the clarification of the behaviors, it is shown that highly coherent lasers are sufficiently durable in coherent transmission systems