H. Hosomatsu

Purely gain‐coupled distributed feedback semiconductor lasers

We propose a new distributed feedback (DFB) laser structure in which almost pure gain coupling can be embodied in principle, without sacrificing low threshold operation. An analysis of coupling coefficients has revealed the condition for canceling index‐coupling component. Utilizing organometallic vapor phase epitaxy, we have fabricated GaAlAs/GaAs ridge waveguide distributed feedback lasers having this structure. Excellent single longitudinal mode oscillation independent of facet reflection has been obtained along with low threshold current. The single‐mode spectrum has exhibited distinctive characters of purely gain‐coupled DFB lasers.

Fabrication and characteristics of gain-coupled distributed feedback semiconductor lasers with a corrugated active layer

A distributed feedback (DFB) semiconductor laser structure in which pure gain coupling can be realized is proposed. The fabrication procedure of this structure makes use of a special feature of organometallic vapor phase epitaxy. Lasers of this structure were fabricated to show the validity of the proposal. Coupling coefficients of DFB lasers were calculated considering the gain-coupling component. Using the results, a DFB laser was designed to obtain pure gain coupling. The parameters of the actual structure observed under a scanning electron microscope showed good agreement with those designed. Device characteristics predicted for purely gain-coupled DFB lasers were achieved. For greater gain coupling and lower threshold current operation, an optimization of the Al composition of the pattern-providing layer and the thickness of the active layer in the structure has been carried out. >

Gain-coupled distributed feedback semiconductor lasers with an absorptive conduction-type inverted grating

A gain-coupled (GC) distributed feedback (DFB) semiconductor laser with an absorptive conduction-type-inverted grating is proposed. Devices based on GaAlAs/GaAs materials are fabricated using two-step OMVPE. By inverting the conduction type of the absorptive region, threshold current is lowered by 10 mA, which is to compensate for the threshold increase due to extra absorption. In addition, nonlinear output property associated with the saturable nature of the absorption is eliminated. An ultralow chirping capability under gain switching high speed modulation and the narrow linewidth nature of this laser are experimentally studied.<<ETX>>

Optimization of grating duty factor in gain-coupled DFB lasers with absorptive grating-analysis and fabrication

Grating duty factor strongly affects the performance of gain-coupled (GC) distributed feedback (DFB) laser diodes with an absorptive grating. Through numerical analysis the authors have found an optimum value in the duty factor for their low threshold operation. The minimum threshold gain achievable at this optimum duty factor is found to be almost independent of the order of the grating. According to this prediction, the authors have fabricated GaAlAs/GaAs GC DFB lasers with a third-order absorptive grating where the grating duty factor has been made close to the theoretical optimum value. In 200- mu m-long devices with both facets as-cleaved, low CW threshold current of 25 mA, external efficiency of 0.5 mW/mA, and SMSR as high as 45 dB have been obtained, which is qualitatively consistent with the analysis. High yield of single mode oscillation seems to be the result of the gain coupling.<<ETX>>

Absorptive-Grating Gain-Coupled Distributed-Feedback MQW Lasers with Low Threshold Current and High Single-Longitudinal-Mode Yield

Although gain-coupled (GC) distributed-feedback (DFB) lasers of the absorptive-grating type have several merits over those of gain-grating type, there have been disadvantages which include threshold increase due to excess absorption, parasitic index coupling, and nonlinear operation. In this paper we describe some new concepts to cope with these problems, namely, grating duty-factor optimization, a conduction-type inverted absorptive grating, and an anti-phase index grating. We have applied these schemes to GaAlAs/GaAs multiple-quantum-well (MQW) GC DFB lasers. Consequently, low threshold current, high slope efficiency, high side-mode suppression railo, narrow linewidth, and high single-mode yield have been achieved concurrently in as-cleaved devices.

Self-suppression effect of longitudinal spatial hole burning in absorptive-grating gain-coupled DFB lasers

Longitudinal spatial hole burning (LSHB) induces degradation of longitudinal-mode stability in distributed-feedback (DFB) lasers. Measurement of frequency modulation characteristics has revealed that, in absorptive-grating gain-coupled DFB lasers, the LSHB diminishes as power increases. This anomalous behavior has been qualitatively explained by a theoretical analysis that took into account the saturable nature of the absorption of the gain-coupled grating. This LSHB suppression effect is advantageous for high-power single-longitudinal-mode operation of DFB lasers.<<ETX>>

Demonstration of a waveguide lens monolithically integrated with a laser diode by compositional disordering of a quantum well

An elliptic waveguide lens that is monolithically integrated with a GaAs/AlGaAs distributed feedback laser diode and a grating output coupler by electron beam lithography, ion implantation, and two-step metalorganic vapor phase epitaxy is described. The waveguide lens is formed by the etching of a patterning layer to ensure that the fabrication process of the waveguide lens is compatible with that of the laser diode. Collimated output light is directly obtained from a semiconductor substrate using this waveguide lens and a grating output coupler.<<ETX>>

GaAlAs/GaAs single quantum well gain-coupled distributed feedback laser

A novel gain-coupled distributed feedback laser with a single quantum well (SQW) active layer is proposed and fabricated. The gain perturbation for the gain coupling is due to the periodically perturbed SQW active layer. The characteristics under continuous-wave (CW) operation are presented, included an excellent single-mode property and a very high yield in the single-mode oscillation. A CW threshold current of 31 mA and a high side-mode suppression ratio of 47 dB were achieved. The high single-mode-oscillation yield near 100% indicated the dominance of the gain-coupled optical feedback in the lasers.<<ETX>>

Gain-coupled DFB laser diode using novel absorptive conduction-type-inverted grating

A gain-coupled DFB laser diode with an absorptive conduction-type-inverted grating has been fabricated. Low threshold current and linear operation are obtained by the conduction-type inversion. Moreover, ultralow chirping and narrow linewidth properties are demonstrated.

Frequency modulation characteristics of absorptive-grating gain-coupled DFB lasers: spatial hole burning self-suppression effect

The spatial hole burning (SHB) in distributed feedback (DFB) lasers limits longitudinal-mode stability and occasionally results in spectral linewidth broadening or multiple-mode oscillation. Introduction of gain coupling is one of effective methods to reduce SHB. However, SHB may still exist in the gain-coupled (GC) DFB laser due to inappropriate coupling strength, parasitic index coupling, and facet reflection in practical devices. This paper describes self-suppression effect of SHB in absorptive-grating GC DFB lasers which is observed through the measurement of frequency modulation (FM) characteristics. This effect is attributed to the photon-density-dependent coupling coefficient in the gain-coupled cavity.<<ETX>>