Roy Lang

External optical feedback effects on semiconductor injection laser properties

Influences on the semiconductor laser properties of external optical feedback, i.e., return of a portion of the laser output from a reflector external to the laser cavity, have been examined. Experimental observations with a single mode laser is presented with analysis based on a compound cavity laser model, which has been found to explain essential features of the experimental results. In particular, it has been demonstrated that a laser with external feedback can be multistable and show hysteresis phenomena, analogous to those of non-linear Fabry-Perot resonator. It has also been shown that the dynamic properties of injection lasers are significantly affected by external feedback, depending on interference conditions between returned light and the field inside the laser diode.

Injection locking properties of a semiconductor laser

Injection locking properties of a semiconductor laser have been analyzed, taking into account the injected carrier density dependent refractive index in the active region. It has been found that this dependence significantly affects the injection locking properties, giving rise to a peculiar asymmetric tuning curve and dynamic instability. The instability originates from the intermode interaction via the modulation in the injected carrier density caused by intensity beat. The detuning dependence of the direct modulation response characteristics inside and outside of the locking range have also been examined.

Conditions for self‐sustained pulsation and bistability in semiconductor lasers

Conditions have been analyzed for the occurrence for self‐sustained pulsations and bistabilities in semiconductor lasers having a saturable absorber region. It is shown that there are three crucial parameters, namely ratios of differential gain and carrier lifetime between amplifying region and absorbing region, and ratio of absorbing magnitude in absorbing region to the cavity loss. Ranges of these three parameters are determined in which bistabilities and self‐sustained pulsations take place.

Suppression of the relaxation oscillation in the modulated output of semiconductor lasers

Practical rates of direct pulse modulation of semiconductor lasers have so far been limited to below several hundred megahertz, owing to serious distortion in the output signal caused by the relaxation oscillation of the light intensity. Based on theoretical analysis of the dynamic properties of lasing under constant injection of external radiation into the cavity modes, light injection is proposed as a method for suppressing the relaxation oscillation. The effectiveness of this method has been confirmed in a series of preliminary experiments which employed GaAs injection lasers both as the modulated laser and as the external source of the injected radiation.

Unstable horizontal transverse modes and their stabilization with a new stripe structure

Kinks in the light output and other anomalous characteristics in stripe-geometry lasers were studied. It was found that these anomalies were caused by unstable horizontal, parallel to the junction, transverse modes. Introduction of a refractive-index guiding by Zn-diffusion stabilized the horizontal mode and removed the kink and other anomalies completely.

Intensity Pulsation Enhancement by Self-Focusing in Semiconductor Injection Lasers

It is shown that self-focusing enhances intensity pulsations in injection lasers with little or no built-in refractive index waveguiding. The effect will promote onset of self-sustaind pulsations, when there is a saturable absorber in the laser cavity. Introduction of a lateral mode stabilizing refractive index waveguide structure reduces the enhancement effect and improves the direct modulation response characteristics, by reducing relaxation oscillations and making sustained pulsations less likely to occur.

InGaAsP InP current confinement mesa substrate buried heterostructure laser diode fabricated by one-step liquid-phase epitaxy

New InGaAsP/InP buried heterostructure laser diodes fabricated by one-step liquid-phase epitaxy are described, in which the InGaAsP active region completely embedded in InP is grown on the top of the mesa stripe formed on the InP substrate while, simultaneously, current confinement structure is automatically formed on both sides of the mesa stripe. These current confinement mesa substrate buried heterostructure laser diodes (CCM-LDs) have current confinement structure which very effectively blocks unwanted leakage current bypassing the light emitting region which has enabled laser operation with a threshold current as low as 20 mA, 70-mW maximum CW output at room temperature, and 125°C maximum CW operation temperature. Life tests over 6000-h CW operation at 70°C and 5 mW/facet have confirmed good reliability of these devices.

Study on decrease in lasing threshold current of GaAs/AlGaAs single quantum‐well lasers through introduction of superlattice waveguide layers

A systematic study for evaluating the effect of introducing short‐period superlattice waveguide layers on the threshold current of single quantum‐well diode lasers is reported. The waveguide layers which consist of short‐period GaAs/AlAs superlattices, in place of AlGaAs alloys, resulted in a decrease by a factor of 4–6 in the lasing threshold current density of GaAs/AlGaAs single quantum‐well diode lasers of separate confinement heterostructure. A time‐resolved photoluminescence study of those laser structures revealed that the threshold current reduction was the result of an improvement of overall nonradiative carrier recombination lifetime of single quantum wells by more than a factor of 10.

Invited: Novel Optical Methods for High Speed Direct Modulation of Semiconductor Lasers

In high speed direct modulation of semiconductor lasers, serious drawbacks arise from relaxation oscillation in the light output. Several possible schemes have been examined to attain the high speed direct pulse modulation. Electronic band reduction in the detection system improves the waveform distortion caused by the relaxation oscillation at modulation rates up to about 400 Mb/sec in practice. Novel optical methods are proposed to overcome the limit of the modulation rates. Feedback of light from a laser into itself with a time delay (optical feedback) and injection of external radiation into laser modes (optical injection) have been found theoretically and experimentally to be quite effective for reducing the waveform distortion. These methods are expected to extend the direct modulation capability of semiconductor lasers.

Saturation behavior of the optical gain in GaAs injection lasers

A recent analysis of the saturation behavior of the optical gain in semiconductor lasers has predicted a strong spectral hole-burning effect. With an improved treatment of the intraband relaxation in the analysis the effect disappears entirely.