N.B. Patel

Refractive index dependence on free carriers for GaAs

This work reports on the influence of the injected free carriers on GaAs refractive index N at 297 °K. The variation of N caused by injected free carriers was theoretically calculated in a more complete way than has been performed earlier. New results were obtained rather than those of a reducing effect on N in a linearlike dependence on the injected free carrier concentration n. They are: (1) linearlike dependence of N on n occurs only beyond a certain value n1 and (2) an increasing effect on N is caused by the injected free carriers for concentrations in the range between n=0 and a certain value of n=nc (nc<n1) . In the range nc<n<n1 a nonlinear decreasing effect was obtained. Effect (2) has not been noticed up to now, so far as the authors know.

Behavior of threshold current and polarization of stimulated emission of GaAs injection lasers under uniaxial stress

The effect of uniaxial pressure perpendicular to the junction on the threshold current of GaAs double-heterostructure lasers and homostructure lasers operated at room temperature was studied. The threshold either first increases with pressure up to a certain critical pressure P o and then decreases, or decreases with pressure from the beginning, depending on whether the laser is operating in a TE or a TM mode with zero pressure. In the first case, the change in the threshold current behavior at P o is accompanied by a change of modes from TE to TM. This behavior is explained by a model, taking into account the splitting of the valence bands of GaAs on application of uniaxial pressure.

Threshold behavior of (GaAl)As‐GaAs lasers at low temperatures

The temperature dependence of the threshold current, differential quantum efficiency, and internal loss have been measured in the temperature range 10–293 °K. The threshold current increases relatively slowly with temperature above 100 °K and is independent of the impurity concentration. Theoretical calculation shows that this behavior is to be expected for a band‐to‐band transition that follows k selection. The threshold behavior at low temperatures (? 80 °K) depends strongly on the type and concentration of the impurity. The relatively fast decrease in threshold below 100 °K shows saturation for an active layer with n‐type impurities or with high‐concentration p‐type impurities. The saturation is attributed to the carrier diffusion length becoming smaller than the active‐layer thickness. The internal differential quantum efficiency is near unity and is independent of temperature. The internal loss, however, decreases with temperature due to reduction in free‐carrier absorption.

A theory on long time delays and internal Q switching in GaAs junction lasers

In this paper we present a theory that explains long time delays and internalQswitching in GaAs junction lasers, using only processes known to occur in these lasers: perturbation of refractive index of the active region by injected carriers, joule heating, and gain guiding.

Variation of spontaneous emission with current in GaAs homostructure and double-heterostructure injection lasers

Partial saturation and change of the slope of the spontaneous emission are observed in CW homostructure GaAs lasers and pulsed homostructure and DH GaAs lasers at the onset of the laser modes. An important new effect that we have observed is a reduction of the spontaneous emission above threshold for homostructure lasers. The results are discussed in comparison with the previously published work and a possible explanation for the difference of behavior between homostructure and DH lasers is proposed.

Behavior of spontaneous emission across threshold in GaAs junction lasers

The spontaneous emission behavior of a cw stripe‐geometry GaAs laser is examined in detail. Contrary to recently published results, a sharp discontinuity of the spontaneous emission growth occurs when the laser reaches threshold. Above threshold, slow increases in the spontaneous emission are observed, coming mostly from the regions where the lasing light intensity is small. Other discontinuity points were, at higher currents, observed corresponding to the onset of higher‐order modes and second‐order mode locking.

Optical coupling of two injection lasers: A new experimental approach to study the gain broadening mechanism

A new type of experiment, the optical coupling of two injection lasers, is presented. This method is used to study the gain broadening mechanism for three different types of laser structures: proton bombarded stripe geometry, V -groove, and channeled substrate planar stripe (CSP) lasers. The gain reduction in the presence of a strong injected light is detected with high sensitivity by a differential measurement. Independent of the laser structure, the homogeneous gain broadening mechanism is by far dominating.

Filaments in semiconductor lasers

Filament profiles in cw stripe‐geometry homostructure GaAs lasers are measured from below to well above threshold through their optical spectra. It is shown that the filament appears well below threshold and its dimensions are not a strong function of injection level and consequently of the optical field intensity. These results are at variance with previously published theories.

Narrow stripe graded barrier single quantum well lasers-threshold current considerations

It is observed that narrow stripe graded barrier single quantum well lasers are relatively high threshold current devices ( > 50 mA), even though made from wafers with extremely low broad area threshold current density ( n = 2 and n = 1 quantum levels. It is shown here that these properties are inherent to narrow stripe gain-guided SQW lasers and are dictated by the nature of the optical gain curve of the quantum well.

Effect of uniaxial pressure on the threshold current of double‐heterostructure GaAs lasers

On application of uniaxial pressure perpendicular to the junction, the threshold current of GaAs double‐heterostructure injection lasers at room temperature is observed to increase to a certain critical pressure P0, and then decrease with further increase in pressure. A flip of the polarization of laser light occurs at P0. This behavior can be explained using a model previously proposed to explain the reduction in threshold of homostructure GaAs lasers upon application of uniaxial pressure.