Abstract

Low-cost gain-guided stripe-geometry diode lasers are the first diode lasers to be used on a wide scale due to their relative ease of fabrication. However, these devices are characterized by a large threshold current (70-120 mA for AlGaAs lasers) and a poor optical quality of the beam in the lateral direction. In looking for configurations alleviating these drawbacks, we performed numerical simulations of diode lasers with a cylindrically concave output facet. The radius of curvature lies only in the junction plane. Simulations of diode lasers with a phase-conjugate mirror set close to the laser back facet were also carried out. Low-cost gain-guided stripe-geometry diode lasers are the first diode lasers to be used on a wide scale due to their relative ease of fabrication. However, these devices are characterized by a large threshold current (70-120 mA for AlGaAs lasers) and a poor optical quality of the beam in the lateral direction. In looking for configurations alleviating these drawbacks, we performed numerical simulations of diode lasers with a cylindrically concave output facet. The radius of curvature lies only in the junction plane. Simulations of diode lasers with a phase-conjugate mirror set close to the laser back facet were also carried out. The 1-D model consists in solving the charge- carrier diffusion equation of the active layer, taking into account the current spreading in the p-type cladding layer¹ as well as several recombination mechanisms.² A FFT-based beam-propagation method² is used for the wave propagation, since the aforementioned configurations lead to axially varying lateral-mode and charge-carrier profiles. We found that the curved output facet and the phase-conjugate mirror can reduce the threshold current and output beam lateral divergence. For standard gain-guided diode lasers, the lateral mode width is nearly constant along the axial direction but is larger than the lateral gain width. Therefore, an important fraction of the optical energy propagates in the lossy regions. The threshold-current reductions obtained with both configurations are due to their ability to decrease the lateral mode width over a certain distance in the laser cavity, thus improving the matching between the lateral-mode and charge-carrier density profiles. When the threshold current is decreased, the peak charge-carrier density is lowered, leading to a reduction of the antiguiding effect responsible for the far-field patten spreading of narrow-stripe lasers. The effectiveness of the curved output facet depends strongly on its radius of curvature (see Fig. 1). For a cavity length of 250 fim, the best results are obtained for values of the radius of curvature lying between 150 and 250 fim, depending on the stripe width. The lateral divergence can be reduced by 70% for an 8-fim stripe laser, while a threshold-current reduction of 30 % is obtained with a 3-pm stripe diode laser. In the latter case, the initially twin-lobed far-field pattern becomes multilobed (see Fig. 2), but 45% of the optical energy is contained in a narrow central lobe. The threshold current of narrow-stripe diode lasers can be decreased by 45 % by using a phase-conjugate mirror. WM2 Fig. 1. Threshold current of a diode laser with a curved output facet as a function of the radius of curvature. L is the cavity length. WM2 Fig. 2. Calculated far-field intensity profiles of a 3-yim stripe diode laser with a curved output facet for different radii of curvature.

© 1988 Optical Society of America