Abstract

External-cavity schemes have been recognized as efficient ways for controlling the lateral-mode content of the output beam emitted from high-power broad-area (BA) semiconductor lasers.^1,2 The external cavity can act as a mode-selecting device provided that the beam fed back into the BA laser is allowed to be wider than that of the free-running BA laser. Due to the limited lateral dimension of the gain medium, a wider beam is expected to experience higher losses, so that the fundamental lateral mode will be favored, compared to the higher-order lateral modes having larger sizes. We have investigated, both theoretically and experimentally, the lateral-mode behaviour of a BA GaAlAs semiconductor laser (stripe width W = 75 μm, length L = 370 μm) operated with an anamorphic external cavity comprising a collimation lens system (f₁ = 8 mm) and a cylindrical lens (f₂ = 25 mm). This lens is spaced from the collimation lens by the distance d = 30 mm, and the external cavity is bounded by a uniform 30%-reflecting plane mirror M₁. Figure 1 illustrates schematically the external-cavity configuration. For the lateral direction parallel to the junction plane, the size of the beam incident upon the antireflection (AR) coated facet of the BA laser can be adjusted by changing the distance L₂ between the cylindrical lens and the external mirror. The curvature of the cylindrical lens lies along the lateral direction, so that the beam size along the transverse direction, perpendicular to the junction plane, remains unchanged after propagation in the external cavity.

© 1994 IEEE