Intracavity Fourier-optical transverse mode selection in an AlGaInP broad-area laser

Abstract

A Fourier-optical set-up within the external cavity of a commercially available broad-area laser diode to select certain transverse modes is investigated experimentally. The external cavity consists of a lens and a spatial frequency filter, the latter consisting of a reflecting slit (perpendicular to the active layer) surrounded by a darkened background. This is a Fourier-optical 4f set-up (2f plus reflection). The laser employed first is a partially antireflection coated 0.25 W 655 nm AlGaInP broad area laser. The free running laser shows non-stationary filamentation. Placing the slit directly onto the optical axis gives cw fundamental mode operation and a transverse shift of the spatial filter allows for selective excitation of higher order modes at low pump currents. Next, a highly antireflection coated 1.2 W 811 nm AlGaAs broad area laser is utilized in the 4f set-up. Selective excitation of higher order modes is achieved at high pump currents. Another approach to support fundamental mode operation uses a 2f set-up (1f plus reflection) consisting of a concave mirror at a distance f away from the laser facet. Here, the laser facet with its active region dimensions is the spatial filter. Also, the feasibility of autocatalytic mode coupling for reduction of unstable filamentation of broad-area lasers is examined experimentally.