Gain-Induced Modes in Planar Structures

Modes in cylindrical structures with refractive index boundaries are well known. Their existence and basic properties are generally visualized by superposition of plane waves reflected at the index boundaries. Much less is known about modes in structures with spatially nonuniform gain or attenuation. 1 ' 2 However, it seems intuitively possible that some kind of mode should also exist in that case; indeed, Kogelnik3 showed that a cylindrical structure with a radial gain profile can support a Gaussian beam of constant diameter even if there are no refractive index differences present. Evidently, the nonuniform transparency of the medium counteracts the natural tendency of the beam to spread. In this paper we wall consider a planar structure with stepwise discontinuities of gain or absorption. This geometry is of considerable practical interest. As will be shown in this paper, the lateral confinement of the optical field in a stripe-geometry GaAs laser is due to the gain-loss interface at the edge of the stripe. Furthermore, it is easy to create nonuniform gain distributions in planar structures either by masking and optically pumping or by nonuniform injection current distribution. The modes induced by these gain distributions can be easily influenced from the outside by changing the pumping intensity or injection current. 887