Power Coupling from GaAs Injection Lasers into Optical Fibers

This paper describes experiments performed to determine the efficiency of coupling light power from GaAs injection lasers* (A ^ 9000 A) into the cores of 3.2-^m and 3.7-^m single-mode fibers and several sizes of multimode fibers.1 At one end of a fiber the light from a laser was coupled in across a small air gap which separated the laser from the fiber. The power coupling efficiency was computed by extrapolating the lasing light power emitted from a 30-cm length of fiber back to its input tip and comparing it with the total laser power. Permanent coupling of injection lasers and fibers was accomplished by applying epoxy between structures which supported the fiber and the GaAs chip. One other coupling technique was tried. It involved bonding (with epoxy) a fiber onto one of the laser mirrors. However, this technique was abandoned because application of epoxy to the active stripe on a GaAs chip raised the lasing threshold by 30 percent. Only 30 percent of this rise was attributable to the decrease in mirror reflectivity at the epoxy interface. The remaining discrepancy may have been caused by strains introduced to the mirror as the epoxy hardened. The body of this paper is divided into three sections. In Section 2.1 we describe measurements to determine the properties of light beams emanating from double-heterostructure (DH) lasers and also from diffused junction (DJ) lasers of striped geometry. Guiding stripes on DH lasers were formed by oxide masking or proton bombardment. In proton-bombarded DH lasers the current is confined within a stripe by high-resistivity regions produced by proton bombardment at the edges of the stripe.