9.5.1 Gain- and Index-Guided Lasers

Stripe width can be limited in two ways: by restricting the flow of current to a narrow stripe, or by fabricating stripes of material with different refractive indexes in the junction plane. The two can be combined in a single laser.

In the gain-guided laser of Figure 9-12A, insulating regions at the top of the laser chip block current from flowing to either side in a complex double-heterojunction laser. The only path for the current is through a narrow stripe at the middle, which runs the length of the chip between the two cavity mirrors. Thus, recombination of current carriers and a population inversion occur only in the narrow stripe through which the current flows, so only that

zone has gain. Because there is no gain at the sides, those regions do not emit light, even though no physical boundary separates the stripe from the rest of the active layer.

Index-guided lasers add another level of confinement by surrounding the stripe in the active layer with a material of a lower refractive index. As shown in Figure 9-12B, the resulting structure can be complex. In this case, the laser has been etched during fabrication to leave only a narrow stripe or mesa containing the GaAs junction layer that runs the length of the chip. Then n-type GaAlAs was deposited on either side of the stripe and covered with an insulator before depositing a metal contact. The insulator confines current flow through the mesa; the boundary with the GaAlAs confines light in the GaAs active layer the same way a double heterojunction confines light in the central active layer. The design shown in Figure 9-12B is an example of a buried heterostructure laser, in which the light-emitting strips are buried entirely by other materials, except at the light-emitting facets.

Index-guided lasers are used for most diode laser applications because they confine light better, producing better beam quality. Gain-guided lasers are simple to make, and their poorer confinement of light can be an advantage in generating high powers because spreading light over a larger area reduces the chance of optical damage to the emitting surface.