Grating-coupled, surface-emitting DFB lasers are desirable for high-power applications in the form of coupled arrays and, along with vertical-cavity lasers, as individual elements for optical interconnects. The latter have seen a great deal of development as they offer small size, low threshold and extremely large-scale integration possibilities but remain plagued by problems related to heating, series resistance, and require a very demanding layer structure. Grating lasers, while larger in size, also offer the potential for very low threshold operation while using a less demanding layer structure. In addition, by fabricating more complex grating structures, one may control the shape of the beam to a great degree in order to achieve, for example, steered and/or focused output beams. For these reasons, single-element, grating-coupled, surface-emitting DFB lasers offer great potential as sources for optical interconnects and large-scale optoelectronic integrated circuits.
A conventional single-element, grating-coupled, GaAs/GaAlAs surface-emitting DFB laser shown in FIG. 1 consists of a GaAs layer between AlGaAs cladding layers grown over a GaAs substrate. A cap layer of AlGaAs is etched on both ends down to or into the upper cladding layer. A diffraction grating is then etched in the upper cladding layer with a period (groove spacing) .LAMBDA.=.lambda., where .lambda. is the wavelength of light in the cladding layer, to provide a second-order grating that will reflect light back into the active region and diffract light perpendicularly out.
Such a grating-coupled, surface-emitting laser suffers from low efficiency and high-threshold current because the same second-order grating with a groove spacing equal to the wavelength of the light in the laser material serves to diffract light upwardly out from the surface and to also diffract it downwardly into the substrate of the laser as indicated by arrows in FIG. 1. The portion of the second-order diffraction component of light diffracted downwardly into the substrate is absorbed and lost. This loss contributes to the low efficiency of grating-coupled, surface-emitting DFB lasers.
Attempts have been made to reduce these losses by removing the substrate from the laser and forming a partial reflector on the side of the waveguide previously occupied by the substrate to reflect light back up and vertically out. However, this is technologically difficult, and it decreases the mechanical stability of the device. A better solution is to provide a multilayer dielectric reflector between the substrate and the waveguide. However, full exploitation of the potential of single-element, grating-coupled, surface-emitting DFB lasers requires fabrication of multiple grating periods and shapes to optimally provide the functions of feedback, output coupling, and beam shaping to the laser.