Gratings used for silicon photonic waveguides were demonstrated with outcoupling efficiency greater than 60% in a short grating length of 13 µm at a wavelength of 1550 nm. Due to problems of generating light from silicon, light has to be generated by either growing III-V material, wafer bonding III-V material to silicon, or externally generated light has to be coupled into and out of silicon photonic waveguides. Gratings have been used in semiconductor lasers to obtain single frequency operation. Some applications such as telecommunications require a narrow and single spectral line. And such single frequency lasers are typically distributed Bragg reflector (DBR) lasers or distributed feedback (DFB) lasers. Gratings used for both DBR and DFB lasers in III-V materials are generally hundreds of micrometers long due to a low index contrast at the grating interface, a low grating confinement factor, or a low value of the electric field at the grating interface. If the short gratings demonstrated in silicon photonic waveguides could be obtained in III-V semiconductor waveguides, lasers could incorporate such short gratings to couple light into silicon photonic circuits.
Addition of a thin, low-index liner layer over a surface grating combined with a high-index layer can result in short, efficient couplers for III-V waveguides with performance similar to grating couplers in silicon photonic waveguides. The same low-index liner and high index cover layer can also minimize reflections at transitions between sections of photonic integrated circuits or between a laser region and a DBR or grating coupler region. A detailed design of an enhanced coupling strength (ECS) grating integrated with a 9-QW and 5-QW laser structure emitting at a wavelength near 1550 nm has been presented. Both designs allow outcoupling of greater than 70% of the emitted light in a distance of about 20 µm in a single pass, which is a reduction of outcoupler grating length in such III-V waveguides by a factor of 50 or more compared to conventional grating outcouplers. Moreover, the ECS approach also allows designing a second-order grating that has near 100% reflection with insignificant outcoupling. Such a grating can provide equivalent performance at twice the grating period as a first order DBR grating, allowing the holographic fabrication of low-loss reflective gratings to laser wavelengths below 400 nm from the previous limit of about 800 nm.
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He, Ruo-Hua, "Enhanced Coupling Strength Gratings for Outcouplers in Optical Waveguides" (2018). Electrical Engineering Theses and Dissertations. 13.