Surface coupling adjusts red-laser-diode wavelength
By applying a contradirectional surface-mode-coupling (SMC) technique, researchers at the Institut für Festkörperelektronik, the Technichische Universität Wien (Vienna, Austria), and the Physikalisches Institut of the Universität Stuttgart (Stuttgart, Germany) have achieved single-longitudinal-mode surface emission from a horizontal-cavity red-emitting gallium indium phosphide/aluminum gallium indium phosphide (GaInP/AlGaInP) laser diode. The scientists also demonstrated the capability to adjust wavelength characteristics just by changing the optical thickness of a surface waveguide through hydrogen fluoride etching. In addition, if the Fabry-Pérot mirrors are etched and not cleaved, the SMC technique allows monitoring and adjusting the emission wavelength of laser groups on the chip, providing a fairly straightforward way to create multiwavelength laser-diode arrays.
According to the researchers, the SMC principle is based on a coupling mechanism between the laser mode and the transverse electrically polarized surface mode that exists in a semitransparent metal/dielectric waveguide structure on the top of the laser diode. Phase-matching of the laser and the surface modes is achieved with a surface-relief grating in the top cladding of the waveguide. Either co- or contradirectional coupling is possible, depending on the grating period.—Paula Noaker Powell