Narrowing the linewidth of 852nm diode lasers

Abstract

Tunable semiconductor lasers with narrow spectral linewidths at specific wavelengths are of interest for both communication and spectroscopy applications. In communications, narrowlinewidth lasers are necessary to reduce optical dispersion, enabling high-speed data rates. Such lasers can also be used for spectroscopic applications that require tuning to the specific wavelengths of an atomic transition. The optical pumping of a cesium transition at 852nm is of particular interest for applications like optical inertial guidance systems. Both types of applications require lasers that operate with a single optical mode and that exhibit spectral linewidths narrower than 1MHz. Most edge-emitting semiconductor lasers, however, have multiple longitudinal modes spaced closely together. Incorporating a distributed Bragg reflector (DBR) or distributed feedback (DFB) grating selects a single longitudinal mode, which allows the laser to operate in a single spectral mode. DBR and DFB gratings are typically located at the interface between the core and cladding of a laser to provide the necessary feedback for narrow-linewidth performance. However, locating the grating between the core and cladding requires epitaxial regrowth, which is difficult for devices with AlGaAs barriers due to the rapid oxidation of Al-containing compounds. Surface-grating ridge-waveguide DBR lasers have been developed as a method for achieving narrow-linewidth lasers with a single epitaxial-growth step. By incorporating an asymmetric cladding, one can reduce the etch depth and form first-order gratings in the DBR section. 3 These devices exhibit narrow-linewidth operation with a minimum spectral linewidth of 36kHz, as determined by the self-heterodyning measurement technique. We recently reported fabricating narrow-linewidth 852nm asymmetric-cladding ridge-waveguide DBRs with firstorder gratings in the AlGaAs/GaAs material system. Figure 1. Scanning electron micrographs of the (a) top view and (b) cross section of the first-order distributed Bragg reflector (DBR) grating. The schematic diagram shows structure of the asymmetriccladding DBR laser diode.