Spectral Stabilization of Laser Diodes by External Bragg Resonator

Abstract

Volume Bragg laser (VOBLA) with feedback produced by a reflective photo-thermo-refractive volume Bragg grating is used for narrowing spectral width and angular divergence of laser diodes. High-efficient, single mode and spectral width of 60 pm at power of 800 mW are demonstrated from 35 and 100 µm-stripe laser. The problem of high-brightness, high-efficient semiconductor laser sources with stable narrow emission spectra is important for different kinds of applications. Various constructions of external resonators for laser diodes (LD) were proposed and investigated, but up to now no successful commercial product resulted from those researches. First of all, no diffraction limited single lobe emission was obtained for external cavity semiconductor lasers under high pumping current (1-5). The other reason restraining external cavity applications is an additional number of optical elements, which should be aligned with very high accuracy. Application of novel photo-thermo-refractive (PTR) glass Volume Bragg Laser (VOBLA) concept to semiconductor lasers allows simplifying an external cavity construction and obtaining narrow spectrum and near-diffraction limited divergence laser beam with no losses of original efficiency of LD. A conventional method for selection of a single transverse mode with narrow emission spectrum is increasing of the aspect ratio (length/aperture) of a resonator by placing an aperture to provide a single Fresnel zone on a wavelength selective feedback element. Actually, this diaphragm in geometrical space means limiting of a maximum angle of light propagation in a resonator. Volume Bragg gratings because of their strong angular selectivity provide selection directly in space of wave vectors. In other words, transmitting and reflecting volume Bragg gratings serve as a slit and a diaphragm in angular space, respectively. It is important that both of them provide additional spectral selectivity. The current level of PTR-glass technology enables fabrication of transmitting and reflective volume gratings with angular selectivity better than 0.1 and 10 mrad, respectively in spectral region from 400 to 2700 nm (6, 7). High diffraction efficiency (better than 95%) of PTR-glass Bragg-grating, low intrinsic losses (below 2%) and high optical damage threshold (more than 100 kW/cm 2 ) create opportunity for replacement the space and wavelength selective elements and mirrors in the laser cavity by a single volume Bragg grating.