In apparatus for wavelength stabilizing and spectrally narrowing an output beam of a diode-laser, a cylindrical lens is arranged to collimate the beam in the fast axis of the diode laser without reducing divergence in the slow axis of the diode-laser. A length of optical fiber is arranged to receive the fast-axis collimated beam from the lens. The optical fiber has a core surrounded by a first cladding, the first cladding being surrounded by a second cladding. The core of the optical fiber has an elongated cross section and includes a wavelength selective Bragg grating. The core functions as a low-mode core in the width direction of the cross section. The length direction of the core is aligned with the fast axis of the diode-laser. The fast-axis collimated beam and the low mode width of the core provides that the Bragg grating only reflects light that propagates parallel the longitudinal axis of the fiber. Light reflected from the grating is fed back to the diode-laser for stabilizing the wavelength and spectrally narrowing the diode-laser output beam.