External cavity laser with reflector in optical wave guide, particularly HDBR laser, with an active element comprising a semiconductor optical amplifying cavity having a low-reflectivity facet (3), for example a Semiconductor Optical Amplifier (SOA) with a facet (30) opposite to said low-reflectivity facet treated with a reflecting coating, or a Fabry-Perot laser, and an external reflector comprising a Bragg grating (70) formed in an optical wave guide (4) near a termination (5), facing said facet, of a segment (5, 6, 7) of said optical wave guide coupled with the facet. The grating has a spatial profile of modulation of the refraction index such that a corresponding optical reflectivity spectrum (A) has an optical bandwidth (W) sufficiently small around a prescribed laser oscillation mode wavelength (c) for the laser to oscillate only on the prescribed mode and not on other oscillation modes even in conditions of high-frequency direct modulation.