Bragg gratings with efficient and wavelength-selective fiber out-coupling

A new type of out-coupling Bragg grating has been fabricated in a standard depressed-cladding telecommunication fiber (3% Ge). In addition to the usual Bragg reflection, a narrow shorter wavelength-band is coupled out of the core guide. High out-coupling efficiency, more than 50%, and high spectral selectivity, 3 dB bandwidths as small as 0.4 mm, have simultaneously been achieved. The performances of such outcoupling elements are increased because of strong variations of the index change perpendicularly to the fiber axis. These are due to the fact that the gratings are narrower than the fiber core diameter. Based on the same principle, enhancement of the out-coupling efficiency is also obtained for gratings fabricated in fibers having a triangular core index profile.