1 Department of Photonics Engineering, Technical University of Denmark2 Rector’s office, Administration, Technical University of Denmark3 Department of Micro- and Nanotechnology, Technical University of Denmark
This research project has focused on gratings in optical waveguides. These gratings may be produced by UV photon imprinting in optical fibers or planar technology waveguides. The gratings are optical waveguide equivalents of bulk dielectric mirrors or diffraction grating. For a grating in a waveguide, the response results from the interplay between the reflection from the grating and the guiding properties of the structure. This gives effects not found in bulk gratings. A variation of the waveguide width changes the effective refractive index, which in turn changes the effective grating period. Thus, in waveguides, variations in the guiding properties have influence on the reflection spectrum. This, so called grating chirp, has been investigated mumerically and experimentally. An equivalent chirp of 0.1% of the grating period is realistic for silica waveguides. The grating in a waveguide will not only couple to the backward propagating fundamental mode, but also to cladding modes. Cladding modes are strongly bound, but slightly leaky, higher-order modes in the core-cladding-air index structure. If the waveguide is not surrounded by air, but by a recoating the cladding modes become highly attenuated. In either case the cladding mode coupling gives loss on the short wavelength side of the reflection band. The cladding mode coupling loss is a major problem for the utilization of fiber Bragg gratings in wavelength division multiplexed (WDM) system. In this project, a numerical model for cladding mode coupling has been developed. The model can predict the spectral location and size of coupling, for various fiber designs. By the aid of this modeling tool, a fiber has been optimized to give low cladding-mode losses. The optimized fiber has been produced and the predicted reduction of cladding mode losses confirmed. An elaborated grating model, including the detailed shape of the index modulation, has been developed. This model improves the interpretation of grating growth dynamic, which is of value to both; analysis of the UV imprinting set-ups, and to the investigation of photosensitivity mechanisms in silica. An other important part of the project aimed at perfection of distributed feed back DFB fiber lasers. At the outset of this project, DFB fiber lasers had been demonstrated, now the DFB fiber lasers are in commercial production. One of the problems that had to be overcome was to secure stable single-polarization-mode lasing. Detailed experimental investigation of polarization discrimination in the seemingly cylindrical fiber DFB have been carried out. It has been found that the UV-induced birefringence, in a distributed phaseshift, is capable of dominating over other effects. The results have lead to a fabrication procedure for single polarization mode DFB fiber lasers. New applications of the DFB fiber lasers are being explored and there is an interest for higher output powers. In this project, the power limitations for erbium/ytterbium codoped DFB fiber lasers have been investigated. Pump induced temperature gratdient in the DFB structure has been found to degrade the output power. Selective cooling to compensate the temperature gradient more than doubled the output power. The experiment indicates that ouput power in excess of 10mW, with 80 mW of 980 nm pumping is feasible, in an optimized DFB design.