Photosensitivity and luminescence of chromium and bismuth doped fibers

Optical fiber materials with broad-band gain in the visible or in the telecommunication window are of great interest for optical communication or the biomedical domain in order to built integrated tunable lasers, amplifiers, ultra-short pulse lasers, or broadband light sources. Possible activator ions in compact solid-state host materials for the generation of broadband luminescence, gain, and laser emission are transition-metal (TM), bismuth and rare-earth (RE) ions. Emission lines in TM and bismuth exhibit luminescence with typically several hundred of nanometers of spectral bandwidth. This is a much broader range than can be obtained from the emission of RE ions, which is restricted to less than hundred nanometers. Chromium and bismuth-doped fibers are of great interest as they are potential candidates for broadband light sources and tunable lasers. The idea of this work was to investigate the photosensitivity and luminescence properties of Cr and Bi-doped silica optical fibers in collaboration with the Fiber Optic Research Center at the Russian Academy of Sciences (FORC-RAS) who fabricated the fibers and glass samples. The photosensitivity of Cr-and Bi-doped fibers with different dopant concentrations was investigated using an ArF excimer laser emitting at 193 nm. No refractive index change was observed in pristine Cr-doped fibers. After H2-loading, refractive index change up to 2.82×10-3 was obtained in Cr-Ga-doped fiber. The hydrogen loaded annealed fiber showed an index change of 1.4×10-3. The Bi-Al-doped fibers showed permanent photo-induced index changes up to 2.0×10-4 (highest Bi concentration). Using H2-loading index changes up to 2.2×10-3 were measured without reaching saturation. Such index changes are almost comparable to index changes in H2-loaded Ge-silicate fibers and allow fabricating very strong Bragg grating fiber laser reflectors directly in the doped fibers. Stress measurements were performed to distinguish between color center and compaction that contribute to the photo-induced refractive index change. Stress changes in SMF-28 standard telecom optical fibers irradiated by a femto-second laser emitting at 264 nm showed a contribution of compaction of 70% for non hydrogen loaded fibers whereas no stress change was observed in hydrogen loaded fiber for the same photo-induced refractive index. No stress change indicates the absence of compaction leaving color centers solely responsible for the refractive index change. Irradiated annealed H2-loaded Ga-Cr-doped fiber showed no stress change as well. Although generally assumed for H2-loaded fibers, these are the only two examples that prove a color center based photosensitivity responsible for fiber Bragg gratings reported so far. In contrary, in H2-loaded Bi-Al-doped fibers the refractive index change is related to color centers and compaction. A contribution of compaction of about 41 % and 11 %, was evaluated for the pristine and the H2-loaded Bi-Al-doped fiber. Strategies to increase the luminescence, i.e. the number of active centers include e.g. temperature annealing (Cr3+ to Cr4+), changing concentrations of dopants (Bi) and/or the co-dopants (Al, P, Ge), and fiber fabrication conditions. In this work new strategies were followed: UV irradiation (193 nm), hydrogen loading, and H2-loading followed by UV irradiation. Measurements of absorption and luminescence of Bi-doped fibers with different treatments were performed. The absorption was measured from 360 to 1700 nm. The spectrum was decomposed into 8 Bi-related and 1 OH band. The irradiation increased the absorption by ∼100 dB/m and induced two supplementary absorption bands. UV irradiation combined with hydrogen loading induced three additional bands as compared to the pristine fiber. The luminescence was investigated using visible and infra-red pump lasers. The emission takes place mainly from 600 to 900 nm (3 bands) and from 900 to 1300 nm (3 bands). The luminescence shape and intensities of the pristine and irradiated fibers were similar. H2-loading combined with irradiation increased the ratio of NIR to visible fluorescence intensity depending on the pump wavelength. A side luminescence measurement showed 18 and 13 dB luminescence increase of the 1130 nm and 1390 nm bands. This luminescence enhancement might be due to an increase of the number of active bismuth centers. Up to 1.8 dB/m on-off gain was observed in the visible band around 700 nm under 676 nm excitation in the pristine fiber. However, the absorption increases simultaneously. As a result the net-gain was always negative with a trend towards a positive value below 700 nm. High reflective fiber gratings and a narrow band transmission filter were fabricated in a single mode fiber with cut-off around 920 nm. All gratings were used to realize a Bi-all-fiber laser operating at 1179 nm with a reduced linewidth of < 100 pm. Narrow linewidth lasers have a great potential for frequency doubling to the yellow.