Photosensitivity and Photodarkening of UV-induced Phosphate Glass Fiber Bragg Gratings and Application in Short Fiber Lasers

Phosphate glass fiber is an ideal choice for building compact high gain fiber lasers, as the fiber can host high concentrations of rare earth ions. However, the low photosensitivity of phosphate glass fibers prevents the integration of UV-written intra-core fiber Bragg gratings (FBGs) into the fiber laser cavity. The research presented in this thesis is focused on the investigation of photosensitivity of UV-written Bragg gratings in phosphate glass fibers and their application in direct-written short monolithic fiber lasers. A systematic investigation of photosensitivity of Er/Yb doped and undoped phosphate glass fibers is carried out using irradiation of intense 193 nm ArF excimer laser light through a phase mask. Thermal annealing experiment results demonstrate that a thermal growth of grating reflectivity of an FBG in phosphate fiber can be obtained upon heating it at temperatures between 100 – 250 °C. A recipe that can reliably obtain strong FBGs with final index modulation amplitudes between 5 and 10×10-5 in phosphate fibers through the photo-thermal process is proposed. An undesirable side effect of FBGs fabrication in a phosphate fiber is discovered that a photodarkening loss of about 1 dB/cm at C-band is also induced by UV irradiation. The photodarkening loss has been identified as the result of formation of a color center at visible band. Photo-bleaching and thermal-bleaching processes are found to be effective in fully erasing the UV-induced photodarkening loss under condition of not diminishing the grating strength. The application of direct-written strong FBGs in building short monolithic phosphate fiber lasers is investigated. A 3.5-cm-long single-wavelength single-longitudinal-mode distributed feedback (DFB) fiber laser has been implemented in the 1%-2% Er/Yb doped phosphate glass fiber. A laser signal with an output power of 27.5 mW at a linewidth of 20 MHz is achieved. In addition, a 5-cm-long monolithic dual-wavelength single-longitudinal-mode distributed Bragg reflector (DBR) all-phosphate fiber laser is also demonstrated. By exploiting the polarization hole burning effect and the spatial hole burning effect, stable narrow-linewidth dual-wavelength lasing emission with 38 pm wavelength spacing and a total emitted power of 2.8 mW is obtained from the DBR fiber laser.