Phosphate glass optical fibers for compact fiber lasers

Soft glass optical fibers are of great interest for the scientific community, since they can be used for compact short pulsed optical fiber lasers operating in the near infrared wavelength region. Indeed soft glasses such as phosphate and tellurite can incorporate large amounts of rare earth ions while keeping a high absorption and emission cross sections, without clustering effects. This research work aimed at developing rare earth doped phosphate glass optical fibers doped with Yb3+ and Er3+ ions suitable for short pulsed fiber lasers. The phosphate glasses were prepared by melt quenching technique in a controlled atmosphere furnace at temperature above 1300 °C and then cast on brass molds. The glasses were thermally characterized by differential scanning calorimetry in order to measure the characteristic temperatures. Optical characterization was carried out to measure the refractive index of the prepared glasses. The characteristic temperatures (glass transition temperature and crystallization temperature) and the refractive index of the fabricated glasses are reported in Table I. The core glass was doped with Er3+ and Yb3+ ions (2.5 x 1026 ions/m3) of and the calculated numerical aperture (NA) between core and 1st cladding was 0.11, while between 1st and 2nd cladding was equal to 0.36. An optical fiber could then be fabricated by the prepared glasses by preform drawing. The preforms were fabricated by rod-in-tube technique with the tubes manufactured using the rotational casting technique. Drawing was carried out using an in-house custom optical fiber drawing tower. In-line measurement of fiber diameter and tension allowed obtaining a stable and reproducible optical fiber suitable for the demonstration of amplification and laser action. The optical gain of the so-obtained optical fiber was measured by cladding pumping a 9 cm-long fiber using a 976 nm pump diode. A net gain of 16 dB at 1535 nm was measured. Laser action was also demonstrated (Fig. 1) by an all-fiber arrangement where the active fiber was spliced to a fiber Bragg grating and core-pumped through a wavelength division multiplexer (WDM). While amplification and lasing experiments yielded an assessment of the quality of the prepared optical fiber, further activities will concern the development of a pulsed compact fiber laser for LIDAR sensing.