Femtosecond laser-written waveguides in thulium-doped fluoroindate glass for S-band amplification

Channel waveguides were written in a fluoroindate bulk glass containing thulium, with a femtosecond laser operating at 800 nm, 100 fs pulses and repetition rate 1 kHz. Formation of waveguides occurred for average powers from 1 to 8 mW (corresponding to energies of 1 to 8 microJ) and scan velocities in the range 0.1 to 4 mm/s. Passive optical characterization included visual inspection by optical microscope, insertion loss and mode profile. Active characterization was performed by co-propagating pumping in a dual-pump scheme that included a 808 nm and a 1054 nm laser diode for an efficient inversion of the $^3$H$_4$ level which is responsible for stimulated emission in the 1460-1530 nm spectral range. Preliminary tests show a net gain of 2.5 dB at 1487 nm, for straight waveguides 1 cm long. Applications include the fabrication of lossless components working in the S-band region of the optical communication spectrum