Spectral broadening in femtosecond laser written waveguides in chalcogenide glass

Nonlinear spectral broadening to 200 nm, from an initial width of 50 nm, has been demonstrated in gallium lanthanum sulphide glass waveguides from 1540 nm, 200 fs pulses at 30 nJ/pulse. A formation mechanism is presented for these femtosecond laser written waveguides, based on optical characterization and comparisons to previous work. Two different types of waveguide are identified. One has a characteristic long narrow structure and is formed through filamentation caused by self-focusing. The other has a characteristic “teardrop” structure, which is formed by a type IIA photosensitivity mechanism and cumulative heating of glass around a central laser-exposed region.