Saturable Bragg reflector improves modelocked laser pulse shape
A novel saturable Bragg reflector (SBR) created by scientists at Cornell University (Ithaca, NY) has allowed them to self-start an additive-pulse modelocked (APM) laser without placing restrictions on cavity parameters. An APM laser consists of two strongly coupled cavities, one containing the gain medium and the other a nonlinear element such as a section of single-mode fiber. The laser, a source of subpicosecond pulses that can be used to produce solitons for telecommunications research, is normally started by pumping with an external modelocked laser, resulting in a train of less-than-ideal sawtooth-shaped pulses. The production of symmetric pulses requires enough phase chirp in the system to allow self-starting.
Rather than increase phase chirp by constraining fiber parameters such as length or power density, Clifford R. Pollock and Eric J. Mozdy replaced an end mirror in the gain cavity of an OH-:NaCl APM laser with an SBR consisting of multiple-quantum-well structures bonded to a reflective Bragg stack. The bonded SBR introduces enough phase chirp for self-starting, yet allows additive-pulse modelocking induced by the fiber to be the dominant modelocking mechanism. Pulses produced are symmetric with widths of 200 fs; output power is 300 mW. The laser can be tuned over a range of 13 nm while maintaining its APM properties.