Semiconductor saturable absorber mirror (SESAM)

A semiconductor saturable absorber mirror is a Bragg mirror with one or a few quantum-well absorber layers grown on top, so its reflectivity rises as the incident pulse fluence rises. Weak light sees the absorber and is attenuated; an intense pulse bleaches the absorber and is reflected nearly in full. Placed as an end mirror in a laser cavity, that intensity-dependent loss favors pulsed over continuous operation, which is why the SESAM has been the standard way to self-start and stabilize passive mode locking in solid-state, fiber, and semiconductor disk lasers since the 1990s.

Four numbers on a SESAM datasheet do almost all the work. Modulation depth is the reflectivity swing between unbleached and bleached states, typically 0.4–3% for solid-state mode locking and up to tens of percent for fiber lasers. Saturation fluence, typically 10–120 µJ/cm², sets how hard the spot on the mirror must be driven; the usual design point is several times the saturation fluence at the absorber. Non-saturable loss is the part that never bleaches and turns into heat. Recovery time, from sub-picosecond to tens of picoseconds depending on growth temperature and ion implantation, sets how fast the shutter closes and bounds the pulse shaping.

On the bench, a SESAM is characterized by measuring nonlinear reflectivity against pulse fluence with a calibrated spot size, then fitting the standard saturation model; getting the spot area wrong is the classic way to misread saturation fluence. The other practical failure modes are damage from operating many times above saturation and Q-switched mode locking when the intracavity pulse energy is too low for the chosen modulation depth.

References: U. Keller et al., IEEE J. Sel. Top. Quantum Electron. 2, 435 (1996); U. Keller, Nature 424, 831 (2003).