Resonant saturable absorber mirror (RSAM)

A resonant saturable absorber mirror is a SESAM whose absorber sits inside a low-finesse cavity that is resonant at the design wavelength, formed between the rear Bragg mirror and the semiconductor top surface or an added partial reflector. The resonance concentrates the field in the absorber, so the same quantum wells produce a far stronger intensity-dependent response than in the standard anti-resonant design.

The trade is stated directly by the structure. An RSAM offers large modulation depth, tens of percent rather than a few percent, and a saturation fluence one to two orders of magnitude lower than an anti-resonant SESAM, but only over the narrow bandwidth of its resonance, typically a few nanometers, and with correspondingly larger group-delay dispersion and temperature sensitivity near the design wavelength. The resonance also shifts with angle of incidence, which a datasheet specifies and a mount design has to respect.

Those properties select the applications. RSAMs are used where a strong nonlinear response at low pulse energy matters more than bandwidth: Q-switching and mode locking of low-gain microchip and waveguide lasers, pulse cleanup and suppression of inter-pulse background at fixed telecom wavelengths, and optical noise discrimination in regenerative links. For broadly tunable or few-cycle oscillators the anti-resonant SESAM remains the right tool, which is why the two entries are best read as a pair.

On the bench the same nonlinear-reflectivity measurement used for a SESAM applies, with two additions: the probe wavelength must be stepped across the resonance to locate its center against the operating wavelength, and the temperature coefficient of the resonance, typically of order 0.1 nm/K for semiconductor cavities, is worth measuring before committing to a fixed operating point.

References: U. Keller, Nature 424, 831 (2003); manufacturer design and application data for resonant absorber mirrors (BATOP GmbH).