Optical characterization of semiconductor saturable absorbers

Semiconductor saturable absorber mirror (SESAM) devices have become a key component of ultrafast passive mode-locked laser sources. Here we describe in more detail how the key SESAM parameters such as saturation fluence, modulation depth, and nonsaturable losses are measured with a high accuracy. These parameters need to be known and controlled to obtain stable pulse generation for a given laser. A high-precision, wide dynamic range setup is required to measure this nonlinear reflectivity of saturable absorbers. The challenge to measure a low modulation depth and key measures necessary to obtain an accurate calibration are described in detail. The model function for the nonlinear reflectivity is based on a simple two-level travelling wave system. We include spatial beam profiles, nonsaturable losses and higher-order absorption, such as two-photon absorption and other induced absorption. Guidelines to extract the key parameters from the measured data are given.