The saturable absorber also functions as an integrated photodetector.
Using the gain, gain modulation and saturable absorber segments, three different mode-locking techniques are possible, including active, passive and hybrid.
Passive mode-locking using a saturable absorber produces a shorter pulse width than active mode-locking, since passive gain modulation is a more effective pulse shortening mechanism than active gain modulation.
The upper repetition rate is reached with passive mode-locking techniques and is limited by the saturable absorber and gain recovery time constants.
In passive mode-locking, the device's repetition rate can be electrically tuned by varying the current in the gain region or by varying the reverse bias voltage on the saturable absorber.
The detected output from the saturable absorber serves as the input to phase locking circuitry and the frequency error signal is applied to the saturable absorber bias voltage.
The saturation energy is a measure of the energy necessary to saturate the gain of a gain segment or the absorption of a saturable absorber segment.