SAM – saturable absorber mirror

The transmittance of the saturable output coupler is mainly governed by the reflectance of the partial reflectorand the absorbance of the absorber layer. The number of film pairs in the quarter-wave stack of the AlAs/GaAspartial reflector (Bragg-mirror) determines the reflectance. It follows from the energy conservation law T + R +A = 1 (T - transmittance, R -reflectance, A - absorbance), that the transmittance is given by T = 1 - R - A.The reflectance of the Bragg mirror increases with increasing number of the high and low index film pairs. AnAlAs/GaAs multilayer stack of 10 film pairs has at the design wavelength a reflectance of ~ 96 % andconsequently a transmitttance of ~ 4%.> Saturable absorptionThe absorbance A of the SOC consists of two parts:●●saturable absorptionnon-saturable absorption.Both parts are proportional to the square of the electric field strength of the standing wave at the position of theabsorber layer in front of the Bragg-mirror. Therefore the absorbance of the SOC can be adjusted by changingthe field distribution due to the design of the thin film stack.The dependency of the saturableabsorption on the optical pulse fluence candescribed bywithΑ absorptionA 0 small signal absorption (saturable absorption)Φ pulse fluence (J/cm 2 )Φ sat saturation fluenceA typical value of the saturation fluence is70 µJ/cm 2 .The figure above shows the saturable absorption A in dependencyof the fluence in case of A 0 = 1 % and Φ sat = 0.1 mJ/cm 2 .The non-saturable absorption is the biggest part of the non-saturable losses. Due to the two-photon absorption itdepends on the power density. A typical ratio of the saturable and non-saturable absorption is around 1.The modulation depth ∆T of the SOC transmittance is ~ A 0 (saturable absorption).The pulse fluence Φ can be derived from the mean output power P of the laser as follows:Φ = P / (T . f . a)withPTfamean output power of the lasertransmittance of the output couplerrepetition rate of the laserilluminated area on the SOC.> Relaxation timeSeongKyeong Photonics 14 info@skphotonics.com