SAM – saturable absorber mirror
SAM – saturable absorber mirror
SAM – saturable absorber mirror
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The transmittance of the <strong>saturable</strong> output coupler is mainly governed by the reflectance of the partial reflectorand the absorbance of the <strong>absorber</strong> layer. The number of film pairs in the quarter-wave stack of the AlAs/GaAspartial reflector (Bragg-<strong>mirror</strong>) 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 <strong>mirror</strong> 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:●●<strong>saturable</strong> absorptionnon-<strong>saturable</strong> absorption.Both parts are proportional to the square of the electric field strength of the standing wave at the position of the<strong>absorber</strong> layer in front of the Bragg-<strong>mirror</strong>. 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 <strong>saturable</strong>absorption on the optical pulse fluence candescribed bywithΑ absorptionA 0 small signal absorption (<strong>saturable</strong> 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 <strong>saturable</strong> absorption A in dependencyof the fluence in case of A 0 = 1 % and Φ sat = 0.1 mJ/cm 2 .The non-<strong>saturable</strong> absorption is the biggest part of the non-<strong>saturable</strong> losses. Due to the two-photon absorption itdepends on the power density. A typical ratio of the <strong>saturable</strong> and non-<strong>saturable</strong> absorption is around 1.The modulation depth ∆T of the SOC transmittance is ~ A 0 (<strong>saturable</strong> 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