SAM – saturable absorber mirror

The absorbance A of the SA consists of two parts:●●saturable absorptionnon-saturable absorption.The ratio between the saturable and the non-saturable part of the absorption dependsmainly on the relaxation time of the excited carriers in the absorbing quantum wells.For a fast absorber with a relaxation time ~ 300 fs, this ratio is about one. It means,that in this case 50% of the absorbance is saturable and the other 50% non-saturable.For absorbers with a relaxation time of about 10 ps the saturable part of the absorptionis about 70%. This part incrises further with increasing saturation time. The saturablepart of the absorption is also known as modulation depth ∆ R.> Relaxation timeThe saturable absorber layer consists of a semiconductor material with a direct bandgap slightly lower than the photon energy. During the absorption electron-hole pairsare created in the film. The relaxation time τ of the carriers has to be a little bit longerthan the pulse duration. In this case the back side of the pulse is still free ofabsorption, but during the whole period between two consecutive pulses the absorberis non saturated and prevents Q-switching.Because the typical relaxation time due to the spontaneous photon emission in a directsemiconductor is about 1 ns, some precautions has to be done to shorten it drastically.Two technologies are used to introduce lattice defects in the absorber layer for fast nonradiativerelaxation of the carriers:●●low-temperature molecular beam epitaxy (LT-MBE)ion implantation.The parameters to adjust the relaxation time in both technologies are the growthtemperature in case of LT-MBE and the ion dose and annealing parameters in case ofion implantation.Typical values of the relaxation time of SAs are between τ = 1 .. 10 ps.SeongKyeong Photonics 27 info@skphotonics.com