SAM – saturable absorber mirror
SAM – saturable absorber mirror
SAM – saturable absorber mirror
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The absorbance A of the SA consists of two parts:●●<strong>saturable</strong> absorptionnon-<strong>saturable</strong> absorption.The ratio between the <strong>saturable</strong> and the non-<strong>saturable</strong> part of the absorption dependsmainly on the relaxation time of the excited carriers in the absorbing quantum wells.For a fast <strong>absorber</strong> with a relaxation time ~ 300 fs, this ratio is about one. It means,that in this case 50% of the absorbance is <strong>saturable</strong> and the other 50% non-<strong>saturable</strong>.For <strong>absorber</strong>s with a relaxation time of about 10 ps the <strong>saturable</strong> part of the absorptionis about 70%. This part incrises further with increasing saturation time. The <strong>saturable</strong>part of the absorption is also known as modulation depth ∆ R.> Relaxation timeThe <strong>saturable</strong> <strong>absorber</strong> 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 <strong>absorber</strong>is 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 <strong>absorber</strong> 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