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SA - Saturable Absorber in transmission

Aim of SA  

The saturable absorber in transmission can be used to realize a mode-locked fiber ring laser.
On other application is the use as mode-locking device in a diode pumped solid-state laser for longer lasing wavelength > 1600 nm, where the preparation of a saturable absorber mirror (SAM) with an AlAs/GaAs Bragg-mirror is too expensive.

SA parameters  

A SA consists of a group of absorbing InGaAs quantum wells on a semiconductor wafer like GaAs, covered on both sides with an antireflection coating.

The most important parameters of a SA are:
  • transmittance
  • saturable absorption
  • relaxation time
  • saturation fluence

The transmittance of the saturable absorber is mainly governed by the absorbance of the quantum well stack. Ideally the reflectance of the device is zero because of the antireflection coating on both sides of the semiconductor chip. It follows from the energy conservation law T + R + A = 1 (T - transmittance, R -reflectance, A - absorbance), that the transmittance is T ~ 1 - A.

Saturable absorption  
  The absorbance A of the SA consists of two parts:
  • saturable absorption
  • non-saturable absorption.

The ratio between the saturable and the non-saturable part of the absorption depends mainly 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 absorption is about 70%. This part incrises further with increasing saturation time. The saturable part of the absorption is also known as modulation depth D R.

Relaxation time  

The saturable absorber layer consists of a semiconductor material with a direct band gap slightly lower than the photon energy. During the absorption electron-hole pairs are created in the film. The relaxation time t of the carriers has to be a little bit longer than the pulse duration. In this case the back side of the pulse is still free of absorption, but during the whole period between two consecutive pulses the absorber is non saturated and prevents Q-switching.

Because the typical relaxation time due to the spontaneous photon emission in a direct semiconductor 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 non-radiative relaxation of the carriers:
  • low-temperature molecular beam epitaxy (LT-MBE)
  • ion implantation.
The parameters to adjust the relaxation time in both technologies are the growth temperature in case of LT-MBE and the ion dose and annealing parameters in case of ion implantation.

Typical values of the relaxation time of SAs are between t = 1 .. 10 ps.