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SOC - Saturable Output Coupler

Aim of SOC  

RSAM scheme Using a saturable output coupler (SOC), a self-starting, passively mode-locked diode pumped solid-state laser with a very simple layout can be arranged. A SOC is a combination of the well known saturable absorber mirror (SAM) with an output coupler.
In case of using a SOC instead of a SAM for passive mode-locking the optical pump power can be provided through the end mirror of the laser cavity.
Mode locking produces stable and coherent pulsed lasers by forcing the faces of the modes to maintain constant values relative to one another. These modes then combine coherently. Fundamental mode-locking results in a train of optical pulses with a period of 2L/c, where L is the cavity length and c the speed of the light in free space.
Mode locking occurs when laser losses are modulated at a frequency equal to the reverse of the pulse period c/2L. The SOC is a passive mode locking device without the use of an external drive signal, which spontaneously locks the modes with fast material response time.

SOC parameters  

A SOC consists of a Bragg-mirror on a semiconductor wafer like GaAs, covered by an absorber layer and a more or less sophisticated top film system, determining the saturable loss. The back side of the SOC wafer is antireflection coated.

The most important parameters of a SOC are:
  • transmittance
  • saturable absorption
  • relaxation time
  • saturation fluence
  • reflectance bandwidth.

The transmittance of the saturable output coupler is mainly governed by the reflectance of the partial reflector and the absorbance of the absorber layer. The number of film pairs in the quarter-wave stack of the AlAs/GaAs partial 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. An AlAs/GaAs multilayer stack of 10 film pairs has at the design wavelength a reflectance of ~ 96 % and consequently a transmitttance of ~ 4%.

  The absorptance A of the SOC consists of two parts:
  • saturable absorption
  • non-saturable absorption.

Both parts are proportional to the square of the electric field strength of the standing wave at the position of the absorber layer in front of the Bragg-mirror. Therefore the absorptance of the SOC can be adjusted by changing the field distribution due to the design of the thin film stack.

  The absorption A depends on the pulse fluence FP. If the pulse duration τp is shorter than the relaxation time τ of the absorber material, the time dependent absorption is given by  
  Formula absorption with Formula fluence eq. (1) down
  A(t) time dependent absorption  
  A0 small signal absorption (saturable absorption)  
  I time dependent light intensity (measured in W/m2)  
  F(t) time dependent fluence  
  Fsat saturation fluence  
  The effective absorption A of a pulse is the result of an averaging over the fluence F(t) of a pulse:  
  Formula absoption averaging Formula pulse fluence eq. (2)  
  FP pulse fluence  
  absorption as a function of fluence_1 up
  A typical value of the saturation fluence Fsat is 70 µJ/cm2.
  The non-saturable absorption is the biggest part of the non-saturable losses. Due to the two-photon absorption it depends on the power density. A typical ratio of the saturable and non-saturable absorption is around 1.

The modulation depth DT of the SOC transmittance is ~ A0 (saturable absorption).
  The pulse fluence F can be derived from the mean output power P of the laser as follows:  
  F = P / (T . f . a)  
  P mean output power of the laser  
  T transmittance of the output coupler  
  f repetition rate of the laser  
  a illuminated area on the SOC.  
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 SOCs are between t = 1 .. 10 ps.
Reflectance bandwidth  

The reflectance bandwidth of the SOC has to be larger than the pulse bandwidth. The reflectance bandwidth is determined by the ratio of the refractive indices nH/nL of the layers in the Bragg mirror thin film stack. More about Bragg-mirrors ...

The relative spectral width w = Dl/l of the high reflectance zone of a common semiconductor AlAs/GaAs thin film stack is about 0.1. Therefore the width of the high reflectance zone of an AlAs/GaAs Bragg-mirror with a centre wavelength of 1000 nm is about 100 nm. This results in a minimum pulse duration of about 20 fs.

Wavelength dependency  

An ideal SOC has a constant saturable absorption for all wavelengths of the pulse spectrum. But the absorption of a direct semiconductor increases heavily with increasing photon energy, starting at the gap energy of the semiconductor material. In case of a quantum well structure the absorption increases as a step-like dependency on the photon energy due to the one-dimensional quantisation of free carriers.
In any case the result is an increasing saturable absorption of a SOC with decreasing wavelength (increasing photon energy). Consequently, the reflectance versus wavelength curve of a SOC reveals under non-saturated conditions a decreasing reflectance for shorter wavelengths.

SOC spectral reflectance/transmittance up