|> Energy band gap > GaAs | AlxGa1-xAs | InxGa1-xAs|
|> Refractive index > GaAs | AlAs | AlxGa1-xAs | InxGa1-xAs|
|> Devices > Bragg mirror | SAM | RSAM | SA | SANOS | SOC | Microchip laser | PCA|
SOC - Saturable Output Coupler
|>||Aim of SOC|
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.
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:
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 absorptance A of the SOC consists of two parts:
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|
|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|
|The effective absorption A of a pulse is the result of an averaging over the fluence F(t) of a pulse:|
|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.|
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:
Typical values of the relaxation time of SOCs are between t = 1 .. 10 ps.
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.
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.