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> Energy band gap > GaAs  Al_{x}Ga_{1x}As  In_{x}Ga_{1x}As  
> Refractive index > GaAs  AlAs  Al_{x}Ga_{1x}As  In_{x}Ga_{1x}As  
> Devices > Bragg mirror  SAM  RSAM  SA  SANOS  SOC  Microchip laser  PCA  
SANOS™  Saturable Noise Suppressor 

>  Contents  
>  How does a SANOS work?  
The active element of a SANOS is a resonant
saturable absorber mirror (RSAM) with
zero reflectance for a low power signal at the resonance wavelength.
The RSAM is a nonlinear optical device, having a low reflectance for week optical
signals like noise and a high reflectance for high power signals like optical pulses. 

A SANOS is mainly characterized by the following
parameters:


>  SANOS applications  
The main applications for SANOS are:


>  Free space SANOS (FSSANOS)  
The free space SANOS is devoted to clean a pulsed optical beam from noise. One possible
application is after a pulse picker to suppress the residual pulses, which has been
passed the picker with a low intensity. An other application is to suppress the amplified spontaneous
emission (ASE) of an optical amplifier. 

The transmittance T of the FSSANOS depends on the peak power density I of the input beam according to the nonlinear reflectance of the RSAM. The output beam intensity I_{out} is related to the input beam intensity I by  
I_{out} = T(I) I  
with  
T(I)  intensity dependent transmittance.  
A typical transmittance curve of a FSSANOS with one RSAM inside shows the figure above.  
>  Fibre coupled SANOS (FCSANOS)  
The fibre coupled SANOS can be used for noise suppression in optical fibre channels.
To reshape an optical signal the passive FCSANOS can be simply insert into a fibre
channel after an EDFA. Due to the working principle of the SANOS this device reshapes
only the amplitude of one wavelength. 

>  Noise suppression ratio x  
The SANOS can be used to increase the signal/niose ratio SNR of optical pulses at the resonance wavelength of the used RSAM by a certain faxtor x > 1 .This means, that the SNR_{out} after the SANOS is larger then the input SNR_{in}, if some conditions are fulfilled. 

In the figure left the SNR improvement factor x is shown in dependency on the input pulse fluence F for two input SNR_{in} ratios 10 (blue curves) and 20 (magenta curves). A further parameter is the low intensity reflectance R(0) of the RSAM. The following statements can be deduced from these model calculations:


In the figure right the SNR improvement factor x is shown for an input pulse fluence F = F_{sat} in dependence on the input signal/noise ratio SNR_{in}. The following statements can be deduced from this figure:


>  Saturation fluence F_{sat}  
The saturation fluence F_{sat} depends on the absorber material and on the RSAM design. In dependency on the finesse of the RSAM the saturation fluence F_{sat} of a SANOS can be smaller than the saturation fluence of the absorber material inside the RSAM. The finesse also limits the bandwidth FWHM of the resonance dip. The figure below shows the resulting effective saturation fluence F_{sat} as a function of the bandwidth full width of half maximum (FWHM). By decreasing the FWHM the effective saturation fluence F_{sat} is also decreased, if the cavity thickness remains constant. On the other hand for a fixed FWHM the effective saturation fluence F_{sat} increases with increasing optical thickness of the RSAM cavity. 

The effective saturation fluence F_{sat} of an RSAM (or SANOS) as a function of the full width at half maximum (FWHM) of the RSAM resonance dip plotted for different RSAM cavity thicknesses.  
>  Relaxation time constant t  
The low temperature grown saturable absorber layer inside the SANOS has a
relaxation time constant t, which can be
varied over a large region from about 100 fs up to 100 ps. A typical value
of the relaxation time t is 1 ps. 
Decrease of the transmittance after saturation with t = 10 ps 

T(t) = T_{max}exp(t/t )]  
with  
T(t)  time dependent transmittace  
T_{max}  saturated transmittance  
t  time  
t  relaxation time constant  
>  Bandwidth  
The spectral bandwidth of the SANOS is gouverned by the used RSAM bandwidth. A compromise is needed between a large bandwidth and a low saturation fluence F_{sat}, because the saturation fluence decreases together with the bandwidth. A typical bandwidth (FWHM) of ~ 20 nm is possible for a SANOS with F>_{sat} = 10 µJ/cm^{2}. The usuable spectral bandwidth Dl around the lowintensity minimum transmittance is by a factor of 5 ... 10 smaller than the FWHM and is therefore only some nanometers.  