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SANOS™ - Saturable Noise Suppressor
  >  Contents  
   
>  How does a SANOS work?  
  SANOS transfer function

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 typical non-linear transfer function of a SANOS is shown in the figure left. The transmittance of the SANOS is shown as a function of the peak puls intensity I. The typical effective saturation intensity Isat,eff is ~2 MW/cm2.

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  A SANOS is mainly characterized by the following parameters:
  • the effective saturation fluence Fsat,eff
  • the relaxation time constant t
  • the effective saturation intensity Isat,eff
  • the usuable spectral bandwidth Dl
  • the insertion loss L
  
 
>  SANOS applications up
  The main applications for SANOS are:
  • noise suppression in free space optics, for example after a pulse picker
  • reshaping of fibre guided optical signals
  • opto-optical wavelength conversion.
For these two applications the following devices has been developed: 		 
 
>  Free space SANOS (FS-SANOS)  
  SANOS transfer curve

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 optical beam is twofold reflected inside the FS-SANOS. The first mirror is a nonlinear RSAM. The second mirror is either a common linear high reflectance mirror or a RSAM.

  
 
  The transmittance T of the FS-SANOS depends on the peak power density I of the input beam according to the nonlinear reflectance of the RSAM. The output beam intensity Iout is related to the input beam intensity I by down
 
  Iout = T(I) I    
  with  
  T(I) intensity dependent transmittance.  
 
  A typical transmittance curve of a FS-SANOS with one RSAM inside shows the figure above.  
 
 
>  Fibre coupled SANOS (FC-SANOS)  
  SANOS transfer curve

The fibre coupled SANOS can be used for noise suppression in optical fibre channels. To reshape an optical signal the passive FC-SANOS 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.

The active device inside the FC-SANOS is a RSAM, mounted on a circulator.

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>  Effective saturation fluence Fsat,eff  
 

The effective saturation fluence Fsat,eff of a SANOS can be defined in such a way, that the transmittance T at Fsat,eff is 50% of the saturated value at a very large fluence F >> Fsat.
Corresponding to the finesse of the RSAM Fsat,eff of a SANOS is smaller than the saturation fluence Fsat of the absorber material inside the RSAM. The finesse also limits the bandwidth FWHM of the resonance dip at the operation wavelength. The figure below shows the relation between the effective saturation fluence Fsat,eff as a function of the bandwidth full width of half maximum (FWHM). For decreasing the FWHM the effective saturation fluence Fsat,eff is also decreasing if the cavity thickness remains constant. On the other hand for a fixed FWHM the effective saturation fluence Fsat,eff is increasing if the optical thickness of the RSAM cavity is increased.

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  The effective saturation fluence Fsat,eff as a function of the full width at half maximum (FWHM) of the RSAM resonance dip plotted for different RSAM cavity thicknesses. Scaling factor V
 
 
>  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.

The relaxation of the carriers and the change of the transmittance T(t) after the saturation can be described as

 Decrease of the transmittance after
saturation with t = 10 ps
Transmittance relaxation 		up
 
  T(t) = Tmaxexp(-t/t )]  
 
  with    
 
  T(t) time dependent transmittace  
  Tmax saturated transmittance  
  t time  
  t relaxation time constant  
 
 
>  Effective saturation intensity Isat,eff  
  The effective saturation intensity Isat,eff is related to the effective saturation fluence Fsat,eff by
Fsat,eff = Isat,eff t.
With Fsat,eff = 7 mJ/cm2 and t = 10 ps the effective saturation intensity is Isat,eff = 700 kW/cm2. 		 
 
 
>  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 Fsat, because the saturation fluence decreases together with the bandwidth. A typical bandwidth (FWHM) of ~ 20 nm is possible for a SANOS with Fsat = 5 µJ/cm2. The usuable spectral bandwidth Dl around the low-intensity minimum transmittance is by a factor of 5 ... 10 smaller than the FWHM and is therefore only some nanometers.  
 
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