home products infos about contact terms offices jobs
    > Energy band gap  > GaAs | AlxGa1-xAs | InxGa1-xAs
BATOP GmbH   > Refractive index   > GaAs | AlAs | AlxGa1-xAs | InxGa1-xAs
  > Devices   > Bragg mirror | SAM | RSAM | SA | SANOS | SOC | Microchip laser | PCA

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 reflectance of the RSAM or the transmittance of the SANOS is shown as a function of the pulse fluence F of the signal.
The saturation fluence Fsat has a typical value of 20 µJ/cm2.
In many cases the low intensity reflectance of a RSAM can be above zero as a result of a non-zero angle of incidence, deviation of the pulse wavelength from the RSAM resonance wavelength or also an imperfect RSAM.

down
 
  A SANOS is mainly characterized by the following parameters:
  • the saturation fluence Fsat
  • the relaxation time constant t
  • 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)  
  free space SANOS

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)  
  fiber coupled SANOS

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.

up
 
 
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 SNRout after the SANOS is larger then the input SNRin, if some conditions are fulfilled.

 
  signal improvement factor x as a function of input pulse fluence

In the figure left the SNR improvement factor x is shown in dependency on the input pulse fluence F for two input SNRin 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:
  • The improvement factor x increases with increasing SNRin
  • For an ideal RSAM is x = SNRin for F < Fsat
  • For non-zero low intensity RSAM reflectance the optimum input pulse fluence is about Fsat
  • To get a high improvement factor x the input pulse fluence Fin must be <= Fsat and is therefore attenuated by a factor of about 5
 
 

In the figure right the SNR improvement factor x is shown for an input pulse fluence F = Fsat in dependence on the input signal/noise ratio SNRin.

The following statements can be deduced from this figure:
  • The improvement factor x increases with increasing input SNRin
  • A minimum SNRin of ~ 5 is needed to get a substantial SNR improvement factor x
  • It is very important to use an RSAM with a small low intensity reflectance value R(0). This means also, that the wavelength of the optical pulse has to be adjusted to the RSAM resonance and the input beam aperture must be low because of the angle dependent RSAM resonance wavelength.
signal improvement factor x as a function of input SNR<sub>in</sub> up
 
 
Saturation fluence Fsat  
 

The saturation fluence Fsat depends on the absorber material and on the RSAM design. In dependency on the finesse of the RSAM the saturation fluence Fsat 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 Fsat as a function of the bandwidth full width of half maximum (FWHM). By decreasing the FWHM the effective saturation fluence Fsat is also decreased, if the cavity thickness remains constant. On the other hand for a fixed FWHM the effective saturation fluence Fsat increases with increasing optical thickness of the RSAM cavity.

 
 
  The effective saturation fluence Fsat 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. RSAM saturation fluence as a function if FWHM
 
 
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  
 
 
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 F>sat = 10 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.  
 
BATOP GmbH