To achieve high quality measurements several main factors have to be optimized.
- Light source with low noise and no polarization
- Polarization rotator, fast and symetric
- Perfectly birefringent FBGs with huge peak separation and low temperature sensitivity
- Steep edge filters with optimized bandwidth
The used light should be unpolarized and low noised to perform high accuracy measurements, what brings a ASE source into play.
But a more important factor is the polarization rotator. This part has a strong influence on the system´s performance. The rotation speed of polarization directly implies the bandwidth of the sensor and the intensitivity to low-speed environmental influences like temperature, vibration or intensity fluctuations of the light source. Symetric lightning of both FBG peaks influence the used spectral FBG bandwidth and in combination with the edge filter accuracy and sensitivity. An asymetric lighning, an imperfect FBG, a non linear filter curve or a too spectrally non linear light source leads to a deformed sine at photo-detector, what complicates phase measurement and therefore degrades the whole system performance.
Also the free-space polarization rotator shows a view effects that yet are not fully understood. It was recongnized, that the tilt of the glass sheet polarizer has a strong influence on the whole system performance. The polarization rotator of choice would be an integrated optic one, for example from Photline PL-LM series or the twin SLD/ASE methode. The steepness and spectral width of edge filter curve depends strongly on temperature induced of FBG reflection signal, operating temperature range and FBG peak separation.
A huge peak separation at simultaneous low temperature sensitivity is desired. For achieveing high rotation angles materials having large Verdet constant are of interest. For example Terbium doped fibers or planar waveguides made of magneto-optical glass MOS-04/MOS-10, Terbium Gallium garnet (TGG) or Ytrium Ion granet (YIG) are the best choices.
Another method for optimization of SNR by a factor of two lies in the right alignment of fiber main axes respectively to the polaritzaion rotator. This can be avoided by using PM fibers (polarization maintaining fibers) instead of SM fibers (single mode fibers) for the whole setup.
The illustration above shows the best setup which consists of an ASE light source, an integrated optics polarization rotator and an AWG based interrogator unit. Using a sensor array can also reduce the number of needed FBGs from 2N to N-1, where N is the number of embedded sensors. This is possible by using each FBG clearance as sensor.