Another relatively simple approach is to set up a free space line in which a glass sheet polarizer is rotated.
In this setup the beam is coupled out by a fiber collimator, sent through a rotating glass sheet polarizer and by a second fiber collimator coupled back into the fiber. The glass sheet polarizer is mounted in a ball-bearing what is driven by a DC motor and a V-belt power train.
This solution on the one hand is very cost effective and provides the feasibility to demonstrate the proper operation of the general sensing scheme. On the other hand drawbacks like huge damping, measured to 10 dB, slow rotation speeds of about 200 Hz, a very critical alignment of the fiber collimators and of course a non-negligible EMI generation because of the DC motor make this rotator only attractive for laboratory experiments.
The figure above illustrates both orthogonal polarization states of the free-space polarization rotator.
A strong damping of one polarization mode can be noticed, whose origin is estimated to a tilt of both collimator lenses respectively to the rotating glass sheet polarizer. Optimizing the alignment leaded to the figure above, showing a significant advancement but also no satisfying solution.
Surveying the rotator preferences with PAX polarization analyzer shows an extensive rotating linear output state and an almost zero Stokes 3 vector as illustrated in Figure 5.10. The non equal magnitude of both polarization modes degrades the performance of the whole system because of the induced non-ideal lighting of both FBG peaks.
Figure 5.11 shows a top view of the free-space polarization rotator constructed for this thesis.
- Polarization rotators – Piezo based polarization rotator
- Polarization rotators – Integrated LiNbO3 based high speed polarization rotator