The polarization rotator based on four piezo squeezers applying force and hence birefringence to the fiber is one of the most common devices for dynamic polarization control.
According to its construction this device provides low insertion loss, low back-reflection, low activation loss and is independent of wavelength, working equally well for signals ranging from 1280 to 1650 nm. Stress induced birefringence generates four wave plates. The retardation of each plate varies with the pressure of each fiber squeezer. In the used PolaRlTE lll from General Photonics, four piezo electric actuators drive the fiber squeezer for high speed.
In principle, increasing the voltage (increasing the squeezing pressure) of one fiber squeezer (F1 or F3) effectively causes the polarization state to rotate about the S1 axis (refer to Poincare sphere) clockwise while decreasing the voltage causes the point to rotate counter-clockwise.
On the other hand, increasing the voltage in a second fiber squeezer (F2) oriented 45° from the first one cause the polarization state to rotate clockwise about an orthogonal axis (S2 axis) from the first one. While decreasing the voltage rotates the polarization counterclockwise.
It is conceivable that one may generate any polarization state from arbitrary input polarization state by using minimum two such fiber squeezers if the input principle state is not parallel to any of the first and second fiber squeezer directions.
It is possible to operate a device with four piezos inducing birefringence of a limited range in such a way, endless operation (reset fee rotation) is possible. They all make use of a feedback loop to dynamically adjust the fiber squeezers.
Figure 5.5 and Figure 5.6 illustrate the evolution of Stoke‘s parameters respectively the reflected FBG centroid signal according to applied force of bthe piezo squeezer.
Two problems are observable, one when the applied piezo force reaches an saturation point and the other when reaching the zero point. In each case reset free operation is not possible. This problem occurs due to an unpredictable shift of the input polarization state. That means that the input state of polarization randomly rotates due to temperature, stress, etc. dependent influences on single mode feed line.
Although the output state of polarization was monitored by a polarization analyzer and feed back to control the fiber squeezer it was not possible to generate a reset free operation. Locking the fiber squeezer control to Stokes 1, representing linear polarization states generates continuity problems of Stokes 2 and Stokes 3, having strong impact on the shape of FBG reflection signal.
This can be seen on the Figure above where the squeezing force was controlled using Stokes 1 as feedback. Stokes 1 gives an acceptable shine shape, but both other Stokes parameter cut off, what directly influences the FBG centroid signal.
- Polarization rotators – Manuel polarization rotator (free-space)
- Polarization rotators – Integrated LiNbO3 based high speed polarization rotator