With the growing field of silicon photonics and optical waveguide structures, it is often desirable to be able to use a polarization controller to align the output of an OBR 4600 to the TE or TM mode of the waveguide structure.
Older OBR 4600's used to leave the laser on in between sweeps. The downside to this is that the laser could be left of for significant amounts of time when not being used, and thus the laser's life expectancy would be reached faster than necessary. However, having the laser stay on allowed the user to easily align the OBR to the waveguide while the laser is parked, and then take a measurement sweep. This was not a documented feature or supported use-case, but it did work well.
With the latest revision of laser, the OBR 4600 now turns the laser off in between measurements in order to preserve the laser life. This will significantly increase the usable life of the 4600. However, the downside to this is that users cannot easily align the output of the 4600 to the desired polarization within their DUT.
Luna has figured out a work-around to this, which leaves the laser on in a steady state at a predictable wavelength. Attached to this article are a few slides documenting how to do this, as well as demonstrating using the 4600 to measure the PER of a simple polarization-selective DUT.