Retroreflectors Lab Facts
Thorlabs Lab Fact: Retroreflectors Alter Polarization State
We present laboratory measurements of the polarization state of a beam retroreflected through a Thorlabs retroreflector. In a polarization-dependent experiment, it's important to understand how the polarization of the input beam is altered during retroreflection. While input beams normal to the base strike each face of the retroreflector at a roughly 55° angle of incidence , the s and p polarization components experience different phase delays and are split differently, depending on the order of surfaces they reflect from. The base of the retroreflector is imagined to be divided into sextants; a beam incident on any one sextant will be retroreflected through the sextant sharing the same vertical angle (see figure to the right). We find that the change in polarization is dependent upon initial polarization of the beam and input sextant.
For our experiment we used the HRS015 stabilized HeNe laser. The beam was retroreflected by a Ø1" N-BK7 prism retroreflector and propagated through a polarizer, after which its power was recorded. We measured beam power with the polarizer oriented horizontally, vertically, or at ±45°. Next, we inserted a quarter-wave plate into the beam path before the polarizer with the fast axis of the λ/4 wave plate aligned horizontally. The power of the beam was recorded with the polarizer set at ±45°. From this set of six measurements, the Stokes parameters were calculated, which yielded the parameters for the electric field polarization ellipse.
The two figures below summarize the measured results for the retroreflected polarization. The lower left figure shows the output beam by sextant for vertical input polarization; the lower right figure shows the output beam by sextant for horizontal input polarization. In both enlarged figures, A and B denote the major and minor axes respectively for the polarization ellipse. Θ is the angle between the major axis and the horizontal. Arrow heads mark the handedness of the polarization. Both Θ and handedness are reported as seen by an observer looking into the retroreflector. These measured results demonstrate that the polarization state of the retroreflected beam is dependent not only on the initial polarization of the incident beam, but also the sextant of the retroreflector that the beam in incident upon. For details on the experimental setup employed and the results summarized here, please click here.
Resources for the Interested Reader
The effects of retroreflectors on polarization state have been investigated via various methods: eigenpolarization states [2 - 4], internal incidence angles using transformations between internal reflections , and analytic geometry . We present experimental results of polarization state changing through retroreflection and compare it to the theory developed in Ref.  though examination of the proper Jones and Rotation matrixes.
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