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Introduction

These sheet polarizers can be cut into any desired size with a pair of scissors. For use with a typical photodiode, 5x5 mm or even less is large enough. And the piece can be glued directly to the face of the photodiode.

Diagrams showing each type are shown below:

• Linear Polarizer (LP) sheet:

  The polarization axis is parallel to one of the edges and orthogonal to the other. It is symmetric in the sense that the input and output sides are interchangeable subject to the restrictions discussed below with respect to the protective film.

  

  The polarization axis will probably not be labeled (but will be parallel to one of the edges as noted). So if that matters, testing will be required. This can use a linearly polarized laser if its polarization axis is known or a linear camera lens polarizer which should be labeled. Or view the oblique reflection of a light source like the Sun from a diffuse non-metallic surface: The reflection will enhance the polarization of reflected light parallel to the surface.

  Of course the LP can be used at any angle, though +45 or -45 degrees would be most common after 0 or 90 degrees.

• Circular Polarizer (CP) sheet:

  This is functionally a sandwich of a linear polarizer oriented at +45 or -45 degrees with a Quarter WavePlate (QWP) with its optical axes at 0/90 degrees.

  

  For our purposes, the CP sheet can be used in several ways. Here are 2 of them:

  1. As a linear polarizer to combine two orthogonal detected polarization components from an interferometer as described above except that the polarization axis is at plus or minus 45 degrees. The output will be circular polarized but that is irrelevant since the destination is likely a photodiode which is polarization-insensitive.

  2. As a QWP to shift the phase of one selected polarization component by 90 degrees with respect to the other. In this case, the input is the QWP-side and the output is the LP side. The output may go directly to a photodiode.

  These create a so-called Type III Quad-Sin-Cos detector.

There is a thin transparent protective film covering both sides of the LP and CP sheets. The film must be removed on either side where the polarization matters as it is highly birefringent. One side of the sheet has an adhesive backing. The protective film can be left in place on that side if the polarization doesn't matter, as where the beam goes directly to a photodiode. Or in that case it can be removed and the LP or CP sheet can be stuck directly to the photodiode. Or it can be stuck to something that is NOT birefringent like a glass window or microscope cover slip. However, if the protective film is removed from the sticky-side without replacing it with something, dust may collect there.

To confirm if a specific sheet is LP or CP, identify the sticky side and remove the protective film. Then rotate a known LP in front of it or pass the beam from a linearly polarized laser through it. The transmitted intensity will be fairly constant for the CP-side of the CP sheet regardless of relative orientation, but will go to nearly zero for the LP when the polarization axes are orthogonal.

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