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I'm doing an undergraduate research project, part of which requires me to build an extended cavity diode laser (ECDL). The new diode mounts take receive the diode in a 90 degree rotation of the old ones, so the polarization is rotated. My mentor says that we need to find a work around for this, since the direction of the polarization of the laser light will effect the efficiency of the diffraction, but I can't seem to find material that explains this. I've found a paper that mentions the fact that the polarization has more or less feedback depending on whether or not it's parallel or perpendicular to the grating lines, but I can't find any explanation further than that. Can someone explain this effect?

Paper referenced: http://www.moglabs.com/uploads/2/4/2/1/24212474/manual_ecd_rev4.20.pdf section 3.3

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  • $\begingroup$ Since reflectivity depends on polarization, this seems pretty straightforward... $\endgroup$ – Jon Custer Jun 6 at 1:34
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I’d say look info Fresnel’s equations for s and p polarized incident light. The direction of the electric field will affect the reflection

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Well first to the workaround: use a half wave plate.

Yes you will get polarisation dependence same as any non normal reflection at a dialectric interface due to the difference in P wave and S wave reflectance. If the angle is just right at brewsters angle only one polarisation transmits. Gas lasers will typically have their plasma tubes cut at brewsters angle to select the one polarisation state. If your lasers polarisation has rotated with respect to the blazed grating then that is not good. It will change the reflected and transmitted amplitudes at each interface and thus the diffraction efficiencies. If your angle is small then there won't be much polarisation dependence and you should be in the clear. I used to work with blazed fibre Bragg gratings which have a cladding ghost mode when etched at 4 degrees. Polarisation dependence was minimal for standard fibre types.

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