Hi,
My setup includes a horizontally linearly polarized laser beam that passes through a polarizing beam splitter and then enters a polarimeter in order to check its polarization properties. I've noticed that with some of the polarizing beam splitters I've tested the polarization elipticity value increased after passing through the optical element. This means that the output polarization is now partially elliptic and was slightly modified by the optical element.
Any idea what can cause this behavior?
Thanks,
Assaf
Looks like some of them have rotator properties. They rotate linearly polarized light and combination of linear and rotated will give you elliptic output. It can have either broadband (Fresnel rhomb) or specific wavelength properties (quarter or half wave plates). Some of crystals can effect angle of polarization by applying electric current to them.
There are many kinds of polarizing beamsplitters, and they behave differently. The 'true' beamsplitters use birefringent materials and should not produce elliptically polarized light when used correctly.
However, most polarizers (and especially those in form of cubes) use normal glass and special interference coating (MacNeille cubes) and their properties may vary with wavelength.
Make sure you are using your polarizers in the correct arrangement, orientation and wavelength range.
From practical work, and as Zdenek Svindrych was mentioned, the polarization direction of light is essential, if you use a cubic beam splitter,make sure that the incident beam is perpendicular to one of the two split beams
Hello,
from my point of view, the ellipticity you find at the output can be due to linear retardance induced by the cube : it can be because the glass used is intrinsically a little bit birefringent, or it can be also because your mount creates stress induced birefringence.
Generally when we characterize the beamsplitter cube in our setup, we found that it induces around 10° linear retardance.
I don't know how your cube is maintained, but you can try to low the mechanical pressure the mount creates on it if there is any.
You can also check if there is any rotation effect on the azimuth of the polarization ellipse.
Some times the dielectric coatings used in polarizers are quite complicated with many high and low index layers. It's been my experience with thin film Brewster polarizers that sometimes the coatings themselves induce significant stress-induced birefringence on the substrates - this might be what's happening to you? Coatings on two sides of the optic tend to introduce more birefringence, and I've also recently found that IBS coatings which have been annealed have lower stress-induced birefringence. You should also watch out for etalon effects if you have very parallel sides on a part - this might create an eigenpolarization problem.
To my knowledge, there is no solid correlation between temporal or spatial coherence and the polarization of light (linear, circular or elliptical) because the coherence arises from the nature of stimulated emission whereas the polarization is the selective vector property of light. No matter, the light is originated from spontaneous or stimulated emission. In fact, polarization may be formed during propagation in the resonator or through suitable optical elements.
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