[Physics] Decoherence of a Coherent Beam

laseropticspolarizationvisible-light

If retarding films (i.e. wave plates) can retard the electric components of light (based on the films thickness) without affecting it's direction, couldn't I use a retarding film of randomly varying thickness to convert a Coherent laser beam into an incoherent laser beam to improve eye safety?

For example, say I have a 632nm laser source. I make a retarding film that is optically flat on one side, but the other side contains nanoscale smooth bumps between 0 and 635nm tall (above the surface) and I pass the laser through it (perhaps a series of these films), wouldn't an incoherent collimated source be generated. Alternatively, in lieu of bumps, I think you could also use a retarder comprised of randomly oriented liquid crystal molecules in a polymer matrix.

Thanks.

Best Answer

Couldn't I use a retarding film of randomly varying thickness to convert a Coherent laser beam into an incoherent laser beam...?

If the film is not changing (in time) you are not changing the coherence properties of beam at all. You can think of putting a slab of something in the way as putting a really bad lens (possibly with no optical power) in the path of the beam. If you pick any two points (either at different points in space if you mean spatial coherence or two points in time if you mean temporal coherence) and you take the light from these two points and combine them, you will still see interference. The only thing you have changed is the phase of the fringes has changed (which means the fringes might shift, but you haven't changed the visibility and therefore the coherence of the fringes).

Now if you could produce random phases that were constantly changing faster than your "detector" is "measuring" the light (e.g. if you are thinking of eye safety then this might mean change fast enough that a single coherent "instant" is too fast for your eye to respond and/or become damaged), then your detector will average over the changes and you effectively have partially coherent or incoherent light.

This is actually done in experiments. For example there are experiments that will shine a laser through a ground glass plate that is rotating [Ref 1], or you can quickly (and precisely) change the phases across the beam on a spatial light modulator [Ref 2] (note they use a micro-mirrored array or DMD like those found in projectors, as normal LCD displays are usually not fast enough to do this).

  1. A. Gatti et al. "Coherent imaging with pseudo-thermal incoherent light." Journal of Modern Optics, 53 (2006). doi:10.1080/09500340500147240 arXiv:quant-ph/0504082

  2. B. Rodenburg et al. "Experimental generation of an optical field with arbitrary spatial coherence properties." JOSA B 31 (2014). doi:10.1364/JOSAB.31.000A51 arXiv:1312.6878

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