New answers tagged experimental-technique
The laser may be lasing on multiple modes (multiple longitudinal modes, multiple transverse modes, or both). Each of the individual modes may have a very long coherence length, while the many modes taken together would have a much shorter coherence length. Suppose that one of the modes has, say, 40% of the power, and that four other modes have 15% of the ...
Use an ellipsometer. It's a common tool in semi-conductor processing and thin layer coating specialists for optics. It uses polarized light and measures the relative change in polarization angle with depth. From this you can get index, thickness of a layer stack.
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