In interference filters, light can disobey Snell’s law

While it may be expected that light passing through an optical coating would follow a path dictated by Snell’s Law, this is not always so, say researchers at JDSU (Santa Rosa, CA).

May 1st, 2006

While it may be expected that light passing through an optical coating would follow a path dictated by Snell’s Law, this is not always so, say researchers at JDSU (Santa Rosa, CA). In fact, while Snell’s Law is a good approximation for the path through antireflection coatings, other coatings produce a different path (for light impinging at an angle) as a result of interference in the coating layers, at least according to the researchers’ model.

In one numerical example, a 400 µm wide Gaussian laser beam with a 1550 nm wavelength entered a 50 µm thick, five-cavity narrow-band-pass dense-wavelength-division-multiplexing filter at an angle of 3° to the normal. Constructive interference in the five cavities progressively caused a lateral shift in the beam; at the point where the beam exited the coating, it had laterally shifted by a distance of almost 300 µm from the point it would have exited if it had obeyed Snell’s Law. Such a difference can be of practical importance in optical systems operating with narrow beams. Contact Karen Hendrix at karen.hendrix@jdsu.com.

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