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). 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.