B Wavelength of a laser beam emerging from a prism

[Andrew Wright]

Summary:: If you send a laser beam through a prism, can you measure any shift in wavelength at the other side of it?

This sounds like a high school experiment and the concept is simple. I feel the laser should emerge monochromatic and at the same wavelength it went in.

Do you get this result if you set it up in a university lab with accurate frequency detection tech?

 

[Twigg]

No, there won't be a shift of the laser frequency. You may lose some monochromaticity (coherence), depending on the prism.

At a minute level, there is some phase noise from variations in the index of refraction within the prism. It won't shift the wavelength, but it will add some fluctuations to the wavelength. Even in the absence of thermal drifts, there's thermodynamic fluctuations of the temperature within the bulk of the prism, which will cause some loss of coherence of the laser beam. However, these effects aren't really noticeable for a prism. Where you really see it is in an optical fiber.

You could do a demonstration of these phase fluctuations by taking advantage of the birefringence of the fiber with a couple hundred dollars worth of budget. You'd need a single mode fiber, a visual fault indicator or other fiber laser, a fiber collimator, and some polarizing film. If you want to get fancier, throw in a photodiode and an oscilloscope. You can cause a large change in the phase shift of the light in the fiber (and thus its output polarization, after traveling through the birefringent fiber) by bending or heating the fiber. Just don't bend it too far or it'll break and cut you! Think the radius of a Sharpie marker or bigger.