I have heard that diode lasers won’t work in ring laser gyroscopes because these lasers are not single frequency. If this is true, will a prism or a diffraction grating isolate one of the frequencies? – M
While most diode lasers operate at several frequencies simultaneously, it’s possible to make lasers that function at only one frequency. In fact, such “single mode” diode lasers are extremely stable light sources and the basis for much current research in optical science. For example, the recent observations of Bose condensation in vapors of alkali metal atoms were made with the help of single mode diode lasers.
The phrase “single mode” refers to a single longitudinal wave that travels back and forth through the device while it is operating. This single wave has one frequency and one wavelength. It is selected from other possible waves through the use of interference effects. For the wave to be stable inside the laser cavity (the laser is bounded at each end by a mirror, thus forming an optical cavity), the cavity’s length must be an integer or half integer multiple of the light’s wavelength. While that criterion alone will allow several possible waves to form, coupling a second cavity to this laser cavity further restricts the wave so that only a single wave can operate inside the laser. The diode laser will then have only a single mode of operation and will emit a single frequency of light.