Littrow Dispersing Prisms

Littrow prisms are retro-reflecting Brewster prisms. They are arranged to that a beam entering at the Brewster angle falls normal to the flat surface and is reflected back along the same path. They are useful in laser cavities where the wavelength of the return beam can be selected by tilting the prism slightly. In this way the gain of the cavity can be tuned to a specific laser line.