Littrow prism

A Littrow prism is an optical component that reflects light almost directly back toward its source while separating it into its constituent wavelengths. It consists of a single prism, typically with a 30 to 60 degree apex angle, mounted with its hypotenuse or back surface silvered or coated to act as a mirror. Light enters through one of the prism faces at Brewster's angle, travels through the prism, reflects off the silvered surface, and exits back through the same face it entered.

The defining feature is the geometry: when light hits the prism at Brewster's angle, the angle of incidence equals the complement of the prism's refractive index. This arrangement produces minimal deviation, meaning the reflected ray travels nearly parallel to the incident ray but spatially displaced. The dispersive power comes from the prism material itself, which refracts different wavelengths at slightly different angles before and after reflection, so red and blue light separate as they pass through.

Littrow prisms appear in spectrometers, wavelength selectors, and optical feedback systems where retroreflection with dispersion is needed. They are common in tunable dye lasers, where the prism acts as a feedback element in the laser cavity; the silvered back surface reflects light back into the gain medium, and rotation of the prism selects which wavelength experiences maximum feedback and oscillates. The prism material, often glass or fused silica, must be transparent across the wavelength range of interest and must have sufficient refractive index to reach Brewster's angle without requiring impractically steep incident angles.

The angle of incidence and exit angle are nearly equal only when the light path is optimized for Brewster's incidence. If the prism is rotated away from this condition, deviation increases and retroreflection degrades. The silvered surface is the weak point; the coating can degrade, scatter light, or absorb wavelengths selectively, so performance depends on coating quality and maintenance. Dust or condensation on the silvered face causes significant signal loss.

The prism is named after the 19th-century inventor and bears his name in optical engineering despite its relatively narrow application. It occupies a niche between the simple retroreflector and the full spectrograph: simpler than a grating spectrometer but more precise than a broadband mirror, and more compact than a rotating dispersing prism not intended for retroreflection.