Lyot filter

From Wikipedia, the free encyclopedia

A Lyot filter, named for its inventor Bernard Lyot, is a type of optical filter that uses birefringence to produce a narrow passband of transmitted wavelengths. Lyot filters are often used in astronomy, particularly for solar astronomy.

A Lyot filter is made from one or more birefringent plates (usually quartz), with (in multi-plate filters) each plate being half the thickness of the previous one. Because the plates are birefringent, light travelling through them is split into two rays (the ordinary and extraordinary rays), each experiencing a different refractive index and thus having a different phase velocity.

Only wavelengths at which the ordinary and extraordinary rays have optical path lengths equal to an integer multiple of the wavelength exit the plates in the same polarization state as they entered the plates. If the plates are surrounded by polarizers, this produces a filter with a transmission function with a comb of peaks. Rotating the plates shifts the wavelengths of the transmission peaks, allowing the filter to be tuned.

If electrically tunable birefringent elements are inserted (liquid crystal or electrooptic), then we obtain an "Electrically Tunable Lyot Filter".

The separation and narrowness of the transmission peaks depends on the number, thicknesses, and orientation of the plates.

Single and multi-plate Lyot filters are often used inside the optical cavity of lasers to allow tuning of the laser. In this case, Brewster losses from the plate and other intracavity elements are usually sufficient to produce the polarizing effect and no additional polarizers are required. Although their mechanisms are different, modelocking lasers and Lyot-filter lasers both produce a comb of multiple wavelengths which can be placed on the ITU grid for Dense Wave Division Multiplexing (DWDM) or used to give each suburban home its own return-signal laser wavelength in a passive optical network (PON) used to provide FTTH (Fiber To The Home).