Definition: optical filter devices based on birefringence, exhibiting a wavelength-dependent transmission

German: Lyot-Filter

A Lyot filter is a kind of optical filter, i.e. an optical device with a wavelength-dependent power transmission. It consists of a sequence of birefringent crystalline plates (e.g. of quartz) and polarizers. The birefringent axis of each crystal is oriented at 45° to the axis direction of the polarizers. The light propagating in a crystal can be considered as containing two different linear polarization components, which experience a different phase delay. The relative phase delay for the two polarization components depends on the wavelength. Therefore, the loss of optical power at the subsequent polarizer is wavelength-dependent.

For a device with a single birefringent crystal, the power transmission versus optical frequency can be described with an approximately sinusoidal oscillation. (Chromatic dispersion causes some deviation from an exact sinusoidal oscillation.) By combining multiple crystals with different thickness, a sharper filter function can be realized. According to the Lyot design (invented by Bernard Lyot), the thickness of each crystal is half the thickness of the previous one (Figure 1). In this way, a small transmission bandwidth combined with a large period of the transmission peaks is possible.

A Lyot-type filter with electrically tunable transmission peaks can be realized by using a Pockels cells instead of the passive birefringent crystals.

For wavelength tuning of lasers, one mostly uses birefringent tuners based on a similar principle, but not containing polarizers, since the losses via the Fresnel reflection for s-polarized light are sufficiently strong.

Bibliography

See also: birefringent tuners, optical filters

Category: photonic devices