Lyot Filters

Author: the photonics expert Dr. Rüdiger Paschotta

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.

In the simplest configuration, the birefringent axis of each crystal is oriented at 45° to the axis direction of the polarizers, and the beam is perpendicular to the plates. 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 that way, a small transmission bandwidth combined with a large period of the transmission peaks (i.e., a large free spectral range) is possible.

A Lyot-type filter with electrically tunable transmission peaks can be realized by using Pockels cells or electrically controlled liquid crystal modulators instead of the passive birefringent crystals. It is a kind of tunable optical filter.

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.

Fiber-based Lyot Filters

Lyot filters are generally built from bulk-optical elements as described above. However, one can realize optical filters based on the same operation principle with fiber optics. Here, the birefringent crystals are replaced with birefringent (polarization-maintaining) fibers, and combined with some fiber-based polarizers [3, 4]. Tuning can be accomplished with a fiber polarization controller or through temperature changes.

More to Learn

Encyclopedia articles:

• birefringent tuners
• optical filters