Lyot filters (original) (raw)

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

Category: photonic devices

• wavelength tuning elements

  • birefringent tuners
    * Lyot filters

• optical elements

  • optical filters
    * interference filters
    * dichroic mirrors
    * rugate filters
    * etalons
    * Fabry–Pérot interferometers
    * diffraction gratings
    * birefringent tuners
    * acousto-optic tunable filters
    * cold mirrors
    * hot mirrors
    * arrayed waveguide gratings
    * Lyot filters
    * neutral density filters
    * monochromators
    * (more topics)

Related: birefringent tunersoptical filters

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DOI: 10.61835/tkp Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Lyot Filters?

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.

Figure 1: A Lyot filter, consisting of a sequence of birefringent crystals (BC) and polarizers (P).

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.

Figure 2: Transmission function of a Lyot filter containing three quartz plates, with thickness values of 5, 2.5, and 1.25 mm.

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 reflections 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 by 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.

Frequently Asked Questions

What is a Lyot filter?

How does a Lyot filter work?

A birefringent crystal splits light into two polarization components, which experience a relative phase delay that depends on the wavelength. A subsequent polarizer then causes a wavelength-dependent power loss, creating the filtering effect.

What is the purpose of using multiple crystals in a Lyot filter?

By combining multiple crystals where each is half the thickness of the previous one, a filter with a small transmission bandwidth and a large free spectral range can be realized.

Can Lyot filters be tunable?

How does a Lyot filter differ from a birefringent tuner used in lasers?

A birefringent tuner works on a similar principle but typically omits the polarizers. It instead relies on the polarization-dependent Fresnel reflections at the surfaces of tilted birefringent plates to create the filtering effect.

Can Lyot filters be made with optical fibers?

Yes, fiber-based versions can be constructed using polarization-maintaining fibers as the birefringent elements, combined with fiber-optic polarizers.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains three suppliers for Lyot filters. Among them:

⚙ hardware

Lyot (birefringent) filters made of crystal quartz for wavelength tuning of lasers or use as spectral selection devices with standard thicknesses ranging from 0.5 mm to 10 mm and diameter 25.4 mm (1 inch) are available from stock. The transmission range for quartz plates is 200 nm … 2700 nm, and optionally made of MgF2 Lyot filters cover the spectral range 150 nm … 6000 nm. Both uncoated or optionally AR-coated can be provided. Use at Brewster angle avoids Fresnel reflection losses. Customer-specific designs are available on request.

Bibliography

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