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Optical Filters

Definition: devices with a wavelength-dependent transmission or reflectance

More specific terms: interference filters, dichroic mirrors, rugate filters, etalons, Fabry–Pérot interferometers, diffraction gratings, birefringent tuners, acousto-optic tunable filters, cold mirrors, hot mirrors

German: optische Filter

Category: photonic devices

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URL: https://www.rp-photonics.com/optical_filters.html

An optical filter is usually meant to be a component with a wavelength-dependent (actually frequency-dependent) transmittance or reflectance, although there are also filters where the dependence is on polarization or spatial distribution, or some uniform level of attenuation is provided. Filters with particularly weak wavelength dependence of the transmittance are called neutral density filters.

Types of Optical Filters

There are many different types of optical filters, based on different physical principles:

Absorption Filters

Absorbing glass filters, dye filters, and color filters are based on intrinsic or extrinsic wavelength-dependent absorption in some material such as e.g. a glass, a polymer material or a semiconductor. For example, one may exploit the intrinsic short-wavelength absorption of a semiconductor, or extrinsic absorption caused by certain ionic impurities or by semiconductor nanoparticles in a glass. As the absorbed light is converted into heat, such filters are usually not suitable for high-power optical radiation.

Interference Filters

Various kinds of optical filters are based on interference effects, combined with wavelength-dependent phase shifts during propagation. Such filters – called interference filters – exhibit wavelength-dependent reflection and transmission, and the light which is filtered out can be sent to some beam dump, which can tolerate high optical powers.

An important class of interference-based filters contains dielectric coatings. Such coatings are used in dielectric mirrors (including dichroic mirrors), but also in thin-film polarizers, and in polarizing and non-polarizing beam splitters. Via thin-film design it is possible to realize edge filters, low-pass, high-pass and band-pass filters, notch filters, etc.

The same physical principle is used in fiber Bragg gratings and other optical Bragg gratings such as volume Bragg gratings.

Apart from step-index structures, there are also gradient-index filters, called rugate filters. That approach allows one to make high-quality notch filters, for example.

Fabry–Pérot interferometers, etalons and arrayed waveguide gratings are also based on interference effects, but sometimes exploiting substantially larger path length differences than monolithic devices. Therefore, they can have sharper spectral features.

Lyot Filters

Lyot filters are based on wavelength-dependent polarization changes. Similar devices are used as birefringent tuners in tunable lasers.

Refractive and Diffractive Filters

Other filters are based on wavelength-dependent refraction in prisms (or prism pairs) or on wavelength-dependent diffraction at gratings, combined with an aperture.

Acousto-optic Filters

There are acousto-optic tunable filters, where it is exploited that Bragg reflection at an acoustic wave works only within a narrow frequency range.

Tunable Optical Filters

While most types of optical filters exhibit fixed optical characteristics, some types are tunable, i.e., their optical characteristics can be actively modified. Some examples:

  • The resonances of an optical resonator can be tuned by modifying the resonator length with a piezo-controlled mirror. That way, one can tune the optical transmission peaks.
  • Etalons can simply be tilted to shift their transmission peaks.
  • Acousto-optic filters can be tuned through their electrical input, which can affect the amplitude or frequency of the generated acoustic wave. See the article on acousto-optic tunable filters.
  • The principle of liquid crystal modulators can be used.

See the article on tunable optical filters for more details.

Different Filter Shapes

Concerning the shape of the transmission curve, there are

  • bandpass filters, transmitting only a certain wavelength range
  • notch filters, eliminating light of a certain wavelength range, e.g. by reflecting it
  • edge filters, transmitting only wavelengths above or below a certain value (high-pass and low-pass filters)

Of course, a wide range of filter shapes can also be realized, particularly with interference filters.

edge filter
Figure 1: Wavelength-dependent reflectance of a dielectric edge filter with high transmission below 980 nm and high reflectance above 1030 nm. Starting from an analytically formulated design, the performance has been further optimized numerically (using the software RP Coating). Such a filter can be used for injecting pump light into the ytterbium-doped crystal of a laser.

Applications

Some examples for the many applications of optical filters are:

Suppliers

The RP Photonics Buyer's Guide contains 243 suppliers for optical filters. Among them:

Questions and Comments from Users

2021-05-20

We have a laser beam with several wavelength components. For example, we want to separate 780.00 nm from 780.04 nm, and can afford to discard one of the two beams; the other beam we want to use. I know the wavelengths are very close to each other, but is there a way to achieve this in practice? It would be really great to have a bandpass filter with 0.04 nm bandwidth.

Answer from the author:

Such a narrow bandwidth is not feasible with a filter based on dielectric coatings; you will need some kind of larger resonator – for example, a Fabry–Perot interferometer made of two dielectric mirrors.

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See also: interference filters, neutral density filters, rugate filters, tunable optical filters, volume Bragg gratings, acousto-optic tunable filters, hot mirrors, cold mirrors, wavelength tuning, gain equalization, optical fiber communications
and other articles in the category photonic devices

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