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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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Author: Dr. Rüdiger Paschotta

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 wavelength-dependent absorption in some material such as a glass dopant, dye, pigment or semiconductor. 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 involve 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.

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.

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:

Filters can eliminate some unwanted light. For example, eye protection against laser radiation is often done with filters which can eliminate e.g. infrared laser light while transmitting visible light (→ laser safety). Similarly, sun glasses attenuate visible light and filter out ultraviolet light. Green laser pointers are often equipped with filters for removing residual infrared light. Heat control filters in the form of cold mirrors are used to transmit visible light while removing intense infrared radiation, as it is emitted e.g. by hot surfaces. Similarly, hot mirrors can remove infrared light from a beam path by reflecting it. Sharp edge filters or bandpass filters can be used in fluorescence microscopes for removing pump light from the fluorescence signal light.
Wavelength-dependent losses are useful for gain equalization of fiber amplifiers, as used in optical fiber communications. Similarly, filters can be used for balancing a photodetector response or the non-uniform optical spectrum of a light source.
In the image sensors of photo cameras, for example, RGB filters allow for separate detection of the intensity in different colors, so that color images are obtained.
Filters in the form of fiber-optic add–drop multiplexers can extract or inject single channels in wavelength division multiplexing optical data transmission systems.
Intracavity filters in lasers can be used for wavelength tuning and for single-frequency operation of lasers, or for suppressing lasing at unwanted wavelengths.
Filters can suppress effects of amplified spontaneous emission in amplifier chains.
The combination of a tunable filter and a broadband photodetector can be used for the spectral analysis of optical signals.
Neutral density filters are used for attenuating optical signals without modifying their spectral shape.

Suppliers

The RP Photonics Buyer's Guide contains 238 suppliers for optical filters. Among them:

Dynasil

Optometrics, a Dynasil company, designs and manufactures long wavepass filters from 500 nm to 1500 nm in increments of 50 nm. These long wavepass filters are mounted in a black anodized aluminium ring. The cut-on wavelengths can be fine tuned by changing the angle of incidence. Optometrics also has short wavepass filters from 450 nm to 1000 nm in increments of 50 nm and otherwise similar characteristics. These filter types can also be used in combination for custom bandpass filtering.

EMF, also a Dynasil company, manufactures a custom line of Optivex™ dichroic filters that block 99% of harmful UV radiation while transmitting high-quality visible light with virtually no distortion. Optivex™ has been the product of choice for over 25 years, and its name is synonymous with absolute protection.

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