Optical Filters
Definition: devices with a wavelength-dependent transmission or reflectance
German: optische Filter
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Author: Dr. RĂ¼diger 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. Some examples of optical filters are:
- 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.
- Various kinds of optical filters are based on interference effects, combined with wavelength-dependent phase shifts during propagation. Such 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 typically exploiting larger path length differences. Therefore, they can have sharper spectral features.
- Lyot filters involve wavelength-dependent polarization changes. Similar devices are used as birefringent tuners in tunable lasers.
- Other filters are based on wavelength-dependent refraction in prisms (or prism pairs) or on wavelength-dependent diffraction at gratings, combined with an aperture.
- There are acousto-optic tunable filters, where it is exploited that Bragg reflection at an acoustic wave works only within a narrow frequency range.
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)

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 226 suppliers for optical filters. Among them:


VisiMax Technologies
VisiMax process color filters selectively and accurately pass light of a small range of colors while reflecting other colors. Our heat-resistant dichroic filter coating technology produces durable interference filters that render light with precise spectral control.
Accelerate your project by choosing a standard VisiMax color filter, or utilize the expertise of our skilled design team to create a custom filter to meet your specific spectral requirements.


Iridian Spectral Technologies
Iridian Spectral Technologies is a leading global supplier for applications in telecommunications, spectroscopy (Raman, fluorescence, and flow cytometry), biomedical imaging, endoscopy imaging, sensing, display and entertainment (glasses for 3D cinema, display coatings). Our optical filters and coatings cover the spectral range from UV 300 nm to LWIR 10 μm. Our products include narrow bandpass filters, steep edge long- or shortpass filters, dichroic edge filter, notch filter, multi-zone filter and other special coating filters.

EKSMA OPTICS
Crystalline material UV band pass filters, neutral density reflective type filters designed to operate at 400–2000 nm range, neutral density absorption type filters designed to operate at VIS (450–650 nm) and Schott color glass filters.


Edmund Optics
Edmund Optics offers a variety of optical filters for many applications, including bandpass interference, notch, edge, dichroic, color substrate, or ND. Edmund Optics also offers highly durable hard coatings for applications that require high optical densities with maximum performance.


Technica Optical Components
World leader in Fiber Bragg grating and Fabry Perot filters. Product portfolio includes athermal Fabry–Perot etalons (TWR30) and athermal FBGs (TWR50), as well as tunable FBGs (T10-T980) and tunable Fabry–Perot filters (TFP10-TFP50).

TeraXion
Ultra narrow tunable optical filters for Brillouin filtering in remote sensing applications or for the isolation of a modulated RF signal in RF communications.


HBM FiberSensing
The FS02 – BraggMETER tunable optical filter features a proprietary cavity design without guiding optics resulting in a compact, robust and vibration-resistant device.
It is most suited for integrating high end products or for research and education purposes in Universities and R & D laboratories.


Knight Optical
Our stock and custom-made optical filters are employed for a wide range of high-tech applications by discerning engineers and R&D professionals within sectors such as military & defence, electronics and research markets. With both stock and custom-made filters available, our collection includes bandpass, longpass, shortpass, neutral-density (ND), dichroic, IR-cut, colour glass, heat glass and ANPR/ALPR filters.
See also: 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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