Optical Filters

Author: the photonics expert 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 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.

Such a filter can be used for injecting pump light into the ytterbium-doped crystal of a laser.

Applications

Some examples of the many applications of optical filters are:

• Filters can eliminate some unwanted light. Some examples: – Eye protection against laser radiation with laser safety glasses 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.

More to Learn

Encyclopedia articles:

• interference filters
• neutral density filters
• rugate filters
• tunable optical filters
• volume Bragg gratings