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
• edge filters, transmitting only wavelengths above or below a certain value (high-pass and low-pass filters)

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.