Interference filter
Encyclopedia
An interference filter or dichroic filter
is an optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection.
An interference filter consists of multiple thin layers of dielectric
material having different refractive indices. There also may be metallic layers. In its broadest meaning, interference filters comprise also etalons that could be implemented as tunable interference filters. Interference filters are wavelength
-selective by virtue of the interference effects that take place between the incident and reflected waves at the thin-film boundaries. The important characteristic of the filter is the form of the leaving signal. It is considered that the best form is a rectangle.
Bandpass filters are normally designed for normal incidence. However, when the angle of incidence
of the incoming light is increased from zero, the central wavelength of the filter decreases, resulting in partial tunability. In addition, the transmission band widens and the maximum transmission decreases. If λc is the central wavelength, λ0 is the central wavelength at normal incidence, and n* is the filter effective index of refraction, then:
For example, for λ0=1550 nm, n*=1.5, Δλ = λ0−λc=32 nm, the rotation angle is
θ = 12.4°. This corresponds to C-band or L-band in 1550 nm fiber-optic communications
window. Equipped with a stepper motor
and electronics, a tunable optical filter that tunes center transmission wavelength over C-band or L-band by remote control can be achieved. See diagram below for its working principle and tunable optical filter devices
Dichroic filter
A dichroic filter, thin-film filter, or interference filter is a very accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the color of light that they...
is an optical filter that reflects one or more spectral bands or lines and transmits others, while maintaining a nearly zero coefficient of absorption for all wavelengths of interest. An interference filter may be high-pass, low-pass, bandpass, or band-rejection.
An interference filter consists of multiple thin layers of dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...
material having different refractive indices. There also may be metallic layers. In its broadest meaning, interference filters comprise also etalons that could be implemented as tunable interference filters. Interference filters are wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
-selective by virtue of the interference effects that take place between the incident and reflected waves at the thin-film boundaries. The important characteristic of the filter is the form of the leaving signal. It is considered that the best form is a rectangle.
Bandpass filters are normally designed for normal incidence. However, when the angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
of the incoming light is increased from zero, the central wavelength of the filter decreases, resulting in partial tunability. In addition, the transmission band widens and the maximum transmission decreases. If λc is the central wavelength, λ0 is the central wavelength at normal incidence, and n* is the filter effective index of refraction, then:
For example, for λ0=1550 nm, n*=1.5, Δλ = λ0−λc=32 nm, the rotation angle is
θ = 12.4°. This corresponds to C-band or L-band in 1550 nm fiber-optic communications
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
window. Equipped with a stepper motor
Stepper motor
A stepper motor is a brushless, electric motor that can divide a full rotation into a large number of steps. The motor's position can be controlled precisely without any feedback mechanism , as long as the motor is carefully sized to the application...
and electronics, a tunable optical filter that tunes center transmission wavelength over C-band or L-band by remote control can be achieved. See diagram below for its working principle and tunable optical filter devices