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Polychromators

Definition: an optical device which sends light with different wavelengths to different directions

Opposite term: monochromator

German: Polychromatoren

Categories: light detection and characterization, optical metrology

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URL: https://www.rp-photonics.com/polychromators.html

A polychromator is an optical device which deflects light into different directions, depending on the optical wavelength. It may transmit all light in a wide spectral region, or alternatively be equipped with additional output slits such that only light in certain spectral bins is transmitted to the corresponding outputs. These outputs may be called spectral channels. A five-channel polychromator, for example, is one having five distinct channels. One may place a photodetector at each output in order to measure the optical power of each channel. In other cases, the light outputs are used differently. A simple kind of two-channel polychromator is a harmonic separator, which separates frequency-doubled light from residual pump light.

Technical Realization of Polychromators

Two typical optical arrangements used as polychromators are the following:

  • An optical prism can be used, since it exhibits wavelength-dependent refraction at its surfaces.
  • One may also use a diffraction grating, where any reflected or transmitted light, except that in the zero diffraction order, obtains a wavelength-dependent direction.

In order to obtain a clear correlation between wavelength and propagation direction, the above-mentioned devices need to be operated such that the input light propagates in one well-defined direction. (Typically, it is delivered as a collimated beam.) However, one may also use additional optical elements such as lenses or mirrors to construct polychromators which can work with strongly divergent input light, but in that case that light usually must be restricted spatially, e.g. by transmission through an optical slit.

Instead of output slits, one may use optical fibers for collecting the output light and sending it to different photodetectors, for example.

Extreme spectral regions, e.g. the far UV, require adapted technologies for polychromators.

Devices Containing Polychromators, Applications

A spectrograph contains a polychromator and some kind of extended photodetector, e.g. a focal plane array, for recording optical spectra.

A monochromator exhibits high light transmission only in a narrow range of wavelengths. It often consists of a polychromator and a narrow output slit.

Some polychromators are used for Raman spectroscopy. Special care needs to be taken to avoid significant contamination of the weak Raman-scattered light from residual pump light.

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See also: polychromatic light, monochromators, spectrographs, optical filters
and other articles in the categories light detection and characterization, optical metrology

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