White Light Sources
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A white light source is a light source with a broad optical bandwidth (usually 100 nm or more). However, two substantially different meanings of the term are in use:
- In some cases, a white light source really emits visible white light. Such sources are required e.g. for lighting applications and for colorimetry. They may contain something such as an incandescent lamp (e.g., a tungsten-halogen lamp), emitting light with a smooth and very broad optical spectrum. (Spectra of fluorescent lamps or other gas discharge lamps (e.g., xenon lamps) can also look white but are much more structured.) Typically, such kinds of white light sources have a low spatial coherence, making it difficult to tightly focus the light.
- In other cases, a broadband light source is meant, which does not necessarily emit in the visible spectral region. Such sources can be superluminescent sources, e.g. superluminescent diodes, and typically exhibit a high spatial coherence, making it easy to tightly focus the output or to deliver it through an optical fiber, even a single-mode fiber. On the other hand, the temporal coherence is low according to the large optical bandwidth. Typical applications of this kind of white light sources are white light interferometry, characterization of optical components, and spectroscopy.
- Supercontinuum generation is a method for obtaining even broader optical spectra. It is based on strongly nonlinear interactions e.g. in an optical fiber. Most widely used are photonic crystal fibers. The input light can be supplied in the form ultrashort pulses, nanosecond pulses, or continuously.
For some applications, the optical bandwidth specified as a full-width half-maximum value is important, whereas weak tails in the spectrum are not relevant. In other cases, it is only important to have at least some moderate level of power spectral density over a broad range, even if the power spectral density varies a lot within that range.
See also: bandwidth, supercontinuum generation, white light interferometers, interferometers, superluminescent sources, amplified spontaneous emission, spectroscopy
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