Definition: light with a single optical frequency
More general term: light
Opposite term: polychromatic light
German: monochromatisches Licht
Author: Dr. Rüdiger Paschotta
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Monochromatic light is light (optical radiation) where the optical spectrum contains only a single optical frequency. The associated electric field strength at a certain point in space, for example, exhibits a purely sinusoidal oscillation, having a constant instantaneous frequency and a zero bandwidth. Light sources can also be called monochromatic, if they emit monochromatic light.
The antonym of monochromatic is polychromatic. A typical example for polychromatic light is life created as thermal radiation, e.g. in an incandescent lamp; such light exhibits a broad range of optical frequencies.
Many calculations in optics and photonics are performed for monochromatic light. For example, the evolution of laser beams is usually calculated that way; there is just one given optical wavelength or frequency.
Real light sources can of course never be exactly monochromatic, i.e., have a zero optical bandwidth. However, particularly laser sources are often quasi-monochromatic, i.e., the optical bandwidth is small enough that certain behavior of the light can hardly be distinguished from that of truly monochromatic light. Some examples:
- Laser light used for laser absorption spectroscopy can be regarded as quasi-monochromatic if its bandwidth is far below that of the spectral features of interest.
- When a light beam should be intensity-enhanced in an optical resonator (for example, for resonant frequency doubling), its bandwidth should be well below the bandwidth of the resonator.
- For the operation of interferometers, the finite bandwidth of light is not relevant if the coherence length is well above any path length differences in the apparatus.
Obviously, the permissible optical bandwidth for quasi-monochromatic light depends very much on the circumstances.
The term monochromatic originally means having only a single color. Different optical wavelengths of visible light are associated with different perceived colors. However, light colors are rarely a criterion for monochromaticity in practice, and non-monochromatic light can also have specific colors. Also, the term is applied to infrared and ultraviolet light as well as to visible light.
Lasers are the primary sources of quasi-monochromatic light. In contrast to narrow-band light obtained by bandpass filtering light from a broadband source (see below), lasers can generate quasi-monochromatic light with high optical powers. Some lasers even exhibit extreme degrees of monochromaticity, i.e., an extremely small optical bandwidth. The highest degree of monochromaticity is achieved with carefully stabilized single-frequency lasers (sometimes with a bandwidth well below 1 Hz).
Before the advent of the laser, it was quite difficult to produce monochromatic light. One possibility was to use certain gas discharge lamps and metal vapor lamps (e.g. mercury vapor lamps and sodium vapor lamps), emitting light dominantly in certain narrow spectral lines, and isolating one such line with a suitable monochromator. The achieved optical powers and intensities were quite low.
A monochromator is essentially a kind of optical filter which allows one to isolate light in a narrow spectral interval from other light. Its output will therefore be quasi-monochromatic. However, light at all other wavelengths is then lost.
See also: optical frequency, polychromatic light, narrow-linewidth lasers, bandwidth, linewidth, monochromators
Questions and Comments from Users
How can you measure the monochromaticity of a light wave?
The author's answer:
Monochromaticity is not a quantity, but you may judge it by considering the spectral bandwidth. You may measure that with and optical spectrum analyzer or with an interferometer, for example.
Is the sodium light truly monochromatic?
The author's answer:
It depends on what you mean was “truly monochromatic”. From a theoretical standpoint, it might mean and exactly constant instantaneous optical frequency, which is never achieved in reality. In practice, one determines what level of optical linewidth is still relevant for a specific application.
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If the electric field from a monochromatic light source varies sinusoidally, shouldn't its power detected on a square law detector that is sufficiently fast give a squared sinusoidal signal? The detectors for optical frequencies would be too slow for this, but I have been told that what is recorded at lower frequencies is a constant power level instead of a squared sinusoid. Is this somehow related to that practical light sources are never perfectly monochromatic, or how is this reconciled?
The author's answer:
If you define optical power as energy delivered by unit time, then it indeed oscillates in the way you described. However, that oscillation is by far too fast to be measured with any electronic detector. Therefore, optical power or optical intensity (as the magnitude of the Poynting vector) is often defined to be without that oscillation because that is usually of no practical relevance.