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What are the coherence lengths of the different types of lasers?
Dave1235
Fiber optic lasers can have coherence lengths greater than 100 km.
Helium-neon lasers can produce light with coherence lengths greater than 5 m but 20 cm is typical.
Laser diode chips are a fraction of a mm on a side and so coherence lengths on that order are expected, however some of the cheapest laser pointers can produce coherence lengths of 20 cm for short intervals of time and have been used to create holograms. In general the length depends on many variables.
The typical red light laser diode (λ= 650 nm) with a frequency stabilizer can have a coherence length of over 1 m.
LEDs have a spectra width Δλ of about 50 nm, and may have a coherence length of 10's to 100's of μms.
As a side note, because the exited states of the atoms in a tungsten filament are short lived, the coherence length is only a few micrometers (μm).
Some notes about coherence lengths:
Interference is only visible if the coherence length of the light is at least as long as the path-length difference that creates the interference.
Spectral width in optics is related to coherence length by the formula
L = λ²/(nΔλ)
where λ is the central wavelength, n is the index of refraction and Δλ is the spectral width.
The coherence time is the above coherence length divided by the light's phase velocity in the medium or..
τ = λ²/(cΔλ)
Refer to the Related link below for Wikipedia's article on coherence length
Wiki User
∙ 14y ago
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