The theoretical limit is set by the Schawlow–Townes linewidth, based on unavoidable quantum noise. For example, 9.5 km coherence length results for a Lorentzian spectrum with a linewidth of 10 kHz. Some lasers, particularly single-frequency solid-state lasers subject to certain methods of laser stabilization, can have very long coherence lengths. Calculating the Coherence Length Center wavelength:Įnter input values with units, where appropriate.Īfter you have modified some values, click a “calc” button to recalculate the field left of it.Īttention: The buttons do not work, as Javascript is turned off in your browser! Lasers with Long Coherence Lengths Generally, the shape and width of the optical spectrum alone does not fully determine the coherence properties. Note that such relations are not valid in cases where the coherence function has a more complicated shape, as is the case for, e.g., a frequency comb. In the literature, one often finds the above equation without the factor in the denominator – often just for estimating the order of magnitude of the coherence length, without referring to a precise definition of optical bandwidth (e.g., as full width at half-maximum) and coherence length. This coherence length is the propagation length after which the magnitude of the coherence function has dropped to the value of. Where is the (full width at half-maximum) linewidth (optical bandwidth). Calculationsįor light with a Lorentzian optical spectrum resulting from a random walk of the optical phase, the coherence length can be calculated as ![]() In addition to holography, a number of other applications may require a certain coherence length see the article on coherence. Also, in a setup for making holographic recordings, coherence between two beams with a somewhat different optical path length is required, so that the coherence length of the light source should be longer than the maximum occurring path-length difference. ![]() For example, the interferometer in Figure 1 shows pronounced interference fringes only if the coherence length of the laser light is at least as long as the path-length difference of the two arms. The reason for often using the term coherence length instead of coherence time is that the optical time delays involved in some experiment are often determined by optical path lengths. ![]() Figure 1: Setup of an interferometer, where the coherence length of light is important. It is defined as the coherence time times the vacuum velocity of light.įor example, if the coherence length of a laser is 1 km, the phase relationship between two points along the laser beam, with a distance of 1 km between them, is still significant, but already substantially degraded by phase fluctuations of the laser. The coherence length can be used for quantifying the degree of temporal (not spatial!) coherence as the propagation length (and thus propagation time) over which coherence degrades significantly, i.e., the optical phase undergoes substantial random changes. How to cite the article suggest additional literature Encyclopedia > letter C > Coherence length Coherence Lengthĭefinition: a measure of temporal coherence, expressed as the propagation distance over which the coherence significantly decaysĬategories: general optics, nonlinear optics
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