
In physics, coherence length is the propagation distance over which a coherent wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves differ by less than the coherence length. A wave with a longer coherence length is closer to a perfect sinusoidal wave. Coherence length is important in holography and telecommunications engineering. This article focuses on the coherence of classical electromagnetic fields. In quantum mechanics, there is a mathematically analogous concept of the quantum coherence length of a wave function. ==Formulas== In radioband systems, the coherence length is approximated by :$L=,$ where ''c'' is the speed of light in a vacuum, ''n'' is the refractive index of the medium, and $\backslash Delta\; f$ is the bandwidth of the source. In optical communications, assuming that the source has a Gaussian emission spectrum, the coherence length $L$ is given by 〔Akcay, C., Parrein. P., and Rolland, J. P. (2002). Estimation of longitudinal resolution in optical coherence imaging. Applied Optics 41 (25), 52565262 〕 :$L=,$ where $\backslash lambda$ is the central wavelength of the source, $n$ is the refractive index of the medium, and $\backslash Delta\backslash lambda$ is the spectral width of the source. If the source has a Gaussian spectrum with FWHM spectral width $\backslash Delta\backslash lambda$, then a path offset of ±$L$ will reduce the fringe visibility to 50%. ''Coherence length'' is usually applied to the optical regime. The expression above is a frequently used approximation. Due to ambiguities in the definition of spectral width of a source, however, the following definition of coherence length has been suggested: The coherence length can be measured using a Michelson interferometer and is the optical path length difference of a selfinterfering laser beam which corresponds to a $1/e=37\backslash \%$ fringe visibility, where the fringe visibility is defined as :$V\; =\; ,\backslash ,$ where $I$ is the fringe intensity. In longdistance transmission systems, the coherence length may be reduced by propagation factors such as dispersion, scattering, and diffraction. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「coherence length」の詳細全文を読む スポンサード リンク
