The time distance tc where significant interference signal is observed is the coherence
time of the light source. Several definitions of tc are found in the literature and the two principal ones are given
hereafter. One possibility is to take the power equivalent width
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(4-13) |
The other way is to take from the g(t) function the
full width at half maximum (FWHM) t3dB or at
1/e of the maximum t1/e. For a Gaussian light source with
FWHM spectral range of Dn, the relation between tc, t3dB and Dn is
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(4-14) |
where l and Dl are the
central wavelength and the FWHM wavelength range of the source respectively.
The detailed algebraic manipulations are found in [4-6] for tc and in appendix D for tc,3dB.
The light source coherence length Lc
is defined as the travel distance for a time corresponding to the coherence
time, that is Lc = tc,3dB×c0 in
vacuum. For a Gaussian light source centered at 1300nm with a spectral width of
40 nm, the corresponding coherence length is 37 mm.
The spectral power density S(n) can be
obtained from the experimental determination g(t) by a Fourier transform.
This spectroscopic method is widely used in the infrared and it is known as the
Fourier spectroscopy.
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