We report on the spectral intensity interferometer (SII) which is a frequency-domain variant of the fourth-order interferometry. In the SII, the power spectrum of the intensity is acquired for light fields of an interferometer. It produces a fringed spectral interferogram which can be acquired by means of an electric spectrum analyzer in keeping the relative time delay constant during the acquisition. Through both theoretical and experimental investigations, we have found that the SII interferogram provides the intensity correlation information without concern of field-sensitive disturbances which are vulnerable to minute variations of the optical paths. As an application example, a precision time-of-flight measurement was demonstrated by using a fiber-optic SII with an amplified spontaneous emission (ASE) light source. A large delay of 4.1-km long fiber was successfully analyzed from the fringe period. Its wavelength-dependent group delay or the group velocity dispersion (GVD) was also measured from the phase shift of the cosine fringe with a sub-picosecond delay precision.
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