Time-domain and spectral-domain low-coherence interferometry applied to dispersion measurements of optical fibers

Successful applications of low-coherence interferometry both in the time and spectral domains are demonstrated in the evaluation of the group optical path difference (OPD) between two linearly polarized (LP) modes of an optical fiber excited by a multimode laser diode TOLD 9201. Firstly, the visibility of spatial interference fringes is measured in a tandem configuration of a Michelson interferometer and a two-mode optical fiber under test as a function of the OPD in the interferometer. From the visibility dependence, the group OPD between two LP modes of the optical fiber is obtained. Secondly, using the same exciting source TOLD 9201, the wavelength-dependent modulation of the source spectrum is obtained at the output of the two-mode optical fiber. By processing ten measured spectral modulations using the Fourier-transform method, the wavelength dependence of the group OPD between two LP modes is obtained and good agreement with time-domain measurement is confirmed. When a halogen lamp is used as an exciting source, the wavelength- dependent modulation is resolved only in the vicinity of the equalization wavelength, where the period of modulation is sufficiently large to be resolved by the spectrometer. From the corresponding dependence, a value of the equalization wavelength of two LP modes of the optical fiber is obtained.