An intermodal fiber interferometer using the light from an incoherent broadband source has been considered analytically and implemented as a laboratory device. It was shown that this optical scheme could be used to measure external perturbations that cause a change in the optical length of a multimode fiber. The use of an optical spectrum analyzer and correlation functions approach in extracting the utility signal made it possible to achieve a linear response to the measured external perturbation and effective fading mitigation. A pair of integral coefficients was introduced: the contrast coefficient for characterization of the coherency of the operation regime, and the fading coefficient for estimating the signal stability against non-signal parasitic influences. Analytical expressions for the utility signal parameters were derived in dependence on the parameters of the light source, multimode fiber, and optical spectrum analyzer. The relationships among fiber length, width of the light source spectrum, and frequency resolution of the optical spectrum analyzer were stated for the optimum regime of interferometer operation. The simulation of the external perturbations performed at the elaborated device proved the applicability of the proposed scheme as a sensor of various physical quantities.