An extended interference formula has been derived permitting simultaneous determination of geometrical and optical parameters of optical fibers whether the cores or stress-applying parts are nonconcentric to cladding. This formula can be applied for normal and oblique transverse interferometric techniques. In this paper, this interference formula is combined with computer-aided Mach–Zehnder microinterferometer for characterization of these optical fibers. The advantage of the presented interference formula is that it can be applied for all types of optical fibers. Also, it is used to minimize any uncertainty of the thickness measurement. In comparison with the well-known conventional interference formula, the measurement accuracy of this extended formula is discussed.