Intermodal beat lengths in birefringent two-mode fibers

We use an accousto-opitc method to measure the beat lengths between the LP(01) and the LP(11) modes propagating both in an elliptical-core polarization-maintaining fiber and in a bow-tie high-birefringence (Hi-Bi) fiber for a wide range of optical wavelengths. The Hi-Bi fiber exhibits considerable polarization splitting in the measured beat lengths at all measured wavelengths. According to theory for circular-core fibers, the beat length should have a minimum at the wavelength at which the two modes have the same group velocity in the fiber. We find that for the elliptical-core fiber such a minimum does not exist in the two-mode region. Based on our measurements we discuss the use of different fiber types in various two-mode devices.