Two-mode fiber modal coupler.

When a single-mode fiber is used at a wavelength below the cutoff wavelength, the fiber guides second-order modes, which travel at different phase velocities from the fundamental mode. Periodically stressing this two-mode fiber once per beat length can cause coherent coupling between the modes. Such a modal coupler has been developed and is described here. Coupling to one of the second-order modes has been achieved, leaving less than -40-dB residual power in the fundamental mode. Two couplers have been mounted on a single strand of fiber to construct a Mach-Zehnder interferometer with better than a 30-dB on/off ratio. The coupler is polarization sensitive and can be used as an in-line polarizer. A 36-dB extinction ratio between polarizations has been observed.