Two-mode fiber interferometer/amplitude modulator

Single-mode fiber used at a wavelength below its cutoff wavelength guides second-order modes which travel at different phase velocities than the fundamental modes. Such two-transverse-mode fiber forms a single-strand interferometer, and studies have shown that the output may be set at quadrature by using all-fiber polarization controllers. Furthermore, modulation of the two-mode fiber with a piezoelectric stretcher produces amplitude modulation. Two-amplitude modulators have been constructed and studied operating at 633 and 830 nm, and a modulation depth of 97% has been observed at 23.4 kHz for an excess loss of ~6 dB.