HELICAL-GRATING 2-MODE FIBER SPATIAL-MODE COUPLER

Helical microbending induced by winding wire around a two-mode fiber results in wavelength-dependent coupling between the fundamental LP01 spatial mode and a hybrid LP11 doughnut mode. The feasibility of making fiber grating filters based on this geometry is investigated. By adding mode strippers at the input and output of a helical-grating spatial-mode coupler, an optical notch filter is demonstrated with 0.3-dB waveguide loss, 24-dB extinction, and 24-nm spectral width. The effect of small ellipticity in the fiber core is also investigated and it is shown that polarization insensitive coupling can be achieved in a fiber with a sufficiently circular core.