The specification describes optical mode converters wherein coupling is
made between a fundamental, or near fundamental, propagation mode and the
next, or closely adjacent, higher order mode (HOM). Both modes propagate
in the core of the optical fiber, thus maintaining efficient transmission
through the mode converter. Mode coupling is effected using a long period
grating (LPG) and the strength of the mode coupling is dynamically varied
by changing the period of the grating or by varying the propagation
constants of the two modes being coupled. The period of the grating is
varied by physically changing the spacing between grating elements, for
example by changing the strain on the grating to physically stretch the
LPG. The propagation constants of the modes can be varied using any method
that changes the refractive index of the fiber containing the LPG, for
example, by changing the temperature, or electrically changing the index
using the electro-optic effect. In every case the two modes being coupled
are core modes with high propagation efficiency.
