Fabrication of heat-induced long-period gratings for mode conversion in few-mode fibers

We present recent progress on fabrication of heat induced long period gratings (LPGs) in few mode fibers for stable mode conversion between the fundamental mode LP01 to asymmetric and symmetric modes, such as LP11 and LP02. A simple, effective, and low-cost method is demonstrated for fabricating heat induced LPGs, as an alternative method compared to more complicated approaches that require UV lasers and hydrogen loading or CO2 lasers. The LPGs are written point-by-point by periodically translating and heating the fiber. The heating filament is realized by conducting electrical current through an omega shaped 0.25 mm electrical platinum wire that enclose the fiber. We expect that the physical mechanisms for the refractive index change are caused by a combination of residual stress relaxation and tapering of the fiber. A grating period down to 622 μm for coupling between the LP01 and LP02 is demonstrated, however, we believe grating periods of a few hundreds of microns are feasible.

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