Step-index optical fibers with wide parameter tolerance for third-harmonic generation

Intermodally phase matched nonlinear frequency conversion processes in optical fibers provide a convenient and compact approach to generate new wavelengths, but efficiency is limited by the strict diameter tolerance of the fiber. Here, we propose a practical and realistic design framework that explicitly incorporates fabrication uncertainties and nonlinear phase detuning for solid step-index fiber. This framework is explained in detail with a specific example of chalcogenide-tellurite hybrid optical fiber which has large core-cladding refractive index difference as well as highly nonlinear core. It is clarified that fabrication tolerance could be relaxed through joint optimization of fiber parameters and input pump power. The results show that conversion efficiency above $\sim 5\text{\%}$ is possible in a chalcogenide-tellurite fiber with mean core diameter deviation up to $\pm 25\mathrm{nm}$ along with lenient random fluctuation of a few nm. This work indicates that efficient fiber-based third-harmonic generation could be feasibly achieved under realistic conditions provided that fibers are properly designed and fabricated. The proposed framework could be applied for preparation of such fibers with a variety of core and cladding materials with suitable characteristics, and helps to bring all-fiber optical frequency conversion to practical exploitation.