Design of Highly Nonlinear Few-Mode Fiber for C-Band Optical Parametric Amplification

We present a thorough description of the dominating intramodal and intermodal four-wave mixing interactions occurring in a highly nonlinear few-mode fiber and describe their phase matching conditions. Those interactions that result in few-mode parametric amplification using a single-frequency pump are of particular interest. Thus, based on the phase matching conditions of such interactions, we outline the dispersion properties that a fiber should possess in order to achieve few-mode parametric amplification, while having minimal modal crosstalk. Accordingly, we design and optimize two fibers such that they meet the dispersion requirements for parametric amplification of two and four spatial modes, respectively. The two-mode fiber provides a maximum differential modal gain (DMG) of 0.21 dB across the C-band with a minimum gain of 9.5 dB per mode, while the four-mode fiber provides a maximum DMG of 1.51 dB with a minimum gain of 6.5 dB per mode over a 19 nm bandwidth in the C-band. The designed fibers are highly nonlinear dispersion-shifted few-mode fibers that provide both high nonlinearity and low dispersion for several modes in the C-band, which have not been demonstrated simultaneously to date. We also take practical fabrication issues into account and analyze and compare the tolerances of the structural parameters of both fibers to small deviations from their optimal values.