Efficient conversion between counter-propagating fundamental and high-order modes in optical fiber with tilted gratings being incorporated for applications in nonlinear optics

Efficient conversion between fundamental and high-order modes propagating in multimode optical fiber is important for applications in nonlinear optics. For example, up-conversion in a nonlinear waveguide can be exploited for reaching single-photon detection levels [1]. However, high-order modes could result in destructive interferences among the up-converted modes [2]. On the other hand, it is equally important for efficient conversion from the fundamental mode to a high-order mode propagating in multi-mode optical fiber for wavelength conversion [3].