Proposal for sum-frequency generation based on modal phase-matching in photonic crystal fibers

In this paper, we report on a feasible phase matching between three low-order modes guided in an appropriately designed photonic crystal fiber (PCF). The phase matching condition for sum-frequency generation (SFG) can be realized over a large wavelength range by modifying the air holes size and the lattice pitch. This can be achieved in the proposed photonic crystal fiber because the sum-frequency wave propagating in a LP02 mode leaks more into the low refractive index cladding than does the fundamental wave. By “feeling the cladding”, the LP02 mode compensates for the increased refractive index of the silica at the sum-frequency wavelength. The numerical results show that the SFG in the proposed nonlinear PCF can be tunable, and the pump and sum-frequency waves are well-confined in the fiber core region.