Octave-wide supercontinuum generation of light-carrying orbital angular momentum.

Abstract

Nonlinear frequency generation of light-carrying orbital angular momentum (OAM), which facilitates realization of on-demand, frequency-diverse optical vortices, would have utility in fields such as super-resolution microscopy, space-division multiplexing and quantum hyper-entanglement. In bulk media, OAM beams primarily differ in spatial phase, so the nonlinear overlap integral for self-phase matched χ(3) processes remains the same across the 4-fold degenerate subspace of beams (formed by different combinations of spin and orbital angular momentum) carrying the same OAM magnitude. This indistinguishable nature of nonlinear coupling implies that supercontinuum generation, which substantially relies on self/cross-phase modulation, and Raman soliton shifting of ultrashort pulses typically results in multimode outputs that do not conserve OAM. Here, using specially designed optical fibers that support OAM modes whose group velocity can be tailored, we demonstrate Raman solitons in OAM modes as well as the first supercontinuum spanning more than an octave (630 nm to 1430 nm), with the entire spectrum in the same polarization as well as OAM state. This is fundamentally possible because spin-orbit interactions in suitably designed fibers lead to large effective index and group velocity splitting of modes, and this helps tailoring nonlinear mode selectivity such that all nonlinearly generated frequencies reside in modes with high spatial mode purity.