Boyin Tai

Intermodal nonlinear mixing with Bessel beams in optical fiber

Nonlinear frequency mixing as a means to coherently convert light to new frequencies is widely used in many branches of optics. This process requires momentum conservation through phase matching (PM). In free-space optics, PM is achieved through angle tuning the medium with respect to the incoming light—here we explore an in-fiber analogue: PM using spatial modes of the fiber. We demonstrate over two octaves (400–1700 nm) of coherent spectral translation generated by intermodal four-wave mixing between subsets of 11 different Bessel-like fiber modes. These interactions are facilitated by the unique mode-coupling resistance of this subset of azimuthally symmetric, zero orbital angular momentum fiber modes. Their stability allows overcoming previous limitations of multimode nonlinear-optical systems imposed by mode coupling, hence enabling long interaction lengths, large effective mode areas, and a highly multimode basis set with which a new degree of freedom for versatile PM can be obtained.

Broadband parametric wavelength conversion at 1 μm with large mode area fibers

Fiber-optic parametric wavelength conversion (PWC) below the zero-dispersion wavelength of silica is typically constrained by the requirement of a small, tightly confined mode with anomalous dispersion to achieve phase matching. This limits the ability to power scale PWC at arbitrary wavelengths. However, the constraint is lifted for higher-order modes. We demonstrate PWC in the 1 μm band via degenerate four-wave mixing pumped in a large effective area (>600  μm²) LP(0,7) mode of a double-clad fiber. We obtain up to 25% conversion in to the Stokes line with 0.5 ns pump pulses, corresponding to ~20  kW peak power at the converted wavelength.