Shift of zero-dispersion wavelength in bent optical fibers.

The understanding of how bending modifies the dispersion of optical fibers, in particular, the zero-dispersion wavelength (λ), is essential in the development of compact nonlinear optical devices such as parametric amplifiers, wavelength converters, soliton lasers and frequency comb generators. Typically, substantial variations in the parametric gain and/or conversion efficiency are significant for changes in λ of ~0.1 nm, which occur for variations on the bending radius (Rb) of 1 cm or less. Measuring λ as a function of bending radius (Rb) is challenging, as it requires detecting changes < 0.1 nm and in short fibers. By using a method based on four-wave mixing (FWM) generated by an incoherent-pump with relatively broad spectrum and a weak laser, we report measurements of λ as a function of Rb in a dispersion-shifted fiber with <0.1 nm accuracy on λ. This method is sensitive enough to measure small variations in λ of ~0.04 nm in very short fibers (~20 m). We observe that λ increases by 12 nm when Rb is decreased from 10 cm to 1 cm, and a change of 1 nm is obtained for Rb = 3 cm. We also present numerical simulations of the bent fiber that are in good agreement with our measurements, and help us to explain the observations and to predict how high-order dispersion is modified with bending. This study can provide insights for dispersion engineering, in which bending could be used as a tuning, equalization, or tailoring mechanism for λ, which can be used in the development of compact nonlinear optical devices based on fibers or other bent-waveguide structures.