Optimum silicon taper structures with minimum temporal walk-off for nonlinear optical signal processing applications

Optical signal processing applications such as amplification, wavelength conversion and all-optical switching in silicon are usually based on short optical pulses with high peak power because these pulses provide a high peak intensity, which enables nonlinear Kerr interactions while keeping the average optical power small to avoid free carriers absorption [1, 2]. However, short optical pulses may have a limited interaction length due to a limited temporal walk-off (mismatch of the group velocities of pump and signal). In low loss waveguides, e.g. optical fibers, at a certain pump-signal detuning the Kerr induced phase shift would cancel out the group velocity dispersion, leading to a small walk-off over a long propagation length [3]. However, in silicon waveguides the loss is much larger which leads to the change of nonlinear phase shift along the waveguide. This severely limits the phase matched propagation distance [4]. Previously, silicon taper structures have been considered to reduce the pulse broadening [5]. In this paper we show that with certain optimally designed silicon taper structures it is also possible to reduce the walk-off for nonlinear pump-signal applications such as wavelength conversion.