40 GHz nonlinear all optical switching in a Mach-Zehnder interferometer integrated device

Abstract

Summary form only given. Silicon photonics is a well established platform for the development of fully integrated optical devices for optical communication systems. In particular, the strong modal confinement typical of silicon waveguides enables the exploitation of nonlinear effects with much lower optical power levels. Different structures for all-optical signal processing have been reported in the last years, such as all optical modulators [1], wavelength converters [2-3], and optical switches [4].Here we propose a fully integrated Mach-Zehnder interferometer for all-optical switching. The device is composed of two arms: a nonlinear arm that introduces a beam-intensity dependent phase term, and a linear arm, introducing a fixed phase term. The nonlinear arm is a 7-mm long standard silicon serpentine waveguide (500×220 nm cross section), while the linear arm is made by a SU-8 polymer waveguide with a cross section that is 15 times larger than the silicon waveguide. Since the third order nonlinearities are inversely proportional to the modal effective area, the Kerr nonlinearities in the SU-8 arm are negligible. We note that a similar device has been proposed in [5] where the linear arm is obtained through free-space propagation. If a balanced interferometer is realized, two different operation regimes can be observed (see Fig. 1a). When the input power is low there is no Kerr-induced phase term and the signal propagates through the cross port. When the power is increased, the Kerr effect in the silicon arm produces a nonlinear phase shift, and as a consequence a fraction (directly proportional to the nonlinear induced phase change) of the input beam switches to the bar port, producing the desired switching behaviour.A pump-probe experiment was implemented in order to test the switching performance of the fabricated device. An intense pump (2 ps FWHM, 40 GHz rep. rate) and a weak CW signal were injected from the same input port. The pump pulses induces the switching at the bar port of the CW co-propagating low power signal. Fig. 1b shows the switching traces for three different pump peak power levels: a 40% switching level, at f = 40 GHz, was achieved using a Pin=27 dBm peak power level. Fig.1c shows the autocorrelation output trace, which reveals that the pulse duration is preserved and no-degradation effects are introduced by the device operating in the nonlinear regime. No two-photon-absorption and free carrier effect limitations were observed at this pump power level as predicted from our simulations.