Nonlinearities in silicon photonics: something to exploit or to counteract?

In the 1550-nm wavelength range, silicon waveguides exhibit a plethora of non-linear phenomena arising when the optical power is sufficiently intense. Inside optical resonators the efficiency of wave mixing effects is largely enhanced and can be effectively exploited for on-chip all-optical signal processing. By cascading silicon microring resonators, we demonstrated the concept of travelling-wave resonant four-wave mixing (FWM) and we realized a 630-μm-long wavelength converter with a conversion gain 28-dB higher than a bare waveguide of the same physical length. Both the conversion efficiency and the bandwidth of the device are enhanced with respect to a single silicon resonator. However, one of the major impairments associated with nonlinearity in silicon is the TPA-induced local heating of the waveguides. In coupled resonator architectures, this produces not simply a rigid red-shift of the spectral response, but a resonance spread disrupting the frequency response of the devices. Active thermal control of the individual resonators is proposed as a viable strategy to adaptively compensate for intensity-dependent distortions and cross-talk increase in silicon coupled resonator filters, demonstrating that device performance can be preserved regardless of the aggregate power transmitted through the waveguide.