Photonic crystal Fano structures for all-optical signal processing

The capacity of optical communication links has been growing very fast as a result of the deployment of dense modulation formats, various multiplexing schemes, the use of multi-core fibres, and other developments. However, switching data packets in data centres between such high speed optical links is still one of the main causes of bottlenecks.

This is mainly due to the need for conversion from optical domain to electrical domain and vice versa for switching and signal processing. The use of all-optical switches is believed to enable signal processing in the optical domain without the need for conversion to the electrical domain, hence avoiding bottlenecks and large energy dissipation.

Our work focuses on the experimental demonstration of fast and energy-efficient all-optical switching using photonic integrated circuits. We have investigated coupled cavity-waveguide structures realized in indium phosphide photonic crystal membranes. The interference between the discrete modes of the cavity and the continuum of waveguide modes results in an asymmetric Fano resonance lineshape, which is characterized by having its transmission maximum and minimum within a small spectral range.

Two types of Fano resonances with different spectral shapes have been demonstrated. By combining these characteristic line shapes with carrier-induced nonlinear resonance shifts, devices potentially suitable for all-optical signal processing applications have been designed. Considerable effort has been put into fabrication and platform development such as implementation of grating couplers for efficient light coupling into and out of the devices, selective membranization of the photonic crystal membrane, and implementation of a p-i-n junction for carrier sweep-out mechanisms.

The fabricated devices are shown to perform well in various signal processing experiments. We have experimentally demonstrated the use of Fano resonances for carvingout short pulses from long-duration input pulses. Input pulses as long as ∼ 500 ps and ∼ 100 ps can be shortened to ∼ 30 ps and ∼ 20 ps pulses, respectively.

This self-pulse carving feature is also implemented for low-duty cycle return-to-zero on off keying (RZ-OOK) signal generation at 2 Gbit/s with energy consumption down to ∼ 1 pJ/bit. Reshaping of optical data signals using Fano resonances is another application that we have been investigating. The combination of an asymmetric lineshape with a nonlinear resonance shift can be used to realize nonlinear power transfer functions suitable for suppression of amplitude fluctuations of data signals.

Using these transfer functions, we have demonstrated reshaping of 10 Gbit/s RZ-OOK data signals with energy consumption down to 41 fJ/bit. Furthermore, we have investigated the switching performances of the devices in a pump-probe measurement scheme in which the pump triggers the resonance shift, hence inducing the switching action.

This allowed demonstration of error-free 10 Gbit/s wavelength conversion and 40 Gbit/s to 10 Gbit/s optical time domain demultiplexing applications. All these functionalities are compared to the use of the conventional Lorentzian-shaped resonances using coupled-mode theory. Moreover, prospects of improving device performances and future perspectives are discussed.