Characterisation and Light Manipulation in Photonic Integrated Circuits

Ultrafast photomodulation spectroscopy (UPMS) is a far-field technique for non-destructive characterisation of individual integrated photonic components at the wafer scale. In UPMS, externally incident femtosecond optical pump pulses create highly localised perturbations in the refractive index profile of a silicon-on-insulator (SOI) waveguide via the plasma dispersion effect. Due to the ultrafast nature of the pump light, free-carrier concentrations far exceeding those achievable by electrical effects can be obtained, which can be exploited to modulate the transmission of the device. UPMS is a diverse tool for photonic chip characterisation, allowing for pump-probe measurements in either the temporal or spatial domain. Fine control of the delay time between probing and pumping pulses allow for ultrafast responses to be observed, as well as direct time of flight measurements within the device. While through the recording of the change in transmission as a function of the perturbation position it is possible to build up a two-dimensional spatial photomodulation map. Such maps provide a direct visualisation of the propagation of light through a device, and in the case of low loss devices can be used to recover information on the electric field without requiring direct access to the near-field. Furthermore, patterns made up of multiple perturbations can be used to shape the wavefront of light, allowing for photonic structures with complex and reconfigurable functionalities, such as variable modulators/switches, (de)multiplexers and mode converters.