'Dial Up' Photonic Integrated Circuit Filter

Photonic integrated circuits (PICs) are capable of providing advanced signal processing functions in small footprints; however, their operation is affected by fabrication tolerances in addition to dynamically changing thermal and acoustic effects. Thus, it can be difficult to tune PICs to accurately implement desired functionality, even those with a relatively small number of photonic components (e.g. 10). In this contribution, we demonstrate a ‘dial-up’ PIC filter that can accurately and reliably implement a wide range of filter functions. It uses an on-chip optical reference path, which allows the phase and amplitude of each tap in a finite impulse response (FIR) filter to be individually determined. The fractional delay of the reference path is chosen to be half the delay between the FIR filter's taps, which upon Fourier transformation enables us to uniquely identify the phase and amplitude of each tap in the FIR filter. This information is used in an independent feedback algorithm to control each tap to ensure that a desired (‘dialed-up’) response is accurately implemented. To demonstrate the flexibility of the method, we experimentally demonstrate functions of a sinc-shaped filter with a phase step, a Hilbert transform and low- and high-pass filters.