Microwave photonic links provide low loss transmission of wide bandwidth analog signals by leveraging fiber-optic technology. In addition, microwave signal processing can be accomplished in the optical domain where widely tunable photonic filters provide versatile signal processing at center frequencies unattainable by electronic solutions. Recent developments in photonic integrated circuit (PIC) technology has rapidly improved the design and fabriation of photonic chips, on which stable coherent signal processing circuits can be created.

In this work, we utilize the active InP material system to provide on-chip optical gain. On-chip gain adds a high level of design versatility and tolerance. Using a coupled-ring geometry, high-quality bandpass filter PICs are designed, fabricated, and demonstrated. Near-ideal filter shapes with bandwidths in the few GHz range and up to 40 dB stop-band rejection are shown to be fully tunable in bandwidth and center frequency without degradation to filter characteristics.

In order for a photonic filter to be useful, it must not negatively impact the microwave signal fidelity. Quantitatively, this signal fidelity is described by the spurious-free dynamic range (SFDR). In order to investigate the impact of a signal processing PIC on a microwave link, a theoretical analysis of PIC SFDR is carried out. In particular, the noise and distortion characteristics of semiconductor optical amplifiers (SOAs) are derived. A general model for PIC-limited SFDR is developed for signal processing circuits with arbitrary transfer functions integrating multiple SOAs. Using this model, the SFDR of coupled-ring bandpass filters is investigated, and design trade-offs are identified. The analysis indicates the possibility of high quality filters with SFDR in the 105-120 dB · Hz2/3 range.