Multicore Fiber Nonuniformly-Spaced Optical Delay Line for Microwave Signal Processing

We propose and experimentally demonstrate, for the first time, the use of nonuniformly-spaced optical delay lines for microwave photonic discrete-time signal processing, leveraging the spatial diversity inherent in a multicore fiber (MCF). We present the design of a novel 32-core heterogeneous MCF with a tailored chromatic dispersion profile for each core to offer tunability over a 30-nm optical wavelength range, providing flexibility for eight specific microwave signal applications. As a proof of concept, we experimentally demonstrate a nonuniformly-spaced optical delay line based on a 5-km homogeneous MCF and variable delay lines. Our results show that the time delay offsets result in additional phase shifts, leading to an effective conversion to equivalent negative coefficients, all within a compact solution. Our experimental findings include the successful implementation of a bandpass filter with a flat frequency response of around 10 GHz using 5 and 7 taps/cores. Additionally, we present a fractional Hilbert transformer with variable <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ρπ</i> phase shifts at 8 GHz, validated through the temporal Hilbert transform of a Gaussian pulse. Furthermore, we showcase a microwave pulse phase encoder for Barker and Quaternary Barker sequences, demonstrating accurate encoding of the input signal and a compression ratio consistent with theoretical predictions. Overall, we introduce a compact solution for microwave photonic applications that require negative coefficients. Our approach benefits from the ability of MCFs to provide parallel distributed signal processing with a reduction of size, weight, and power consumption.