Abstract
The availability of a tunable delay line with a chip-size footprint is a crucial step towards the full implementation of integrated microwave photonic signal processors. Achieving a large and tunable group delay on a millimetre-sized chip is not trivial. Slow light concepts are an appropriate solution, if propagation losses are kept acceptable. Here we use a low-loss 1.5 mm-long photonic crystal waveguide to demonstrate both notch and band-pass microwave filters that can be tuned over the 0-50-GHz spectral band. The waveguide is capable of generating a controllable delay with limited signal attenuation (total insertion loss below 10 dB when the delay is below 70 ps) and degradation. Owing to the very small footprint of the delay line, a fully integrated device is feasible, also featuring more complex and elaborate filter functions.
Highlights
The availability of a tunable delay line with a chip-size footprint is a crucial step towards the full implementation of integrated microwave photonic signal processors
Typical delay line footprints range from several tens of centimetres when using fibre Bragg gratings (FBGs) to a few kilometres when using standard optical fibres
A solution to the above limitations of Microwave photonics (MWPs) filters is contingent on the possibility of incorporating this functionality into an application-specific photonic integrated circuit, which makes the former approaches based on extended delay lines inapplicable
Summary
The availability of a tunable delay line with a chip-size footprint is a crucial step towards the full implementation of integrated microwave photonic signal processors. Preliminary efforts towards integration have relied on resonant delay lines provided by ring cavities and coupled resonant optical waveguides[17,18,19,20] These approaches suffer from an intrinsic trade-off between dispersion and bandwidth, preventing broadband operation. The salient features include: notch and band-pass operation; a footprint reduced by several orders of magnitude compared to filters based on FBG or fibre delay lines; broadband operation up to 50 GHz; and tunable free spectral ranges over 70 GHz. In addition, there is clear potential for the improvement of the selectivity, the main to secondary side-lobe (MSSL) ratio and the Q factor, thereby leading to highperformance MWP filtering owing to compatibility with sources of optical frequency combs[23]
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