High-speed demodulation of weak fiber Bragg gratings based on microwave photonics and chromatic dispersion.

A high-speed demodulation technique based on microwave photonics and chromatic dispersion is proposed for distributed weak fiber Bragg gratings (FBGs). A broadband light modulated with a frequency-swept microwave is reflected by FBGs, and the reflected signal mixes with the original microwave to produce a beat frequency. A dispersion compensation fiber (DCF) changes the beat frequency within the FBG wavelength range. With a crossing microwave sweep, all wavelengths of cascade FBGs can be quickly decoded by measuring the change of the beat frequency. The Chirp-Z + Hanning window algorithm helps to analyze the beat frequency in a single cycle. By adopting the single-peak filter as a reference, the DCF thermal drift can be also eliminated. Therefore, the wavelength detection accuracy is improved, and a higher demodulation speed of 40 kHz can be realized. We have demonstrated 105 FBGs with 0.1% reflectivity and 1 m interval, with the achieved accuracy of 8 pm.