Slope-Assisted VBOTDA Based on Simultaneous Double-Sideband Modulation for Pump and Probe Waves

Brillouin optical time domain analysis (BOTDA) is an attractive solution to temperature and strain measurement for structural health monitoring. Here, we report a slope-assisted vector BOTDA system without optical filters based on simultaneous double-sideband modulations: the pump wave is intensity-modulated into a double-sideband frequency-agile optical pulse train while the probe wave is phase-modulated into an optical carrier and double-sidebands. The double sidebands of the phase-modulated probe wave will experience the Brillouin gain and loss from that of the pump wave through the vector SBS effect. With the phase cancellation broken in the phase-modulated probe wave, the resulting RF signal of the Brillouin-induced beat signal theoretically doubles the signal-to-noise ratio relative to conventional schemes. The phase-shift spectrum of the RF signal is immune to optical power fluctuation and has a monotonous vector slope of 108-MHz. Therefore, a single-slope-assisted vector BOTDA is implemented with a sampling rate of 500 Hz to measure a mechanical vibration containing a 74.36 Hz component and a dynamic range of 1500 μϵ. Then, the dynamic range is expanded to 3000μϵ using a multi-slope-assisted scheme. We believe the proposed scheme is a competitive solution to rapid, wide-dynamic range distributed temperature and strain sensing.