// Operating principle
Full-spectrum scanning interferometric demodulation
The interrogator employs a full-spectrum scanning interferometric demodulation architecture based on the Fabry-Pérot cavity principle. The instrument continuously and densely samples the interference spectrum across the C-band or, in the extended configuration, the combined C+L band (1525–1610 nm). By extracting the positions of multiple interference fringes (peaks and troughs) within the acquired spectrum, the system reconstructs the variation in FP cavity length, from which pressure and temperature are derived with high precision.
Relative to conventional electronic transducers, fibre-optic FP sensors offer superior mechanical stability, stronger immunity to electromagnetic and environmental interference, and enhanced high-temperature tolerance, enabling high-precision, low-drift measurement over extended deployment periods under extreme downhole conditions. The full-spectrum scanning architecture offers the following advantages over narrowband, single-peak-tracking demodulation schemes:
- Multi-fringe joint demodulation — multiple interference fringes are captured simultaneously across the scanning band, and multi-point fitting across the free spectral range (FSR) improves demodulation accuracy and noise immunity
- Robustness against loss-of-lock — even if a single spectral peak is distorted or obscured by environmental disturbance, cavity-length demodulation can still be completed using the remaining fringes, avoiding the loss-of-lock failure mode common to conventional peak-tracking algorithms
- Wide dynamic range — coverage of a wide scanning band accommodates a correspondingly wide range of cavity-length variation (i.e., a larger pressure measurement range) without additional disambiguation logic when the fringe order changes
- Synchronous multi-parameter demodulation — both temperature-induced and pressure-induced cavity-length shifts are reflected in the movement of the full interference spectrum, enabling pressure–temperature decoupling through a dual-parameter compensation algorithm
- High-speed pressure signal acquisition — the system accurately captures millisecond-scale transients, such as those occurring during hydraulic fracturing or well-kick events, as well as high-frequency equipment vibration, supporting safety assurance and precise fault diagnosis under extreme operating conditions
The downhole probe employs a compact, highly integrated design that combines high reliability with ease of installation, while the surface interrogator supports raw spectral data export, extensive data-processing capability, and remote-monitoring functionality to meet the long-term production-monitoring requirements of modern oil and gas operations.
