Intrinsic distributed sensing using wavelength-multiplexed QPSK signals in fiber-optic communication

The integration between fiber-optic communication and sensing has gained enormous interest worldwide due to the potential for utilizing existing communication cable infrastructure for low cost structural-health monitoring and security applications. Forward distributed acoustic sensing is particularly promising for such integration, as the sensing and communication signals co-propagate, and grants advantages including long sensing distance, high sensitivity and wide frequency response. In this work, an integrated architecture is proposed and experimentally demonstrated for unidirectional fiber-optic communication and distributed sensing, which solves the detector synchronization problem that has hindered practical application. The sensing demodulator is separated from the communication demodulator by choice, in order to address practical deployment requirements regarding a closed-source commercial communication system and data security. The adoption of the self-referencing forward interferometry scheme improves optical stability, streamlines design and reduces cost. As demonstration, a quadrature phase-shift keying (QPSK) communication signal at a 1 Gbps modulation rate is multiplexed onto the optical carrier, with a transmission fiber length of ∼90 km. The experimental results show that enabling the communication modulation does not cause significant degradation in sensing performance. The system demonstrated a sensitivity of 0.83 rad/V at 50 MHz, facilitating high-frequency ultrasonic detection in the 1MHz-50 MHz range, with a spatial resolution of 0.64 m at 5 MHz. This integrated architecture enables distributed sensing within the existing framework of communications while providing a means for data security, does not occupy additional bandwidth, and has the potential to unlock massively scalable sensing solutions.