The integration of high-speed optical communication and distributed sensing could bring intelligent functionalities to ubiquitous optical fibre networks, such as urban structure imaging, ocean seismic detection, and safety monitoring of underground embedded pipelines. This work demonstrates a scheme of integrated sensing and communication in an optical fibre (ISAC-OF) using the same wavelength channel for simultaneous data transmission and distributed vibration sensing. The scheme not only extends the intelligent functionality for optical fibre communication system, but also improves its transmission performance. A periodic linear frequency modulation (LFM) light is generated to act as the optical carrier and sensing probe in PAM4 signal transmission and phase-sensitive optical time-domain reflectometry (Φ-OTDR), respectively. After a 24.5 km fibre transmission, the forward PAM4 signal and the carrier-correspondence Rayleigh backscattering signal are detected and demodulated. Experimental results show that the integrated solution achieves better transmission performance (~1.3 dB improvement) and a larger launching power (7 dB enhancement) at a 56 Gbit/s bit rate compared to a conventional PAM4 signal transmission. Meanwhile, a 4 m spatial resolution, 4.32-nε/ $$\sqrt {Hz}$$H z strain resolution, and over 21 kHz frequency response for the vibration sensing are obtained. The proposed solution offers a new path to further explore the potential of existing or future fibre-optic networks by the convergence of data transmission and status sensing. In addition, such a scheme of using shared spectrum in communication and distributed optical fibre sensing may be used to measure non-linear parameters in coherent optical communications, offering possible benefits for data transmission.

A novel distributed fiber vibration sensing technique based on phase extraction from time-gated digital optical frequency domain reflectometry (TGD-OFDR) which can achieve quantitative vibration measurement with high spatial resolution and long measurement range is proposed. A 90 degree optical hybrid is used to extract phase information. By increasing frequency sweeping speed, the influence of environmental phase disturbance on TGD-OFDR is mitigated significantly, which makes phase extraction in our new scheme more reliable than that in conventional OFDR-based method, leading to the realization of long distance quantitative vibration measurement. By using the proposed technique, a distributed vibration sensor that has a measurement range of 40 km, a spatial resolution of 3.5 m, a measurable vibration frequency up to 600 Hz, and a minimal measurable vibration acceleration of 0.08g is demonstrated.

An approach of phase-shift transform is proposed and demonstrated to suppress the interference fading in phase sensitive optical time domain reflectometry (-OTDR). The phase-shift transform is executed in the digital domain and the complicated frequency modulation is not required. The operation principle is carefully analyzed and a signal with complementary amplitude can be obtained with this method. Through synthesizing the complementary signals, the false phases can be corrected. Benefiting from the flexibility of this approach, multiple transforms are implemented to make sure that all phases are retrieved accurately. Experimental results show that the intensity fluctuation over 60 dB is reduced to 15 dB, which decreases the standard deviation of differential phase to 0.0224. Besides, 30 dynamic measurements are tested and all distorted phases have been corrected, indicating the high reliability of this method.

The frequency division multiplexing (FDM) technique is firstly introduced into a direct-detection phase-sensitive OTDR to improve the distributed acoustic sensing (DAS) performance by using a frequency step sweeping laser source and a dual-pulse heterodyne detection scheme. A raised-cosine-shaped pulse is used to suppress the crosstalk in FDM technique. By using this technique, a 40-kS/s sampling rate to vibration is realized with a 10-km measurement range, which implies the tradeoff relationship between the frequency response and the measurement range is broken. In the experiment, vibrations with different frequencies are measured to validate the effectiveness of the proposed technique. A 20-kHz frequency response is achieved over a 10-km measurement distance, and the frequency response shows a good flatness with a fluctuation ∼0.5 dB.

In this Letter, we demonstrate a highly sensitive quasi-distributed fiber-optic acoustic sensing system. This system interrogates a weak reflector array by using phase-sensitive optical time domain reflectometry with coherent detection. A phase-noise-compensated configuration is proposed to reduce the influence of $1/f$ noise, which is the main limitation when coherent detection is used in this high-sensitivity system. In the experiment to simulate the scenario with a link loss of 20 km fiber and 10 m array interval, the noise level shows an acoustic signal sensitivity of $3.84\mathrm{pϵ}/\surd \mathrm{Hz}$ at 10–2500 Hz.

In this paper, we propose and demonstrate a millimeter-resolution long-range optical frequency domain reflectometry (OFDR) using an ultra-linearly 100-GHz swept optical source realized by injection-locking technique and cascaded four-wave-mixing (FWM) process. The ultra-linear sweep is realized using an external modulation method with a linearly swept radio frequency (RF) signal. The RF signal sweeps from 16 GHz to 19.3 GHz, and the slave laser is injection-locked to the 8th-order sideband of the master laser, achieving a frequency sweeping span of ~25 GHz. By using the injection-locked frequency-swept laser as the optical source of OFDR, we obtain a spatial resolution of 4.2 mm over 10-km measurement range. A polarization beat length of 10.5 cm is measured benefiting from the high spatial resolution. To improve the spatial resolution further, FWM process is used to broaden the frequency sweeping span. Frequency sweeping span of ~100 GHz is achieved with cascaded FWM. We demonstrate a 1.1-mm spatial resolution over 2-km measurement range with the proposed ultra-linearly swept optical source.