Time-gated Digital Optical Frequency Domain Research Articles

Silicon photonic integrated interrogator for fiber-optic distributed acoustic sensing

Distributed acoustic sensing (DAS) technology has been a promising tool in various applications. Currently, the large size and relatively high cost of DAS equipment composed of discrete devices restrict its further popularization to some degree, and the photonic integration technology offers a potential solution. In this paper, we demonstrate an integrated interrogator for DAS on the silicon-on-insulator (SOI) platform. The design of the chip revolves around a Mach–Zehnder modulator (MZM) transmitter and a dual-quadrature and dual-polarization coherent receiver. The integrated interrogator supports multiple DAS schemes, including the time-gated digital optical frequency domain reflectometry (TGD-OFDR), which is adopted for system performance evaluation. 59 pε/Hz strain resolution in 12.1 km sensing fiber with 1.14 m spatial resolution (SR) is realized. Besides, along 49.0 km sensing fiber, 81 pε/Hz strain resolution with 3.78 m SR is achieved. The results show that the integrated interrogator has comparable performance to the discrete DAS system. To the best of our knowledge, this is the first dedicated on-chip DAS interrogator, which validates the effectiveness of the blend of photonics integration and DAS technology.

Scanning-free hybrid Rayleigh-Brillouin distributed fiber-optic sensing system.

Hybrid systems based on Brillouin optical time domain analysis (BOTDA) utilizing Rayleigh backscattering light wave as a probe have enabled single-end and long-range distributed sensing for multiple parameters. However, the spatial resolution for dynamic parameter measurement is limited, and the frequency scanning process of BOTDA is time-consuming. To address these challenges, we propose a hybrid system that combines BOTDA and time-gated digital optical frequency domain reflectometry (TGD-OFDR), aiming to enhance the spatial resolution of dynamic measurements without compromising the system's signal-to-noise ratio and eliminate the frequency scanning process of BOTDA. In the experimental setup, we conducted measurements on a 9.52 km single-mode fiber. A sinusoidal vibration with a frequency of 3 kHz was measured with a spatial resolution of 3 m, achieving a noise floor of 0.05 nε/√Hz. Furthermore, temperature measurements with a spatial resolution of 10 m and a Brillouin frequency shift (BFS) measurement accuracy of 0.74 MHz were successfully obtained using the scanning-free single-end BOTDA technique. This hybrid system shows promising potential for various applications in distributed fiber-optic sensing.

Multipath Quasi-Distributed Acoustic Sensing Based on Space Division Multiplexing and Time-Gated OFDR

Quasi-distributed optic sensing system has been widely utilized in various applications for its simple configuration, high sensing resolution, and long sensing range. In large sensing network, multipath sensing arrays are needed to be monitored simultaneously. This paper proposes a multi-path quasi-distributed acoustic sensing system based on time-gated digital optical frequency domain reflectometry (TGD-OFDR) and spatial division multiplexing (SDM) technique. The single mode fiber segment between weak reflectors is efficiently used. In experiments, vibrations on two weak reflectors array are detected simultaneously, and crosstalk between pathes is analyzed. This multi-path system does not require additional hardware equipment and the bandwidth of the system is not sacrificed, which has great potential in large-scale sensing network.

Distributed Fiber-Optic Acoustic Sensor for Sparse-Wideband Vibration Sensing With Time Delay Sampling

This paper proposes a novel distributed fiber-optic acoustic sensor, which can solve the trade-off between the measurable distance and the maximum detectable frequency. The system is based on time-gated digital optical frequency domain reflectometry combined with time delay sampling method and frequency division multiplexing. In experiments, vibration with frequency up to 13 kHz and 47 kHz are detected simultaneously with 10 kHz pulse repetition rate. The system reached a high signal-to-noise ratio of 25 dB and spatial resolution of 8 m. This system does not require additional hardware equipment and has high flexibility, which could be utilized in specific occasions.

108-km Distributed Acoustic Sensor With 220-p$\varepsilon /\surd$Hz Strain Resolution and 5-m Spatial Resolution

The measurement distance is one of the most important parameters for distributed acoustic sensor (DAS). In this paper, we report a long-distance and high-sensitivity DAS system based on time-gated digital optical frequency domain reflectometry. The bi-directional distributed Raman amplification is adopted to realize long measurement distance. The shape of optical pulse is pre-distorted to be a standard Hanning window, which provides a theoretical peak-side lobe ratio of 46 dB to suppress the crosstalk. The interference fading and polarization fading are well suppressed, and hence phase-demodulation method is adopted, instead of intensity demodulation method. As a result, the sensitivity is enhanced and the full information (amplitude, phase, and frequency) of the vibration can be obtained. In experiments, the fiber length is about 108 km, whereas the spatial resolution is 5 m. A weak vibration with peak-peak amplitude of 14.7 nε is correctly located at the distance of 98 km with a high SNR of 30 dB. It is the first time that 220-pε/√Hz strain sensitivity is realized over 100-km-level fiber and the vibration waveform is retrieved linearly without harmonics.

High-fidelity distributed fiber-optic acoustic sensor with fading noise suppressed and sub-meter spatial resolution.

In order to solve fading problem and realize sub-meter spatial resolution in DAS, this paper proposes a novel configuration of time-gated digital optical frequency domain reflectometry (TGD-OFDR) based on optical intensity modulator (IM). IM has a large modulation bandwidth and the positive and negative harmonics can be fully used to suppress fading while the spatial resolution remains unchanged. In experiments, with fading suppressed, the spatial resolution of DAS is 0.8 m and the strain resolution is about 245.6 pε√Hz along the total 9.8-km sensing fiber. The response bandwidth of vibration is 5 kHz, only/limited by the fiber length.

Distributed Fiber-Optic Acoustic Sensor With Enhanced Response Bandwidth and High Signal-to-Noise Ratio

This paper proposes a novel distributed fiber-optic acoustic sensor, which can solve both the tradeoff between the maximum measurable distance and the spatial resolution, and that between the measurement distance and the vibration response bandwidth. The system is based on frequency-division-multiplexing time-gated digital optical frequency domain reflectometry, which consecutively injects linear-frequency-modulated probe pulses with different frequency ranges. Undersampling method is introduced to reduce the sampling rate of the analog-to-digital converter and the data size, which can reduce the cost of the system and facilitate real-time data processing. In experiments, two simultaneous vibrations with frequency up to 9 kHz are detected over the 24.7-km-long fiber, with a sign-to-noise ratio of 30 dB and spatial resolution of 10 m.

Distributed fiber-optic vibration sensing based on phase extraction from time-gated digital OFDR.

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.

Time-gated digital optical frequency domain reflectometry with 1.6-m spatial resolution over entire 110-km range.

A novel time-gated digital optical frequency domain reflectometry (TGD-OFDR) technique with high spatial resolution over long measurement range is proposed and experimentally demonstrated. To solve the contradictory between the tuning rate of lightwave frequency, which determines the spatial resolution, and the measurable distance range in traditional OFDR, our proposed scheme sweeps the frequency of probe beam only within a time window, while the local reference remains a frequency-stable continuous lightwave. The frequency-to-distance mapping is digitally realized with equivalent references in data domain. In demonstrational experiments, a 1.6-m spatial resolution is obtained over an entire 110-km long fiber link, proving that the phase noises of the laser source as well as environmental perturbations are well suppressed. Meanwhile, the dynamic range was 26 dB with an average of only 373 measurements. The proposed reflectometry provides a simple-structure and high-performance solution for the applications where both high spatial resolution and long distance range are required.