Interference fading suppression in φ-OTDR using space-division multiplexed probes.

We propose and experimentally demonstrate a novel interference fading suppression method for phase-sensitive optical time domain reflectometry (φ-OTDR) using space-division multiplexed (SDM) pulse probes in a few-mode fiber. The SDM probes consist of multiple different modes, and three spatial modes (LP01, LP11a, and LP11b) are used in this work for the proof of concept. Firstly, the Rayleigh backscattering light of different modes is experimentally characterized, and it turns out that the waveforms of the φ-OTDR traces for distinct modes are all different and independent. Thanks to the spatial difference of the fading positions for distinct modes, multiple probes from spatially multiplexed modes can be used to suppress the interference fading in φ-OTDR. Then, the performances of the φ-OTDR systems using a single probe and multiple probes are evaluated and compared. Specifically, the statistical analysis shows that the fading probabilities over both the fiber length and the time scale are reduced significantly by using multiple SDM probes, which verifies the significant performance improvement on fading suppression. By introducing the concept of SDM to φ-OTDR, the proposed novel interference fading suppression method avoids the complicated frequency or phase modulation, which has the advantages of simplicity, good effectiveness and high reliability.

Enabling long range distributed vibration sensing using multicore fiber interferometers.

We report a novel long range distributed fiber optic vibration sensor using two counter-propagating interferometers, which are space-division multiplexed in different cores of a seven-core fiber. The proposed system requires only one laser source. Thanks to the spatially separated structure of the two interferometers, it no longer suffers from the coherent Rayleigh backscattering noise, and, since forward transmitting CW light instead of weak backscattering light is used, as a result, the sensing range can be significantly extended. On the other hand, because the two interferometers are implemented in one single compact multicore fiber, the output temporal waveforms have good correlation; therefore, the sampled waveforms can be used directly to determine the vibration location through cross correlation, and no additional complicated data processing (e.g., time-frequency analysis methods) is required. For proof of concept, we achieved distributed vibration detection with 38.5 km sensing range, and the root mean square error of positioning is estimated to be about 54.9 m with 120 measurements. The proposed system has the unique advantages of simple setup, high sensitivity, large dynamic range, ultra-long sensing range, etc.

BOTDA using channel estimation with direct-detection optical OFDM technique.

A novel Brillouin optical time-domain analysis (BOTDA) system without frequency sweep operation is proposed using intensity-modulated direct detection optical orthogonal frequency division multiplexing (IM-DD-OOFDM) probe signal. The influence of peak to average power ratio (PAPR) of OFDM probe signal on the recovery of Brillouin gain spectrum (BGS) is analyzed in theory and experiment. The complex BGS is reconstructed by channel estimation algorithm and Brillouin frequency shift (BFS) is located by curve fitting of intensity spectrum. The IM-DD-OOFDM BOTDA is demonstrated experimentally with 25 m spatial resolution over 10 km standard single mode fibers within much less measurement time.

Towards large dynamic range and ultrahigh measurement resolution in distributed fiber sensing based on multicore fiber.

Featuring a dependence of Brillouin frequency shift (BFS) on temperature and strain changes over a wide range, Brillouin distributed optical fiber sensors are however essentially subjected to the relatively poor temperature/strain measurement resolution. On the other hand, phase-sensitive optical time-domain reflectometry (Φ-OTDR) offers ultrahigh temperature/strain measurement resolution, but the available frequency scanning range is normally narrow thereby severely restricts its measurement dynamic range. In order to achieve large dynamic range and high measurement resolution simultaneously, we propose to employ both the Brillouin optical time domain analysis (BOTDA) and Φ-OTDR through space-division multiplexed (SDM) configuration based on the multicore fiber (MCF), in which the two sensors are spatially separately implemented in the central core and a side core, respectively. As a proof of concept, the temperature sensing has been performed for validation with 2.5 m spatial resolution over 1.565 km MCF. Large temperature range (10 °C) has been measured by BOTDA and the 0.1 °C small temperature variation is successfully identified by Φ-OTDR with ~0.001 °C resolution. Moreover, the temperature changing process has been recorded by continuously performing the measurement of Φ-OTDR with 80 s frequency scanning period, showing about 0.02 °C temperature spacing at the monitored profile. The proposed system enables the capability to see finer and/or farther upon requirement in distributed optical fiber sensing.

Few-mode fiber based Raman distributed temperature sensing.

We proposed and experimentally demonstrated a few mode fiber (FMF) based Raman distributed temperature sensor (RDTS) to extend the sensing distance with enhanced signal-to-noise ratio (SNR) of backscattered anti-Stokes spontaneous Raman scattering. Operating in the quasi-single mode (QSM) with efficient fundamental mode excitement, the FMF allows much larger input pump power before the onset of stimulated Raman scattering compared with the standard single mode fiber (SSMF) and mitigates the detrimental differential mode group delay (DMGD) existing in the conventional multimode fiber (MMF) based RDTS system. Comprehensive theoretical analysis has been conducted to reveal the benefits of RDTS brought by QSM operated FMFs with the consideration of geometric/optical parameters of different FMFs. The measurement uncertainty of FMF based scheme has also been evaluated. Among fibers being investigated and compared (SSMF, 2-mode and 4-mode FMFs, respectively), although an ideal 4-mode FMF based RDTS has the largest SNR enhancement in principle, real fabrication imperfections and larger splicing loss degrade its performance. While the 2-mode FMF based system outperforms in longer distance measurement, which agrees well with the theoretical calculations considering real experimental parameters. Using the conventional RDTS hardware, a 30-ns single pulse at 1550nm has been injected as the pump; the obtained temperature resolutions at 20km distance are estimated to be about 10°C, 7°C and 6°C for the SSMF, 4-mode and 2-mode FMFs, respectively. About 4°C improvement over SSMF on temperature resolution at the fiber end with 3m spatial resolution within 80s measuring time over 20km 2-mode FMFs have been achieved.

Spatial-division multiplexed Brillouin distributed sensing based on a heterogeneous multicore fiber.

We have experimentally investigated spatial-division multiplexed (SDM) Brillouin optical time-domain analysis in a heterogeneous multicore fiber whose central core and six outer cores are made from different preforms, showing a ∼70 MHz Brillouin frequency shift (BFS) difference between them. It reveals that the heterogeneous central core and the outer cores have different temperature sensitivities, but their strain sensitivities are almost the same. By making use of the distinct temperature coefficients of these two kinds of cores, simultaneous and discriminative temperature and strain measurements are achieved. The bending-induced Brillouin gain spectrum (BGS) broadening issue in off-center cores has been clarified, and a solution has been proposed to eliminate the uncertainty caused by a bending-induced BFS shift, by averaging the BFS variations of two symmetrical outer cores. We show a new perspective for discriminative measurement in Brillouin distributed sensors based on SDM solutions.

Spatial-division multiplexed hybrid Raman and Brillouin optical time-domain reflectometry based on multi-core fiber.

Spatial-division multiplexed (SDM) hybrid Raman- and Brillouin- optical time-domain reflectometry (RODTR and BODTR) utilizing the multi-core fiber has been proposed and experimentally demonstrated. The solution is proposed in order to overcome the incompatible input pump power required for hybrid ROTDR and BOTDR in single mode fiber (SMF), while ensuring the capability of discriminative measurement between temperature and strain. What's more, the central core has been intentionally chosen to implement BOTDR so as to avoid bending-induced cross-sensitivity on Brillouin frequency shift (BFS) measurement. The proposed system utilizes a single laser source, shared pump generation devices, but separate interrogation fiber channels, thus enabling efficient input power management for the two reflectometry, allowing for simultaneous measurement of spontaneous Raman scattering and Brillouin scattering. The worst temperature and strain resolutions are estimated to be about 2.2 °C and 40 µÎµ respectively in 6 km sensing range with 3 m spatial resolution.