Distributed fiber sensing possesses the unique ability to measure the distributed profile of an environmental quantity along many tens of kilometers with spatial resolutions in the meter or even ...centimeter scale. This feature enables distributed sensors to provide a large number of resolved points using a single optical fiber. However, in current systems, this number has remained constrained to a few hundreds of thousands due to the finite signal-to-noise ratio (SNR) of the measurements, which imposes significant challenges in the development of more performing sensors. Here, we propose and experimentally demonstrate an ultimately optimized distributed fiber sensor capable of resolving 2100000 independent points, which corresponds to a one-order-of-magnitude improvement compared to the state-of-the-art. Using a Brillouin distributed fiber sensor based on phase-modulation correlation-domain analysis combined with temporal gating of the pump and time-domain acquisition, a spatial resolution of 8.3 mm is demonstrated over a distance of 17.5 km. The sensor design addresses the most relevant factors impacting the SNR and the performance of medium-to-long range sensors as well as of sub-meter spatial resolution schemes. This step record in the number of resolved points could be reached due to two theoretical models proposed and experimentally validated in this study: one model describes the spatial resolution of the system and its relation with the sampling interval, and the other describes the amplitude response of the sensor, providing an accurate estimation of the SNR of the measurements.
This paper proposes and experimentally validates a distributed temperature alarm system based on carbon dioxide (CO 2 )-filled 18 m side air-holes fiber (SAHF), interrogated through a conventional ...(incoherent) optical time-domain reflectometer (OTDR). Customizable alarm threshold temperatures can be designed and set by adjusting the pressure of the CO 2 filling the air-hole region, which in turn determines a threshold temperature under which CO 2 liquefies. The observed slopes of the backscattered Rayleigh intensity trace as a function of the position along the fiber serve as indicators of the CO 2 states, allowing the identification of both gaseous and liquid phases through variations in the optical attenuation. Utilizing a comparative analysis of curve slopes, the proposed method enables the distributed identification and localization for both cold and hot spots along the fiber. The spatial resolution and the required loss accuracy are subject to the classical tradeoff inherent to OTDR interrogation. In this study, two interrogation wavelengths (namely, 1310 nm and 1550 nm) are exploited. Results point out that the selection of the operating wavelength gives rise to optical attenuation factors with distinct contrasts along the OTDR traces, allowing the optimization of the system to meet specific measurement objectives. In particular, the use of an optical source at 1550 nm demonstrates superior measurement accuracy within short fiber lengths, revealing a higher loss contrast between gas and liquid phases. Conversely, a 1310 nm light source is deemed more suitable for long-distance monitoring due to the lower optical loss in cold spots (CO 2 in liquid phase), providing relevant measurements over extended cooled fiber sections. Based on our experimental results using a spatial resolution of 30 cm, the maximum detection length is limited to 36.0 m or 16.0 m at 1310 nm or 1550 nm, respectively, which take place when all CO 2 liquefies inside the fiber.
Modulation instability is thoroughly investigated and a simple analytical model for its power critically modifying the wave properties in terms of system parameters is derived and experimentally ...validated. The differences on the modulation instability gain spectrum in lossless and lossy optical fibers are analyzed based on theoretical models and numerical simulations. In particular the impact of background noise on the behavior of modulation instability is studied analytically and verified by measurements and simulations. The proposed analytical model is experimentally validated by monitoring the wave propagation along an optical fiber using a Brillouin optical time-domain analyzer. This way, the evolution of the optical signal traveling through optical fibers, especially, the pump depletion and the recurrence phenomenon are investigated.
The signal-to-noise ratio (SNR) of Brillouin optical time-domain analyzers (BOTDA) is modelled and experimentally validated, using direct detection with and without the use of optical ...pre-amplification. The behavior of SNR as a function of the Brillouin gain and the probe power reaching the photo detection is analyzed in depth using this developed model and checked using two photodetectors with different specifications. It proves that a pre-amplification associated to a good-quality photodetector and a well-matched post-processing filtering can secure the highest SNR for direct-detection BOTDA. Such an optimal SNR presents only a 2.3 dB penalty compared to the ideal shot-noise-limited case that can only be reached using rather sophisticated configurations. In addition, the model here established predicts the SNR at any fiber position in any given experimental condition.
Brillouin scattering has been widely exploited for advanced photonics functionalities such as microwave photonics, signal processing, sensing, lasing, and more recently in micro- and nano-photonic ...waveguides. Most of the works have focused on the opto-acoustic interaction driven from the core region of micro- and nano-waveguides. Here we observe, for the first time, an efficient Brillouin scattering generated by an evanescent field nearby a single-pass sub-wavelength waveguide embedded in a pressurised gas cell, with a maximum gain coefficient of 18.90 ± 0.17 m
W
. This gain is 11 times larger than the highest Brillouin gain obtained in a hollow-core fibre and 79 times larger than in a standard single-mode fibre. The realisation of strong free-space Brillouin scattering from a waveguide benefits from the flexibility of confined light while providing a direct access to the opto-acoustic interaction, as required in free-space optoacoustics such as Brillouin spectroscopy and microscopy. Therefore, our work creates an important bridge between Brillouin scattering in waveguides, Brillouin spectroscopy and microscopy, and opens new avenues in light-sound interactions, optomechanics, sensing, lasing and imaging.
The robustness of bipolar pulse coding against pump depletion issues in Brillouin distributed fiber sensors is theoretically and experimentally investigated. The presented analysis points out that ...the effectiveness of bipolar coding in Brillouin sensing can be highly affected by the power unbalance between -1's and + 1's elements resulting from depletion and amplification of coded pump pulses. In order to increase robustness against those detrimental effects and to alleviate the probe power limitation imposed by pump depletion, a technique using a three-tone probe is proposed. Experimental results demonstrate that this method allows increasing the probe power by more than 12.5 dB when compared to the existing single-probe tone implementation. This huge power increment, together with the 13.5 dB signal-to-noise enhancement provided by 512-bit bipolar Golay codes, has led to low-uncertainty measurements (< 0.9 MHz) of the local Brillouin peak gain frequency over a real remoteness of 100 km, using a 200 km-long fiber-loop and 2 m spatial resolution. The method is evaluated with a record figure-of-merit of 380'000.
The performance of unipolar unicolor coded Brillouin optical time-domain analysis (BOTDA) is evaluated based on both Simplex and Golay codes. Four major detrimental factors that limit the system ...performance, including decoded-gain trace distortion, coding pulse power non-uniformity, polarization pulling and higher-order non-local effects, are thoroughly investigated. Through theoretical analysis and an experimental validations, solutions and optimal design conditions for unipolar unicolor coded BOTDA are clearly established. First, a logarithmic normalization approach is proposed to resolve the linear accumulated Brillouin amplification without distortion. Then it is found out that Simplex codes are more robust to pulse power non-uniformity compared to Golay codes; whilst the use of a polarization scrambler must be preferred in comparison to a polarization switch to mitigate uncompensated fading induced by polarization pulling in the decoded traces. These optimal conditions enables the sensing performance only limited by higher-order non-local effects. To secure systematic errors below 1.3 MHz on the Brillouin frequency estimation, while simultaneously reaching the maximum signal-to-noise ratio (SNR), a mathematical model is established to trade-off the key parameters in the design, i.e., the single-pulse Brillouin amplification, code length and probe power. It turns out that the optimal SNR performance depends in inverse proportion on the value of maximum single-pulse Brillouin amplification, which is ultimately determined by the spatial resolution. The analysis here presented is expected to serve as a quantitative guideline to design a distortion-free coded BOTDA system operating at maximum SNR.
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