Long-range Brillouin Optical Time-domain Analysis Research Articles

Towards fast sensing along ultralong BOTDA: flatness enhancement by utilizing injection-locked dual-bandwidth probe wave.

Brillouin optical time-domain analysis (BOTDA) using distributed Brillouin amplification (DBA) only requires a milliwatt-level pump to achieve a sensing range beyond 100 km, which provides a powerful tool for temperature/strain sensing. However, similar to the majority of other long-range BOTDAs, the state-of-the-art reports require > 1000 times average, severely restricting the sensing speed. The blind area over tens of kilometers caused by the nonuniform Brillouin response and parasitic amplitude modulation (AM) are crucial factors affecting the signal-to-noise ratio (SNR). Here, a comprehensive performance optimization and substantial enhancement for BOTDA sensors was presented by the direct demodulation of an injection-locked dual-bandwidth probe wave. Injection locking (IL) can completely eliminate the impact of AM noise; dual-bandwidth probe enables self-adaptive pulse loss compensation, thereby intensifying the SNR flatness along the ultralong fiber, and direct probe demodulation can overcome nonlocal effects and allows ∼19.7 dB enhancement of probe input power. Therefore, using only 100 times average, ∼148.3 km sensing, and ∼5 m spatial resolution were achieved with < ∼0.8 MHz standard deviation of Brillouin frequency shift (BFS) over a broad range (∼131.7 km). The reduction in averages was more than 10 times that of the reported majority of long-range BOTDAs. Such performances were achieved without using time-consuming or post-processing techniques, such as optical pulse coding and image denoising. Because this approach is compatible with optical chirp chain technique without frequency sweeping, fast acquisition (0.3 s) was also realized, which has the potential for fast sensing at 3.3 Hz along a ∼150 km fiber.

150 km fast BOTDA based on the optical chirp chain probe wave and Brillouin loss scheme.

Distributed long-range Brillouin optical time domain analysis (BOTDA) is an extremely time-consuming sensing scheme, which requires frequency mapping of the Brillouin spectrum and a large number of average times. Here, we propose a fast long-range BOTDA based on the optical chirp chain (OCC) probe wave and Brillouin loss scheme. The OCC-modulated probe wave is enabled by cascading fast-frequency-changing microwave chirp segments head-to-tail, which covers a large frequency range around the anti-Stokes frequency relative to the pump wave. The combination of the OCC technique and Brillouin loss scheme provides several advantages, i.e., fast measurement, a high Brillouin threshold, no additional amplification scheme, and freedom from the nonlocal effect. In the experiment, 6m spatial resolution, 3.2s measurement time, and 3MHz measurement precision were achieved over a 150km single-mode fiber.

Comparative study on reduced impacts of Brillouin pump depletion and nonlinear amplification in coded DBA-BOTDA

The impacts of Brillouin pump depletion and nonlinear amplification in coded long-range Brillouin optical time-domain analysis (BOTDA) based on distributed Brillouin amplification (DBA) were studied. The error of Brillouin frequency shift (BFS) due to Brillouin pump depletion was compared for DBA-BOTDA using non-cyclic and cyclic coding. For non-cyclic coding, significant over- and under-shoots of BFS were found in the range with larger BFS variation, such as hot spot. The impact of Brillouin pump depletion can be reduced considerably by cyclic coding. Furthermore, to compensate the BFS error due to nonlinear amplification, a simple and effective log linearization was proposed and demonstrated.

Effects of pump pulse extinction ratio in Brillouin optical time-domain analysis sensors.

We report on two previously unknown non-local effects that have been found to impair Brillouin optical time-domain analysis (BOTDA) sensors that deploy limited extinction ratio (ER) pump pulses. The first one originates in the increased depletion of the pedestal of the pump pulses by the amplified probe wave, which in turn entails a reduced amplification of the probe and a measurement distortion. The second effect is due to the interplay between the transient response of the erbium-doped fiber amplifiers (EDFA) that are normally deployed to amplify the pump and the pedestal of the pump pulses. The EDFA amplification modifies the pedestal that follows the pulses in such a way that it also leads to a distortion of the measured gain spectra after normalization. Both effects are shown to lead to non-local effects in the measurements that have similar characteristics to those induced by pump pulse depletion. In fact, the total depletion factor for calculations of the Brillouin frequency shift (BFS) error in BOTDA sensors is shown to be the addition of the depletion factors linked to the pump pulse as well as the pedestal. A theoretical model is developed to analyze both effects by numerical simulation. Furthermore, the effects are investigated experimentally in long-range BOTDA sensors. The pedestal depletion effect is shown to severely constrain the probe power as well as the minimum ER of the pulses that can be deployed in BOTDA sensors. For instance, it is shown that, in a long-range dual-probe BOTDA, an ER higher that 32-dB, which is above that provided by standard electro-optic modulators (EOM), is necessary to be able to deploy a probe power of -3 dBm, which is the theoretical limit for that type of sensors. Even more severe can be the limitation due to the depletion effect induced by the EDFA transient response. It is found that the impairments brought by this effect are independent of the probe power, hence setting an ultimate limit for the BOTDA sensor performance. Experimentally, a long-range BOTDA deploying a 26-dB ER EOM and a conventional EDFA is shown to exhibit a BFS error higher than 1 MHz even for very small probe power.

High-accuracy and long-range Brillouin optical time-domain analysis sensor based on the combination of pulse prepump technique and complementary coding

A Brillouin optical time-domain analysis (BOTDA) sensor that combines the conventional complementary coding with the pulse prepump technique for high-accuracy and long-range distributed sensing is implemented and analyzed. The employment of the complementary coding provides an enhanced signal-to-noise ratio (SNR) of the sensing system and an extended sensing distance, and the measurement time is also reduced compared with a BOTDA sensor using linear coding. The combination of pulse prepump technique enables the establishment of a preactivated acoustic field in each pump pulse of the complementary codeword, which ensures measurements of high spatial resolution and high frequency accuracy. The feasibility of the prepumped complementary coding is analyzed theoretically and experimentally. The experiments are carried out beyond 50-km single-mode fiber, and experimental results show the capabilities of the proposed scheme to achieve 1-m spatial resolution with temperature and strain resolutions equal to ∼1.6°C and ∼32 μϵ, and 2-m spatial resolution with temperature and strain resolutions equal to ∼0.3°C and ∼6 μϵ, respectively. A longer sensing distance with the same spatial resolution and measurement accuracy can be achieved through increasing the code length of the prepumped complementary code.

Long-range BOTDA sensor over 50 km distance employing pre-pumped Simplex coding

We propose and demonstrate a long-range Brillouin optical time-domain analysis (BOTDA) sensor based on the pre-pumped Simplex coding technique. This approach combines Simplex coding with the pulse pre-pump technique, which takes full advantage of the signal-to-noise ratio enhancement provided by optical pulse coding and achieves meter-scale spatial resolution with an unbroadened Brillouin gain spectrum by the use of pre-pumped short pump pulse. Compared to the widely used differential pulse-width pair technique, a comparable performance can be realized while the measurement time is reduced by half. The theoretical analysis of pre-pumped Simplex coding applied to BOTDA systems is presented and a proof-of-concept experiment is carried out along a ∼51 km single-mode fiber composed of two fiber segments with slightly different Brillouin frequency shift values. With the proposed technique, Brillouin frequency shifts of the two different fiber sections at the end of the sensing fiber are clearly distinguished, and the obtained spatial resolution and temperature/strain accuracy along the sensing fiber are respectively 1 m and . According to the experimental results, we believe that the BOTDA employing pre-pumped Simplex coding is able to realize extra-long distance sensing without Raman amplification, while keeping a high spatial resolution and measurement accuracy.

Overcoming Nonlocal Effects and Brillouin Threshold Limitations in Brillouin Optical Time-Domain Sensors

We demonstrate, for the first time to our knowledge, a Brillouin optical time-domain analysis (BOTDA) sensor that is able to operate with a probe power larger than the Brillouin threshold of the deployed sensing fiber and that is free from detrimental nonlocal effects. The technique is based on a dual-probe-sideband setup in which an optical frequency modulation of the probe waves along the fiber is introduced. This makes the optical frequency of the Brillouin interactions induced by each probe wave on the pump vary along the fiber so that two broadband Brillouin gain and loss spectra that perfectly compensate are created. As a consequence, the pulse spectral components remain undistorted, avoiding nonlocal effects. Therefore, very large probe power can be injected, which improves the signal-to-noise ratio (SNR) in detection for long-range BOTDA. Moreover, the probe power can even exceed the Brillouin threshold limit due to its frequency modulation, which reduces the effective amplification of spontaneous Brillouin scattering in the fiber. Experiments demonstrate the technique in a 50-km sensing link in which 8 dBm of probe power is injected.

Non-local effects in dual-probe-sideband Brillouin optical time domain analysis.

According to recent models, non-local effects in dual-probe-sideband Brillouin Optical Time Domain Analysis (BOTDA) systems should be essentially negligible whenever the probe power is below the Stimulated Brillouin Scattering (SBS) threshold. This paper shows that actually there appear non-local effects in this type of systems before the SBS threshold. To explain these effects it is necessary to take into account a full spectral description of the SBS process. The pump pulse experiences a frequency-dependent spectral deformation that affects the readout process differently in the gain and loss configurations. This paper provides a simple analytical model of this phenomenon, which is validated against compelling experimental data, showing good agreement. The main conclusion of our study is that the measurements in gain configuration are more robust to this non-local effect than the loss configuration. Experimental and theoretical results show that, for a total probe wave power of ~1 mW (500 μW on each sideband), there is an up-shifting of ~1 MHz in the Brillouin Frequency Shift (BFS) retrieved from the Brillouin Loss Spectrum, whereas the BFS extracted from the measured Brillouin Gain Spectrum is up-shifted only ~0.6 MHz. These results are of particular interest for manufacturers of long-range BOTDA systems.

Phasorial differential pulse-width pair technique for long-range Brillouin optical time-domain analysis sensors.

We introduce a novel phasorial differential pulse-width pair (PDPP) method for Brillouin optical time-domain analysis (BOTDA) sensors that combines spatial resolution enhancement with increased tolerance to non-local effects. It is based on the subtraction of the complex time-domain traces supplied by a sensor configuration that uses a phase-modulated probe wave and RF demodulation. The fundamentals of the technique are first described theoretically and using numerical simulation of the propagating waves. Then, proof-of-concept experiments demonstrate the measurement of the Brillouin frequency shift distribution over 50-km. The system is shown to withstand large variations of the pump power generated by its interaction with a powerful probe wave along the fiber; hence, highlighting the potential of the PDPP technique to increase the detected signal-to-noise ratio in long-range BOTDA. Moreover, the PDPP is also shown to increase the measurement contrast by allowing the use of relatively long duration pulses while retaining 1-m spatial resolution.

Cyclic pulse coding for fast BOTDA fiber sensors

A cyclic pulse coding technique is proposed and experimentally demonstrated for fast implementation of long-range Brillouin optical time-domain analysis (BOTDA). The proposed technique allows for accurate temperature and strain measurements with meter-scale spatial resolution over kilometers of standard single-mode fiber, with subsecond measurement times.

Distributed Brillouin Fiber Sensor With Enhanced Sensitivity Based on Anti-Stokes Single-Sideband Suppressed-Carrier Modulation

We demonstrate an alternative technique to reduce pump depletion and to improve sensitivity in the long-range Brillouin optical time-domain analysis (BOTDA) sensor. Our BOTDA system uses an anti-Stokes single-sideband suppressed-carrier modulation technique that balances the pump depletion due to fiber loss. With this technique, we show both numerically and experimentally a great improvement compared to the dual sideband standard technique.

Simplex-Coded BOTDA Sensor Over 120-km SMF With 1-m Spatial Resolution Assisted by Optimized Bidirectional Raman Amplification

Bidirectional low-noise Raman amplification and simplex coding based on the return-to-zero modulation format are optimized through numerical simulations for long-range Brillouin optical time-domain analysis sensing. Experimental results are reported on sensing capabilities along 120-km distance with 1-m spatial resolution, and worst-case temperature and strain resolution values of 1.3 and 26 , respectively.

Long-range distributed Brillouin fiber sensors by use of an unbalanced double sideband probe

We propose and demonstrate a long-range Brillouin Optical Time-Domain Analysis (BOTDA) distributed sensing system making use of an unbalanced double sideband probe formed by a Stokes and an anti-Stokes line. In particular, we show that for each measuring condition an optimal Stokes /anti-Stokes input power ratio exists, allowing a larger suppression of nonlocal effects induced by pump depletion. Experiments on a 50 km single-mode sensing fiber with 5 meters spatial resolution are reported.

Optimization of long-range BOTDA sensors with high resolution using first-order bi-directional Raman amplification

In this paper we perform an optimization of Brillouin optical time-domain analysis (BOTDA) sensors for achieving high resolution over long sensing ranges using distributed Raman amplification. By employing an optimized first-order bi-directional Raman amplification scheme and combining high-power fiber-Raman lasers and Fabry-Pérot lasers with low relative-intensity-noise (RIN), we demonstrate distributed sensing over 120 km of standard single-mode fiber with 2 meter spatial resolution and with a strain/temperature accuracy of 45με/2.1°C respectively.

Time-division multiplexing-based BOTDA over 100km sensing length

We propose and demonstrate a high-performance and long-range Brillouin optical time-domain analysis (BOTDA) based on time-division multiplexing measurement, where a probe pulse and a pump pulse are used to perform the measurement on a selected sensing section, and the measurement of the entire sensing fiber is realized by combining the series measurements over different sections through changing the delay time between the two pulses. In experiment, a 100 km sensing fiber is divided into 11 sections based on the gain-controlled principle, and spatial resolutions of 0.6 m and 2 m are obtained at the end of 75 km and 100 km, respectively.

Long-range Brillouin optical time-domain analysis sensor employing pulse coding techniques

In this paper we describe and implement a long-range Brillouin optical time-domain analysis (BOTDA) sensor, for both temperature and strain measurements, using optical pulse coding techniques. A theoretical analysis of Simplex coding applied to BOTDA systems is presented and experimentally demonstrated for both Brillouin loss and Brillouin gain configurations. With the proposed technique, ∼7.1 dB and ∼10.3 dB of signal-to-noise ratio improvements are demonstrated in BOTDA measurements using 127-bit and 511-bit Simplex codes, respectively. This feature allows us to extend the dynamic range of the measurements, overcoming the limitations to the maximum usable optical power imposed by pump depletion and modulation instability; thus, the sensing range can be extended by several tens of kilometers while keeping meter-scale spatial resolution. Experimental results show the capabilities of optical pulse coding techniques to achieve 1 m spatial resolution over 50 km of standard single-mode fiber enabling temperature and strain resolutions equal to 2.2 °C and 44 µε, respectively.