Optical Time-domain Analysis Sensors Research Articles

Determination of average times for Brillouin optical time domain analysis sensor denoising by non-local means filtering

Integration of signal average and image restoration can improve the performance of distributed optical fiber sensors. Here we illustrate the denoising behavior of non-local means filtering (NLM) affected by number of average. On the one hand, the signal average can effectively remove the high-frequency noise but be time-consuming; on the other hand, the NLM can accelerate decent in noise but is dependent on level of noise contained in raw sensing image. To best of our knowledge, this is the first time to apply NLM to conventional Brillouin optical time domain analysis sensor (BOTDA) with theoretically spatial resolution of 10 ns. Moreover, we propose an optimal averaging number determination method for further denoising by NLM to achieve trade-off between accuracy and average times. The results indicated that the optimal-times for BOTDA configured 10 ns and 10km fiber-loop is 13 with Brillouin frequency shift uncertainty of 0.85MHz. The proposed method has potential to apply in engineering applications and commercialized software for BOTDA denoising by image restoration.

Feasibility Study of a Fiber Ring Laser Working on the SLM Regime in a BOTDA Sensor

A feasibility study of the employment of a fiber laser working as the laser source of a Brillouin distributed sensor is presented in this paper. The erbium fiber ring laser structure designed works on the single longitudinal mode regime. The main parameters of the ring cavity are analyzed in terms of the laser requirements and the performance of a Brillouin optical time domain analysis sensor using the fiber laser as light source.

Nonlocal Effect Compensation in Frequency-Fixed Probe Based BOTDA Sensor

The robustness of a recently proposed Brillouin optical time-domain analysis sensor based on dual-tone probe wave with fixed frequency separation is deeply studied. It is discovered that when the distribution of Brillouin frequency shift (BFS) along the sensing fiber is not entirely uniform, non-local effect might be imposed at the middle of sensing fiber, which gives rise to systematic frequency error on the local estimated BFS. Aiming at solving this problem, we propose an approach that acquires both the upper and lower probe sidebands simultaneously using two photodiodes, and the average between the Brillouin gain and loss spectral is calculated to eliminate the detrimental impact of the non-local effect. Moreover, the impact of probe power on the distortions of both pump pulse and Brillouin gain/loss spectrum is experimentally investigated, thus the maximum allowable power of the injection probe wave is determined.

Channel Capacity of Wavelength Division Multiplexing-Based Brillouin Optical Time Domain Sensors

We propose and demonstrate a novel WDM-based Brillouin optical time domain analysis (BOTDA) sensor using Brillouin loss configuration, and theoretically analyze channel capacity of WDM-based brillouin optical time domain reflectometry (BOTDR) and BOTDA sensors by considering the fiber dispersion and nonlinear effects in sensing fibers. A three-wavelength WDM-based BOTDA senor is experimentally validated in the distributed temperature measurement of a 23-km-long standard telecom fiber (SMF-28), demonstrating an electrical signal-to-noise ratio enhancement of 9.2 dB. The numerical calculation results of the channel capacity of WDM-based Brillouin optical time domain sensors show that, when compared with the conventional single-wavelength BOTDR and BOTDA sensors, 11-wavelength BOTDR and BOTDA sensor using large effective area fiber can respectively achieve 8.4 and 20.8 dB signal-noise-ratio improvement (SNRI), and 7-wavelength BOTDR and BOTDA sensor using SMF-28 can respectively achieve 7.2 B and 16.9 dB SNRI without evident spatial resolution degradation and nonlinear impairment.

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.

Second-Order Nonlocal Effects Mitigation in Brillouin Optical Time-Domain Analysis Sensors by Tracking the Brillouin Frequency Shift Profile of the Fiber

We report on an additional limitation that has been found in Brillouin optical time-domain analysis (BOTDA) sensors due to the so-called second-order nonlocal effects (NLE). Second-order NLE appear in BOTDA setups that deploy a double probe waves to compensate the transfer of energy between the pump pulse and the probe wave, and are related to a spectral distortion of the pump pulse that leads to measurement errors and an effective limit on the maximum probe power that can be deployed in the sensor. We theoretically and experimentally demonstrate that the techniques that have been presented so far in the literature to compensate second-order NLE are only effective in the case that the Brillouin frequency shift (BFS) along the sensing fiber is uniform. However, this requirement for uniformity is not realistic in real world scenarios in which a variety of fibers with different BFS and subjected to different environmental conditions are typically deployed. Therefore, we demonstrate a new method to mitigate the effects of BFS variation in the BOTDA setups that compensate second-order NLE. This method is based on introducing an additional wavelength modulation to the probe wave so as to track the mean BFS changes along the sensing fiber link. With this method, we demonstrate a BOTDA setup that, without coding, distributed amplification, or any other form of performance enhancement, achieves a sensing length of 120 km with 3-m spatial resolution and 2-MHz measurement precision. Moreover, the setup demonstrates, to our knowledge, the largest probe power ever injected in a BOTDA sensing link.

Non-Local Effects in Brillouin Optical Time-Domain Analysis Sensors

Brillouin optical time-domain analysis (BOTDA) sensors have great potential to provide distributed measurements of temperature and strain over large structures with high spatial resolution and measurement precision. However, their performance ultimately depends on the amount of probe and pump pulse power that can be injected into the sensing fiber, which determines the signal-to-noise ratio of the detected measurement signal. The probe wave power is constrained by the generation of noise induced by spontaneous Brillouin scattering and at lower power by the so-called non-local effects. In this work, we focus on the latter. We review the physical origins of non-local effects and analyze the performance impairments that they bring. In addition, we discuss the different methods that have been proposed to counteract these effects comparing their relative merits and ultimate performance. Particularly, we focus on a technique that we have devised to compensate non-local effects which is based on introducing an optical frequency modulation or dithering to the probe wave. This method is shown to provide a comprehensive solution to most of the impairments associated with non-local effects and also to enable some side benefits, such as amplification of the pump pulses to compensate the attenuation of the fiber.

Temperature extraction in Brillouin optical time-domain analysis sensors using principal component analysis based pattern recognition.

We propose and experimentally demonstrate the use of principal component analysis (PCA) based pattern recognition to extract temperature distribution from the measured Brillouin gain spectra (BGSs) along the fiber under test (FUT) obtained by Brillouin optical time domain analysis (BOTDA) system. The proposed scheme employs a reference database consisting of relevant ideal BGSs with known temperature attributes. PCA is then applied to the BGSs in the reference database as well as to the measured BGSs so as to reduce their size by extracting their most significant features. Now, for each feature vector of the measured BGS, we determine its best match in the reference database comprised of numerous reduced-size feature vectors of the ideal BGSs. The known temperature attribute corresponding to the best-matched BGS in the reference database is then taken as the extracted temperature of the measured BGS. We analyzed the performance of PCA-based pattern recognition algorithm in detail and compared it with that of curve fitting method. The experimental results validate that the proposed technique can provide better accuracy, faster processing speed and larger noise tolerance for the measured BGSs. Therefore, the proposed PCA-based pattern recognition algorithm can be considered as an attractive method for extracting temperature distributions along the fiber in BOTDA sensors.

Enhancement of the Dynamic Range in Slope-Assisted Coherent Brillouin Optical Time-Domain Analysis Sensors

We present two techniques that provide an extension of the dynamic range of coherent Brillouin optical time-domain analysis (BOTDA) sensors for dynamic measurements. These types of BOTDA sensors rely on self-heterodyne detection of a phase-modulated probe wave, and the dynamic range for fast measurements is limited to the linear region of the radio-frequency (RF) phase-shift spectrum measured. The first method for range extension that we introduce is based on launching pump pulses containing multiple frequency components. This makes the Brillouin spectra generated by each component overlap, providing a wider linear region of the detected RF phase-shift spectrum and allowing measurement of larger Brillouin frequency shift variations. The second method relies on shortening the length of the pump pulses, which leads to the broadening of the detected RF spectra. The theoretical fundamentals of both range enhancing techniques are presented. Moreover, we experimentally demonstrate that they provide a threefold to fourfold enhancement in the dynamic range. Finally, the factors limiting their performance are determined: For the multi-frequency pump pulse technique, it is the worsening of Kerr nonlinear effects due to the simultaneous propagation of multiple spectral components in the fiber, and for the pulse-shortening method, it is the signal-to-noise ratio (SNR) penalty linked to the reduction of the magnitude of the Brillouin interaction.

Cyclic coding for Brillouin optical time-domain analyzers using probe dithering.

We study the performance limits of mono-color cyclic coding applied to Brillouin optical time-domain analysis (BOTDA) sensors that use probe wave dithering. BOTDA analyzers with dithering of the probe use a dual-probe-sideband setup in which an optical frequency modulation of the probe waves along the fiber is introduced. This avoids non-local effects while keeping the Brillouin threshold at its highest level, thus preventing the spontaneous Brillouin scattering from generating noise in the deployed sensing fiber. In these conditions, it is possible to introduce an unprecedented high probe power into the sensing fiber, which leads to an enhancement of the signal-to-noise ratio (SNR) and consequently to a performance improvement of the analyzer. The addition of cyclic coding in these set-ups can further increase the SNR and accordingly enhance the performance. However, this unprecedented probe power levels that can be employed result in the appearance of detrimental effects in the measurement that had not previously been observed in other BOTDA set-ups. In this work, we analyze the distortion in the decoding process and the errors in the measurement that this distortion causes, due to three factors: the power difference of the successive pulses of a code sequence, the appearance of first-order non-local effects and the non-linear amplification of the probe wave that results when using mono-color cyclic coding of the pump pulses. We apply the results of this study to demonstrate the performance enhancement that can be achieved in a long-range dithered dual-probe BOTDA. A 164-km fiber-loop is measured with 1-m spatial resolution, obtaining 3-MHz Brillouin frequency shift measurement precision at the worst contrast location. To the best of our knowledge, this is the longest sensing distance achieved with a BOTDA sensor using mono-color cyclic coding.

Damage evaluation of concrete based on Brillouin corrosion expansion sensor

To evaluate the damage degree of reinforced concrete due to steel bar corrosion, a damage factor is proposed to quantitatively evaluate the damage degree of concrete before initial cracking and during the development of cracks. Brillouin optical fiber time domain analysis (BOTDA) sensors are fabricated to monitor the expansion strain of steel corrosion. Two concrete specimens embedded with corrosion sensors are cast. An accelerated corrosion experimental program is used to accelerate the process of steel corrosion. The experimental results show that the corrosion sensor can be used to monitor the expansion strain of steel corrosion in real-time. At last, to map the monitoring results with the damage factor, finite element analysis is used to simulate the process of steel corrosion to determine the cracking strain of the interfacial concrete and concrete cover.

Frequency-comb-based BOTDA sensors for high-spatial-resolution/long-distance sensing.

Frequency-comb-based Brillouin optical time-domain analysis (BOTDA) sensors were developed to achieve acquisition-time reduction and high-spatial-resolution/long-distance sensing simultaneously. We found that, for the standard frequency-comb-based BOTDA, the use of a double-sideband (DSB) pulse generates a series of pulse pairs that simultaneously propagate along the sensing fiber, leading to a nonlinear interaction between the two sidebands of each frequency comb pulse, and a significant splitting of the Brillouin gain spectrum (BGS). This problem prevents its application in high-spatial-resolution sensing due to the higher pulse power requirement. Thus, one of the sidebands of DSB pulse was proposed for greatly suppressing the BGS distortion. In combination with the phonon pre-excitation technique based on phase-shifted pulse, a sensor with a spatial-resolution approximately 60 cm along a fiber approximately 592 m in length was demonstrated. Furthermore, we explored the detailed performance of long-distance sensing by frequency- comb-based BOTDA. The use of a frequency comb for the probe wave can suppress the pulse distortion and non-local effect, which is helpful for extending the sensing distance. A spatial resolution of approximately 6 m along a sensing fiber approximately 74.2 km in length was successfully demonstrated.

Enhanced performance for differential detection in coherent Brillouin optical time-domain analysis sensors

Logarithmic detectors (LogDs) have been used in coherent Brillouin optical time-domain analysis (BOTDA) sensors to reduce the effect of phase fluctuation, demodulation complexities, and measurement time. However, because of the inherent properties of LogDs, a DC component at the level of hundreds of millivolts that prohibits high-gain signal amplification (SA) could be generated, resulting in unacceptable data acquisition (DAQ) inaccuracies and decoding errors in the process of prototype integration. By generating a reference light at a level similar to the probe light, differential detection can be applied to remove the DC component automatically using a differential amplifier before the DAQ process. Therefore, high-gain SA can be employed to reduce quantization errors. The signal-to-noise ratio of the weak Brillouin gain signal is improved from ∼11.5 to ∼21.8 dB. A BOTDA prototype is implemented based on the proposed scheme. The experimental results show that the measurement accuracy of the Brillouin frequency shift (BFS) is improved from ±1.9 to ±0.8 MHz at the end of a 40-km sensing fiber.

Distributed monitoring method for upheaval buckling in subsea pipelines with Brillouin optical time-domain analysis sensors

A method for monitoring the upheaval buckling of subsea buried pipelines by distributed optical fiber sensors is introduced. The proposed method originates from the fact that the bucked pipe has the deterministic patterns of the bending-induced and axial force–induced strains, which can be used to identify the pre- and post-buckling states of the pipe. The challenge for utilizing these features is that the distributed fiber optic sensor can only measure the compound strain created by both bending and axial compression. A monitoring scheme by placing three sensors with [Formula: see text] space around the circumference of the pipeline was proposed in this article. An approach was presented to extract the bending-induced and axial force–induced strains from the longitudinal strains measured by the distributed fiber optic sensors. The experimental programs were then designed to investigate the feasibility of the proposed method. It is demonstrated that the proposed method is possible to detect the occurrence of buckling in the pipe. However, further research is necessary in order to examine the efficiency of the proposed approach under the complex loading patterns at subsea levels.

Gain Dependence of the Phase-Shift Spectra Measured in Coherent Brillouin Optical Time-Domain Analysis Sensors

We report on the effects of large pump pulse powers on Brillouin optical time-domain analysis (BOTDA) sensors based on phase-modulated probe wave and coherent self-heterodyne detection. These sensors are particularly suitable to perform dynamic strain and temperature measurements because the radio-frequency (RF) signal that is obtained when the probe wave is detected has a phase-shift spectrum that is independent to first order of the Brillouin gain. Therefore, a fixed optical frequency separation between pump and probe wave can be deployed and uses the stable RF phase-shift spectrum to obtain the Brillouin frequency shift (BFS) from measured changes in the probe RF phase-shift. However, in this paper, it is found that there is a narrowing of the RF phase-shift spectrum that depends on the Brillouin gain. This effect becomes significant when very high power pulses are used and the resultant large gain induces a narrowing of the RF phase-shift spectrum. This narrowing leads to a BFS measurement error when the sensor is configured for dynamic measurements. We analyze, theoretically and experimentally, the origins and the magnitude of the narrowing of RF phase-shift spectra for high pump pulses in a coherent BOTDA sensor. Furthermore, this spectral shape change is compared to the broadening of the gain spectrum that has been recently discovered in conventional direct-detection BOTDA sensors, which is linearly dependent on the pulse peak power injected to the fiber, finding that the spectral shape change is less significant in coherent BOTDA sensors. Finally, we quantify the BFS measurement error that it can induce and find the trade-offs to keep it below a certain threshold. It is found that, from a practical point of view, this effect is significant for short fibers, where nonlinear effects are negligible and large pump pulses can be used.

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.

Brillouin optical time-domain analysis sensor with pump pulse amplification.

We demonstrate a simple technique to provide conventional Brillouin optical time-domain analysis sensors with mitigation for pump pulse attenuation. The technique is based on operating the sensor in loss configuration so that energy is transferred from the probe wave to the pump pulse that becomes amplified as it counter-propagates with the probe wave. Furthermore, the optical frequency of the probe wave is modulated along the fiber so that the pump pulse experiences a flat total gain spectrum that equally amplifies all the spectral components of the pulse, hence, preventing distortion. This frequency modulation of the probe brings additional advantages because it provides increased tolerance to non-local effects and to spontaneous Brillouin scattering noise, so that a probe power above the Brillouin threshold of the fiber can be safely deployed, hence, increasing the signal-to-noise ratio of the measurement. The method is experimentally demonstrated in a 100-km fiber link, obtaining a measurement uncertainty of 1 MHz at the worst-contrast position.

DBA-Based BOTDA Using Optical Comb Pump and Pulse Coding With a Single Laser

We proposed and demonstrated a new structure, which employs a single laser in Brillouin optical time-domain analysis sensor assisted by distributed Brillouin amplification. This structure can avoid the stability problem caused by laser frequency drift. In addition, we used an optical comb technique to suppress the distortion of pump pulse. Moreover, the feasibility of an optical pulse coding technology in this system was studied. In particular, we explored the influence of coding parameters on the transient gain saturation of the coded pulse. Results show that the coded pulse space and coding length must be optimized to suppress its transient gain saturation, by which the sensing with lower decoding error can be achieved.

Application of distributed optical fiber sensor for monitoring the mechanical performance of a driven pile

This paper proposes a special application of Brillouin Optical Time Domain Analysis (BOTDA) sensors for monitoring the mechanical performance of a steel H-pile during driving process in a field in Hong Kong. First, basic calibration and the related installation method of optical fiber sensors (OFS) on pile body were introduced. Second, distributed strain profiles along the H-pile during driving process were successfully obtained from optical fiber sensors. Finally, measured strain distributions of the H-pile subjected to two loading/unloading cycles monitored by OFS were used to interpret field performance of the driven pile. Monitoring results indicate that the vertical load applied on the pile head generated significant compressive deformation, which was however mostly released by the corresponding unloading process. The 1st unloading and 2nd unloading processes released around 66% and 72% of the total compressive pile deformation, respectively. The shear stress levels mobilized between H-pile and surrounding soils were limited after each loading/unloading cycle according to the monitoring results. In addition, loading locations on the driven pile can be clearly identified from strain/stress distribution profiles. It is found from the monitoring study that BOTDA based sensors are more useful than some traditional or other point OFS (such as strain gauges or fiber Bragg grating sensors) to evaluate the average strain distribution of large scale structures.

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.