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Abstract
Utilizing the large effective area non-zero dispersion-shifted fiber (LEAF), a multi-parameter optical-fiber sensor has been proposed and experimentally demonstrated for distributed simultaneous temperature and strain measurement, which is based on multiple acoustic modes in spontaneous Brillouin scattering (SpBS) effect. Proof-of-concept experiments demonstrate 3 m spatial resolution over 2.5 km sensing LEAF with 2°C temperature accuracy and 60µɛ strain accuracy. The proposed distributed Brillouin optical fiber sensor allows simultaneously temperature and strain measurement, thus opening a door for practical application such as superconducting cable.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Distributed Brillouin sensor has been a subject of interest because of its ability to achieve the information of strain and temperature with high spatial resolution and high accuracy along the sensing fiber [1–9]. It has potential applications in aging degradation and seismic damage of civil infrastructures [5,6]. In 1990, the Brillouin frequency shift (BFS) in a Brillouin scattering-based distributed optical fiber sensor was found to be dependent on the combined effects of strain and temperature [10]. Since then, extensive research and testing have been performed to exploit Brillouin scattering to develop fully distributed optical fiber sensors. One important category of Brillouin scattering-based optical fiber sensors is Brillouin optical time-domain reflectometry (BOTDR) which typically employ a modulated pulse light to interrogate the sensing fiber and determine position information from the time-of-flight of the backscattered Brillouin from the fiber under test (FUT). The BOTDR is known as one of the most promising techniques that can simultaneously achieve operation by single-end light injection, cost efficiency, random accessibility, and high spatial resolution. The BFS relies on the temperature and strain conditions of the sensing fiber, which provides the basis for Brillouin-based sensing technique capable of detecting these two parameters. However, it also brings the difficulty to distinguish these two cross effects by a single BFS. This cross effects, which exists in almost all the optical fiber sensors, would introduce errors when monitoring surrounding environment changes along the length of the sensing fiber. The cross sensitivity severely limits the practice of distributed optical fiber sensors. The challenge, then, is to develop a technique to separate the temperature and strain individual contributions to the shift in the Brillouin frequency of the sensing fiber and to avoid the cross sensitivity problem. Many alternative measuring approaches have been reported to accomplish the simultaneous distributed measurement of fiber strain and temperature in the past few years [11–15]. In these methods, the one that utilizes a half of the fiber isolated from the effects of strain for temperature sensing [11], and another one that uses the fiber Bragg gratings (FBG) combined with fiber [12]. Some groups combined stimulated Brillouin scattering with other optical nonlinearities effect such as the Rayleigh scattering and Raman scattering to solve this cross issue by comparing the difference [13–15]. Obviously, above mentioned methods require rather complicated sensing structure. On the other hand, another technique that simultaneously measures the Brillouin power and frequency shift of the backscattered Brillouin can accomplish the fully distributed measurement of strain and temperature, but the temperature resolution is affected by the power measurement fluctuation and random change of state of polarization in the measurement, which is very difficult to reach 0.05 dB over a considerable length of fiber and is easily influenced by surrounding environment [16].
In this letter, we have proposed a novel resolution based on BOTDR scheme, which employed a large effective area non-zero dispersion-shifted fiber (LEAF) with different temperature coefficients in core as the sensing fiber to solve the cross sensitivity problem. The LEAF has been reported to have several Brillouin gain spectrum (BGS) with multi-peak in the fiber core [17–19], in which multiple optical modes can exist and contribute two effective BFSs at least to realize the cross insensitivity measurement only utilizing BOTDR system without a lot of measurements. This proposed technique needs only the measurement of BFSs of the BGSs and can simultaneously achieve the high spatial resolution and accuracy of temperature and strain measurement without modifying the sensing fiber. Additionally, the experiment results also demonstrated that the proposed method was suitable to achieve multi-parameter sensing in a relatively simple scheme, and a strain accuracy about 60µɛ and a temperature measurement accuracy about 2°C were obtained in 2.5km sensing range.
2. Operation principle and experimental setup
From a previous experiment [11], in the case of an sensing fiber with compound compositions in the core, the optical refractive index and the acoustic velocity vary with temperature or strain. The change of BFS is related to the temperature change and strain variation by the following equations:
It is apparent that at least two acoustic modes are necessary for the discriminative measurement of temperature and strain. Therefore, this proposed method can distinguish temperature and strain cross effects along the length of the LEAF link through the frequency analysis of the measured BFS.
The experimental setup was implemented as shown in Fig. 1. It was basically a classical BOTDR scheme, where, in this case, the reference branch was scanned by an arbitrary waveform generator (AWG) for coherent detection. A distributed feedback (DFB) fiber laser with 10kHz ultra-narrow linewidth outputted a continuous wave (CW) laser, which operated at the wavelength of 1550nm. Then, a 3dB coupler (OC1) was used to split the output CW into two branches. One part was used for the generation of the probe pulsed signal. It was modulated by an electro-optic modulator (EOM1) with 45dB high extinction ratio (ER) to form 30ns Gaussian modulated pulse corresponding to 3m spatial resolution. The peak power of the probe pulse was amplified by an erbium-doped fiber amplifier (EDFA) at 27dBm, and injected into the FUT (NZJDBSSM655) through a circulator (CIR). The backscattered sensing signal was collected in the same access. The other part was used as the reference light. It was offset in frequency with the Brillouin frequency shift (BFS) of ∼10.70GHz by an intensity modulator (EOM2), which worked at the carrier suppression mode. Besides, the driving microwave signal was swept from 10.2GHz to 10.9GHz with a frequency step of 5MHz. The output of EOM2 was filtered by a fiber Bragg grating (FBG) to retain the lower sideband. A polarization scramblers (PS) was employed to change the polarization state of the chosen modulated signal to suppress the polarization fading noise. Finally, the sensing signal of the back-scattered spontaneous Brillouin scattering and the reference wave were mixed by means of a 2×2 coupler, and then the beat sensing signals were captured by an electrical signal with a 350MHz balanced photo detector (PD).Meanwhile, the demodulation results were recorded by a high-speed data acquisition (DAQ) for follow-up data processing to get the temperature and strain information. It was noteworthy that the modulator and FBG of the lower-branch could be also placed in the upper-branch for generating an up-convertor modulated probe light, which was an alternative method for the experimental setup.
Fig. 1. Schematic diagram of experimental setup. EOM, electro-optic modulator; EDFA, erbium doped fiber amplifier; FUT, fiber under test; PC, polarization controller; AWG, arbitrary waveform generator; OI, optical isolator; PD, photodetector; AWG, arbitrary waveform generator; FBG, fiber Bragg grating, VOA, variable optical attenuator; OC, optical coupler; DAQ, data acquisition; CIR, circulator.
3. Experimental results and discussion
In the experiment, the used LEAF has the features with loss coefficient of 0.22dB/km, effective area of 70µm2 at 1550nm. A BOTDR system was used to measure the BGS to obtain the BFS along the entire length of LEAF. For the temperature measurement, a 4m segment of the 2.5km LEAF (C) was placed in a thermally insulated oven, which was located at the position between 2.466km and 2.47km as arranged in Fig. 2(a). It is worth mentioning that the length of heated section is about 4m which is longer than the spatial resolution. The entire LEAF sensing fiber contained four separate sections. The first 2.416km sensing fiber remained on the original spool as supplied by the manufacturer, the following 50m sensing fiber was free of strain as a reference segment, followed by the 4m heated fiber with two fixed ends, and the final spool (30m) was also free of strain.
Fig. 2. The detail arrangement diagram of sensing fiber for the sensing measurement: (a) Temperature measurement; (b) Strain measurement.
The time trace of Brillouin signal was obtained with 215 times of averaging, performing the temperature measurement with a high signal-to-noise (SNR). Figure 3 highlights the measured multi-peak BGS at one location of heated C section when the temperature is 30°C and strain is about 0µɛ. It can be observed from Fig. 3 that the Brillouin central frequencies of four peaks were 10.700GHz, 10.880GHz, 11.040GHz, and 11.155GHz for Peak1 – Peak4, respectively. Based on the theory analysis as mentioned above, only two peaks of BGS can be used to resolve the obstacle of cross effects. Therefore, the Peak1 and Peak2 were chosen as the main two peaks. The measured Brillouin spectra of the heated LEAF at Peak1 and Peak2 from 30°C to 70°C with 10°C per step were depicted in Fig. 4 and Fig. 5, respectively.
Fig. 3. Measured multi-peak BGS of LEAF after Lorentz fitting under heated temperature of 30°C and strain of 0µɛ.
It can be seen from Fig. 4 and Fig. 5 that the BGSs were changed when temperature changed, and moved to the higher frequency as the temperature increased. From the data, the temperature coefficients of the FUT were determined to be 1.02MHz/°C and 0.974MHz/°C, respectively, by the linear fitting method as displayed in Fig. 6. Meanwhile, a temperature accuracy of 2°C can be obtained along the 2.5km fiber, given by standard deviation (SD) of the five continuous measurement results. A spatial resolution of 3m was demonstrated in Fig. 7. The BFS in Fig. 7 belongs to the Peak1.
For the strain measurement, the strain was applied to the rear of the LEAF by a fiber stretching unit to change the sensing fiber strain only. The 2.5km-long LEAF was divided into four sections as displayed in Fig. 2(b): A1 (2.416km, low-level tension); B1 (50m, free); C1 (5m, stretched); and D1 (30m, free). Section A1 remained on the original spool as supplied by the manufacturer. Sections B1 and D1 were ensured zero strain. A motor-driven fiber stretching unit was used to accurately stretch Section C1 to control the strain with a step of 1000µm (220µɛ). The BGSs can be measured using the same procedure as described above mentioned. The stretched LEAF changes of BFS were measured in the range of 200µɛ to 2200µɛ for Peak1 and Peak2 at one location of C1. The strain coefficients of the FUT were therefore measured to be the same of 0.0574MHz/µɛ. The BFS dependence of different acoustic modes on strain is presented in Fig. 8. The measured strain accuracy was calculated by the standard deviation (SD) of the BFS at the stretched section of the LEAF, which was about 3.45MHz (corresponding to an accuracy of ±60µɛ). As the temperature and strain sensitivity of the two peaks contributed by different acoustic modes is unequal, the influence of temperature and strain on the fiber can be distinguished.
Additionally, an experiment was implemented to verify the feasibility of this proposed approach for simultaneous strain and temperature measurement. The testing case of strain and temperature was 50.8°C/220µɛ, and the room temperature for reference was 23°C; the strain for reference was about 0µɛ. Figure 9 depicted the experimental results of the distributed fiber link in frequency shift of Peak1 and Peak2 versus distance in the testing condition. By using (3) and (4), the calculated temperature/strain were 53.69°C/161.08µɛ at the length of 2491.76m and 52.28°C/186.14µɛ at the length of 2494.84m, respectively. Therefore, the corresponding results shown that the distributed temperature and strain can be simultaneously distinguished with a temperature accuracy of 2°C, a strain accuracy of 60µɛ.
Fig. 9. The measured change in BFS of Peak1 and Peak2 versus distance in the testing case of 50.8°C/220µɛ.
4. Conclusions
In this work, a BOTDR sensor was upgraded to realize simultaneously the distributed measurement of strain and temperature, which was based on a large effective area non-zero dispersion-shifted fiber having multiple acoustic modes with different sensing coefficients in SpBS effect. The capability of discriminative measurement between temperature and strain has been experimentally investigated in detail, as the experimental results proved that the LEAF has multiple-peak BGSs. Furthermore, the temperature/strain coefficient along the sensing fiber showed a great difference to each other. As a result, the simultaneous discrimination of temperature and strain were successfully demonstrated by analyzing the coefficients of the BFSs introduced by different acoustic modes with a temperature accuracy of 2°C, a strain accuracy of 60µɛ, and a spatial resolution of 3m in 2.5km sensing range. With the help of this proposed approach and experimental setup, some promising work in large structures can be also made in the future such as a railway or a superconducting cable.
Funding
Taishan Series Talent Project (2017TSCYCX-05); Science and Technology on Electronic Test & Measurement Laboratory Foundation (41Q1313-5).
Disclosures
The authors declare no conflicts of interest.
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