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Abstract
Fiber-optic distributed acoustic sensing (DAS) technology with high spatial and strain resolutions has been widely used in many practical applications. New methods to enhance the phase sensitivity of sensing fiber are worth exploring to further improve DAS performances, although the standard single-mode fiber (SSMF) has been widely used for DAS technology. In this work, we propose and demonstrate the concept of enhancing the phase sensitivity of DAS by softening the cladding of the sensing fiber, for the first time. The theoretical analysis indicates that softening sensing fiber cladding is an effective way to improve phase sensitivity. Thus, we fabricated cladding softened fibers (CSFs) and tested their phase sensitivities experimentally. According to the results, it is found that the phase sensitivity of the CSF with 0.48 WT% phosphorus-doping concentration and 80 µm cladding diameter is 22% and 54% higher than that of the non-phosphorus-doping fiber with 80 µm cladding diameter and SSMF, respectively. The results show that by reducing fiber cladding Young's modulus with higher phosphorus-doping concentration, the DAS phase sensitivity can be enhanced effectively, verifying the theoretical analysis. Also, we found that the phase sensitivity enhancement of the sensing fiber has a linear relationship with the cladding phosphorus-doping concentration, i.e. Young’s modulus. In conclusion, the reported CSF paves a way for improving the DAS phase sensitivity and would be applied to other major optical fiber sensing systems as a better sensing element over SSMF due to the enhancement in the elasto-optical effect of the sensing fiber.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
DAS technology can detect acoustic wave quantitively by demodulating phase information of coherent Rayleigh backscattering light in optical fiber with long sensing range, precise localization accuracy and high efficiency in real-time [1–4]. At present, DAS technology is widely deployed in seafloor detection [5], oil/gas exploration [6], border security [7], seismic surveillance [8], train speed monitoring [9], etc.
In recent years, sensitivity enhancement of DAS has become a very important issue in order to detect weak acoustic signal in many practical applications. Reducing coherent fading noise in Rayleigh backscattering [10,11], adopting Raman amplification [12,13] and improving demodulation methods [14] are popular ways to achieve the goal. By using repeated exposures of continuously distributed weak-gratings, a new kind of special optical fiber is proposed to increase the backscattering light intensity by more than 10 dB, while the attenuation coefficient is controlled within 0.4 dB/km [15]. Similarly, the technique of improving the intensity of backscattering light by introducing a series of localized point reflectors is proposed [16]. By using a highly birefringent photonic crystal fiber (PCF), Mikhailov et al. demonstrated ∼3.8 to ∼8.8 times pressure sensitivity enhancement than that of other PCFs [17]. Among these approaches, the post-processing methods are normally slow and complicated with much higher cost. Nevertheless, there is little study on phase sensitivity-enhancement of low-cost SSMF during manufacturing process for DAS uses.
In this paper, a novel method for enhancing the fiber phase sensitivity of DAS by softening the cladding of the sensing fiber is proposed and demonstrated, for the first time. First, the theoretical analysis is given, showing that the phase sensitivity-enhancement fiber should have low cladding Young’s modulus and smaller diameter. Then, the experimental results indicate that the CSFs with phosphorus-doping cladding and smaller diameter have higher phase sensitivity than SSMF, which proves the correctness of the proposed method. Furthermore, the results also show that the higher the doping concentration is, the higher the phase sensitivity will be. Finally, it should be pointed out that the CSFs are also adoptable for other major optical fiber sensing systems.
2. Theoretical analysis
Mechanical properties of optical fiber can be regarded as isotropic. When mechanical environment of optical fiber is changed by way of acoustic pressure, fiber strain will change accordingly. With perturbation by acoustic pressure P, photo-elastic effect causes optical fiber deforming and refractive index n changing, further resulting in phase difference of Rayleigh backscattering light changing. The change in phase difference Δφ resulted from the disturbance is defined by [18]:
According to Refs. [19–21], phase sensitivity of optical fiber can be expressed by:
where E1 represents the Young’s modulus of fiber core.In order to simplify the model, the simulation in this paper is mainly about the optical fibers with the most common structure, including core, cladding and coating, as shown in Fig. 1.
It is assumed that the axial load Pz, and the radial load Pr are uniform in the directions z and r. ɛz and ɛr are the axial and radial strain caused by Pz and Pr. According to the elasto-optical effect, Δφ can be expressed by [18]:
According to Saint-Venant’s principle, the fiber strain distribution is given by [21]:
According to Eq. (1), core diameter changing caused by Pr is so small and can be ignored. Therefore, P in Eq. (2) takes the value of Pz. For different optical fibers under the same pressure, the corresponding phase sensitivity can be obtained from Eqs. (1)-(4).
The phase sensitivity calculated as the functions of each parameter for individual layer with 40 kPa axial pressure and 10 kPa radial pressure is shown in Fig. 2. It can be seen that if the cladding diameter is reduced from 125 µm to 80 µm, the phase sensitivity of the fiber can be improved by ∼62%. In addition, the phase sensitivity of the fiber can be further enhanced with lower Young’s modulus. When the Young’s modulus of the fiber cladding with 80 µm cladding diameter decreases from 80 GPa to 40 GPa, the phase sensitivity can be improved by ∼88%. According to Eq. (4), when the Young’s modulus of fiber cladding decreases, ɛz2 and ɛr2 will be larger. As the boundary strain between the cladding and the core is equal, ɛz1 and ɛr1 will increase accordingly. And since the change of ɛz1 is much larger than that of ɛr1, Δφ is enlarged according to Eq. (3). Since the sensitivity of the 80 µm fiber is relatively higher, our research mainly focuses on cladding softening of the 80 µm fiber.
3. Fabrication of CSF
To reduce Young’s modulus of silicon, phosphorus doping has been proved to be an effective way due to high concentration of free carries in heavily phosphorus-doped silicon [22]. In addition, elastic strain may change energy bands structure and raise degeneracy of band extrema where free carriers are contained, and further cause carriers redistribution [23]. Carriers redistribution reduces free energy of strain silicon, so that part of elastic deformation can be restored and effective elastic constant is reduced. According to this, we have fabricated five kinds of 80 µm CSFs with different P2O5 concentrations using the modified chemical vapor deposition (MCVD) process [24]. Chemical reagents are entrained in a gas stream in controlled amounts by passing carrier gases such as O2 through liquid dopants. When the fluoride-doped (F-doped) siliceous loose layer is depositing, the concentration of POCl3 carried in the gas is gradually increased without affecting the fiber transmission performance, as shown in Fig. 3(a). The cross-section photograph of the CSF is showed in Fig. 3(b).
By measuring the Young’s modulus of the preforms of CSFs as shown in Fig. 4(a), the softening effect by phosphorus-doping is verified. The Young’s modulus of the phosphorus-doping layer of the CSFs with phosphorus-doping concentration of 0.08 WT% and 0.24 WT% are measured by using Nanoindentation [Anton Paar NHT2, as shown in Fig. 4(b)], respectively. The results show that the Young’s modulus of the CSFs with 0.08 WT% and 0.24% concentration is 78 GPa and 73GPa, respectively, which means that the Young’s modulus of the cladding is reduced by 5 GPa as expected when the phosphorus-doping concentration is increased by 2 times. It is reasonably derived that the higher the phosphorus concentration is, the smaller the cladding Young’s modulus will be.
Fig. 4. (a) CSF preform specimen and (b) Young’s modulus of CSFs test setup by using Nanoindentation.
4. Experimental setup
The relevant parameters of the fibers under test (FUTs) are shown in Table 1. Fiber 1 is the non-phosphorus-doping fiber with 80 µm cladding diameter. The others are all CSFs.
Table 1. Parameters of FUT
The experimental set-up for phase sensitivity testing is shown in Fig. 5. In order to ensure that the distance between the FUTs and the vibration source is the same, the FUTs are attached to the floor in concentric circles with a diameter of 1.5 m. Since the spacing between each fiber is only ∼5 mm, the difference between the minimum and maximum radius is only ∼25 mm, which is negligible when compared with the radius of the test circle (750 mm). Different kinds of FUTs are connected in series with the first cable that is interrogated by an ultra-sensitive DAS (uDAS) instrument with ∼18 pɛ/√Hz strain resolution for SSMF at very low frequency of up to 1 Hz and very high spatial resolution of <1 m developed in order to experience the same vibration and simultaneously measure the demodulated phase amplitudes. The seismic wave is generated when a free-fall basketball hits the ground at the center of the circle and mainly propagates through the ground to the FUTs, causing the phase difference change of Rayleigh backscattering light in the FUTs. The uDAS instrument then carries out the phase demodulation of the FUTs.
5. Experimental results and discussion
Figure 6(a) shows the time-domain traces of the demodulated phase difference change of the FUTs by using the uDAS instrument during once basketball free-fall. When the free-falling basketball hits the ground, the phase difference changes of the CSFs are always larger than fiber 1 as the descending height of the basketball decreasing. It is indicated that the CSFs responses are more sensitive.
Fig. 6. (a) Time-domain traces of the demodulated phase difference changes of fibers 1 to 6 and (b) normalized phase sensitivity v.s. phosphorus-doping concentration of CSF.
The ratio between the phase difference change of the certain CSF and that of fiber 1 obtained in the same shot is used as the normalized phase sensitivity. The comparison of the normalized phase sensitivities with 30 times measurements and averaging is shown in Fig. 6(b). It can be seen clearly from Fig. 6 that fiber 6 has the maximum response amplitude. According to Fig. 6(b), the phase sensitivity of the CSF with phosphorus-doping concentration of 0.48 WT% is increased by 22%. As the phosphorus-doping concentration increasing, the normalized phase sensitivity increases linearly, which is due to cladding Young’s modulus reduction. According to Eq. (1), if Young’s modulus of fiber cladding is lower, ɛr caused by the same Pr is relatively larger. Then the cladding exerts radial pressure on the core, resulting in greater variation of ΔL and n, further resulting in higher phase sensitivity of fiber. We also compared the performance of SSMF in the same test set-up and found that fiber 6 has a 54% higher normalized phase sensitivity than SSMF.
It can be seen that when cladding Young’s modulus of the fiber with 80 µm cladding diameter decreases from 78 GPa to 73 GPa, the phase sensitivity can be improved by ∼7% in the simulation. Moreover, the phase sensitivity of the CSF with phosphorus-doping concentration of 0.24 WT% (73 GPa) is increased by ∼8.9% than that of 0.08 WT% (78GPa) in the experiment. Thus, the experimental data is coincided with the simulation results as shown in Figs. 2 and 6(b).
By linear fitting of the experimental normalized phase sensitivity, we can obtain the relationship between the normalized phase sensitivity and phosphorus-doping concentration of the CSF as shown in Fig. 6(b). It can be seen that the linearity between the fiber cladding Young's modulus and phase sensitivity improvement is quite good. It could be predicted that by further increasing the phosphorus-doping concentration to 2 WT%, the fiber phase sensitivity could be enhanced by ∼99.5% over 80 µm fiber and ∼153.3% over SSMF correspondingly. Furthermore, it can be seen from Table 1 that the alteration of P2O5 concentration in cladding has little influence on the CSF attenuation, thus the CSF can be used for long-distance sensing applications. In addition, the fusion splicing loss of the CSF with SSMF is less than 0.03 dB, so that it can be well integrated with SSMF.
6. Conclusions
In conclusion, we explored the possibility of realizing novel CSFs for DAS phase sensitivity enhancement during manufacturing process, for the first time. Our studies indicate that the phase sensitivity of fiber with low cladding Young’s modulus and small cladding diameter can be improved effectively. To verify the idea, we fabricated and tested the CSFs with phosphorus doping in fiber cladding. The experimental results show that the phase sensitivity of the CSF with 0.48 WT% phosphorus-doping concentration can reach 54% higher than SSMF. Moreover, there is a linear relationship between fiber cladding Young's modulus decrease and phase sensitivity improvement, which means that by further increasing the phosphorus-doping concentration, the fiber phase sensitivity can be enhanced correspondingly. Therefore, this work may open a door for design of new sensitivity-enhancement fibers not only for DAS but also other major optical fiber sensing systems, due to the enhancement in the elasto-optical effect of sensing fiber, such as like Brillouin optical time-domain reflectometer (BOTDR), polarization optical time-domain reflectometer (POTDR), optical frequency domain reflectometer (OFDR), fiber Bragg grating (FBG) sensing systems, etc.
Funding
Major Instrument Project of Natural Science Foundation of China (41527805); 111 project (B14039); UESTC-ZTT joint laboratory project (H04W180463).
Disclosures
The authors declare no conflicts of interest.
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