Quasi-acoustic impedance matching distributed optomechanical sensor with Aluminized coating optical fiber

doi:10.21203/rs.3.rs-1638116/v1

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Quasi-acoustic impedance matching distributed optomechanical sensor with Aluminized coating optical fiber

https://doi.org/10.21203/rs.3.rs-1638116/v1

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The uncoated optical fiber has been extensively researched as an optomechanical sensor since it can achieve substance identification of the surrounding media by exciting and detecting transverse acoustic waves via forward stimulated Brillouin scattering (FSBS), yet within danger of being easily broken. Although polyimide-coating fibers are reported to allow transverse acoustic waves transmission to the coating for reaching the ambient while maintaining the mechanical properties of the fiber, it still suffers from the problems of humidity sensitivity and spectral instability. Here, we propose for the first time a robust distributed FSBS-based optomechanical sensor using an aluminized coating optical fiber. Benefiting from the quasi-acoustic impedance matching condition of the aluminized coating and silica core-cladding, aluminized coating optical fibers not only have stronger mechanical properties and higher transverse acoustic wave transmission efficiency but also has a higher signal-to-noise ratio, compared to the polyimide-coating fibers. The distributed measurement ability is verified by identifying air and water around the aluminized coating optical fiber with a spatial resolution of 2 m. In addition, the proposed sensor is immune to the external relative humidity changes, which is a preponderance for liquid acoustic impedance measurements.

Nonlinear optics

Forward stimulated Brillouin scattering

Optomechanical sensor

Acoustic impedance sensing

Aluminized coating optical fiber

Forward stimulated Brillouin scattering (FSBS) is an intriguing nonlinear acousto-optic interaction between laser beams and electrostriction-induced transverse acoustic waves [1]. Since the excited transverse acoustic waves could low-loss transmit and reflect at the boundaries between the cladding, coating, and the ambient media, FSBS has been investigated for diverse acoustic metrologies such as fiber diameter monitoring [2,3], temperature [4,5], strain [6,7], ionizing radiation [8] and acoustic impedance sensing [9–12] in standard fibers. By measuring the frequency shift or linewidth of FSBS spectra, the information about the medium outside the optical fibers under test (FUTs) is obtained. The phonon scattering is also applied to the study of dual-mechanism sensing and functional excitonic devices [13,14]. Recently, distributed optomechanical sensor has been realized by merging FSBS with location resolved Rayleigh scattering or backward Brillouin scattering, increasing the practicality, yet with spatial resolutions of tens of meters [15,16]. In order to improve the spatial resolution, optomechanical time-domain analysis (OMTDA) is proposed which enhances the signal-to-noise ratio (SNR) of FSBS spectra via coherent probing technique, reaching a spatial resolution up to 2 m [17]. Furthermore, protocols such as polarization separation and frequency shift extraction have been introduced to improve the spatial resolution of the sub-meter scale [18,19].

However, ordinary single mode fiber (SMF) coatings such as acrylate tend to constrain the acoustic waves within the cladding, preventing them from passing through the coating to reach the surrounding media. Thus, it is necessary for the FSBS-based sensing methods to strip the coating of the sensing fiber, which is greatly reduced the mechanical strength and limited the industrial and engineering application. In order to overcome the drawback, both point and distributed optomechanical sensing based on FSBS have been proposed by involving polyimide-coated fibers (PI-fibers) as the sensing fibers [20,21]. Nonetheless, the PI-fibers are prone to undermine the accuracy and reliability of the measurements considering that the water absorption characteristic and the thickness nonuniformity of the polyimide coatings leave their acoustic performance susceptible to influences and spectral instability [22,23]. Hence, an optical fiber coating material with more robust adaptation to environmental changes is starved for solving the above problems, especially for liquid acoustic impedance sensing.

In this article, we proposed a novel distributed optomechanical sensing with stable sensing performance and high robustness by using an Aluminized coating optical fiber (Al-fiber). The quasi-acoustic impedance matching condition of the aluminized coating and silica core-cladding makes the Al-fibers have stronger mechanical properties and higher transverse acoustic wave transmission efficiency. Experimental results demonstrate that the FSBS spectra of Al-fibers are steady and insensitive to changes in relative humidity (RH). Meanwhile, the distributed FSBS over a 15-m Al-fiber has been investigated with a spatial resolution of 2 m, and the air and water outside the FUTs are identified. The proposed distributed optomechanical sensor under quasi-acoustic impedance matching condition has a broader potential application.

Acoustic impedance matching condition

The transverse acoustic waves are activated to steady-state by launching an intense activating pulse via electrostriction effect. Once the pulse is interrupted, the transverse acoustic waves can still maintain the phase continuity with the driven acoustic field and decay exponentially [19]. The reflection and transmission of the transverse acoustic waves which propagate within multi-layered structures are determined by the acoustic impedances of each layer. Assuming that the reflection and transmission of the wave at the interface are energy-conserving, the acoustic wave reflectivity at the boundary is expressed as [20]

$$r=\left| {\frac{{{Z_s} - {Z_i}}}{{{Z_s}+{Z_i}}}} \right|,$$

where Z_s and Z_i are the acoustic impedance of silica and coating respectively. Note that when the impedance of the cladding and coating layers are equal (Z_s = Z_i), the acoustic wave reflectivity is zero (r = 0), which means that the transmitted power of the acoustic wave is maximum, i.e., the acoustic impedance matching condition. Concurrently, the acoustic impedance of the outer material of the fiber is directly related to the resonance linewidth Γ_m of the FSBS spectra [24]

$${\Gamma _m}={\Gamma _s}+\frac{{{V_L}}}{{\pi d}}\ln \frac{1}{{\left| R \right|}},$$

where Γ_s is the intrinsic linewidths of FBSS resonance, V_L = 5996 m/s is denoted as the longitudinal acoustic velocity, R is the total reflectance of sound waves. The boundary condition of the acoustic wave can be represented by [1,25]

$$(1 - \frac{{V_{T}^{2}}}{{V_{L}^{2}}}){J_0}(y) - \frac{{V_{T}^{2}}}{{V_{L}^{2}}}{J_2}(y)=0,$$

where V_T = 3740 m/s is the transverse acoustic velocity. J_n is the n^th Bessel function of the first kind. With the m^th-order root y_m, the boundary condition gives rise to an inverse relationship between the resonance frequency f_m and the cladding diameter d [9]

$${f_m}=\frac{{{V_L} \cdot {y_m}}}{{\pi d}}.$$

Fortunately, the acoustic impedance of the Aluminum (~ 17.21×10⁶ kg/(m²⋅s)) [26] is close to that of Silica (~ 13.19×10⁶ kg/(m²⋅s)) [20], realizing a quasi-acoustic impedance matching (r_{si − Al} = 0.13). Thus, the acoustic waves tend to oscillate within the cross-section of the coating, rather than merely cover the area of cladding. Consequently, the dispersion relation given by Eq. (4) is no longer applicable which is expected to be replaced by the more general elastic dynamic equation [3]

$$\rho \frac{{{\partial ^2}{u_i}}}{{\partial {t^2}}} - {\left[ {{c_{ijkl}}{u_{kl}}+{\eta _{ijkl}}\frac{{\partial {u_{kl}}}}{{\partial t}}} \right]_j}={\left[ {{\varepsilon _0}{\chi _{klij}}E_{k}^{{(1)}}E_{l}^{{(2) * }}} \right]_j},$$

where${\chi _{klij}}={\varepsilon _{im}}{\varepsilon _{jn}}{p_{klmn}}$, $\rho$is the medium density, ${c_{ijkl}}$and${\eta _{ijkl}}$are stiffness matrix and viscosity matrix,$\varepsilon$and${p_{klmn}}$are dielectric constant and polarization tensor, respectively. According to Eq. (5), the acoustic field distribution , along with the resonance frequency can be calculated. The exemplary intensity and displacement distribution of R_0,6 mode in both the conventional SMF and the used Al-fiber are present in Fig. 1(a), and (b), respectively. The significant difference is that the intensity of acoustic wave distribution in the coating of Al-fiber maintains a high level while that of SMF is rapidly attenuated to a negligible level, which is also valid for acoustic modes of other orders. Under the quasi-acoustic impedance matching between the cladding and Al-coating layer, the acoustic waves have the chance to directly interact with the outside media, realizing the chemical substance identification or liquid acoustic impedance sensing.

Experimental setup of OMTDA system

Incorporating the Al-fiber, a high resolution OMTDA configuration is used to realize distributed optomechanical sensing, which is demonstrated in Fig. 2. A narrow linewidth laser of 1550 nm is divided into two branches. In the upper branch with 90% laser, an activation pulse and a probing pulse end to end are generated to excite and probe the transverse acoustic waves successively in a coherent way using an arbitrary waveform generator (AWG). The sequential radio-frequency pulse signals are imposed onto the light wave through a double-sideband electro-optic modulator (EOM1) working under the suppressed carrier condition, and the light power is amplified by an erbium-doped fiber amplifier (EDFA). Note a frequency interval f₁ of 1.5 GHz is added to the activation pulse to remove the backward stimulated Brillouin scattering (BSBS) interaction between the activation pulse and the detecting light. After being modulated, the activation pulse contains four frequency components distributed symmetrically in pairs with the carrier component. According to Ref. [17], the probing pulse contains two frequency components with a frequency difference around the FSBS resonance frequency. Meanwhile, in the lower branch, another EOM2 cooperating with a tunable optical filter (OF2) is employed to generate the continuous detecting light. By switching the frequency loaded on the EOM2 between the Brillouin gain regions of the two sidebands contained by the probing pulse, the intensity evolution along the sensing fiber of the upper and lower sidebands are recorded via BSBS successively. Then, with the frequency difference between the probing pulse swept around the resonance frequency of the acoustic wave, the localized FSBS spectrum can be scanned and demodulated by applying differential calculation of the energy accumulation.

Sensing fibers spectra analysis

The acoustic mode distributions of specially coated optical fibers are investigated. The experimental results of Al-fiber and uncoated-SMF are first compared. The diameter of the Al-fiber core and cladding are 8.9 and 125.5 µm, respectively, and the aluminized coating thickness is ~ 20 µm. The measured FSBS spectra with the frequency range from 10 to 500 MHz of Al-fiber and uncoated-SMF are shown in Fig. 3(a). Since the aluminized coating acts as an acoustic field propagation medium with quasi-acoustic impedance matching condition corresponding to a larger acoustic wave oscillation diameter, the FSBS in Al-fiber has a discrete spectra distribution similar to that of the uncoated-SMF but with a relative decrease in resonance frequency spacing (~ 36.76 MHz) of each acoustic mode. Note that the spectrum becomes narrower, as the resonance frequency decreases. The resonance frequencies corresponding to R_0,1~R_0,14 modes obtained from both calculations and experiments are deposited in Table 1. A decent consistency between the calculations and experiments verifies the feasibility of replacing the FUT of OMTDA with Al-fiber.

Table 1

Frequencies of resonant modes of Al-fiber
m	Calculation (MHz)	Measurement (MHz)
1	25.05	22.37
2	62.92	62.45
3	99.67	101.32
4	136.24	138.65
5	172.75	173.49
6	209.24	209.32
7	245.71	247.68
8	282.17	286.28
9	318.62	322.15
10	355.07	357.02
11	391.51	394.23
12	427.96	433.38
13	464.41	470.54
14	500.84	504.69

For comparison, the PI-fibers with the same cladding diameter but with distinct coating thicknesses (~ 17.1 µm for PI-fiber1 and ~ 13.8 µm for PI-fiber2) are chosen as FUTs and shown in Fig. 3(b). Severe intensity distortions of the spectra distribution are inspected resulted from the coating thickness difference of ~ 3 µm. Specifically, the acoustic impedance mismatch between silica and polyimide results in a relatively equivalent acoustic reflection (r_{si − PI} = ~ 0.57) and transmission at the cladding-coating interface, which eventually results in the interference induced acoustic wave cancellation or enhancement. Consequently, the thickness and uniformity of the coating can interfere with the performance of FSBS sensors using PI-fibers. Nonetheless, the reflection at the interface is negligible when the aluminized layer is applied as the optical fiber coating, avoiding the interference of the acoustic wave, leaving the system with higher robustness to coating parameters.

Humidity characteristics of aluminized optical fiber

Apart from the irregular FSBS spectra distribution, the PI-coatings tend to absorb water when humidity is considered, which may further impact the FSBS spectra measurements. These drawbacks are fully conquered by involving Al-fiber. In order to verify the non-sensitivity of FSBS in Al-fiber to RH, the 15 m FUTs are placed in a temperature and humidity-controlled device with a stable temperature of 30 °C. The RH is changed from 4 %RH to 9 %RH with a step of 1 %RH. The FSBS spectrum of R_0,6 mode is measured by the OMTDA system, as depicted in Fig. 4. Note that the acoustic mode R_0,6 is chosen in accordance with the previous investigation by reason of its higher FSBS efficiency and suitable linewidth in Al-fiber. The central frequency and the linewidth of each spectrum are calculated to be around 209.41 MHz and 1.73 MHz, respectively. The six sets of FSBS spectra show a high degree of consistency with slight fluctuations around the base. The average SNR of them reaches 34.01 dB for the strictly constrained acoustic wave within the whole cross-section resulting from the severe acoustic impedance mismatch between Al-coating and air. The SNR fluctuation between the measurement sets is blown at 7.58 dB. Furthermore, the measurements of FSBS spectra at each humidity are repeated 5 times. And their central frequencies and linewidths extracted through Lorentz fitting are demonstrated in Fig. 5. Because of the enhanced SNR of OMTDA, the uncertainty values in frequency shift and linewidth are controlled below ±0.01 MHz and ±0.02 MHz, respectively. Meanwhile, the average frequency difference between the measurements under different humidity is 15.6 kHz, while the maximum linewidth difference equals 35.2 kHz. Both are smaller than the intrinsic measurement uncertainties. Hence, the FSBS in Al-fiber is considered to be independent of humidity variation owing to the promptly formation of a dense oxidation film outside the aluminum coating that protects the sensing fiber from external relative humidity variations, which reveals the optomechanical sensing more robust adaptation to chemical or liquid sensing.

Distributed substance identification

The distributed measurement of the external environmental substance acoustic impedance is investigated. Firstly, Al-fibers with the length of 15 m are used as the FUTs, placed loosely in the air without intrinsic torsional strain. The frequency of the beat created by both activation pulse and probing pulse is swept from 204 MHz to 215 MHz to cover the resonance frequency of R_0,6 induced FSBS with a fine step of 0.01 MHz. In detail, the pulse width of the activation pulse is set as 1.5 µs to avoid the spectrum broadening from transient excitation of the transverse acoustic wave. The width of the probing pulse followed is 20 ns for relatively high-resolution sensing. OMTDA records the energy transfer accumulated over the propagation of the probing pulse. The local FSBS spectra can be demodulated via a differential process where a smoothing window is applied to enhance the SNR. Here, a smoothing window of 2 m is applied to represent the spatial resolution of the measurement.

The scanned 3D spectra distribution is shown in Fig. 6(a). The reflection ratio between the Al coating and air interface approaches r_{Al − air} ≈ 1. The linewidth of the FSBS spectra of the air environment is determined mainly by the intrinsic acoustic loss. Then, the middle 4 m of the Al-fibers are laid in deionized water while the rest are kept in the air. The FSBS-induced energy transfer efficiency in water is decreased due to a larger acoustic impedance of water (1.483×10⁶ kg/(m²⋅s)) [16], resulting in modest peak intensity and a broadened line shape. The distributed linewidth of the FSBS spectra is calculated by Lorentz-fitting and plotted in Fig. 6(c), in which an abrupt step advent at the center of the FUTs indicating the change of the external environment. The spectrum linewidth representing air surrounding is below 1.9 MHz while that in the water environment is around 3.3 MHz. The linewidth fluctuations in the two regions are both confined below 0.22 MHz, indicating a reliable distinction in acoustic impedance sensing. In addition, the FSBSs at 2.5 m and 7.5 m are visualized in Fig. 6(d). The representative linewidths of FSBS spectra corresponding to air and water surroundings are 1.72 MHz and 3.41 MHz respectively.

In conclusion, a distributed optomechanical sensor with stable performance and high robustness has been proposed for the external environmental substance and acoustic impedance identification. we use the Aluminized coating optical fiber, for the first time, to excite transverse acoustic waves via the FSBS effect, which could low-loss transmit through the cladding-coating boundary to the coating layer or even the ambient media due to the quasi-acoustic impedance matching. The Aluminized coating is not only significantly enhanced the sensing fiber’s mechanical properties and strength but also to protect the sensing fiber from changes in external relative humidity. Meanwhile, the coherent stimulated probing process and the more significant difference in acoustic impedance between the Al-fiber and the external material both enhance the signal-to-noise ratio of the measured FSBS signal. The experimental results clearly show that the resonance frequency and linewidth of the FSBS spectra of Al-fiber are extremely insensitive to RH, which is an obvious advantage over the PI-fiber. Finally, the material recognition ability of the distributed acoustic impedance sensing is demonstrated by identifying the air and water surrounding the fiber under test with a spatial resolution of 2 m. Such a novel sensor will be suitable for chemical sensing fields such as oil pipeline leakage and marine pollutant detection, and simultaneous temperature and acoustic impedance sensing.

FSBS

forward stimulated Brillouin scattering

FUT

fiber under test

OMTDA

optomechanical time domain analysis

SNR

signal-to-noise ratio

SMF

single mode fiber

PI-fiber

polyimide-coated fiber

Al-fiber

Aluminized coating optical fiber

relative humidity

AWG

arbitrary waveform generator

EOM

electro-optic modulator

microwave generator

BSBS

backward stimulated Brillouin scattering

optical filter

optical fiber coupler

polarization controller

EDFA

erbium-doped optical fiber amplifier

orthogonal polarization scrambler

CIR

circulator

photoelectric detector

ISO

isolator

Acq

data acquisition card.

Acknowledgements

Not applicable at this moment.

Authors’ contributions

Y. D. guided the whole work. Y. R. performed major experiments, analyses and wrote the manuscript. A. S., D. Z, T. L., H. Z., D. L., D. B. were involved in the discussion of the manuscript. All authors contributed to the discussion and manuscript checking. The authors read and approved the final manuscript.

Funding

This work is sponsored by the National Natural Science Foundation of China (62075051); the National Natural Science Foundation of China (62005067); the National Postdoctoral Program for Innovative Talents of China (BX20200104); the Postdoctoral Science Foundation (2020M681088); the Heilongjiang Postdoctoral Fund to pursue scientific research in Heilongjiang Province (LBH-Z20067).

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Ethics approval and consent to participate

There is no ethics issue for this paper.

Consent for publication

All authors agreed to publish this paper.

Competing interests

The authors declare that they have no competing interests.

Shelby RM, Levenson MD, Bayer PW. Guided acoustic-wave Brillouin scattering. Phys Rev B.1985; 31(8): 5244-52.
Ohashi M, Shibata N, Shiraki K. Fibre diameter estimation based on guided acoustic wave Brillouin scattering. Electronics Lett. 1992; 28(10): 900-2.
Hua Z, Ba D, Zhou D, Jia Y, Wang Y, Bao X, et al. Non-destructive and distributed measurement of optical fiber diameter with nanometer resolution based on coherent forward stimulated Brillouin scattering. Light: Adv Manuf. 2021; 2(4): 373 − 84.
Tanaka Y, Ogusu K. Temperature coefficient of sideband frequencies produced by depolarized guided acoustic-wave Brillouin scattering. IEEE Photon Technol Lett. 1998; 10(13): 1769-71.
Zhang Z, Lu Y, Peng J, Ji Z. Simultaneous measurement of temperature and acoustic impedance based on forward Brillouin scattering in LEAF. Opt Lett. 2021; 46(7): 1776-79.
Tanaka Y, Ogusu K. Tensile-strain coefficient of resonance frequency of depolarized guided acoustic-wave Brillouin scattering. IEEE Photon Technol Lett. 1999; 11(7): 865 − 67.
Sanchez LA, Diez A, Cruz JL, Amdres MV. Strain and temperature measurement discrimination with forward Brillouin scattering in optical fibers. Opt Express. 2022; 30(9): 14384-92.
London Y, Sharma K, Diamandi HH, Hen M, Bashan G, Zehavi E, et al. Opto-mechanical fiber sensing of gamma radiation. J Lightwave Technol. 2021; 39(20): 6637-45.
Antman Y, Clain A, London Y, Zadok A. Optomechanical sensing of liquids outside standard fibers using forward stimulated Brillouin scattering. Optica. 2016; 3(5): 510-6.
Hayashi N, Mizuno Y, Nakamura K, Set SY, Yamashita S. Experimental study on depolarized GAWBS spectrum for optomechanical sensing of liquids outside standard fibers. Opt Express. 2017; 25(3): 2239-44.
Liang Y, Sun H, Cheng L, Jin L, Guan B. High spatiotemporal resolution optoacoustic sensing with photothermally induced acoustic vibrations in optical fibres. Nat Commun. 2021; 12: 4139.
Zheng Z, Li Z, Fu X, Wang L, Wang H. Multipoint acoustic impedance sensing based on frequency-division multiplexed forward stimulated Brillouin scattering. Opt Lett. 2020; 45(16): 4523-26.
Wang B, Ba D, Chu Q, Qiu L, Zhou D, Dong Y. High-sensitivity distributed dynamic strain sensing by combining Rayleigh and Brillouin scattering. Opto-Electron Adv. 2020; 3: 20001301-15.
Qi P, Luo Y, Shi B, Li W, Liu D, Zheng L, Liu Z, Hou Y, Fang Z. Phonon scattering and exciton localization: molding exciton flux in two dimensional disorder energy landscape. eLight. 2021; 1: 6.
Bashan G, Diamandi HH, London Y, Preter E, Zadok A. Optomechanical time-domain reflectometry. Nat Commun. 2018; 9: 2991.
Chow DM, Yang Z, Soto MA, Thévenaz L. Distributed forward Brillouin sensor based on local light phase recovery. Nat Commun. 2018; 9: 2990.
Pang C, Hua Z, Zhou D, Zhang H, Chen L, Bao X, et al. Opto-mechanical time-domain analysis based on coherent forward stimulated Brillouin scattering probing. Optica. 2020; 7(2): 176 − 84.
Ba D, Hua Z, Li Y, Dong Y. Polarization separation assisted optomechanical time-domain analysis with submeter resolution. Opt Lett. 2021; 46(23): 5886-89.
Zaslawski S, Yang Z, Thévenaz L. Distributed optomechanical fiber sensing based on serrodyne analysis. Optica. 2021; 8(3): 388 − 95.
Chow DM, Thévenaz L. Forward Brillouin scattering acoustic impedance sensor using thin polyimide-coated fiber. Opt Lett. 2018; 43(21): 5467-70.
Diamandi HH, London Y, Bashan G, Zadok A. Distributed opto-mechanical analysis of liquids outside standard fibers coated with polyimide. APL Photon. 2019; 4: 016105.
Xu Y, Zhao X, Li Y, Qin Z, Pang Y, Liu Z. Simultaneous measurement of relative humidity and temperature based on forward Brillouin scattering in polyimide-overlaid fiber. Sens Actuators B chem. 2021; 348: 130702.
Diamandi HH, London Y, Bashan G, Shemer K, Zadok A. Forward stimulated Brillouin scattering analysis of optical fibers coatings. J Lightwave Technol. 2021; 39: 1800-7.
Zheng Z, Li Z, Fu X, Gui X. Coherent-detection-based distributed acoustic impedance sensing enabled by a chirped fiber Bragg grating array. Photonics Research.2022. https://doi.org/10.1364/PRJ.450127.
Sanchez LA, Diez A, Cruz JL, Andres MV. High accuracy measurement of Poisson s ratio of optical fibers and its temperature dependence using forward-stimulated Brillouin scattering. Opt Express. 2022; 30(1): 42–52.
Physics-Sound-Acoustic Impedance. NDT Education Resource Center. https://www.nde-ed.org/Physics/Sound/acousticimpedance.xhtml.

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Quasi-acoustic impedance matching distributed optomechanical sensor with Aluminized coating optical fiber

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Version 1

Abstract

Figures

Introduction

Materials And Methods

Acoustic impedance matching condition

Experimental setup of OMTDA system

Results And Discussion

Sensing fibers spectra analysis

Humidity characteristics of aluminized optical fiber

Distributed substance identification

Conclusion

Abbreviations

Declarations

References

Status:

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