Optical fiber defect detection using Brillouin optical time domain analyser

Transcription

1 Indian Journal of Pure & Applied Physics Vol. 54, September 2016, pp Optical fiber defect detection using Brillouin optical time domain analyser M Kasinathan a *, Aleksander Wosniok b, Katerina Krebber b, C Babu Rao a, N Murali a & T Jayakumar a a Indira Gandhi Centre for Atomic Research, Kalpakkam , India b Federal Institute for Materials Research and Testing (BAM), Berlin, Germany Received 31 January 2015; revised 2 December 2015; accepted 4 February 2016 A new technique for optical fiber defect detection using Brillouin distributed fiber optic sensor (DFOS) has been proposed and experimentally demonstrated in this paper. This technique is based on stimulated Brillouin scattering (SBS), which offers three wave interaction in single mode optical fiber (SMF -10 µm/125 µm acrylic coated fiber). The nonlinear effect of SBS is manipulated to locate the defect in optical fiber using distributed sensing technology. Various kind of defects may be present in optical fibers. This paper details a case study on observation of a defect, which manifests its presence in certain temperature values. The detail of defect detection through distributed fiber sensor using the SBS has been brought out. SBS is sensitive to temperature and strain. In order to study the effect of defect in distributed fiber sensor as function of temperature and strain, the distributed pre-strained and unstrained optical fiber is subjected to temperature variation and corresponding measurements are obtained with Brillouin optical time domain analyser (BOTDA). This technique enables the utilization of Brillouin parameters, such as decreased amplitude, frequency and increased linewidth in the defect region of the fiber length. The fiber defect location can be determined with spatial resolution accuracy of less than 50 cm of using BOTDA technique. Keywords: Defect, Brillouin, Distributed sensor, Optical fiber, Strain, temperature, Time domain 1 Introduction Optical fibers with low transmission loss and wide bandwidth have been developed. The recent communication systems and sensing applications use the optical fibers. The application of optical fiber in communication and structural health monitoring (SHM) are requires the quality of fiber is very high. It is important to monitor the quality of optical fiber, which is used in critical applications such as temperature measurement in the pipeline 1, leak detection 2, storage tanks 3, reactor vessels 4, power cables 5 and strain measurements in mechanical 6 and civil structures 7. Optical time domain reflectometers (OTDR) have been used for measuring the optical fiber conditions. Barnoski et al 8 have shown that the OTDR is a viable technique by measuring fiber loss and locating optical fiber faults during installation at ambient condition. This technique manipulates the reflected signals in the optical fiber due to the defects or impurities and the difference of indices occurred along the propagation of the light. It is because of Rayleigh backscattering and Fresnel reflection 9. These defects may arise at *Corresponding author ( mkasi@igcar.gov.in) different stages of fiber manufacturing process such as preform fabrication, fiber drawing and fiber laying. The defects can be due to the presence of dopants, OH ions, residual stresses and micro cracks developed at different stage of fiber fabrication Even if the optical fibers have been so intensively investigated over the years, the interest of the scientific community is still alive: in fact the Nobel Price in Physics 2009 was awarded to C K Kao, whose discoveries have paved the way for optical fiber modern technology. In 1966, Kao understood that it was not imperfection in the fiber thread that was the main responsible for losses, instead it was the glass that to be purified, because of the presence of defects 14. One of the important factors that decides the usage of fiber for sensor and communication link is its attenuation. Optical fibers that are used for long line communication link fails often in tensile strength. Analysis of tensile properties of optical fiber, which reveals that the variation in tensile strength with fiber length, pointing to uncommon structural defects randomly occurring along the fiber length. Failures occur from crack propagation at these defects. The defects can be revealed according to the light attenuation 15. Attenuation profiling is usually carried

2 566 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 2016 out immediately after drawing the fiber, which is part of the fiber characterization process. This is normally done at room temperature. The fiber defects can be unnoticed in overall fiber profile during the normal fiber testing and it is carried out after installation of distributed fiber for sensor applications. The attenuation profile of the optical fiber at room temperature is well documented in earlier studies. Point defects and their precursors in the amorphous silica network are introduced during the fabrication process, through dopants and the interaction with ionizing radiation (high energy, photons including UV laser irradiation and particles). Optical properties and structure of defects in silica glass are reviewed in literatures 16,17. The major factors constituting the fiber attenuation are absorption, scattering, bending and coupling in fiber 18. Defects can be distinguished in intrinsic, when they are due to a variation of the basic silica elements (silicon or oxygen) and extrinsic, if they are related to presence of impurities in the silica matrix (H, Ge, P, etc). Extrinsic defects due to the presence of impurities (Cl, H and so on) are always present in variable concentrations in the material. The various absorption defects can be formed during fiber fabrication such as, intrinsic material defects (Ultra Violet and Infrared wavelength dependent) in the fused silica fiber 19,20 and extrinsic material defects (impurities) in fiber fabricated due to the presence of dopants, OH ions and metallic ions (Cr 3+, C 2+, Cu 2+, Fe 2+, Fe 3+, Ni 2+, Mn 3+, V 4+ ) are trapped during preform fabrication through modified chemical Vapour deposition (MCVD) process 10. These ion impurities are the defects, having differential thermal coefficients which manifests with function of temperature. Most of defects have optical absorption and luminescence bands and could be detected by optical absorption in the visible, UV,or IR spectral range, Raman and photoluminescence spectroscopies 16,17. The optical fiber is used as distributed sensor and light scattering plays the major role. The low power level is used for linear scattering studies such as Rayleigh and Mie scattering. Rayleigh scattering is wavelength dependent and it is the backbone of OTDR operations. It is used for detecting the location of local loss and fault through intensity reflection profiles, whereas reflections are not point defects and nonpropagating density fluctuations in the fiber is referred as a defect 3. However, these techniques are not sensitive to the temperature/strain. If optical fiber carries high power, non-linear scattering occurs, such as Raman and Brillouin effects. Raman scattering arises from the interaction of light with the vibrational modes of the constituent molecules in the fiber medium and scattering of light from optical phonons. The anti Stokes shift of Raman scattering is sensitive to temperature. The Raman Distributed Temperature Sensor (RDTS) works based on this method, which is widely used for distributed temperature measurement 21. Earlier works have been demonstrated to identify a dormant thermally stimulated defect in Multimode fiber using RDTS 22. Stimulated Brillouin scattering (SBS) is recognized as the dominant optical fiber nonlinearity, which involves three wave interaction in the fiber 23,24. It arises from the interaction of light with propagating acoustic phonons. Since the Brillouin frequency shift in an optical fiber depends on the fiber strain as well as temperature, this approach has difficulty of temperature cross-sensitivity problem. Brillouin scattering based distributed fiber sensor is used for temperature/strain measurement with single mode silica fiber in the order of GHz range 25,26. Brillouin based technique relies on measurement of frequency shift is inherently more accurate and more stable, compared to intensity based Raman technique, which is prone to drift. The detection of fiber break location is demonstrated using the Brillouin technique, where the intensity of the reflected signal is analysed 27. The optical fibers are dielectric waveguide in which light undergoes scattering. The defects in fiber waveguide are following the direction oriented scattering, which directly affect the light transmission that is forward (high frequency generation) and back scattered (acoustic frequency generation) therefore, significantly decreasing the performance. The fiber bending and coupling lead to attenuation due to evanescent modes, leaky modes, radiation losses, connector losses and splicing losses 18. The presence of defects in clad or surface of the connectors may have an impact on the fiber connector integrity. An optical fiber connector is inspected for its defect by magnifying visual examination. When the defects are visualized, fiber surface defects can be identified and categorized such as micro scratches and cracks 28. Accurate thermal or thermally-related properties of the active core, e.g. any change of its properties, is particularly important because it is the source of quantum defect heating as well as the path for the signal 29. In order to detect such defects in optical fiber, optical/digital joint transform correlator method is used 30.

3 KASINATHAN et al: OPTICAL FIBER DEFECT DETECTION USING BRILLOUIN ANALYSER 567 The Brillouin parameters such as amplification/ frequency/full Width Half Maximum (FWHM) and their effects with temperature/strain were studied 24,31. The Brillouin frequency shift due to the phonon-defect interaction was studied at various temperature by Mitra Dutta 35. Fabien Ravet et al 32 demonstrated that the defect structure with Brillouin frequency shift assess the structure through BOTDA. In this paper, we present a case study of a dormant defect in an optical fiber using BOTDA. To the best of the authors` knowledge, this approach for defect detection in optical fiber has not been reported earlier. It is necessary to detect any defects in the optical fiber. These defects can be further investigated and classified with their specific feature. This work is part of a project undertaken to deploy the distributed fiber sensor for structural health monitoring studies. 2 Experimental Details The basic principle of temperature and strain measurement using SBS is extensively discussed in the literatures In this study, BOTDA which works on the principle of Optical time domain reflectometer (OTDR) is used 33. The Brillouin shift is given by: v B 2 n va = (1) λ i where n is the refractive index of the fiber, ν a is the velocity of the acoustic wave and λ i is the wavelength of the input laser. Since n and ν a are dependent on temperature and strain, the above equation can be expressed as: ( ) ( ) vb = vb 0 + αt T Tr + βε ε ε r (2) where ν B0 is the peak frequency of optical fiber due to Brillouin scattering, α T is the frequency-temperature coefficient and β ε is the frequency-strain coefficient. α T and β ε are the proportional constants, which are determined by the fiber material composition, laser wavelength and additional fiber coatings or jackets. The temperature or strain information can be calculated from the Brillouin shift. The relation between the absolute temperature T and the absolute strain ε were studied 23,24. The Brillouin system is coupled to optical fiber, it provides a pulsed pump signal and continuous Stokes signal. The Brillouin shift depends on the velocity of the acoustic signal, which is due to strong pump pulse electrostriction. The Brillouin frequency profile of the optical fiber is obtained using BOTDA. It is configured to follow 1 m spatial resolution with a sampling interval of 50 cm. The BOTDA uses a laser source of wavelength 1.55 µm and its associated Brillouin shift is in the order of GHz range depending on the fiber type. The temperature and strain measurement is performed in terms of Brillouin frequency shift, which is in the order of GHz 24. The experimental structure is a cylindrical SS drum of 12 cm long, 20 cm diameter and a 0.5 mm pitch to keep the fiber under strain with the help of friction (Fig. 1). The size of the metal drum is chosen appropriately to perform the temperature measurement comfortably. In order to evaluate the effect of defect in optical fiber as a function of temperature, a single fiber with a defect is wounded on the metal drum with a pattern of distributed strain (pre calculated strain = L/L, where L is the change in length and L is the original length of the fiber). A 39 m acrylic coated single mode step index silica optical fiber is deployed spanning of five separate strain segments with a length distribution of 5 m. The first and second segments are maintained at 0.2% of strain ( L = 0.01 m). The third and fourth segments are maintained at 0.4% of strain ( L = 0.02 m). The fifth segment is maintained at 0.6% of strain ( L = 0.3 m). In this setup alternate fiber segments are maintained as unstrained segments. The induced defect in the fiber segment is in unstrained segment, so that the fiber integrity is ensured and fiber break can be avoided. The basic configuration of BODTA system is shown in Fig. 2. Sensor fiber is connected to both ports of the BOTDA system to form a loop for obtaining the frequency profile Fig. 1 Cylindrical metal drum with strained fiber Fig. 2 Schematic of the BODTA system

4 568 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 2016 at room temperature (Fig. 3) and also verify the strained and unstrained segments on the metal drum. The structure is subjected to temperature variation from -40 o C to 80 o C and the fiber operating temperature is 100 o C. Brillouin parameters are monitored over the temperature range and analyzed. 3 Results and Discussion 3.1 Defect detection using Brillouin frequency In this work the measurements are performed with a spatial resolution (SR) of 1 m and sample interval of 0.5 m using BOTDA system 31. As the temperature of the metal drum was varied from -40 o C to 80 o C, the Brillouin frequency were found to undergo changes according to Eq. (2). The measured Brillouin frequency shifts for strained and unstrained fiber sensor segments with respect to temperature are shown in Fig. 4 and Fig. 5, respectively. However, the frequency shifts of unstrained fiber and that of the strained fiber segments are not matching with each other. Also the unstrained fiber frequency shift is well deviated from the strained fiber. Whereas the defect segment (23 rd m-unstrained) frequency shift neither follow the strained fiber nor unstrained fiber. The frequency shifts are linear in strained and unstrained fibers in the range of -40 o C to 10 o C and 50 o C to 80 o C, the defect manifests at 20 o C, 30 o C and 40 o C. Defects and colour centres inside the fiber core can also contribute to second harmonic generation under certain conditions 36, which leads to frequency shift. Some intrinsic defects owing the frequency shift characteristics of an electro-optic intensity modulators 37, such as instability output and generation of multi-sidebands, introduce adverse effects to the signal-to-noise ratio (SNR) of these systems. From the result, it is found that the fiber defect alters the frequency shift monotonically. 3.2 Defect detection using Brillouin amplitude The presence of defects in optical fibers often causes the appearance of new energy levels located inside the band gap of the dielectric 16,38. Brillouin amplitude has weak dependence on temperature, so that the combined measurement of Brillouin frequency shift and amplitude can provide the additional information required for temperature/strain discrimination 24. As a consequence, the fiber material absorbs more important part of the transmitted signal. It leads to an attenuation of the light guided inside and consequently degradation of the fibers themselves. The BOTDA based technique measures the integrated Brillouin amplitude for the signal beam, which is broadened by the temperature variation along the fiber length induced by the quantum defect heating 39. In Fig. 3 Fiber frequency profile at room temperature Fig. 4 Frequency shift of fiber strain segments as a function of temperature Fig. 5 Frequency shift for unstrain fiber function of temperature

5 KASINATHAN et al: OPTICAL FIBER DEFECT DETECTION USING BRILLOUIN ANALYSER 569 this spatially-resolved thermal diagnosis of high-power fiber laser system is used for in-situ measurement. Brillouin amplitude profile follows the attenuation characteristics of the fiber, as a function of the length of the fiber. From the temperature profile, the 23 rd m of unstrained region amplitude drop is observed, which is shown in Fig. 6 for low temperature and in Fig. 7 for high temperature. The amplitude drop is found to vary monotonically in strained fiber and normal in the unstrained fiber segments. This amplitude drop and subsequent raising behaviour are attributed to defects in the fiber. The Brillouin amplitude for the unstrained fiber segment's response is matched, due to the defect fiber amplitude drop as a function of temperature. The Brillouin amplitude drop clearly indicates that the fiber defect which affects the fiber integrity in the distributed fiber sensor. 3.3 Defect detection using FWHM The SBS amplitude is maximum at specific Brillouin frequency and apart from frequency shift it has minimum and maximum frequency values. The Brillouin frequency shows the limitation of linewidth as well as width of the amplitude spectrum. SBS amplitude spectrum shape is with lorentzian function 40. Brillouin frequency shift and amplitude are linear with respect to variation in temperature and the FWHM is obtained Brillouin amplitude spectrum, which includes the amplitude and frequency. The Brillouin amplitude is influenced by the pump laser spectral linewidth. Brillouin spectral linewidth measurement is a complement to the amplitude and frequency 24. In order to study the fiber defect, Brillouin scattering linewidth is also considered apart from frequency and amplitude. Thus evaluating the spectral information is for the defect detection by suppressing or exploiting Brillouin scatter attributes in a fiber. Figure 8 shows the linewidth response with respect to temperature cycle from 20 o C to 80 o C. Figure 9 shows the linewidth response for the temperature cycle of -40 to 20 o C. From the Figs 8 and 9, the peaks of the line width profile show the strained and unstrained fiber segments. In FWHM profile, the peak is an indication of the rising and lowering segment of the fiber and the Fig. 6 Amplitude profile for high temperature Fig. 8 FWHM profile at high temperature Fig. 7 Amplitude profile for low temperature Fig. 9 FWHM profile at low temperature

6 570 INDIAN J PURE & APPL PHYS, VOL 54, SEPTEMBER 2016 plateau region corresponds to strained and unstrained segments. From the experimental results, it has been observed at 23 rd m length, the fiber sensor shows monotonic response. For the rest of the unstrained segments line width as a function of temperature agreed well with the estimated values of unstrained segments fiber. The Brillouin spectral characterization for SMF, frequency depends on temperature, the amplitude increases linearly with temperature and the linewidth decreases with temperature 17. In the case of Brillouion spectral profile, the frequency is linear, with decrease in amplitude and unchanged linewidth 38. In this length 23 rd m is unstrained segment and observed the attenuation of amplitude, an increase in frequency shift and peak rise in linewidth. It indicates the presence of a thermally stimulated defect in the unstrained segment of the fiber. 4 Conclusions The fiber defect was observed using BOTDA for the temperature cycle. The results are verified with Brillouin parameters, i.e., frequency shift, variation in amplitude and line width. The defect has been noticed and its temperature dependence has been observed. Brillouin parameters are not altered by temperature and the performance is matched for unstrained fiber. For the strain and unstrained fiber segments, Brillouin parameter changes are well in line with the estimated and experimental results. Whereas the Brillouin parameters compared for the defective fiber segment is not matching with normal fiber results. Drop in Brillouin amplitude, increase in frequency and in line width peaks are due to defect, whereas the change in performance is due to the effect of temperature. Simultaneous analysis of Brillouion parameters shows the thermally stimulated defect, which might have gone unnoticed if the frequency profile as a function of temperature was only studied. The measurement of the Brillouin frequency factor alone can characterize the Brillouin spectral such as amplitude and linewidth, as well as determine the defect as function of a tempearture. This type of analysis could be recommended as a procedural testing of fibers deployed for structural health monitoring purposes. The results demonstrate that BOTDA is a promising tool in non destructive technique for detecting the defect in fiber sensor. 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