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Abstract
A distributed refractive index (RI) sensor based on high-performance optical frequency domain reflectometry was developed by bending a piece of standard single-mode fiber to excite sets of higher-order modes that penetrate the surrounding medium. External variations in RI modifies the profiles of the sets of excited higher-order modes, which are then partially coupled back into the fiber core and interfere with the fundamental mode. Accordingly, the fundamental mode carries the outer varied RI information, and RI sensing can be achieved by monitoring the wavelength shift of the local Rayleigh backscattered spectra. In the experiment, an RI sensitivity of 39.08 nm/RIU was achieved by bending a single-mode fiber to a radius of 4 mm. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Fiber-optic sensors are used extensively in the chemical [1], physical [2], and biological applications [3] fields to measure the refractive index (RI) of an external medium because of their high sensitivity, compact size, and immunity to electromagnetic interference. Many fiber-optic RI sensors [4–6] based on various sensing elements, such as tilted fiber Bragg grating [7], fiber tapers [8], D-shaped fibers [9], thin core fibers [10], and fiber in-line interferometers [11], have been proposed to achieve high sensitivity in measuring RI in combination with schemes such as surface plasmon resonance or sensitive label film [12,13]. For RI sensing, one should use a technique that couples the light propagating in the fiber core to the external boundary. This, however, requires a complicated fabrication process involving high cost, which in turn reduces the possibility of high-volume fabrication. Moreover, distributed RI sensing is even more challenging, but if achieved, will have multiple practical applications.
Distributed fiber-optic sensors utilize a backward scattering scheme in the optical fiber, e.g., Rayleigh [14], Brillouin [15], and Raman scattering [16], combined with optical time-, frequency-, or correlation-domain techniques, to sense basic physical parameters such as temperature [17,18] and strain [19–21]. As the light propagates within the fiber core, these techniques are limited to detect physical parameters inside the fiber core. To overcome this limitation, Zadok et al. [22] innovatively proposed the use of forward Brillouin scattering based on optic-acoustic resonance with transverse acoustic waves to identify the surrounding medium. Several distributed forward Brillouin fiber-optic sensors have been proposed using this concept [23–25]. However, the spatial resolution of such sensors is on a several-meter level and their sensitivity is too low. Minardo et al. [26] proposed the use of a side-polished optical fiber to measure the outer RI of a surrounding solution using the backward Brillouin-scattering-based Brillouin optical frequency domain analysis technique. They achieved a sensitivity of less than 2.4 × 10−3 nm/RIU in the index range of 1–1.42. Its sensitivity was low because of weak interaction between the evanescent field and backward Brillouin scattering, and the sensing length was limited by the side polishing technique.
Optical fiber frequency reflectometry (OFDR) [27,28] is a more competitive candidate for the detection of distributed RI of the surrounding medium in comparison to Brillouin-scattering-based distributed optical fiber sensors, as it has a high signal to noise ratio (SNR), high spatial resolution, and takes a relatively short duration for measurement. Recently, Tosi et al. [29] proposed the use of an etched MgO-doped optical fiber to measure RI based on OFDR, where a sensitivity of 1.53 nm/RIU was achieved. Although distributed RI measurement was achieved, the sensitivity was low because of weak interaction of the evanescent wave with the outer medium. Ding et al. [30] proposed the use of a tapered optical fiber to measure the external RI based on OFDR, where a sensitivity of 62.52 nm/RIU was achieved because of large evanescent field coupling and a multimodal interference effect. This is the highest sensitivity achieved to date, but the proposed RI sensor is fragile as its diameter is much thinner than that of the standard optical fiber and lacks protective coating. The aforementioned distributed RI sensors show that, to achieve high sensitivity, side polishing, etching, or tapering techniques are necessary for the light wave propagating in the fiber core to radiate out of the fiber boundary. Such methods would inevitably alter the original mechanical properties, introduce unstable factors, and lower their long-term stability.
In this study, we developed a distributed RI sensor by bending a single-mode fiber (SMF) into a U shape to excite sets of higher-order modes. Some of these higher-order modes radiate out of the fiber boundary and penetrate the surrounding medium for RI sensing. The excited higher-order modes are then recoupled into the fiber core and interfere with the fundamental mode. Thus, the fundamental mode carries the varied information of the external RI, and distributed index sensing is achieved by measuring the wavelength shifts of the local Rayleigh backscattered spectra via OFDR. Owing to the high SNR of the OFDR, which compensates the bending-induced loss, the proposed sensor can be bent in a small bending radius to achieve high sensitivity. An RI sensitivity of 39.08 nm/RIU in the RI range of 1.3330–1.3773 was experimentally achieved by imposing a bending radius of 4 mm. The spatial resolution was 2 mm, and the sensing length was 13.86 m. Additionally, the proposed sensor maintains its buffer coating intact, which boosts its practicability and application adaptability.
2. Operating principle and experimental setup
A schematic diagram of the proposed U-bent fiber, which consists of an intact coating, is illustrated in Fig. 1. When the light travels from the straight portion of the sensing fiber into the U-bent region, part of the light of the core mode is coupled into the cladding and generates many sets of higher-order modes. We classify the optical field into two parts: one part represents the fundamental mode ${E_F}(\lambda )$, and the other part is a set of higher-order modes ${E_H}(\lambda )$. These two parts are respectively expressed as
Fig. 1. Schematic diagram of the U-bent fiber structure made of standard SMF that consists of core, cladding, and coating.
Rayleigh scattering originates from core index fluctuations induced by inhomogeneous density and structure perturbations. The amplitude of each scattering center is randomly distributed but fully deterministic in time and presents a noise-like scattering pattern. The schematic of a typical OFDR network for the Rayleigh scattering measurement is shown in Fig. 2. It comprises a tunable laser source (TLS), two interferometers (one measurement/main interferometer and one trigger/auxiliary interferometer), and a data acquisition (DAQ) unit. The light from the TLS is split into two branches using a 1:99 coupler (OC1). The upper branch (auxiliary interferometer) adopts the Michelson configuration, which takes 1% of the laser power from the TLS. It generates clock pulses at equidistant optical frequency points to trigger the DAQ, which is known as the frequency-sampling method and is widely used to address the laser tuning nonlinearity issue. Two Faraday rotating mirrors (FRMs) effectively reduce the polarization sensitivity of the auxiliary interferometer. The lower branch of the schematic is the Mach-Zehnder interferometer used for the measurements. A laser power of 99% from the TLS is launched to the measurement Mach-Zehnder interferometer. The measurement path of the interferometer is composed of a fiber under test (FUT) with three U-bent fiber sensors. Finally, the optical beat signals are received by a balanced photo detector (BPD). The measurement results were recorded using a high-speed DAQ for data processing [30]. The spatial resolution of the OFDR is determined by the spectral bandwidth $\Delta f$ of the tunable laser scan range, and is expressed as follows:
where n is the RI of the fundamental mode of the FUT. Owing to the large scanning range of the TLS, the spatial resolution can be easily reduced to the order of millimeters.Fig. 2. Schematic of the OFDR network for RI measurement based on Rayleigh scattering. TLS: tunable laser source; FRM: Faraday rotating mirror; PC: polarization controller; OC: optical coupler; BPD: balanced photo detector; DAQ: data acquisition card; CIR: circulator; FUT: fiber under test; CG: clock generator. The U-bent fiber is immersed in a tank with different concentrations of sucrose solution.
In the experiment, we adopted a high-performance OFDR system (Luna, Inc., OBR 4600), whose wavelength was repeatedly scanned from 1530.000 to 1571.737 nm at a tuning rate of 80 nm/s, corresponding to a frequency of 5217 GHz. Theoretically, a spatial resolution of 20 μm could be achieved, but we set the spatial resolution to 2 mm to maintain a sufficiently high SNR. The sensing fiber is a standard telecom SMF (Corning, SMF-28e). This ensures compatibility between the U-bent RI sensor and other commercial optical fiber devices. The proposed fiber structure is fabricated as follows: first, radially squeeze the SMF into a U shape; second, fix the two straight edges of the U-shape on steel plate with 3M industrial tape; last, adjust the distance between the two steel plates to form the expected bending radius. Using this method, the three U-bent RI sensors were fabricated mechanically with bending radii of 6, 5, and 4 mm. The proposed method is easy to build and has a robust structure that can be implemented easily on any section along the entire fiber link for distributed RI sensing. The length of the FUT was approximately 14 m, and the U-bent fiber sections were immersed in sucrose solution of different concentrations. The change in RI to be measured will result in a change in the local Rayleigh scattering spectrum, which can be measured by a cross-correlation between the measurement and reference Rayleigh scattering spectra.
3. Results and discussions
We randomly fabricated three U-shaped fiber structures to verify their distributed sensing capability. The backward Rayleigh scattering signal of the FUT is shown in Fig. 3. The spatial domain scattering pattern after Fourier transform of the original OFDR signal is highlighted in Fig. 3(a). The first 5 m of the optical fiber is the standard optical fiber patch cord, and the rest, which extends to 13.86 m long, constitutes the FUT. The reflection of the connector and the end of fiber were clearly observed. Figure 3(b) shows the local zoomed-in results of the three U-bent regions. It can be seen that the U-bent fiber sensor causes a loss of approximately 5.65 dB at the 6.78-m position with a bending radius of 6 mm, 7.81 dB at the 7.67-m position with bending radius of 5 mm, and 8.97 dB at the 9.35-m position with a corresponding bending radius of 4 mm. The bending loss mainly originates from the radiation leakage of the light wave at the sensing point and can be clearly observed. We can see that the signal shown in Fig. 3(b) depicts a sensitivity greater than 140 dB, and an SNR greater than 20 dB, which is attributed to the excellent detection performance of the OFDR system used. Based on this fact, the Rayleigh scattering spectra of the three U-bent RI sensors are clearly observed, and their intensities are stronger than those of the straight sections owing to bending-induced stress.
Fig. 3. Measured backscattered light signals of the FUT in the spatial domain. (a) Three U-bent fiber structures with bending radii of 6, 5, and 4 mm located at the 6.78-, 7.67-, and 9.35-m positions, respectively. (b) Local zoom-in of (a). (c), (d), and (e) are the local zoom-in of the Rayleigh backscattered spectrum of the corresponding U-bent fiber.
The local zoom-in of the backward Rayleigh scattering signals of the three U-bent regions are shown in Figs. 3(c), 3(d), and 3(e), respectively. From Figs. 3(c)–3(e), we can see that the relative intensity of the signal is higher than that of the other parts, and this phenomenon is increasingly clearer with a smaller bending diameter. This is mainly because of the fact that bending-induced stress effectively elevates the RI of the fiber core through the elasto-optical effect. It can be seen that the signal in this region exhibits a hump shape, where the signal intensity is high at both ends of each U-bent region and shows a decrease in the middle region. This is mainly due to the fact that the bending stress is strong at the beginning of the U-bent region, owing to the deformation of the straight fiber, and this stress decreases gradually in the middle region. We note that such a backward Rayleigh scattering signal enhancement cannot be observed in [31], which is probably because of the relatively low sensitivity of the OFDR used. Based on the superior performance of the OFDR system, the proposed U-bent RI sensor can afford a larger bending radius loss, which indicates that a larger optical field of the higher-order modes can radiate out from the fiber boundary.
Then, we show the Rayleigh scattering spectra and cross-correlation spectra of the U-bent fiber RI sensors, which were immersed in a sucrose solution. The change in RI can be achieved by changing the concentration of the sucrose solution, and the index of the solution was monitored using a commercial refractometer (SGW-731, Shanghai Physico-Optical Instrument Ltd.). The reference condition is 0% sucrose solution, which corresponds to an RI of 1.3330, and the changing condition is a 0.7% sucrose solution that corresponds to an RI of 1.3343. It should be noted that the temperature was kept constant to avoid any additional disturbances, and there was no extra strain. Figures 4(a)–4(c) depict the Rayleigh backward scattering spectrum of the three U-bent fiber RI sensors with an RI variation of 0.0013 for bending radii of 6, 5, and 4 mm, and their corresponding cross-correlation spectra. It was found that the Rayleigh scattering spectra exhibited a random noise-like pattern. More specifically, periodic peaks and troughs appear on these Rayleigh backward scattered signals, and this phenomenon is more obvious for small bending diameters [see Fig. 4(c)]. We attribute this to the bending-induced excitation of higher-order modes that produce multimodal interference. From Figs. 4(a)–4(c), we can see that the Rayleigh backward scattered signal shifts to longer wavelengths when the external RI changes, while maintaining the same signal pattern. After creating a cross-correlation algorithm between the reference signal and 0.7% sucrose solution, we obtained cross-correlation peaks as shown in Figs. 4(d)–4(f), as 0.0255, 0.0357, and 0.0509 nm, respectively.
Fig. 4. Rayleigh backward scattering spectra of three U-bent fiber RI sensors exposed to an RI variation of 0.0013 for bending radii of (a) 6, (b) 5, and (c) 4 mm, and their corresponding cross-correlation spectra showed wavelength shifts of (d) 0.0225, (e) 0.0357, and (f) 0.0509 nm, respectively.
We then characterized the RI sensitivity of the proposed U-bent RI sensor. We simultaneously immersed the above three U-bent sensors in a tank filled with sucrose solutions, as schematically presented in Fig. 2, and the RI changed from 1.3330 (0% sucrose) to 1.3737 (22% sucrose) in steps of 2% mass fraction. We obtain the wavelength shifts by making cross-correlation between each consecutive measurement, as shown in Fig. 4, and the results of the wavelength shifts for U-bent fiber RI sensors with bending radii of 6, 5, and 4 mm are shown in Fig. 5. It was found that the optical wavelength of the U-bent sensor increases linearly with the surrounding RI. The wavelength shifts appear only at the position of the U-bent fiber sensing heads, indicating that there is no other significant crosstalk from nearby sections without curved structures.
Fig. 5. Wavelength shifts for U-bent fiber RI sensors with bending radii of 6, 5, and 4 mm in the RI range of 1.3330–1.3737.
Finally, we measured the RI sensitivity of the proposed three U-bent RI sensors by recording the peak shifts and applying linear fitting. The optical wavelength and frequency shifts of the proposed U-bent RI sensors with different RIs are shown in Fig. 6. A linear fitting yields an RI sensitivity of 19.61, 27.59, and 39.08 nm/RIU for the U-bent RI sensor with bending radii of 6, 5, and 4 mm, respectively. The fitting results of all bending structures demonstrated a good linear relationship, with R-squared values of up to 0.99530, 0.99847, and 0.99895, respectively. Specifically, there are two single points of wavelength shifts that deviate from the linearly fitting line for the U-bent structure with a bending radius of 4 mm, which are probably due to the temperature fluctuations induced by RI variation. In addition, all U-bent structures showed a good linear response to RI, which verified the feasibility of the proposed sensor.
Fig. 6. Optical wavelength/frequency shifts of the U-bent RI sensors with bending radii of 6, 5, and 4 mm in the RI range of 1.333–1.3737, and their linear fitting.
Compared with Brillouin-scattering-based fiber-optic sensors for external substance detection, the proposed U-shaped distributed RI sensor based on OFDR exhibits superior advantages in terms of RI sensitivity, spatial resolution, and measurement duration. It is worth noting that a similar method is used by Huang et al. [31], but our developed distributed RI sensor exhibits a different measurement result and a higher sensitivity, which are attributed to the high performance OFDR. We also note that the RI sensitivity is lower than that reported in [10] (62.52 nm/RIU), which uses a tapered fiber that encounters the issue of breakage. In contrast, the proposed RI sensor maintains the buffer coating intact and can detect a longer sensing length. Furthermore, the measured RI sensitivity based on the proposed method is approximately 25.5 times higher than that reported in [29], which is due to the bending-excited higher-order modes bridging the gap between the surrounding medium and the fundamental mode that propagates in the fiber core. Overall, the proposed distributed RI sensor is demonstrated for high sensitivity and high spatial resolution with practicability, which could be considered as a potential candidate in chemical and biological applications.
4. Conclusion
In this paper, a distributed refractive index sensor is demonstrated by monitoring Rayleigh backward scattering spectra in a standard SMF. The proposed sensor is bent into a U shape to excite high-order modes that are sensitive to surrounding medium, which are then recouple into the fiber core and yield an interferogram with the fundamental mode. A high performance OFDR system is used to characterize the proposed U-bent RI sensor. The spatial resolution is 2 mm, and the sensing length is 13.86 m. An RI sensitivity of 39.08 nm/RIU in the RI range of 1.3330–1.3773 is experimentally achieved by imposing a bending radius of 4 mm. The proposed sensor maintains its buffer coating intact, which allows to keep its original mechanical property. It could multiplex multiple RI sensors on a single fiber that covers a large sensing range, which exhibits the distributed measurement ability of RI. Combined with various functional materials based on RI modulation, the proposed sensor shows the potential applications in the fields of chemistry and biology, such as NH3, pH, biomarker and ion detection, etc.
Funding
Taishan Series Talent Project (2017TSCYCX-05); Science and Technology on Electronic Test & Measurement Laboratory Foundation (41Q1313-5, KDW03012003); Guangdong Basic and Applied Basic Research Foundation (2021A1515011891); Guangdong Provincial Key Laboratory of Photonics Information Technology (2020B121201011); Science and Technology Planning Project of Guangdong Province (2018B010114002); National Natural Science Foundation of China (61901426, 61925501, 62005052, 62005055, U2001601).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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