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Abstract
In this paper, we report the demonstration of an optical fiber sensor based on the cascaded up-down-tapered (UDT) hetero-core fiber structures in a sequence for simultaneous refractive index (RI) measurement at dual spatial locations for the first time. The two hetero-core fiber structures, namely sensor structures A and B, were fabricated by sandwiching a section of no-core fiber (NCF) with lengths of 45 and 60 mm between two standard single-mode fibers (SMF), respectively. The UDT hetero-core fiber structure was formed by introducing a down taper between two adjacent up tapers in the NCF section by using the over-fusion splicing method. The dual up tapers respectively serve as a fiber splitter/combiner, and the down taper acts as an optical attenuator. The proposed dual point sensing head (sensing point A and sensing point B) was engineered by cascading the two sensor structures A and B to generate two separate interference dips and achieve wavelength division multiplexing. Experimental results show that for the dual point sensing structure, sensing point A and sensing point B are sensitive to RI independently and simultaneously, and exhibit sensitivities of 99 nm/RIU and 93.7 nm/RIU for simultaneous measurement of RI ranging from 1.33 to 1.38 for each point, respectively. During measurement, there is no mutual interference to each sensing head. The results point out that the proposed dual point sensor can be potentially applied for the simultaneous detection of RI in any environment and at different locations.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The accurate measurement of refractive index (RI) is a key requirement in a variety of measurement applications such as in medical, biological, and chemical industries [1–3]. Information about RI is widely employed to identify unknown samples, examine the purity of samples or measure the concentration of certain compounds within a solution [4]. Several techniques have been developed for measuring RI [5,6]. The more traditional technique is based on bulk optics to conclude the RI based on information obtained about the critical and Brewster angles [7]. For example, conventional hand-held refractometers and digital refractometers are devices based on this technique. However, because of size and weight limitations, these methods are not always appropriate in many real-life situations. These restrictions can be avoided by using optical-fiber sensors instead [4].
In recent years, optical fibers are considered the basis for extraordinary growth in worldwide applications such as telecommunications, fiber lasers, sensors, short pulse generation, etc [8–12]. In this sense, optical fiber-based monitoring methods have been widely investigated owing to their significant features such as their portability, simplicity, versatility, security, and immunity to outside electromagnetic interference. Additionally, optical fiber enables the transmission of signals over great distances with a minimum loss, enabling the operation of a remote monitoring system and difficult settings [13–15]. So far, to diversify the RI sensing applications and enhance their sensitivity, more complex tapered fiber structures can be developed. different optical fiber RI sensors configurations have been studied and reported in the literature, such as Mach-Zehnder (MZIs) [16–18], fiber Bragg grating [19–21], Fabry-Pérot (FPIs) [22–24], and multi-mode optical fiber (MMF), [25,26] and so on. However, the complex tapered fibers with FBG still suffer from high fabrication costs. The multimode interference (MMI) effect and the generation of self-imaging phenomena have been extensively investigated in multimode fiber (MMF) spliced between two SMFs, forming a single mode–multimode–single mode (SMS) fiber structure. In SMS fiber structures, the MMI effect in the MMF segment leads to the formation of a self-image of the input single-mode excitation onto the output SMF aperture. The MMI effect defines the field characteristics at the output SMF, which is a function of various parameters such as the core and cladding refractive indices, core diameter, and multimode section length [25–27].
In recent years, hetero-core optical fiber structures based on multimode interference in NCF which is functioning similarly to MMF have gained a lot of interest in sensing applications where NCF can replace the MMF and acts as the sensing head [15,28]. No-core fiber (NCF) diameter has been widely used as a substitute for an etched and tapered MMF due to its high index relative to the surroundings which results from the lack of cladding.
The hetero-core structure consists of optical fibers with a smaller core diameter fiber sandwiched between two segments of fiber with a larger core diameter. This structure permits the light to propagate out from the fiber core and propagate in the cladding of the sandwiched fiber portion, being sensitive to the external RI changes through evanescent [15]. When compared to the MMF, the NCF has a more sensitive response to external environmental changes due to the lack of cladding. Moreover, it is well known that tapering is a convenient technical means to improve the RI sensitivity since it can effectively increase the contact area between the in-fiber light power and the external liquid, creating an enhanced evanescent field as a medium for power exchange at the same time [29,30]. However, optical fiber RI sensors available today can only perform single-point sensing, making them unsuitable for applications requiring a high number of sensor arrays. In addition, for multiple sensing applications, each sensor demands an individual light source and detection component. Therefore, the cost of the sensor would be prohibitively high. Therefore, the multiplexing wavelength method effectively reduces the cost, time, and maintenance of the sensor arrays [31].
To enable multi-point sensing, many multiplexing approaches for sensors have been suggested, including spatial division multiplexing (SDM) [32], temporal division multiplexing (TDM) [33], and spatial frequency division multiplexing (SFDM) [34]. However, the SDM necessitates the use of many detectors, which is costly. TDM needs pulsed light and a time-gated detector, as well as tight reflectivity criteria. With a restricted wavelength spread, WDM can only multiplex a few sensors. The SFDM needs varying cavity lengths between sub-sensors, which is difficult to build. Today, Frequency-modulated continuous-wave (FMCW) interferometry has become a common technique for distributed optical fiber sensing and is widely used in sensing temperature, strain, vibration, and RI [31]. Recently, multi-point RI sensing is achieved by combining the FMCW interferometry with microstructure fibers [35], etched fiber [36], and macro bending fiber [37]. However, the sensing points are very few and the RI sensitivities are low.
In this paper, a dual-point optical fiber RI sensor based on multimode interference from two cascaded UDT hetero-core fiber structures was demonstrated. Tapering the NCF region is significant in increasing the interaction between the evanescent field and the surrounding medium. The transmission spectrum of the two cascaded structures was adjusted to achieve the multiplexed wavelength modulation-based measurement. To achieve high RI sensitivity, each point (point A and point B) is made up of a compact up-down tapered NCF with selected lengths. The dual-up tapers act as optical beam splitters and combiners respectively, while the down taper acts as an optical attenuator. Experimental results of the dual point cascaded structures show that sensing point A and sensing point B are sensitive to RI with maximum RI sensitivities of 99 nm/RIU and 93.7 nm/RIU for simultaneous measurement in the range of 1.33 to 1.38 for each point, respectively. The proposed UDT hetero-core fiber structure can be used as an all-fiber type device integrating double optical beam splitters/combiners and an optical attenuator. The dual up tapers act as optical beam splitter and combiner, respectively. The down taper acts as an optical attenuator to monitor the value of the light intensity. The reported results provide promising perspectives for dual-point sensing applications to monitor the RI of liquid samples at two different spatial locations.
2. Dual point sensor structure operating principle and fabrication
2.1 Operating principle
Figure 1 illustrates the proposed dual point sensing head based on two cascaded UDT-SMF-NCF-SMF structures. Each UDT-SMF-NCF-SMF served as the one point of the dual point sensing head.
From Fig. 1, the principle of the proposed UDT-SMF-NCF-SMF structure operation can be described as follows: core modes are excited from the input light of the broadband source and propagate along the input of the lead-in SMF section. when the propagated light reaches the first up taper which acts as a splitter, the higher-order cladding modes are excited as a result of the core mismatch of the mode field diameter and propagates along the NCF section. The function of the down taper at the middle of the NCF is to act as an optical attenuator which is employed to excite more evanescent waves to enhance the sensing performance of the sensor structure with surrounding environments. When these lights reach the second up taper, the core and cladding modes will recouple and interfere with each other and then propagate back into the core of the lead-out SMF.
The input optical field of the NCF can be expressed using the coupled-mode theory and linearly polarized modes as follows [38,39]:
The radial coordinate in the lead-in SMF and NCF cross-section is represented as r. while ${\psi _m}$(r) is the optical field of the mth excited mode in the NCF and M is the total number of excited modes in the NCF. Where ${x_m}$ represents the excitation coefficient from the LP01 mode in the SMF to the LP0n mode in the NCF, which can be expressed as follows:
These propagating modes will interact with one another while they are transmitted in the NCF. The optical field profile can be described by the following after propagating a distance z in the NCF:
In Eq. (5) n is integer even number (2,4,6…….) and m is integer odd number (1,3, 5,….). Where n and m are the order eigenmode of optical fiber. While N is an integer number (N = 1,2, 3,….) and L represents the length of the NCF. The transmission spectrum of the UDT-SMF-NCF-SMF structure is very sensitive to the surrounding RI where the surrounding media acts as the NCF's cladding. The effective RI of the fundamental mode of the NCF increases as a result of the tapers and producing more evanescent fields, which further increases the evanescent field's penetration of the surrounding media. As a result, as the RI increase, the dip wavelength of the sensor shows redshifts [41,42].
2.2 Sensing structure fabrication
The fabrication process for the single-point RI sensor based on the UDT-SMF-NCF-SMF structure included three steps, as shown in Fig. 2. The sensing structure was fabricated by splicing a section of an NCF between two spherical-shaped segment ends of commercial telecommunication single-mode fibers (Corning SMF-28). The spherical-shaped end of SMF was fabricated by using a commercial fusion splicer (Fujikura FSM-60 s) with manual splicing mode. First, arc discharge was applied twice at one end of the lead-in SMF as illustrated in Fig. 2(a). The end of the lead-in SMF was softened and become a spherical shape and a fused up-taper was added to the SMF end as shown in Fig. 2(b). The microscopic image of the spherical-shaped SMF end is shown in Fig. 2(c), and the diameter of the sphere is about 162.135 µm. The discharge power and discharge time were set to 50 bits and 9000 ms respectively, by standard splicing mode after manual alignment. Second, NCF (FG125LA from Thorlabs) with length L was cleaved into two equal sections: NCF1 with length L1 and NCF with length L2. Then, one end of NCF1 was spliced to the spherical-shaped SMF by standard splicing mode after manual alignment as shown in Fig. 2(d) and Fig. 2(e). The following criteria have been established for the discharge process: the overlap is 10 m, the discharge strength is 25 bits, and the discharge length is 1200 ms. Then another up-taper of the lead-out SMF with the other end side of the NCF2 was fused. The microscope image of the fusion up taper SMF-SNF structure is shown in Fig. 2(f). Finally, the other end of NCF1 was positioned with NCF2, and a down taper between NCF1 and NCF2 was achieved by using the arc discharge method as shown in Fig. 2(g). The arc parameters are set up as follows: discharge intensity of 75 bits, and 5000 ms of taper discharge time. The manual fusion splicing process captured from the splicer machine is illustrated in Fig. 3.
Fig. 2. The fabrication process for the UTD structures (a) conventional SMF, (b) spherical-shaped SMF, (c) the microscope image of the spherical-shaped SMF, (d) schematic of the fusion up taper, (e) up-tapered SMF-NCF structure, (f) the microscope image of the fusion up taper, (g) schematic of the fusion down taper, (h) up-down tapered SMF-NCF-NCF structure, (i) the microscope image of the fusion down taper NCF.
Fig. 3. The manual fusion splicing process captured from the splicer machine (a) spherical-shaped SMF, (b) manual alignment of the spherical-shaped SMF with NCF, and (c) up-tapered SMF-NCF structure.
The length of the tapered area L4 is 413.933 µm with a small waist diameter of about 53.753 µm. The outer diameter of the up-tapered and tapered length L3 were about 159.95 µm and 288.013 µm, respectively. The fabricated UDT is illustrated in Fig2(h). Figure 2(i) displays the microscopic image of the fabricated UDT structure. In this work, first, two hetero-core fiber sensing configurations based on UDT-SMF-NCF-SMF structures were proposed, fabricated, and analyzed. a- UDT-SMF-NCF-SMF structure (so-called sensor A) with NCF length (L) of 45 mm long, b- UDT-SMF-NCF-SMF (so-called sensor B) structures with NCF length (L) of 60 mm. In this experiment, because the two up-tapers act as the beam splitter and the combiner, and the down-taper acts as an attenuator, various diameters of them cause different extinction ratios (ERs) of the interference fringe. Therefore, to investigate these characteristics, several UDT-shaped structures with different parameters were fabricated to detect and investigate the transmission spectra. In this work, in order to determine the optimum length of the NCF, many trial-and-error attempts were carried out to get an interference dip with a better extinction ratio (ER). The transmission spectrum of the proposed structure with different NCF lengths of 15 mm, 30 mm,45 mm, and 60 mm was examined as shown in Fig. 4. From this figure, a clear interference dip with a good ER of 24 dB and 22.4 dB were observed with NCF lengths of 45, and 60 mm respectively. This might be attributed to most of the light modes being coupled into the cladding modes as the NCF lengths were increased. Moreover, it is clear that the ERs are almost increased with the increasing of NCF length. Thus, structures with 45 mm and 60 mm due to the proper spectrum contrast were selected. Figure 5 illustrates the experimental setup used to characterize the transmission spectra of the proposed individual UDT-SMF-NCF-SMF structures. A broadband light source (BBS, Thorlabs: SLD1550SA1) with an operating wavelength from 1400 to 1600 nm was utilized to measure the light propagation characteristics of the proposed structures. The transmission spectra were detected using an optical spectrum analyzer (OSA, Yokogawa: AQ6370C). The transmission spectra of the individual sensor A and sensor B are shown in Fig. 6. It can be seen that dip A located at 1518.18 nm and dip B located at 1539.9 nm have the highest ER values and correspond to sensor A and sensor B structures. For the clarity of the discussion, the two dips that correspond to sensor A and sensor B were named (dip A and dip B) according to their spectral position.
Figure 6 shows the transmission spectra of the fiber structures with NCF length (45 and 60) mm for sensor A and B, respectively. In the experimental situation described for each sensor, dip A located at 1518.18 nm, and dip B located at 1539.9 nm corresponding to sensor A and sensor B may also represent the presence of destructive interference between the propagation modes. According to Eq. (5), the resonant dip wavelength can be controlled by changing the effective indices of the core mode and cladding mode, and by the length of the effective interference region resulting from the ambient environment which can induce the interference attenuation peak shift. In addition, in the tapered fiber, the phase-matching condition coupling can also be controlled by controlling the diameter of the taper.
In this work, the selected diameter of the sphere is of utmost importance. Changing the diameter of the sphere influences the length of the sensor structure and hence the transmission spectrum. Therefore, using a microsphere of unsuitable diameter results in a vastly different spectrum of measured signals. For making the suitable sphere, the spherical-shaped structure is fabricated by a commercial fusion splicer with manual splicing mode. The shape and size were controlled by the applied arc discharge, the discharge duration, and the discharge intensity. The microscopic image of the spherical structure with different sphere diameters is illustrated in Fig. 7. First, The SMF was mounted on the right holder of the splicer machine which is fixed on a stepper motor-controlled stage using V-groove clamps. Then, arc discharge was applied twice at one end of an SMF; the end of the SMF would soften and become a spherical-shaped structure. Second, the lead-out SMF is spliced to the spherical-shape structure by normal splicing after manual alignment. The distance and angle of the two SMF spheres are manually adjusted so that they stay on the same horizontal line. Completing the fabrication of cascaded UDT structure: Another SMF fiber sphere with the above splicing parameters was used and the splicing process was repeated.
Then, the wavelength tracking method was determined by searching the resonant dip wavelength with a higher extension ratio. Figure 8 shows the transmission spectrum of the fiber structure with a fixed NCF length and different sphere diameters. It can be found that the spectral noise is large for diameter values above and lower than 162.135 µm. Therefore, in this work, the sphere diameter of 162.135 µm was selected as the optimal diameter. In this work, the difference in the diameter of the spheres is mainly caused by the error of discharging intensity and manual alignment welding. However, the size and shape of the sensor head structure only have a little difference after manual splicing of ±5 µm therefore the fabrication process is repeatable.
Second, the dual point sensing structure was fabricated by series-connecting of the two fabricated sensors A and B to investigate the multiplexing of the wavelength’s dips generated by sensors A and B. The dual point sensing structure is shown in Fig. 9. The transmission spectrum of the dual point sensing structure is shown in Fig. 10. From this figure two spatial sensing points can be observed. Sensing point A corresponds to sensor A, and sensing point B corresponds to sensor B. Also, from this figure, it is obvious that the wavelength dips are matching those in the individual measurements illustrated previously in Fig. 6, i.e., at 1518.18, and 1539.9 nm for sensing point A and sensing point B respectively. The UDT structure induces an excess of losses into the sensor cavity. The values of the insertion losses of the proposed sensor structures are illustrated in Table 1.
Fig. 10. The transmission spectrum of the dual point sensing structure based on cascaded sensors A and B.
Table 1. Insertion loss of the proposed sensor structures
3. Experimental setup of the RI measurement
The schematic experimental setups with the individual and dual sensing points based on cascaded sensors configuration are shown in Fig. 11. A BBS is used as an input light source and an optical spectra analyzer (OSA) is utilized to monitor the transmission spectra. All fiber sensor structures are placed straight with the help of adhesive tape. With the fabricated UDT structures, we conduct a series of experiments for the RI solution measurements. The RI of solutions ranged from 1.33 to 1.38. The solutions were prepared by mixing various concentrations of sodium chloride (NaCl) into deionized water. The concentration of NaCl was varied (from 0% to 25% in steps of 5%) to evaluate the sensitivity of these sensing structures. The RI of the prepared solutions was measured by an Abbe refractometer. Five experiments were carried out with the fabricated sensor heads to investigate sensor performance against RI changes. The sensing heads were immersed into the RI solution which was injected by using a pipette. Before each measurement, the sensor head was cleaned with deionized water and dried in air.
4. Results and discussion
A. Single sensing point
Firstly, for an individual UDT-SMF-NCF-SMF hetero-core structure, the transmission spectra of each structure of NCF lengths 45 mm and 60 mm were evaluated separately. The experiment setup was described in Section 3, Fig. 11. During the measurement, all fiber structures were aligned along the same axis and kept straight. NaCl solutions of different concentrations in deionized water were used as samples with different RIs. Before each measurement, the sensor head was cleaned with deionized water and let to dry in the air. In this experiment, the sensor head is totally immersed in the solution. The measured transmission spectral response of individual sensors with NCF lengths of 45 mm and 60 mm when subjected to RI variation from 1.33 to 1.38 at a constant temperature of 25°C are shown in Fig. 12. It can be seen that the wavelengths of the dip A and dip B exhibit drift with the increasing of the surrounding solutions RIs.
Fig. 12. Spectral response of the proposed UDT-SMF-NCF-SMF based RI sensor for (a) Sensor A (b) Sensor B.
Figure 13 illustrates the wavelength of dip A and dip B as a function of RI increasing. From the linear fitting results, sensor A and sensor B exhibit an excellent linear response to external RI variation, and the RI sensitivities within the RI range were calculated to be 89.42 nm/RIU with an R2 of 0.997, and 90.57 nm/RIU with an R2 of 0.998 for sensor A and B respectively.
B. Cascaded dual sensing point
Next, both sensors (sensors A and B) were cascaded in series and the resultant spectrum has two wavelength dips as illustrated previously in Fig. 10. The dips are matching those in the individual sensing measurement, i.e., at 1518.18 and 1539.9 nm. The RI sensing was examined by immersing only one sensing point in the varying RI solution, whereas the other point was left in the air at all times.
First, point A under the test of RI changed from 1.33 to 1.38 while point B was in the air all the time, Fig. 14 shows the evolution of the transmission spectrum of dip A against RI variation. It is obvious that dip A shows a clear shift with the increases in RI. And, it is clearly demonstrated that dip B is immune to external RI variation, which means the RI sensitivity at dip B can be expressed as 0. For this case, the linear fitting results show that dip A exhibits an excellent linear response to external RI changes, and sensing point A exhibits maximum sensitivity of 89.71 nm/RIU with R2 of 0.996 as illustrated in Fig. 15.
Fig. 14. The evolution of the transmission spectrum of the dual point sensor when only sensing point A was immersed in varying RI solutions.
In between consecutive measurements, the sensing point was cleaned with deionized water and dried in the air before dropping the solutions with different RIs. Similarly, Fig. 16 demonstrates the spectrum evolution when only sensing point B is immersed in the RI solutions. Dip B resulting from sensor B shows a clear shift as RI increases, whereas dip A resulting from sensing point A did not show unnoticeable shifts with RI, which means the RI sensitivity at dip A can be expressed as 0. Figure 17 illustrates the wavelength of dip B as a function of RI increasing respectively. From this figure, the calculated sensitivity of the sensing point is 119.7 nm/RIU with an R2 of 0.986. From Figs. 14, and 16 it was proven that the two sensing points have no influence on each other during RI measurements.
Fig. 16. The evolution of the transmission spectrum of the dual point sensor when only sensing point B was immersed in varying RI solutions.
Finally, RI variation measurements in cascaded UDT hetero-core structure (i.e., cascaded sensing points A and B) were performed simultaneously by placing the dual sensing point A and point B in the same solution of NaCl with the RI ranging from 1.33 to 1.38. Figure 18 shows the transmission spectra with two dips in response to RI variation. From this figure, it is demonstrated that the two dips of sensing point A and point B simultaneously shift to longer wavelengths as the RI increases. Figure 19 shows that sensing point A and sensing point B are sensitive to RI simultaneously, and exhibit sensitivities of 99 nm/RIU and 93.7 nm/RIU for simultaneous measurement of RI ranging from 1.33 to 1.38 respectively. The obtained results indicate that there is no interference between sensing point A and sensing point B, and both points work independently.
Fig. 18. The evolution of the transmission spectrum of the dual point sensor when sensing point A and sensing point B were immersed simultaneously in varying RI solutions.
In comparison with the previously published multi-point RI fiber sensor, the proposed sensor is competitive with the other sensors in terms of linearity and measurement points as illustrated in Table 2. It is worth noting that the sensitivity of the proposed dual-point sensor could be further enhanced by reducing the taper NCF diameter and increasing the RI interaction length.
Table 2. Performance of various multi-point fiber optic RI sensors
5. Conclusions
In this work, a new optical fiber dual-point sensor based on cascaded two UDT-hetero core fiber structures has been demonstrated experimentally and proven for dual-point RI measurements. The hetero-core fiber structures were fabricated by splicing a section of NCF between two SMFs. The UDT hetero-core fiber structure was formed by introducing a down taper between two adjacent up tapers in the NCF section by using the over-fusion splicing method that involves simple cleaving and manual fusion splicing processes. The NCF lengths of 45 and 60 mm were carefully to have a distinct and nonoverlapping dip wavelength in order to achieve wavelength division multiplexing. The dip wavelengths of the UDT-hetero core fiber structures-based sensor A and sensor B were located at two different spectral positions, and they exhibit independent RI responses, making multiplexed RI sensing possible for simultaneous measurement of RI. Experimental results of the dual point cascaded structures show that sensing point A and sensing point B are sensitive to RI with maximum RI sensitivities of 99 nm/RIU and 93.7 nm/RIU for simultaneous measurement in the range of 1.33 to 1.38 for each point respectively. The obtained results validate the use of the proposed dual point sensor for the simultaneous measuring of RI at two different spatial locations. At the same time, it has the advantages of a compact structure, low cost, and dual-parameter measurement capability.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper may be obtained from the authors upon reasonable request.
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