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Abstract
How to couple the light in the fiber core to the cladding is an urgent issue that need to be done for the fabrication of the fiber-cladding SPR sensor, and there is no report about the fiber SPR strain sensor. Hereby, we propose and demonstrate a high sensitivity fiber cladding SPR strain sensor based on V-groove structure. By CO2 laser, the V-groove is fabricated on the single-mode fiber, and the light in the fiber core is effectively coupled to the cladding. The cladding 2cm behind the V-groove is coated with sensing gold film, and a multimode fiber is spliced with the sensing probe to construct the novel fiber cladding SPR sensor. On the basis of the investigation of the effects of different V-groove depth, number and period on the performance of fiber SPR refractive index sensor, a high sensitivity strain SPR sensor is designed and fabricated by employing the characteristic that the V-groove will deform with strain. The testing results indicate that the average refractive index sensitivity of the sensor is 2896.4nm/RIU, and the strain wavelength sensitivity is 25.92pm/µε which is much higher than that of the fiber interference and grating strain sensors, and the strain light intensity sensitivity is -4.4×10−4 a.u./µε. The proposed fiber cladding SPR strain sensor has the advantages of simple structure and convenient manufacture, and can be used for working in a narrow space.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Surface plasmon resonance (SPR) has become a research hotspot in recent years because of its advantages of high sensitivity, label-free and real-time measurement. In principle, the formation condition of SPR is that the evanescent wave needs to enter into the metal film, that is the total reflected light needs to contact the metal film directly. For optical fiber, transmission light will totally reflect at the interface between the fiber core and cladding, the light will be bound in the fiber core, and there is no transmission light in the fiber cladding. When design and construct the structure of fiber SPR sensor, the major problem to be solved is how to make the transmission light contact the metal film during total reflection. There are two methods to solve this problem, one is to remove the fiber cladding and coat metal film outside the fiber core. And another is to make the light transmitted in the fiber core leak into fiber cladding, and the sensing metal film is coated outside the cladding directly. Based on this, the fiber SPR sensor can be divided into two types according to the sensing substrate, fiber core SPR sensor and fiber cladding SPR sensor.
Fiber core SPR sensor needs to remove the fiber cladding to make the evanescent field contact the metal film, the most widely used methods are corrosion [1,2], optical fiber side polishing [3–5] and optical fiber grinding [6–9], etc. However, these processing methods have problems of machining difficulty, deterioration of fiber mechanical strength and poor reproducibility, etc. Fiber cladding SPR sensor needs to couple the transmission light in the fiber core to the cladding, so that to make the evanescent field contact the metal film outside the fiber cladding. Its research results are relatively few. At present, the sensing structures include tapered structure [10,11], heterocore structure [12–15] and U-shaped structure [16]. However, the fiber SPR sensor with tapered structure is easy to break and has poor reusability. Repeated fabrication of U-shaped structure is difficult, and it also has bending loss which will affect the measurement accuracy of the sensor. Aiming at the problem that there are few types of fiber cladding SPR sensor, the major problem to be settled is how to effectively couple the light in the fiber core to the cladding on the basis of maintaining the mechanical strength of the optical fiber.
In the field of fiber grating, many scholars employ CO2 lasers to form periodic V-grooves on the surface of fiber to fabricate long-period fiber gratings (LPFG) [17–21]. When the core and cladding modes in the fiber meet the phase matching conditions determined by the grating period, they can realize energy exchange, that is the coupling is realized. By employing fiber V-grooves, the high-order cladding mode is effectively excited. Therefore, we employ the fiber V-grooves to construct the fiber cladding SPR sensor. At present, there are few reports about the SPR sensor based on fiber V-grooves structure. Tsujiuchi of Japan proposed V-type silicon prism array SPR sensor in 2006 and tested three samples of air, water and ethanol, but the prism SPR has a large volume [22]. In 2013, Ken et al. fabricated surface plasma V-shaped fluid channel to realize the function of prism SPR sensor. The system has no prism, no noise and requires few samples [23]. In 2019, Liu et al. proposed a V-shaped microstructure optical fiber (MOF) SPR sensor for simultaneous measurement of refractive index and temperature [24]. The sensor employed the grinding and welding of the eccentric core fiber to fabricate V-shaped MOF, one side of the MOF is used for refractive index sensing and the other side is used for temperature sensing. It has high sensitivity and temperature compensation. However, grinding is difficult and special optical fiber is required. In 2020, Omri proposed a high-sensitivity SPR biosensor [25]. The sensor is composed of gold nanowires placed below the V-channel area, which can form high-sensitivity SPR biosensors with single V-channel and double V-channel. The sensitivity of double V-channel is up to 15384nm / RIU, but the structural is complex and the processing is difficult. Lo et al. employed the phase difference between P wave and S wave of light wave to test the change of fiber refractive index caused by strain applied to fiber [26].
At present, there are many reports about the strain measurement by employing fiber interference sensors [27–33] and grating sensors [34–40]. But the research on the application of highly sensitive fiber SPR technology to strain sensing has not been reported. Based on this, we firstly propose the construction of the fiber cladding SPR sensor by using fiber V-groove processing technology. And by the characteristic that strain will deform V-groove, the high sensitivity strain SPR sensor is designed and fabricated. Its principle is that: the light is propagating in the core of the single-mode fiber, when the transmitting light reaches the V-groove, the cladding modes are excited. The metal sensing film is coated outside the cladding to constitute SPR sensing probe. When the physical morphology and effective refractive index of the V-groove change under strain, the SPR incident angle of the sensing zone changes. And then the strain can be measured by the shift of SPR resonance valley, and the high sensitivity strain sensing with a wavelength sensitivity of 25.92pm/µε is realized.
2. Sensing principle and probe fabrication
2.1 Sensing principle
When a V-groove is fabricated on the fiber, the effective refractive index and light field distribution of the fiber will change. When light is injected into the core of V-groove fiber, the cladding mode is excited. By the software of Rsoft [41], we study the transmitted light field of single-mode fiber with V-groove structure theoretically. The diameter of the fiber core and cladding is 9µm and 125µm, respectively. The refractive index of the core and cladding is set to be 1.465 and 1.455, respectively. As shown in Fig. 1(a), when there is a V-groove in the single-mode fiber, the fiber effective index near the V-groove structure will change, the cladding mode is excited when the light in the core passes through the V-groove structure. Figure 1(b) shows the photograph of the side light field of the single-mode fiber with a V-groove depth of 38µm, the V-groove structure couples the transmission light in the core to the cladding, and the simulation result tallies well with the test result. Figure 1(c) shows end light field of the fiber with a V-groove structure, and the illustration is the photograph of end face light filed distribution in the experiment. And Fig. 1(d) shows the output optical field distribution of the fiber with a V-groove. According to Fig. 1, the cladding mode will be excited when the light in the core passes through the V-groove structure.
Fig. 1. Sketch diagram of single-mode fiber with a V-groove structure, (a) simulation results of side light field distribution, (b) experiment photograph of (a), (c) end face light field distribution, (d) output optical field distribution.
Based on the above analysis, we design a novel high sensitivity cladding SPR strain sensor based on fiber V-groove. By CO2 laser, the V-groove was fabricated on the single-mode fiber. A 50nm gold film was coated outside the cladding with the position about 2cm after the V-groove, and a multimode fiber was spliced with the single-mode fiber to receive the cladding light of the sensing probe. As shown in Fig. 2, The sensing probe contains four parts: (a) single-mode fiber, (b) V-groove, (c) sensing zone, (d) multimode fiber, and Fig. 2(e) shows the side photograph of the V-groove. In principle, the cladding mode was excited when the transmitting light passing through the V-groove, the light in the cladding contacted the gold film coated outside the cladding because of the total reflection. When the phase matching condition is satisfied, the evanescent waves will resonate with the surface plasmon waves, which leads to the resonance energy transferring, and the SPR resonance valley will be observed in the exiting spectrums of sensing probe. The working range of the SPR resonance valley changes with the external refractive index, so we can employ the offset of the SPR resonance valley to realize the measurement of the external refractive index. By a multimode fiber with the diameter of 105µm, the output light of the sensing probe was received and sent to optical spectrum analyser and computer for offspring data processing.
Fig. 2. Sketch diagram of sensing probe, radial section views of (a)single-mode fiber, (b) V-groove, (c) sensing zone, (d) multimode fiber. (e) Side photograph of the V-groove.
2.2 Sensing probe fabrication
The V-groove and sensing zone are fabricated by single-mode fiber, the light-receiving fiber is fabricated by step index multimode fiber, the manufacture procedures are as follows:
- (1) As shown in Fig. 3(a), the coating layer of the middle of the single-mode fiber was peeled off mechanically by fiber stripper, and the fiber was then cleaned by alcohol.
- (2) As shown in Fig. 3(b). The bare fiber part was placed on the three-dimensional micro motion table under the CO2 laser(MC-E-B, Yueming Laser). One end of the bare fiber was fixed with a fiber clamp, and the other end of the fiber was suspended with a light weight, so that the fiber maintained constant axial stress during heating and is always straight. we adjusted the micro stage so that the focal spot of the bare fiber coincided with that of the laser beam. The number and period of V-grooves were designed by computer. By CO2 laser, the V-grooves were fabricated, the processing speed is 800mm/s, the power is 50% and the frequency is 5KHz. The output laser from CO2 laser is focused by cylindrical mirror and illuminated on the surface of the fiber. The fiber was deformed by heating to form V-groove. By controlling processing times, the depth of the V-groove was changed.
- (3) In Fig. 3(c), The fiber with V-groove was taken down from the micro stage, the fiber was cut at the position of 2cm after the V-groove by a fiber fixed-length cutter.
- (4) As shown in Fig. 3(d).The single-mode fiber after cutting and the step index multimode fiber with the diameter of 105µm and numerical aperture of 0.22 were put into the fiber fusion splicer (NT-200H, Notevio) to weld coaxially, so that the cladding light of the single-mode fiber was received by the multimode fiber.
- (5) The spliced fiber was taken down from the fiber fusion splicer, we covered the V-groove zone by tape. The fiber was fixed by the clamps of the magnetron sputtering coater (ETD-650MS, YLBT), a 50nm gold film was rotatably coated outside the cladding 2cm behind the V-groove. Thus far, the fabrication of the sensing probe was finished as shown in Fig. 3(e).
Fig. 3. Sketch diagram of sensing probe fabrication, (a)removing the coating layer (b) putting the fiber and fabricating the V-groove, (c) cutting to length, (d) splicing the fibers. (e) coating the gold film.
The experimental setup of the cladding SPR sensor based on fiber V-groove is shown in Fig. 4. The sensing probe was sealed in the liquid flow-through cell. The solution to be measured was injected into the flow-through cell by a syringe, and the waste liquid was discharged into a waste reservoir. The light from the light source (HL-2000, Ocean Optics) was launched into the left side of the sensing probe, the SPR effect was excited in the sensing zone, the transmitting light was received by the multimode fiber and sent to optical spectrum analyser (USB2000+, Ocean Optic) and computer for offspring data processing.
3. Effects of V-groove structure parameters on the coupling condition of cladding light and sensing performance of the SPR sensor
The depth, number and period of the V-groove on the fiber have indispensable impact on the coupling efficiency from fiber core mode to cladding mode and the total reflection angle of cladding mode. The power ratio of cladding mode increases, the proportion of light that can excite SPR effect increases with it, and the SPR resonance valley will be deeper and easier to identify. In addition, different cladding modes correspond to different total internal reflection angles (SPR incidence angles), which will affect the SPR sensing performance. In order to obtain the SPR sensor with better sensitivity and easily identifiable SPR resonance valley, we sequentially studied the effect of depth, number and period of the V-groove on the coupling condition of cladding light and sensing performance of the SPR sensor, and the optimized fabrication parameters of the cladding SPR sensor based on fiber V-groove were obtained.
3.1 Effect of V-groove depth on the coupling condition of cladding light and sensing performance of the SPR sensor
The processing speed, processing power and frequency of CO2 laser kept constant, by controlling the processing times, the depth of the V-groove was changed. A V-groove was fabricated on four same single-mode fiber, the depth of the V-groove was 38µm, 45µm, 56µm and 75µm, respectively. A light source with the wavelength of 532nm was employed to inject light for the single-mode fiber. The end light field of the fiber was observed by microscope, and the output optical field was observed by dropping rhodamine solution. As shown in Fig. 5, the light in the fiber core was coupled and leaked into the cladding, with the increase of the V-groove depth, the proportion of energy in the cladding increases. However, when the V-groove depth reaches 75µm, as the V-groove depth penetrates the core, most of the energy in the core leaks into the air, resulting in a significant reduction in the energy of the core and cladding. By the single-mode fibers with V-groove with different depth, we fabricated four different cladding SPR sensor, and performed the index testing, respectively. The range of the refractive index to be measured is 1.333-1.385RIU, and the testing results are shown in Fig. 6.
Fig. 5. Photograph of fiber side, end filed, output optical field of the fiber with V-groove depth of (a) 38µm, (b) 45µm, (c) 56µm, (d) 75µm.
Fig. 6. Testing results of the sensing probe with V-groove depth of (a) 38µm, (b) 45µm, (c) 56µm, (d) 75µm. (e)Relationship curves of resonance wavelength and refractive index.
As shown in Fig. 6(a-d), the lager the refractive index of the solution is, the larger the SPR resonance wavelength is, which demonstrates that the SPR resonance wavelength of the cladding fiber SPR sensor based on fiber V-groove can realize the sensing measurement of the refractive index of the solution. According to Fig. 6(e), with the increase of the V-groove depth, the sensing sensitivity increases first and then decreases, and when the depth is too deeper, the sensing probe is easy to break. Beyond that, when the depth is too deeper, the resonance valley is broadened which will influence the detection accuracy. Together, the sensing probe with the fiber V-groove depth of 45µm has a higher sensitivity and good mechanical strength, hence we select 45µm as the best V-groove depth parameter.
3.2 Effect of V-groove number on the coupling condition of cladding light and sensing performance of the SPR sensor
Similarly, we studied the effect of V-groove number on the coupling condition of cladding light and sensing performance of the SPR sensor. The depth of the V-groove was kept at 45µm, the period was 571µm, three fiber probes with V-groove number of 1, 10 and 40 were fabricated, respectively. The photographs and optical field distribution of the sensing probes are shown in Fig. 7 (when the V-groove number is 10 and 40, the sensing zone is too long to take photograph by microscope, we only show a single V-groove). In which, with the increase of the V-groove number, the total light intensity maintained in the fiber decreases gradually, but the proportion of the cladding light increases. We employed the fibers with different V-groove number to fabricate three different sensing probes, and performed the index testing, the detection range of the refractive index is 1.333-1.385RIU as shown in Fig. 8.
Fig. 7. Photograph of fiber side, end filed, output optical field of the fiber with V-groove number of (a) 1, (b) 10, (c) 40.
Fig. 8. Testing results of the sensing probe with V-groove number of (a) 1, (b) 10, (c) 40. (d) Relationship curves of resonance wavelength and refractive index.
According to Fig. 8(d), with the increase of the V-groove number, the average sensitivity of the sensing probe increases. As shown in Fig. 8(a-c), when the V-groove number is 40, the resonance valley of the sensing probe is widened and the SPR curve is seriously deteriorated. And the sensing probe with the V-groove number of 10 has the best full width half maximum(FWHM). Therefore, we selected the sensing probe with the V-groove depth of 45µm and V-groove number of 10 to conduct the follow-up study.
3.3 Effect of V-groove period on the coupling condition of cladding light and sensing performance of the SPR sensor
Four different fibers with V-groove depth of 45µm, V-groove number of 10 and V-groove period of 335µm, 571µm, 797µm and 1094µm were fabricated, respectively. The photographs and optical field distribution of the sensing probes are shown in Fig. 9. In which the light energy in the fiber cladding increases with the period of the V-groove. The fibers with different V-groove period were employed to fabricate the sensing probes, and by the four different sensing probes, the refractive index of the solution to be measured was detected. The detection range of the refractive index is 1.333-1.385RIU, the testing results are shown in Fig. 10.
Fig. 9. Photograph of fiber side, end filed, output optical field of the fiber with V-groove period of (a) 335µm, (b) 571µm, (c) 797µm, (d) 1094µm.
Fig. 10. Testing results of the sensing probe with V-groove period of (a) 335µm, (b) 571µm, (c) 797µm, (d) 1094µm. (e) Relationship curves of resonance wavelength and refractive index.
As shown in Fig. 10, with the increase of the V-groove period, the sensitivity increases first, and then decreases, the FWHM also decreases. The optimal V-groove period is 797µm which has the highest sensitivity and a better FWHM, and the sensitivity reaches 2896.4nm/RIU.
Based on the above testing results and analysis, we got the following conclusions:
- (1) The deeper of the V-groove is, the larger the wavelength sensitivity of the sensing probe is. However, when the depth of the V-groove is too large, the stability and mechanical strength of the sensor will be affected, so the depth selection range of V-groove is 38-45µm.
- (2) The more the number of V-groove, the wavelength sensitivity of the sensing probe is larger. But when the number is too many, the SPR curves deteriorate, the resonance valleys was widened, and the sensor is easy to break. Therefore, the selection range of the number of V-groove is 1-30.
- (3) The larger the period of the V-groove is, the larger the wavelength sensitivity of the sensing probe is. However, when the period is too large, the sensitivity decreases, and part of the SPR curves deteriorates. Therefore, the selection range of V-groove period is 500-800µm.
4. Research on the properties of strain sensing by the cladding SPR sensor based on fiber V-groove
Strain is a parameter that cannot be ignored in the fields of structural health monitoring, aircraft structural inspection and environmental monitoring. By Rsoft, we simulated the transmission light field and power distribution of the fiber with V-groove. The Fig. 11(a) shows the optical field distribution of the complete single-mode fiber, Fig. 11(b) shows that of the single-mode fiber with a V-groove, and Fig. 11(c) shows that of fiber with a V-groove which is deformed by axial strain. The Fig. 11(e-g) show the fiber core monitoring power, corresponding to Fig. 11(a-c), respectively. The physical construction of the complete single-mode fiber basically does not change with the axial strain, the fiber keeps the transmission for fundamental mode in the core, and the fiber core energy does not decay. When the V-groove is fabricated on the single-mode fiber, the cladding mode is excited and the energy in the core is attenuated as shown in Fig. 11(b) and (f). When the fiber with a V-groove is subjected to axial strain, the shape of the V-groove changes and the angle of the V-groove increases from ${\theta _1}$ to ${\theta _2}$, the optical mode coupled to the cladding changes and more fiber core energy leaks as shown in Fig. 11(c) and (g). In a word, when the fiber V-groove structure is subjected to axial strain, the shape of the V-groove will change, the optical mode and light intensity of the fiber core coupled to the cladding will change, resulting in the change of SPR incident angle and SPR intensity. Therefore, we can employ this cladding SPR sensor based on the fiber V-groove to detect the axial strain.
Fig. 11. Simulation results of optical field distribution and power monitoring of (a) and (e) complete single-mode fiber, (b) and (f) single-mode fiber with a V-groove, (c) and (g) single-mode fiber with a V-groove deformed by axial strain.
Figure 12 shows the strain testing platform. We employed the optimized parameters to fabricate the cladding SPR sensor based on V-groove, the depth of the V-groove was 45µm, the number the V-groove was 10, and the period length of the V-groove was 797µm. The left side of the sensing probe was connected with the light source and placed in the left fixture of the testing platform, and the right side of the sensing probe was placed in the right fixture and connected with the spectrometer. The sensing probe was soaked in the solution with the refractive index of 1.375RIU. The left and right fixture were fixed on the motors. By controlling the motors, the left and right fixture were driven to move in the three-dimensional space, keeping the left fixture stationary, and keeping the right fixture stationary in the Y-axis and Z-axis directions. By controlling the right motor, the right fixture was quantitatively moved away from the left fixture in the X-axis, so that the fiber is subject to axial micro strain, the testing results are shown in Fig. 13.
Fig. 13. Testing results of strain detection, (a) resonance curves of wavelength shift, (b) resonance curves of light intensity variation, (c) the relationship curves of the resonance wavelength and strain, resonance light intensity and strain.
According to Fig. 13(a), with the increase of the strain, the SPR resonance wavelength moves from 734.73nm to 750.05nm, a total of 15.32nm. As shown in Fig. 13(b), the resonance light intensity increases from -0.18a.u. to -0.45a.u. with the strain. In Fig. 13(c), the blue curve represents the fitting curve of the relationship between resonance wavelength and strain, and the black curve represents the fitting curve of the relationship between the depth pf the resonance valley and strain. The wavelength sensitivity and light intensity sensitivity are 25.92pm/µµ and -4.4×10−4a.u./µε, respectively.
In Table 1, we contrasted the strain detection sensitivity and detection range of the cladding SPR sensor based on V-groove, fiber optic interferometric sensor and grating sensor. In which, the strain wavelength sensitivity of the cladding SPR sensor based on V-groove is 13 times higher than that of the fiber optic interferometric sensor, and 21 times higher than that of the grating sensor. Therefore, the proposed SPR sensor has the advantage of high strain sensitivity. Besides that, the research on the detection of the strain physical quantity by SPR technology has not been reported, the research in this paper will open a new direction of multi physical quantity measurement in the field of SPR.
Table 1. Strain detection sensitivity and range comparison results of different fiber sensors
5. Conclusion
In this paper, a novel cladding SPR sensor based on fiber V-groove was proposed and demonstrated. By CO2 laser, the V-groove was fabricated on the single-mode fiber, which solved the problem that the light in the fiber core was difficult to couple to fiber cladding. The sensing gold film was coated outside the, and a multimode fiber was spliced with the single-mode fiber to receive the output light of the sensing probe, the novel fiber cladding SPR sensor had been realized so far. Based on the research on the effects of different the V-groove depth, V-groove number and V-groove period length on the coupling condition of the cladding light and performance of fiber SPR refractive index sensor, the optimized parameters were obtained. The optimized sensor with the V-groove depth of 45µm, V-groove number of 10 and V-groove period length of 797µm has an index sensitivity of 2896.4nm/RIU with the detection range of 1.333-1.385RIU. When the fiber V-groove structure is subjected to axial strain, the shape of the V-groove will change, resulting in the change of the optical mode and light intensity coupled to the cladding. Therefore, the proposed cladding SPR sensor based on V-groove can realize the axial strain detection. The wavelength sensitivity of the strain detection is 25.92pm/µε which is 13 times or more higher than that of the traditional interference fiber sensor. The light intensity sensitivity of the strain detection is -4.4×10−4a.u./µε. The proposed cladding SPR sensor has the advantages of simple structure and convenient manufacture, and can be used for working in a narrow space
Funding
Fundamental Research Funds for Chongqing Three Gorges University of China (19ZDPY08); Chongqing Key Laboratory of Geological Environment Monitoring and Disaster Early-Warning in Three Gorges Reservoir Area (ZD2020A0102, ZD2020A0103); Science and Technology Project Affiliated to the Education Department of Chongqing Municipality (KJ1710247, KJQN201801217, KJQN201901226); Chongqing Natural Science Foundation (cstc2018jcyjAX0817, cstc2019jcyj-msxmX0431); National Natural Science Foundation of China (61705025).
Disclosures
The authors declare no conflict of interest.
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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