Cascaded plastic optical fiber based SPR sensor for simultaneous measurement of refractive index and temperature

A cascaded side-polish plastic optical fiber (POF) and FONTEX optical fiber based surface plasmon resonance (SPR) sensor is proposed for simultaneous measurement of refractive index (RI) and temperature. The side-polish POF and FONTEX optical fiber are connected by using the UV glue in a Teflon plastic tube. The SPR phenomenon can be excited at both of the side-polish region and the FONTEX fiber cladding. The polydimethylsiloxane (PDMS) is coated on the side-polish POF to get a temperature sensing channel. Due to the low RI sensitivity of the FONTEX optical fiber, the cascaded fiber sensor can obtain a broader RI measurement range with a low crosstalk. An RI sensitivity of 700 nm/RIU in the RI measurement range of 1.335-1.39 and a temperature sensitivity of −1.02 nm/°C measured in deionized water with a range of 20-60 °C are obtained. In addition, the cascaded POF based SPR sensor has potential application prospects in the field of biochemical sensing.


Introduction
Surface plasmon resonance (SPR) is a physical phenomenon formed by the oscillating charge on the metal surface and the light wave resonance acting on the metal surface, which is very sensitive to the slight change of the refractive index (RI) at the interface between the metal and the medium [1].Because of its high sensitivity characteristic [2][3][4], SPR sensors are often used in the field of biochemical sensing.Compared with the traditional prism-based SPR sensors, the fiber-based SPR sensors have many advantages, such as small size, low cost, high integration and the ability to achieve remote sensing [5].So far, different kinds of optical fibers including the multicore fiber, single mode fiber, hollow core fiber etc. have been used to fabricate the SPR sensors [6][7][8][9][10].However, most of the reported fiber-optic SPR sensors are based on glass fibers (GOFs), the GOF is very fragile after processing, which limits its application in the field of biochemical sensing.
Plastic optical fiber (POF) is a kind of optical fiber composed of polymer.Compared with GOF, POF has the advantages of large diameter, good flexibility, easy processing, and good biocompatibility, which is more suitable for the application in vivo biochemical detection.In recent years, POF-based SPR sensors have received increasingly attention, and different types of POF-based SPR sensors have been reported using for variety of physical, chemical, and biomass measurements [11][12][13][14][15].For biochemical detection, besides the RI, temperature is also a very important parameter, since the measured RI is usually disturbed by temperature changes, therefore, it is necessary to compensate the temperature influence when measuring the RI.In recent years, many fiber-based SPR sensors with temperature compensation function have been proposed [16][17][18].For POF-based SPR sensors, it is an effective method to obtain temperature compensation function by structural modification of POF and introducing the temperature sensitive materials.For example, Teng et al. fabricated two symmetrical SPR sensing regions by parallel polishing the POF and obtained the temperature sensing channel by covering one of them with polydimethylsiloxane (PDMS) [19].They also polished two sides of the U-shaped POF and obtained two adjacent RI and temperature sensing channels [20].However, due to the large diameter of the POF, the width of the resonance peak is often wide.Especially, the U-shaped structure can induce the macrobending loss, which could further enlarge the resonance peak width and distort the spectrum.The enlarged peak width could also reduce the space between the two resonance peaks wavelength, increasing the wavelength crosstalk and limiting the RI measurement range.In order to narrow the resonance peak and decrease the crosstalk, a V-groove POF [21] and a step-side polished POF with differentiated Au film thickness [22] were designed as the SPR sensors, respectively.And recently, a dual-channel POF-based SPR sensor was proposed by using an inkjet 3D printing waveguides and photocurable resin technology [23], however, the manufacturing process is complicated and difficult to operate.
In this paper, we propose a POF-based dual-parameter SPR sensor with a cascaded sidepolished step index POF and FONTEX optical fiber.The side-polished step index POF and FONTEX optical fiber were inserted into a Teflon plastic tube and connected by using the UV glue.The SPR can be excited at both of the side-polish region and the FONTEX optical fiber cladding after coating a layer of the Au film on their surfaces.In order to compensate the temperature variations and enlarge the wavelength space between the RI and temperature sensing channels, the PDMS was coated on the side-polish POF.The structure of the side-polish POF and the length of the FONTEX optical fiber were optimized for fabricating the dual-parameters SPR sensor.The results show that by combining the two structures, a wider RI measurement range, as well as a low crosstalk between the RI and temperature resonance peaks can be obtained.An RI sensitivity of 700 nm/RIU in the RI measurement range of 1.335 to 1.39, and a temperature sensitivity of -1.02 nm/°C measured in deionized water with range of 20 to 60 °C were obtained.In addition, this structure is helpful for the development of SPR sensors for multi-parameter detection and has potential applications in the field of biochemical sensing.

Fabrication and principle
The schematic diagram of the fiber optic sensing probe is shown in Fig. 1.The FONTEX optical fiber (AGC) and step index POF (ESKA, CK-20) were cascaded to fabricate the probe.The core material of FONTEX optical fiber is CYTOP (AGC's unique fluorine-containing material) with a diameter of 50 µm and graded RI, and the cladding material is a perfluorinated polymer with a thickness of 225 µm.The core material of POF is polymethyl methacrylate (PMMA) with a diameter of 490 µm, n co = 1.49, and the cladding material is a fluorinated polymer with a thickness of 5 µm, n cl = 1.41.The two ends of the FONTEX optical fiber are connected with the step index POF through a Teflon tube by using the UV glue in the tube.One of the step index POFs is side polished, and a layer of 50 nm thick Au film is deposited on the upper and lower surfaces of the FONTEX optical fiber and the surface of the side polished structure of the step index POF as the sensing area to excite the SPR phenomenon.The PDMS is coated on the side polishing area as the temperature sensing channel, and the FONTEX optical fiber with Au film is used as the RI sensing channel.
A wheel polishing process was performed on the step index POF, and the polishing length and depth can be controlled by this process [19].After the wheel polishing process, the polished surface should be finely polished with the alumina polishing paste to get a smooth surface.After that, an Au film with a thickness of 50 nm was sputtered on the side polishing area by magnetron sputtering equipment.A Teflon plastic tube with a length of 5 mm and inner diameter of 660 µm was used to connect the step index POF and FONTEX fiber.For this process, the Teflon tube is filled with UV glue by using the syringe firstly, then insert the FONTEX optical fiber and step index POF into about half length of the Teflon tube from its two ends, respectively.After irradiating the Teflon tube for about 5 minutes by the ultraviolet light, the UV glue is cured and the FONTEX fiber and step index POF are connected.After a similar step, the two ends of the FONTEX fiber can be connected to the step index POFs.Finally, the upper and lower surfaces of the FONTEX optical fiber section are sputtered with Au film with a thickness of 50 nm.
When the incident light is coupled from the left POF to the FONTEX optical fiber, parts of the light will go into the FONTEX fiber cladding due to the mismatched fiber core, and some of them will be totally reflected at the interface between the fiber cladding and the Au film.Since the thickness of the Au film is less than the penetration depth of the evanescent wave, an evanescent wave is generated at the boundary between the surface of the Au film and the external medium.The evanescent wave vector can be expressed as: where ω is the angular frequency of the incident light, θ is the incident angle, ε 0 is the dielectric constant of the cladding, and c is the speed of light.At the effect of evanescent wave, the plasma oscillation can be generated and confined to the surface of the Au film, which is called surface plasma wave.The wave vector of the surface plasma wave can be expressed as: where ε 1 is the dielectric constant of the Au film and ε 2 is the dielectric constant of the external medium.
When K ew = K spw , the SPR effect is excited, and the SPR resonance peak appears at a specific wavelength in the spectrum.When the RI of the surrounding environment changes, the position of the resonance peak shifts, so the RI can be detected by observing the movement of the resonance peak.Similarly, when the light is coupled from the FONTEX fiber to the step index POF, the evanescent wave generated can also stimulate the surface plasmon wave at the interface between the fiber core and the Au film.Since PDMS has a high RI and a negative thermo-optic coefficient (TOC), another SPR resonance peak will appear, and as the temperature changes, the position of the resonance peak will be changed accordingly.

Results and discussion
Figure 2 shows the schematic diagram of the experimental setup.The fabricated probe is fixed on the heating stage.One end of the probe is connected to the halogen lamp as the incident light source (ideaoptics HL2000, wavelength range 360-2500 nm), and the other end is connected to the spectrometer for monitoring the transmission spectrum (Shanghai Simtrum Technology Co., Ltd, China, wavelength detection range 300-1100 nm).The sample solution is a glycerol-water mixed solution with an RI step of 0.01, and the RI of the sample solution is calibrated by an Abbe refractometer.

Study on the optimal FONTEX fiber length
Figure 3(a)-(c) shows the transmission spectra of the probes when the length of the sensing area of the splicing structure is 10,15 and 20 mm, respectively.It can be seen that as the sensing length of the probe increases, the depth of the resonance peak increases, which is due to the increased number of the excited SPR. Figure 3(d) shows the resonance wavelength as a function of the sample RI, it can be seen that the sensitivity for the probe with sensing length of 15 mm is the highest.Figure 3(e) shows the transmission spectrum in deionized water, and it can be seen that the probe with a sensing length of 10 mm has the smallest full width at half maximum (FWHM).It can be also seen that both of the FWHM and depth of the resonance peak will increase with the increase of the sensing length.The probe with a sensing length of 10 mm is selected to prepare a dual-parameter probe considering its comparable sensitivity and the better FWHM. Figure 3 (f) shows the photographs of the probes before and after coating Au film.

Study on the optimal side polishing depth
Figure 4(a)-(c) show the transmission spectra of the probes when the side polishing length is 20 mm and the side polishing depth are 100, 150 and 200 µm, respectively.It can be seen that with the increase of the side polishing depth, the resonance peak depth increases.Figure 4(d) shows the resonance wavelength as a function of the sample RI, it can be seen that the sensitivities of the probes are almost the same.Figure 4(e) shows the transmission spectrum in deionized water, and it can be seen that the probe with a side polishing depth of 150 µm has the smallest FWHM.Considering the better FWHM, the probe with a side polishing depth of 150 µm was selected to prepare a two-parameter probe.Figure 4(f) shows the photographs of the probe before and after Au film coating.

Study on the optimal side polishing length
Figure 5(a)-(b) show the transmission spectra of the probes when the side polishing depth is 150 µm and the side polishing length are 15 and 25 mm, respectively.It can be seen that with the increase of the side polishing length, the resonance peak depth of the probe increases.Figure 5 (c) shows the resonance wavelength as a function of the sample RIs, it can be seen that the sensitivities of the probes are almost the same.Figure 5(d) shows the transmission spectrum in deionized water, it can be seen that the FWHM difference of the probes is very small.In the  following experiment, the probe with a side polishing length of 15 mm was selected to prepare a two-parameter probe.By comparing the transmission spectra of the FONTEX fiber and the side-polish step index POF in Fig. 3 and Fig. 5, it can be seen clearly that the FONTEX fiber has a lower RI sensitivity and the side-polish POF has a higher RI sensitivity, therefore, in order to obtain a boarder RI sensing range and a lower crosstalk between the two sensing channels, the PDMS should be chose to coat on the side-polished structure as a temperature sensing channel.

RI measurement and temperature sensing
The normalized transmission spectrum for the cascaded FONTEX fiber and the side-polish step index POF in a solution with different RI is shown in Fig. 6(a).It can be seen that when the probe is immersed in deionized water with an RI of 1.335, the transmission spectrum has two SPR peaks of λ RI and λ PDMS , and the wavelengths are at about 600 nm and 750 nm, which is similar with the reports in [19] and [21].For the RI sensing process, however, it can be seen that when RI increases to 1.39, the contrast of the two SPR resonance peaks is still very obvious due to the low RI sensitivity of the FONTEX fiber, which exhibits a better sensing performance.It can be also seen that when RI increases, λ RI shifts to the red wavelength, while λ PDMS is almost unchanged.However, if the RI is further increased, it will also cause a serious crosstalk between the λ RI and λ PDMS and the linearity of sensor will be declined.During the RI measurement, the temperature of the heating stage is kept at room temperature and the probe is immersed in a solution with different RI in turn.Figure 6(b) is the change of resonance wavelength with sample RI.From a linear fitting, it can obtain an RI sensitivity of 700 nm/RIU for λ RI , and the RI sensitivity of λ PDMS is nearly 0 nm/RIU in the measurement range of RI from 1.335 to 1.39.For the temperature sensing process, the probe was immersed in deionized water (RI =1.335).
The temperature was increased from 20 to 60 °C, and the transmission spectrum was recorded every 10 °C. Figure 6(c) shows the variation of the transmission spectrum with temperature.It can be seen from Fig. 6(c) that as the temperature changes, λ PDMS undergoes a blue shift, while λ RI is almost unchanged.Figure 6(d) shows the change of resonance wavelength with temperature.From the linear fitting, it can be seen that the temperature sensitivity of λ PDMS is −1.02 nm/°C, and the temperature sensitivity of λ RI is almost 0 nm/°C.
The changes of RI and temperature result in the movement of λ RI and λ PDMS , respectively.After obtaining RI and temperature sensitivity, their relationship can be expressed as follows: Among them, ∆n and ∆T represent the change of RI and temperature respectively.After a matrix transformation, the change of RI and temperature can be obtained by using the following formula: The sensing performance comparison between the proposed sensor probe and other POF-based SPR sensor probes with different structures is shown in Table 1.It can be seen from Table 1 that although the RI sensitivity of the proposed sensor is relatively low, it has a wider RI measurement range, a higher temperature sensitivity, as well as a narrow FWHM.These advantages make the sensor more suitable for large-range, low crosstalk multi-parameter measurement.

Conclusion
In summary, a side-polish POF cascaded with a FONTEX optical fiber based SPR sensor is proposed and implemented for simultaneous measurement of RI and temperature sensing.The cascaded structure was obtained by inserting the side-polish POF and the FONTEX optical fiber into a Teflon plastic tube and connected with UV glue.The SPR can be excited at the side-polished region and the FONTEX fiber cladding.Before constructing the dual-channel SPR sensor, the structure optimization was carried out, the sensing length of the FONTEX fiber and the sensing depth and length of the side-polish POF were optimized experimentally.In order to obtain a dual-channel SPR sensor with a better sensing performance, the PDMS was coated on the side-polish POF region.Attributed to the lower RI sensitivity of FONTEX fiber, the proposed sensor exhibits a lower crosstalk and a wider RI measurement range.An RI sensitivity of 700 nm/RIU in the RI measurement range of 1.335 to 1.39, and a temperature sensitivity of -1.02 nm/°C in the temperature range of 20 to 60 °C were obtained.The sensor is easy to make and can be used for the development of multi-parameter measurement sensing system, which has potential applications in the field of biochemical sensing.

Fig. 3 .
Fig. 3.The normalized transmission spectra of the probes with different sensing lengths of (a) 10 mm; (b) 15 mm; (c) 20 mm; the change in resonance wavelength as a function of the sample RI (d); the normalized transmission spectra of the probes in deionized water (e); Photographs of the probes before and after Au-film coating (f).

Fig. 4 .
Fig. 4. The normalized transmission spectra of the side-polish probe with different side polishing depths of (a) 100 µm; (b) 150 µm; (c) 200 µm; the change of resonance wavelength as a function of RI of the sample solution (d); the normalized transmission spectra of probes in deionized water (e).Photographs of the side-polish probes before and after Au-film coating (f).

Fig. 5 .
Fig. 5.The normalized transmission spectra of the side-polish probes with different side polishing lengths of (a) 15 mm; (b) 25 mm; The change of resonance wavelength as a function of RI of the sample solution (c); The normalized transmission spectra of probes in deionized water (d).

Fig. 6 .
Fig. 6.(a) The change of transmission spectrum with RI; (b) SPR peak changes with the sample RIs; (c) The change of transmission spectrum with temperature; (d) SPR peak changes with temperature.