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Abstract
In this work, we fabricated and investigated lossy mode resonance (LMR) based fiber-optic refractometers, using a niobium pentoxide coated optical fiber as a sensitive element. In order to do that, thin Nb2O5 films were deposited on the surface of chemically thinned optical fibers by metalorganic chemical vapor deposition (MOCVD). The sensitivities of the first transverse electric (TE) and transverse magnetic (TM) LMRs to the surrounding medium refractive index (SMRI) were measured and compared. Aqueous solutions of glucose and sodium chloride were used as test liquids. The sensor sensitivity to a change in the SMRI enhanced with an increase in the dissolved substance concentration and was greater for glucose solution. The maximum response of the 1-st TE and TM LMRs was 6580 and 6120 nm per refractive index unity (RIU), respectively.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It is impossible to imagine the modern world without the existing wide variety of sensors. They have become our kind of artificial sense organs, which allow us to receive a large amount of information about the external environment [1,2]. This list of parameters includes, but is not limited to: temperature [3,4], humidity [5,6], pressure [7,8], strain [9], bend [10], electromagnetic field [11], refractive index [12,13], including solutions of concentrated acids and alkalis [14], chemical and biological composition [15], and even radiation [16,17]. Among all types of detectors, the sensors made on the basis of optical fiber should be singled out separately. Such devices have a number of advantages over their counterparts: small size and weight, physical and chemical durability, the ability to remotely monitor the probed parameters, short response time and high sensitivity, as well as immunity to external electromagnetic fields.
To date, probably the most popular fiber-optic sensors are ones based on the surface plasmon resonance (SPR) effect [18–22]. To implement this phenomenon, it is necessary to synthesize a thin film or nanoparticles on the optical fiber surface. The coating material must have a negative real part of the permittivity that exceeds the absolute value of the same parameter for the dielectric waveguide and the environment, as well as the imaginary part of the thin layer dielectric constant [23]. This imposes quite strict restrictions on the possible choice of coating material, which, in fact, narrows down to a number of metals, among which gold is most often used. Despite the recent progress in the SPR area in terms of the variety of utilized coating materials [24–27], this is still a serious disadvantage of this sensor type. Another drawback of SPR-based sensors is the ability to excite surface plasmons exclusively with TM polarized light. However, if a material with a positive real part of the permittivity is utilized as a coating for an optical waveguide, while complying with all the other conditions necessary for the SPR implementation, all the aforesaid shortcomings can be eliminated. In this case, the conditions for observing the lossy mode resonance (LMR) [28–32] will be met, which, unlike its SPR counterpart, is sensitive to both TM and TE modes. In addition, such conditions correspond to a wide range of materials such as polymers, semiconductors, dielectrics and various combinations of them.
Despite all its advantages, LMR-based fiber-optic sensors have been actively developing only in the last fifteen years [33–63]. To date, it has already been experimentally shown that the following materials can be applied as coatings for LMR-based optical fiber sensors: silicon [33], indium oxide [48], indium tin oxide (ITO) [49,50], titanium dioxide [44], various combinations of polymers with or without titanium dioxide [51–54], diamond-like carbon [55], tin dioxide [14,56,57], silicon nitride [58], aluminum oxide [59], zinc oxide [34–37,60], polymers loaded with silver or gold nanoparticles [39–41], copper oxide [43], hafnium dioxide, zirconium dioxide, tantalum oxide [38,42,61], aluminum-doped zinc oxide (AZO) [62], indium gallium zinc oxide (IGZO) [63], and nanocomposites with α-Fe2O3 or MoS2 [45–47]. Niobium pentoxide (Nb2O5) is one of the substances that has the necessary optical parameters [64,65], but until this study was not utilized to create LMR-based fiber-optic sensors. This material has such advantages as good chemical stability [66,67], low film stress [68], resistance to temperature and external physical influence [69–72]. In addition, it demonstrates tremendous electrochemical properties [73–75] and complete safety for health [76], thanks to which it is used for the manufacture of both gas [77–79] and biological sensors [80,81]. All this makes Nb2O5 a promising material for application in LMR-based fiber-optic sensors. Our goal was primarily to demonstrate the possibility of using this type of sensor in the field of biology and medicine. For this purpose, it was shown to be used as a refractometer for measuring the refractive index of aqueous solutions of table salt (saline solution) and glucose (dextrose solution), which are on the one hand one of the simplest and most important medicines, and on the other hand can serve as media for other biological objects, for example, cyanobacteria or yeast, respectively.
To strengthen the coating effect on the distribution of light propagating inside the optical waveguide core, the fiber is preliminarily undergone to one of the following procedures: chemical thinning [14,57], thermal pulling [59], or side polishing [63]. The main advantages of the first approach are the geometry preservation of the optical fiber light-guiding core and the constancy of the distance between the core and the thin film. At the same time, using the metalorganic chemical vapor deposition (MOCVD) technology, it is achievable to synthesize coatings uniform in thickness over the entire thinned section of the cylindrical fiber. Due to these advantages of chemical thinning and MOCVD, in the transmission spectra of fabricated optical waveguides, the separation of transverse electric (TE) and transverse magnetic (TM) LMRs is observed for several resonance orders without applying a polarizer [14].
In this work, we first show the possibility of synthesizing thin Nb2O5 films on the optical fiber surface by the MOCVD method using a commercially available organometallic compound niobium ethoxide (Nb(OC2H5)5) as a precursor. Secondly, we manufacture fiber-optic lossy mode resonance sensors based on chemically thinned optical fiber coated with niobium pentoxide. Finally, using aqueous solutions of glucose and sodium chloride (NaCl), we compare the sensitivity of the 1-st TE and TM LMRs to the surrounding medium refractive index (SMRI).
2. Sensor fabrication
2.1. Chemical etching of optical fibers
The manufacturing procedure for the basis of fiber-optic sensors was carried out in several stages. First of all, a standard single-mode fiber SMF-28 Corning (core/cladding diameter = 8.2/125 µm) without a jacket was mechanically cleaned from the polymer coating on a section, the size of which further determined the taper length. The deviation from the planned value of this parameter typically does not exceed 0.5 mm. To reduce the likelihood of fiber breakage and/or damage to its surface during stripping, the treated area was previously immersed in acetone for 30-45 s. In the second stage, the optical fiber was fixed on a hydrophobic coated plank, which was mounted on a specially designed rocking mechanism [82] (Fig. 1). The right block shown in the schematic is necessary to stabilize the position of the slat, which prevents its movement in the horizontal plane. An aqueous solution of ammonium fluoride (NH4F) and ammonium sulfate ((NH4)2SO4) was utilized as this fluid, providing an etching rate of 5 nm/s at a temperature of 25 °C. Note that they can be stored indefinitely in fluoroplastic and glass containers, respectively. In both cases, 10 g of dry substance per 25 ml of water was utilized for their preparation. The etchant solution used by us is made immediately before the etching procedure and was utilized to fabricate only one thinned fiber. For its manufacture, aqueous solutions of NH4F and (NH4)2SO4 were mixed in volumes of 1 and 0.5 ml, respectively. Approximately half of the prepared polishing etchant was dropped by a pipette onto the surface of the cleaned fiber section. Its main advantage over hydrofluoric acid (HF) is a relatively low toxicity. During the rotation of the eccentric around the axis, there is a periodic change in the inclination angle of the plate, which leads to a reciprocating motion of the liquid drop along the optical fiber. The continuous movement of the etchant solution ensures its high uniformity and makes it possible to minimize surface defects in the thinning area. Due to the hydrophobic coating, the small angle of inclination of the bar (less than 2°) and the small volume of the drop, its maximum deviation from the central position during the rotation of the eccentric was no more than 2 mm.
Fig. 1. Schematic representation of the experimental setup for thinning optical fibers by chemical etching.
The second stage lasted about 2 hours and 45 minutes. At the end of this time, the etchant was drained and the fiber light guide was repeatedly rinsed with bidistilled water. Using an optical microscope, the outer diameter of the thinned fiber was determined, which was usually 75 ± 5 µm. Based on the obtained data on the etching rate, the expected time of the next step was adjusted, which on average was equal to the time of the second phase. In the third stage, the optical fiber was stripped from the polymer coating at a distance of about 7 cm from the edges of the initially thinned section, in order to avoid polymer decomposition when the optical waveguide was subsequently located in a heated MOCVD reactor. The cleaning from the polymer coating was absolutely identical to this procedure at the first stage: with its preliminary softening in acetone. After this operation, the taper was again poured with the etchant and the thinning process continued until the optical fiber waist external diameter reached the desired value (18–30 µm). The difference between the intended and actually obtained external diameter of the taper usually does not overstep 1 µm.
The conception we implement [14,82] provides a way to produce fiber-optic tapers with a high optical quality of the etched surface and a smooth transition between the cylindrical part of the thinned area and the untreated fiber (Fig. 2). The transition zone and its geometric parameters are formed under the influence of a number of factors. Initially, at the second stage, when a small bare section of the optical fiber is etched, the surface of this area is protected by a polymer coating and remains intact. That is why there is a difference in the external diameters of this region and the cylindrical segment. Further, at the third stage, when the total length of the fiber section undergoing the thinning process is defined by the diameter of the dribble and the amplitude of its movement, the etching process of this area begins. One part of this zone (the one that is closer to the cylindrical segment) is constantly immersed in the etchant solution and therefore its diameter along the fiber almost does not change. The second part of the transition zone (the one that is closer to the unetched fiber) is formed under the influence of periodic changes in the etching region due to the drop movement.
Fig. 2. Simplified view of the optical fiber after the chemical etching procedure (top) and a photographic image of the central part of the thinned region of the fiber light guide obtained using an optical microscope (bottom).
2.2. MOCVD setup
An optical fiber with a thinned section was placed along the axis in the center of a special tubular silica glass reactor, which was then hermetically sealed on both sides (Fig. 3). The process of synthesizing thin Nb2O5 films on the taper cylindrical surface was carried out by the MOCVD method. The required temperature in the deposition area was provided by a resistance furnace. The carrier gas for the organometallic precursor Nb(OC2H5)5 was argon, the volumetric flow rate and linear flow velocity of which at the reactor inlet were 550 cm3/min and 50 cm/s, respectively. Since niobium ethoxide at room temperature is a liquid with an extremely low vapor pressure [64,83,84], the container with it, as well as all gas pipelines and connections through which its fumes flew, were heated to a temperature of 90 °C. A small amount of air (from 0 to 2 cm3/min) was supplied to the reactor through a separate entrance. During the coating deposition process, the fiber light guide was integrated into the optical circuit, in which a halogen lamp LS-1 and a spectrometer NIRQUEST-512 OceanOptics (wavelength resolution = 3.1 µm) served as a radiation source and receiver, respectively. Thanks to this, we were able to observe the optical transmission spectra and record them in the spectral range from 900 to 1700 nm directly during the synthesis.
Fig. 3. (a) – General view of the MOCVD experimental setup, with the realization of the transmission spectra measurement during the deposition of thin Nb2O5 film on the optical fiber taper; (b) – scheme of gas flows in the tubular reactor.
2.3. Film thickness uniformity
At the first stage of searching the optimal conditions for the synthesis of niobium pentoxide coatings, a series of thin film deposition processes was carried out on fiber-optic tapers of various lengths (2–4 mm) and diameters (16–24 µm). The temperature of the tubular MOCVD reactor was varied in the range from 420 to 610 °C. During the deposition process, dips were observed in the transmission spectra, which were transverse electric (TE) and transverse magnetic (TM) LMRs of different orders. The designations TE and TM LMRs were chosen because of the currently well-established terminology in this domain [85]. Note that a feature of our sensors is that there is no need for a polarizer to separate TE and TM resonances. If we consider the optical fiber and the thin film separately, we can say that the TE and TM LMRs are caused by the interaction of the fundamental mode of the optical fiber (HE1,1) with the TE and TM modes of the coating, respectively. As is known, this coupling mainly occurs under circumstances corresponding to the cut-off conditions of modes appearing in a thin layer [86]. And since these ones are met at the same wavelength for TE and TM modes at different film thicknesses, only a high uniformity of the coating is necessary for the separation of TE and TM LMRs.
We will consider two deposition processes: “ideal” (Fig. 4, top) and “real” (Fig. 4, bottom). In the first case, the coating thickness (d) in any two randomly selected points will always coincide with each other. Thus, with an increase in the film synthesis time (t), the separation of TE and TM LMRs for each resonance order will be observed until the width and depth of the individual LMR allow this to be fulfilled. However, in almost any “real” process of film growth, there is a deposition zone within which the coating is synthesized. The layer growth rate in different points in this area may differ, which will lead to variation in the coating thickness (Fig. 4, bottom). Assume that dmin and dmax are the minimum and maximum layer thickness, respectively; dTEn and dTMn are the film thicknesses at which TE and TM LMRs of the n-th order are observed, respectively. We assume that in each individual point the deposition rate is constant and directly proportional to the deposition time, then dmax – dmin ∝ t. We will say that TE and TM resonances of the same order are not separated when the condition dmax – dmin > dTMn – dTEn is satisfied. In this case, due to the various thickness of the layer on different sections of the optical fiber, the conditions for observing both types of resonances of the same order will be fulfilled simultaneously. If the condition dmax – dmin > dTEn+1 – dTEn is met (or, what is the same, dmax – dmin > dTMn+1 – dTMn), then we will talk about the merging of two resonances of neighboring orders, since the requirement for their concurrent observation will be met.
Fig. 4. Schematic representation of the change in layer thickness during the “ideal” (top) and “real” (bottom) process of thin film deposition on the optical fiber surface.
The quality of the synthesized layer was evaluated by three main parameters. First, the optical transmission of the fiber must be restored to 100% at any particular wavelength in the spectral range under study after the 1-st TE LMR is shifted to the long-wavelength part of the spectrum and before the 1-st TM LMR occurs. This criterion characterizes the film in terms of the thickness uniformity. Second, the dependence of the spectral position of all observed resonances on the process duration should be linear. Satisfaction of this condition indicates that the material synthesis process is stationary. Third, the possibility of observing high-order resonances, which concurrently indicates the high thickness uniformity and optical quality of the coating. Note that in all our experiments, with an increase in the resonance order, its depth decreased. The best correspondence to all three signs of high quality of the layer was achieved with a volumetric air flow equal to 0.38 cm3/min and a temperature of 484 ℃. Figure 5 shows the spectral response obtained as a function of the duration of the deposition process carried out under these conditions. The coating was synthesized on the surface of a fiber-optic taper with a length of 2.9 mm and a diameter of 19.1 µm. From the presented graphic data, it can be seen that the manufactured sample meets all three quality criteria. First, there is a separation into TE and TM components up to the third order of LMR. Secondly, the wavelength of the resonances depends on the process duration in a linear way. Third, it is possible to distinguish LMRs up to the 12-th order. Separately, it is worth noting that, despite the large thickness of the deposited layer, we were able to extract optical fibers with a thinned section after the process from the MOCVD reactor, avoiding their destruction. This feature favorably distinguishes the compound obtained in this work in comparison with tin dioxide. When using the latter as a coating material, the manufactured samples broke when the gaskets were weakened, even if only the 1-st TM LMR was observed in the transmission spectra [14].
Fig. 5. Dependence of the spectral response on the deposition process duration for a coated fiber-optic taper in the wavelength range (a) – from 900 to 1700nm and (b) – from 900 to 1000 nm. The area represented in the bottom image is highlighted by a white dotted line in the upper one. The integration time was 350 ms. Every second spectrum was recorded.
Using the data presented in Fig. 5, it is possible to estimate the thickness of the deposited layer. For this purpose, the finite element method (FEM) and the well-known approach [87,88] were used: the resonance position was computed from the equality of the imaginary parts of the effective refractive index of the optical fiber fundamental mode and the corresponding coating one. Available data on optical constants for Nb2O5 were used for calculations [89]. Initially, we estimated the difference in layer thicknesses required to observe one type of resonance of neighboring orders at wavelengths of 930 and 1400 nm. This value, as expected [33], did not depend on the LMR number and was approximately 267.5 and 409.2 nm, respectively. Taking into account that there are 8 resonances at a wavelength of 1400 nm, and 12 ones at a wavelength of 930 nm (Fig. 5) and calculating the coating thickness at which the first TE and TM LMRs are observed at the corresponding wavelength, the total film thickness was computed. The results of the performed estimates at the wavelengths of 930 and 1400 nm, yielded values equal to 3029 and 2996 nm, respectively. Thus, the average film deposition rate was approximately 0.6 nm/s, with a total thickness of about 3 µm.
2.4. Coating chemical composition
At the second stage, in order to characterize the deposited films, a long (for two hours) deposition process of the layer on the optical fiber surface stripped of the polymer coating, but not undergone the chemical thinning procedure, was carried out. The surface of the synthesized film and its chemical composition were studied using a Jeol JSM-6480LV a Scanning Electron Microscope (SEM) with a tungsten thermionic cathode and Energy Dispersive X-Ray Analysis (EDX), respectively. Figure 6 shows SEM images of the optical fiber coating synthesized in the central region of the MOCVD reactor, and the EDX spectrum obtained from the same area.
Fig. 6. (a), (b) – SEM images of the side surfaces of an optical fiber coated with a synthesized film; (c) – EDX spectrum of the presented area. The reactor temperature during the deposition process was 484 °C.
Despite the presence of individual defects on the sample surface, in general, the film is continuous without visible individual grains of the structure (Fig. 6(a), (b)). Chemical analysis data showed that the atomic concentrations of niobium and oxygen are related as 2:5. Before the calculation, it was taken into account that part of the detected oxygen is included in the SiO2 structure. Thus, the material we obtained is Nb2O5 (Fig. 6(c)).
3. Sensor characterization
This section presents a comparison of Nb2O5-coated fiber-optic sensors based on the principle of determining the position displacement of the 1-st TE and TM LMRs. Tapers with lengths of 2.5 and 2.1 mm and diameters of 19.5 and 20.4 µm, respectively, were utilized as a basis for their fabrication. The difference in the geometric parameters of the thinned optical fibers is due to the method features of their manufacturing. The deposition times of the thin films were chosen so that when the thinned region of the fiber was immersed in water, the maxima of the 1-st TE and TM LMRs would be in the wavelength range from 1300 to 1350 nm. This allows you to provide a wide operating range of the detector, while keeping the spectral position of the resonance at wavelengths larger than the optical fiber cut-off wavelength. To achieve this, the coating synthesis process on the taper surface continued until the maximum of the corresponding LMR reached a wavelength of ∼900 nm. After the deposition process was completed, the optical fibers extracted from the reactor were fixed to a plank with a hydrophobic coating and re-integrated into the same transmission spectrum measurement scheme.
Two series of aqueous solutions with gradually increasing concentration of glucose or NaCl were used as test fluids. During the transmission spectrum measurement, the thinned region of the fiber covered with a Nb2O5 layer was poured with the prepared solution. Since the main contribution to the change in the transmission spectrum is made by the cylindrical segment (Fig. 2) with a film deposited on its surface, it can be considered as a sensitive region of the refractometer. Before the experiment with the next liquid, the previous aqua was drained, and the fiber-optic sensor was rinsed in bidistilled water until the original transmission spectrum was completely restored. All the experimental data presented in this section were obtained at the same temperature equal to 20 ℃. Figure 7 demonstrates the optical transmission spectra of the LMR sensors in the region of the 1-st TE (Fig. 7(a), (b)) and TM (Fig. 7(c), (d)) resonances, when they are in aqueous solutions of glucose (Fig. 7(a), (c)) and table salt (Fig. 7(b), (d)). The wavelengths of the TE and TM LMRs maxima when the tapers were in pure water amounted 1349 and 1310 nm, respectively. Knowing the geometric parameters of fiber-optic tapers and the spectral position of the resonances, the thickness of the coatings in both cases was estimated using the FEM. Their values were 48.5 nm when observing TE LMR and 111.2 nm when detecting TM LMR. Figure 8 shows the azimuthal distribution of the light intensity and the direction of the electric (Fig. 8(a)) and magnetic (Fig. 8(b)) fields for the fundamental mode propagating in the waveguide for tapers coated with Nb2O5 with a thickness of 48.5 and 111.2 nm, respectively. Note that in both cases, the corresponding component of the electromagnetic field is directed along the thin film, which confirms the interaction of the optical fiber fundamental mode with the TE (Fig. 8(a)) and TM (Fig. 8(b)) layer modes.
Fig. 7. Optical transmittance of fiber-optic sensors with niobium pentoxide coating depending on the wavelength when immersed in glucose (left) and sodium chloride (right) aqueous solutions of various concentrations in the case of observing the 1-st TE (top) and TM (bottom) LMR. The insets show the dependences of the resonance maximum wavelength on the environment refractive index and their approximation by an exponential function.
Fig. 8. Azimuthal distribution of the light intensity and (a) – electric and (b) – magnetic fields for the main mode of a fiber light guide when observed in the transmission spectrum of 1-st TE and TM LMRs, respectively.
The relatively small difference between the spectral positions of the resonances suggests a relatively high (∼3%) accuracy of comparing the response of different sensors. The insets show the dependence of the spectral position of the corresponding resonance maximum on the environment refractive index (nD at a wavelength of 589 nm), which was determined on the basis of reference data [90]. The average error in determining the position of the TE and TM LMRs was 2.1 and 0.55 nm, respectively. By definition, the sensor SMRI sensitivity is the ratio of the shift in the spectral position of the maximum LMR (Δλ) to the change in the refractive index of the external medium (Δn). Thus, to evaluate the detector response, it is necessary to calculate the first-order derivative (dλ/dn) for the experimental sensor characteristic curve. However, in order to reduce the random error influence, we first performed an approximation of the empirical data. Since the sensor response to changes in environmental conditions is clearly non-linear (especially in the case of glucose solution), the exponential distribution was used as the fit function: λ=λ0+k·exp((n-n0)/t), where λ0, n0, k and t are the approximation parameters. For all the specimens under consideration, the coefficient of determination R2 differs from unity by no more than 10−13. The type of the proposed function was chosen based on the accuracy of the obtained approximation, provided that the sensitivity curves do not have local extrema.
Figure 9(a) shows the dependence of the sensor sensitivity on the external environment refractive index, obtained by differentiating the approximation curves (Fig. 7, insets). It is clearly seen that, first, the sensitivity enhances with an increase in the SMRI, which is a well-known fact [85]. Second, with the same chemical composition of the environment, the response of the 1-st TE LMR is greater than one of the 1-st TM LMR. Third, the detector sensitivity to changes in the refractive index of a glucose solution is higher than that of a table salt one, and it magnifies faster with an increment in the liquid refractive index. For example, with an increase in SMRI for the 1-st TE LMR, the sensor sensitivity for the glucose solution is higher than for the NaCl solution from 15 to 30%, and for the 1-st TM LMR – from 0 to 40%. This dissimilarity is most likely associated with distinct dispersion curves of these substances (Fig. 9(b)). For definiteness, consider two media with the same refractive index at the resonance wavelength, but with different values of normal dispersion. As the concentration of the solution increases, the refractive index of the liquid will also build up (from n0 to n at a wavelength of λ) and the resonance will shift to the long wavelength part of the spectrum (from λ0 to λ). However, in the entire spectral range located to the right of the initial resonance wavelength, the refractive index of a liquid with a large absolute value of chromatic dispersion will be lower (Fig. 10). We denote the sensitivity of a fiber-optic LMR refractometer when it is used to measure the refractive index of a solution without (Fig. 10(a)) and with (Fig. 10(b)) chromatic dispersion (Dch) as S0 and S, respectively. S0 = Δλ/Δn, by definition. In the presence of chromatic dispersion, it is important to note that in order to change the spectral position of the LMR from the wavelength λ0 to λ, the condition n(λ) – n(λ0) = Δn must be fulfilled. In this case, the refractive index at the wavelength λ will change to Δn + Δn′. Since Δn′ = |Dch|·Δλ, then S = Δλ/(Δn + Δn′) = Δλ/(Δn + |Dch|·Δλ). Dividing the numerator and denominator by Δn, we get: S = S0/(1 + |Dch|·S0). Obviously, S < S0, and S0 decreases with increasing |Dch|. Thus, it is not quite correct to compare the sensitivities of LMR-based optical fiber sensors if liquids of different chemical composition were exploited for their characterization.
Fig. 9. (a) – Dependence of the sensitivity of fiber-optic sensors with niobium pentoxide coating on the refractive index of glucose and sodium chloride aqueous solutions when observing the 1-st TE and TM LMRs; (b) – dispersion curves for water solutions of table salt [91] and glucose [92].
Fig. 10. The schematic dependence of the refractive index on the wavelength for a liquid with (a) – zero and (b) – negative chromatic dispersion.
The highest sensitivity to the SMRI was demonstrated by a sensor with a Nb2O5 coating based on the 1-st TE LMR when testing a glucose solution with a refractive index of ∼1.36. However, the obtained value (6580 nm/RIU) was only 8% higher than the response of the detector based on the 1-st TM LMR (6120 nm/RIU). When a table salt solution with a refractive index of ∼1.36 was applied as the ambient medium, the difference in the sensitivities for TE (5070 nm/RIU) and TM (4420 nm/RIU) LMRs was more significant and amounted to 15%. As far as we know (Table 1), the refractometer manufactured in this work has the greatest sensitivity to changes in the refractive index of aqueous solutions of sodium chloride and glucose, among its counterparts.
Table 1. Comparison of the sensitivities of the LMR-based refractometer manufactured in this work with its analogues
4. Conclusions
In the present study, we have demonstrated lossy mode resonance fiber-optic sensors based on niobium pentoxide thin film. Nb2O5 layers were synthesized on the surface of chemically thinned optical fibers by MOCVD. During the coating deposition process, the optical transmission spectra of the optical fiber were measured, which made it possible to observe the LMR phenomenon. The wavelength of the maximum resonances increased in direct proportion to the film synthesis duration. We have fabricated and characterized fiber-optic sensors operating on the principle of the relation of the 1-st TE and TM LMR position on the external medium refractive index, which was water with different content of glucose or sodium chloride. According to our data, the comparison of the TE and TM LMR sensitivities in the same spectral region was carried out for the first time, for which two separate refractometers with different thicknesses of niobium pentoxide coatings were specially manufactured. It is shown that the sensitivity of LMR-based optical fiber sensors grows with an increase in the mass concentration of a dissolved substance and depends on their chemical composition. The second effect is probably related to different dispersion curves of solutions, which was not noticed in previous works on LMR-based sensors. The maximum response values were obtained for a glucose solution with a refractive index of ∼1.36 and amounted to 6580 and 6120 nm/RIU using fiber-optic sensors based on the 1-st TE and TM LMRs, respectively. These parameters are higher than those of the previously presented LMR refractometers designed to measure the refractive index of aqueous glucose solutions.
Funding
Russian Science Foundation (21-19-00259).
Disclosures
The authors declare no conflicts 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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