Numerical and experimental analysis of side polished single mode fiber for refractive index sensing

doi:10.21203/rs.3.rs-4014736/v1

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Numerical and experimental analysis of side polished single mode fiber for refractive index sensing

https://doi.org/10.21203/rs.3.rs-4014736/v1

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In this paper, we introduced a comprehensive study, based on both numerical and experimental analyses, of side polished (SP) single mode fibers (SMF) to investigate their evanescent field interaction with air and liquid analytes (water, isopropanol). In particular, the finite element method (FEM) and the beam propagation method (BPM) are employed to predict the optical properties of the SP-SMF. The alteration of analytes at the sensing region enhances the change in refractive index of the materials. An ad-hoc experimental setup has been designed and built to characterize the side polished single mode fiber when air, water, and isopropanol are set in the side polished region. The sensitivity of the analytes is obtained as 1.207 V/RIU. The performance in terms of effective refractive index and transmittance are reported to show how these SP-SMF can be efficiently used for calculating the liquid refractive index. The simulation and experimental results display the significant performance of the SP-SMF as a sensing element.

SP-SMF

refractive index

analytes

optical sensor

In the past few decades, optical fiber technology has fascinated attention in medical, industrial engineering, and environmental monitoring [1]. This technology has been widely examined for the measurements of several parameters, such as refractive index (RI), temperature, magnetic field, stress, strain, and liquid sensing due to the remarkable benefits of high sensitivity, flexibility, electromagnetic interference, anti-corrosion properties, efficient size, and low cost as compared to the conventional sensors [2, 3].

Recently many types of optical fibers have been utilized in sensing technology such as single mode fiber (SMF), multimode fiber (MMF), microstructure fiber, and photonic crystal fiber (PCF) [4]. These sensing techniques rely on the Mach-Zehnder interferometer (MZI) [5], Fabry-Perot interferometer (FPI) [6], and Michelson interferometer (MI) [7]. Moreover, surface plasmon resonance (SPR) sensors have earned substantial concern, due to their real-time supervising abilities and high sensitivity. They implicate a fluctuation of charge intensity that exhibits at the boundary between the metal and dielectric layer, and utilizing several ultimate methods permits the manipulation of this charge intensity oscillation [8]. However, the implementation of plasmonic materials with the side polished fiber requires a specific procedure to fabricate, which could be complex and lengthy[9].

SMF structures are highly attractive to environmental parameters, particularly side polished SMF design has obtained an extensive devotion and been occupied in many kinds of optical sensing devices thanks to evanescent filed extended outside to core into the side-polished region [10]. Side polished single mode fiber has been broadly utilized in various applications, containing attenuators, switches, polarizers, and evanescent optical fiber sensor, due to the well-recognized of transient field on the tapered surface of the fiber. SP-SMF structure play a key role in the measurements of RI through a precise relation to the intensity of analytes [11].

In this regard, two schemes of side polished arrangement can be considered: the first one is based on the filling of liquids or analytes inside the fibers, where these analytes are inserted into the core or the cladding regions of the fibers [12]. Mostly microstructure fibers have been exploited for this method [13]. The benefit of this scheme is related to its high sensitivity and manageability because the filled analyte inside the fiber directly impacts the dispersion of the fiber [14]. However, the major drawback is related to the fact that filling the fiber with liquids can be very complex or difficult as related to other approaches in which analyte keep on the exterior part of the fiber [15]. Therefore, D-shaped fibers, with sensing or tapered region located outside the fiber, can be considered as mainstream for real time measurements [16].

In recent years, D-shaped fiber structures are reported, in which half part of the cladding section is polished, which allows a stronger interaction between the optical mode and the target analytes [14, 15]. Wang et al. demonstrated the temperature sensor based on helical core fiber which is polished as a D-type fiber coated with the layer of gold [19]. Studies of Ag/TiO2 plasmonic formation combined with side polish fiber has been described by Yousuf et al. to build the humidity sensor [20]. Further, Xu et al. designed the magnetic field sensor based on the magnetic fluid and side polished hollow core optical fiber [21]. A film of cholesteric liquid quartz, employed to the side polished fiber to give the capability of measurement of volatile organic compound (VOC) gases [22]. A recent work reports the measuring the liquid level and refractive index by using the side polished plastic optical fiber [23]. Moreover, a humidity sensor with tremendous sensitivity employing a side-polished in-fiber directional coupler coated with gelatin [24].

Moreover, for the manufacturing method, more polishing intensity indicates extra refined input development which creates fabrication cost of fiber sensor more expensive. As far as sensitivity factor, sensors with numerous sensing paths are more responsive than the specific sensing channel over the same circumstances [25]. The operating wavelength range is also a significant element for the sensing devices. Therefore, it is possible to achieve a higher sensitivity and wider range of detection at certain conditions with different analytes placed outside the D-type SMF sensors.

In this study, the characterization of commercially available SP-SMF associated with air and analytes has been proposed. The simulation study based on the finite element and beam propagation technique has been exploited to examine the phenomena of light propagation. On the other hand, for the practical application experimental studies have been analyzed to describe the matching results of numerical output. On the bases of optical transmittance spectrum, air, water, and isopropanol (IPA) have been taken into account. The characteristic of light confinement depends on the refractive index of the analytes, which leads to the change in optical output power. The performance of the proposed model is achieved as 1.207 V/RIU for the experimental study. This type of structure could be used as a building block in liquid sensing applications as well as refractive index sensing.

The cross section and side view of the proposed SP-SMF structure has been shown in Fig. 1(a) and (b) respectively. The commercial SP-SMF (manufactured by Phoenix Photonics Ltd) has been taken to perform the experimental study. The geometrical parameters for the simulation study have been set up according to the commercial fiber.

The central part of the fiber consists of the core region with the diameter (2*r₁) of 9.5µm which is surrounded by the cladding region. The whole cladding diameter (2*r₂) of the fiber is equal to 62.5µm in which the length (L) of the polished region is equal to 17mm, which behaves as an active region through adding the analytes. The “Etch depth” refers to the depth at which the cladding of a SMF is removed during the fabrication process of polishing, and the distance between the core and the tapered section is about 2µm. The outer layer of fiber consists of a perfectly matched layer which absorbs the reflected light from different angles. Figure 2 indicates the wheel side polishing technique which is used for manufacturing the SP fiber [26]. The coated section of SMF is held compactly over the fixed motor driven wheel by two fiber clampers. In this method, a PC controller is associated to the polished layer for observing remaining fiber depth till the fiber core is revealed. The connection of light source and optical spectrum analyzer monitors the overall polishing process and attenuation of light transmission along the fiber [27].

In the simulation analysis, two different studies have been carried out based on (i) COMSOL Multiphysics and (ii) Beam PROP implementing (i) the Finite Element Method (FEM) and (ii) the finite differences beam propagation method (FD-BPM), respectively [28]. The computation of FEM model develops to examine and evaluate of light transmission through the proposed SP-SMF sensor. The physics-controlled way of fine mesh analysis is studied which consists of 1512 domain elements and 184 boundary elements. The wavelength range is taken from 1460 to 1580 nm. Pure silica is used as a background material of the fiber with a refractive index of 1.445 at the wavelength of 1500nm.

The dispersion relation of the refractive index of pure silica and analytes have been calculated by using the Sellimeier equation [24, 25]. For silica the following equation has been used [31].

$$silica \left(n\right)=\sqrt{1+\frac{{A}_{1}{\lambda }^{2}}{{{\lambda }^{2}-B}_{1}}+\frac{{A}_{2}{\lambda }^{2}}{{{\lambda }^{2}-B}_{2}}+\frac{{A}_{3}{\lambda }^{2}}{{{\lambda }^{2}-B}_{3}}}$$

where n is the effective refractive index of the pure silica glass, the values for the constant variables are A₁ = 0.6961663, A₂ = 0.4079426, A₃ = 0.8974794, B₁ = 0.0684043, B₂ = 0.1162414, B₃ = 98.96161 and λ is the operating wavelength in (nm).

In the FEM, the light transmission of the SP-SMF has been examined which is employed to evaluate and calculate the propagating modes profile of the proposed structure. During the simulation assessment an external coating covers a perfectly matched layer (PML). The refractive index of analytes like air, water, and isopropanol have been assumed at the polished region of the fiber. The different mode profiles of light spectra with respect to change in refractive index have been shown in Fig. 3.

The chosen analytes have lower refractive indexes than 1.45 (silica) in which the most part of light confines into the core region. From the inspection of Fig. 3, it seems that a small portion of light is gradually moving from the core to tapered region as the refractive index of the analyte n increases. At the lower refractive index of analytes like (air), the light is totally confined into the core region. After changing the analytes from air to water, the light is slightly transferred from the core region to polished section. Similarly with the analytes of IPA light is transferred more as compared to air and water. The propagation of light totally depends on the refractive index of the materials. When the signals pass through the tapered region, the light spectrum moves to the polished surface due to the effect of analytes refractive index. Thus, the transient impulse is created, and light modes travel through the analytes section. In this way, the impacts of the SP-SMF arrangements converted the digital signals into physical parameters involving the silica refractive index.

Similarly, a simulation study based on the beam propagation method is carried out to analyses the mode distribution when the refractive index of the analyte is changing. The mode profile in the tapered region with the analytes of (air, water, and IPA) is depicted in Fig. 4. The beam propagation method shows the electric field distribution on the transmitting mode varies with the change of analytes. It seems that there is a little conversion of light mode to the tapered region as the refractive index increases from 1.0 to 1.37. When it reaches up to 1.45 (silica), the confinement of light fully transferred from core region to tapered region due to the higher refractive index of analytes.

The transmitted signals of the side polished SMF composition have been simulated, which can be determined by using the following equation [32]:

$$T\left(\lambda \right)=\text{e}\text{x}\text{p}\left[-\frac{4\pi }{\lambda }Im\left({n}_{eff}\right)L\right]$$

where $\lambda$ refers the operating wavelength, $Im\left({n}_{eff}\right)$ considers the imaginary part of the effective refractive index of the fundamental mode, and L indicates the length of the sensing area of the side-polished fiber.

Moreover, the transmittance when air, water, and isopropanol are set in the polished region has been studied by using the FEM and FD-BPM as shown in Fig. 5(a) and 5(b), respectively. The plot in Fig. 4(a) shows that, at the wavelength of 1530nm, the transmittance of isopropanol (T = 0.869) is higher than water (T = 0.859) and air (T = 0.842) respectively. The transmittance also depends on the wavelength, which is going to increase as wavelength increases.

Similarly, the same trend has been obtained in the case of the FD-BPM method leading to a good agreement between these two methods. It seems that isopropanol (IPA) has the maximum transmittance spectrum as compared to air and water due to the higher refractive index analytes. The transmittance of light is going to increase as the wavelength is increasing.

To validate the above simulation studies, the experimental setup has been designed to exploit the side polished SMF-28 (from Phoenix Photonics Ltd). The overall experimental schematic diagram has been shown in Fig. 6, which comprises the necessary instruments for the optical characterization. A tunable laser (TL) with a tunable wavelength range of 1460nm to 1580nm has been used as a source and connected to the fiber. The output signal was sent to a photodetector (New Focus-2033, USA) that was connected to an oscilloscope to detect the transmitted signal. Both ends of sensing structure has been connected between the input (light source) and output (photodetector). The whole experimental process is taken under the room temperature (25^oC).

The microscopic view of tapered or polished region has been shown in Fig. 7(a). Here the sensing and polished section of the SP fiber is shown along with the tapered regions (on the left and right sides). Both ends of SP fiber are connectorized by FC/PC connectors to lead in (light source) and out (photodetector) the light. A substrate has been designed on the COMSOL Multiphysics after that it has been fabricated by using the 3D printer (Photon Mono Se Anycubic). A valuable attribute in a sensor is its stability. Stability allocates with the degree to which the sensor abilities stay stable over time. Alterations in stability are due to elements aging, a decline in the sensitivity of components, or a change in the signal-to-noise ratio. To achieve the stability and minimize the signal noise, SP fiber has been kept inside a 3D printed reservoir which is fixed with optical table to avoid any movement. The shape of the reservoir is shallow deep in the middle, where the analytes or liquids have been filled at the upper part of the tapered region. Figure 7(b) highlights the installed experimental setup with lead in SP-SMF to lead out SP-SMF.

In the wavelength range of interest (1460nm to 1580nm), the photodetector sensitivity was set to low, and the input power was set to 4mW. Figure 8(a) indicates the experimental results with respect to wavelength and voltage by analyzing the analytes air, water, and isopropanol, respectively. This graph illustrates that isopropanol (blue curve) shows a higher voltage as it has a higher refractive index than air and water. The black and red curve shows the voltage value of air and water respectively. The maximum voltage curve is achieved at the wavelength of 1530nm. After that there is a decline in the voltage with the increase of wavelength. Here, the effect of some noise can be seen which depends on the optical setup and instruments (photodetector and oscilloscope). Furthermore, the plotted graph indicates the highest voltage at the wavelength of 1530nm at which the maximum voltages of air, water and IPA are 4.350 V, 4.708 V, and 4.797 V, respectively. The voltage differences between air, water, and isopropanol are 0.44 V, 0.35 V, and 0.089 V as the refractive index of air water and isopropanol are 1.0, 1.33, and 1.37 respectively. The voltage of isopropanol is higher than the water and air. However, there is not much difference in voltage between the isopropanol and water, due to the close refractive indexes.

Moreover, repeatability assigns the reliability of a particular sensor. It can be utilized to demonstrate the capability of a sensor to present the same result under the same circumstances repeatedly. All the effective optical parameters for the proposed sensor have been tuned several times to observe the results. The transmittance and voltage outcomes indicate the sensor gives good responses, ensuring stability and repeatability with same results of the proposed sensor.

On the other hand, the analysis of air, water, and isopropanol has been explored with respect to time variation. The wavelength has been fixed at 1530nm, where the maximum power is achieved. The same parameters as the input power equal to 4 mW and low sensitivity was used for the photodetector. The overall time duration was set equal to 0 to 180s, which was equally distributed for the whole analytes (air, water, IPA). Each analyte has been investigated for the 60s. The experimental graph with respect to voltage and time variation has been outlined in Fig. 8(b). This graph indicates the evaluation of air, water and isopropanol with real time measurement. It has been seen that with the variation of time and change in the analytes, the voltage is changing accordingly, the average voltage for each liquid is quite constant. IPA gained the maximum voltage, due to the higher refractive index rather than air and water. This accounts for the repeatability and validation of SP-SMF fiber effect with various liquids and air. Hence it is confirmed that the voltage with respect to wavelength and time variations of analytes (air, water, and isopropanol) have the same behavior.

Furthermore, to shed the light comparison with the simulation study, the experimental results are normalized and fitted for air, water, and IPA as a surrounding media. Notably, Fig. 9(a), shows a discernible difference in transmittance, revealing a relative change of 6% between air and water and 2% between water and IPA at the wavelength of 1530 nm where the maximum curve is achieved. To check the performance of the proposed SP-SMF model, the sensitivity has been calculated which is the variation of refractive index occurs with the change of analytes and the voltage. The sensitivity (S) of the proposed model with respect to wavelength and the voltage has been measured by the following equation [33]:

$$S\left(V\right)= \varDelta V/{\Delta }n$$

where ${\Delta }n$ is the variation of the refractive index, and $\varDelta V$ is the change of voltage.

Correspondingly, Fig. 9(b) shows the linear fitting curve of experimental study by relating the voltage with the change of refractive index of considered analytes. The sensitivity of the analytes is achieved as 1.207 V/RIU with the adjacent R² is 0.988. By comparing the simulations and experimental work, the results are in good agreement since the same behavior has been achieved.

In conclusion, the SP-SMF structure for assessing the refractive index of different analytes (air, water, and isopropanol) has been investigated through numerical and experimental study. Two different numerical tools, based on FEM and FD-BPM, have been explored to verify the proposed geometry and calculate the effective refractive index and the transmittance. Moreover, an experimental setup has been realized to compare the experimental results with the numerical findings when the tapered region of SPF structure is filled with different analytes. The obtained sensitivity of the analytes is received as 1.207 V/RIU with the adjacent R2 is 0.988.This designed model and setup could be exploited for the realization of building blocks for sensing applications. Furthermore, the investigation of different sizes of core region, tapered layer, and sensing region could be adjusted and optimized to improve the sensing performance of new types of SP-SMF. The simulation and experimental results display the significant performance of the SP-SMF sensor and can be a useful measuring device for future research study.

Disclosures:

The authors declare that there is no conflict of interest.

Funding:

This research is funded by Instituto Tecnologico y de Estudios Superiores de Monterrey, Mexico.

Author Contribution

Conceptualization, S.L.-A and M.G.; Methodology M.M.B and A.T.; Validation, M.G and S.L.-A.; Formal analysis, M.M.B.; Data curation, A.T.; Writing—original draft, M.M.B.; Writing—review & editing, A.T.; Supervision, S.L.-A and M.G. All authors have read and agreed to the published version of the manuscript.

Acknowledgment:

We are thankful by the support and help of Conacyt Organization, Mexico. We also acknowledge the fruitful discussion with Liam O’ Faolain, Tyndall National Institute, T12 PX46 Cork, Ireland.

Data availability:

Data implicit in the results submitted in this report are not publicly accessible at this time but may be obtained from the authors upon reasonable request.

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No competing interests reported.

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Numerical and experimental analysis of side polished single mode fiber for refractive index sensing

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Abstract

Figures

1. Introduction

2. Geometrical structure

3. Numerical results

4. Experimental setup

5. Results and discussion

6. Conclusion

Declarations

Disclosures:

Funding:

Author Contribution

Acknowledgment:

Data availability:

References

Additional Declarations

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