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Abstract
A negative axicon micro-cavity polydimethylsiloxane (PDMS) filled Fabry-Perot interferometer (FPI) based sensor for accurate temperature sensing is proposed and demonstrated. The micro-cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm are employed for temperature sensing from 27 °C to 80 °C. The sensing probe responses were measured in terms of wavelength shift resulting from the thermo-induced change in the FP cavity length and in the refractive index of the PDMS filled in the cavity. However, the effect of thermal expansion/contraction of PDMS is found to be more dominating over the change in its refractive index. The highest and least sensitivity of the order of 59 pm/ °C and 24 pm/ °C are observed for 130 µm and 260 µm cavity lengths. The linear relationship between the change in spatial frequency and cavity length with respect to temperature variation are also studied. These miniaturized and stable sensor probes are capable of measuring small change in temperature variation with high accuracy and sensitivity and can be used for remote sensing measurements.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Accurate temperature sensing is being keen interest for researcher’s due to its various applications in bioscience, pharmaceutical and food science etc. To fulfill their requirements a highly sensitive, stable, biocompatible, reliable and durable sensor is always on anvil. Fiber optic-based sensors could meet such demands and proved to be suitable for such measurement due to its inert nature, immune to electromagnetic interference (EMI), and distributed sensing ability. Various fiber optic configurations were proposed and fabricated for temperature measurement including Mach-Zehnder [1], Sagnac loop [2], and Fabry-Perot interferometric (FPI) [3]. Certain geometries including thin film inside the single mode fiber (SMF) [4], polymer coating at the distal end of the fiber [5,6], long period gratings (LPGs) [7], photonic crystal fiber-Fiber Bragg gratings (PCF-FBGs) [8], femto-second laser micromachining [9], hollow core PCF-SMF [10], and liquid filled PCF [11] were demonstrated for the temperature sensing.
Among the various interferometers, FPI based geometry could be the most suitable for temperature measurement due to its simplicity, high sensitivity and remote sensing ability. Some advancement has been done in the field of FPI-based tip sensors such as cantilever-based [12], polymer coated [13,14], monolithic photonic crystal based [15] and nano-patterned based [16] for temperature sensing. However, some of these sensors possesses low thermal expansion coefficient, low mechanical stability and durability and thus limits it uses in on-field temperature sensing applications. Therefore, to overcome these limitations there is a need of highly sensitive, mechanically stable and a reliable sensor which could fulfill the requirement of the industry.
Considering the above limitations in fabricating the FPI cavity at the tip of an optical fiber, we have proposed and fabricated a negative axicon micro-cavity filled with Polydimethylsiloxane (PDMS) for temperature measurement. PDMS is chosen for sensing due to its characteristics such as biocompatible, non-toxic, optically transparent and possess very high thermal expansion coefficient of the order of 4.66 × 10−4/ °C [17]. Hence, PDMS filled negative axicon cavity could be a promising sensing platform for temperature measurement. This type of cavity is existing inside the optical fiber and thus mechanically stable. As the PDMS is filled inside the cavity, there will be relatively less impact of humidity on it and becomes mainly sensitive to the temperature variation. This type of cavity possesses large cavity volume, so that we can tune the cavity length accordingly. Therefore, this sensor possesses many interesting features like cost-effectiveness, simple to construct and mechanically stable and thus may improve the accuracy of the temperature measurement.
2. Fabrication and experiment
The schematic of a negative axicon tip micro-cavity fabrication of the PDMS (SYLGARD 184) filling inside an air-cavity is shown in Fig. 1. The sensor tips were fabricated by using simple hydrofluoric (HF) acid treatment in which the Cladding Mode Offset Photosensitive Single Mode Fiber (CMOP-SMF) (highly Ge doped optical fiber with core diameter ∼3.5 µm, Nufern, USA) was immersed in HF acid solution (concentration 48%) as shown in Fig. 1(a). The total etching time of fabricating the air-cavity inside the fiber was around 40 minutes. The core of the highly Ge doped fiber etched faster than the cladding and hence it forms an air-cavity inside the fiber. The quality of the fabricated micro-cavity in terms of surface roughness and uniform etching was confirmed through beam profile before PDMS filling [18]. The cavity length can be controlled by varying the etching time. As shown in Fig. 1(b), the PDMS was taken at the tip of a positive axicon fiber and filled inside an air cavity of a negative axicon fiber with the help of high precision XYZ translation stage (561D-XYZ ULTRAlign 3 axis stage, Newport). It observed that the lengths of the PDMS filled micro-cavities were of the order of ∼130 µm, ∼160 µm, ∼240 µm and ∼260 µm respectively.
Fig. 1. Schematic of negative axicon micro-cavity fabrication (a) etching (inset shows the fabricated probe) and (b) PDMS filing (inset shows the micro-graph of fabricated sensor probe).
The schematic of the proposed temperature sensor set up is shown in Fig. 2. It consists of an Amplified Spontaneous Emission (ASE) light source (1525-1565 nm, Output power 28 mW, Optolink Corp. Ltd., Hong Kong), a circulator (SMF-28, 60:40 ratio, Optolink Corp. Ltd., Hong Kong), an Optical Spectrum Analyzer (OSA) (AQ6370C, wavelength resolution 20 pm, 600-1700 nm, Yokogawa, Japan) and a temperature oven. The negative axicon filled with PDMS sensor was spliced at the distal end of the circulator. The light from PDMS-Air interface reflects and gets coupled in to the OSA to generate an interference pattern. The temperature response of all sensor probes were measured by placing it inside the temperature oven with gradually increasing the temperature from 27 °C to 80 °C with step of 10 °C. During each measurement, the temperature of the oven was verified by placing a thermometer inside the oven. With respect to temperature variation, the corresponding interference spectrums were measured and studied for further analysis.
Fig. 2. Schematic diagram of an experimental set up of FPI-based PDMS filled micro-cavity for temperature sensing. Inset shows micrographs of micro-cavity lengths of i) 130 µm, ii) 160 µm, iii) 240 µm and iv) 260 µm respectively.
3. Theoretical discussion
Due to temperature effect, the properties of the PDMS changes which results into the change in cavity length. The optical path length (OPL) inside the FP cavity for round trip and free spectral range (FSR) is given by [4,19]:
where n is the refractive index of the FP cavity and L is the length of the FP cavity. The change in OPL due to temperature variation inside the FP cavity is the function of the refractive index of PDMS and the cavity length is given by [4,10]:Furthermore, the change in length of the cavity either due to thermal expansion or contraction of PDMS, which induces change in dip wavelength ($\Delta {\lambda _{dip}}$) of an interference spectrum is given by [4,13]:
Similarly, change in the refractive index of PDMS further induces the dip wavelength shift in an interference spectrum. Thus, the change in wavelength shift in terms of temperature variation is given by [10]: The temperature also induces change in spatial frequency of wavelength spectrum and therefore, the spatial frequency shift due to temperature variation can be calculated by [19]: where $\lambda $ is the laser source wavelength and $\Delta \xi $ is the amount of spatial frequency shift due to temperature variation.4. Results and discussion
The reflection spectrums of all sensor probes were recorded on the OSA in the temperature range from 27 °C to 80 °C. Figure 3 (a, c, e, g) shows that as the temperature increases from 27 °C to 80 °C the corresponding dip wavelength shift also increases. The dip wavelength shift was attributed due to the change in the properties of the PDMS in terms of change in cavity length resulting from the temperature effect. To understand the behavior of an interference fringes in detail, the spatial frequencies were calculated by applying the Fast Fourier Transform (FFT) for all micro-cavity of lengths 130 µm, 160 µm, 240 µm and 260 µm as shown in Fig. 3 (b, d, f, h). The theoretical FSR [13] value was calculated by using equation as $FSR = \;\ \frac{{{\lambda ^2}}}{{{l_{OPL}}}}$ and found around 6.79 nm, 5.46 nm, 4.14 nm and 3.45 nm which corresponds to the theoretical spatial frequencies around 0.14 nm−1, 0.18 nm−1 0.24 nm−1 and 0.28 nm−1 for cavity length of 130 µm, 160 µm, 240 µm and 260 µm respectively. Experimentally, the spatial frequency peaks in FFT spectrums were observed around 0.14 nm−1, 0.18 nm−1, 0.23 nm−1 and 0.29 nm−1 as shown in Fig. 3 (b, d, f, h), which are nearly match with the theoretical values. Thus, the theoretical and experimental study confirms that the FP cavity is present inside the fiber.
Fig. 3. The spectral response and its spatial frequency spectrum obtained by taking the FFT for each PDMS filled micro-cavity FPI with temperature variation from 27 °C to 80 °C for each cavity length of (a, b) for 130 µm, (c, d) for 160 µm, (e, f) for 240 µm and (g, h) for 260 µm respectively.
The change in dip wavelength shift with respect to change in temperature variation is shown in Fig. 4, which shows the linear relation of dip wavelength shift with temperature variation from 27 °C to 80 °C for micro-cavity of lengths 130 µm, 160 µm, 240 µm and 260 µm respectively. The observed sensitivity of the fabricated sensors in the heating and cooling mode from temperature range of 27 °C to 80 °C were observed around 59.44 pm/ °C and 58.56 pm/ °C, 50.50 pm/ °C and 45.85 pm/ °C, 44.01 pm/ °C and 36.09 pm/ °C, 24.09 pm/ °C and 22.97 pm/ °C for micro-cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively. The approximate maximum error observed in the measurement were of the order of ± 0.54 nm, ±0.16 nm, ±0.16 nm and ± 0.16 nm for micro-cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively.
Fig. 4. Relationship between dip wavelength shift Vs temperature in heating and cooling mode for micro-cavity lengths of a) 130 µm, b) 160 µm, c) 240 µm and d) 260 µm respectively within temperature range of 27 °C to 80 °C.
In cooling mode, all sensors shown linear response with small systematic deviation compared with the results obtained from the heating mode. The reason for systematic deviation during cooling mode can be attributed due to the change in temperature gradient formed inside the cavity. The structure of the micro-cavity allows heating and cooling of the cavity in one direction only. It means during heating mode, the temperature gradient flows from air to PDMS and PDMS to air during cooling mode. The unequal temperature gradient formed inside the cavity during heating and cooling mode could be the reason for this systematic error in the measurement. Such systematic error was observed in three repeated experiment followed the similar trend during heating and cooling mode. However, overall sensitivity for all sensors in heating and cooling mode is found nearly of the same order.
The sensing response of the fabricated probes were tested in the temperature range from 27 °C to 80 °C. The limit of detection (LOD) of these sensors were observed around 0.33 °C, 0.39 °C, 0.45°C and 0.83 °C for cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively and it was limited due to the OSA resolution (wavelength resolution ≈ 20 pm). The response and recovery time of the sensors were around 8-10 seconds. From Eq. (3), it is confirmed that the wavelength shift change was large in case of 130 µm followed by 160 µm, 240 µm and 260 µm cavity lengths respectively. Theoretically, the PDMS thermally expands or contract due to thermal expansion effect and thermo-optic effect and hence it induces the wavelength shift in an interference spectrum. Such expansion/contraction can be confirmed from the cavity length changes which results in change in the sensitivity [17].
As discussed, the PDMS showed good characteristics relation with external temperature variation measured from the thermal and optical effect corresponding to the change in the cavity length. To understand the effect of temperature on the refractive index of PDMS at different temperature range, we measured the actual refractive index of PDMS in temperature variation from 27 °C to 80 °C by using digital refractometer (ATAGO, Rx-7000i, Japan) (shown in Fig. 5(a)). The linear response was observed with change in refractive index value of PDMS from 1.4104 to 1.3915 RIU in the applied temperature range from 27 °C to 80 °C. Thus, as discussed in theory the change in PDMS refractive index with temperature variation is also contributing into the change in wavelength shift.
Fig. 5. (a) Refractive index variation of PDMS with change in temperature, (b) Change in length of micro-cavity for 130 µm, 160 µm, 240 µm and 260 µm in temperature variation from 27 °C to 80 °C.
The change in cavity length in terms of either due to thermal expansion or contraction was calculated from Eq. (3). Figure 5(b) shows the change in PDMS filled micro-cavity lengths due to temperature variation from 27 °C to 80 °C (during heating mode) for cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively. All sensing probes show the linear response with temperature variation. The corresponding sensitivity values were observed around 0.043 nm/ °C, 0.037 nm/ °C, 0.030 nm/ °C and 0.016 nm/ °C in temperature range from 27 °C to 80 °C for the sensing probes of cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively.
Figure 6 shows the spatial frequency shift with temperature variation for different cavity lengths. The maximum spatial frequency shift of around 0.0020/nm, 0.0025/nm, 0.0037/nm and 0.0041/nm were observed with total temperature change 53 °C (temperature range from 27 °C to 80 °C) for cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively. Therefore, corresponding variation in round-trip OPL induced due to the temperature variation were calculated around 4.80 µm, 6.01 µm, 8.88 µm and 9.85 µm for the cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively.
Fig. 6. The spatial frequency shifts Vs change in temperature from 27 °C to 80 °C for cavity lengths of 130 µm, 160 µm, 240 µm and 260 µm respectively.
Though, the effect of temperature on the PDMS through change in OPL study needs to be understood. First, we assume that the change in wavelength shift is induced due to the thermal expansion of PDMS and calculated from Eq. (2) modified as, ${\mathbf \Delta }{l_{OPL}} = {l_{OPL}}\alpha {\mathbf \Delta }T$ and obtained as 2.5 × 10−4/ °C. Therefore, the theoretical value of thermal expansion is 4.66 × 10−4/°C, which is close to the experimentally obtained value [17,20]. Secondly, consider the temperature variation rather than thermal expansion; we obtained thermo-optic coefficient from Eq. (2) as, $\textrm{L}{\mathbf \Delta }\textrm{n} = {l_{OPL}}\beta {\mathbf \Delta }T$ and obtained thermo-optic coefficient as 7.1 × 10−1/ °C. However, actual thermo-optic coefficient of PDMS is reported as −4.5 × 10−4/ °C [21], which is much smaller than the obtained value. Therefore, the thermal expansion of PDMS is more dominating over the change in refractive index due to temperature variation.
The obtained sensitivity is much higher than the sensors reported on FBGs [31] and PCF [8] with comparatively lower fabrication cost. The comparison of performance between the proposed sensor with various reported optical fiber-based sensors are tabulated in Table 1. There is scope to enhance the sensitivity further by changing the length of PDMS cavity. The systematic error can be removed by further changing the thickness of the film to make it more sensitive with optimized parameters.
Table 1. The sensing performance of various developed fiber optic sensors with temperature.
5. Conclusion
The PDMS filled negative axicon tip microcavity-based sensors were proposed and demonstrated for temperature sensing. Due to temperature effect, PDMS thermally expands which changes the cavity length and further induced the wavelength shift in an interference spectrum. The sensors exhibited the sensitivity of the order of 59.44 pm/ °C for 130 µm; 50.50 pm/ °C for 160 µm; 44.01 pm/ °C for 240 µm and 24.09 pm/ °C for 260 µm cavity lengths in the temperature range of 27 °C to 80 °C. We believe that the favorable characteristics of this sensor such as simple, compact, sensitive, and biocompatible nature makes it ideal for temperature sensing for various industrial applications.
Funding
Council of Scientific and Industrial Research (CSIR) (HRDG/CSIR-Nehru PDF/EN, ES&PS/EMR-1/04/2018); Science and Engineering Research Board (SERB) (EMR/2015/001909).
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