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Abstract
In this work, we have shown the capability to produce Fabry-Pérot (FP) cavities based on photopolymerizable resins sandwiched between two single-mode fibers. The process allows easy control of the length of the cavity and an enhancement on the fringe visibility when compared with standard droplet based FP sensors. The method will be employed for the fabrication of four sensors composed of different photopolymerizable resins. Their performance regarding humidity, temperature, pressure and refractive index will be under analysis, revealing that in some cases it is possible to reach better sensitivities than the ones reported in literature.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nowadays, optical fiber sensors (OFS) are widely spread in many areas, such as defense, automotive, biomedical, manufacturing, aerospace, among others. The reason is due to their unique characteristics when compared with traditional electric sensors, which include: immunity to electromagnetic interference, small size, passive/low power, long distance, high resolution, multiplexing capabilities, ability to respond to a variety of measurands, operation in harsh environments [1], etc. OFS can be found in four main categories which comprises the ones based on intensity; spectrometric (wavelength based, i.e. Bragg sensors); polarimetric and interferometric ones. Among those, the interferometric sensors such as the Fabry-Pérot (FP) sensors are very promising since they can be precise, simple, versatile, high sensitive and provide multiparameter opportunities. Generally speaking, a fiber optic FP sensor can be constructed in its simplest form by the use of two in-line fiber reflectors which can be done through the employment of two fiber end faces aligned through their centers and separated by a small distance. The light reflection from each reflector interferes with each other creating an interference spectrum. By measuring the shift of the phase or wavelength spectrum, the sensing parameter applied to the FP sensor can be quantitatively obtained. Sensing opportunities include the ability to measure strain [2], temperature [2–6], pressure [7, 8], refractive index [3, 9, 10], relative humidity (RH) [5, 11, 12], pH [13], magnetic field [14], airflow [15], liquid level [16], etc. Thanks to that, some of these FP sensors are already on the market [17]. Despite that, huge efforts have been developed by the research community in order to improve their fabrication process, sensitivity and selectivity [1, 18]. FP sensors have been fabricated through several methodologies, such as the splicing of hollow core fibers and photonic crystal fibers between single-mode fibers (SMFs) [4]. However, this kind of FP sensors are normally difficult to fabricate, where hand-made processes are essentially employed, leading to have problems of reproducibility, repeatability, and long-term stability [1]. Chemical etching of graded index multimode fibers [2], has also been shown to give good results, yet, they involve the use of dangerous chemicals which require special care and proper tools. The use of other technologies, such as the ones based on femtosecond lasers [9], focused ion beam [3], 157 nm excimer lasers [13], have shown promising results, however the cost of such equipment is relatively high.
The interesting characteristics of polymers such as low Young's modulus (E), ability to absorb water from the environment, operation in high elastic regimes [19], etc., led the scientific community to use different kinds of transparent UV photopolymerizable resins to act as FP cavities [5–7, 12, 15, 16, 20–25]. Additionally, the use of these type of resins allows easy and faster fabrication procedures. Nevertheless, the wide availability of UV photopolymerizable resins and suppliers makes their use appealing. The technology has been mainly employed by incorporating the resin into hollow core fibers or PCFs, that is then harden by UV gun [5–7, 16, 23, 26]; or by dipping an SMF fiber in a resin container, to allow the creation of a droplet at the tip of the fiber [12, 15, 22, 24, 25], which is then hardened by UV exposure. Despite the easy fabrication process for the latter case, the control of the cavity length is difficult to achieve, even considering the deposit of several layers of resin as shown in [21, 22], or by etching the silica fiber as shown in [25]. Additionally, the interferometric fringes of those FP cavities appear with low visibility [12, 21, 22, 25], which is related with the non-perpendicular surface orientation of the droplet resin with respect to the fiber axis, which compromises the sensor resolution. The photopolymerizable resins employed so far are formulations of Norland products, with NOA61 [12, 22, 25] and NOA65 [21, 23], the most used ones. Nevertheless, depending on the chemical composition of the resin, different properties could benefit the application, leading the resin choice for a specific application a tuff task.
In this work it will be shown a new methodology to create FP cavities composed of photopolymerizable resins with possibility to control the cavity length by positioning two SMFs with a specific distance between each other. The two end faces of the fibers will allow the creation of fringe interferences with good quality, which becomes a resolution advantage when compared with droplet resin based FP sensors. The FP cavities will be formed using four different photopolymerizable resins, allowing the comparison between them in what concerns humidity detection in either sensitivity and response time; temperature sensitivity; pressure sensitivity and refractive index sensitivity. This work is thus intended to give an easy and affordable way to create FP sensors with good quality, giving also an overview of the possibilities and properties of different resins acting as FP sensors.
2. Concept
One way to classify a Fabry-Pérot sensor is based on the fiber type, which include single-mode and multimode, as well as microstructured fibers, tapered or microfibers, etc. Among those, the SMF has the lowest transmission loss and also the lowest cost, thanks to the development of optical fiber communications, making it, an excellent candidate for the fabrication of FP sensors. The proposed FP cavity is shown in Fig. 1. It is composed of two standard SMFs sandwiching a small portion of a photopolymerizable resin, with cavity length L, and diameter of 125 μm.
Fig. 1 Schematic of the FP sensor composed by a photopolymerizable resin sandwiched between two SMFs.
For fiber tip surfaces with low reflectivity, the system presented here can be described by the two-beam interference model. Thus, the light beam traveling along the fiber core shown at the left part of Fig. 1, will be reflected in the first interface ((I), before the resin FP cavity) and also at the second interface ((II), after the resin FP cavity), and then recombined in the fiber core, resulting in an interference fringe pattern at the output signal. The optical phase difference between the two beams can be described by:
where λ is the free space wavelength and OPD refers to the optical path difference, described as OPD = 2nL, being n and L the refractive index and length of the medium filling the cavity. Therefore, considering that the intensities of the reflected beams by the interfaces I and II are I1 and I2, respectively, the intensity interference signal as a function of wavelength can be written as:For a certain peak/dip in the spectrum, its phase can be expressed by 2mπ (being m an integer). Thus, the wavelength at the peak/dip λm is expressed as:Thus, considering two adjacent peak/dip wavelengths, the free spectral range can be expressed as:From Eq. (4), it can be seen that ΔλFSR is function of the cavity length, wavelength and refractive index of the medium. Concerning the fringe visibility, it can be calculated from the well-known expression:being Imax and Imin, respectively the maximum and minimum values of I given in (2). Several groups prefer fringe contrast (expressed in dB) instead of visibility and thus, the fringe contrast (FC), is defined in this work as FC = −10log(1−V).When the FP cavity is subjected to an external perturbation, the wavelength shift can then be expressed as:
being Δn and ΔL the induced changes in n and L, respectively, due the external perturbations and εZ = ΔL/L, the axial strain.3. Sensor fabrication
The sensor was constructed by employing a single-mode fiber pigtail fused to a common silica SMF-28e, composed by 8.2 μm core and 125 μm cladding, with a nominal effective refractive index at 1550nm of 1.468. The fiber end is crushed in order to avoid back reflections allowing to reduce the signal background noise. Then, a Fujikura CT-30A fiber cleaver was used to cleave the fiber in a distance of few millimeters from the crushed region. Each end of the fiber is then fixed with magnets to separate v-grooves which are attached to a xyz translation stage (MBT616D/M from Thorlabs), with adjustment screws resolution of 1 μm. For precise alignment of the fibers related to each other, two cameras coupled to telecentric lens with 2X magnification were placed orthogonally to the fibers. A picture of the setup employed for the fabrication of the resin FP cavities may be seen in Fig. 2(a). The alignment procedure is then started by axially aligning the movable fiber, related to the static one. Later, the movable fiber is aligned longitudinally, in order to carefully touch the second fiber end. The reading on the micrometer is taken as the zero reference. This allows the precise control of the distance between the fiber tips (cavity length). The movable fiber is then relocated with the desired distance, which will define the cavity length of the sensor. A small drop of photopolymerizable resin is then taken from the resin container by another fiber tip and is then carefully placed between the fibers. The surface tension between the fibers crated by the resin allows the excess resin to be removed by passing a clean fiber through the droplet, allowing the formation of a FP cavity with a diameter similar to that of the fiber. By doing that, it is allowed the creation of sensors with similar dimensions. To finalize the sensor fabrication, the photopolymerizable resin is harden during ten seconds by side illumination with the help of an hand-held UV source (Opticure LED200 from Norland Products Inc.), with wavelength of 365 nm and power of 2.5 W/cm2. The full fabrication process can be seen on Visualization 1, while an image taken during the fabrication (i.e. UV hardening) and the microscope image of the final sensor can be seen in Fig. 2(b) and (c), respectively.
Fig. 2 (a) Picture of the setup used for the FP cavity fabrication, composed by a xyz translation stage that align the fibers with help of two side cameras. (b) FP cavity during the fabrication process (UV side illumination). (c) Microscope image of the final FP sensor, composed by photopolymerizable resin sandwiched between two SMFs.
It is worth to mention that the fabrication process described here was made through a dedicated setup. However, if proper care is taken, conventional fiber splicers could be employed in a more affordable way, as described in [21] for the creation of a droplet at the tip of a fiber.
Even for fully cured resins, the optimal adhesion can only be achieved by an aging process. For that reason, the fabricated sensors were placed in a thermal chamber at 70 °C during 12 hours.
4. Sensor characterization
The characterization of the fiber sensors was made in reflection using a static optical sensor interrogation module, which comprises an internal swept laser, circulator and detector (sm125 from Micron Optics). This device operates at the 1550 nm region, where most of the devices and technologies are already developed. The device provides a wavelength range from 1510 to 1590 nm, and offers measurements with high resolution (1 pm).For comparison purposes four different photopolymerizable resins were employed as the active element (i.e. NOA78, NOA85; NOA86H and Loctite3525), being the NOAs obtained from the Norland Products, Inc. and Loctite from Henkel. These resins have a cured refractive index at 589 nm of 1.50, 1.46, 1.55, 1.51, a Young’s modulus of 7.9, 64.4, 2484.9, 172.4 MPa and a temperature range of −20 to 60, −15 to 90, −125 to 125, −54 to 149, respectively for the NOA78, NOA85; NOA86H and Loctite3525 [27, 28]. The cavity was defined to be short in length, allowing to reduce the insertion loss and to obtain high fringe contrast which benefits the resolution of the sensing application. All the cavities were fabricated with a length of 30 μm and a diameter of 125 μm, allowing the comparison of their responses to different external parameters. The reflection spectra of the fabricated cavities can be seen in Fig. 3.
Fig. 3 Reflection spectra of the fabricated FP sensors, respectively for the: (a) NOA78; (b) NOA3525; (c) NOA86H and (d) NOA85.
The spectra presented in Fig. 3 appear with good fringe contrast, achieving values of 18.6, 23.2, 19.0 and 31.6 dB, corresponding to a visibility of 0.9862, 0.9952, 0.9874, 0.9993, respectively for NOA78, Loctite3525, NOA86H and NOA85 FP cavities. It is worth to mention that high fringe visibility is preferred in sensing application, as it allows high resolution on the detection of the peak/dip wavelength. Considering the free spectral range, which could be considered constant for the short wavelength range used in this work (80 nm), NOA78, Loctite3525, NOA86H and NOA85, presented values of 22.4, 21.9, 21.5 and 27.7 nm, respectively. The high value obtained for NOA85 when compared with the other FP cavities is related to its lower refractive index value.
The fabricated sensors were characterized to relative humidity, temperature, hydrostatic pressure and external refractive index. For the humidity and temperature tests, the FP sensors were placed in a climatic chamber (Angelantoni CH340), with accuracy of 0.3 °C and 1%RH for the temperature and humidity, respectively. The humidity tests were performed at constant temperature of 25°C, and swept from 30 to 90%RH in 15%RH steps. Each humidity step was conducted during 2 hours, in order to allow a complete absorption of the moisture in the chamber. For the temperature characterization, the relative humidity was kept at 30%RH and the temperature was swept from 25 to 45 °C in steps of 5°C, with 1 hour duration. For both humidity and temperature tests, the acquisition of the spectra was taken every 2 minutes. A picture of the characterization setups can be seen in Fig. 4(a).
Fig. 4 Pictures of the setups used for: (a), (b) humidity and temperature characterization, by placing the FP sensors in a climatic chamber; (b) hydrostatic pressure by introducing each FP sensor in the pipe that is connected to the oil pump; (c) refractive-index characterization by placing the FP sensors in different salt-water solutions (the black lines and red dots were drawn for clarity purposes).
Considering the pressure tests, the FP sensor heads were placed in a pipe connected to an hydraulic pump (p-18, Enerpac), see the picture of the setup used in Fig. 4(b). The sensors were left in the chamber (filled with oil), during 2 hours allowing signal stabilization. The tests were made at constant humidity (sensor surrounded by oil) and at room temperature of 22°C. The tests were done by pressurizing the pipe from 0 to 200 bar in steps of 34.5 bar and with resolution given by the manometer as 3.4 bar.
For the refractive index tests, five solutions of water and salt were prepared. The refractive index was measured at 589 nm and 25 °C, using a refractometer (AbbematTM200), with resolution of 1x10−4. The FP sensors were secured vertically and placed in each solution. The spectral response was then taken after two hours, allowing the stabilization of the optical signal. A picture of the setup used for the refractive index characterization may be seen in Fig. 4(c).
5. Results
As known, polymers are prone to absorb the water from the environment and thus, their response to humidity is a time dependent process. Since the dimensions of the FP cavities are equal, the temporal response of the sensors is only dependent on the resins composition. Thus, different absorption rates can be achieved. In order to know the response time of each fiber sensor, the wavelength change of the dips of the spectra shown in Fig. 3 where measured during the humidity step change from 30 to 45%RH and from 45 to 30%RH. For comparison purposes the dip wavelength shifts were normalized to the maxima and they can be seen in Fig. 5(a) for the humidity increase and Fig. 5(b) for the humidity decrease.
Fig. 5 Normalized wavelength shift evolution for different resin based FPIs, considering constant temperature of 25 °C and a step humidity increase from 30 to 45%RH (a); and a step humidity decrease from 45 to 30%RH (b).
The time taken for each sensor to reach 90% of its final value was considered to calculate the response time. Such condition reveals a time response of 16, 40, 44 and 56 min for the humidity step increase (30 to 45%RH), and values of 15, 36, 43 and 55 min for the humidity step decrease (45 to 30%RH), respectively for the Loctite3525, NOA86H, NOA78 and NOA85. The values obtained here are similar to the ones found for PMMA based fibers [29]. Nevertheless, it is worth to mention that the time response can be further reduced by employing shorter cavity lengths which is an easy task to implement with the proposed fabrication process.
When the cavities are in the presence of moisture, the water tends to drive into the polymer resin. Once this occurs, the medium refractive index is lowered as a consequence of the lower refractive index of water when compared with the ones of the polymer resins. Additionally, the water intake will promote the swelling of the resin, leading to an increase of the length of the FP cavity. Furthermore, the induced axial strain will be dependent on the toughness of the polymer, which acts as a resistance to the length change. Nevertheless, if one considers a decrease of the relative humidity, the opposite to what was described before will be observed. The results concerning the spectra wavelength shift for the different sensors can be seen in Fig. 6.
Fig. 6 Reflection spectra obtained at 25°C for different humidity conditions, considering the FP sensors based on: (a) NOA78; (b) Loctite3525; (c) NOA86H and (d) NOA85.
As can be seen from Fig. 6(a) and (b), the dips undergo a phase shift higher than 2π. Thus, to allow the measurement by the wavelength tracking method a software routine was created in order to search the dip wavelength of the spectrum in a small wavelength range. Then, the calculated dip wavelength shift of the next spectrum is used to shift the wavelength detection range for the subsequent spectrum. By doing that, it was possible to track the dips as shown in Fig. 6(a) and (b). By representing the calculated dip wavelength shifts as function of humidity, as shown in Fig. 10(a), it is possible to verify the existence of a non-linear behavior. This is in accordance with what is described in literature for polymer based materials [30–32]. This occurs because water is clustered in the polymer matrix leading to an absorption increase at higher humidities [32]. Based on that, we decide to adjust the data points to the following quadratic expression:
where Δλ is the wavelength shift of the dip of the FP interference fringe, as a result of changes in RH, being α1, β1 and γ1 the coefficients obtained from the fit shown in Fig. 10(a). Their corresponding values are shown in Table 1:
Table 1. Fitting parameters of FP cavities based on different UV resins, concerning the humidity tests
Nevertheless, due to simplicity reasons, most of the research community has implemented first order linear fits [5, 12, 22]. Therefore, in order to compare our results with the ones reported in literature, we have also implemented linear fits to the experimental data. From that we obtained sensitivity values of 573.8 ± 14.4, 367.5 ± 9.2, 95.3 ± 2.4 and 74.1 ± 1.9 pm/%RH, with a coefficient of determination (R2) of 0.995, 0.985, 0.993 and 0.994, respectively for the NOA78, Loctite 3525, NOA85, and NOA86H. Based on the values provided by the coefficient of determination, we can observe that they are close to the unit, revealing that the linear fits are still acceptable. In order to give an explanation for the obtained sensitivities, we provide here the Young’s modulus of the resins, which are 7.9, 172.4, 64.4, 2484.9 MPa, for the NOA78, Loctite 3525, NOA85, and NOA86H, respectively. Thus it can be seen that the FP based on NOA78 is the one with the highest sensitivity and the lowest E, while the FP based on NOA86H presents the lowest sensitivity with the highest E. Additionally, since the dip wavelength shift follow a red-shift with increasing humidity, we can realize from Eq. (6), that the strain effect induced by the thermal expansion is more pronounced than the negative refractive index change induced by the lower refractive index of water. Comparing the sensitivity values with the ones found in literature for similar FP sensors based on other resins but similar cavity lengths, it can be seen that NOA78 and Loctite3525 offer a much higher sensitivity values than the ones presented in [5, 12, 22].
Regarding the temperature tests, it can be seen on Fig. 7, the reflection spectra obtained for different temperatures at constant humidity.
Fig. 7 Reflection spectra obtained at 30%RH for different temperature conditions, obtained for the FP sensors based on (a) NOA78; (b) Loctite3525; (c) NOA86H and (d) NOA85.
By tracking one of the dips shown in Fig. 7, it was possible to determine the wavelength shift and thus, the sensitivity could be calculated from a first order linear fit as shown in Fig. 10(b). The positive wavelength shift with increasing temperature is the result of the net contribution of the thermo-optic and thermal expansion effects of the polymer resins, described as:
where ξ and α are the thermo-optic and thermal expansion coefficients of the polymers employed, respectively. Since ξ is negative in polymers, we could infer that α is the prominent effect. Concerning the calculated sensitivity values, which were 262.1 ± 2.0, 363.2 ± 2.7, 369.5 ± 2.8, 458.9 ± 3.5 pm/°C, with R2 values of 0.996, 0.997, 0.999 and 0.998 respectively for the NOA86H, NOA85, Loctite3525 and NOA78, we could infer that NOA78 has the higher temperature sensitivity and would be the preferred choice for a temperature sensor. In what concerns the comparison with the sensors reported on literature we can see that NOA78 exhibits higher sensitivity than the sensors based on resins reported in [5, 12, 21, 22, 25], however much smaller than the one found in [6], probably because the wavelength shift in this particular sensor occurs due the combination of the thermal expansion coefficients either from the resin and from the glass capillary. Despite the better sensitivity of the NOA78 FP sensor, the operational temperature is quite low (−20 to 60 °C), when compared with FP sensors based on NOA 86H (−125 to 125 °C); NOA85 (−15 to 90 °C) and Loctite3525 (−54 to 149 °C). For that reason, a compromise between the operational temperature and sensitivity needs to be taken into account when developing a temperature sensor based on UV curable resins.Results concerning the pressure tests, regarding the reflection spectra for the different pressure step conditions, can be seen in Fig. 8.
Fig. 8 Reflection spectra obtained for the pressure tests, for the FP sensors based on (a) NOA78; (b) Loctite3525; (c) NOA86H and (d) NOA85.
As can be seen from Fig. 8(a)-(d), and better visualized on the inset of those figures, the reflection spectrum is blue shifted when the hydrostatic pressure is increased. The wavelength shift is due to two different effects as described in Eq. (6). In fact, when the hydrostatic pressure is applied to the FP cavity a refractive index change will occur due the photoelastic effect and an axial strain owing to the physical compression. Concerning the sensitivity values which were calculated from the linear fits shown in Fig. 10(c), we can see that the sensitivity values are similar between the FP sensors, (−8.5 ± 0.1, −9.9 ± 0.2, −10.7 ± 0.1, −15.7 ± 0.3 pm/bar, with R2 values of 0.998, 0.986, 0.999, 0.993, respectively for NOA86H, NOA78, NOA85 and Loctite 3525 FP cavities). Compared with literature, these values are 10 times lower than the ones shown in [21] for a polymer droplet. The reason is mainly due to the pressure induced strain effect, being more effectively distributed around the droplet resin sensor, than the configuration presented in this work.
Results concerning the refractive index tests can be seen in Fig. 9 for different refractive index solutions.
Fig. 9 Reflection spectra obtained for different refractive index solutions,, obtained for the FP sensors composed of (a) NOA78; (b) Loctite3525; (c) NOA86H and (d) NOA85.
The results shown in Fig. 9(a)-(e), revealed a blue-wavelength shift with increasing concentration of the solution. By tracking one of the dips shown in these figures, it was possible to plot the wavelength shift as function of refractive index, as shown in Fig. 10(d).
Fig. 10 Sensitivities obtained for the four sensors developed, concerning the characterizations to: (a) humidity, (b) temperature; (c) pressure and (d) refractive index.
Regarding literature, silica FP refractive index sensors that operate in wavelength are composed of hollow fiber structures that allow the entrance of the solution to be analyzed. Such configuration leads to the occurrence of a red wavelength shift with increasing refractive index [9]. However, the explanation of the dip wavelength shift for the polymer FP cavities developed in this work cannot be applied. In fact, the process behind the wavelength shift in polymer FP cavities is completely different. Instead, an osmosis process as already reported for PMMA based polymer fibers [33], is more adequate and for that reason it will be employed in the description of these results.
The beginning of the refractive index tests started by placing the FP sensors in pure water during two hours, allowing the polymer matrix to become swollen as a result of an osmotic process. This occurs since the water pressure outside the sensor is higher than the one inside the polymer. As a result of this osmotic pressure, the water will drive to the polymer until the saturation is achieved. However, if a solute is added to the water, then a hydraulic pressure will drive the water from the polymer to the solution, until reaching the equilibrium. In this case the polymer will shrink, and considering the presented FP sensors, it will impose a decrease of the FP cavity length, leading to have a blue dip wavelength shift as presented in Fig. 10(d). Similarly to what occurs in the humidity characterization tests, the dip wavelength shift follows a non-linear behavior with increasing refractive index of the solution. Because of that, we decide to adjust quadratic fits to the experimental data, according to the following equation:
being Δλ2 the dip wavelength shift of the FP interference fringe, as a result of a change in refractive index (RI) of the surrounding solution. The coefficients α2, β2 and γ2 where obtained from the quadratic fits adjusted to the experimental data as shown in Fig. 10(d). Their values can be seen on Table 2.Table 2. Fitting parameters of FP cavities based on different UV resins, concerning the refractive index characterizations
Again, similarly to what was made for the humidity tests, it was decided to also adjust linear fits to the experimental data, allowing better comparison with some works reported on literature [9, 10, 20]. Regarding those results, it can be seen that NOA78 and Loctite3525 FP cavities, are the ones with higher sensitivity, achieving values of −1046 ± 2 nm/RIU, with R2 value of 0.978 and −1320 ± 2 nm/RIU with R2 value of 0.993, respectively, contrasting with the lower sensitivities achieved for NOA86H and NOA85 which revealed values of −36.63 ± 0.04 with R2 value of 0.983 and −45.95 ± 0.05 nm/RIU with R2 value of 0.993, respectively. Again, the R2 value gives values close to the unit, revealing confidence on the adjustment. Concerning the literature results, these sensors, namely the ones based on NOA78 and Loctite3525, present higher sensitivities than the ones presented for NOA68 based FP cavity fabricated on the tip of an etched multimode fiber [20] and also higher than the one presented for hole fiber structures [9]. However, they are much smaller than the sensitivity values of 11500 nm/RIU presented in [10]. Nevertheless, it is worth to mention that these type of sensors have poor response time due the time needed for the water uptake by the polymer as reported for the humidity results. Besides that, the reduction of the cavity length or diameter could be a possible solution to improve the sensors response time.
6. Conclusion
This work reports the fabrication of a new FP sensor based on photopolymerizable resins. The sensor fabrication is easy to execute, allowing the creation of FP cavities with specific length. Additionally, thanks to the use of two SMF terminals acting as mirrors for the FP cavity, it is allowed an enhancement in fringe visibility when compared with conventional droplet resin FP based devices. Furthermore, this work explores the use of four different photopolymerizable resins for the fabrication of FP sensors. A comparison between the different sensors in what concerns humidity, temperature, pressure and refractive index sensitivities was explored and the mechanisms behind the sensitivity values explained. Concerning the humidity results, it was shown that humidity detection is a time dependent process, being their response dependent on the polymer resin employed. In the overall, it was shown that FP sensors composed of NOA78 and Loctite3525 offered the best sensitivities and in some cases also better than the ones reported in literature. Furthermore, it was explored for the first time the capability to measure refractive index in the wavelength domain in a resin based FP sensor. The explanation of the phenomenon behind the sensitivity was explained recurring to a similar phenomenon already described for PMMA based plastic fibers.
In the overall, this work shows a new FP based sensor since their fabrication to their characterization to different external parameters, allowing a comparison between different photopolymerizable resins, paving the way for a more intense use of resin FP based sensors with enhanced characteristics, such as simplicity, versatility, reliability and cost effectiveness.
Funding
FCT-Fundação para a Ciência e Tecnologia through Portuguese national funds hiPOF (PTDC/EEI-TEL/7134/2014, investigator grant IF/01664/2014).
Acknowledgments
The authors would like to acknowledge Cristiano M. B. Cordeiro and Jonas H. Osório from Instituto de Física “Gleb Watagin”, Universidade Estadual de Campinas, from their help on the assembly of the pressure chamber circuits.
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