- Email: [email protected]
Humidity sensor based on optical fiber attached with hydrogel spheres
Optics & Laser Technology 74 (2015) 16–19
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Humidity sensor based on optical fiber attached with hydrogel spheres Zhi Feng Zhang, Yilei Zhang n School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798 Singapore, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 13 May 2015 Accepted 14 May 2015
This paper reports a new type of intensity-based fiber optic humidity sensor formed by solidifying mini hydrogel spheres on bare fiber cores. Upon variation of relative humidity, the refractive index of hydrogel spheres changes and leads to the intensity change of the transmitted light through the fiber. The dependence of the transmission ratio on the refractive index, size, number and separated distance of hydrogel spheres is simulated and optimized by using commercial software program TracePro. Simulation results suggest great dependence of the transmission ratio on the refractive index change of hydrogel spheres compared to even-thickness coating, because the spherical geometry is much more effective to couple light out of the fiber core. For example, the transmission ratio of the fiber core attached with a single hydrogel sphere in diameter of 2 mm could be reduced to 9% by changing refractive index of hydrogel while 2 mm coating could only achieve 87% for the same fiber core. Larger spheres reduce transmission as expected for longer coupling length. Increasing sphere number also cut transmission but the magnitude become minimal for four and more spheres. Preliminary experimental studies were carried out and demonstrated the idea of the sensor. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Fiber optic sensor Humidity sensor Hydrogel
1. Introduction Fiber optic sensing of relative humidity has been extensively studied and there have been a great number of proposed methods [1]. Sensing relative humidity is critical in various applications including food storage, climate monitoring, and chemical process [2]. Among various sensing methods of relative humidity, fiber optic based ones is preferred in many situations because of their minimal size, immunity to electromagnetic interference, and remote sensing capability. Though a large number of sensing schemes have been reported in literature, there is still a need for low-cost sensors, including both the fabrication and operation cost [3]. Intensity type of fiber optic sensors are the low-cost options compared to those based on the fiber Bragg grating [4], the surface plasmon resonance [5], photonic crystal fibers [6] etc. Those intensity based fiber optic sensor for humidity sensing are normally fabricated by coating hydrogels on the etched fibers. Agarose [7–10] and polyvinyl alcohol are the two most commonly used hydrogels and both silica fibers [7] and polymer optical fibers [11] are adopted previously. It had been demonstrated that agarose coated fiber core is sensitive to relative humidity due to refractive index change of the agarose hydrogel depending on the environment humidity [12]. The intensity of the transmitted light decreased at lower relative humidity. n
Corresponding author. Tel.: þ 65 6790 5952. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.optlastec.2015.05.006 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
This paper reports a new type of fiber optic humidity sensor which is also based on hydrogel coated optical fiber. Rather than coating the bare optical fiber core with an even-thickness coating, the reported sensor exploit hydrogel spheres on the fiber core. Its performance is firstly studied by commercial optical engineering software TracePro, which has been a powerful optical engineer tool and successfully applied in sensor design [13–16].
2. Simulation The sensing performance of the sensor is simulated by TracePro. The fiber model is built based on the fiber patch cable (Thorlabs M28L02) used in experiments, which has a silica core in diameter of 400 μm and a TEQS™ cladding in thickness of 12.5 μm. The refractive index of the fiber core is set to be 1.4517 (pure silica at 625 nm). The refractive index of the cladding is calculated to be 1.40 based on the specified numerical aperture of 0.39. The refractive index of the hydrogel sphere is changed from 1.40 to 1.51. Design parameters including the sphere size, number, and separation distance are investigated.
3. Experiment Protective layers of a segment of a fiber patch cable (Thorlabs M28L02) are removed by appropriate tools and the cladding is
Z.F. Zhang, Y. Zhang / Optics & Laser Technology 74 (2015) 16–19
Fig. 2 shows simulated light rays that travel through a single hydrogel sphere attached on a section of fiber core, illustrating the working principle of the sensor. Light travels in the fiber from the left to the right end. The refractive index of the hydrogel is higher than that of the original fiber cladding and hence some light rays refract into the hydrogel sphere. Because of the spherical geometry, refracted light rays have very small incident angle at surface of the front side of the hydrogel sphere and therefore refract again out of the sphere leading to light loss. This is the main reason that spheres are more effective in coupling light out of the fiber core and accordingly high sensitivity of the proposed sensor. As the refractive index of the hydrogel sphere is changed due to varied humidity, the transmission of light will change accordingly. Simulated transmittance of hydrogel spheres coated fibers is shown in Figs. 3–5. Fig. 3 shows the transmittance of the fiber with a single hydrogel sphere as the refractive index of the hydrogel is increased from 1.40 to 1.51. For all sphere sizes, the transmittance starts to sharply decrease above refractive index of 1.43, and reaches a lowest transmittance around 1.46 and then gradually increases. The size of spheres dramatically influences the transmittance. As the diameter of the sphere increases from 0.6 mm to 2 mm, the lowest transmittance decreases from 62.9% to 9.0%. As a comparison, a uniform coating in thickness of 15 μm and length of 2 mm is included in the figure, which only leads to a lowest transmittance of 86.9%. While a single sphere of 2 mm is capable of reducing transmittance to 10%. Evidently, hydrogel spheres are much more effective in coupling out light from the fiber than the coating, suggesting higher sensitivity of sensors based on spheres.
Transmission ratio of the fiber
100 80 60 40 20 0 1.38
0.6 mm sphere 0.8 mm sphere 1.0 mm sphere 1.2 mm sphere 2mm sphere 2mm c oating
1.4
1.42
1.44
1.46
1.48
1.5
1.52
Refractive index of hydrogel spheres Fig. 3. Dependence of transmittance on the refractive index and size of a single hydrogel sphere.
100 Transmittance
4. Results and discussion
Fig. 2. Illustration of light rays that travel through a single hydrogel sphere in diameter of 2 mm and with a refractive index of 1.45. It should be noted that light loss occurs not only at the hydrogel sphere but also along fiber surface after the sphere, though the magnitude along the fiber is lower than that at the spheres.
80 60
1 sphere 2 spheres
40
3 spheres
20
5 spheres 10 spheres
0 1.38
1.4
1.42
1.44
1.46
1.48
1.5
1.52
Refractive index of hydrogel spheres Fig. 4. Dependence of transmittance on the refractive index and number of hydrogel spheres in diameter of 1 mm.
100 Transmission ratio of the fiber
burnt by flame to obtain bare fiber core. To avoid fiber break, the patch cable is fixed on an aluminum block. Then one or more drops of PEGDMA (Poly(ethylene glycol) dimethacrylate, average molecular weight 750, ALDRICH) hydrogel mixed with 0.5 wt% curing agent is dipped onto fiber core by using a micropipette. Hydrogel droplets are crosslinked for 30 s by a UV lamp (UV Flood, Epoxy & Equipment Technology Pte Ltd). Fig. 1 shows a typical sensor and hydrogel spheres and the spheres are not perfectly spherical due to the influence of gravity and surface tension. Humidity sensing tests are done in a temperature and humidity chamber (ESPEC North America Inc., SH-240). The temperature is maintained at 30 °C for all tests while the relative is set to 90% for two hours, then 80% for one hour, 70% for one hour etc. A fiber coupled LED (Thorlabs, M625F1) is used as the light source and a spectrometer (StellarNet, Silver-NOVA spectrometer) is used to measure the relative intensity of the transmitted light at 631 nm. Transmission ratio is calculated by dividing the relative intensity to that of 90% RH. The PEGDMA was selected because it has a refractive index of 1.465 which is the refractive index for hydrogel spheres that leads to lowest transmittance.
17
80 60
2 mm 5 mm 10 mm 20 mm
40 20 0 1.38
1.4
1.42
1.44
1.46
1.48
1.5
1.52
Refractive index of hydrogelspheres Fig. 5. Dependence of transmittance on the refractive index and separation distance of hydrogel spheres (five spheres in diameter of 1 mm).
Fig. 1. Photo of a fabricated sensor with five hydrogel spheres attached on a section of fiber core. (a) The whole sensor; (b) detail view of five spheres with light input from left side. Bright spots away from hydrogel spheres along fiber core are due to slight damage to the core surface during fabrication.
Fig. 4 illustrates the influence of sphere number on the transmittance of sensing fibers. Expectedly more spheres lead to lower transmittance. For a single sphere, the lowest transmission occurs when the refractive index of the hydrogel is 1.46. For multiple spheres, this critical value of refractive index increases slightly. However, the accumulated transmission for N spheres is higher
5 spheres 4 spheres 3 spheres 2 spheres 1 spheres
75% 85% 95% Relative humidity
Transmitance
100% 90% 80% 70% 60% 50% 40% 30% 20% 65%
100% 90% 80% 70% 60% 50% 40% 30% 20% 65%
Transmitance
Z.F. Zhang, Y. Zhang / Optics & Laser Technology 74 (2015) 16–19
Transmitance
18
100% 90% 80% 70% 60% 50% 40% 30% 20% 65%
0.1 μL 0.2 μL 0.4 μL 0.6 μL
75% 85% 95% Relative humidity
2 mm 5 mm 10 mm 20 mm
75%
85%
95%
Relative humidity Fig. 6. Influence of (a) sphere number, (b) size and (c) separation space on the response of the sensor to relative humidity.
than TN (T for the transmission ratio of a single sphere, N for an integer). For example, transmissions for 1, 2 and 3 spheres with a refractive index of 1.460 are 38.0%, 31.2% and 22.4% respectively, suggesting that presence of the first sphere has influence on the transmission ratio of the second and the third ones. The first sphere always leads to the largest transmission loss due to high percent of light rays that have propagating angle with fiber axis close to the angle of total internal reflection. It is always the light rays that refracts into the hydrogel first and lead to light loss. After each sphere, the percent of light rays with large propagating angle decreases and thus decreases transmission loss. The ultimate result is that the transmission of N spheres is always high than TN. This memory effect is more significant when the separation distance is small, as shown in Fig. 5. For example, increasing the separation distance from 2 mm to 5 mm reduces the lowest transmittance from around 20% to 10%, but further increasing produces no apparent effect, suggesting that 5 mm separation is an acceptable distance for sensor fabrication. Large separation distance results long sensor while too close spheres make it difficult to fabricate and lower sensitivity. Fig. 6 shows preliminary experimental results. The sphere number is varied from 1 to 5. The size of hydrogel spheres is tuned by the volume of the hydrogel solution, from 0.1 to 0.6 μL. The separation distance of 2, 5, 10, and 20 mm is studied. The cured hydrogel sphere is not exactly spherical due to gravity and surface tension (Fig. 1), making it difficult to compare to simulation results. Among the three studied parameters, the sphere number demonstrates biggest impact on the sensitivity of the sensor. Contrast to the simulation, the influence of sphere size is small. The main reason might be that the variation of sphere size is small compared to the range in the simulation. Large hydrogel droplets drip from the fiber due to gravity, and besides the fabricated sphere is not perfectly spherical because of the surface tension. The separation distance has no significant influence on the sensor
sensitivity especially above 5 mm. Typically, for the sensor with 5 spheres, 20% variation in relative humidity leads to a transmittance change of 78%. Due to large fluctuation of the output power of the used LED fiber light source, there is large errors in test results as suggested by long error bars in the figure. Large error is also caused by variation from sensor to sensor. Consequently, accuracy of the fabricated sensors still has considerable room to improve.
5. Conclusion A new type of fiber optic humidity sensor based on hydrogel spheres attached on bare fiber core is simulated and verified by experimental studies, which is easy to fabricate and thus low cost. Most importantly, simulation by TracePro has demonstrated its superior sensitivity compared to even-thickness coatings due to the spherical geometry. The bigger the hydrogel sphere, the lower the transmission of the sensing fiber is. With regard to sphere number, three spheres are almost enough to achieve the lowest transmission while the benefit of further increasing sphere number is minimal. The separation distance of hydrogel sphere also plays a noticeable role on the transmission, but the magnitude is negligible compared to the size and the number. Preliminary experiments of curing pure PEGDMA hydrogel droplets on a multimode silica fibers confirm the major trend of simulation, suggesting that using hydrogel spheres rather than coating could enhance the sensitivity of this kind of intensity-based fiber optic humidity sensors. Nevertheless, the sensing range of the current sensor is still very narrow, from 70% to 90%, due to the moisture absorption behavior of the used hydrogel PEGDMA. By using suitable hydrogels in future, the sensing range of relative humidity can be extended and the sensor could become feasible for practical applications.
Z.F. Zhang, Y. Zhang / Optics & Laser Technology 74 (2015) 16–19
Acknowledgment YL Zhang acknowledges the financial support of this research by the AnSTAR AOP Project (1223600005) and the AnSTAR Industrial Robotics Programm (1225100007).
Reference [1] Yeo TL, Sun T, Grattan KTV. Fibre-optic sensor technologies for humidity and moisture measurement. Sens Actuators Phys 2008;144:280–95. http://dx.doi. org/10.1016/j.sna.2008.01.017. [2] Farahani H, Wagiran R, Hamidon M. Humidity sensors principle, mechanism, and fabrication technologies: a comprehensive review. Sensors 2014;14:7881– 939. http://dx.doi.org/10.3390/s140507881. [3] Zhao Z, Duan Y. A low cost fiber-optic humidity sensor based on silica sol–gel film. Sens Actuators B Chem 2011;160:1340–5. http://dx.doi.org/10.1016/j. snb.2011.09.072. [4] Kronenberg P, Rastogi PK, Giaccari P, Limberger HG. Relative humidity sensor with optical fiber Bragg gratings. Opt Lett 2002;27:1385. http://dx.doi.org/ 10.1364/OL.27.001385. [5] Hernáez M, Zamarreño CR, Matías IR, Arregui FJ. Optical fiber humidity sensor based on surface plasmon resonance in the infra-red region. J Phys Conf Ser 2009;178:012019. http://dx.doi.org/10.1088/1742-6596/178/1/012019. [6] Gao R, Jiang Y, Ding W. Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio. Sens Actuators B Chem 2014;195:313–9. http://dx.doi.org/10.1016/j. snb.2014.01.061.
19
́ IR, Arregui FJ, López-Amo M. Optical fiber humidity sensor [7] Bariáin C, Matıas based on a tapered fiber coated with agarose gel. Sens Actuators B Chem 2000;69:127–31. http://dx.doi.org/10.1016/S0925-4005(00)00524-4. [8] Batumalay M, Harun SW, Ahmad F, Nor RM, Zulkepely NR, Ahmad H. Study of a fiber optic humidity sensor based on agarose gel. J Mod Opt 2014;61:244–8. http://dx.doi.org/10.1080/09500340.2013.879937. [9] Mathew J, Semenova Y, Rajan G, Farrell G. Agarose coated single mode fiber bend for monitoring humidity. In: Baldini F, Homola J, Lieberman RA, Kalli K, editors. Proceedings of SPIE, 8073. SPIE; 2011. p. 807317. http://dx.doi.org/10.1117/12.886184. [10] Mathew J, Thomas KJ, Nampoori VPN, Radhakrishnan P. A comparative study of fiber optic humidity sensors based on chitosan and agarose. Sens Transducers J 2007;84:1633–40. [11] Batumalay M, Lokman A, Ahmad F, Arof H, Ahmad H, Harun SW. Tapered plastic optical fiber coated with HEC/PVDF for measurement of relative humidity. IEEE Sens J 2013;13:4702–5. http://dx.doi.org/10.1109/ JSEN.2013.2272329. ́ IR. An experimental study about [12] Arregui FJ, Ciaurriz Z, Oneca M, Matıas hydrogels for the fabrication of optical fiber humidity sensors. Sens Actuators B Chem 2003;96:165–72. http://dx.doi.org/10.1016/S0925-4005(03)00520-3. [13] Di H. Sensing principle of fiber-optic curvature sensor. Opt Laser Technol 2014;62:44–8. http://dx.doi.org/10.1016/j.optlastec.2014.02.009. [14] Fu Y, Di H. Fiber-optic curvature sensor with optimized sensitive zone. Opt Laser Technol 2011;43:586–91. http://dx.doi.org/10.1016/j. optlastec.2010.08.005. [15] Bajić JS, Stupar DZ, Manojlović LM, Slankamenac MP, Živanov MB. A simple, low-cost, high-sensitivity fiber-optic tilt sensor. Sens Actuators Phys 2012. http://dx.doi.org/10.1016/j.sna.2012.07.027. [16] Clarkin JP, Timmerman RJ, Shannon JH. Shaped fiber tips for medical and industrial applications. In: Gannot I, editor. Proceedings of SPIE, 5317. SPIE; 2004. p. 70–80. http://dx.doi.org/10.1117/12.540734.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Humidity sensor based on optical fiber attached with hydrogel spheres Zhi Feng Zhang, Yilei Zhang n School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798 Singapore, Singapore
art ic l e i nf o
a b s t r a c t
Article history: Received 3 March 2015 Received in revised form 13 May 2015 Accepted 14 May 2015
This paper reports a new type of intensity-based fiber optic humidity sensor formed by solidifying mini hydrogel spheres on bare fiber cores. Upon variation of relative humidity, the refractive index of hydrogel spheres changes and leads to the intensity change of the transmitted light through the fiber. The dependence of the transmission ratio on the refractive index, size, number and separated distance of hydrogel spheres is simulated and optimized by using commercial software program TracePro. Simulation results suggest great dependence of the transmission ratio on the refractive index change of hydrogel spheres compared to even-thickness coating, because the spherical geometry is much more effective to couple light out of the fiber core. For example, the transmission ratio of the fiber core attached with a single hydrogel sphere in diameter of 2 mm could be reduced to 9% by changing refractive index of hydrogel while 2 mm coating could only achieve 87% for the same fiber core. Larger spheres reduce transmission as expected for longer coupling length. Increasing sphere number also cut transmission but the magnitude become minimal for four and more spheres. Preliminary experimental studies were carried out and demonstrated the idea of the sensor. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Fiber optic sensor Humidity sensor Hydrogel
1. Introduction Fiber optic sensing of relative humidity has been extensively studied and there have been a great number of proposed methods [1]. Sensing relative humidity is critical in various applications including food storage, climate monitoring, and chemical process [2]. Among various sensing methods of relative humidity, fiber optic based ones is preferred in many situations because of their minimal size, immunity to electromagnetic interference, and remote sensing capability. Though a large number of sensing schemes have been reported in literature, there is still a need for low-cost sensors, including both the fabrication and operation cost [3]. Intensity type of fiber optic sensors are the low-cost options compared to those based on the fiber Bragg grating [4], the surface plasmon resonance [5], photonic crystal fibers [6] etc. Those intensity based fiber optic sensor for humidity sensing are normally fabricated by coating hydrogels on the etched fibers. Agarose [7–10] and polyvinyl alcohol are the two most commonly used hydrogels and both silica fibers [7] and polymer optical fibers [11] are adopted previously. It had been demonstrated that agarose coated fiber core is sensitive to relative humidity due to refractive index change of the agarose hydrogel depending on the environment humidity [12]. The intensity of the transmitted light decreased at lower relative humidity. n
Corresponding author. Tel.: þ 65 6790 5952. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.optlastec.2015.05.006 0030-3992/& 2015 Elsevier Ltd. All rights reserved.
This paper reports a new type of fiber optic humidity sensor which is also based on hydrogel coated optical fiber. Rather than coating the bare optical fiber core with an even-thickness coating, the reported sensor exploit hydrogel spheres on the fiber core. Its performance is firstly studied by commercial optical engineering software TracePro, which has been a powerful optical engineer tool and successfully applied in sensor design [13–16].
2. Simulation The sensing performance of the sensor is simulated by TracePro. The fiber model is built based on the fiber patch cable (Thorlabs M28L02) used in experiments, which has a silica core in diameter of 400 μm and a TEQS™ cladding in thickness of 12.5 μm. The refractive index of the fiber core is set to be 1.4517 (pure silica at 625 nm). The refractive index of the cladding is calculated to be 1.40 based on the specified numerical aperture of 0.39. The refractive index of the hydrogel sphere is changed from 1.40 to 1.51. Design parameters including the sphere size, number, and separation distance are investigated.
3. Experiment Protective layers of a segment of a fiber patch cable (Thorlabs M28L02) are removed by appropriate tools and the cladding is
Z.F. Zhang, Y. Zhang / Optics & Laser Technology 74 (2015) 16–19
Fig. 2 shows simulated light rays that travel through a single hydrogel sphere attached on a section of fiber core, illustrating the working principle of the sensor. Light travels in the fiber from the left to the right end. The refractive index of the hydrogel is higher than that of the original fiber cladding and hence some light rays refract into the hydrogel sphere. Because of the spherical geometry, refracted light rays have very small incident angle at surface of the front side of the hydrogel sphere and therefore refract again out of the sphere leading to light loss. This is the main reason that spheres are more effective in coupling light out of the fiber core and accordingly high sensitivity of the proposed sensor. As the refractive index of the hydrogel sphere is changed due to varied humidity, the transmission of light will change accordingly. Simulated transmittance of hydrogel spheres coated fibers is shown in Figs. 3–5. Fig. 3 shows the transmittance of the fiber with a single hydrogel sphere as the refractive index of the hydrogel is increased from 1.40 to 1.51. For all sphere sizes, the transmittance starts to sharply decrease above refractive index of 1.43, and reaches a lowest transmittance around 1.46 and then gradually increases. The size of spheres dramatically influences the transmittance. As the diameter of the sphere increases from 0.6 mm to 2 mm, the lowest transmittance decreases from 62.9% to 9.0%. As a comparison, a uniform coating in thickness of 15 μm and length of 2 mm is included in the figure, which only leads to a lowest transmittance of 86.9%. While a single sphere of 2 mm is capable of reducing transmittance to 10%. Evidently, hydrogel spheres are much more effective in coupling out light from the fiber than the coating, suggesting higher sensitivity of sensors based on spheres.
Transmission ratio of the fiber
100 80 60 40 20 0 1.38
0.6 mm sphere 0.8 mm sphere 1.0 mm sphere 1.2 mm sphere 2mm sphere 2mm c oating
1.4
1.42
1.44
1.46
1.48
1.5
1.52
Refractive index of hydrogel spheres Fig. 3. Dependence of transmittance on the refractive index and size of a single hydrogel sphere.
100 Transmittance
4. Results and discussion
Fig. 2. Illustration of light rays that travel through a single hydrogel sphere in diameter of 2 mm and with a refractive index of 1.45. It should be noted that light loss occurs not only at the hydrogel sphere but also along fiber surface after the sphere, though the magnitude along the fiber is lower than that at the spheres.
80 60
1 sphere 2 spheres
40
3 spheres
20
5 spheres 10 spheres
0 1.38
1.4
1.42
1.44
1.46
1.48
1.5
1.52
Refractive index of hydrogel spheres Fig. 4. Dependence of transmittance on the refractive index and number of hydrogel spheres in diameter of 1 mm.
100 Transmission ratio of the fiber
burnt by flame to obtain bare fiber core. To avoid fiber break, the patch cable is fixed on an aluminum block. Then one or more drops of PEGDMA (Poly(ethylene glycol) dimethacrylate, average molecular weight 750, ALDRICH) hydrogel mixed with 0.5 wt% curing agent is dipped onto fiber core by using a micropipette. Hydrogel droplets are crosslinked for 30 s by a UV lamp (UV Flood, Epoxy & Equipment Technology Pte Ltd). Fig. 1 shows a typical sensor and hydrogel spheres and the spheres are not perfectly spherical due to the influence of gravity and surface tension. Humidity sensing tests are done in a temperature and humidity chamber (ESPEC North America Inc., SH-240). The temperature is maintained at 30 °C for all tests while the relative is set to 90% for two hours, then 80% for one hour, 70% for one hour etc. A fiber coupled LED (Thorlabs, M625F1) is used as the light source and a spectrometer (StellarNet, Silver-NOVA spectrometer) is used to measure the relative intensity of the transmitted light at 631 nm. Transmission ratio is calculated by dividing the relative intensity to that of 90% RH. The PEGDMA was selected because it has a refractive index of 1.465 which is the refractive index for hydrogel spheres that leads to lowest transmittance.
17
80 60
2 mm 5 mm 10 mm 20 mm
40 20 0 1.38
1.4
1.42
1.44
1.46
1.48
1.5
1.52
Refractive index of hydrogelspheres Fig. 5. Dependence of transmittance on the refractive index and separation distance of hydrogel spheres (five spheres in diameter of 1 mm).
Fig. 1. Photo of a fabricated sensor with five hydrogel spheres attached on a section of fiber core. (a) The whole sensor; (b) detail view of five spheres with light input from left side. Bright spots away from hydrogel spheres along fiber core are due to slight damage to the core surface during fabrication.
Fig. 4 illustrates the influence of sphere number on the transmittance of sensing fibers. Expectedly more spheres lead to lower transmittance. For a single sphere, the lowest transmission occurs when the refractive index of the hydrogel is 1.46. For multiple spheres, this critical value of refractive index increases slightly. However, the accumulated transmission for N spheres is higher
5 spheres 4 spheres 3 spheres 2 spheres 1 spheres
75% 85% 95% Relative humidity
Transmitance
100% 90% 80% 70% 60% 50% 40% 30% 20% 65%
100% 90% 80% 70% 60% 50% 40% 30% 20% 65%
Transmitance
Z.F. Zhang, Y. Zhang / Optics & Laser Technology 74 (2015) 16–19
Transmitance
18
100% 90% 80% 70% 60% 50% 40% 30% 20% 65%
0.1 μL 0.2 μL 0.4 μL 0.6 μL
75% 85% 95% Relative humidity
2 mm 5 mm 10 mm 20 mm
75%
85%
95%
Relative humidity Fig. 6. Influence of (a) sphere number, (b) size and (c) separation space on the response of the sensor to relative humidity.
than TN (T for the transmission ratio of a single sphere, N for an integer). For example, transmissions for 1, 2 and 3 spheres with a refractive index of 1.460 are 38.0%, 31.2% and 22.4% respectively, suggesting that presence of the first sphere has influence on the transmission ratio of the second and the third ones. The first sphere always leads to the largest transmission loss due to high percent of light rays that have propagating angle with fiber axis close to the angle of total internal reflection. It is always the light rays that refracts into the hydrogel first and lead to light loss. After each sphere, the percent of light rays with large propagating angle decreases and thus decreases transmission loss. The ultimate result is that the transmission of N spheres is always high than TN. This memory effect is more significant when the separation distance is small, as shown in Fig. 5. For example, increasing the separation distance from 2 mm to 5 mm reduces the lowest transmittance from around 20% to 10%, but further increasing produces no apparent effect, suggesting that 5 mm separation is an acceptable distance for sensor fabrication. Large separation distance results long sensor while too close spheres make it difficult to fabricate and lower sensitivity. Fig. 6 shows preliminary experimental results. The sphere number is varied from 1 to 5. The size of hydrogel spheres is tuned by the volume of the hydrogel solution, from 0.1 to 0.6 μL. The separation distance of 2, 5, 10, and 20 mm is studied. The cured hydrogel sphere is not exactly spherical due to gravity and surface tension (Fig. 1), making it difficult to compare to simulation results. Among the three studied parameters, the sphere number demonstrates biggest impact on the sensitivity of the sensor. Contrast to the simulation, the influence of sphere size is small. The main reason might be that the variation of sphere size is small compared to the range in the simulation. Large hydrogel droplets drip from the fiber due to gravity, and besides the fabricated sphere is not perfectly spherical because of the surface tension. The separation distance has no significant influence on the sensor
sensitivity especially above 5 mm. Typically, for the sensor with 5 spheres, 20% variation in relative humidity leads to a transmittance change of 78%. Due to large fluctuation of the output power of the used LED fiber light source, there is large errors in test results as suggested by long error bars in the figure. Large error is also caused by variation from sensor to sensor. Consequently, accuracy of the fabricated sensors still has considerable room to improve.
5. Conclusion A new type of fiber optic humidity sensor based on hydrogel spheres attached on bare fiber core is simulated and verified by experimental studies, which is easy to fabricate and thus low cost. Most importantly, simulation by TracePro has demonstrated its superior sensitivity compared to even-thickness coatings due to the spherical geometry. The bigger the hydrogel sphere, the lower the transmission of the sensing fiber is. With regard to sphere number, three spheres are almost enough to achieve the lowest transmission while the benefit of further increasing sphere number is minimal. The separation distance of hydrogel sphere also plays a noticeable role on the transmission, but the magnitude is negligible compared to the size and the number. Preliminary experiments of curing pure PEGDMA hydrogel droplets on a multimode silica fibers confirm the major trend of simulation, suggesting that using hydrogel spheres rather than coating could enhance the sensitivity of this kind of intensity-based fiber optic humidity sensors. Nevertheless, the sensing range of the current sensor is still very narrow, from 70% to 90%, due to the moisture absorption behavior of the used hydrogel PEGDMA. By using suitable hydrogels in future, the sensing range of relative humidity can be extended and the sensor could become feasible for practical applications.
Z.F. Zhang, Y. Zhang / Optics & Laser Technology 74 (2015) 16–19
Acknowledgment YL Zhang acknowledges the financial support of this research by the AnSTAR AOP Project (1223600005) and the AnSTAR Industrial Robotics Programm (1225100007).
Reference [1] Yeo TL, Sun T, Grattan KTV. Fibre-optic sensor technologies for humidity and moisture measurement. Sens Actuators Phys 2008;144:280–95. http://dx.doi. org/10.1016/j.sna.2008.01.017. [2] Farahani H, Wagiran R, Hamidon M. Humidity sensors principle, mechanism, and fabrication technologies: a comprehensive review. Sensors 2014;14:7881– 939. http://dx.doi.org/10.3390/s140507881. [3] Zhao Z, Duan Y. A low cost fiber-optic humidity sensor based on silica sol–gel film. Sens Actuators B Chem 2011;160:1340–5. http://dx.doi.org/10.1016/j. snb.2011.09.072. [4] Kronenberg P, Rastogi PK, Giaccari P, Limberger HG. Relative humidity sensor with optical fiber Bragg gratings. Opt Lett 2002;27:1385. http://dx.doi.org/ 10.1364/OL.27.001385. [5] Hernáez M, Zamarreño CR, Matías IR, Arregui FJ. Optical fiber humidity sensor based on surface plasmon resonance in the infra-red region. J Phys Conf Ser 2009;178:012019. http://dx.doi.org/10.1088/1742-6596/178/1/012019. [6] Gao R, Jiang Y, Ding W. Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio. Sens Actuators B Chem 2014;195:313–9. http://dx.doi.org/10.1016/j. snb.2014.01.061.
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