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Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio
Sensors and Actuators B 195 (2014) 313–319
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio Ran Gao ∗ , Yi Jiang, Wenhui Ding School of Opto-Electronics, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 4 November 2013 Received in revised form 15 January 2014 Accepted 17 January 2014 Available online 26 January 2014 Keywords: Humidity sensor Photonic crystal fiber Agarose gel Interferometry
a b s t r a c t An Agarose gel filled photonic crystal fiber in-line interferometric sensor for the measurement of relative humidity is proposed and experimentally demonstrated. The sensor is constructed by filling the Agarose gel between the aligned single mode fiber (SMF) and the photonic crystal fiber (PCF). A fiber in-line interferometer is fabricated by splicing the other end of the PCF and another SMF. Due to the tunable refractive index property of the Agarose gel, the mode field diameter of the propagation light is changed with the external relative humidity, which induces the change of coupling ratio between the PCF and the SMF. The relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The experimental results show that the sensitivity of up to 2.2 dB/RH is achieved. The proposed method possesses high resolution and low temperature cross-sensitivity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Measurement of relative humidity is increasingly essential in many areas, such as chemical and food processing industry, environmental monitoring and meteorological services [1]. In recent years, fiber optic humidity sensors have been extensively investigated due to their advantages, such as low weight, small size, and long distance signal transmission for remote operation [2]. Many forms of humidity sensors based on optical fibers have been developed, such as long period gratings [3], fiber Bragg gratings [4], side polished fibers [5], plastic optical fibers [6], surface plasmon resonance [7], and tapered optical fibers [8]. Most optical fiber based humidity sensors are constructed by using the hygroscopic material. Based on the properties of these hygroscopic material, fiber optic sensors for the measurement of the relative humidity can be approximately classified into two types: the size method and the refractive index (RI) method. In the size method, the hygroscopic material will expand in size with the increase of the humidity, which would change the optical path difference (OPD) of the interferometer, and the humidity can be measured by tracking the wavelength valley in the interference fringe. A fiber optic humidity sensor based on the size method can be constructed by coating a chitosan film or producing a Fabry–Perot cavity [9,10]. However, such sensors always suffer
from serious temperature cross-sensitivity. On the other hand, the RI technique can be realized in many ways, such as a fiber Bragg grating deposited with the N-ethyl-4-vinylpyridinium chloride layer [11], a long period grating coated with the polyvinyl alcohol [12], or a hetero-core structure optical fiber filmed with porous sol–gel silica (PSGS) [13]. The relative humidity is measured by utilizing the tunable RI property of the hygroscopic material. The RI method possesses many attractive characteristics, such as high sensitivity, linear response, and low temperature crosssensitivity. In this paper, we propose a compact relative humidity sensor based on the PCF and the Agarose gel. The humidity sensor is constructed by aligning a SMF and a PCF, and the Agarose gel is filled into the gap between the SMF and the PCF. A fiber in-line interferometer is fabricated by splicing the other end of the PCF to another SMF. Due to the tunable RI property of the Agarose gel, the mode field diameter of the propagation light is changed with the increase of the relative humidity, and the relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The proposed method possesses high sensitivity and low temperature cross-sensitivity.
2. Operation principle 2.1. The RI response of the Agarose gel for the water molecules
∗ Corresponding author at: 5 South Zhongguancun Street, Haidian District, Beijing 100081, China. Tel.: +86 010 68913586; fax: +86 010 68913586. E-mail address: [email protected] (R. Gao). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.061
In general, the optic fiber humidity sensors are based on the humidity – sensitive materials, such as polyvinyl alcohol (PVA), chitosan and cyclic olefin copolymer. Although these materials
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Fig. 2. (a) Schematic diagram of the PCF based humidity sensor and (b) the MFD of the light in the Agarose gel.
Fig. 1. The RI of the Agarose gel response to the ambient humidity.
have many attractive characteristics, such as high sensitivity, small size, and low temperature cross-sensitivity, some of materials are not suitable for sensing a wide humidity range; some of other materials provide a non-linear response because these materials are soluble in water very easily [14]. Agarose gel is a high humidity–sensitivity material which offers a wide operating humidity range with a simple producing procedure. When the Agarose gel is exposed to the humidity with high level, water molecules enter pores of the Agarose gel due to the hydrophilic nature and capillary forces. The air in the pores of the Agarose gel is replaced by water. Therefore, Agarose gel shows a linear change in its RI with respect to ambient humidity. The thermal expansion coefficient and thermo optical coefficient of the Agarose gel is ∼4 × 10−3 /◦ C and ∼−5 × 10−4 /◦ C [15,16]. Due to the transparent property of the Agarose gel, we measured the RI of the Agarose gel response to the ambient humidity by using Abbe Refractometer (DR-M2/1550(A), ATAGO) at the wavelength of 1550 nm, as shown in Fig. 1. The RI of the Agarose gel is in linear proportion to the external humidity. On the other hand, since the Agarose is soluble in hot water [17], the producing procedure of the Agarose gel is easy. According to these factors, Agarose gel is a suitable material for fabricating the optic fiber humidity sensor. 2.2. Sensor principle The schematic diagram of the PCF based humidity sensor is presented in Fig. 2(a), which is formed by two SMFs, a PCF and the
Agarose gel. A SMF and a PCF with cleaved ends are aligned to each other. The gap between the PCF and the SMF is filled with the Agarose gel. The other end of the PCF is spliced to a SMF by using the technique reported in [18], and the splicing loss is ∼1.2 dB. At the fiber splice, a collapsed region is formed between the PCF and the SMF due to strong electric arc discharges [19]. The operation principle of the sensor is shown in Fig. 2(b). When the propagation light travels from the SMF to the Agarose gel, the mode field of the propagation light begins to expand according to the Gaussian beam approximation. Then both the fundamental core mode and cladding modes are excited when the transmission light propagates from the Agarose gel to the PCF. Two excited modes propagate along the PCF, and re-couple back into the core of the SMF at the collapsed region, which forms an in-line Mach–Zender interferometer (MZI). The PCF is essential in the proposed sensor because the collapse region is naturally formed in the splicing between the SMF and the PCF. However, the collapse region is hard to be fabricated by splicing two SMFs, which is difficult to couple cladding modes back into the core of the SMF. Although some techniques for fabricating the combiner in the SMF have been presented, such as the misaligned spliced joint [20], the peanut-shape structure [21], the tapered region [22], and the laser irradiation [23], these methods involve many complicated procedures and are very fragile in harsh environment. The mode field diameter (MFD) of the light depends on the spot size at the end face of the SMF ω0 , the wavelength of the light , the length of the Agarose gel gap z, and the RI of the Agarose gel n, which is [24]
MFD = 2ω0
1+
z 2 nω0 2
.
(1)
Fig. 3. Simulations for (a) the MFD of the light and the fringe visibility of the spectrum with different RI of Agarose gel and (b) intensity distributions of the propagation mode field in the Agarose gel with different RIs.
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Given that ω0 = 9 m, = 1550 nm, and z = 300 m, the simulation results indicate that the MFD decreases with the increase of the RI of the Agarose gel, as shown in Fig. 3(a) (blue dash dot line). Due to the MFD mismatch, the partial guided light, of which the MFD is larger than the core size of the PCF, is coupled into the cladding of the PCF. With the decrease of the MFD, the light which couples into the core is increased, and the light which couples into the cladding is decreased simultaneously, as shown in Fig. 3(b). Therefore, the coupling ratio between the core and cladding is changed with the increase of the RI of the Agarose gel. The light intensity of the core mode and cladding modes, Ico and Icl , can be expressed according to the Gaussian beam approximation [25] Ico =
d ω0 2
0
Icl =
∞
MFD
ω 2 0 MFD
d
exp
−
exp
2z 2 MFD2
−
2z 2 MFD2
zdz.
(2)
zdz.
(3)
where d is the diameter of the core size, which is 11.4 m in the all-solid PCF. The interference visibility of the white-light interferogram can be expressed as [26] V=
Imax − Imin = Imax + Imin
2 Ico /Icl +
Icl /Ico
(4)
.
When the RI of the Agarose gel increases with the increase of the humidity, the MFD of the light is decreased simultaneously, and induces the change of the light intensity distribution between the core and cladding. The relationship between the fringe visibility of the white-light interferogram and the MFD can be calculated by substituting Eqs. (2) and (3) into Eq. (4), which is given by V=
exp
2 2ω2 /MFD2 0 1−exp(2ω2 /MFD2 0
1−exp(2ω2 /MFD2 ) . +
(5)
0
exp(2ω2 /MFD2 ) 0
It can be seen that the relationship is nonlinearity according to Eq. (5). Fig. 3(a) (red dash line) shows the relationship between the fringe visibility of the white-light interferogram and the RI of the Agarose gel by calculating Eq. (5). With the increase of the RI of the Agarose gel, the fringe visibility decreases simultaneously. Thus the relative humidity can be measured by interrogating the fringe visibility. 3. Fabrication of the humidity sensor The configuration of the fiber optic humidity sensor is shown in Fig. 4(a). Firstly, a notch is machined on a ceramic tube with an inner diameter of 2.5 mm. The length and depth of the notch are 2.2 mm and 1.2 mm, respectively. Secondly, a SMF and a PCF with cleaved ends are inserted into two ceramic ferrules and are polished. The SMF is aligned to the PCF in the ceramic tube automatically. The other end of the PCF with the length of 5 cm is spliced with a SMF by using the technique reported in [18]. Finally, two ceramic ferrules are inserted into the notched ceramic tube, and an air-gap cavity is formed between the two ends of the SMF and PCF. The cavity locates at the middle of the notch. In this way, the water molecule can freely diffuse to the internal cavity through the notch of the ceramic tube. In the sensor, the all-solid PCF (Yangtze Optical Fiber and Cable Company) used in our experiment includes five rings of holes arranged in a regular hexagonal pattern, as shown in Fig. 4(b). The PCF has a center-to-center distance between the holes of = 8.5 m and an average hole diameter of d = 2.9 m. The core diameter is 11.4 m, and the outer diameter of the PCF is 125 m. Unlike the traditional PCF, the PCF used in this experiment is solid
Fig. 4. (a) The configuration of the humidity sensor and (b) the close view of the sensor with Agarose gel.
without any air holes. The PCF core is also solid and constructed by the pure silica. Then the Agarose gel is filled into the internal cavity. The solution is prepared by dissolving the 2.1 wt% Agarose into the distilled water in a beaker. Under the beaker a heater combined with a magnetic stirrer is fixed, of which the temperature is set to be 65 ◦ C. To dissolve the Agarose in the distilled water, the mixture of the Agarose and the distilled water inside the beaker is stirred by using the magnetic stirrer until the Agarose is completely dissolved. Then the sensor is immersed into the beaker, and the Agarose solution is flowed into the cavity because the mixture is still in the liquid form. After this, the mixture is cooled by removing the heater. When the mixture reaches the gelling point of 35 ◦ C, it becomes the form of hydrogel. In this way, the Agarose gel is filled into the cavity of the sensor, as shown in Fig. 4(c). Next step the sensor is rinsed with the distilled water to remove the residual Agarose gel on the surface of the sensor. The fiber sensor is kept for one day at room temperature until it is partially dehydrated and reaches the equilibrium with the ambient environment. 4. Experiment and discussion The experimental setup is shown in Fig. 5. An unflattened amplified spontaneous emission (ASE) source with wavelength covering 1525–1565 nm and output power of 20 mW is used as a broadband source to illuminate the humidity sensor. The transmission spectra of the sensor are interrogated by using a CCD based optical spectrum analyze (OSA) (BaySpec FBGA-F-1525-1565). The sensor was put inside the airtight humidity controlled chamber. The humidity and the temperature inside the chamber can be adjusted by using microprocessor based control unit and the feedback sensors. Fig. 6 shows transmission spectra of the proposed sensor with and without the Agarose gel, of which the distance of the gap is
Fig. 5. The experimental setup.
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Fig. 8. The spatial frequency of the interference fringe.
Fig. 6. Transmission spectra of the proposed sensor with and without the Agarose gel.
314 m. The transmission intensity with the Agarose gel is larger than that without the Agarose gel, because the MFD of the light in the air is much larger than the core size of the PCF, which results in the large mode field mismatch between the SMF and the PCF. The insertion loss of the Agarose gel filled into the gap between the SMF and the PCF is 6.3 dB. The influence of the distance of the gap to the humidity sensor is investigated. One ceramic ferrule of the sensor is fixed on a fixed stage, and the other ceramic ferrule is fixed on a single-Axis stage with the resolution of 1 m. The length of the gap could be changed precisely by adjusting the single-Axis stage with one ceramic ferrule. Then three sensors were fabricated, of which distances of gaps are 153 m, 204 m, and 256 m, respectively, and all gaps were filled with the Agarose gel. Given that ω0 = 9 m, = 1550 nm, and n = 1.46, the simulation results is shown in Fig. 7(a) according to Eq. (1). When the distance of the Agarose gel gap is smaller than 193 m, the MFD of the light is smaller than the core size of the PCF (11.4 m), which results in the low light intensity of the cladding mode. Thus the fringe visibility of the transmission spectrum is almost zero. Then the MFD, which is larger than the core size of the PCF, increases with the increase of the length of the gap, and the fringe visibility also increases simultaneously. Transmission spectra with different length of gap in a room RH of 50 ± 2% are shown in Fig. 7(b). The fringe visibility of the transmission spectrum is almost zero when the Agarose gel gap is 153 mm. With the
increase of the distance of the gap, the fringe visibility of transmission spectrum increases simultaneously. Therefore, when it is used as a sensing element, the length of the Agarose gel gap should be adjusted in order to keep the maximal fringe visibility. The proposed PCF based sensor was tested for relative humidity measurement over the range from 20%RH to 90%RH with the step of 10%RH. For the in-line MZI, many modes with different effective refractive indices may be excited when the light is coupled into the PCF from Agarose gel [27]. The homogeneous spectrum of the proposed sensor with the Agarose gel was Fourier transformed to obtain the spatial frequency of the interference fringe, as shown in Fig. 8. The spatial frequency confirms that the interference is mainly produced by two modes: one core mode and one dominant cladding mode [28]. Because other higher-order modes are very weak, they have little influence on the interference. Therefore, Eq. (4) can still be used to investigate the evolution of interference visibility. The transmission spectra which correspond to different relative humidity are shown in Fig. 9(a). For the increase of the humidity, the fringe visibility decreases simultaneously. The fringe visibility of the transmission spectrum can be expressed as [26] V=
Imax − Imin Imax + Imin
(6)
where Imax and Imin are wavelength peak and valley in the whitelight interferogram. To deal with these variations of fringe visibility, a wavelength valley at 1531.38 nm and an adjacent wavelength peak at 1532.37 nm are chosen to calculate the fringe visibility, as shown in Fig. 9(b). The response of the sensor keeps a good quadratic relationship between the fringe visibility and the relative humidity. The polynomial fitting curve can be expressed as
Fig. 7. (a) Simulations for the MFD of the light and the fringe visibility of the spectrum with different length of gap and (b) transmission spectra of the sensor with different length of gap.
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Fig. 9. (a) Transmission spectra of the sensor at different relative humidity and (b) average fringe visibilities at different relative humidity.
y = −1.5 × 10−4 x2 + 1.9 × 10−2 x + 11.1. Although the response is a nonlinear, the good quadratic curve also keeps an accurate relationship between the fringe visibility and the relative humidity in the practical measurement. The maximum sensitivity reaches 0.022 dB/% at the relative humidity of 90%. Because the resolution of the OSA is 0.001 dB, the resolution of 0.044% of the sensor is achieved. The resolution of the proposed method is higher than previous results of 0.4% which is based on the method of the long period grating [3], 0.78% which is based on the method of the FBG [11], or 1.2% which is based on the method of the a fiber bend [14]. Due to the unflattened background of the ASE, the sensitivity of the sensor to humidity is different at different wavelengths valley, as shown in Fig. 10. At the relative humidity of 50%, the visibility of the transmission spectrum at wavelength valley of 1526.72 nm is 4.62 dB. Then it reaches the peak of 7.84 dB at the wavelength of 1531.38. After that, the fringe visibility decreases with the increase of the wavelength. However, all of response with different wavelength valley of the sensor keeps a good quadratic relationship between the fringe visibility and the relative humidity. The sensor was evaluated for its stability and repeatability. To test the stability of the humidity sensor, a sensor with the Agarose gel gap distance of 314 m was fixed in the chamber, in which the humidity and temperature are 60%RH and 20 ◦ C, respectively. The fringe visibility with the resonance wavelength of 1531.48 nm was measured for 60 times over duration of 3 h. As shown in Fig. 11(a), the standard variation of the fringe visibility is 0.92%, which indicates the good stability of the sensor. Besides, the repeatability of the sensor was also experimentally tested. The humidity in the chamber was adjusted in an ascending and descending order, respectively, and the corresponding fringe visibility is shown in
Fig. 10. The fringe visibilities with different wavelength valley at different relative humidity.
Fig. 11(b). Results in the ascending and descending order are in good agreement with each other; the maximum variation is 0.3% at the humidity of 40%RH. The temperature dependence of the sensor is investigated by adjusting the temperature range of the chamber from 20 ◦ C to 55 ◦ C. The humidity was set with 30%RH, 50%RH, and 80%RH, respectively. The variation of the fringe visibility with the temperature change is shown in Fig. 12(a). The temperature sensitivities of the sensor are 0.045 dB/◦ C, 0.038 dB/◦ C, and 0.035 dB/◦ C in the relative humidity of 30%RH, 50%RH, and 80%RH, respectively. Because the thermo optical coefficient of the Agarose gel is negative, the fringe visibility is increased with the increase of temperature. It is observed that the proposed sensor is insensitive to temperature change with a high relative humidity. Although the sensor is suffered from a little temperature cross-sensitivity at the low relative humidity, the proposed sensor is much less sensitive to temperature change compared with some other fiber optic humidity sensors which are suffered from temperature cross-sensitivity seriously, such as long period gratings (LPGs) [3], or fiber bend humidity sensor [14]. Besides, a FBG can be fabricated in the SMF, and the temperature effect of the sensor at low humidity can be compensated by measuring the wavelength shift of the FBG [29]. On the other hand, the temperature sensitivity of the sensor at low humidity is higher than that at high humidity, because the form of the Agarose gel would be transformed from hydrogel to liquid with the increase of temperature [16]. When the humidity is high, pores in the Agarose gel are filled with water molecules, of which the RI is closed to the Agarose gel. Thus when the form of the Agarose gel is transformed with the temperature, the RI of the Agarose gel is changed slightly. However, pores in the Agarose gel are filled with air at low humidity, of which the RI is far away from the Agarose gel. Thus the RI of the Agarose gel is changed seriously when the form is transformed, and the visibility of the spectrum is more sensitive with temperature at high humidity. The time response of the sensor has been analyzed. The sampling frequency of the OSA is 5000 Hz. Thus the time resolution of the sensor is 0.2 ms, and the time response of the sensor is measured by calculating the sum of the sampling points. However, the humidity controlled chamber is not suitable for studying the response time of the sensor, because the chamber is adjusted very slowly. Instead, we lead the humid air from the chamber to the sensor through a rubber pipe. The response time is shown in Fig. 12(b). When the RH jumps from 56%RH (room humidity) to 80%RH, the estimated response time of the sensor is ∼200 ms. Then when the rubber pipe was removed, the estimated recovery time of the sensor is ∼230 ms. For the application, all of fibers are sealed into ceramic ferrule, and each component of the sensor, such as ceramic ferrule or tube,
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Fig. 11. (a) The stability of the humidity sensor and (b) the repeatability of the sensor.
Fig. 12. (a) The temperature dependence of the sensor and (b) the time response of the sensor.
is fixed with each other by using epoxy. Thus the mechanical property of the proposed sensor is stable and reusable robust. The RI of the Agarose gel is affected by the water molecules in pores, which would influence on the calibration of the sensor. In Ref. [14], Agarose gel shows a linear change and reversible response in its RI with respect to humidity, which presents a stable property of the Agarose gel. Therefore, the proposed sensor could be reused to measure the humidity repeatedly even without the calibration. Besides, in order to improve the calibration of the sensor precisely, the sensor could be fixed into the airtight humidity controlled chamber before each measurement. When the humidity in the chamber is fixed, the water molecules in pores of the Agarose gel can be evaporated by heating the sensor, and the visibility of the spectrum can be calibrated by adjusting the temperature. Moreover, the intensity of the spectrum would decreases when the length of the PCF is increased because of the highly loss of cladding modes [30]. However, once the proposed sensor is fabricated, the length of the PCF is almost fixed. According to Eqs. (2–4), the fringe visibility only depends on the property of Agarose gel, such as the RI or the length of the gap. Thus although the temperature fluctuant or vibration may induce the change of the PCF with a small range, the intensity of the spectrum is independent with the length of the PCF, and the fringe visibility is unaffected with the length variation of the PCF, which improve the stability and repeatability of the sensor. 5. Conclusion In conclusion, a compact relative humidity sensor based on the all-solid PCF is proposed and experimentally demonstrated. The sensor is constructed by filling the Agarose gel between the aligned SMFs and PCFs. A fiber in-line interferometer is fabricated
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Biographies Ran Gao received his MS degree in Photoelectron from Beijing University of Technology, China, in 2010. He is currently working toward the PhD degree in Electronic Science and Technology Beijing Institute of Technology. His current research interest is focused on the optical fiber sensors. Yi Jiang received the BA degree in 1987 and PhD degree in 1996, respectively, from Chongqing University, China. He is a professor in Beijing Institute of technology. His research interests include fiber optical sensors, smart structures, and measurement instruments. Wenhui Ding is currently working toward the PhD degree in Electronic Science and Technology Beijing Institute of Technology. His current research interest is focused on the optical fiber sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Agarose gel filled temperature-insensitive photonic crystal fibers humidity sensor based on the tunable coupling ratio Ran Gao ∗ , Yi Jiang, Wenhui Ding School of Opto-Electronics, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e
i n f o
Article history: Received 4 November 2013 Received in revised form 15 January 2014 Accepted 17 January 2014 Available online 26 January 2014 Keywords: Humidity sensor Photonic crystal fiber Agarose gel Interferometry
a b s t r a c t An Agarose gel filled photonic crystal fiber in-line interferometric sensor for the measurement of relative humidity is proposed and experimentally demonstrated. The sensor is constructed by filling the Agarose gel between the aligned single mode fiber (SMF) and the photonic crystal fiber (PCF). A fiber in-line interferometer is fabricated by splicing the other end of the PCF and another SMF. Due to the tunable refractive index property of the Agarose gel, the mode field diameter of the propagation light is changed with the external relative humidity, which induces the change of coupling ratio between the PCF and the SMF. The relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The experimental results show that the sensitivity of up to 2.2 dB/RH is achieved. The proposed method possesses high resolution and low temperature cross-sensitivity. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Measurement of relative humidity is increasingly essential in many areas, such as chemical and food processing industry, environmental monitoring and meteorological services [1]. In recent years, fiber optic humidity sensors have been extensively investigated due to their advantages, such as low weight, small size, and long distance signal transmission for remote operation [2]. Many forms of humidity sensors based on optical fibers have been developed, such as long period gratings [3], fiber Bragg gratings [4], side polished fibers [5], plastic optical fibers [6], surface plasmon resonance [7], and tapered optical fibers [8]. Most optical fiber based humidity sensors are constructed by using the hygroscopic material. Based on the properties of these hygroscopic material, fiber optic sensors for the measurement of the relative humidity can be approximately classified into two types: the size method and the refractive index (RI) method. In the size method, the hygroscopic material will expand in size with the increase of the humidity, which would change the optical path difference (OPD) of the interferometer, and the humidity can be measured by tracking the wavelength valley in the interference fringe. A fiber optic humidity sensor based on the size method can be constructed by coating a chitosan film or producing a Fabry–Perot cavity [9,10]. However, such sensors always suffer
from serious temperature cross-sensitivity. On the other hand, the RI technique can be realized in many ways, such as a fiber Bragg grating deposited with the N-ethyl-4-vinylpyridinium chloride layer [11], a long period grating coated with the polyvinyl alcohol [12], or a hetero-core structure optical fiber filmed with porous sol–gel silica (PSGS) [13]. The relative humidity is measured by utilizing the tunable RI property of the hygroscopic material. The RI method possesses many attractive characteristics, such as high sensitivity, linear response, and low temperature crosssensitivity. In this paper, we propose a compact relative humidity sensor based on the PCF and the Agarose gel. The humidity sensor is constructed by aligning a SMF and a PCF, and the Agarose gel is filled into the gap between the SMF and the PCF. A fiber in-line interferometer is fabricated by splicing the other end of the PCF to another SMF. Due to the tunable RI property of the Agarose gel, the mode field diameter of the propagation light is changed with the increase of the relative humidity, and the relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The proposed method possesses high sensitivity and low temperature cross-sensitivity.
2. Operation principle 2.1. The RI response of the Agarose gel for the water molecules
∗ Corresponding author at: 5 South Zhongguancun Street, Haidian District, Beijing 100081, China. Tel.: +86 010 68913586; fax: +86 010 68913586. E-mail address: [email protected] (R. Gao). 0925-4005/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2014.01.061
In general, the optic fiber humidity sensors are based on the humidity – sensitive materials, such as polyvinyl alcohol (PVA), chitosan and cyclic olefin copolymer. Although these materials
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Fig. 2. (a) Schematic diagram of the PCF based humidity sensor and (b) the MFD of the light in the Agarose gel.
Fig. 1. The RI of the Agarose gel response to the ambient humidity.
have many attractive characteristics, such as high sensitivity, small size, and low temperature cross-sensitivity, some of materials are not suitable for sensing a wide humidity range; some of other materials provide a non-linear response because these materials are soluble in water very easily [14]. Agarose gel is a high humidity–sensitivity material which offers a wide operating humidity range with a simple producing procedure. When the Agarose gel is exposed to the humidity with high level, water molecules enter pores of the Agarose gel due to the hydrophilic nature and capillary forces. The air in the pores of the Agarose gel is replaced by water. Therefore, Agarose gel shows a linear change in its RI with respect to ambient humidity. The thermal expansion coefficient and thermo optical coefficient of the Agarose gel is ∼4 × 10−3 /◦ C and ∼−5 × 10−4 /◦ C [15,16]. Due to the transparent property of the Agarose gel, we measured the RI of the Agarose gel response to the ambient humidity by using Abbe Refractometer (DR-M2/1550(A), ATAGO) at the wavelength of 1550 nm, as shown in Fig. 1. The RI of the Agarose gel is in linear proportion to the external humidity. On the other hand, since the Agarose is soluble in hot water [17], the producing procedure of the Agarose gel is easy. According to these factors, Agarose gel is a suitable material for fabricating the optic fiber humidity sensor. 2.2. Sensor principle The schematic diagram of the PCF based humidity sensor is presented in Fig. 2(a), which is formed by two SMFs, a PCF and the
Agarose gel. A SMF and a PCF with cleaved ends are aligned to each other. The gap between the PCF and the SMF is filled with the Agarose gel. The other end of the PCF is spliced to a SMF by using the technique reported in [18], and the splicing loss is ∼1.2 dB. At the fiber splice, a collapsed region is formed between the PCF and the SMF due to strong electric arc discharges [19]. The operation principle of the sensor is shown in Fig. 2(b). When the propagation light travels from the SMF to the Agarose gel, the mode field of the propagation light begins to expand according to the Gaussian beam approximation. Then both the fundamental core mode and cladding modes are excited when the transmission light propagates from the Agarose gel to the PCF. Two excited modes propagate along the PCF, and re-couple back into the core of the SMF at the collapsed region, which forms an in-line Mach–Zender interferometer (MZI). The PCF is essential in the proposed sensor because the collapse region is naturally formed in the splicing between the SMF and the PCF. However, the collapse region is hard to be fabricated by splicing two SMFs, which is difficult to couple cladding modes back into the core of the SMF. Although some techniques for fabricating the combiner in the SMF have been presented, such as the misaligned spliced joint [20], the peanut-shape structure [21], the tapered region [22], and the laser irradiation [23], these methods involve many complicated procedures and are very fragile in harsh environment. The mode field diameter (MFD) of the light depends on the spot size at the end face of the SMF ω0 , the wavelength of the light , the length of the Agarose gel gap z, and the RI of the Agarose gel n, which is [24]
MFD = 2ω0
1+
z 2 nω0 2
.
(1)
Fig. 3. Simulations for (a) the MFD of the light and the fringe visibility of the spectrum with different RI of Agarose gel and (b) intensity distributions of the propagation mode field in the Agarose gel with different RIs.
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Given that ω0 = 9 m, = 1550 nm, and z = 300 m, the simulation results indicate that the MFD decreases with the increase of the RI of the Agarose gel, as shown in Fig. 3(a) (blue dash dot line). Due to the MFD mismatch, the partial guided light, of which the MFD is larger than the core size of the PCF, is coupled into the cladding of the PCF. With the decrease of the MFD, the light which couples into the core is increased, and the light which couples into the cladding is decreased simultaneously, as shown in Fig. 3(b). Therefore, the coupling ratio between the core and cladding is changed with the increase of the RI of the Agarose gel. The light intensity of the core mode and cladding modes, Ico and Icl , can be expressed according to the Gaussian beam approximation [25] Ico =
d ω0 2
0
Icl =
∞
MFD
ω 2 0 MFD
d
exp
−
exp
2z 2 MFD2
−
2z 2 MFD2
zdz.
(2)
zdz.
(3)
where d is the diameter of the core size, which is 11.4 m in the all-solid PCF. The interference visibility of the white-light interferogram can be expressed as [26] V=
Imax − Imin = Imax + Imin
2 Ico /Icl +
Icl /Ico
(4)
.
When the RI of the Agarose gel increases with the increase of the humidity, the MFD of the light is decreased simultaneously, and induces the change of the light intensity distribution between the core and cladding. The relationship between the fringe visibility of the white-light interferogram and the MFD can be calculated by substituting Eqs. (2) and (3) into Eq. (4), which is given by V=
exp
2 2ω2 /MFD2 0 1−exp(2ω2 /MFD2 0
1−exp(2ω2 /MFD2 ) . +
(5)
0
exp(2ω2 /MFD2 ) 0
It can be seen that the relationship is nonlinearity according to Eq. (5). Fig. 3(a) (red dash line) shows the relationship between the fringe visibility of the white-light interferogram and the RI of the Agarose gel by calculating Eq. (5). With the increase of the RI of the Agarose gel, the fringe visibility decreases simultaneously. Thus the relative humidity can be measured by interrogating the fringe visibility. 3. Fabrication of the humidity sensor The configuration of the fiber optic humidity sensor is shown in Fig. 4(a). Firstly, a notch is machined on a ceramic tube with an inner diameter of 2.5 mm. The length and depth of the notch are 2.2 mm and 1.2 mm, respectively. Secondly, a SMF and a PCF with cleaved ends are inserted into two ceramic ferrules and are polished. The SMF is aligned to the PCF in the ceramic tube automatically. The other end of the PCF with the length of 5 cm is spliced with a SMF by using the technique reported in [18]. Finally, two ceramic ferrules are inserted into the notched ceramic tube, and an air-gap cavity is formed between the two ends of the SMF and PCF. The cavity locates at the middle of the notch. In this way, the water molecule can freely diffuse to the internal cavity through the notch of the ceramic tube. In the sensor, the all-solid PCF (Yangtze Optical Fiber and Cable Company) used in our experiment includes five rings of holes arranged in a regular hexagonal pattern, as shown in Fig. 4(b). The PCF has a center-to-center distance between the holes of = 8.5 m and an average hole diameter of d = 2.9 m. The core diameter is 11.4 m, and the outer diameter of the PCF is 125 m. Unlike the traditional PCF, the PCF used in this experiment is solid
Fig. 4. (a) The configuration of the humidity sensor and (b) the close view of the sensor with Agarose gel.
without any air holes. The PCF core is also solid and constructed by the pure silica. Then the Agarose gel is filled into the internal cavity. The solution is prepared by dissolving the 2.1 wt% Agarose into the distilled water in a beaker. Under the beaker a heater combined with a magnetic stirrer is fixed, of which the temperature is set to be 65 ◦ C. To dissolve the Agarose in the distilled water, the mixture of the Agarose and the distilled water inside the beaker is stirred by using the magnetic stirrer until the Agarose is completely dissolved. Then the sensor is immersed into the beaker, and the Agarose solution is flowed into the cavity because the mixture is still in the liquid form. After this, the mixture is cooled by removing the heater. When the mixture reaches the gelling point of 35 ◦ C, it becomes the form of hydrogel. In this way, the Agarose gel is filled into the cavity of the sensor, as shown in Fig. 4(c). Next step the sensor is rinsed with the distilled water to remove the residual Agarose gel on the surface of the sensor. The fiber sensor is kept for one day at room temperature until it is partially dehydrated and reaches the equilibrium with the ambient environment. 4. Experiment and discussion The experimental setup is shown in Fig. 5. An unflattened amplified spontaneous emission (ASE) source with wavelength covering 1525–1565 nm and output power of 20 mW is used as a broadband source to illuminate the humidity sensor. The transmission spectra of the sensor are interrogated by using a CCD based optical spectrum analyze (OSA) (BaySpec FBGA-F-1525-1565). The sensor was put inside the airtight humidity controlled chamber. The humidity and the temperature inside the chamber can be adjusted by using microprocessor based control unit and the feedback sensors. Fig. 6 shows transmission spectra of the proposed sensor with and without the Agarose gel, of which the distance of the gap is
Fig. 5. The experimental setup.
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Fig. 8. The spatial frequency of the interference fringe.
Fig. 6. Transmission spectra of the proposed sensor with and without the Agarose gel.
314 m. The transmission intensity with the Agarose gel is larger than that without the Agarose gel, because the MFD of the light in the air is much larger than the core size of the PCF, which results in the large mode field mismatch between the SMF and the PCF. The insertion loss of the Agarose gel filled into the gap between the SMF and the PCF is 6.3 dB. The influence of the distance of the gap to the humidity sensor is investigated. One ceramic ferrule of the sensor is fixed on a fixed stage, and the other ceramic ferrule is fixed on a single-Axis stage with the resolution of 1 m. The length of the gap could be changed precisely by adjusting the single-Axis stage with one ceramic ferrule. Then three sensors were fabricated, of which distances of gaps are 153 m, 204 m, and 256 m, respectively, and all gaps were filled with the Agarose gel. Given that ω0 = 9 m, = 1550 nm, and n = 1.46, the simulation results is shown in Fig. 7(a) according to Eq. (1). When the distance of the Agarose gel gap is smaller than 193 m, the MFD of the light is smaller than the core size of the PCF (11.4 m), which results in the low light intensity of the cladding mode. Thus the fringe visibility of the transmission spectrum is almost zero. Then the MFD, which is larger than the core size of the PCF, increases with the increase of the length of the gap, and the fringe visibility also increases simultaneously. Transmission spectra with different length of gap in a room RH of 50 ± 2% are shown in Fig. 7(b). The fringe visibility of the transmission spectrum is almost zero when the Agarose gel gap is 153 mm. With the
increase of the distance of the gap, the fringe visibility of transmission spectrum increases simultaneously. Therefore, when it is used as a sensing element, the length of the Agarose gel gap should be adjusted in order to keep the maximal fringe visibility. The proposed PCF based sensor was tested for relative humidity measurement over the range from 20%RH to 90%RH with the step of 10%RH. For the in-line MZI, many modes with different effective refractive indices may be excited when the light is coupled into the PCF from Agarose gel [27]. The homogeneous spectrum of the proposed sensor with the Agarose gel was Fourier transformed to obtain the spatial frequency of the interference fringe, as shown in Fig. 8. The spatial frequency confirms that the interference is mainly produced by two modes: one core mode and one dominant cladding mode [28]. Because other higher-order modes are very weak, they have little influence on the interference. Therefore, Eq. (4) can still be used to investigate the evolution of interference visibility. The transmission spectra which correspond to different relative humidity are shown in Fig. 9(a). For the increase of the humidity, the fringe visibility decreases simultaneously. The fringe visibility of the transmission spectrum can be expressed as [26] V=
Imax − Imin Imax + Imin
(6)
where Imax and Imin are wavelength peak and valley in the whitelight interferogram. To deal with these variations of fringe visibility, a wavelength valley at 1531.38 nm and an adjacent wavelength peak at 1532.37 nm are chosen to calculate the fringe visibility, as shown in Fig. 9(b). The response of the sensor keeps a good quadratic relationship between the fringe visibility and the relative humidity. The polynomial fitting curve can be expressed as
Fig. 7. (a) Simulations for the MFD of the light and the fringe visibility of the spectrum with different length of gap and (b) transmission spectra of the sensor with different length of gap.
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Fig. 9. (a) Transmission spectra of the sensor at different relative humidity and (b) average fringe visibilities at different relative humidity.
y = −1.5 × 10−4 x2 + 1.9 × 10−2 x + 11.1. Although the response is a nonlinear, the good quadratic curve also keeps an accurate relationship between the fringe visibility and the relative humidity in the practical measurement. The maximum sensitivity reaches 0.022 dB/% at the relative humidity of 90%. Because the resolution of the OSA is 0.001 dB, the resolution of 0.044% of the sensor is achieved. The resolution of the proposed method is higher than previous results of 0.4% which is based on the method of the long period grating [3], 0.78% which is based on the method of the FBG [11], or 1.2% which is based on the method of the a fiber bend [14]. Due to the unflattened background of the ASE, the sensitivity of the sensor to humidity is different at different wavelengths valley, as shown in Fig. 10. At the relative humidity of 50%, the visibility of the transmission spectrum at wavelength valley of 1526.72 nm is 4.62 dB. Then it reaches the peak of 7.84 dB at the wavelength of 1531.38. After that, the fringe visibility decreases with the increase of the wavelength. However, all of response with different wavelength valley of the sensor keeps a good quadratic relationship between the fringe visibility and the relative humidity. The sensor was evaluated for its stability and repeatability. To test the stability of the humidity sensor, a sensor with the Agarose gel gap distance of 314 m was fixed in the chamber, in which the humidity and temperature are 60%RH and 20 ◦ C, respectively. The fringe visibility with the resonance wavelength of 1531.48 nm was measured for 60 times over duration of 3 h. As shown in Fig. 11(a), the standard variation of the fringe visibility is 0.92%, which indicates the good stability of the sensor. Besides, the repeatability of the sensor was also experimentally tested. The humidity in the chamber was adjusted in an ascending and descending order, respectively, and the corresponding fringe visibility is shown in
Fig. 10. The fringe visibilities with different wavelength valley at different relative humidity.
Fig. 11(b). Results in the ascending and descending order are in good agreement with each other; the maximum variation is 0.3% at the humidity of 40%RH. The temperature dependence of the sensor is investigated by adjusting the temperature range of the chamber from 20 ◦ C to 55 ◦ C. The humidity was set with 30%RH, 50%RH, and 80%RH, respectively. The variation of the fringe visibility with the temperature change is shown in Fig. 12(a). The temperature sensitivities of the sensor are 0.045 dB/◦ C, 0.038 dB/◦ C, and 0.035 dB/◦ C in the relative humidity of 30%RH, 50%RH, and 80%RH, respectively. Because the thermo optical coefficient of the Agarose gel is negative, the fringe visibility is increased with the increase of temperature. It is observed that the proposed sensor is insensitive to temperature change with a high relative humidity. Although the sensor is suffered from a little temperature cross-sensitivity at the low relative humidity, the proposed sensor is much less sensitive to temperature change compared with some other fiber optic humidity sensors which are suffered from temperature cross-sensitivity seriously, such as long period gratings (LPGs) [3], or fiber bend humidity sensor [14]. Besides, a FBG can be fabricated in the SMF, and the temperature effect of the sensor at low humidity can be compensated by measuring the wavelength shift of the FBG [29]. On the other hand, the temperature sensitivity of the sensor at low humidity is higher than that at high humidity, because the form of the Agarose gel would be transformed from hydrogel to liquid with the increase of temperature [16]. When the humidity is high, pores in the Agarose gel are filled with water molecules, of which the RI is closed to the Agarose gel. Thus when the form of the Agarose gel is transformed with the temperature, the RI of the Agarose gel is changed slightly. However, pores in the Agarose gel are filled with air at low humidity, of which the RI is far away from the Agarose gel. Thus the RI of the Agarose gel is changed seriously when the form is transformed, and the visibility of the spectrum is more sensitive with temperature at high humidity. The time response of the sensor has been analyzed. The sampling frequency of the OSA is 5000 Hz. Thus the time resolution of the sensor is 0.2 ms, and the time response of the sensor is measured by calculating the sum of the sampling points. However, the humidity controlled chamber is not suitable for studying the response time of the sensor, because the chamber is adjusted very slowly. Instead, we lead the humid air from the chamber to the sensor through a rubber pipe. The response time is shown in Fig. 12(b). When the RH jumps from 56%RH (room humidity) to 80%RH, the estimated response time of the sensor is ∼200 ms. Then when the rubber pipe was removed, the estimated recovery time of the sensor is ∼230 ms. For the application, all of fibers are sealed into ceramic ferrule, and each component of the sensor, such as ceramic ferrule or tube,
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Fig. 11. (a) The stability of the humidity sensor and (b) the repeatability of the sensor.
Fig. 12. (a) The temperature dependence of the sensor and (b) the time response of the sensor.
is fixed with each other by using epoxy. Thus the mechanical property of the proposed sensor is stable and reusable robust. The RI of the Agarose gel is affected by the water molecules in pores, which would influence on the calibration of the sensor. In Ref. [14], Agarose gel shows a linear change and reversible response in its RI with respect to humidity, which presents a stable property of the Agarose gel. Therefore, the proposed sensor could be reused to measure the humidity repeatedly even without the calibration. Besides, in order to improve the calibration of the sensor precisely, the sensor could be fixed into the airtight humidity controlled chamber before each measurement. When the humidity in the chamber is fixed, the water molecules in pores of the Agarose gel can be evaporated by heating the sensor, and the visibility of the spectrum can be calibrated by adjusting the temperature. Moreover, the intensity of the spectrum would decreases when the length of the PCF is increased because of the highly loss of cladding modes [30]. However, once the proposed sensor is fabricated, the length of the PCF is almost fixed. According to Eqs. (2–4), the fringe visibility only depends on the property of Agarose gel, such as the RI or the length of the gap. Thus although the temperature fluctuant or vibration may induce the change of the PCF with a small range, the intensity of the spectrum is independent with the length of the PCF, and the fringe visibility is unaffected with the length variation of the PCF, which improve the stability and repeatability of the sensor. 5. Conclusion In conclusion, a compact relative humidity sensor based on the all-solid PCF is proposed and experimentally demonstrated. The sensor is constructed by filling the Agarose gel between the aligned SMFs and PCFs. A fiber in-line interferometer is fabricated
by splicing the other end of the PCF and another SMF. Due to the tunable RI property of the Agarose gel, the mode field diameter of the propagation light is changed with the external relative humidity, and the relative humidity is measured by interrogating the fringe visibility of the white-light interferogram. The experimental results show that the sensitivity of up to 2.2 dB/RH is achieved. The proposed sensor offers advantages of easy fabrication, good repeatability, and low temperature cross-sensitivity, and the technique appears to have potential applications in researches of chemistry, medicine, and biology. Acknowledgments This work is supported by Natural Scientific Foundation of China (51075037), Defense Equipments Foundation of China (9140A0206041213Q1028), Aeronautics Key Foundation of China (20110343004), and the Doctoral Foundation of Education Ministry of China (20101101110014). References [1] T.L. Yeo, T. Sun, K.T.V. Grattan, Fibre-optic sensor technologies for humidity and moisture measurement, Sens. Actuators A: Phys. 144 (2008) 280–295. [2] K. Milenko, D.J.J. Hu, P.P. Shum, T. Zhang, J.L. Lim, Y. Wang, T.R. Wolinski, H. Wei, W. Tong, Photonic crystal fiber tip interferometer for refractive index sensing, Opt. Lett. 37 (2012) 1373–1375. [3] Y. Liu, L.W. Wang, M. Zhang, D.S. Tu, X.H. Mao, Y.B. Liao, Long-period grating relative humidity sensor with hydrogel coating, IEEE Photon. Technol. Lett. 19 (2007) 880–882. [4] C. Zhang, W. Zhang, D.J. Webb, G.D. Peng, Optical fibre temperature and humidity sensor, Electron. Lett. 46 (2010) 643–644. [5] A. Alvarez-Herrero, H. Guerrero, D. Levy, High-sensitivity sensor of low relative humidity based on overlay on side-polished fibers, IEEE Sens. J. 4 (2004) 52–56.
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Biographies Ran Gao received his MS degree in Photoelectron from Beijing University of Technology, China, in 2010. He is currently working toward the PhD degree in Electronic Science and Technology Beijing Institute of Technology. His current research interest is focused on the optical fiber sensors. Yi Jiang received the BA degree in 1987 and PhD degree in 1996, respectively, from Chongqing University, China. He is a professor in Beijing Institute of technology. His research interests include fiber optical sensors, smart structures, and measurement instruments. Wenhui Ding is currently working toward the PhD degree in Electronic Science and Technology Beijing Institute of Technology. His current research interest is focused on the optical fiber sensors.