Intensity-modulated relative humidity sensing with polyvinyl alcohol coating and optical fiber gratings Jingyi Yang,1 Xinyong Dong,1,* Kai Ni,1 Chi Chu Chan,2 and Perry Ping Shun3 1
Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China
2
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457, Singapore
3
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 637457, Singapore *Corresponding author: [email protected] Received 5 January 2015; revised 3 March 2015; accepted 3 March 2015; posted 4 March 2015 (Doc. ID 231747); published 23 March 2015
A relative humidity (RH) sensor in reflection mode is proposed and experimentally demonstrated by using a polyvinyl alcohol (PVA)-coated tilted-fiber Bragg grating (TFBG) cascaded by a reflectionband-matched chirped-fiber Bragg grating (CFBG). The sensing principle is based on the RH-dependent refractive index of the PVA coating, which modulates the transmission function of the TFBG. The CFBG is properly designed to reflect a broadband of light spectrally suited at the cladding mode resonance region of the TFBG, thus the reflected optical signal passes through and is modulated by the TFBG again. As a result, RH measurements with enhanced sensitivity of ∼1.80 μW∕%RH are realized and demodulated in the range from 20% RH to 85% RH. © 2015 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.0060) Fiber optics and optical communications. http://dx.doi.org/10.1364/AO.54.002620
1. Introduction
Relative humidity (RH) is an essential environmental factor so the measurement of RH for the purpose of monitoring and control has attracted much attention. Compared with conventional humidity sensors based on the measurement of electrical resistivity or capacitance of various humidity-sensitive materials, optical fiber-based RH sensors possess many advantages such as electrically passive operation, long lifetime, and immunity to electromagnetic interference, which makes them quite qualified for use in some harsh circumstances [1–3]. Their operation normally relies on the use of moisture sensitive materials such as polyvinyl alcohol (PVA) [4–8], chitosan [9], agar, and so on to induce a secondary effect like a change in the refractive index or strain [4–8] that modulates 1559-128X/15/102620-05$15.00/0 © 2015 Optical Society of America 2620
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
the optical signal of a specific optical fiber structure [3,7,10–14]. Tilted-fiber Bragg gratings (TFBGs), which are named after their tilted grating planes related to the perpendicular of the fiber axis, have been widely investigated for RH measurements in recent years [14–18]. The TFBGs have not only Bragg mode like fiber Bragg gratings (FBGs), but also various cladding modes that respond differently to RH variation. However, most of the reported TFBG-based sensors were operated in transmission mode and demodulated by wavelength measurements [15,16], and this causes an inconvenience in practical applications and adds costs to the measurement system. To solve these problems, several intensity-modulated designs in reflection operation mode were proposed by using offset fiber fusion splicing [17], a long-period fiber grating, or a fiber taper [18] in front of the TFBG to couple the backward propagating cladding modes generated by the TFBG into the backward propagating core mode. Intensity of the recoupled
Fig. 1. (a) Transmission spectrum of the TFBG; (b) reflection spectrum of the CFBG; (c) reflection spectrum of the CFBG-cascaded TFBG.
signal is modulated by the measurands and collected by the lead-in fiber so that intensity-modulated measurements in reflection operation mode are realized. However, these mode coupling structures usually cause high insertion loss, introduce cross effects, and weaken the physical strength of the fiber. An optimized design based on a TFBG cascaded by a CFBG was developed by us in recent years and measurements of the refractive index [19] and magnetic field [20] were realized. In this paper, an intensity-modulated RH measurement is achieved by using a PVA-coated TFBG sensor cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The RH-dependent refractive index of the PVA coating modulates transmission of the TFBG. The CFBG is well designed to reflect the spectral region of cladding mode resonances of the TFBG, which is more sensitive to surrounding refractive index changes. The reflected optical signal of the CFBG passes through and is modulated by the PVAcoated TFBG again, thus contributing to an enhanced sensitivity. By optimizing the tilt angle of the TFBG based on the linear response refractive index range of PVA, RH measurements with a linear response in a wide range from 20% RH to 85% RH can be realized. The sensitivity is up to 180 μW∕%RH. 2. Sensor Fabrication and Principle
The gratings were manufactured by using the phase mask method in a hydrogen-loaded Germania-doped single mode fiber with a frequency-doubled argon laser emitting at 244 nm. The TFBG, which is achieved with a uniform phase mask with a 1085 nm period, is 10 mm long and has a tilt angle of 4°. Figure 1(a) shows the measured optical transmission spectrum of the TFBG. The Bragg mode resonance of the TFBG is located at 1585 nm with a transmission loss of ∼1.88 dB, while the cladding mode resonances are located between 1520 and 1583 nm with a maximum transmission loss of ∼28 dB at ∼1566 nm. The CFBG is 40 mm long, and it has a reflectivity of ∼14 dB in a broad and well-designed reflection band of ∼35 nm (1533–1568 nm), which covers most regions of the cladding mode resonances. The reflection spectra of the CFBG and the TFBG–CFBG combination are shown in Figs. 1(b) and 1(c), respectively.
The PVA was coated on the outside of the TFBG through a dip-coating process with a 3% PVA solution. After being cleaned with deionized water and dried with nitrogen gas, the bared sensing part was immersed into the PVA solution for several seconds and then taken out slowly and dried with nitrogen gas blowing. This process was repeated five times for one dip coating operation. The formed PVA coating film with a thickness of 6 μm can absorb moisture and swell physically; hence, its refractive index may decrease from 1.453 to 1.358 linearly with RH when the later varies from 20% RH to 90% RH [3,10,14]. When the refractive index of PVA is equal or greater than the effective refractive index of a certain cladding mode of the TFBG, this mode is no longer guided and the coupling now occurs with a continuum of radiation mode, as marked by a smoothing of the cladding mode resonance curve in the spectrum and reduction of the transmitted optical power. By cascading the reflection-band-matched CFBG to the TFBG, the operation mode of the proposed sensor is changed from transmission to reflection, which makes it more convenient in practical applications. Moreover, the optical signal is reflected by the CFBG and modulated by the TFBG again in a manner that enhances the sensitivity. A similar sensor head for refractive index measurements consisting of a bared TFBG and a CFBG was reported recently by us [19]. The measurement range is from 1.33 to ∼1.46, but the response is not linear especially when the refractive index is over 1.42. For achieving a linear response in this study, we first optimized the TFBG by changing its tilt angle from 6° to 4°. Notably, the grating planes of a TFBG are tilted in relation to the perpendicular of the fiber axis and the value of the tilt angle affects the coupling wavelengths of the cladding modes. By reducing the tilt angle, we can move the cladding mode resonance region toward the longer wavelength direction, i.e., closer to the Bragg mode resonance wavelength [21,22], thus including more cladding mode resonances within the reflection band of the CFBG. That will increase the refractive index measurement range and improve the linear response range correspondingly. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
2621
Fig. 2. (a) Evolution of the reflection spectra with the surrounding refractive index. (b) Reflected optical power versus refractive index.
3. Experimental Results and Discussion
The experimental setup for RH sensing is shown in Fig. 3. Light from a broadband source (BBS), without polarization dependence, centered at a wavelength of 1550 nm was launched into the TFBG–CFBG through an optical fiber circulator. The reflected light is guided into either an optical power meter (OPM) or an optical spectrum analyzer (OSA, Yokogawa, AQ6370B) for measurements of reflected power and spectrum, respectively. In the experiment, we tested the response of the bared TFBG (4°)–CFBG combination to the refractive index first by using glycerin–water solutions with different concentrations. Figure 2(a) shows the evolution of the reflection spectrum against the surrounding refractive index. It can been seen that by cascading the CFBG, the cladding mode resonance modulated wavelength region becomes attenuated gradually from the short wavelength. The reflected optical power decreases rapidly when the refractive index varies from 1.348 to 1.453, as shown in Fig. 2(b). The data fit well to a linear function, y −1228.7x 1947.2, and a relatively high R2 of 0.9912 was obtained. The sensitivity is −1228.7 μW∕R:I:U. For the purpose of comparison, the results reported in [19], which were derived by using a 6°-TFBG, are also shown in this figure. It is obvious that the linearity is improved significantly, as predicted in the last part. The humidity measurements are conducted in a half-opened humidity chamber as shown in Fig. 3. The PVA-coated sensor head, free from any bending
Fig. 3. Experimental setup of the proposed humidity sensor. 2622
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
and vibration, was mounted and fixed in the chamber. The humidity level in the chamber was controlled by inputting both dry and humid air with a changeable ratio between them. Increasing the ratio of humid air (air bubbled through the water) will increase the RH value and vice versa. In this experiment, the RH value was tuned into a range from 20% RH to 90% RH. A calibrated commercially available humidity meter with a sensitivity of 0.1% RH was mounted close to the sensor head to provide reference. The average response time is ∼2 s, which agrees well with the result reported in [10]. Figure 4(a) shows the measured reflection spectrum evolution of the proposed sensor with RH from 20% RH to 90% RH. When the atmosphere is at 20% RH, the cladding mode resonance region has been weakened already because the relatively high initial refractive index (∼1.45) of the PVA coating changes the guided cladding modes into radiation mode, which is very lossy and absorbed by the PVA coating finally [22]. When RH increases, the PVA expands with absorbing moisture and its refractive index is reduced gradually so the cladding mode resonances recover correspondingly from the lower order (longer wavelength) side and the reflection power becomes stronger and stronger. When RH reaches 85%, nearly all the cladding mode resonances within the reflected region are recovered. Figure 4(b) shows the measured reflected optical power by the OPM against RH. It increases from 172.5 to 287.3 μW within the entire range, i.e., 20% RH to 90% RH. From 20% RH to 85% RH, the data fit well to the linear function y 1.8048x 134.59 with a R2 of 0.9889, so the sensitivity is 1.805 μW∕%RH. When the RH exceeds 85% RH, the reflection power trends to a constant. This is caused by the saturation effect of the PVA film for moisture absorbing. After moisture is evaporated, the sensor can be operated properly as usual. During the humidity test over a period of 90 min at 60% RH, the maximum fluctuation observed is only 1.9 μW. That corresponds to ∼1% RH. The experiment has been repeated several times and all the data fit well.
Fig. 4. (a) Reflection spectrum evolution of the sensor with the relative humidity. (b) Measured reflected optical power versus relative humidity.
In principle, the proposed sensor is not sensitive to temperature because the temperature-caused wavelength shifts for both the TFBG and the CFBG are the same, which ensures that the reflected optical power is not changed with temperature. However, temperature measurements based on our sensor design are possible. They can be achieved by monitoring the wavelength shift of the Bragg mode of the TFBG [23]. Since the RH measurements are based on intensity demodulation, not wavelength demodulation, we did not include this aspect of the work. Besides, performance of the proposed RH sensor can be improved by some methods such as increasing the power level of the light source and reducing the insertion loss of the optical signal. Both can increase the signal-to-noise ratio and improve the measurement resolution. The transmission of the TFBG is also known to depend on the polarization state of the input light, especially when the tilt angle is relatively large. In this case, the tilt angle of 4° is quite small, but for high accuracy purposes, the polarized light source should be avoided in the sensing system as the reflected signal power may change if the polarization state changes because of environmental factors. 4. Conclusion
In summary, an intensity-modulated RH sensor operated in reflection mode has been reported, and it is formed by a PVA-coated TFBG with a tilt angle of 4° and cascaded by a matched CFBG whose reflection band covers most of the cladding mode resonances region of the TFBG. The reflected optical power is modulated twice by the RH through the PVA-coatedTFBG. Linear response with an enhanced sensitivity of ∼1.80 μW∕%RH has been achieved within the range from 20% RH to 85% RH. It also possesses advantages of robustness, low cost, and ease of operation in applications. This work was funded by the National Natural Science Foundation (NSF) (61475147) and the National Natural Science Foundation of Zhejiang Province, China (Z13F050003, LY13F050004).
References 1. S. Q. Zhang, X. Y. Dong, T. Li, C. C. Chan, and P. P. Shum, “Simultaneous measurement of relative humidity and temperature with PCF-MZI cascaded by fiber Bragg grating,” Opt. Commun. 303, 42–45 (2013). 2. J. F. Wang, H. H. Liang, X. Y. Dong, and Y. X. Jin, “A temperature-insensitive relative humidity sensor by using polarization maintaining fiber-based Sagnac interferometer,” Microwave Opt. Technol. Lett. 55, 2305–2307 (2013). 3. T. Li, C.-L. Zhao, X. Dong, W. Qian, Y. Jin, and S. Jin, “Relative humidity sensor based on photonic crystal fiber with tapered and filled in polymer,” in 2010 Asia Communications and Photonics Conference and Exhibition (ACP) (Institute of Electrical and Electronics Engineers, 2010), pp. 288–289. 4. A. Gastón, F. Pérez, and J. Sevilla, “Optical fiber relativehumidity sensor with polyvinyl alcohol film,” Appl. Opt. 43, 4127–4132 (2004). 5. B. Gupta, “A novel probe for a fiber optic humidity sensor,” Sensors Act. B Chem. 80, 132–135 (2001). 6. T. Li, X. Dong, C. C. Chan, C.-L. Zhao, and P. Zu, “Humidity sensor based on a multimode-fiber taper coated with polyvinyl alcohol interacting with a fiber Bragg grating,” IEEE Sens. J. 12, 2205–2208 (2012). 7. W. C. Wong, C. C. Chan, L. H. Chen, T. Li, K. X. Lee, and K. C. Leong, “Polyvinyl alcohol coated photonic crystal optical fiber sensor for humidity measurement,” Sens. Actuators B 174, 563–569 (2012). 8. D. Su, X. Qiao, Q. Rong, H. Sun, J. Zhang, Z. Bai, Y. Du, D. Feng, Y. Wang, M. Hu, and Z. Feng, “A fiber Fabry–Perot interferometer based on a PVA coating for humidity measurement,” Opt. Commun. 311, 107–110 (2013). 9. L. H. Chen, T. Li, C. C. Chan, R. Menon, P. Balamurali, M. Shaillender, B. Neu, X. M. Ang, P. Zu, W. C. Wong, and K. C. Leong, “Chitosan based fiber-optic Fabry–Perot humidity sensor,” Sens. Actuators B 169, 167–172 (2012). 10. L. Tao, D. Xinyong, C. Chi Chiu, Z. Chun-Liu, and Z. Peng, “Humidity sensor based on a multimode-fiber taper coated with polyvinyl alcohol interacting with a fiber Bragg grating,” IEEE Sens. J. 12, 2205–2208 (2012). 11. P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, “Relative humidity sensor with optical fiber Bragg gratings,” Opt. Lett. 27, 1385–1387 (2002). 12. T. Venugopalan, T. L. Yeo, T. Sun, and K. T. V. Grattan, “LPGbased PVA coated sensor for relative humidity measurement,” IEEE Sens. J. 8, 1093–1098 (2008). 13. L. Honglin, L. Houhui, S. Mingming, N. Kai, and J. Yongxing, “Simultaneous measurement of humidity and temperature based on a long-period fiber grating inscribed in fiber loop mirror,” IEEE Sens. J. 14, 893–896 (2014). 14. X. Dong, T. Li, Y. Liu, Y. Li, C. L. Zhao, and C. C. Chan, “Polyvinyl alcohol-coated hybrid fiber grating for relative humidity sensing,” J. Biomed. Opt. 16, 077001 (2011). 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
2623
15. Y. P. Miao, B. Liu, H. Zhang, Y. Li, H. B. Zhou, H. Sun, W. H. Zhang, and Q. D. Zhao, “Relative humidity sensor based on tilted fiber Bragg grating with polyvinyl alcohol coating,” IEEE Photon. Technol. Lett. 21, 441–443 (2009). 16. X. Yu, P. Childs, M. Zhang, Y. Liao, J. Ju, and W. Jin, “Relative humidity sensor based on cascaded long-period gratings with hydrogel coatings and Fourier demodulation,” IEEE Photon. Technol. Lett. 21, 1828–1830 (2009). 17. T. Guo, H.-Y. Tam, P. A. Krug, and J. Albert, “Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling,” Opt. Express 17, 5736–5742 (2009). 18. T. Guo, L. Y. Shao, H. Y. Tam, P. A. Krug, and J. Albert, “Tilted fiber grating accelerometer incorporating an abrupt biconical taper for cladding to core recoupling,” Opt. Express 17, 20651– 20660 (2009).
2624
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
19. J. Zheng, X. Dong, J. Ji, H. Su, and P. Ping Shum, “Powerreferenced refractometer with tilted fiber Bragg grating cascaded by chirped grating,” Opt. Commun. 312, 106–109 (2014). 20. J. Zheng, X. Dong, P. Zu, J. Ji, H. Su, and P. Ping Shum, “Intensity-modulated magnetic field sensor based on magnetic fluid and optical fiber gratings,” Appl. Phys. Lett. 103, 183511 (2013). 21. J. Albert, L.-Y. Shao, and C. Caucheteur, “Tilted fiber Bragg grating sensors,” Laser Photon. Rev. 7, 83–108 (2013). 22. Y. P. Miao, B. Liu, and Q.-D. Zhao, “Refractive index sensor based on measuring the transmission power of tilted fiber Bragg grating,” Opt. Fiber Technol. 15, 233–236 (2009). 23. T. Guo, A. Ivanov, C. Chen, and J. Albert, “Temperatureindependent tilted fiber grating vibration sensor based on cladding-core recoupling,” Opt. Lett. 33, 1004–1006 (2008).
Institute of Optoelectronic Technology, China Jiliang University, Hangzhou 310018, China
2
School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637457, Singapore
3
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 637457, Singapore *Corresponding author: [email protected] Received 5 January 2015; revised 3 March 2015; accepted 3 March 2015; posted 4 March 2015 (Doc. ID 231747); published 23 March 2015
A relative humidity (RH) sensor in reflection mode is proposed and experimentally demonstrated by using a polyvinyl alcohol (PVA)-coated tilted-fiber Bragg grating (TFBG) cascaded by a reflectionband-matched chirped-fiber Bragg grating (CFBG). The sensing principle is based on the RH-dependent refractive index of the PVA coating, which modulates the transmission function of the TFBG. The CFBG is properly designed to reflect a broadband of light spectrally suited at the cladding mode resonance region of the TFBG, thus the reflected optical signal passes through and is modulated by the TFBG again. As a result, RH measurements with enhanced sensitivity of ∼1.80 μW∕%RH are realized and demodulated in the range from 20% RH to 85% RH. © 2015 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (060.0060) Fiber optics and optical communications. http://dx.doi.org/10.1364/AO.54.002620
1. Introduction
Relative humidity (RH) is an essential environmental factor so the measurement of RH for the purpose of monitoring and control has attracted much attention. Compared with conventional humidity sensors based on the measurement of electrical resistivity or capacitance of various humidity-sensitive materials, optical fiber-based RH sensors possess many advantages such as electrically passive operation, long lifetime, and immunity to electromagnetic interference, which makes them quite qualified for use in some harsh circumstances [1–3]. Their operation normally relies on the use of moisture sensitive materials such as polyvinyl alcohol (PVA) [4–8], chitosan [9], agar, and so on to induce a secondary effect like a change in the refractive index or strain [4–8] that modulates 1559-128X/15/102620-05$15.00/0 © 2015 Optical Society of America 2620
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
the optical signal of a specific optical fiber structure [3,7,10–14]. Tilted-fiber Bragg gratings (TFBGs), which are named after their tilted grating planes related to the perpendicular of the fiber axis, have been widely investigated for RH measurements in recent years [14–18]. The TFBGs have not only Bragg mode like fiber Bragg gratings (FBGs), but also various cladding modes that respond differently to RH variation. However, most of the reported TFBG-based sensors were operated in transmission mode and demodulated by wavelength measurements [15,16], and this causes an inconvenience in practical applications and adds costs to the measurement system. To solve these problems, several intensity-modulated designs in reflection operation mode were proposed by using offset fiber fusion splicing [17], a long-period fiber grating, or a fiber taper [18] in front of the TFBG to couple the backward propagating cladding modes generated by the TFBG into the backward propagating core mode. Intensity of the recoupled
Fig. 1. (a) Transmission spectrum of the TFBG; (b) reflection spectrum of the CFBG; (c) reflection spectrum of the CFBG-cascaded TFBG.
signal is modulated by the measurands and collected by the lead-in fiber so that intensity-modulated measurements in reflection operation mode are realized. However, these mode coupling structures usually cause high insertion loss, introduce cross effects, and weaken the physical strength of the fiber. An optimized design based on a TFBG cascaded by a CFBG was developed by us in recent years and measurements of the refractive index [19] and magnetic field [20] were realized. In this paper, an intensity-modulated RH measurement is achieved by using a PVA-coated TFBG sensor cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The RH-dependent refractive index of the PVA coating modulates transmission of the TFBG. The CFBG is well designed to reflect the spectral region of cladding mode resonances of the TFBG, which is more sensitive to surrounding refractive index changes. The reflected optical signal of the CFBG passes through and is modulated by the PVAcoated TFBG again, thus contributing to an enhanced sensitivity. By optimizing the tilt angle of the TFBG based on the linear response refractive index range of PVA, RH measurements with a linear response in a wide range from 20% RH to 85% RH can be realized. The sensitivity is up to 180 μW∕%RH. 2. Sensor Fabrication and Principle
The gratings were manufactured by using the phase mask method in a hydrogen-loaded Germania-doped single mode fiber with a frequency-doubled argon laser emitting at 244 nm. The TFBG, which is achieved with a uniform phase mask with a 1085 nm period, is 10 mm long and has a tilt angle of 4°. Figure 1(a) shows the measured optical transmission spectrum of the TFBG. The Bragg mode resonance of the TFBG is located at 1585 nm with a transmission loss of ∼1.88 dB, while the cladding mode resonances are located between 1520 and 1583 nm with a maximum transmission loss of ∼28 dB at ∼1566 nm. The CFBG is 40 mm long, and it has a reflectivity of ∼14 dB in a broad and well-designed reflection band of ∼35 nm (1533–1568 nm), which covers most regions of the cladding mode resonances. The reflection spectra of the CFBG and the TFBG–CFBG combination are shown in Figs. 1(b) and 1(c), respectively.
The PVA was coated on the outside of the TFBG through a dip-coating process with a 3% PVA solution. After being cleaned with deionized water and dried with nitrogen gas, the bared sensing part was immersed into the PVA solution for several seconds and then taken out slowly and dried with nitrogen gas blowing. This process was repeated five times for one dip coating operation. The formed PVA coating film with a thickness of 6 μm can absorb moisture and swell physically; hence, its refractive index may decrease from 1.453 to 1.358 linearly with RH when the later varies from 20% RH to 90% RH [3,10,14]. When the refractive index of PVA is equal or greater than the effective refractive index of a certain cladding mode of the TFBG, this mode is no longer guided and the coupling now occurs with a continuum of radiation mode, as marked by a smoothing of the cladding mode resonance curve in the spectrum and reduction of the transmitted optical power. By cascading the reflection-band-matched CFBG to the TFBG, the operation mode of the proposed sensor is changed from transmission to reflection, which makes it more convenient in practical applications. Moreover, the optical signal is reflected by the CFBG and modulated by the TFBG again in a manner that enhances the sensitivity. A similar sensor head for refractive index measurements consisting of a bared TFBG and a CFBG was reported recently by us [19]. The measurement range is from 1.33 to ∼1.46, but the response is not linear especially when the refractive index is over 1.42. For achieving a linear response in this study, we first optimized the TFBG by changing its tilt angle from 6° to 4°. Notably, the grating planes of a TFBG are tilted in relation to the perpendicular of the fiber axis and the value of the tilt angle affects the coupling wavelengths of the cladding modes. By reducing the tilt angle, we can move the cladding mode resonance region toward the longer wavelength direction, i.e., closer to the Bragg mode resonance wavelength [21,22], thus including more cladding mode resonances within the reflection band of the CFBG. That will increase the refractive index measurement range and improve the linear response range correspondingly. 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
2621
Fig. 2. (a) Evolution of the reflection spectra with the surrounding refractive index. (b) Reflected optical power versus refractive index.
3. Experimental Results and Discussion
The experimental setup for RH sensing is shown in Fig. 3. Light from a broadband source (BBS), without polarization dependence, centered at a wavelength of 1550 nm was launched into the TFBG–CFBG through an optical fiber circulator. The reflected light is guided into either an optical power meter (OPM) or an optical spectrum analyzer (OSA, Yokogawa, AQ6370B) for measurements of reflected power and spectrum, respectively. In the experiment, we tested the response of the bared TFBG (4°)–CFBG combination to the refractive index first by using glycerin–water solutions with different concentrations. Figure 2(a) shows the evolution of the reflection spectrum against the surrounding refractive index. It can been seen that by cascading the CFBG, the cladding mode resonance modulated wavelength region becomes attenuated gradually from the short wavelength. The reflected optical power decreases rapidly when the refractive index varies from 1.348 to 1.453, as shown in Fig. 2(b). The data fit well to a linear function, y −1228.7x 1947.2, and a relatively high R2 of 0.9912 was obtained. The sensitivity is −1228.7 μW∕R:I:U. For the purpose of comparison, the results reported in [19], which were derived by using a 6°-TFBG, are also shown in this figure. It is obvious that the linearity is improved significantly, as predicted in the last part. The humidity measurements are conducted in a half-opened humidity chamber as shown in Fig. 3. The PVA-coated sensor head, free from any bending
Fig. 3. Experimental setup of the proposed humidity sensor. 2622
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
and vibration, was mounted and fixed in the chamber. The humidity level in the chamber was controlled by inputting both dry and humid air with a changeable ratio between them. Increasing the ratio of humid air (air bubbled through the water) will increase the RH value and vice versa. In this experiment, the RH value was tuned into a range from 20% RH to 90% RH. A calibrated commercially available humidity meter with a sensitivity of 0.1% RH was mounted close to the sensor head to provide reference. The average response time is ∼2 s, which agrees well with the result reported in [10]. Figure 4(a) shows the measured reflection spectrum evolution of the proposed sensor with RH from 20% RH to 90% RH. When the atmosphere is at 20% RH, the cladding mode resonance region has been weakened already because the relatively high initial refractive index (∼1.45) of the PVA coating changes the guided cladding modes into radiation mode, which is very lossy and absorbed by the PVA coating finally [22]. When RH increases, the PVA expands with absorbing moisture and its refractive index is reduced gradually so the cladding mode resonances recover correspondingly from the lower order (longer wavelength) side and the reflection power becomes stronger and stronger. When RH reaches 85%, nearly all the cladding mode resonances within the reflected region are recovered. Figure 4(b) shows the measured reflected optical power by the OPM against RH. It increases from 172.5 to 287.3 μW within the entire range, i.e., 20% RH to 90% RH. From 20% RH to 85% RH, the data fit well to the linear function y 1.8048x 134.59 with a R2 of 0.9889, so the sensitivity is 1.805 μW∕%RH. When the RH exceeds 85% RH, the reflection power trends to a constant. This is caused by the saturation effect of the PVA film for moisture absorbing. After moisture is evaporated, the sensor can be operated properly as usual. During the humidity test over a period of 90 min at 60% RH, the maximum fluctuation observed is only 1.9 μW. That corresponds to ∼1% RH. The experiment has been repeated several times and all the data fit well.
Fig. 4. (a) Reflection spectrum evolution of the sensor with the relative humidity. (b) Measured reflected optical power versus relative humidity.
In principle, the proposed sensor is not sensitive to temperature because the temperature-caused wavelength shifts for both the TFBG and the CFBG are the same, which ensures that the reflected optical power is not changed with temperature. However, temperature measurements based on our sensor design are possible. They can be achieved by monitoring the wavelength shift of the Bragg mode of the TFBG [23]. Since the RH measurements are based on intensity demodulation, not wavelength demodulation, we did not include this aspect of the work. Besides, performance of the proposed RH sensor can be improved by some methods such as increasing the power level of the light source and reducing the insertion loss of the optical signal. Both can increase the signal-to-noise ratio and improve the measurement resolution. The transmission of the TFBG is also known to depend on the polarization state of the input light, especially when the tilt angle is relatively large. In this case, the tilt angle of 4° is quite small, but for high accuracy purposes, the polarized light source should be avoided in the sensing system as the reflected signal power may change if the polarization state changes because of environmental factors. 4. Conclusion
In summary, an intensity-modulated RH sensor operated in reflection mode has been reported, and it is formed by a PVA-coated TFBG with a tilt angle of 4° and cascaded by a matched CFBG whose reflection band covers most of the cladding mode resonances region of the TFBG. The reflected optical power is modulated twice by the RH through the PVA-coatedTFBG. Linear response with an enhanced sensitivity of ∼1.80 μW∕%RH has been achieved within the range from 20% RH to 85% RH. It also possesses advantages of robustness, low cost, and ease of operation in applications. This work was funded by the National Natural Science Foundation (NSF) (61475147) and the National Natural Science Foundation of Zhejiang Province, China (Z13F050003, LY13F050004).
References 1. S. Q. Zhang, X. Y. Dong, T. Li, C. C. Chan, and P. P. Shum, “Simultaneous measurement of relative humidity and temperature with PCF-MZI cascaded by fiber Bragg grating,” Opt. Commun. 303, 42–45 (2013). 2. J. F. Wang, H. H. Liang, X. Y. Dong, and Y. X. Jin, “A temperature-insensitive relative humidity sensor by using polarization maintaining fiber-based Sagnac interferometer,” Microwave Opt. Technol. Lett. 55, 2305–2307 (2013). 3. T. Li, C.-L. Zhao, X. Dong, W. Qian, Y. Jin, and S. Jin, “Relative humidity sensor based on photonic crystal fiber with tapered and filled in polymer,” in 2010 Asia Communications and Photonics Conference and Exhibition (ACP) (Institute of Electrical and Electronics Engineers, 2010), pp. 288–289. 4. A. Gastón, F. Pérez, and J. Sevilla, “Optical fiber relativehumidity sensor with polyvinyl alcohol film,” Appl. Opt. 43, 4127–4132 (2004). 5. B. Gupta, “A novel probe for a fiber optic humidity sensor,” Sensors Act. B Chem. 80, 132–135 (2001). 6. T. Li, X. Dong, C. C. Chan, C.-L. Zhao, and P. Zu, “Humidity sensor based on a multimode-fiber taper coated with polyvinyl alcohol interacting with a fiber Bragg grating,” IEEE Sens. J. 12, 2205–2208 (2012). 7. W. C. Wong, C. C. Chan, L. H. Chen, T. Li, K. X. Lee, and K. C. Leong, “Polyvinyl alcohol coated photonic crystal optical fiber sensor for humidity measurement,” Sens. Actuators B 174, 563–569 (2012). 8. D. Su, X. Qiao, Q. Rong, H. Sun, J. Zhang, Z. Bai, Y. Du, D. Feng, Y. Wang, M. Hu, and Z. Feng, “A fiber Fabry–Perot interferometer based on a PVA coating for humidity measurement,” Opt. Commun. 311, 107–110 (2013). 9. L. H. Chen, T. Li, C. C. Chan, R. Menon, P. Balamurali, M. Shaillender, B. Neu, X. M. Ang, P. Zu, W. C. Wong, and K. C. Leong, “Chitosan based fiber-optic Fabry–Perot humidity sensor,” Sens. Actuators B 169, 167–172 (2012). 10. L. Tao, D. Xinyong, C. Chi Chiu, Z. Chun-Liu, and Z. Peng, “Humidity sensor based on a multimode-fiber taper coated with polyvinyl alcohol interacting with a fiber Bragg grating,” IEEE Sens. J. 12, 2205–2208 (2012). 11. P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, “Relative humidity sensor with optical fiber Bragg gratings,” Opt. Lett. 27, 1385–1387 (2002). 12. T. Venugopalan, T. L. Yeo, T. Sun, and K. T. V. Grattan, “LPGbased PVA coated sensor for relative humidity measurement,” IEEE Sens. J. 8, 1093–1098 (2008). 13. L. Honglin, L. Houhui, S. Mingming, N. Kai, and J. Yongxing, “Simultaneous measurement of humidity and temperature based on a long-period fiber grating inscribed in fiber loop mirror,” IEEE Sens. J. 14, 893–896 (2014). 14. X. Dong, T. Li, Y. Liu, Y. Li, C. L. Zhao, and C. C. Chan, “Polyvinyl alcohol-coated hybrid fiber grating for relative humidity sensing,” J. Biomed. Opt. 16, 077001 (2011). 1 April 2015 / Vol. 54, No. 10 / APPLIED OPTICS
2623
15. Y. P. Miao, B. Liu, H. Zhang, Y. Li, H. B. Zhou, H. Sun, W. H. Zhang, and Q. D. Zhao, “Relative humidity sensor based on tilted fiber Bragg grating with polyvinyl alcohol coating,” IEEE Photon. Technol. Lett. 21, 441–443 (2009). 16. X. Yu, P. Childs, M. Zhang, Y. Liao, J. Ju, and W. Jin, “Relative humidity sensor based on cascaded long-period gratings with hydrogel coatings and Fourier demodulation,” IEEE Photon. Technol. Lett. 21, 1828–1830 (2009). 17. T. Guo, H.-Y. Tam, P. A. Krug, and J. Albert, “Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling,” Opt. Express 17, 5736–5742 (2009). 18. T. Guo, L. Y. Shao, H. Y. Tam, P. A. Krug, and J. Albert, “Tilted fiber grating accelerometer incorporating an abrupt biconical taper for cladding to core recoupling,” Opt. Express 17, 20651– 20660 (2009).
2624
APPLIED OPTICS / Vol. 54, No. 10 / 1 April 2015
19. J. Zheng, X. Dong, J. Ji, H. Su, and P. Ping Shum, “Powerreferenced refractometer with tilted fiber Bragg grating cascaded by chirped grating,” Opt. Commun. 312, 106–109 (2014). 20. J. Zheng, X. Dong, P. Zu, J. Ji, H. Su, and P. Ping Shum, “Intensity-modulated magnetic field sensor based on magnetic fluid and optical fiber gratings,” Appl. Phys. Lett. 103, 183511 (2013). 21. J. Albert, L.-Y. Shao, and C. Caucheteur, “Tilted fiber Bragg grating sensors,” Laser Photon. Rev. 7, 83–108 (2013). 22. Y. P. Miao, B. Liu, and Q.-D. Zhao, “Refractive index sensor based on measuring the transmission power of tilted fiber Bragg grating,” Opt. Fiber Technol. 15, 233–236 (2009). 23. T. Guo, A. Ivanov, C. Chen, and J. Albert, “Temperatureindependent tilted fiber grating vibration sensor based on cladding-core recoupling,” Opt. Lett. 33, 1004–1006 (2008).
