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Improving the sensitivity of a humidity sensor based on fiber bend coated with a hygroscopic coating
Optics & Laser Technology 43 (2011) 1301–1305
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Improving the sensitivity of a humidity sensor based on fiber bend coated with a hygroscopic coating Jinesh Mathew n, Yuliya Semenova, Ginu Rajan, Pengfei Wang, Gerald Farrell DIT Photonics Research Centre, School of Electronic and Communications Engineering, Dublin Institute of Technology, Dublin 8, Ireland
a r t i c l e i n f o
abstract
Article history: Received 10 June 2010 Received in revised form 24 March 2011 Accepted 28 March 2011 Available online 15 April 2011
A highly sensitive all-fiber humidity sensor is demonstrated. The sensor behaves as a humidity dependent optical switch between 85% and 90% RH. This sensor also offers the advantages of simple structure and low cost and its response is fast and reversible in nature. The typical humidity response of the sensor is suitable for using it as a human breath rate monitor. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Macro bend fibre Humidity sensor Poly(ethylene oxide)
1. Introduction The ratio of the percentage of water vapour present in air at a particular temperature to the maximum amount of water vapour the air can hold at that temperature is called relative humidity. Measurement of humidity is required in a range of areas, including meteorological services, the chemical and food processing industry, civil engineering, air-conditioning, horticulture, and electronic processing. Therefore, development of a low-cost humidity sensor with a fast response is very important. Optical fibre humidity sensors offer specific advantages, such as small size and weight, immunity to electromagnetic interference, corrosion resistance and remote operation, by comparison to their conventional electronic counterparts. A wide range of optical fibre humidity sensors have been reported in the literature. Most fibre optic humidity sensors work on the basis of a hygroscopic material coated over the optical fibre to modulate the light propagating through the fibre [1]. The available fibre optic humidity sensors require complex processing of the fibre, such as fabrication of gratings [2], cladding removal [3,4], side polishing [5,6], tapering [7], formation of a fibre Fabry–Perot cavity [8], or photonic crystal fibre interferometer [9] to allow the light to interact with the hygroscopic coating. The use of a simple bent optical fibre has been previously demonstrated by Bownass et al. [10] for humidity state detection (dry or humid) in telecommunications networks using a bent standard single mode fibre coated with an organic film polyethylene oxide (PEO). A bend
n
Corresponding author. Tel.: þ35314024812; fax: þ 35314024690. E-mail address: [email protected] (J. Mathew).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.03.028
loss based humidity sensor is attractive because the fabrication of the sensor head does not require special treatment of the fibre. In Ref. [10], conventional low bend loss SMF fibre is used, which results in low sensitivity, while the selection of an optimum wavelength to maximise the sensitivity was also not considered. In this paper we present a detailed study of such a bend loss based sensor with a focus on significantly improving the sensitivity. In our study we observe that in a buffer-stripped bent single mode fibre, due to the coupling of the fundamental mode to cladding modes, resonant peaks will occur in the transmission response [11]. This response is oscillatory with respect to the bend radius and wavelength and also varies with ambient refractive index. If the surrounding refractive index of the bend fibre is changed, it will lead to a change in the coupling conditions and results in a shift in the wavelength of the resonant peaks. In order to achieve improved sensitivity for the humidity sensor we used a high bend loss fibre (1060XP). We characterised the spectral response of this bent fibre as a function of surrounding refractive index (RI) to establish the effect of wavelength on the sensitivity of the sensor. A bend radius of 15 mm is used in our investigation, which is an additional advantage of using a high bend loss fibre in that the sensor bend radius can be much larger than the 6 mm used in Ref. [10] with standard SMF, resulting in a much lower probability of stress-induced breakage of the fibre. As a confirmation of the suitability of using a high bend loss fibre bend as a sensitive humidity sensor, we implement and study a sensor using a 1060XP fibre bend coated with PEO. We show that it is possible to utilise a simple ratiometric power measurement scheme for interrogation and we also determine the optimum operating wavelength to maximise sensitivity using such a scheme. The sensor offers a competitive humidity
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sensitivity utilising a cost effective ratiometric power measurement system. The response of the demonstrated humidity sensor is reversible and shows good long term mechanical stability. Finally, we also study for the first time the time response of such a sensor and we show that the response to a humidity change is very fast by comparison with existing electronic sensors.
2. Operating principle of the sensor When an uncoated fibre is bent the effective refractive index profile is changed [12–14]. Light propagating on the outside of the bend has further to travel and this effect can be modelled as a tilt applied to the refractive index profile of the fibre [14], given as nðxÞ ¼ n0 ðxÞ½1 þ ð1 þ gÞx=R
ð1Þ
where n0(x) is the refractive index profile when the fibre is straight, R is the bend radius, x is a transverse coordinate along a line joining the centre of curvature and the centre of the fibre, with the origin in the centre of the fibre and with x increasing in a positive fashion toward the fibre outer surface. The factor g ¼ 0.22 for silica fibre [14] accounts for the strain-optic effect. According to coupled mode theory the fundamental core-guided mode can couple with a cladding mode with a peak intensity at the outer rim of the fibre bend [12,13], which propagates along the fibre bend as a Whispering Gallery mode. With increasing bend curvature the refractive index profile of the bend fibre becomes more tilted and the effective index for the cladding modes increases, reaching a maximum at the outer edge. At some critical bend radius, the effective index of the cladding modes equals the effective index for the core-guided mode. If there is sufficient overlap between the core mode and a cladding mode at this point, the core mode and a cladding mode will be coupled together. At tighter bends the core mode will couple to other cladding modes. In the straight portion of the fibre following the bend region, the conventional fibre buffer coating suppresses the cladding modes so that the power coupled to the cladding mode is absorbed and is not re-coupled to the core mode. Therefore at certain bend radii and wavelengths, there are dips in the transmission spectrum of the macro bend fibre that are a function of the degree of coupling of the fundamental mode to the cladding modes [11]. The phase matching condition for mode coupling also depends on the ambient refractive index at the bend. If the surrounding refractive index of the bent fibre is changed, it will lead to a change in the coupling conditions and will result in a shift in the resonant wavelength. To utilise such a bent fibre as a humidity sensor the buffer-stripped fibre bend can be coated with a hygroscopic material, whose refractive index changes with respect to the ambient humidity. In practice the simplest mechanical configuration for such a sensor is a simple fibre bend where the number of turns is less than one. Because of the low bend sensitivity of standard fibre such as SMF28, a sensor fabricated from such a fibre will need to utilise very low values of bend radius to achieve a reasonable sensitivity, increasing the risk of stress-induced fibre breakage. To improve sensitivity and increase the bend radius so as to reduce the risk of fibre breakage, a high bend loss fibre such as 1060XP is much more suitable.
the fabrication of a compact sensor head. Since it is known that suitable humidity sensitive coating materials can have RI values that range from below the fibre cladding RI (1.456) to above the cladding RI, it was necessary in this experimental study to utilise RI values from circa 1.340 to 1.478. A 35 mm length of the buffer coating was stripped from the middle section of a longer length of the fibre and the fibre was fixed on to a semi-circular rod of outer diameter 30 mm forming a bend angle (y) of about 1331. We selected this bend radius and bend angle experimentally as it was found that this is the highest bend radius and smallest bend angle that can give strong resonant dips in the measurable wavelength range of our experiment. A high bend radius is desirable in order to minimise the risk of fibre breakage. Light from a Super Luminescent Diode was launched into the fibre from the input side of the bend and the output from the bend was connected to an Optical Spectrum Analyser. The transmission response of this bent fibre was studied by immersing it in different RI solutions prepared by precisely mixing Dimethyl sulfoxide (DMSO) and water with different volume concentrations. The RI of the prepared solutions is obtained from an Abbe refractometer. The transmission responses of the bent fibre for different ambient RIs are shown in Fig. 1 for the measurable wavelength range in our experiment. For ambient RIs below the value of the refractive index of the fibre cladding, resonant dips are observed in the transmission response. As shown in Fig. 2, when the RI increases the resonant wavelength shifts to a shorter wavelength. When the surrounding RI matches or exceeds the RI of the fibre cladding the loss dips disappear from the transmission spectrum; the surrounding liquid acts as an infinite cladding, effectively absorbing the leaky modes. In this range of ambient RIs, resonant peaks are suppressed and the transmission loss increases monotonically with wavelength and the ambient RI as expected.
Fig. 1. Transmission response of bent fibre for different ambient refractive indices.
3. Experimental RI characterisation of the fibre bend Initially an experimental characterisation of the fibre bend was carried out to enable the selection of a suitable polymer coating for humidity sensing. The fibre used in the experiment was a 1060XP single mode fibre, a high bend loss fibre that allows for
Fig. 2. Resonance wavelength shift as a function of the ambient refractive index.
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4. Experimental characterisation of the fibre bend as a humidity sensor To utilise this structure as a humidity sensor the bufferstripped fibre bend was coated with a hygroscopic material, whose refractive index depends on the ambient humidity. A humidity sensitive polymer, polyethylene oxide (PEO), was chosen as a polymer coating with a refractive index in the range 1.4–1.5 with respect to humidity [10]. Such a range of RIs makes it a suitable choice for coating, given the range of RI values, which can be measured using the single mode fibre bend sensor. The refractive index of the PEO decreases with increasing humidity and this change is reversible. PEO also has the advantage that it can be easily coated on to the fibre due to its high adhesion with silica [2]. The sensor head is fabricated by dip coating the buffer-striped portion of the 1060XP fibre in a PEO solution prepared by dissolving 6 wt% PEO (Sigma-Aldrich) in distilled water. No thickness control was used or required, since the function of the coating was simply to act as a variable refractive index layer. The coated fibre is kept for one day at room temperature until it is partially dehydrated and reaches equilibrium with the ambient room humidity. To make it mechanically stable the probe is then fixed on to a semi-circular rod of outer diameter 30 cm forming a bend of about 1331. A schematic diagram of a PEO coated fibre bend is shown in Fig. 3. The humidity response of the fabricated sensor head is studied at room temperature and normal atmospheric pressure by placing the sensing element in a customised climate chamber shown in the Fig. 4. It consists of a sealed acrylic chamber with a dry/wet air flow system that can vary the internal humidity in the chamber. A calibrated electronic humidity sensor (model: 11-661-21, make: Control Company) was used for monitoring the temperature and humidity inside the chamber. We have observed the transmission response of the PEO coated fibre bend for a range of humidity values from 30% RH to 95% RH. No resonant dips were observed in the transmission spectrum for ambient humidity values below 85% RH, which is shown in Fig. 5. As concluded from in Fig. 1, this is due to the fact that the RI of the coated PEO film is above the RI of the fibre
Fig. 3. Poly(ethylene oxide) coated fibre bend.
Fig. 4. Experimental setup for studying the humidity response of the PEO coated fibre bend.
Fig. 5. Humidity response of the PEO coated fibre bend for RH below 85%.
Fig. 6. Appearance of loss dips in the transmission spectrum in a PEO coated fibre bend at RH above 85% due to resonant mode coupling at particular RI values of the coating.
cladding and therefore the PEO coating is acting as an absorption coating. As surrounding humidity value increases above 85% RH resonant dips appear in the transmission spectrum. The appearance of these loss dips in the transmission spectrum in a PEO coated fibre bend is due to resonant mode coupling at particular RI values as shown in Fig. 6. The reason for these resonant dips is that around this humidity value PEO undergoes a physical phase change from a semi-crystalline structure to a gel with a large decrease in its index of refraction to a value below the RI of the fibre cladding and as we have shown in Fig. 1 this will result in resonant dips in the transmission spectrum as the surrounding RI of the fibre bend is below the cladding RI. At higher humidity values, the RI of PEO is below the refractive index of the fibre cladding (1.456) and the resonant wavelength shifts to a higher wavelength with respect to humidity as shown in Fig. 7 and as predicted by our experimental RI characterisation of the fibre bend in Fig. 1. The variation in the RI of the PEO layer modulates the intensity of the light propagating through a fibre bend with respect to ambient humidity. From Fig. 6 it is observed that at 85% relative humidity the refractive index of the PEO layer is just above the cladding refractive index of the fibre and the loss will be higher at higher wavelengths. At 87% RH the PEO layer RI value is below the cladding RI and mode coupling takes place and hence resonant
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Fig. 8. Humidity response of the PEO coated fibre bend.
Fig. 7. Resonant wavelength shifts to higher wavelengths with the increase of humidity.
dips appear in the transmission spectrum at around 1550 nm. Furthermore it is observed that between RH values from 85% to 87.35% a large power variation occurs in the transmission loss over the wavelength range from 1590 to 1630 nm. While it is possible to utilise an OSA to determine the shift in the resonant dip wavelength in this RH range in order to monitor RH, the large power variation in the 1590–1630 nm range suggests a much simpler, more economic, approach to interrogating the sensor based on power measurement. To demonstrate the feasibility of this an experimental scheme was developed based on a ratiometric power measurement scheme, with a tunable laser as a source. A ratiometric scheme is used to avoid errors induced by fluctuations in the optical source power, resulting in more stable and accurate results. Experimentally it is found that the optimum wavelength that offers the highest RH sensitivity using this interrogation scheme is 1620 nm. The tunable source was therefore set to 1620 nm for this experiment. The input signal from the tunable source using a 50:50 coupler is split into two equal power signals, one goes to the fibre sensor bend and the other is the reference signal. Two photodiodes and associated electronics and data logging are used to measure the power at the outputs of the corresponding arms. By measuring the power ratio of the two signals the transmission loss of the PEO coated fibre bend can be obtained. The output power variation of the PEO coated fibre bend is shown in Fig. 8. The two curves represent the measurements taken with a time gap of one week, which shows the good repeatability and long term mechanical stability of the sensor head. A large power variation of approximately 20 dB is observed above 85% RH so that the sensor as expected behaves as a humidity dependant optical switch. The developed PEO coated bent single mode fibre based humidity sensor can be applied to monitor changes in humidity in the range from 80 to 95% with a very high sensitivity. To estimate the response time of the sensor, the coated fibre was exposed to an environment with rapid changes of the RH. First the RH in the chamber was maintained at 70% RH and then the cover of the chamber was removed quickly in order to rapidly expose the sensor to a measured environmental RH of 90%. The measured time-dependant response of the sensor is shown in Fig. 9. The sensor has a very fast response to humidity variations and the estimated response time from the Fig. 9 is 785 milliseconds when the RH changes from 70% to 90%. Our results show that the sensor has a reversible, fast and repeatable response to humidity. The experimental results
Fig. 9. Time response of the fibre-bend based humidity sensor.
presented in this work suggest a practical motivation to investigate inexpensive and disposable PEO-based fibre optic sensors with a simple fabrication technique and fast response for relative humidity control. For example, relative humidity above 85% must be maintained in food refrigerators to protect vegetables and fruit against rotting and to preserve their freshness for an extended time period. The demonstrated sensor behaves as a humidity dependant optical switch in this region with a large power variation of about 20 dB at around 85% RH. Because of this large power variation at around 85% RH and since we know the normal room humidity is below 85% RH and the relative humidity of the human breath is above 90% RH it can be used as a human breath rate monitor. To prove the feasibility of using it as a breath rate monitor we placed the PEO coated fibre bend at a distance of about 2 cm from the tip of the nose and the resulting breath RH response of the sensor is as shown in Fig. 10, for a time span of 60 s. This result also confirms the fast recovery and repeatability of the sensor. A fibre optic humidity sensor is suitable as a breathing monitor for patients during an MRI scan because of its immunity to magnetic field interference. Additionally given that humidity is being monitored using an intensity variation, there is the possibility of utilising an OTDR based interrogation scheme to allow for multiplexing of several sensors along a single fibre for distributed RH sensing. Finally it is recognised that a limitation of a fibre bend based humidity sensor is that the sensor size is limited to the diameter of the bend. However the bend
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the same configuration can be used for sensing humidity alternative RH ranges if different hydrophilic polymers are used as coatings. References
Fig. 10. Continuous human breath response of the sensor.
diameter can be reduced with a reduced fibre diameter, which can be achieved by chemical etching of the fibre using HF acid.
5. Conclusion We have demonstrated a highly sensitive all-fibre humidity sensor based on power measurement at an optimised wavelength and bend radius. The demonstrated sensor behaves as a humidity dependant optical switch between 85% and 90% RH and its response is fast and reversible in nature. The sensor also offers the advantages of simple structure and low cost. Since the demonstrated fibre bend responds to refractive index variation
[1] Yeo TL, Sun T, Grattan KTV. Fibre-optic sensor technologies for humidity and moisture measurement. Sensors Actuators A Phys 2008;144:280–95. [2] Konstantaki M, Pissadakis S, Pispas S, Madamopoulos N, Vainos NA. Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating. Appl Opt 2006;45(19):4567–71. [3] Mathew J, Thomas KJ, Nampoori VPN, Radhakrishnan P. A comparative study of fiber optic humidity sensors based on chitosan and agarose. Sensors Transducers J 2007;84(10):1633–40. [4] Gupta BD, Ratnanjali. A novel probe for a fiber optic humidity sensor. Sensors Actuators B Chem 2001;80:132–5. [5] Acikgoz S, Bilen B, Demir MM, Menceloglu YZ, Skarlatos Y, Aktas G, Inci MN. Use of polyethylene glycol coatings for optical fibre humidity sensing. Opt Rev 2008;15(2):84–90. [6] Bownass DC, Barton JS, Jones JDC. Serially multiplexed point sensor for the detection of high humidity in passive optical networks. Opt Lett 1997;22(5):346–8. [7] Zhang L, Gu F, Lou J, Yin X, Tong L. Fast detection of humidity with a subwavelengthdiameter fiber taper coated with gelatin film. Opt Express 2008;16(17):13349–53. [8] Corres JM, Matias IR, Hernaez M, Bravo J, Arregui FJ. Optical fiber humidity sensors using nanostructured coatings of SiO2 nanoparticles. IEEE Sensors J 2008;8(3):281–5. [9] Mathew J, Semenova Y, Rajan G, Farrell G. Humidity sensor based on a photonic crystal fiber interferometer. Electron Lett 2010;46(19):1341–3. [10] Bownass DC, Barton JS, Jones JDC. Detection of high humidity by optical fibre sensing at telecommunications wavelengths. Opt Commun 1998;146:90–4. [11] Mathew J, Rajan G, Semenova Y, Farrell G. All Fiber tunable loss filter. Proc SPIE 2009;7503. 75037G–75037G-4. [12] Love JD, Durniak C. Bend loss, tapering, and cladding-mode coupling in single-mode fibers. IEEE Photonics Technol Lett 2007;19(16):1257–9. [13] Yao L, Birks A, Knight JC. Low bend loss in tightly-bend fibers through adiabatic bend transitions. Opt Express 2009;17(4):2962–7. [14] Taylor HF. Bending effects in optical fibers. J Lightwave Technol 1984; LT-2(5):617–28.
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Improving the sensitivity of a humidity sensor based on fiber bend coated with a hygroscopic coating Jinesh Mathew n, Yuliya Semenova, Ginu Rajan, Pengfei Wang, Gerald Farrell DIT Photonics Research Centre, School of Electronic and Communications Engineering, Dublin Institute of Technology, Dublin 8, Ireland
a r t i c l e i n f o
abstract
Article history: Received 10 June 2010 Received in revised form 24 March 2011 Accepted 28 March 2011 Available online 15 April 2011
A highly sensitive all-fiber humidity sensor is demonstrated. The sensor behaves as a humidity dependent optical switch between 85% and 90% RH. This sensor also offers the advantages of simple structure and low cost and its response is fast and reversible in nature. The typical humidity response of the sensor is suitable for using it as a human breath rate monitor. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Macro bend fibre Humidity sensor Poly(ethylene oxide)
1. Introduction The ratio of the percentage of water vapour present in air at a particular temperature to the maximum amount of water vapour the air can hold at that temperature is called relative humidity. Measurement of humidity is required in a range of areas, including meteorological services, the chemical and food processing industry, civil engineering, air-conditioning, horticulture, and electronic processing. Therefore, development of a low-cost humidity sensor with a fast response is very important. Optical fibre humidity sensors offer specific advantages, such as small size and weight, immunity to electromagnetic interference, corrosion resistance and remote operation, by comparison to their conventional electronic counterparts. A wide range of optical fibre humidity sensors have been reported in the literature. Most fibre optic humidity sensors work on the basis of a hygroscopic material coated over the optical fibre to modulate the light propagating through the fibre [1]. The available fibre optic humidity sensors require complex processing of the fibre, such as fabrication of gratings [2], cladding removal [3,4], side polishing [5,6], tapering [7], formation of a fibre Fabry–Perot cavity [8], or photonic crystal fibre interferometer [9] to allow the light to interact with the hygroscopic coating. The use of a simple bent optical fibre has been previously demonstrated by Bownass et al. [10] for humidity state detection (dry or humid) in telecommunications networks using a bent standard single mode fibre coated with an organic film polyethylene oxide (PEO). A bend
n
Corresponding author. Tel.: þ35314024812; fax: þ 35314024690. E-mail address: [email protected] (J. Mathew).
0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.03.028
loss based humidity sensor is attractive because the fabrication of the sensor head does not require special treatment of the fibre. In Ref. [10], conventional low bend loss SMF fibre is used, which results in low sensitivity, while the selection of an optimum wavelength to maximise the sensitivity was also not considered. In this paper we present a detailed study of such a bend loss based sensor with a focus on significantly improving the sensitivity. In our study we observe that in a buffer-stripped bent single mode fibre, due to the coupling of the fundamental mode to cladding modes, resonant peaks will occur in the transmission response [11]. This response is oscillatory with respect to the bend radius and wavelength and also varies with ambient refractive index. If the surrounding refractive index of the bend fibre is changed, it will lead to a change in the coupling conditions and results in a shift in the wavelength of the resonant peaks. In order to achieve improved sensitivity for the humidity sensor we used a high bend loss fibre (1060XP). We characterised the spectral response of this bent fibre as a function of surrounding refractive index (RI) to establish the effect of wavelength on the sensitivity of the sensor. A bend radius of 15 mm is used in our investigation, which is an additional advantage of using a high bend loss fibre in that the sensor bend radius can be much larger than the 6 mm used in Ref. [10] with standard SMF, resulting in a much lower probability of stress-induced breakage of the fibre. As a confirmation of the suitability of using a high bend loss fibre bend as a sensitive humidity sensor, we implement and study a sensor using a 1060XP fibre bend coated with PEO. We show that it is possible to utilise a simple ratiometric power measurement scheme for interrogation and we also determine the optimum operating wavelength to maximise sensitivity using such a scheme. The sensor offers a competitive humidity
1302
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sensitivity utilising a cost effective ratiometric power measurement system. The response of the demonstrated humidity sensor is reversible and shows good long term mechanical stability. Finally, we also study for the first time the time response of such a sensor and we show that the response to a humidity change is very fast by comparison with existing electronic sensors.
2. Operating principle of the sensor When an uncoated fibre is bent the effective refractive index profile is changed [12–14]. Light propagating on the outside of the bend has further to travel and this effect can be modelled as a tilt applied to the refractive index profile of the fibre [14], given as nðxÞ ¼ n0 ðxÞ½1 þ ð1 þ gÞx=R
ð1Þ
where n0(x) is the refractive index profile when the fibre is straight, R is the bend radius, x is a transverse coordinate along a line joining the centre of curvature and the centre of the fibre, with the origin in the centre of the fibre and with x increasing in a positive fashion toward the fibre outer surface. The factor g ¼ 0.22 for silica fibre [14] accounts for the strain-optic effect. According to coupled mode theory the fundamental core-guided mode can couple with a cladding mode with a peak intensity at the outer rim of the fibre bend [12,13], which propagates along the fibre bend as a Whispering Gallery mode. With increasing bend curvature the refractive index profile of the bend fibre becomes more tilted and the effective index for the cladding modes increases, reaching a maximum at the outer edge. At some critical bend radius, the effective index of the cladding modes equals the effective index for the core-guided mode. If there is sufficient overlap between the core mode and a cladding mode at this point, the core mode and a cladding mode will be coupled together. At tighter bends the core mode will couple to other cladding modes. In the straight portion of the fibre following the bend region, the conventional fibre buffer coating suppresses the cladding modes so that the power coupled to the cladding mode is absorbed and is not re-coupled to the core mode. Therefore at certain bend radii and wavelengths, there are dips in the transmission spectrum of the macro bend fibre that are a function of the degree of coupling of the fundamental mode to the cladding modes [11]. The phase matching condition for mode coupling also depends on the ambient refractive index at the bend. If the surrounding refractive index of the bent fibre is changed, it will lead to a change in the coupling conditions and will result in a shift in the resonant wavelength. To utilise such a bent fibre as a humidity sensor the buffer-stripped fibre bend can be coated with a hygroscopic material, whose refractive index changes with respect to the ambient humidity. In practice the simplest mechanical configuration for such a sensor is a simple fibre bend where the number of turns is less than one. Because of the low bend sensitivity of standard fibre such as SMF28, a sensor fabricated from such a fibre will need to utilise very low values of bend radius to achieve a reasonable sensitivity, increasing the risk of stress-induced fibre breakage. To improve sensitivity and increase the bend radius so as to reduce the risk of fibre breakage, a high bend loss fibre such as 1060XP is much more suitable.
the fabrication of a compact sensor head. Since it is known that suitable humidity sensitive coating materials can have RI values that range from below the fibre cladding RI (1.456) to above the cladding RI, it was necessary in this experimental study to utilise RI values from circa 1.340 to 1.478. A 35 mm length of the buffer coating was stripped from the middle section of a longer length of the fibre and the fibre was fixed on to a semi-circular rod of outer diameter 30 mm forming a bend angle (y) of about 1331. We selected this bend radius and bend angle experimentally as it was found that this is the highest bend radius and smallest bend angle that can give strong resonant dips in the measurable wavelength range of our experiment. A high bend radius is desirable in order to minimise the risk of fibre breakage. Light from a Super Luminescent Diode was launched into the fibre from the input side of the bend and the output from the bend was connected to an Optical Spectrum Analyser. The transmission response of this bent fibre was studied by immersing it in different RI solutions prepared by precisely mixing Dimethyl sulfoxide (DMSO) and water with different volume concentrations. The RI of the prepared solutions is obtained from an Abbe refractometer. The transmission responses of the bent fibre for different ambient RIs are shown in Fig. 1 for the measurable wavelength range in our experiment. For ambient RIs below the value of the refractive index of the fibre cladding, resonant dips are observed in the transmission response. As shown in Fig. 2, when the RI increases the resonant wavelength shifts to a shorter wavelength. When the surrounding RI matches or exceeds the RI of the fibre cladding the loss dips disappear from the transmission spectrum; the surrounding liquid acts as an infinite cladding, effectively absorbing the leaky modes. In this range of ambient RIs, resonant peaks are suppressed and the transmission loss increases monotonically with wavelength and the ambient RI as expected.
Fig. 1. Transmission response of bent fibre for different ambient refractive indices.
3. Experimental RI characterisation of the fibre bend Initially an experimental characterisation of the fibre bend was carried out to enable the selection of a suitable polymer coating for humidity sensing. The fibre used in the experiment was a 1060XP single mode fibre, a high bend loss fibre that allows for
Fig. 2. Resonance wavelength shift as a function of the ambient refractive index.
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4. Experimental characterisation of the fibre bend as a humidity sensor To utilise this structure as a humidity sensor the bufferstripped fibre bend was coated with a hygroscopic material, whose refractive index depends on the ambient humidity. A humidity sensitive polymer, polyethylene oxide (PEO), was chosen as a polymer coating with a refractive index in the range 1.4–1.5 with respect to humidity [10]. Such a range of RIs makes it a suitable choice for coating, given the range of RI values, which can be measured using the single mode fibre bend sensor. The refractive index of the PEO decreases with increasing humidity and this change is reversible. PEO also has the advantage that it can be easily coated on to the fibre due to its high adhesion with silica [2]. The sensor head is fabricated by dip coating the buffer-striped portion of the 1060XP fibre in a PEO solution prepared by dissolving 6 wt% PEO (Sigma-Aldrich) in distilled water. No thickness control was used or required, since the function of the coating was simply to act as a variable refractive index layer. The coated fibre is kept for one day at room temperature until it is partially dehydrated and reaches equilibrium with the ambient room humidity. To make it mechanically stable the probe is then fixed on to a semi-circular rod of outer diameter 30 cm forming a bend of about 1331. A schematic diagram of a PEO coated fibre bend is shown in Fig. 3. The humidity response of the fabricated sensor head is studied at room temperature and normal atmospheric pressure by placing the sensing element in a customised climate chamber shown in the Fig. 4. It consists of a sealed acrylic chamber with a dry/wet air flow system that can vary the internal humidity in the chamber. A calibrated electronic humidity sensor (model: 11-661-21, make: Control Company) was used for monitoring the temperature and humidity inside the chamber. We have observed the transmission response of the PEO coated fibre bend for a range of humidity values from 30% RH to 95% RH. No resonant dips were observed in the transmission spectrum for ambient humidity values below 85% RH, which is shown in Fig. 5. As concluded from in Fig. 1, this is due to the fact that the RI of the coated PEO film is above the RI of the fibre
Fig. 3. Poly(ethylene oxide) coated fibre bend.
Fig. 4. Experimental setup for studying the humidity response of the PEO coated fibre bend.
Fig. 5. Humidity response of the PEO coated fibre bend for RH below 85%.
Fig. 6. Appearance of loss dips in the transmission spectrum in a PEO coated fibre bend at RH above 85% due to resonant mode coupling at particular RI values of the coating.
cladding and therefore the PEO coating is acting as an absorption coating. As surrounding humidity value increases above 85% RH resonant dips appear in the transmission spectrum. The appearance of these loss dips in the transmission spectrum in a PEO coated fibre bend is due to resonant mode coupling at particular RI values as shown in Fig. 6. The reason for these resonant dips is that around this humidity value PEO undergoes a physical phase change from a semi-crystalline structure to a gel with a large decrease in its index of refraction to a value below the RI of the fibre cladding and as we have shown in Fig. 1 this will result in resonant dips in the transmission spectrum as the surrounding RI of the fibre bend is below the cladding RI. At higher humidity values, the RI of PEO is below the refractive index of the fibre cladding (1.456) and the resonant wavelength shifts to a higher wavelength with respect to humidity as shown in Fig. 7 and as predicted by our experimental RI characterisation of the fibre bend in Fig. 1. The variation in the RI of the PEO layer modulates the intensity of the light propagating through a fibre bend with respect to ambient humidity. From Fig. 6 it is observed that at 85% relative humidity the refractive index of the PEO layer is just above the cladding refractive index of the fibre and the loss will be higher at higher wavelengths. At 87% RH the PEO layer RI value is below the cladding RI and mode coupling takes place and hence resonant
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Fig. 8. Humidity response of the PEO coated fibre bend.
Fig. 7. Resonant wavelength shifts to higher wavelengths with the increase of humidity.
dips appear in the transmission spectrum at around 1550 nm. Furthermore it is observed that between RH values from 85% to 87.35% a large power variation occurs in the transmission loss over the wavelength range from 1590 to 1630 nm. While it is possible to utilise an OSA to determine the shift in the resonant dip wavelength in this RH range in order to monitor RH, the large power variation in the 1590–1630 nm range suggests a much simpler, more economic, approach to interrogating the sensor based on power measurement. To demonstrate the feasibility of this an experimental scheme was developed based on a ratiometric power measurement scheme, with a tunable laser as a source. A ratiometric scheme is used to avoid errors induced by fluctuations in the optical source power, resulting in more stable and accurate results. Experimentally it is found that the optimum wavelength that offers the highest RH sensitivity using this interrogation scheme is 1620 nm. The tunable source was therefore set to 1620 nm for this experiment. The input signal from the tunable source using a 50:50 coupler is split into two equal power signals, one goes to the fibre sensor bend and the other is the reference signal. Two photodiodes and associated electronics and data logging are used to measure the power at the outputs of the corresponding arms. By measuring the power ratio of the two signals the transmission loss of the PEO coated fibre bend can be obtained. The output power variation of the PEO coated fibre bend is shown in Fig. 8. The two curves represent the measurements taken with a time gap of one week, which shows the good repeatability and long term mechanical stability of the sensor head. A large power variation of approximately 20 dB is observed above 85% RH so that the sensor as expected behaves as a humidity dependant optical switch. The developed PEO coated bent single mode fibre based humidity sensor can be applied to monitor changes in humidity in the range from 80 to 95% with a very high sensitivity. To estimate the response time of the sensor, the coated fibre was exposed to an environment with rapid changes of the RH. First the RH in the chamber was maintained at 70% RH and then the cover of the chamber was removed quickly in order to rapidly expose the sensor to a measured environmental RH of 90%. The measured time-dependant response of the sensor is shown in Fig. 9. The sensor has a very fast response to humidity variations and the estimated response time from the Fig. 9 is 785 milliseconds when the RH changes from 70% to 90%. Our results show that the sensor has a reversible, fast and repeatable response to humidity. The experimental results
Fig. 9. Time response of the fibre-bend based humidity sensor.
presented in this work suggest a practical motivation to investigate inexpensive and disposable PEO-based fibre optic sensors with a simple fabrication technique and fast response for relative humidity control. For example, relative humidity above 85% must be maintained in food refrigerators to protect vegetables and fruit against rotting and to preserve their freshness for an extended time period. The demonstrated sensor behaves as a humidity dependant optical switch in this region with a large power variation of about 20 dB at around 85% RH. Because of this large power variation at around 85% RH and since we know the normal room humidity is below 85% RH and the relative humidity of the human breath is above 90% RH it can be used as a human breath rate monitor. To prove the feasibility of using it as a breath rate monitor we placed the PEO coated fibre bend at a distance of about 2 cm from the tip of the nose and the resulting breath RH response of the sensor is as shown in Fig. 10, for a time span of 60 s. This result also confirms the fast recovery and repeatability of the sensor. A fibre optic humidity sensor is suitable as a breathing monitor for patients during an MRI scan because of its immunity to magnetic field interference. Additionally given that humidity is being monitored using an intensity variation, there is the possibility of utilising an OTDR based interrogation scheme to allow for multiplexing of several sensors along a single fibre for distributed RH sensing. Finally it is recognised that a limitation of a fibre bend based humidity sensor is that the sensor size is limited to the diameter of the bend. However the bend
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the same configuration can be used for sensing humidity alternative RH ranges if different hydrophilic polymers are used as coatings. References
Fig. 10. Continuous human breath response of the sensor.
diameter can be reduced with a reduced fibre diameter, which can be achieved by chemical etching of the fibre using HF acid.
5. Conclusion We have demonstrated a highly sensitive all-fibre humidity sensor based on power measurement at an optimised wavelength and bend radius. The demonstrated sensor behaves as a humidity dependant optical switch between 85% and 90% RH and its response is fast and reversible in nature. The sensor also offers the advantages of simple structure and low cost. Since the demonstrated fibre bend responds to refractive index variation
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