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OPTICS LETTERS / Vol. 39, No. 14 / July 15, 2014
Nanoscale TiO2-coated LPGs as radiationtolerant humidity sensors for high-energy physics applications Marco Consales,1 Gaia Berruti,1,2 Anna Borriello,3 Michele Giordano,3 Salvatore Buontempo,4 Giovanni Breglio,5 Alajos Makovec,6 Paolo Petagna,2 and Andrea Cusano1,* 1 2
Optoelectronics Group, Department of Engineering, University of Sannio, I-82100 Benevento, Italy
PH Department, DT Section, CERN—European Organization for Nuclear Research, CH-1211 Geneva, Switzerland 3
4
IMCB—National Research Council, I80055 Portici, Italy INFN—Istituto Nazionale di Fisica Nucleare, I 80126 Napoli, Italy
5
Department of Electrical Engineering and Information Technology, I-80138 Napoli, Italy 6
University of Debrecen, HU-4032, Debrecen, Hungary *Corresponding author: [email protected]
Received March 19, 2014; revised May 27, 2014; accepted May 28, 2014; posted May 28, 2014 (Doc. ID 208608); published July 7, 2014 This Letter deals with a feasibility analysis for the development of radiation-tolerant fiber-optic humidity sensors based on long-period grating (LPG) technology to be applied in high-energy physics (HEP) experiments currently running at the European Organization for Nuclear Research (CERN). In particular, here we propose a high-sensitivity LPG sensor coated with a finely tuned titanium dioxide (TiO2 ) thin layer (∼100 nm thick) through the solgel deposition method. Relative humidity (RH) monitoring in the range 0%–75% and at four different temperatures (in the range −10°C–25°C) was carried out to assess sensor performance in real operative conditions required in typical experiments running at CERN. Experimental results demonstrate the very high RH sensitivities of the proposed device (up to 1.4 nm/% RH in correspondence to very low humidity levels), which turned out to be from one to three orders of magnitude higher than those exhibited by fiber Bragg grating sensors coated with micrometerthin polyimide overlays. The radiation tolerance capability of the TiO2 -coated LPG sensor is also investigated by comparing the sensing performance before and after its exposure to a 1 Mrad dose of γ-ionizing radiation. Overall, the results collected demonstrate the strong potential of the proposed technology with regard to its future exploitation in HEP applications as a robust and valid alternative to the commercial (polymer-based) hygrometers currently used. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (120.0280) Remote sensing and sensors. http://dx.doi.org/10.1364/OL.39.004128
The Compact Muon Solenoid (CMS) is one of the two large general-purpose particle detectors built on the Large Hadron Collider (LHC) accelerator, running at the European Organization for Nuclear Research (CERN) in Geneva. Its innermost detector, immersed in a 4 T magnetic field, is a particle tracker where silicon pixel and microstrip sensors measure the momentum and trajectory of particles emerging from the LHC collisions. In order to reduce the radiation-induced loss of the sensors, the subdetectors are cooled at −20°C∕ − 25°C. To avoid water condensation and ice formation, the volume needs to be kept sealed and dry. A distributed thermal and hygrometric monitoring of the air is, however, required in the external area surrounding the tracker, where cold services are distributed in complex geometries and a safe control of the environmental parameters is more difficult. In addition, each tracking detector needs to be as much as possible transparent to particles, requiring mass minimization of the devices to be installed in situ, radiation resistance from 1 to 100 Mrad doses, and magnetic field insensitivity. All the commercial miniaturized relative humidity (RH) sensors used by CMS are capacitive and multiwired and not designed with radiation hard characteristics [1]. On the other hand, fiber optic sensors (FOSs) are able to provide many attractive features that overcome the electrical sensors’ limitations. In addition, modern fabrication technologies allow us to obtain fibers tolerating high 0146-9592/14/144128-04$15.00/0
radiations levels [2], and this promoted renewed interest in their use in high-energy physics (HEP) environments. An extensive review concerning the use of FOSs for humidity monitoring has been reported by Yeo et al. [3]. Among others, fiber Bragg grating (FBG) sensor technology has particularly attracted the attention of many researchers in recent years [4–6] due to their advantages in terms of wavelength encoding, multiplexing capability, linear output, etc. [7]. Recently, investigations carried out by our research group at CERN provided a clear demonstration of polyimide-coated FBG radiation tolerance up to 9 Mrad ionizing radiation exposure, domonstrating good potential for their future exploitation in HEP environments [8]. However, RH sensors based on coated FBGs were found to suffer from some limitations, including the influence of the coating adhesion on the sensors’ performance and their low RH sensitivity (1.0∕1.5 pm∕% RH). Moreover, the inherent sensitivity to temperature of FBGs (∼10 pm∕°C) requires the development of a very precise temperature compensation scheme to decouple the T cross-sensitivities in real applications (e.g., a temperature reading error of 1°C produces 7% RH/10% RH error if no compensation is applied) [6]. Alternatively, coated long-period gratings (LPGs) were proposed for RH monitoring to overcome these limitations [9,10]. These are photonic devices fabricated by inducing a periodic refractive index (RI) modulation (typical period 100–500 μm) of a single-mode optical fiber core, allowing © 2014 Optical Society of America
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
the power transfer from the fundamental guided core mode to a discrete number of forward-propagating cladding modes and to each of them at a distinct wavelength where the so‐called phase-matching condition is satisfied: λres;0i neff;co − n0i eff;cl Λ;
(1)
where neff;co and n0i eff;cl are the core and ith cladding mode effective indices, respectively, and Λ is the grating period. As a result of this process (referred to as mode coupling), the LPG transmission spectrum shows several attenuation bands or dips related to the different excited optical fields: the cladding modes. A portion of the electromagnetic field of the cladding modes penetrates the surrounding medium in the form of an evanescent wave, thus making n0i eff;cl (and so λres;0i ) sensitive to the chemical and physical properties of the surrounding environment. Among the different parameters able to influence the spectral features of LPGs, the surrounding refractive index (SRI) sensitivity can be considered as a key feature enabling the use of this kind of sensors in chemical and biological sensing applications through the integration of functional overlays [11,12]. Moreover, the SRI sensitivity can be optimized for a specific application, by acting on the thickness and optical properties of the functional overlay [10]. Several kinds of materials have been explored as coating for the development of LPG-based RH sensors, including polymers [9,10], hydrogels [13], gelatine [14], cobalt-chloride-based materials [15], and SiO2 nanospheres [16]. In all these cases, the water absorption/ desorption in the hygrosensitive coating in presence of RH variations modifies its RI (and thickness), thus creating a spectral variation and amplitude change in the LPG attenuation bands, independently of the adhesion properties of the coating onto the grating itself. However, in the literature there is no evidence of the characterization of coated LPGs at low humidity values (below 20% RH) and temperatures below 15°C. Moreover, while the radiation effects have been investigated in the case of bare LPGs up to 154 Mrad [17,18], their influence on the sensing performance of coated LPGs seems to be completely unexplored. In this work, titanium dioxide (TiO2 ) has been used for the first time, to the best of our knowledge, as a sensitive element for the development of high-sensitivity LPGbased humidity sensors, due to its high RI (n 1.96) [19] and hygrosensitive characteristics [20,21]. Furthermore, the use of an oxide is expected to avoid the typical aging problems characteristic of polymeric overlays. The solgel dip coating method has been selected for the integration of nanoscale TiO2 layers onto the LPG surface, mainly due to its ability to guarantee a good optical quality, a ring-shaped symmetry, and an adequate longitudinal uniformity over the grating length. The deposition procedure mainly consisted of the solgel preparation (an alkoxide gel of titanium) followed by a standard dip-coating procedure [12]. More specifically, the fiber was first immersed into the solgel solution and then withdrawn with a controlled speed of 100 mm/min
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in order to allow the formation of a coherent fluid film. The final coating material is typically obtained by a curing step at 150°C for 10 min. Several samples with different TiO2 thicknesses have been successfully produced. In what follows, data related to a LPG characterized by a period Λ 404 μm, henceforth named LPG1, are discussed. Figure 1(a) reports a 20× optical microscope image of LPG1 after the deposition procedure, from which the smoothness and homogeneity of the deposited layer can be appreciated. The wavelength shift associated with the investigated attenuation band was monitored during all the deposition phases. This guarantees efficient on-line control of the targeted final overlay thickness of ∼100 nm. Figure 1(b) shows the transmittance spectra of LPG1, before and after the TiO2 deposition. The bare device exhibits an attenuation band (related to the fifth cladding mode) centered at λres;05 1589.0 nm, while the deposition of the TiO2 layer causes a resonance blueshift of ∼24.1 nm together with an ∼5 dB increase in its depth. The experimental results provide the first evidence of the optical quality of the deposited overlay; indeed, if an optical lossy overlay is deposited onto the LPG surface, a decrease in the visibility of the attenuation band combined with a spectral broadening would be expected. Once we assessed the fabrication step, the sensor performance of LPG1 was investigated in the range 0%–75% RH, at the temperatures of −10°C, 0°C, 10°C, and 25°C (pre-irradiation stage). In addition, the effects induced by the sensor’s exposure to a ϒ-ionizing radiation dose up to 1 Mrad were also studied (post-irradiation stage). The results reported were collected during an extensive experimental campaign carried out in the laboratories at CERN. To analyze the performance of the fabricated probe, a flexible apparatus was used to create stable and controlled humidity conditions in the range 0%– 100% RH and −20∕ 30°C, as described in [6]. A chilled mirror dew point hygrometer (Dew Master) was used to provide the RH reference during the sensor calibrations.
Fig. 1. (a) Typical microscope image of a TiO2 -coated LPG and (b) transmitted spectra of the bare and coated LPG.
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Furthermore, four reference resistive sensors (RTD), very close to the LPGs, have been installed to monitor the homogeneity and stability conditions of the temperature. The transmission spectra of the LPGs (and thus their resonant wavelengths) during the experiments were monitored by using a compact LPG interrogator, characterized by a measurement range from 1510 to 1590 nm and a wavelength resolution of 1 pm. Figure 2(a) shows the typical spectral variations of the TiO2 -coated LPG sensor (LPG1) during a characterization test carried out at 25°C. As theoretically expected, increasing the humidity content inside the test chamber causes a blueshift of λres;05 . This is due to the TiO2 coating RI increase caused by the higher amount of adsorbed water molecules, which in turn leads to an increase of n0i eff;cl [20]. Figure 2(a) also reveals that by increasing the RH value, the attenuation band visibility also increases. This is an important aspect as, in most cases, LPGs coated with “lossy” overlays experience partial or full resonance fading when operating in transition mode [22]. Figure 2(b) reports the λres;05 variations of the TiO2 coated LPG sensor (LPG1) during the characterization test of Fig. 2(a). For comparison, the response from the reference humidity sensor is also provided. It is worth emphasizing that the response provided by the optical sensor is in agreement with the reference device readings. Similar results have also been obtained at 10°C, 0°C, and −10°C, underlying the capability of the device to work properly also at low temperatures in the full RH range of interest. Figure 3(a) shows the calibration curves of LPG1 obtained at the four temperatures during the pre-irradiation stage. In contrast with what was observed with polyimide-coated FBGs [6], they have been found to be nonlinear, as a larger λres;05 shift was
registered at low RH levels. This can also be observed in Fig. 3(b), where the RH sensitivity (S RH ) curves of LPG1 versus RH are plotted. At 25°C, S RH values are between 1.4 and 0.11 nm/% RH in the range 0%–10% RH and decrease to 0.01 nm/% RH at very high humidity levels. This has to be compared with a uniform S RH in the range 1 · 10−3 nm∕% RH∕1.5 · 10−3 nm∕% RH typical of polyimide-coated FBG sensors [6]. Obtained results suggest that TiO2 -coated LPGs are able to provide RH sensitivities one to three orders of magnitude higher than those exhibited by FBG-based sensors coated with micrometer-thin polyimide overlays. From Fig. 3(b), a slight dependence of S RH on temperature (a small decrease occurs when temperature decreases) can be also appreciated. In addition, starting from the characteristic curves reported in Fig. 3(a), working at constant RH, the temperature sensitivity (S T ) of the LPG probe can also be retrieved by differentiating the λ-T curves (found to be linear), obtained by extrapolating the λres;05 values for each temperature from the fitting curves. As a result, the S T values (obtained at different humidity levels) were found to slightly decrease with RH; however, a mean S T value of ∼0.250 0.015 nm∕°C has been evaluated in the full RH range. It is worth noting that a temperature compensation scheme is required in order to reduce the cross sensitivity to unavoidable temperature modifications occurring in real applications. However, in the critical operative range 0%–10% RH, assuming for the LPG a S RH mean of ∼0.5 nm∕%RH, a temperature change of 1°C would induce a 0.5% RH/1% RH error if no compensation is applied. This value is significantly better than the thermal cross talk observed in the case of coated FBGs (7%–10% RH error for a noncompensated temperature change of 1°C). The radiation effects on coated LPG operation have also been investigated by exposing the sample to a 1 Mrad o
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low humidity values), which turned out to be one to three orders of magnitude higher than those exhibited by FBG sensors coated by micrometer-thin polyimide overlays. In addition, the sensing performance of LPG-based sensors turned out to be almost unaffected by the exposure to a 1 Mrad dose of γ-ionizing radiation. Overall, the results collected demonstrate the strong potential of the proposed technology with regard to its future exploitation as a robust and valid alternative to the commercial (polymer-based) hygrometers currently used in HEP applications. Fig. 4. LPG1 (Δλres;05 , RH) points at 25°C before (black points) and after (red points) the 1 Mrad radiation exposure. Inset: (λres;05 , RH) points referring to the same tests.
dose of γ-ionizing radiation. The irradiation process has been performed using a Co60 source. Preliminary results indicate a radiation-induced dip wavelength shift of about 4.4 nm in correspondence to 30% RH (inset in Fig. 4). Even if further irradiation campaigns at progressively higher doses are currently in progress in order to confirm these preliminary results, we point out that the observed shift is almost in agreement with radiation hardness investigations conducted on different types of bare LPGs [18]. In particular, the observed shift is of the same order of magnitude of the shift observed with bare chiral LPGs [18] and is also similar to that recorded after a dose of 6 kilogray (0.6 Mrad) with low-order modes (8th, 9th, and 10th) in the case of a bare turnaround point LPG written in a B/Ge-doped fiber [23]. In Fig. 4, Δλres;05 versus RH at 25°C, before (black curve) and after (red curve) the irradiation campaign, is also depicted. Interestingly, the LPG sensor does not lose its excellent ability to respond to RH changes, and apart from some variations (∼16%), its characteristic curve still exhibits the same behavior shown before irradiation. We draw attention to the fact that the data reported provide the first experimental demonstration of the radiation tolerance of TiO2 -coated LPG sensors for RH monitoring and demonstrate good prospects for an effective exploitation of LPG technology for RH monitoring in HEP applications. The results obtained, however, suggest the necessity of taking into account the γ-ionizing radiation effects on the LPG sensor response in order to fully exploit the extremely high sensitivity of these devices in HEP applications. In conclusion, we reported the development and the first experimental demonstration of high-sensitivity TiO2 coated LPG sensors for RH monitoring at temperatures below 0°C as well as after high γ-radiation dose exposures (up to 1 Mrad). In particular, the humidity detection performance of a LPG sensor coated with a 100 nm thick TiO2 overlay has been analyzed in the range 0%–75% RH and at four different temperatures (in the range −10°C–25°C) during the experimental campaign carried out at CERN laboratories in Geneva. The results obtained are very encouraging and demonstrate the capability of TiO2 -coated LPG sensors to provide very high RH sensitivities (up to 1.4 nm/% RH in correspondence to very
References 1. M. Fossa and P. Petagna, “Use and calibration of capacitive RH sensors for hygrometric control of the CMS tracker,” CMS Note 2003/24 (2003). 2. A. Cusano, A. Cutolo, and J. Albert, Fiber Bragg Grating Sensors: Recent Advancements, Industrial Applications and Market Exploitation (Bentham Science, 2011), Chap. 12, pp. 218–237. 3. T. L. Yeo, T. Sun, and K. T. V. Grattan, Sens. Actuators A 144, 280 (2008). 4. T. L. Yeo, T. Sun, K. T. V. Grattan, D. Parry, R. Lade, and B. D. Powell, Sens. Actuators B 110, 148 (2005). 5. P. Kronenberg, P. K. Rastogi, P. Giaccari, and H. G. Limberger, Opt. Lett. 27, 1385 (2002). 6. G. Berruti, M. Consales, M. Giordano, L. Sansone, P. Petagna, S. Buontempo, G. Breglio, and A. Cusano, Sens. Actuators B 177, 94 (2013). 7. Y. J. Rao, Meas. Sci. Technol. 8, 355 (1997). 8. A. Makovec, G. Berruti, M. Consales, M. Giordano, P. Petagna, S. Buontempo, G. Breglio, Z. Szillasi, N. Beni, and A. Cusano, J. Instrum. 9, C03040 (2014). 9. T. Venugopalan, T. Sun, and K. T. V. Grattan, Sens. Actuators A 148, 57 (2008). 10. T. Venugopalan, T. L. Yeo, T. Sun, and K. T. V. Grattan, IEEE Sens. J. 8, 1093 (2008). 11. I. Del Villar, I. Matías, F. Arregui, and P. Lalanne, Opt. Express 13, 56 (2005). 12. A. Cusano, A. Iadicicco, P. Pilla, L. Contessa, S. Campopiano, A. Cutolo, and M. Giordano, Opt. Express 14, 19 (2006). 13. Y. Liu, L. Wang, M. Zhang, D. Tu, X. Mao, and Y. Liao, IEEE Photon. Technol. Lett. 19, 880 (2007). 14. K. M. Tan, C. M. Tay, S. C. Tjin, C. C. Chan, and H. Rahardjo, Sens. Actuators B 110, 335 (2005). 15. M. Konstantaki, S. Pissadakis, S. Pispas, N. Madamopoulos, and N. A. Vainos, Appl. Opt. 45, 4567 (2006). 16. D. Viegas, J. Goicoechea, J. M. Corres, J. L. Santos, L. A. Ferreira, F. M. Araújo, and I. R. Matias, Meas. Sci. Technol. 20, 034002 (2009). 17. S. Kher, S. Chaubey, S. M. Oak, and A. Gusarov, IEEE Photon. Technol. Lett. 25, 2070 (2013). 18. A. Gusarovand and S. K. Hoeffgen, IEEE Trans. Nucl. Sci. 60, 3 (2013). 19. E. Davies, R. Viitala, M. Salomäki, S. Areva, L. Zhang, and I. Bennion, J. Opt. A Pure Appl. Opt. 11, 015501 (2009). 20. G. Montesperelli, A. Pumo, E. Traversa, G. Gusmano, A. Bearzotti, A. Montenero, and G. Gnappi, Sens. Actuators B 25, 705 (1995). 21. B. C. Yadav, N. K. Pandey, A. K. Srivastava, and P. Sharma, Meas. Sci. Technol. 18, 260 (2007). 22. I. Del Villar, I. R. Matias, F. J. Arregui, and M. Achaerandio, J. Lightwave Technol. 23, 4192 (2005). 23. S. Kher, S. Chaubey, R. Kashyap, and S. M. Oak, IEEE Photon. Technol. Lett. 24, 742 (2012).
OPTICS LETTERS / Vol. 39, No. 14 / July 15, 2014
Nanoscale TiO2-coated LPGs as radiationtolerant humidity sensors for high-energy physics applications Marco Consales,1 Gaia Berruti,1,2 Anna Borriello,3 Michele Giordano,3 Salvatore Buontempo,4 Giovanni Breglio,5 Alajos Makovec,6 Paolo Petagna,2 and Andrea Cusano1,* 1 2
Optoelectronics Group, Department of Engineering, University of Sannio, I-82100 Benevento, Italy
PH Department, DT Section, CERN—European Organization for Nuclear Research, CH-1211 Geneva, Switzerland 3
4
IMCB—National Research Council, I80055 Portici, Italy INFN—Istituto Nazionale di Fisica Nucleare, I 80126 Napoli, Italy
5
Department of Electrical Engineering and Information Technology, I-80138 Napoli, Italy 6
University of Debrecen, HU-4032, Debrecen, Hungary *Corresponding author: [email protected]
Received March 19, 2014; revised May 27, 2014; accepted May 28, 2014; posted May 28, 2014 (Doc. ID 208608); published July 7, 2014 This Letter deals with a feasibility analysis for the development of radiation-tolerant fiber-optic humidity sensors based on long-period grating (LPG) technology to be applied in high-energy physics (HEP) experiments currently running at the European Organization for Nuclear Research (CERN). In particular, here we propose a high-sensitivity LPG sensor coated with a finely tuned titanium dioxide (TiO2 ) thin layer (∼100 nm thick) through the solgel deposition method. Relative humidity (RH) monitoring in the range 0%–75% and at four different temperatures (in the range −10°C–25°C) was carried out to assess sensor performance in real operative conditions required in typical experiments running at CERN. Experimental results demonstrate the very high RH sensitivities of the proposed device (up to 1.4 nm/% RH in correspondence to very low humidity levels), which turned out to be from one to three orders of magnitude higher than those exhibited by fiber Bragg grating sensors coated with micrometerthin polyimide overlays. The radiation tolerance capability of the TiO2 -coated LPG sensor is also investigated by comparing the sensing performance before and after its exposure to a 1 Mrad dose of γ-ionizing radiation. Overall, the results collected demonstrate the strong potential of the proposed technology with regard to its future exploitation in HEP applications as a robust and valid alternative to the commercial (polymer-based) hygrometers currently used. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (120.0280) Remote sensing and sensors. http://dx.doi.org/10.1364/OL.39.004128
The Compact Muon Solenoid (CMS) is one of the two large general-purpose particle detectors built on the Large Hadron Collider (LHC) accelerator, running at the European Organization for Nuclear Research (CERN) in Geneva. Its innermost detector, immersed in a 4 T magnetic field, is a particle tracker where silicon pixel and microstrip sensors measure the momentum and trajectory of particles emerging from the LHC collisions. In order to reduce the radiation-induced loss of the sensors, the subdetectors are cooled at −20°C∕ − 25°C. To avoid water condensation and ice formation, the volume needs to be kept sealed and dry. A distributed thermal and hygrometric monitoring of the air is, however, required in the external area surrounding the tracker, where cold services are distributed in complex geometries and a safe control of the environmental parameters is more difficult. In addition, each tracking detector needs to be as much as possible transparent to particles, requiring mass minimization of the devices to be installed in situ, radiation resistance from 1 to 100 Mrad doses, and magnetic field insensitivity. All the commercial miniaturized relative humidity (RH) sensors used by CMS are capacitive and multiwired and not designed with radiation hard characteristics [1]. On the other hand, fiber optic sensors (FOSs) are able to provide many attractive features that overcome the electrical sensors’ limitations. In addition, modern fabrication technologies allow us to obtain fibers tolerating high 0146-9592/14/144128-04$15.00/0
radiations levels [2], and this promoted renewed interest in their use in high-energy physics (HEP) environments. An extensive review concerning the use of FOSs for humidity monitoring has been reported by Yeo et al. [3]. Among others, fiber Bragg grating (FBG) sensor technology has particularly attracted the attention of many researchers in recent years [4–6] due to their advantages in terms of wavelength encoding, multiplexing capability, linear output, etc. [7]. Recently, investigations carried out by our research group at CERN provided a clear demonstration of polyimide-coated FBG radiation tolerance up to 9 Mrad ionizing radiation exposure, domonstrating good potential for their future exploitation in HEP environments [8]. However, RH sensors based on coated FBGs were found to suffer from some limitations, including the influence of the coating adhesion on the sensors’ performance and their low RH sensitivity (1.0∕1.5 pm∕% RH). Moreover, the inherent sensitivity to temperature of FBGs (∼10 pm∕°C) requires the development of a very precise temperature compensation scheme to decouple the T cross-sensitivities in real applications (e.g., a temperature reading error of 1°C produces 7% RH/10% RH error if no compensation is applied) [6]. Alternatively, coated long-period gratings (LPGs) were proposed for RH monitoring to overcome these limitations [9,10]. These are photonic devices fabricated by inducing a periodic refractive index (RI) modulation (typical period 100–500 μm) of a single-mode optical fiber core, allowing © 2014 Optical Society of America
July 15, 2014 / Vol. 39, No. 14 / OPTICS LETTERS
the power transfer from the fundamental guided core mode to a discrete number of forward-propagating cladding modes and to each of them at a distinct wavelength where the so‐called phase-matching condition is satisfied: λres;0i neff;co − n0i eff;cl Λ;
(1)
where neff;co and n0i eff;cl are the core and ith cladding mode effective indices, respectively, and Λ is the grating period. As a result of this process (referred to as mode coupling), the LPG transmission spectrum shows several attenuation bands or dips related to the different excited optical fields: the cladding modes. A portion of the electromagnetic field of the cladding modes penetrates the surrounding medium in the form of an evanescent wave, thus making n0i eff;cl (and so λres;0i ) sensitive to the chemical and physical properties of the surrounding environment. Among the different parameters able to influence the spectral features of LPGs, the surrounding refractive index (SRI) sensitivity can be considered as a key feature enabling the use of this kind of sensors in chemical and biological sensing applications through the integration of functional overlays [11,12]. Moreover, the SRI sensitivity can be optimized for a specific application, by acting on the thickness and optical properties of the functional overlay [10]. Several kinds of materials have been explored as coating for the development of LPG-based RH sensors, including polymers [9,10], hydrogels [13], gelatine [14], cobalt-chloride-based materials [15], and SiO2 nanospheres [16]. In all these cases, the water absorption/ desorption in the hygrosensitive coating in presence of RH variations modifies its RI (and thickness), thus creating a spectral variation and amplitude change in the LPG attenuation bands, independently of the adhesion properties of the coating onto the grating itself. However, in the literature there is no evidence of the characterization of coated LPGs at low humidity values (below 20% RH) and temperatures below 15°C. Moreover, while the radiation effects have been investigated in the case of bare LPGs up to 154 Mrad [17,18], their influence on the sensing performance of coated LPGs seems to be completely unexplored. In this work, titanium dioxide (TiO2 ) has been used for the first time, to the best of our knowledge, as a sensitive element for the development of high-sensitivity LPGbased humidity sensors, due to its high RI (n 1.96) [19] and hygrosensitive characteristics [20,21]. Furthermore, the use of an oxide is expected to avoid the typical aging problems characteristic of polymeric overlays. The solgel dip coating method has been selected for the integration of nanoscale TiO2 layers onto the LPG surface, mainly due to its ability to guarantee a good optical quality, a ring-shaped symmetry, and an adequate longitudinal uniformity over the grating length. The deposition procedure mainly consisted of the solgel preparation (an alkoxide gel of titanium) followed by a standard dip-coating procedure [12]. More specifically, the fiber was first immersed into the solgel solution and then withdrawn with a controlled speed of 100 mm/min
4129
in order to allow the formation of a coherent fluid film. The final coating material is typically obtained by a curing step at 150°C for 10 min. Several samples with different TiO2 thicknesses have been successfully produced. In what follows, data related to a LPG characterized by a period Λ 404 μm, henceforth named LPG1, are discussed. Figure 1(a) reports a 20× optical microscope image of LPG1 after the deposition procedure, from which the smoothness and homogeneity of the deposited layer can be appreciated. The wavelength shift associated with the investigated attenuation band was monitored during all the deposition phases. This guarantees efficient on-line control of the targeted final overlay thickness of ∼100 nm. Figure 1(b) shows the transmittance spectra of LPG1, before and after the TiO2 deposition. The bare device exhibits an attenuation band (related to the fifth cladding mode) centered at λres;05 1589.0 nm, while the deposition of the TiO2 layer causes a resonance blueshift of ∼24.1 nm together with an ∼5 dB increase in its depth. The experimental results provide the first evidence of the optical quality of the deposited overlay; indeed, if an optical lossy overlay is deposited onto the LPG surface, a decrease in the visibility of the attenuation band combined with a spectral broadening would be expected. Once we assessed the fabrication step, the sensor performance of LPG1 was investigated in the range 0%–75% RH, at the temperatures of −10°C, 0°C, 10°C, and 25°C (pre-irradiation stage). In addition, the effects induced by the sensor’s exposure to a ϒ-ionizing radiation dose up to 1 Mrad were also studied (post-irradiation stage). The results reported were collected during an extensive experimental campaign carried out in the laboratories at CERN. To analyze the performance of the fabricated probe, a flexible apparatus was used to create stable and controlled humidity conditions in the range 0%– 100% RH and −20∕ 30°C, as described in [6]. A chilled mirror dew point hygrometer (Dew Master) was used to provide the RH reference during the sensor calibrations.
Fig. 1. (a) Typical microscope image of a TiO2 -coated LPG and (b) transmitted spectra of the bare and coated LPG.
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Fig. 2. (a) Transmittance spectra of LPG1 acquired at different RH values and (b) typical LPG response (blue curve) during a RH characterization at 25°C. The response provided by the reference device is also reported (black curve).
Furthermore, four reference resistive sensors (RTD), very close to the LPGs, have been installed to monitor the homogeneity and stability conditions of the temperature. The transmission spectra of the LPGs (and thus their resonant wavelengths) during the experiments were monitored by using a compact LPG interrogator, characterized by a measurement range from 1510 to 1590 nm and a wavelength resolution of 1 pm. Figure 2(a) shows the typical spectral variations of the TiO2 -coated LPG sensor (LPG1) during a characterization test carried out at 25°C. As theoretically expected, increasing the humidity content inside the test chamber causes a blueshift of λres;05 . This is due to the TiO2 coating RI increase caused by the higher amount of adsorbed water molecules, which in turn leads to an increase of n0i eff;cl [20]. Figure 2(a) also reveals that by increasing the RH value, the attenuation band visibility also increases. This is an important aspect as, in most cases, LPGs coated with “lossy” overlays experience partial or full resonance fading when operating in transition mode [22]. Figure 2(b) reports the λres;05 variations of the TiO2 coated LPG sensor (LPG1) during the characterization test of Fig. 2(a). For comparison, the response from the reference humidity sensor is also provided. It is worth emphasizing that the response provided by the optical sensor is in agreement with the reference device readings. Similar results have also been obtained at 10°C, 0°C, and −10°C, underlying the capability of the device to work properly also at low temperatures in the full RH range of interest. Figure 3(a) shows the calibration curves of LPG1 obtained at the four temperatures during the pre-irradiation stage. In contrast with what was observed with polyimide-coated FBGs [6], they have been found to be nonlinear, as a larger λres;05 shift was
registered at low RH levels. This can also be observed in Fig. 3(b), where the RH sensitivity (S RH ) curves of LPG1 versus RH are plotted. At 25°C, S RH values are between 1.4 and 0.11 nm/% RH in the range 0%–10% RH and decrease to 0.01 nm/% RH at very high humidity levels. This has to be compared with a uniform S RH in the range 1 · 10−3 nm∕% RH∕1.5 · 10−3 nm∕% RH typical of polyimide-coated FBG sensors [6]. Obtained results suggest that TiO2 -coated LPGs are able to provide RH sensitivities one to three orders of magnitude higher than those exhibited by FBG-based sensors coated with micrometer-thin polyimide overlays. From Fig. 3(b), a slight dependence of S RH on temperature (a small decrease occurs when temperature decreases) can be also appreciated. In addition, starting from the characteristic curves reported in Fig. 3(a), working at constant RH, the temperature sensitivity (S T ) of the LPG probe can also be retrieved by differentiating the λ-T curves (found to be linear), obtained by extrapolating the λres;05 values for each temperature from the fitting curves. As a result, the S T values (obtained at different humidity levels) were found to slightly decrease with RH; however, a mean S T value of ∼0.250 0.015 nm∕°C has been evaluated in the full RH range. It is worth noting that a temperature compensation scheme is required in order to reduce the cross sensitivity to unavoidable temperature modifications occurring in real applications. However, in the critical operative range 0%–10% RH, assuming for the LPG a S RH mean of ∼0.5 nm∕%RH, a temperature change of 1°C would induce a 0.5% RH/1% RH error if no compensation is applied. This value is significantly better than the thermal cross talk observed in the case of coated FBGs (7%–10% RH error for a noncompensated temperature change of 1°C). The radiation effects on coated LPG operation have also been investigated by exposing the sample to a 1 Mrad o
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Fig. 3. (a) LPG1 characteristic curves obtained at different temperatures (the straight lines represent the best fits of the points collected during the experimental tests) and (b) RH sensitivity of LPG1 as a function of RH at different temperatures.
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low humidity values), which turned out to be one to three orders of magnitude higher than those exhibited by FBG sensors coated by micrometer-thin polyimide overlays. In addition, the sensing performance of LPG-based sensors turned out to be almost unaffected by the exposure to a 1 Mrad dose of γ-ionizing radiation. Overall, the results collected demonstrate the strong potential of the proposed technology with regard to its future exploitation as a robust and valid alternative to the commercial (polymer-based) hygrometers currently used in HEP applications. Fig. 4. LPG1 (Δλres;05 , RH) points at 25°C before (black points) and after (red points) the 1 Mrad radiation exposure. Inset: (λres;05 , RH) points referring to the same tests.
dose of γ-ionizing radiation. The irradiation process has been performed using a Co60 source. Preliminary results indicate a radiation-induced dip wavelength shift of about 4.4 nm in correspondence to 30% RH (inset in Fig. 4). Even if further irradiation campaigns at progressively higher doses are currently in progress in order to confirm these preliminary results, we point out that the observed shift is almost in agreement with radiation hardness investigations conducted on different types of bare LPGs [18]. In particular, the observed shift is of the same order of magnitude of the shift observed with bare chiral LPGs [18] and is also similar to that recorded after a dose of 6 kilogray (0.6 Mrad) with low-order modes (8th, 9th, and 10th) in the case of a bare turnaround point LPG written in a B/Ge-doped fiber [23]. In Fig. 4, Δλres;05 versus RH at 25°C, before (black curve) and after (red curve) the irradiation campaign, is also depicted. Interestingly, the LPG sensor does not lose its excellent ability to respond to RH changes, and apart from some variations (∼16%), its characteristic curve still exhibits the same behavior shown before irradiation. We draw attention to the fact that the data reported provide the first experimental demonstration of the radiation tolerance of TiO2 -coated LPG sensors for RH monitoring and demonstrate good prospects for an effective exploitation of LPG technology for RH monitoring in HEP applications. The results obtained, however, suggest the necessity of taking into account the γ-ionizing radiation effects on the LPG sensor response in order to fully exploit the extremely high sensitivity of these devices in HEP applications. In conclusion, we reported the development and the first experimental demonstration of high-sensitivity TiO2 coated LPG sensors for RH monitoring at temperatures below 0°C as well as after high γ-radiation dose exposures (up to 1 Mrad). In particular, the humidity detection performance of a LPG sensor coated with a 100 nm thick TiO2 overlay has been analyzed in the range 0%–75% RH and at four different temperatures (in the range −10°C–25°C) during the experimental campaign carried out at CERN laboratories in Geneva. The results obtained are very encouraging and demonstrate the capability of TiO2 -coated LPG sensors to provide very high RH sensitivities (up to 1.4 nm/% RH in correspondence to very
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