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Temperature and refractive index sensing with Al2O3-nanocoated long-period gratings working at dispersion turning point
Optics and Laser Technology 107 (2018) 268–273
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Temperature and refractive index sensing with Al2O3-nanocoated long-period gratings working at dispersion turning point Mateusz S´mietana a,⇑, Magdalena Dominik a, Predrag Mikulic b, Wojtek J. Bock b a b
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, Warszawa 00-662, Poland Centre de recherche en photonique, Université du Québec en Outaouais, 101 Rue St Jean Bosco, Gatineau, QC J8X 3X7, Canada
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Article history: Received 23 November 2017 Received in revised form 22 April 2018 Accepted 4 June 2018
Keywords: Optical fiber sensors Long-period grating (LPG) Temperature sensing Refractive index sensing Atomic layer deposition (ALD) Thin films
a b s t r a c t In this work we discuss the effect of nanocoating a long-period grating (LPG) with aluminum oxide (Al2O3) on both its refractive index (RI) and its temperature (T) sensitivity. Two LPGs, one coated and one uncoated, were optimized to work at the dispersion turning point (DTP) of the higher-order cladding modes, where the sensitivities are the highest. The DTP was reached by two methods – wet etching of the fiber cladding and use of an optimized Al2O3 nanocoating applied by atomic layer deposition (ALD). In both cases we show a significant increase in RI sensitivity at the DTP. Thanks to an additional mode transition (MT) effect, the RI sensitivity of the coated LPG reaches about 8200 nm/RIU in the range nD = 1.333–1.345 RIU traced for only one resonance of the pair. This is more than 3.5 times higher than for the non-coated LPG. Moreover, T sensitivity for the Al2O3-coated LPG was found to be 2.5 times higher than for the non-coated LPG. When the MT effect alone is applied, i.e., the LPG works away from the DTP, the nanocoated LPG may offer higher RI and lower T sensitivity than a standard non-coated LPG working at the DTP. The differences in sensitivity are attributed to the order of coupled cladding modes and the thermo-optic coefficients of both Al2O3 and the external medium. We believe that by proper selection of the nanocoating material, the T sensitivity can be greatly reduced, a major advantage for future biosensing applications of LPGs. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Optical fiber sensors, due to their numerous advantages including small dimensions, immunity to electromagnetic radiation, and usually relatively simple fabrication processes, have already been widely investigated and applied [1]. The sensors based on optical fiber gratings, i.e., fiber Bragg gratings (FBGs) and long-period gratings (LPGs), are made by periodic modulation of the refractive index within the core of a single-mode optical fiber [2]. In an LPG, the modulation induces coupling between the core mode and the cladding modes, resulting in the appearance of a series of resonance attenuation bands in the grating’s transmission spectrum. When the properties of the core, cladding or external medium vary, the resonances experience a spectral shift [3]. Sensitivity to a certain measurand is then typically defined as a spectral shift of the resonance wavelength vs. the measurand [4]. LPGs have already been used for such applications as temperature (T) [5] and pressure [6] sensing, where the difference between the thermo- and pressure-optic coefficients respectively of the ⇑ Corresponding author. E-mail address: M.S´[email protected] (M. S´mietana). https://doi.org/10.1016/j.optlastec.2018.06.017 0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.
cladding and core materials has a major impact on the sensitivities [6]. Due to coupling of the cladding modes, there is also a significant dependence between the properties of the external medium and the spectral response of the LPG [7]. For this reason, the LPG may be highly sensitive to the external refractive index (RI). Besides the refractometric applications, where high RI sensitivity is reached, the LPG can be used as a universal label-free biosensor targeting biomolecules of different sizes, such as bacteria [8], viruses [9], proteins [10], or toxins [11]. In the label-free sensing approach the sensor is capable of monitoring a biological film growth on the LPG surface, which corresponds to an increase in the RI near the LPG surface [12]. For label-free biosensing applications the LPG must offer high RI sensitivity when working at RI values close to that of water (nD = 1.3330 RIU). When optimized RI sensitivity is considered, high-sensitivity conditions are known to occur when the dispersion curve of the cladding mode for a particular effective RI experiences a turning point [4]. At dispersion turning point (DTP) conditions for each of the cladding modes, a characteristic pair of resonances can be observed in the LPG transmission spectrum. The resonances in the vicinity of the DTP shift in opposite directions under certain external influences. In particular, an increase in the external RI is
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followed by an increase in the spectral distance between the two resonances, while for T the direction of the change depends mainly on the properties of the core and the cladding material and can be positive or negative [13]. In addition to the DTP effect when highRI sensitivity is considered, a mode transition (MT) phenomenon is also known that takes place when the LPG is coated with a high-RI thin coating [14]. Depending on the properties of the coating it can then guide a mode and thus induce transitions of the other cladding modes. At such transition conditions, which depend on the external RI, the RI sensitivity can be greatly enhanced. A high sensitivity in the specified range of RI can be achieved by precise adjustment of the thickness and the optical properties of the coatings [15,16]. Considering both the DTP and the MT effects, the LPG sensitivity to variations in external RI can be improved even more [17,18]. In our previous work [19] we demonstrated how to optimize the RI sensitivity taking into account both these effects by etching the fiber cladding before applying the nanocoating. Both these processes require very high precision and may make the sensor fabrication process relatively complex [20]. However, following results presented in [17], it is known that the DTP and MT effects can also be obtained solely by a nanocoating deposition. In the present work we investigate and compare in terms of RI and T sensitivities the two methods for tuning LPG working towards DTP, namely etching of the fiber cladding and deposition of a nanocoating. These two types of processing are currently the most often employed for various RI sensing purposes, including label-free biosensing. As a nanocoating, we applied an aluminum oxide (Al2O3) thin film with carefully selected properties. The Al2O3 was deposited by the atomic layer deposition (ALD) method which gives precise control of the coating properties on the whole length of the grating [20]. The Al2O3 shows high hardness, as well as high temperature resistance. As discussed above, thanks to nanocoating deposition to induce the MT effect, the RI sensitivity can be greatly enhanced in certain RI ranges. However, the T sensitivity of a nanocoated LPG optimized towards DTP has never been reported or compared to that of an LPG working solely at DTP. The effect of cross-sensitivity to T when the external RI is measured is critical for mature applications of nanocoated LPGs, and that is why we strongly believe that the issue is worth further investigation.
2. Experimental details The LPG sensors in our experiments were manufactured using standard germanium-doped Corning SMF-28 single-mode optical fiber. The process for fabricating the LPGs included: fiber loading with hydrogen, UV irradiation with an amplitude mask (period of 226.8 lm) and annealing in order to stabilize the optical properties of the LPGs [21]. The first set of LPGs was tuned by hydrofluoric (HF) acid etching up to the DTP for an external RI close to that of water (nD = 1.3330 RIU) [11]. On the second set of LPGs, an Al2O3 film was deposited with the ALD method using the Cambridge NanoTech Savannah S100 system [22]. For the deposition process, water and trimethylaluminum (TMA) precursors were used. Thickness of the Al2O3 film was controlled by the number of ALD cycles, which was set up to 2100. The optimized Al2O3 film thickness made it possible to achieve both DTP and MT effects. Spectral response of the LPGs was investigated in the wavelength range 1300–1700 nm using a supercontinuum white light laser source (Leukos SM30) and a spectrum analyzer (Yokogawa AQ6370B). The RI sensitivity was measured by immersing the LPG in a glycerin/water solution with an RI range of nD = 1.333– 1.345 RIU. The nD of the solutions was determined using an automatic refractometer (Reichert AR200). The temperature sensitivity
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was measured for LPGs immersed in water by tuning their T in the range from 5 °C to 50 °C using a home-made thermoblock equipped with Peltier modules. The LPGs were also mounted in a special holder to prevent evaporation of water and to maintain the fiber tension during all the experiments. The sensor’s spectral response was analyzed numerically using Optiwave Optigrating software v4.2.2. The LPG model assumed properties of the LPG and coating as reported in [13] and [22], respectively. The RIs of water and Al2O3 were taken to be 1.318 and 1.611 RIU at k = 1550 nm. No allowance was made for possible change in the optical properties of the coating as a result of change in its thickness within the investigated coating thickness range (200–300 nm) [22]. 3. Results and discussion Three LPGs working in different conditions were designed for this experiment: (1) an etched LPG reaching the DTP for an external RI close to that of water, (2) a non-etched grating with nanocoating thickness optimized to obtain both MT and DPT conditions, and (3) an LPG with a slightly thicker coating, where the MT phenomenon still affects the spectral response, but the structure works away from the DTP [20]. The results of numerical analysis of these three designs, assuming water as the external medium, are shown in Fig. 1. When properties of the fiber and the LPG are assumed as in [13], for the LPG (period of 226.8 mm) in the investigated spectral range just one resonance is observed at k = 1230 nm and this is due to coupling of the LP0,9 cladding mode. As a result of etching the fiber cladding to reduce its radius by about 5 mm, the resonance shifts towards a longer wavelength and the mode experiences its DTP in the wavelength range above 1600 nm. However, when the LPG is instead given a coating with an RI of 1.611 RIU, LP0,9 shifts towards a shorter wavelength and a higher cladding mode (LP0,10) appears in the range between 1500 and 1600 nm. The spectral distance between the resonances for LP0,10 increases significantly with the coating thickness. It is evident that, depending on the approach (etching or deposition), the DTP can be reached, but for a different order of cladding modes. The three LPG variants are further analyzed experimentally. 3.1. RI sensitivity The transmission spectrum of the investigated LPGs before any processing when immersed in water is shown in Fig. 2. The experimental spectrum agrees well with numerical analysis shown in Fig. 1. The effect on spectral response induced by an increase in external RI for the obtained LPGs is compared in Fig. 3. In all cases
Fig. 1. Numerical analysis of spectral response of the LPG (226.8 mm in period) to etching (5 mm reduction in radius) or nanocoating deposition (increase in coating thickness to 202 and 208 nm). Arrows show progress for each of the processes.
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Fig. 2. Transmission spectrum of the investigated LPG before any etching or coating when immersed in water (nD = 1.333 RIU).
a pair of resonances can be seen as is typical near the DTP, and the resonances separate spectrally when the external RI increases. The initial spectral responses measured in water agree well with the results obtained numerically. Experimental results also show that there is a significant difference between the spectral range where DTP can be observed for LPGs obtained as a result of etching and deposition. The LPG with no coating experiences the DTP in water at k 1625 nm (Fig. 3a), whereas with the Al2O3-coated LPG (Fig. 3b. and c) it takes place at k 1470 nm. There is also a slight
difference between the transmitted power at the resonances for the etched and the nanocoated LPGs. However, the coupling coefficient of the modes on which the spectral depth depends can undoubtedly be further optimized during the process of fabricating the coated LPGs. The analyzed response to RI is highly influenced by the fact that an LPG is working near the DTP. It is known that the sensitivity of an etched LPG drops rapidly when resonances separate [11] and further increases when the external RI is close to the RI of the fiber cladding material [9]. When high-RI nanocoating is deposited, the MT phenomenon can be employed in addition to the DTP, and these two effects which take place separately for an uncoated LPG can be combined at an arbitrary RI range [20]. However, the optimization process requires a careful selection of thickness and optical properties of the coating for a particular RI range. In this case the RI sensitivity of the LPG was tuned towards its highest values in water solutions, where label-free biosensing applications of LPGs are possible [23]. The applied Al2O3 thickness was as used in numerical analysis, i.e., 202 and 208 nm for the LPG working close to and slightly away from the DTP, respectively. It can be seen that the roughly estimated RI sensitivity of an optimized Al2O3-coated LPG given for a single resonance may reach about 8200 nm/RIU (Fig. 4). The sensitivity of such an optimized LPG is more than 3.5 times that of an LPG with no coating but also working at the DTP, and double that of the nanocoated one working away from the DTP. Also noticeable is the significant difference in sensitivity between the left- and right-shifting resonances, in favor of the right-shifting one in all cases. To summarize, deposition of Al2O3 nanocoating on an LPG produces a sensing structure with higher RI sensitivity than that of an uncoated LPG, even working away from the DTP. Furthermore, the
Fig. 3. Evolution of transmission spectrum with external RI for the LPGs working close to DTP, where (a) and (b) show the effects for LPGs without and with the Al2O3 coating, respectively. The same effect for the nanocoated LPG, but working slightly away from DTP, is shown in (c).
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disturb RI measurements. Most often discussed is the influence of T [24]. Keeping in mind that the nanocoated LPG offers significantly higher RI sensitivity than an uncoated one, responses of the LPGs to T when immersed in water were investigated next. It can be seen in Fig. 5. that an increase in T induces a shift opposite to the one observed when the external RI is increased. In this case, the resonances shift toward each other with T. The effect can be explained following the analysis presented by Shu et al. in [4]. The T sensitivity of an LPG can be described by Eq. (1), where: th km cladding mode, neff - effective res - resonance wavelength of the m RIs of core and mth cladding mode, ng - group RIs of core and mth cladding mode, and nco and ncl - thermo-optic coefficients of core and cladding materials, respectively. 0m 01 0m n01 dkm eff neff nco neff ncl neff res ¼ dkm res 01 01 0m 0m dT ng ng neff neff
Fig. 4. Resonance wavelength vs. external RI compared for the LPG without (squares) and with (circles) Al2O3 thin film working close to DTP and away from DTP (triangles). Linear fits (dash lines) were applied to the measurement points.
deposition process avoids the hazards of fabrication using HF as the etching agent. 3.2. T Sensitivity High RI sensitivity is not the only characteristic that matters for label-free biosensing applications of any type of optical sensor. Limited cross-sensitivity to other measurands can be expected to
ð1Þ
The first part of the equation represents the waveguide dispersion and the second corresponds to the temperature dependence of waveguide dispersion. The T sensitivity of the LPG can then be explained on the basis of the thermo-optic coefficients of the core and the cladding materials. In the case of standard germanium-doped fiber used in this experiment we can assume 0m nco n01 eff > ncl neff [25]. Since the RI of the cladding increases less with T than does the core RI, the resonances shift towards each other. Comparing the results shown in Fig. 5, we can clearly see the influence of DTP on the T sensitivity of the coated LPGs. When the Al2O3 nanocoating is slightly thicker, mainly the MT effect is induced and the working point is away from the DTP. For this case,
Fig. 5. Evolution of transmission spectrum with T for the LPGs working close to DTP, where (a) and (b) show the effect for the grating without and with the Al2O3 coating, respectively. The same effect for the nanocoated LPG, but working away from DTP is shown in (c).
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Fig. 6. Response to temperature as resonance wavelength shift for the LPG without coating (squares) and with Al2O3 nanocoating (circles), both working close to DTP, and for nanocoated LPG but working away from DTP (triangles). For comparison linear fits (dash lines) were applied to the measurement points.
the T sensitivity in the same T range is significantly lower than for the other LPGs working close to the DTP (Fig. 6). It can be seen that when both DTP and MT effects are employed, as in the case of RI sensitivity, the result is an increase of the T sensitivity. For the nanocoated sample at room temperature, the sensitivity for each resonance is 2.5 times higher than for the non-coated LPG. However, the sensitivities for the uncoated LPG and the nanocoated LPG working away from the DTP are similar. It is noteworthy that for the right resonance and the nanocoated LPG working away from the DTP, the RI to T sensitivity ratio is the highest among all the samples. Three reasons can explain the differences noted in the T sensitivity: the different order of coupled cladding modes observed for nanocoated and non-coated samples, the higher RI sensitivity of the nanocoated LPG and a non-zero thermo-optic coefficient of the Al2O3. The spectral response of an LPG strongly depends on the order of coupled cladding modes [4]. In general, the higher the order, the higher the sensitivity that can be expected. In the case at hand, the observed resonances correspond to the LP0,9 and LP0,10 cladding modes for non-coated and coated LPGs, respectively. The impact of the mode order manifests itself most when the DTP is reached. However, it must be taken into account that on top of the influence of the mode order also the fiber etching may lead to increase of the RI sensitivity [26]. A second reason for T sensitivity is that the nanocoated LPG is highly sensitive to the external RI, which may vary with T. Thirdly, there is the influence of the thermo-optic coefficient of water on the measured response to T. The effect for the cases reported here is significant due to the RI sensitivity of the coated LPG, which has been tuned to be highest at the RI of water. Assuming that for water the thermo-optic coefficient is around 8 105 RIU°C1 [27], its contribution to temperature shift is negative, i.e., it may in fact decrease the T-induced response. This effect in particular is observed for the LPG working away from DTP, which still offers high RI sensitivity. When the LPG is nanocoated, the thermo-optic coefficient of the coating may contribute to the :. The thermo-optic coefficient of Al2O3 highly depends on the deposition method and its parameters, as well as the intended spectral range [28]. For a rough calculation we can assume it to be approx. 4.92 105 RIU°C1 [28], which is almost 5 times higher than the coefficient of fused silica cladding (ncl = 0.78 105 RIU°C1 [25]). This characteristic influences the coupling conditions and makes a positive contribution
Fig. 7. Evolution of resonance wavelength with T of water for the optimized Al2O3coated LPG working close to DTP (circles) and away from DTP (triangles). Arrows and numbers show direction and order of the temperature change.
to the resonance wavelength shift with T. Since other high-RI materials can be used as nanocoatings to tune the RI response, a material with a negative thermo-optic coefficient should be chosen. The goal is to have a coating material that will balance the intrinsic T sensitivity of the LPG and make the device insensitive to temperature and simultaneously highly sensitive to the external RI. At the same time, the material must be stable in watery solutions and resistant to induced temperature variations. To confirm the high stability of Al2O3 to such variations, the nanocoated LPG underwent several temperature cycles in the T range 10–45 °C. The resonance wavelength response to T cycles is shown in Fig. 7. It can be seen that these measurements are highly repeatable. A slight nonlinear trend can be seen when the LPG approaches the DTP at higher T readings, but this effect also occurs with all other measurands investigated in close vicinity to the DTP. 4. Conclusions In this work we compared refractive index and temperature sensitivity for LPGs with and without Al2O3 nanocoating, both working at the DTP, where the sensitivity to a number of measurands is highest. The DTP was reached in two ways: by etching the cladding of the LPGs in HF and by deposition of Al2O3 nanocoating using the ALD method. In both cases we observed a high RI sensitivity induced by the DTP. However, thanks to the MT effect induced by the coating, the RI sensitivity was enhanced up to 8200 nm/RIU for tracing a single resonance and RI in the range 1.333–1.345 RIU. The sensitivity is over 3.5 times higher than that of non-coated LPGs. Moreover, T sensitivity for the two structures was compared and at room temperature was found to be 2.5 times higher for the Al2O3-coated sample than for the uncoated sample. When the RI to T sensitivity ratio is considered, the nanocoated samples working away from the DTP can reach higher values than do those at the DTP. We expect that the T sensitivity of the LPG can be reduced by selection of a nanocoating material whose thermo-optic coefficient compensates for the intrinsic T sensitivity of the LPG. Such nanocoated LPGs can find further applications in label-free biosensing, due to their optimized sensitivity to RI in its range close to that of water. Moreover, the spectral range within which the high RI of nanocoated LPGs was measured fits into the range commonly interrogated by cost-effective FBG analyzers. On top of high RI sensitivity, this is a great advantage of the proposed approach.
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Full length article
Temperature and refractive index sensing with Al2O3-nanocoated long-period gratings working at dispersion turning point Mateusz S´mietana a,⇑, Magdalena Dominik a, Predrag Mikulic b, Wojtek J. Bock b a b
Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, Warszawa 00-662, Poland Centre de recherche en photonique, Université du Québec en Outaouais, 101 Rue St Jean Bosco, Gatineau, QC J8X 3X7, Canada
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Article history: Received 23 November 2017 Received in revised form 22 April 2018 Accepted 4 June 2018
Keywords: Optical fiber sensors Long-period grating (LPG) Temperature sensing Refractive index sensing Atomic layer deposition (ALD) Thin films
a b s t r a c t In this work we discuss the effect of nanocoating a long-period grating (LPG) with aluminum oxide (Al2O3) on both its refractive index (RI) and its temperature (T) sensitivity. Two LPGs, one coated and one uncoated, were optimized to work at the dispersion turning point (DTP) of the higher-order cladding modes, where the sensitivities are the highest. The DTP was reached by two methods – wet etching of the fiber cladding and use of an optimized Al2O3 nanocoating applied by atomic layer deposition (ALD). In both cases we show a significant increase in RI sensitivity at the DTP. Thanks to an additional mode transition (MT) effect, the RI sensitivity of the coated LPG reaches about 8200 nm/RIU in the range nD = 1.333–1.345 RIU traced for only one resonance of the pair. This is more than 3.5 times higher than for the non-coated LPG. Moreover, T sensitivity for the Al2O3-coated LPG was found to be 2.5 times higher than for the non-coated LPG. When the MT effect alone is applied, i.e., the LPG works away from the DTP, the nanocoated LPG may offer higher RI and lower T sensitivity than a standard non-coated LPG working at the DTP. The differences in sensitivity are attributed to the order of coupled cladding modes and the thermo-optic coefficients of both Al2O3 and the external medium. We believe that by proper selection of the nanocoating material, the T sensitivity can be greatly reduced, a major advantage for future biosensing applications of LPGs. Ó 2018 Elsevier Ltd. All rights reserved.
1. Introduction Optical fiber sensors, due to their numerous advantages including small dimensions, immunity to electromagnetic radiation, and usually relatively simple fabrication processes, have already been widely investigated and applied [1]. The sensors based on optical fiber gratings, i.e., fiber Bragg gratings (FBGs) and long-period gratings (LPGs), are made by periodic modulation of the refractive index within the core of a single-mode optical fiber [2]. In an LPG, the modulation induces coupling between the core mode and the cladding modes, resulting in the appearance of a series of resonance attenuation bands in the grating’s transmission spectrum. When the properties of the core, cladding or external medium vary, the resonances experience a spectral shift [3]. Sensitivity to a certain measurand is then typically defined as a spectral shift of the resonance wavelength vs. the measurand [4]. LPGs have already been used for such applications as temperature (T) [5] and pressure [6] sensing, where the difference between the thermo- and pressure-optic coefficients respectively of the ⇑ Corresponding author. E-mail address: M.S´[email protected] (M. S´mietana). https://doi.org/10.1016/j.optlastec.2018.06.017 0030-3992/Ó 2018 Elsevier Ltd. All rights reserved.
cladding and core materials has a major impact on the sensitivities [6]. Due to coupling of the cladding modes, there is also a significant dependence between the properties of the external medium and the spectral response of the LPG [7]. For this reason, the LPG may be highly sensitive to the external refractive index (RI). Besides the refractometric applications, where high RI sensitivity is reached, the LPG can be used as a universal label-free biosensor targeting biomolecules of different sizes, such as bacteria [8], viruses [9], proteins [10], or toxins [11]. In the label-free sensing approach the sensor is capable of monitoring a biological film growth on the LPG surface, which corresponds to an increase in the RI near the LPG surface [12]. For label-free biosensing applications the LPG must offer high RI sensitivity when working at RI values close to that of water (nD = 1.3330 RIU). When optimized RI sensitivity is considered, high-sensitivity conditions are known to occur when the dispersion curve of the cladding mode for a particular effective RI experiences a turning point [4]. At dispersion turning point (DTP) conditions for each of the cladding modes, a characteristic pair of resonances can be observed in the LPG transmission spectrum. The resonances in the vicinity of the DTP shift in opposite directions under certain external influences. In particular, an increase in the external RI is
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followed by an increase in the spectral distance between the two resonances, while for T the direction of the change depends mainly on the properties of the core and the cladding material and can be positive or negative [13]. In addition to the DTP effect when highRI sensitivity is considered, a mode transition (MT) phenomenon is also known that takes place when the LPG is coated with a high-RI thin coating [14]. Depending on the properties of the coating it can then guide a mode and thus induce transitions of the other cladding modes. At such transition conditions, which depend on the external RI, the RI sensitivity can be greatly enhanced. A high sensitivity in the specified range of RI can be achieved by precise adjustment of the thickness and the optical properties of the coatings [15,16]. Considering both the DTP and the MT effects, the LPG sensitivity to variations in external RI can be improved even more [17,18]. In our previous work [19] we demonstrated how to optimize the RI sensitivity taking into account both these effects by etching the fiber cladding before applying the nanocoating. Both these processes require very high precision and may make the sensor fabrication process relatively complex [20]. However, following results presented in [17], it is known that the DTP and MT effects can also be obtained solely by a nanocoating deposition. In the present work we investigate and compare in terms of RI and T sensitivities the two methods for tuning LPG working towards DTP, namely etching of the fiber cladding and deposition of a nanocoating. These two types of processing are currently the most often employed for various RI sensing purposes, including label-free biosensing. As a nanocoating, we applied an aluminum oxide (Al2O3) thin film with carefully selected properties. The Al2O3 was deposited by the atomic layer deposition (ALD) method which gives precise control of the coating properties on the whole length of the grating [20]. The Al2O3 shows high hardness, as well as high temperature resistance. As discussed above, thanks to nanocoating deposition to induce the MT effect, the RI sensitivity can be greatly enhanced in certain RI ranges. However, the T sensitivity of a nanocoated LPG optimized towards DTP has never been reported or compared to that of an LPG working solely at DTP. The effect of cross-sensitivity to T when the external RI is measured is critical for mature applications of nanocoated LPGs, and that is why we strongly believe that the issue is worth further investigation.
2. Experimental details The LPG sensors in our experiments were manufactured using standard germanium-doped Corning SMF-28 single-mode optical fiber. The process for fabricating the LPGs included: fiber loading with hydrogen, UV irradiation with an amplitude mask (period of 226.8 lm) and annealing in order to stabilize the optical properties of the LPGs [21]. The first set of LPGs was tuned by hydrofluoric (HF) acid etching up to the DTP for an external RI close to that of water (nD = 1.3330 RIU) [11]. On the second set of LPGs, an Al2O3 film was deposited with the ALD method using the Cambridge NanoTech Savannah S100 system [22]. For the deposition process, water and trimethylaluminum (TMA) precursors were used. Thickness of the Al2O3 film was controlled by the number of ALD cycles, which was set up to 2100. The optimized Al2O3 film thickness made it possible to achieve both DTP and MT effects. Spectral response of the LPGs was investigated in the wavelength range 1300–1700 nm using a supercontinuum white light laser source (Leukos SM30) and a spectrum analyzer (Yokogawa AQ6370B). The RI sensitivity was measured by immersing the LPG in a glycerin/water solution with an RI range of nD = 1.333– 1.345 RIU. The nD of the solutions was determined using an automatic refractometer (Reichert AR200). The temperature sensitivity
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was measured for LPGs immersed in water by tuning their T in the range from 5 °C to 50 °C using a home-made thermoblock equipped with Peltier modules. The LPGs were also mounted in a special holder to prevent evaporation of water and to maintain the fiber tension during all the experiments. The sensor’s spectral response was analyzed numerically using Optiwave Optigrating software v4.2.2. The LPG model assumed properties of the LPG and coating as reported in [13] and [22], respectively. The RIs of water and Al2O3 were taken to be 1.318 and 1.611 RIU at k = 1550 nm. No allowance was made for possible change in the optical properties of the coating as a result of change in its thickness within the investigated coating thickness range (200–300 nm) [22]. 3. Results and discussion Three LPGs working in different conditions were designed for this experiment: (1) an etched LPG reaching the DTP for an external RI close to that of water, (2) a non-etched grating with nanocoating thickness optimized to obtain both MT and DPT conditions, and (3) an LPG with a slightly thicker coating, where the MT phenomenon still affects the spectral response, but the structure works away from the DTP [20]. The results of numerical analysis of these three designs, assuming water as the external medium, are shown in Fig. 1. When properties of the fiber and the LPG are assumed as in [13], for the LPG (period of 226.8 mm) in the investigated spectral range just one resonance is observed at k = 1230 nm and this is due to coupling of the LP0,9 cladding mode. As a result of etching the fiber cladding to reduce its radius by about 5 mm, the resonance shifts towards a longer wavelength and the mode experiences its DTP in the wavelength range above 1600 nm. However, when the LPG is instead given a coating with an RI of 1.611 RIU, LP0,9 shifts towards a shorter wavelength and a higher cladding mode (LP0,10) appears in the range between 1500 and 1600 nm. The spectral distance between the resonances for LP0,10 increases significantly with the coating thickness. It is evident that, depending on the approach (etching or deposition), the DTP can be reached, but for a different order of cladding modes. The three LPG variants are further analyzed experimentally. 3.1. RI sensitivity The transmission spectrum of the investigated LPGs before any processing when immersed in water is shown in Fig. 2. The experimental spectrum agrees well with numerical analysis shown in Fig. 1. The effect on spectral response induced by an increase in external RI for the obtained LPGs is compared in Fig. 3. In all cases
Fig. 1. Numerical analysis of spectral response of the LPG (226.8 mm in period) to etching (5 mm reduction in radius) or nanocoating deposition (increase in coating thickness to 202 and 208 nm). Arrows show progress for each of the processes.
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Fig. 2. Transmission spectrum of the investigated LPG before any etching or coating when immersed in water (nD = 1.333 RIU).
a pair of resonances can be seen as is typical near the DTP, and the resonances separate spectrally when the external RI increases. The initial spectral responses measured in water agree well with the results obtained numerically. Experimental results also show that there is a significant difference between the spectral range where DTP can be observed for LPGs obtained as a result of etching and deposition. The LPG with no coating experiences the DTP in water at k 1625 nm (Fig. 3a), whereas with the Al2O3-coated LPG (Fig. 3b. and c) it takes place at k 1470 nm. There is also a slight
difference between the transmitted power at the resonances for the etched and the nanocoated LPGs. However, the coupling coefficient of the modes on which the spectral depth depends can undoubtedly be further optimized during the process of fabricating the coated LPGs. The analyzed response to RI is highly influenced by the fact that an LPG is working near the DTP. It is known that the sensitivity of an etched LPG drops rapidly when resonances separate [11] and further increases when the external RI is close to the RI of the fiber cladding material [9]. When high-RI nanocoating is deposited, the MT phenomenon can be employed in addition to the DTP, and these two effects which take place separately for an uncoated LPG can be combined at an arbitrary RI range [20]. However, the optimization process requires a careful selection of thickness and optical properties of the coating for a particular RI range. In this case the RI sensitivity of the LPG was tuned towards its highest values in water solutions, where label-free biosensing applications of LPGs are possible [23]. The applied Al2O3 thickness was as used in numerical analysis, i.e., 202 and 208 nm for the LPG working close to and slightly away from the DTP, respectively. It can be seen that the roughly estimated RI sensitivity of an optimized Al2O3-coated LPG given for a single resonance may reach about 8200 nm/RIU (Fig. 4). The sensitivity of such an optimized LPG is more than 3.5 times that of an LPG with no coating but also working at the DTP, and double that of the nanocoated one working away from the DTP. Also noticeable is the significant difference in sensitivity between the left- and right-shifting resonances, in favor of the right-shifting one in all cases. To summarize, deposition of Al2O3 nanocoating on an LPG produces a sensing structure with higher RI sensitivity than that of an uncoated LPG, even working away from the DTP. Furthermore, the
Fig. 3. Evolution of transmission spectrum with external RI for the LPGs working close to DTP, where (a) and (b) show the effects for LPGs without and with the Al2O3 coating, respectively. The same effect for the nanocoated LPG, but working slightly away from DTP, is shown in (c).
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disturb RI measurements. Most often discussed is the influence of T [24]. Keeping in mind that the nanocoated LPG offers significantly higher RI sensitivity than an uncoated one, responses of the LPGs to T when immersed in water were investigated next. It can be seen in Fig. 5. that an increase in T induces a shift opposite to the one observed when the external RI is increased. In this case, the resonances shift toward each other with T. The effect can be explained following the analysis presented by Shu et al. in [4]. The T sensitivity of an LPG can be described by Eq. (1), where: th km cladding mode, neff - effective res - resonance wavelength of the m RIs of core and mth cladding mode, ng - group RIs of core and mth cladding mode, and nco and ncl - thermo-optic coefficients of core and cladding materials, respectively. 0m 01 0m n01 dkm eff neff nco neff ncl neff res ¼ dkm res 01 01 0m 0m dT ng ng neff neff
Fig. 4. Resonance wavelength vs. external RI compared for the LPG without (squares) and with (circles) Al2O3 thin film working close to DTP and away from DTP (triangles). Linear fits (dash lines) were applied to the measurement points.
deposition process avoids the hazards of fabrication using HF as the etching agent. 3.2. T Sensitivity High RI sensitivity is not the only characteristic that matters for label-free biosensing applications of any type of optical sensor. Limited cross-sensitivity to other measurands can be expected to
ð1Þ
The first part of the equation represents the waveguide dispersion and the second corresponds to the temperature dependence of waveguide dispersion. The T sensitivity of the LPG can then be explained on the basis of the thermo-optic coefficients of the core and the cladding materials. In the case of standard germanium-doped fiber used in this experiment we can assume 0m nco n01 eff > ncl neff [25]. Since the RI of the cladding increases less with T than does the core RI, the resonances shift towards each other. Comparing the results shown in Fig. 5, we can clearly see the influence of DTP on the T sensitivity of the coated LPGs. When the Al2O3 nanocoating is slightly thicker, mainly the MT effect is induced and the working point is away from the DTP. For this case,
Fig. 5. Evolution of transmission spectrum with T for the LPGs working close to DTP, where (a) and (b) show the effect for the grating without and with the Al2O3 coating, respectively. The same effect for the nanocoated LPG, but working away from DTP is shown in (c).
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Fig. 6. Response to temperature as resonance wavelength shift for the LPG without coating (squares) and with Al2O3 nanocoating (circles), both working close to DTP, and for nanocoated LPG but working away from DTP (triangles). For comparison linear fits (dash lines) were applied to the measurement points.
the T sensitivity in the same T range is significantly lower than for the other LPGs working close to the DTP (Fig. 6). It can be seen that when both DTP and MT effects are employed, as in the case of RI sensitivity, the result is an increase of the T sensitivity. For the nanocoated sample at room temperature, the sensitivity for each resonance is 2.5 times higher than for the non-coated LPG. However, the sensitivities for the uncoated LPG and the nanocoated LPG working away from the DTP are similar. It is noteworthy that for the right resonance and the nanocoated LPG working away from the DTP, the RI to T sensitivity ratio is the highest among all the samples. Three reasons can explain the differences noted in the T sensitivity: the different order of coupled cladding modes observed for nanocoated and non-coated samples, the higher RI sensitivity of the nanocoated LPG and a non-zero thermo-optic coefficient of the Al2O3. The spectral response of an LPG strongly depends on the order of coupled cladding modes [4]. In general, the higher the order, the higher the sensitivity that can be expected. In the case at hand, the observed resonances correspond to the LP0,9 and LP0,10 cladding modes for non-coated and coated LPGs, respectively. The impact of the mode order manifests itself most when the DTP is reached. However, it must be taken into account that on top of the influence of the mode order also the fiber etching may lead to increase of the RI sensitivity [26]. A second reason for T sensitivity is that the nanocoated LPG is highly sensitive to the external RI, which may vary with T. Thirdly, there is the influence of the thermo-optic coefficient of water on the measured response to T. The effect for the cases reported here is significant due to the RI sensitivity of the coated LPG, which has been tuned to be highest at the RI of water. Assuming that for water the thermo-optic coefficient is around 8 105 RIU°C1 [27], its contribution to temperature shift is negative, i.e., it may in fact decrease the T-induced response. This effect in particular is observed for the LPG working away from DTP, which still offers high RI sensitivity. When the LPG is nanocoated, the thermo-optic coefficient of the coating may contribute to the :. The thermo-optic coefficient of Al2O3 highly depends on the deposition method and its parameters, as well as the intended spectral range [28]. For a rough calculation we can assume it to be approx. 4.92 105 RIU°C1 [28], which is almost 5 times higher than the coefficient of fused silica cladding (ncl = 0.78 105 RIU°C1 [25]). This characteristic influences the coupling conditions and makes a positive contribution
Fig. 7. Evolution of resonance wavelength with T of water for the optimized Al2O3coated LPG working close to DTP (circles) and away from DTP (triangles). Arrows and numbers show direction and order of the temperature change.
to the resonance wavelength shift with T. Since other high-RI materials can be used as nanocoatings to tune the RI response, a material with a negative thermo-optic coefficient should be chosen. The goal is to have a coating material that will balance the intrinsic T sensitivity of the LPG and make the device insensitive to temperature and simultaneously highly sensitive to the external RI. At the same time, the material must be stable in watery solutions and resistant to induced temperature variations. To confirm the high stability of Al2O3 to such variations, the nanocoated LPG underwent several temperature cycles in the T range 10–45 °C. The resonance wavelength response to T cycles is shown in Fig. 7. It can be seen that these measurements are highly repeatable. A slight nonlinear trend can be seen when the LPG approaches the DTP at higher T readings, but this effect also occurs with all other measurands investigated in close vicinity to the DTP. 4. Conclusions In this work we compared refractive index and temperature sensitivity for LPGs with and without Al2O3 nanocoating, both working at the DTP, where the sensitivity to a number of measurands is highest. The DTP was reached in two ways: by etching the cladding of the LPGs in HF and by deposition of Al2O3 nanocoating using the ALD method. In both cases we observed a high RI sensitivity induced by the DTP. However, thanks to the MT effect induced by the coating, the RI sensitivity was enhanced up to 8200 nm/RIU for tracing a single resonance and RI in the range 1.333–1.345 RIU. The sensitivity is over 3.5 times higher than that of non-coated LPGs. Moreover, T sensitivity for the two structures was compared and at room temperature was found to be 2.5 times higher for the Al2O3-coated sample than for the uncoated sample. When the RI to T sensitivity ratio is considered, the nanocoated samples working away from the DTP can reach higher values than do those at the DTP. We expect that the T sensitivity of the LPG can be reduced by selection of a nanocoating material whose thermo-optic coefficient compensates for the intrinsic T sensitivity of the LPG. Such nanocoated LPGs can find further applications in label-free biosensing, due to their optimized sensitivity to RI in its range close to that of water. Moreover, the spectral range within which the high RI of nanocoated LPGs was measured fits into the range commonly interrogated by cost-effective FBG analyzers. On top of high RI sensitivity, this is a great advantage of the proposed approach.
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