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Bane chemicals detection through photonic crystal fiber in THz regime
Optical Fiber Technology 54 (2020) 102102
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Bane chemicals detection through photonic crystal fiber in THz regime Md. Bellal Hossain a b c
a,b,⁎
a
a,c
, Etu Podder , Abdullah Al-Mamun Bulbul , Himadri Shekhar Mondal
T a
Electronics and Communication Engineering Discipline, Khulna University, Khulna 9208, Bangladesh School of Electrical and Information Engineering, The University of Sydney, NSW, Australia Department of Electronics and Telecommunication Engineering (ETE), Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalganj, Bangladesh
A B S T R A C T
Sarin, Soman, and Tabun are man-made nerve agents which were introduced as vice-regent of chemical battle. As they are extremely dangerous for human health so, it is very worthy to find a precious, safe, and efficient method for detection of these bane chemicals. Accounting this as the key objective, PCF based chemical sensor model is suggested in this research. Zeonex is used as the fiber material and this sensor performance is analyzed in terahertz regime. The reported sensor model provides enhanced sensitivity (94.4%) along with tiny confinement loss (1.71 × 10−14 cm−1) at frequency 1.8 THz. Besides, the fabrication of this model is also probable by exercising subsisting fabrication methods.
1. Introduction Nerve agents are the utmost bane as well as quickly performing well-known chemicals. Tabun, Soman, and Sarin are the dominating chemical agents made by human. All of them are clear or colorless and tasteless liquid. They are most volatile among all the nerve agents which implies that they can simply as well as swiftly evaporate from liquid to vapor. On account of this, one can be exposed by the vapor even if s/he does not come in touch with these chemicals. Initially they were introduced as pesticide. Afterward, they are mostly used in various chemical battles among different countries. Significant hazard to human health be carried out by the exposer of these chemicals. For instance, the exposer of Sarin, Soman, and Tabun into the air, may harm human skin, human eye as well as it can create breathing problem. Besides, mixing up with water and foods, it can turn water and food poisonous, and the use of these water and foods results in immature death of human or animals. The extents of poisoning caused by these chemicals depend on their amount, form as well as time duration of the exposure. Hence, it is very worthy to find out a perfect detection method for of these chemicals. So far numerous methods for different chemicals detection have been displayed for instance electrometric [1], volta-metric [2], chromatographic [3] etc. but all of them have some problems for instance the requirement of large analysis duration, complicated process as well as expert user. The main benefits of the proposed method include cost effectiveness as well as comparatively user friendly. To enhance the accuracy of these dangerous chemicals detection and to simplify the detection process, researchers have proposed numerous new sensing techniques. Currently, PCF sensors have been evolving in the numerous
⁎
applications of chemical sensing, defense purpose, communication applications etc. at terahertz (THz) frequency range [4–9]. Several number of fiber materials are used as bulk materials for instance Silica, Teflon, Topas, and Zeonex [10–15]. Many special characteristics of Topas/Zeonex in chemical as well as bio-sensing has made them unique and best suited for the hosting material of porous fiber [16,17]. Though both Topas and Zeonex have analogous guiding features for instance constant refractive index (RI) of around 1.53 throughout THz regime, lower bulk absorption loss, low dispersion, high transparency, humidity insensitivity, high temperature insensitivity etc. However, Zeonex provides greater chemical resistivity as well as larger bio-compatibility compared to Topas [13–15]. Besides, Topas lags behind Zeonex for another reason that is Topas has smaller glass transition temperature compared to Zeonex [14,15]. As higher glass transition temperature is greatly expected for fiber fabrication, on account of this, in this letter, Zeonex is chosen as the fiber material. Numerous studies followed up for chemical sensing through PCF for the last several years [16–20]. In 2007, the THz reflection spectroscopy method was demonstrated by Jepsen et al. for chemical sensing [21,22]. Still then, terahertz spectra were not achieved easily by that process [23–25]. In 2016, Asaduzzaman et al. preferred a Hybrid Photonic Crystal Fiber (HPCF) structure for chemical detection which exhibited the sensitivity of around 49.2% where birefringence was observed only 0.0015 [26]. Next, a chemical sensor with a variant air hole diameter was offered by Arif et al. to enhance the sensitivity [16]. Therefore, the sensitivity was increased to 59%. Moreover, various models were also presented to detect chemical through PCF [27–33]. Nevertheless, all of these previously proposed sensors were designed to work mainly in infrared regime and failed to gain relative sensitivity
Corresponding author at: School of Electrical and Information Engineering, The University of Sydney, NSW 2006, Australia. E-mail address: [email protected] (Md. B. Hossain).
https://doi.org/10.1016/j.yofte.2019.102102 Received 22 May 2019; Received in revised form 19 October 2019; Accepted 24 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
Optical Fiber Technology 54 (2020) 102102
Md. B. Hossain, et al.
Fig. 1. Cross sectional representation of PCF sensor.
Fig. 3. Sensitivity of optimized-2% design structure in x direction.
Fig. 4. Sensitivity of optimized-2% design structure in y direction. Fig. 2. Mode field polarization direction: a) Sarin, x polarization; b) Sarin, y polarization; c) Soman, x polarization; d) Soman, y polarization; e) Tabun, x polarization; f) Tabun, y polarization.
2. Sensor model design and its fabrication opportunities To design the proposed PCF model, COMSOL Multiphysics v5.2 has been used. Cross-sectional representation of the proposed sensor model for bane chemicals detection is presented in Fig. 1 and Fig. 2 shows the light propagation direction when Sarin, Soman and Tabun are taken as analytes. The hollow core PCF is chosen for its numerous advantages for instance hollow core fiber offers lower effective material loss (EML)
greater than 85% at frequency 1.5 THz [20]. Therefore, in 2018, though few works exhibited high sensitivity, but the fabrication of those sensors were difficult. So, still lots of possibilities are present to develop PCF based chemical sensor. 2
Optical Fiber Technology 54 (2020) 102102
Md. B. Hossain, et al.
Fig. 8. Sensitivity of optimized +2% design structure in y direction.
Fig. 5. Sensitivity of optimized design structure in x direction.
whether the lengths of R3, R4, R9, R10, R11, and R12 are same (1220 μm). Again, for rectangle R5, and R6, the lengths are same 1420 μm) where the lengths of R7, and R8 are same (1640 μm). Same spacing between two rectangular holes is kept in the clad region. The PML is normally chosen between 6% to 10% of fiber radius and in this design we kept it around 9% of fiber radius. Sarin, Soman and Tabun are selected as analyte materials. To get high core power fraction, one needs to keep the core hole as large as possible so that high amount of liquid can be inserted through the core region. But for fabrication complexity, no one can have large core area as per free will. We have kept the clad holes length and width fixed where, we increased and decreased the length as well as width of core hole by 2% to experience the sensor performance. After that we declared the lengths and widths of rectangular core as optimum since by increasing the length as well as width of core, we didn’t find any noteworthy variations in sensitivity. Recently, numerous PCF fabrication methods have been used for instance stacking [37], sol–gel [38], extrusion, 3D printing [39,40] etc. It is worthy to note that stacking as well as sol–gel methods are fit for fabrication of circular air holes, where the 3D printing as well as extrusion method is used to fabricate asymmetric PCF structure. The Max Plank Institute has already done the fabrication of some complex structures which also includes rectangular air hole [40,41]. So, the fabrication of the proposed PCF model is possible by using existing fabrication methods.
Fig. 6. Sensitivity of optimized design structure in y direction.
3. Simulation result and discussions In order to investigate the sensor performance, it is essential to calculate the sensing properties of PCF for instance relative sensitivity, confinement loss etc. The relative sensitivity is the main sensing agent which indicates the existence and amount of any sensing analytes. It can be measured by [20],
r=
nl X neff
Fig. 7. Sensitivity of optimized +2% design structure in x direction.
(1)
Where, nl stands for the liquid or analyte RI which is 1.44 for Tabun, 1.394 for Soman and 1.366 for Sarin and neff stands for effective mode index. Nevertheless, X stands for the quantity of interaction between light and matter that can be calculated by [20],
with higher sensitivity [20]. Again, the proposed sensor model’s asymmetric core and cladding arrangement provide higher birefringence with enlarged sensitivity which was not obtained earlier [34–36]. Total fiber diameter is 3.65 mm which includes a perfectly matched layer (PML) as absorbing boundary after the cladding region, one rectangular core where analytes are inserted, another twelve rectangles which covers clad region guide the light to propagate through the core area. The length and width of rectangular hollow core is reported as Lc = 600 μm and Wc = 400 μm respectively. In cladding region, the widths of all holes are same (400 μm) and the lengths are different. The lengths of rectangle R1, and R2 are same (600 μm),
X=
∫analyte sample Re (Ex Hy − Hx Ey ) dxdy ∫total Re (Ex Hy − Hx Ey ) dxdy
(2)
Here, Ex , Ey and Hx , Hy are electric and magnetic field components respectively. Integration is performed in numerator and denominator of Eq. (2) to have the ratio of light propagation between the chosen analyte and total fiber cross-section, respectively. To experience the sensor performance we kept clad holes length and width fixed, but we 3
Optical Fiber Technology 54 (2020) 102102
Md. B. Hossain, et al.
Table 1 Comparative study of the sensitivity of Sarin, Soman, and Tabun in y polarization mode for different core hole lengths and widths. L c and Wc
Operating Frequency
Relative sensitivity of Sarin
Relative Sensitivity of Soman
Relative Sensitivity of Tabun
588 μm and 392 μm 600 μm and 400 μm 612 μm and 408 μm
1.8 THz 1.8 THz 1.8 THz
90.90% 92.84% 94.28%
91.88% 93.450% 94.87%
93.1% 94.4% 95.5%
varied the core length and width by 2%. We considered 3 different conditions (optimum-2%, optimum, optimum + 2%). The length and width of core are 588 μm and 392 μm respectively for optimum-2% structure. For optimum design structure, the length and width of core is 600 μm and 400 μm respectively where, for optimum + 2% design structure, the length and width of core are 612 μm and 408 μm respectively. The design structure which length is 600 μm and width is 400 μm is given the title “optimum”. Because, this design parameter provides high sensitivity and further increase in the core hole length and width don’t provide any significant change in sensor performance as well as it may overlap core-clad region. The relative sensitivity of Sarin, Soman, and Tabun with respect to THz frequency are shown in Figs. 3–8. From the figures, it is evident that relative sensitivity enlarges with enlarging frequency in both polarization mode and it is logical. As light confinement through the core area becomes higher for increasing frequency so the core power fraction goes higher for increasing frequency which provides high sensitivity for this proposed model. Besides, relative sensitivity is higher for analytes which refractive index is higher in both polarization mode and it is also logical. Because, light confinement through high indexed analytes (Tabun > Soman > Sarin) is better. Again, comperatively higher sensitivity is observed in y polarization mode and it happens because for any operating frequency, neff becomes smaller in y polarization mode than x polarization mode. Also, the physical reason behind this is the proposed structure’s core area is larger in y direction than in x direction and for this reason, more light goes through the analyte in y direction which makes high relative sensitivity in y polarization mode for this PCF model. Table 1 shows comparative study of relative sensitivity for the three different analytes used in this research. For expanding sensor performance, it is worthy to preserve the polarization stuff of any PCF and Birefringence is that stuff which deals with the polarization state of sensor. Birefringence basically comes from the asymmetry between core and cladding holes. The higher the asymmetry between core and clad is, the higher the birefringence will be. It can be measured as follows [27],
Fig. 9. Birefringence of optimum design structure.
Fig. 10. Confinement loss of optimum design structure.
B = |n x − n y|
(3)
Here, n x and n y stands for the mode goes through x and y- polarization direction respectively. From Fig. 9, it is realized that with increasing frequency, birefringence of optimum design structure goes lower and it is logical. When the operating frequency increases then the RI difference between x and y polarization mode becomes lower and as a result, the birefringence quantity becomes smaller with enlarging frequency. The optimum structure provides with birefringence around 0.00682, 0.00645, and 0.00598 for Sarin, Soman and Tabun respectively at frequency 1.8 THz. Confinement loss is another important issue which influences the sensor performance. It comes from the imaginary portion of RI and can be measured by [35].
4πf Im (neff ) ⎞, cm−1 Lc = ⎛ ⎝ c ⎠ Fig. 11. EML curve for optimum design structure.
(4)
Here, f is the operating frequency and c is the light speed in free space. Fig. 10 shows confinement loss profile for optimum structure. Here, confinement loss declines with increasing frequency and it is logical. Because, with increasing operating frequency the RI difference between 4
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Md. B. Hossain, et al.
Table 2 Comparative study of sensing parameter among previous works and this manuscript. References
Operating Region
Relative sensitivity
1.33 μm 1.33 μm 1.55 THz 1.8 THz
Ref. [16] Ref. [17] Ref. [20] Optimum structure
Birefringence – 0.00021 0.0051 0.00682
59% – 85.50% 94.4%
x and y polarization mode becomes lower and as a result, the birefringence quantity becomes smaller with enlarging frequency. Also, confinement loss is lower for higher indexed analyte (Tabun) and it is also logical. As light can confine highly through the core when it gets larger RI index in core region and for this reason, the confinement loss is lower for Tabun. Confinement loss is observed around 1.71 × 10−14 cm−1, 2.15 × 10−14 cm−1, and 2.85 × 10−14 cm−1 for Tabun, Soman, and Sarin respectively at frequency 1.8 THz. EML is a limiting issue of THz sensor performance. So, less EML is expected. The recommended rectangular hollow core reduces the overall background material which reduces the EML and it can be measured by [20].
α eff =
∊0 ⎛ ∫fiber μ0 ⎜ ⎝
material
nmaterial |E|2 αmaterial dA ⎞
∫all Pz dA
⎟ ⎠
1 (E × H ∗) z ̂ 2
−11
EML −1
cm 8.345 × 10 2.7 × 10−12 cm−1 −9 1.67 × 10 cm−1 1.71 × 10−14 cm−1
– – – 0.00859 cm−1
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, cm−1 (5)
Where, ∈0 is the permittivity and μ0 is the permeability in free space, nmaterial stands for RI of zeonex and αmaterial stands for bulk absorption loss of Zeonex. Pz is the pointing vector in z direction which is evaluated by,
Pz =
Confinement Loss
(6)
Here, E is the electric filed and H ∗ is the complex conjugate of the magnetic field. EML profile for the proposed sensor model is shown by Fig. 11. With enlarging frequency EML also becomes larger and it is already theoretically realized [27]. EML is also larger for higher indexed analyte and in this work Tabun has higher index and so it provides higher EML when compared to both Soman and Sarin. EML is observed around 0.00859, 0.00885, 0.0094 cm−1 for Sarin, Soman, and Tabun respectively at frequency 1.8 THz which is much lower. To have the overall idea about the proposed sensor structure performance, we need to have a look on comparative result of the sensing properties and it is presented in Table 2. 4. Conclusion A rectangular core PCF sensor is offered for the detection of bane chemicals (Sarin, Soman and Tabun) in THz frequency region where Zeonex is used as fiber material. The optimum model shows enhanced relative sensitivity of around 92.84% for Sarin, 93.45% for Soman, and 94.4% for Tabun at frequency 1.8 THz. Besides, the reported model offers very small EML as well as very tiny confinement loss which is very worthy for proficient sensing. In addition, this model offers high birefringence which is also very expected in order to enhance sensor performance. Existing fabrication approaches are suitable to fabricate this recommended sensor model. Hence, the manufacture of this proposed sensor model will be applicable in numerous chemicals industries. References [1] Yegor G. Timofeyenko, Jeffrey J. Rosentreter, Susan Mayo, Piezoelectric quartz crystal microbalance sensor for trace aqueous cyanide ion determination, Anal. Chem. 79 (1) (2007) 251–255. [2] Junjun Wu, Lu. Wang, Qinqin Wang, Lina Zou, Baoxian Ye, The novel voltammetric method for determination of hesperetin based on a sensitive electrochemical sensor,
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Optical Fiber Technology 54 (2020) 102102
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6
Contents lists available at ScienceDirect
Optical Fiber Technology journal homepage: www.elsevier.com/locate/yofte
Bane chemicals detection through photonic crystal fiber in THz regime Md. Bellal Hossain a b c
a,b,⁎
a
a,c
, Etu Podder , Abdullah Al-Mamun Bulbul , Himadri Shekhar Mondal
T a
Electronics and Communication Engineering Discipline, Khulna University, Khulna 9208, Bangladesh School of Electrical and Information Engineering, The University of Sydney, NSW, Australia Department of Electronics and Telecommunication Engineering (ETE), Bangabandhu Sheikh Mujibur Rahman Science and Technology University, Gopalganj, Bangladesh
A B S T R A C T
Sarin, Soman, and Tabun are man-made nerve agents which were introduced as vice-regent of chemical battle. As they are extremely dangerous for human health so, it is very worthy to find a precious, safe, and efficient method for detection of these bane chemicals. Accounting this as the key objective, PCF based chemical sensor model is suggested in this research. Zeonex is used as the fiber material and this sensor performance is analyzed in terahertz regime. The reported sensor model provides enhanced sensitivity (94.4%) along with tiny confinement loss (1.71 × 10−14 cm−1) at frequency 1.8 THz. Besides, the fabrication of this model is also probable by exercising subsisting fabrication methods.
1. Introduction Nerve agents are the utmost bane as well as quickly performing well-known chemicals. Tabun, Soman, and Sarin are the dominating chemical agents made by human. All of them are clear or colorless and tasteless liquid. They are most volatile among all the nerve agents which implies that they can simply as well as swiftly evaporate from liquid to vapor. On account of this, one can be exposed by the vapor even if s/he does not come in touch with these chemicals. Initially they were introduced as pesticide. Afterward, they are mostly used in various chemical battles among different countries. Significant hazard to human health be carried out by the exposer of these chemicals. For instance, the exposer of Sarin, Soman, and Tabun into the air, may harm human skin, human eye as well as it can create breathing problem. Besides, mixing up with water and foods, it can turn water and food poisonous, and the use of these water and foods results in immature death of human or animals. The extents of poisoning caused by these chemicals depend on their amount, form as well as time duration of the exposure. Hence, it is very worthy to find out a perfect detection method for of these chemicals. So far numerous methods for different chemicals detection have been displayed for instance electrometric [1], volta-metric [2], chromatographic [3] etc. but all of them have some problems for instance the requirement of large analysis duration, complicated process as well as expert user. The main benefits of the proposed method include cost effectiveness as well as comparatively user friendly. To enhance the accuracy of these dangerous chemicals detection and to simplify the detection process, researchers have proposed numerous new sensing techniques. Currently, PCF sensors have been evolving in the numerous
⁎
applications of chemical sensing, defense purpose, communication applications etc. at terahertz (THz) frequency range [4–9]. Several number of fiber materials are used as bulk materials for instance Silica, Teflon, Topas, and Zeonex [10–15]. Many special characteristics of Topas/Zeonex in chemical as well as bio-sensing has made them unique and best suited for the hosting material of porous fiber [16,17]. Though both Topas and Zeonex have analogous guiding features for instance constant refractive index (RI) of around 1.53 throughout THz regime, lower bulk absorption loss, low dispersion, high transparency, humidity insensitivity, high temperature insensitivity etc. However, Zeonex provides greater chemical resistivity as well as larger bio-compatibility compared to Topas [13–15]. Besides, Topas lags behind Zeonex for another reason that is Topas has smaller glass transition temperature compared to Zeonex [14,15]. As higher glass transition temperature is greatly expected for fiber fabrication, on account of this, in this letter, Zeonex is chosen as the fiber material. Numerous studies followed up for chemical sensing through PCF for the last several years [16–20]. In 2007, the THz reflection spectroscopy method was demonstrated by Jepsen et al. for chemical sensing [21,22]. Still then, terahertz spectra were not achieved easily by that process [23–25]. In 2016, Asaduzzaman et al. preferred a Hybrid Photonic Crystal Fiber (HPCF) structure for chemical detection which exhibited the sensitivity of around 49.2% where birefringence was observed only 0.0015 [26]. Next, a chemical sensor with a variant air hole diameter was offered by Arif et al. to enhance the sensitivity [16]. Therefore, the sensitivity was increased to 59%. Moreover, various models were also presented to detect chemical through PCF [27–33]. Nevertheless, all of these previously proposed sensors were designed to work mainly in infrared regime and failed to gain relative sensitivity
Corresponding author at: School of Electrical and Information Engineering, The University of Sydney, NSW 2006, Australia. E-mail address: [email protected] (Md. B. Hossain).
https://doi.org/10.1016/j.yofte.2019.102102 Received 22 May 2019; Received in revised form 19 October 2019; Accepted 24 November 2019 1068-5200/ © 2019 Elsevier Inc. All rights reserved.
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Fig. 1. Cross sectional representation of PCF sensor.
Fig. 3. Sensitivity of optimized-2% design structure in x direction.
Fig. 4. Sensitivity of optimized-2% design structure in y direction. Fig. 2. Mode field polarization direction: a) Sarin, x polarization; b) Sarin, y polarization; c) Soman, x polarization; d) Soman, y polarization; e) Tabun, x polarization; f) Tabun, y polarization.
2. Sensor model design and its fabrication opportunities To design the proposed PCF model, COMSOL Multiphysics v5.2 has been used. Cross-sectional representation of the proposed sensor model for bane chemicals detection is presented in Fig. 1 and Fig. 2 shows the light propagation direction when Sarin, Soman and Tabun are taken as analytes. The hollow core PCF is chosen for its numerous advantages for instance hollow core fiber offers lower effective material loss (EML)
greater than 85% at frequency 1.5 THz [20]. Therefore, in 2018, though few works exhibited high sensitivity, but the fabrication of those sensors were difficult. So, still lots of possibilities are present to develop PCF based chemical sensor. 2
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Fig. 8. Sensitivity of optimized +2% design structure in y direction.
Fig. 5. Sensitivity of optimized design structure in x direction.
whether the lengths of R3, R4, R9, R10, R11, and R12 are same (1220 μm). Again, for rectangle R5, and R6, the lengths are same 1420 μm) where the lengths of R7, and R8 are same (1640 μm). Same spacing between two rectangular holes is kept in the clad region. The PML is normally chosen between 6% to 10% of fiber radius and in this design we kept it around 9% of fiber radius. Sarin, Soman and Tabun are selected as analyte materials. To get high core power fraction, one needs to keep the core hole as large as possible so that high amount of liquid can be inserted through the core region. But for fabrication complexity, no one can have large core area as per free will. We have kept the clad holes length and width fixed where, we increased and decreased the length as well as width of core hole by 2% to experience the sensor performance. After that we declared the lengths and widths of rectangular core as optimum since by increasing the length as well as width of core, we didn’t find any noteworthy variations in sensitivity. Recently, numerous PCF fabrication methods have been used for instance stacking [37], sol–gel [38], extrusion, 3D printing [39,40] etc. It is worthy to note that stacking as well as sol–gel methods are fit for fabrication of circular air holes, where the 3D printing as well as extrusion method is used to fabricate asymmetric PCF structure. The Max Plank Institute has already done the fabrication of some complex structures which also includes rectangular air hole [40,41]. So, the fabrication of the proposed PCF model is possible by using existing fabrication methods.
Fig. 6. Sensitivity of optimized design structure in y direction.
3. Simulation result and discussions In order to investigate the sensor performance, it is essential to calculate the sensing properties of PCF for instance relative sensitivity, confinement loss etc. The relative sensitivity is the main sensing agent which indicates the existence and amount of any sensing analytes. It can be measured by [20],
r=
nl X neff
Fig. 7. Sensitivity of optimized +2% design structure in x direction.
(1)
Where, nl stands for the liquid or analyte RI which is 1.44 for Tabun, 1.394 for Soman and 1.366 for Sarin and neff stands for effective mode index. Nevertheless, X stands for the quantity of interaction between light and matter that can be calculated by [20],
with higher sensitivity [20]. Again, the proposed sensor model’s asymmetric core and cladding arrangement provide higher birefringence with enlarged sensitivity which was not obtained earlier [34–36]. Total fiber diameter is 3.65 mm which includes a perfectly matched layer (PML) as absorbing boundary after the cladding region, one rectangular core where analytes are inserted, another twelve rectangles which covers clad region guide the light to propagate through the core area. The length and width of rectangular hollow core is reported as Lc = 600 μm and Wc = 400 μm respectively. In cladding region, the widths of all holes are same (400 μm) and the lengths are different. The lengths of rectangle R1, and R2 are same (600 μm),
X=
∫analyte sample Re (Ex Hy − Hx Ey ) dxdy ∫total Re (Ex Hy − Hx Ey ) dxdy
(2)
Here, Ex , Ey and Hx , Hy are electric and magnetic field components respectively. Integration is performed in numerator and denominator of Eq. (2) to have the ratio of light propagation between the chosen analyte and total fiber cross-section, respectively. To experience the sensor performance we kept clad holes length and width fixed, but we 3
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Table 1 Comparative study of the sensitivity of Sarin, Soman, and Tabun in y polarization mode for different core hole lengths and widths. L c and Wc
Operating Frequency
Relative sensitivity of Sarin
Relative Sensitivity of Soman
Relative Sensitivity of Tabun
588 μm and 392 μm 600 μm and 400 μm 612 μm and 408 μm
1.8 THz 1.8 THz 1.8 THz
90.90% 92.84% 94.28%
91.88% 93.450% 94.87%
93.1% 94.4% 95.5%
varied the core length and width by 2%. We considered 3 different conditions (optimum-2%, optimum, optimum + 2%). The length and width of core are 588 μm and 392 μm respectively for optimum-2% structure. For optimum design structure, the length and width of core is 600 μm and 400 μm respectively where, for optimum + 2% design structure, the length and width of core are 612 μm and 408 μm respectively. The design structure which length is 600 μm and width is 400 μm is given the title “optimum”. Because, this design parameter provides high sensitivity and further increase in the core hole length and width don’t provide any significant change in sensor performance as well as it may overlap core-clad region. The relative sensitivity of Sarin, Soman, and Tabun with respect to THz frequency are shown in Figs. 3–8. From the figures, it is evident that relative sensitivity enlarges with enlarging frequency in both polarization mode and it is logical. As light confinement through the core area becomes higher for increasing frequency so the core power fraction goes higher for increasing frequency which provides high sensitivity for this proposed model. Besides, relative sensitivity is higher for analytes which refractive index is higher in both polarization mode and it is also logical. Because, light confinement through high indexed analytes (Tabun > Soman > Sarin) is better. Again, comperatively higher sensitivity is observed in y polarization mode and it happens because for any operating frequency, neff becomes smaller in y polarization mode than x polarization mode. Also, the physical reason behind this is the proposed structure’s core area is larger in y direction than in x direction and for this reason, more light goes through the analyte in y direction which makes high relative sensitivity in y polarization mode for this PCF model. Table 1 shows comparative study of relative sensitivity for the three different analytes used in this research. For expanding sensor performance, it is worthy to preserve the polarization stuff of any PCF and Birefringence is that stuff which deals with the polarization state of sensor. Birefringence basically comes from the asymmetry between core and cladding holes. The higher the asymmetry between core and clad is, the higher the birefringence will be. It can be measured as follows [27],
Fig. 9. Birefringence of optimum design structure.
Fig. 10. Confinement loss of optimum design structure.
B = |n x − n y|
(3)
Here, n x and n y stands for the mode goes through x and y- polarization direction respectively. From Fig. 9, it is realized that with increasing frequency, birefringence of optimum design structure goes lower and it is logical. When the operating frequency increases then the RI difference between x and y polarization mode becomes lower and as a result, the birefringence quantity becomes smaller with enlarging frequency. The optimum structure provides with birefringence around 0.00682, 0.00645, and 0.00598 for Sarin, Soman and Tabun respectively at frequency 1.8 THz. Confinement loss is another important issue which influences the sensor performance. It comes from the imaginary portion of RI and can be measured by [35].
4πf Im (neff ) ⎞, cm−1 Lc = ⎛ ⎝ c ⎠ Fig. 11. EML curve for optimum design structure.
(4)
Here, f is the operating frequency and c is the light speed in free space. Fig. 10 shows confinement loss profile for optimum structure. Here, confinement loss declines with increasing frequency and it is logical. Because, with increasing operating frequency the RI difference between 4
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Table 2 Comparative study of sensing parameter among previous works and this manuscript. References
Operating Region
Relative sensitivity
1.33 μm 1.33 μm 1.55 THz 1.8 THz
Ref. [16] Ref. [17] Ref. [20] Optimum structure
Birefringence – 0.00021 0.0051 0.00682
59% – 85.50% 94.4%
x and y polarization mode becomes lower and as a result, the birefringence quantity becomes smaller with enlarging frequency. Also, confinement loss is lower for higher indexed analyte (Tabun) and it is also logical. As light can confine highly through the core when it gets larger RI index in core region and for this reason, the confinement loss is lower for Tabun. Confinement loss is observed around 1.71 × 10−14 cm−1, 2.15 × 10−14 cm−1, and 2.85 × 10−14 cm−1 for Tabun, Soman, and Sarin respectively at frequency 1.8 THz. EML is a limiting issue of THz sensor performance. So, less EML is expected. The recommended rectangular hollow core reduces the overall background material which reduces the EML and it can be measured by [20].
α eff =
∊0 ⎛ ∫fiber μ0 ⎜ ⎝
material
nmaterial |E|2 αmaterial dA ⎞
∫all Pz dA
⎟ ⎠
1 (E × H ∗) z ̂ 2
−11
EML −1
cm 8.345 × 10 2.7 × 10−12 cm−1 −9 1.67 × 10 cm−1 1.71 × 10−14 cm−1
– – – 0.00859 cm−1
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, cm−1 (5)
Where, ∈0 is the permittivity and μ0 is the permeability in free space, nmaterial stands for RI of zeonex and αmaterial stands for bulk absorption loss of Zeonex. Pz is the pointing vector in z direction which is evaluated by,
Pz =
Confinement Loss
(6)
Here, E is the electric filed and H ∗ is the complex conjugate of the magnetic field. EML profile for the proposed sensor model is shown by Fig. 11. With enlarging frequency EML also becomes larger and it is already theoretically realized [27]. EML is also larger for higher indexed analyte and in this work Tabun has higher index and so it provides higher EML when compared to both Soman and Sarin. EML is observed around 0.00859, 0.00885, 0.0094 cm−1 for Sarin, Soman, and Tabun respectively at frequency 1.8 THz which is much lower. To have the overall idea about the proposed sensor structure performance, we need to have a look on comparative result of the sensing properties and it is presented in Table 2. 4. Conclusion A rectangular core PCF sensor is offered for the detection of bane chemicals (Sarin, Soman and Tabun) in THz frequency region where Zeonex is used as fiber material. The optimum model shows enhanced relative sensitivity of around 92.84% for Sarin, 93.45% for Soman, and 94.4% for Tabun at frequency 1.8 THz. Besides, the reported model offers very small EML as well as very tiny confinement loss which is very worthy for proficient sensing. In addition, this model offers high birefringence which is also very expected in order to enhance sensor performance. Existing fabrication approaches are suitable to fabricate this recommended sensor model. Hence, the manufacture of this proposed sensor model will be applicable in numerous chemicals industries. References [1] Yegor G. Timofeyenko, Jeffrey J. Rosentreter, Susan Mayo, Piezoelectric quartz crystal microbalance sensor for trace aqueous cyanide ion determination, Anal. Chem. 79 (1) (2007) 251–255. [2] Junjun Wu, Lu. Wang, Qinqin Wang, Lina Zou, Baoxian Ye, The novel voltammetric method for determination of hesperetin based on a sensitive electrochemical sensor,
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