Initially in 1966, Optical fiber technology was developed for telecommunication applications. After that this technology got a huge positive feedback among the researchers. Very soon optical fibers were seen to expand its application area like sensing field. The growths of photonic crystal fibers (PCFs) with their significant optical properties have confirmed the prospective benefits of optical fibers in chemical and biological sensing    . The outstanding characteristics of these micro structured fibers such as small size, freedom of design, and relative compatibility make them stand out for sensing applications    . Where in conventional fibers, designing is not so easy, core size is limited, material selection is tough etc. Moreover, these limitations on the geometry hamper the flexibility in recognizing fiber properties such as dispersion   , confinement loss   , birefringence   , relative sensitivity  etc. PCF provides us with long distance light propagation that was not possible previously. Light propagation characteristics can be controlled by adjusting the air holes in the core and cladding region. A lot of research studies are already done in the fields of optical communications   , nonlinear optics  and sensing    .
Sensitivity vastly depends on refractive index of material. The higher the refractive index of a material, the higher the sensitivity. For this reason, Sensing material with lower refractive index is very tough, but with blessing of PCF lower index materials can also be sensed    .
Water and alcohols are considered as the major analytes for these types of applications because they account for the immense majority of biological or chemical solutions   . We need to know how much alcohol is mixed water within any drinking liquid as drinking large amount of alcohol is harmful for human body. Alcohols are also used to make different biomedical solution so sensing alcohol is a great issue. Sensing properties of alcohol highly depend on its concentration as well as in which temperature it is measured    .
In this research work, we proposed a simple hexagonal PCF structure where ethyl alcohol-water mixture is inserted through the core region. In order to realize the sensitivity profile of ethyl alcohol-water mixture, we have varied the concentration of ethyl alcohol and also we have varied the operating temperature to investigate the sensitivity profile.
2. Materials and Methods
In this thesis, we have designed a hexagonal PCF structure in COMSOL Multiphysics (version-5).
Silica glass is used as core material which is surrounded by three layers of air holes with diameter, d1 = 1.6 um and pitch, Ʌ = 2.1 um. Air holes are used to guide the light propagation through the core. In the core area, 6 holes with diameter, d = 1.2 um and another 7 holes with diameter 0.8 um is filled with different concentration of Ethyl Alcohol at different temperature. A perfectly matched layer (PML) of 0.7 um is used. The designed hexagonal PCF structure is shown in Figure 1.
In this work, we have taken different mixture of Ethyl alcohol-water at different temperature and then we have investigated the relative sensitivity of ethyl alcohol at different temperature at wavelength range from 600 nm to 1600 nm. Refractive index of Ethyl Alcohol-Water Mixture changes with its concentration as well as its operating temperature. Tables 1-3 show the refractive index for different concentration of Ethyl Alcohol-Water mixture and at temperature 20˚C, 25˚C, 30˚C respectively.
The relative sensitivity can be calculated by using the below equation    :
Figure 1. Proposed hexagonal PCF structure in COMSOL Multiphysics: (a) Transverse geometry and (b) Fundamental mode field of the PCF.
Table 1. Refractive index of ethyl alcohol-water mixtures at 20˚C.
Table 2. Refractive index of ethyl alcohol-water mixtures at 25˚C.
Table 3. Refractive index of ethyl alcohol-water mixtures at 30˚C.
Where nr is the refractive index of sensed material and neff is the modal effective refractive index which is obtained from Comsol Multiphysics simulation. Again, f is the power ration in percentage which f can be obtained from Comsol Multiphysics simulation directly using the rule given below    :
Here, Ex, Ey and Hx, Hy are the transverse electric field and magnetic field in x and y axies respectively. Then putting the values of nr, neff and f in MATLAB we can get relative sensitivity curve with respect to wavelength.
3. Simulation Results
We have got the simulated result for different concentration of Ethyl Alcohol-Water mixtures as well as for different temperature. Then the simulated result is inserted into MATLAB and then the relative sensitivity curves are obtained with respect to different wavelength. The sensitivity curves of Ethyl Alcohol water mixture for different concentration at temperature 20˚C, 25˚C, 30˚C are shown for wavelength range from 600 nm to 1600 nm. A tuning laser can be used to have light of different wavelength. The relative sensitivity profile for different concentration of Ethyl Alcohol at temperature 20˚C is shown in Figure 2.
For 20˚C temperature, the relative sensitivity is increasing with increasing amount of Ethyl Alcohol water mixture as the refractive index goes higher with increasing amount of Ethyl Alcohol. Again, the relative sensitivity is increasing with increasing wavelength. The relative sensitivity is obtained approximately 37.5, 40.6, 41.5, 42 in percentage at wavelength 1200 nm and 44,44.59, 44.85, 45 in percentage at wavelength 1500 nm when 15%, 40%, 60%, 75% of Ethyl Alcohol-water mixture is inserted through the core region respectively.
We performed the same procedure to investigate the relative sensitivity profile at temperature 25˚C and it is shown in Figure 3.
For 25˚C temperature, the relative sensitivity is also increasing with increasing amount of Ethyl Alcohol water mixture as the refractive index goes higher with increasing amount of Ethyl Alcohol.
Again, the relative sensitivity is also increasing with increasing wavelength. The relative sensitivity is obtained approximately 37, 40, 41.5, 41.8 in percentage at wavelength 1200 nm and 42, 44.2, 44.8, 44.9 in percentage at wavelength 1500 nm when sensing liquid is 15%, 40%, 60%, 75% of Ethyl Alcohol-water mixture respectively.
Again, we performed similar experiment to investigate the relative sensitivity profile at temperature 30˚C and it is shown in Figure 4.
For 30˚C temperature, the relative sensitivity is also increasing with increasing amount of Ethyl Alcohol water mixture as the refractive index goes higher with increasing amount of Ethyl Alcohol. Again, the relative sensitivity is increasing with increasing wavelength. The relative sensitivity is obtained approximately 36.9, 40, 41, 41.2 in percentage at wavelength 1200 nm and 42, 43.8, 44.5, 44.85 in percentage at wavelength 1500 nm for 15%, 40%, 60%, 75% of Ethyl Alcohol-water mixture respectively.
Figure 2. The sensitivity profile of Ethyl Alcohol water mixture for different concentration at temperature 20˚C.
Figure 3. The sensitivity profile of Ethyl Alcohol water mixture for different concentration at temperature 25˚C.
Then we have taken 15% of Ethyl Alcohol-water mixture as sensing liquid and then we performed sensing operation at different temperature and the relative sensitivity profile for different temperature is shown in Figure 5.
Figure 4. The sensitivity profile of Ethyl Alcohol water mixture for different concentration at temperature 30˚C.
Figure 5. The Relative sensitivity profile of 15% Ethyl Alcohol water mixture at different temperature.
The relative sensitivity profile is increasing with increasing wavelength as like before. Again, it is decreasing with increasing temperature which means higher sensitivity will be achieved when we will operate this sensor at lower temperature. The relative sensitivity is obtained approximately 37, 36.5, 36 in percentage at wavelength 1200 nm and 42, 41.5, 41 in percentage at wavelength 1500 nm when sensing liquid is 15% of Ethyl Alcohol-water mixture at temperature 20˚C, 25˚C, 30˚C respectively.
In this research work, a simple hexagonal photonic crystal fiber (PCF) sensor has been designed in Comsol Multiphysics to sense Ethyl Alcohol. The main goal of this research is to investigate the relative sensitivity profile by varying the concentration of liquid as well as operating temperature. At wavelength 1500 nm, the relative sensitivity is obtained approximately 44, 44.59, 44.85, 45 in percentage at temperature 20˚C, 42, 44.2, 44.8, 44.9 in percentage at temperature 25˚C, 42, 43.8, 44.5, 44.85 in percentage at temperature 30˚C for 15%, 40%, 60%, 75% of Ethyl Alcohol-water mixture respectively. Again, lower sensitivity is observed when this sensor is operated at higher temperature.
 Chen, D., Vincent Tse, M.L. and Tam, H.Y. (2010) Optical Properties of Photonic Crystal Fibers with a Fiber Core of Arrays of Subwavelength Circular Air Holes: Birefringence and Dispersion. Progress in Electromagnetics Research, 105, 193-212.
 Olyaee, S., Seifouri, M., Nikoosohbat, A. and Abadi, M.S.E. (2015) Low Nonlinear Effects Index-Guiding Nanostructured Photonic Crystal Fiber. International Journal of Chemical, Nuclear, Materials and Metallurgical Engineering, 9, 253-257.
 Geerthana, S., Raja, A.S. and Sundar, D.S. (2015) Design and Optimization of Photonic Crystal Fiber with Improved Optical Characteristics. Journal of Nonlinear Optical Physics & Materials, 24, Article ID: 1550051.
 Kim, S., Lee, Y.S., Lee, C.G., Jung, Y. and Oh, K. (2015) Hybrid Square-Lattice Photonic Crystal Fiber with Broadband Single-Mode Operation, High Birefringence, and Normal Dispersion. Journal of the Optical Society of Korea, 19, 449-455.
 Wu, B.Q., Lu, Y., Hao, C.J., Duan, L.C., Luan, N.N., Zhao, Z.Q. and Yao, J.Q. (2013) Hollow-Core Photonic Crystal Fiber Based on C2H2 and NH3 Gas Sensor. In: Applied Mechanics and Materials, Vol. 411, Trans Tech Publications, Zürich, 1577-1580.
 Priya, K.R., Raja, A.S. and Sundar, D.S. (2016) Design of a Dual-Core Liquid-Filled Photonic Crystal Fiber Coupler and Analysis of Its Optical Characteristics. Journal of Optical technology, 83, 569-573.
 Hossain, M.B., Kabir, M.A., Bulbul, A.A.M., Podder, E. and Hossen, M.K. (2017) Optical Parameters Analysis of Photonic Crystal Fiber with Rectangular Lattice Geometry. Journal of Scientific Research & Reports, 17, 1-8.
 Hossain, M.B., Bulbul, A.A.M., Mukit, M.A. and Podder, E. (2017) Analysis of Optical Properties for Square, Circular and Hexagonal Photonic Crystal Fiber. Optics and Photonics Journal, 7, 235-243.
 Ademgil, H. (2014) Highly Sensitive Octagonal Photonic Crystal Fiber Based Sensor. Optik-International Journal for Light and Electron Optics, 125, 6274-6278.
 Akowuah, E.K., Gorman, T., Ademgil, H., Haxha, S., Robinson, G.K. and Oliver, J.V. (2012) Numerical Analysis of a Photonic Crystal Fiber for Biosensing Applications. IEEE Journal of Quantum Electronics, 48, 1403-1410.
 Pinto, A.M.R., Baptista, J.M., Santos, J.L., Lopez-Amo, M. and Frazão, O. (2012) Micro-Displacement Sensor Based on a Hollow-Core Photonic Crystal Fiber. Sensors, 12, 17497-17503.
 Liu, S., Wang, Y., Hou, M., Guo, J., Li, Z. and Lu, P. (2013) Anti-Resonant Reflecting Guidance in Alcohol-Filled Hollow Core Photonic Crystal Fiber for Sensing Applications. Optics Express, 21, 31690-31697.
 Cubillas, A.M., Unterkofler, S., Euser, T.G., Etzold, B.J., Jones, A.C., Sadler, P.J. and Russell, P.S.J. (2013) Photonic Crystal Fibres for Chemical Sensing and Photochemistry. Chemical Society Reviews, 42, 8629-8648.
 Yu, Y., Li, X., Hong, X., Deng, Y., Song, K., Geng, Y. and Tong, W. (2010) Some Features of the Photonic Crystal Fiber Temperature Sensor with Liquid Ethanol Filling. Optics Express, 18, 15383-15388.
 Hao, R. and Sun, G. (2015) Design of Photonic Crystal Fiber with Large Negative Dispersion and High Nonlinearity. Optik-International Journal for Light and Electron Optics, 126, 3353-3356.
 Guo, S., Wu, F., Albin, S., Tai, H. and Rogowski, R.S. (2004) Loss and Dispersion Analysis of Microstructured Fibers by Finite-Difference Method. Optics Express, 12, 3341-3352.
 Luo, M., Liu, Y.G., Wang, Z., Han, T., Wu, Z., Guo, J. and Huang, W. (2013) Twin-Resonance-Coupling and High Sensitivity Sensing Characteristics of a Selectively Fluid-Filled Microstructured Optical Fiber. Optics Express, 21, 30911-30917.
 Lee, H.W., Schmidt, M.A., Uebel, P., Tyagi, H., Joly, N.Y., Scharrer, M. and Russell, P.S.J. (2011) Optofluidic Refractive-Index Sensor in Step-Index Fiber with Parallel Hollow Micro-Channel. Optics Express, 19, 8200-8207.