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Highly efficient compact temperature sensor using liquid infiltrated asymmetric dual elliptical core photonic crystal fiber
Optical Materials 64 (2017) 574e582
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Highly efficient compact temperature sensor using liquid infiltrated asymmetric dual elliptical core photonic crystal fiber N. Ayyanar a, R. Vasantha Jayakantha Raja b, *, D. Vigneswaran c, B. Lakshmi b, M. Sumathi a, K. Porsezian d a
Department of Electronics and Communication Engineering, Dhirajlal Gandhi College of Technology, Salem, 636 309, India Centre for Nonlinear Science and Engineering, School of Electrical and Electronics Engineering, SASTRA University, Thanjavur, 613401, India Department of Electronics and Communication Engineering, Sethu Institute of Technology, Kariapatti, 626 115, India d Department of Physics, Pondicherry University, Puducherry, 605 014, India b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2016 Received in revised form 12 December 2016 Accepted 3 January 2017 Available online 25 January 2017
We propose a novel temperature sensor based on asymmetry in dual elliptical core photonic crystal fiber (DECPCF) structure featuring an enhanced sensitivity with wide detecting range over small distances. As we are interested in constructing compact temperature sensor, we put forth a novel design of asymmetric DECPCF where the core is infiltrated by chloroform. To accomplish the proposed aim, we consider the thermo-optic coefficient of chloroform and silica to anlayse the temperature dependent propagation characteristics of the proposed DEPCF. The unique property of temperature dependent effective refractive index has been exploited to tune coupling length and transmission spectrum, using finite element method. The subsequent calculation of transmission spectrum shows a temperature sensitivity of 42.99 nm/ C at 1.41 cm in the proposed asymmetric DECPCF. © 2017 Elsevier B.V. All rights reserved.
Keywords: Elliptical core photonic crystal fiber Asymmetric fiber Temperature sensor Optical liquid materials Optical switching
1. Introduction Optical fiber sensor has always been popular in the temperature sensor field, as they find indispensable advantages such as wide detecting range, electromagnetic interference (EMI), immunity and high sensitivity. Recently, photonic crystal fiber (PCF) sensors have attracted much attention of the researchers compared to the conventional fiber sensor for its design flexibility, easy fabrication and engineered optical properties such as an endlessly single mode, tailorable zero dispersion, enhanced birefringence and large effective mode area [1e3]. Numerous contributions have been made both theoretically and experimentally for achieving efficient temperature sensor using PCF with the variety of structures and methods. For instance, Liu et al. [4] reported an ultrahigh birefringence index-guiding PCF temperature sensor with the sensitivity of 11 pm/ C and 11.2 pm/ C for the fast and slow axis, respectively. In 2010, choi et al. proposed hollow core PCF for simultaneous measurement of refractive index and temperature which has the sensitivity of 8.9 nm/ C at low temperature and
* Corresponding author. E-mail address: [email protected] (R. Vasantha Jayakantha Raja). http://dx.doi.org/10.1016/j.optmat.2017.01.011 0925-3467/© 2017 Elsevier B.V. All rights reserved.
14.6 nm/ C at high temperature [5]. The temperature sensitivity of 3.102 nm/ C was achieved based on droplet like fiber circle at low cost [6]. Besides, several works have been demonstrated through various possibilities to improve the sensitivity and temperature sensing range of sensor by employing alternative materials of PCF fabrication, such as tellurite glasses, chalcogenide glasses and liquids, thus allowing efficient sensor in a fixed setup [7e14]. Along the lines of development of nonsilica PCF technology, there is a motivation to develop liquid core PCFs, due to large variation of refractive index on temperature as compared to silica core PCFs. For instance, Liu et al. [15] reported a PCF temperature sensor based on coupling between liquid core mode and defect mode with a sensitivity of 1.85 nm/ C, where the temperature changed from 20 C to 80 C. Lin et al. [16] reported a liquid filled PCF based multimodal interferometer with sensitivity of 24.757 nm/ C, temperature range of 19.5 Ce22.5 C. Naeem et al. [17] reported high sensitivity temperature sensor based on a selectively polymer filled two core PCF with sensitivity of 1.595 nm/ C, temperature range of 32 Ce38 C. A plasmonic temperature sensor based on a photonic crystal surface plasmon waveguide was obtained a sensitivity of 70 p.m./ C [18]. Plasmonic temperature sensor based on PCF coated
N. Ayyanar et al. / Optical Materials 64 (2017) 574e582
with nanoscale gold film with the sensitivity of 2.15 nm/ C [19] was reported. Even though several authors have reported the different techniques for temperature sensing using PCF, obtaining optical fiber temperature sensor with high sensitivity at the low distance over the wide spectrum window is the main theme of this work. To accomplish highly sensitive compact temperature sensor, the liquid which has high temperature sensitivity is the key requirement for the liquid filled PCF. Among the several liquids, chloroform emerges as an interesting candidate for the development of liquid PCFs [20e22], as their refractive indices are highly dependent on temperature. Also, the chloroform liquid has wide transmission window from visible to mid IR regime which will help to improve the detecting range of temperature. In the present work, at first, we design the dual elliptical core PCF (DECPCF) where the core is filled with chloroform in section 2. From the proposed structures, the essential optical properties, namely, effective refractive index and coupling characteristics are determined by employing the finite element method (FEM) [23] in section 3. In section 4, the transmission studies are carried out numerically for various temperature. The proposed structure is optimized by adjusting size of the asymmetric core, diameter and pitch of the air hole in section 5. From the transmission spectrum, the compact temperature sensor is constructed and its sensitivity is determined. 2. Modeling of DECPCF With an aim to investigate the performance of temperature sensor, we initially designed a twin core PCF with the air hole diameter d ¼ 2.2 mm and pitch constant L ¼ 3.1 mm. The core holes are filled by liquid chloroform and their inter core separation is C ¼ 2L. The experimental techniques to fabricate liquid core PCFs are already proposed, by use of capillary forces or pressure, and selective filling methods for linear and nonlinear pulse propagation [24e26]. The holes of cladding are filled up with air and the background material of cladding is made up of silica. The circular shape of cores are modified to elliptical to enhance the sensitivity. In order to explore the influence of the varying size of the elliptical core on temperature sensor in DECPCF, we put forth three types of chloroform filled PCF. In the first configuration, minor axis of two liquid cores (r1x and r2x) are increased by 50% from the size of air hole and major axis of two cores (r1y and r2y) are chosen as 1.1 times of minor axis. Hence, we designed a symmetric structure where
575
both the minor and major axis of the elliptical cores having same size of r1x ¼ r2x ¼ 1.65 mm and r1y ¼ r2y ¼ 1.815 mm is called symmetric-1. Fig. 1 shows the cross section of the proposed DECPCF of design 1 with two symmetric elliptical cores covered by the triangular lattice of air holes. Even though several research have been carried out using twin core PCF, the symmetrical structures of cores exhibit less sensitivity for temperature. Hence, considering a DECPCF, we have tried to analyze how the asymmetry structures play an increasingly important role in the temperature sensor. Generally, the asymmetric dual core fiber can be achieved by three different ways namely, (i) varying the refractive index with constant core radius (ii) varying the core radius by keeping the constant refractive index and (iii) varying both the core radius as well as the refractive index. Since our aim is to get more sensitivity, we have designed an asymmetric structure in the design 2 where one of the core radius is varied in the y axis. Here, in the asymmetric structure, the major of the first core is adjusted from 1.1 times of minor axis to 1.2 times and hence r1y ¼ 1.815 mm is adjusted to r1y ¼ 1.98 mm. The reason for enhancement of sensitivity in the design 2 may be either asymmetry size of core or large core size in the y axis. To find the reason for having of this high sensitivity, in the design 3, both the core holes are replaced with large size in the y axis. The size of major axis of both the elliptical core is r1y ¼ 1.98 mm and r2y ¼ 1.98 mm. The fiber parameters of designs 1, 2 and 3 tabulated in Table 1. For chloroform liquid, variation of refractive index as a function of wavelength at a temperature of 20 C is given by Ref. [20].
n20 ðlÞ ¼ 1:431364 þ 5632:41 l2 2:0805 108 l4 6 þ1:2613 1013 l
(1)
where l represents wavelength of the propagating light in nm. n20 ðlÞ stands for the refractive index of chloroform liquid as a function of wavelength at a temperature of 20 C. From the obtained refractive indices by Eq. (1) at 20 C, the refractive indices for any other temperature is calculated by the formula,
nðl; TÞ ¼ n20 ðlÞ þ DT
dn ; dT
(2)
where nðl; TÞ is the refractive index of chloroform at a particular wavelength l for the desired temperature T. DT is the difference between the desired temperature and 20 C. dn/dt gives the rate of variation of refractive index as a function of temperature and takes the value of 7:91 104 K 1 for chloroform [22]. In cladding, we employ the Sellmeier-type equation for the refractive index of silica that includes the dependencies of wavelength and temperature as follows [27].
Table 1 The structure parameters of proposed symmetric and asymmetric DECPCF.
Fig. 1. Schematic diagram of DECPCF with geometrical parameters d ¼ 2.2 mm and pitch constant L ¼ 3.1 mm with C ¼ 2 L and that for r1x ¼ r2x ¼ 1.65 mm and r1y ¼ r2y ¼ 1.815 mm.
Pitch (L) Diameter (d) r1x r2x r1y r2y
Design 1
Design 2
Design 3
Symmetric-1
Asymmetry
Symmetric-2
3.1 mm 2.2 mm 1.65 1.65 1.815 1.815
3.1 mm 2.2 mm 1.65 1.65 1.98 1.815
3.1 mm 2.2 mm 1.65 1.65 1.98 1.98
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n2 ðl; TÞ ¼ 1:31552 þ 6:90754 106 T þ 2 0:788404 þ 23:5835 106 T l þ l2 0:0110199 þ 0:584758 106 T 2 0:91316 þ 0:548368 106 T l
(3)
l2 100
effective refractive indexes can be increased by choosing asymmetric core. As the effective refractive index values difference are high in the case of asymmetric DEPCF, it is clear that it affect the coupling length and transmission spectrum when compared with symmetry DECPCF. Coupling length equals to the minimum at which light is easily transfer from one core of the dual elliptical core to another one. The coupling length Lc of x and y-polarization is calculated by the formula as follows,
l
3. Coupling characteristics Here, we are interested in the study of coupling length, transmission spectra and sensitivity in the proposed structure by the way of calculating of effective refractive indices of the coupling modes. The coupling modes inside the dual core PCF can be calculated by two even modes and two odd modes which are called y y super modes. The four basic modes namely nxeven, neven , nxodd and nodd in the designed liquid filled DECPCF are calculated by using FEM and their field distribution are shown in Fig. 2, Fig. 3 and Fig. 4. The effective index difference between the supermodes of x-polariza tion and y-polarization can be calculated by Dneo ¼ nieven niodd , where i is x or y. The variation of effective index difference of super modes with respect to wavelength for x-polarization and y-polarization from 30 C to 34 C are shown in Fig. 5 to Fig. 7. The solid and dashed line denotes the effective index difference of supermodes for x and y-polarization, respectively. It has been observed from Figs. 5e7 that the effective index difference of super modes of asymmetric DECPCF decreases as wavelength increases, whereas the effective index difference of super modes increases in the case of symmetry DECPCF. The calculated refractive indices of symmetric-1 case at wavelength 1.55 mm are nxeven ¼ 1.40761, nxodd ¼ 1.407645, nyeven ¼ 1.4075958, and nyodd ¼ 1.407649 and their corresponding D n values are 3.45 105 and 5.34 105 for xpolarization and y-polarization respectively at 30 C. The symmetry structure of the DECPCF allows the overlapping of lower order modes of one core to the other and it is not confined well within the core as that of the core of asymmetric radius. But, in the asymmetric structure, the continuous energy exchange between the cores takes place at shorter distance as shown by Fig. 6. Thus, the difference of D n for asymmetric case is 3.57 104 for design 2 and symmetric2 case is 3.48 105 for design 3 in the x-polarization. It clearly shows that the symmetric structure results in less index difference when compared with asymmetric structure at various operating wavelengths for x and y-polarization. Thus, the index difference of
Lc ¼ 2 nieven niodd
(4)
Using Eq. (4), we have calculated the coupling length of symmetric and asymmetric DECPCF for temperature range from 30 C to 34 C. The temperature dependence of coupling length as a function of wavelength for x and y-polarization are plotted in Figs. 8e10. For a particular wavelength say 1.55 mm, the coupling length of design 1 of x-polarization is calculated as 22.42 mm and 22.1 mm for the design 3 at 30 C. It is found that, when the radius of the major axis is varied to asymmetric, the coupling length reduces to 2.17 mm which is 10 times smaller than that of symmetric PCF. As our prime aim is to design PCF for temperature sensor, the reduction of coupling length through asymmetric structure supports in designing a compact PCF sensor. Thus, the fast optical field transfer between the cores enables by introducing asymmetry which supports to achieve compact temperature sensor. 4. Transmission spectrum In order to find the temperature of the environment through PCF, we have calculated the intensity variation of pulse propagation. According to the mode coupling theory, the optical power transferred from one core to another core along the proposed PCF can be calculated by
Pout ðlÞ ¼ sin2
jneven nodd jpL
l
¼ sin2
Dneo pL l
(5)
where L is the length of the fiber. Thus, we can calculate the transferred optical power to the second core in the proposed fiber with a fixed length according to Eq. (5), when the light is injected into first fiber core of the proposed PCF. The transmittance can be calculated by
Tr ¼ 10log10
Pout Pin
Fig. 2. Electric field distribution of design 1 (symmetric-1). The arrow indicates the direction of electric field of even and odd mode for x polarization.
(6)
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Fig. 3. Electric field distribution of design 2 (asymmetric). The arrow indicates the direction of electric field of even and odd mode for x polarization.
Fig. 4. Electric field distribution of design 3 (symmetric-2). The arrow indicates the direction of electric field of even and odd mode for x polarization.
Fig. 5. Effective index difference of x and y-polarization as a function of wavelength for various temperatures for symmetric-1 design.
Here, Pin is assumed as maximum power of Pout . The transmission spectrum of the proposed PCF are calculated from Eq. (6). The length of the fiber for calculating transmission spectrum is considered as approximately ten times of the coupling length at 1.55 mm at 30 C. Hence, the length of the PCF has been considered as 16.7 cm, 1.54 cm and 16.6 cm for the design 1, design 2 and design 3 respectively for y polarization. Fig. 11 to Fig. 13 portrays the variation of transmittance as a function of wavelength when the temperature is varied from 30 C to 34 C for x- and y-polarization
of design 1, design 2 and design 3 respectively. It is observed that the dip of the transmittance gets blue shifted in symmetric structure DECPCF and red shifted in asymmetric structure DECPCF when the temperature is increased. From Fig. 11, we have observed a set of five dip wavelengths for x-polarization at 1584, 1565, 1546, 1524 and 1502 nm with their corresponding temperature being 30 C, 31 C, 32 C, 33 C and 34 C for symmetric-1. Thus, the dip wavelength blue shifts from 1584 nm to 1502 nm for symmetric-1 and shifts from 1582 nm to 1496 nm for symmetric-2 as
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Fig. 6. Effective index difference of x and y-polarization as a function of wavelength for various temperatures for asymmetric design.
Fig. 7. Effective index difference of x and y-polarization as a function of wavelength for various temperatures for symmetric-2 design.
Fig. 8. Coupling length as a function of wavelength for various temperatures for symmetric-1 design.
temperature increases from 30 C to 34 C. At the same time, the dip wavelengths are observed at 1420, 1462, 1503, 1544 and 1581 nm for the same temperature range and the total wavelength shift is calculated as 161 nm in asymmetric DECPCF as shown in Fig. 12. Fig. 14 depicts the position of dip wavelength of the transmittance spectrum for various temperature for symmetric and asymmetric DECPCF. The calculation shows the wavelength of the dip changes linearly with increasing temperature from 30 C to
34 C, which is used for temperature sensing applications. It can be clearly seen that the transmission dip wavelength moves toward longer wavelength with increasing temperature and the shift is high in our proposed asymmetric DECPCF than that in symmetric DECPCF. In general the sensitivity of the sensor can be measured through the shift of peak wavelength for variation of temperature. The sensitivity can be defined as
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Fig. 9. Coupling length as a function of wavelength for various temperatures for asymmetric design.
Fig. 10. Coupling length as a function of wavelength for various temperatures for symmetric-2 design.
Fig. 11. Calculated transmission of symmetric-1 design as a function of wavelength for various temperatures.
Dlp S¼ DT
(7)
where lp is the peak wavelength. Fig. 15 depicts the calculated wavelength shift of the transmission spectrum for each 1 C increase of temperature from 30 C in the proposed DECPCF. It is calculated that wavelength shift of 18.3 nm is obtained at 17.1 cm of symmetric DECPCF while it is 41.6 nm of 1.53 cm of asymmetric DECPCF by increasing the temperature 1 C for x-polarization. It has been calculated from Fig. 18 that the sensitivity of the design 1,
design 2 and design 3 are 18.3 nm/ C, 41.6 nm/ C and 18.7 nm/ C respectively for x-polarization and 18.7 nm/ C, 40.2 nm/ C and 18.6 nm/ C respectively for y-polarization. The sensitivity can also be increased by the further optimization of our proposed DECPCF structure for temperature sensing.
5. Optimization Here, our ultimate aim is to achieve maximum sensitivity at the short distance by optimizing asymmetry core radius, diameter and
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Fig. 12. Calculated transmission of asymmetric design as a function of wavelength for various temperatures.
Fig. 13. Calculated transmission of symmetric-2 design as a function of wavelength for various temperatures.
Fig. 14. Calculated dip wavelength of transmission spectrum as a function of temperature for symmetric-1, asymmetric and symmetric-2 design.
pitch of the air hole in the cladding. To achieve maximum wavelength shift for changing temperature, we first optimize the asymmetry design in DECPCF. To check the sensitivity in an asymmetric structure, the major axis of the first elliptical core (r1y) is adjusted by increasing and decreasing with respect to earlier structure. Thus the major axis is adjusted from 1.15 to 1.25 times of minor axis and hence we have chosen two additional structures of r1y ¼ 1.8975 mm and 2.0625 mm. From Fig. 16, it is calculated that the maximum sensitivity 42.99 nm/ C at 1.41 cm and 41.09 nm/ C at 1.43 cm are achieved for x and y polarization respectively when the
major axis is maximum. At the same time, we have not observed any significant changes when the major axis is increased beyond the 2.0625 mm. As the air filling factor increased for large air hole diameter and lower pitch value in the cladding, the mode doesn't confine well into the core. So that the diameter of the air hole is considered to 2 mm and 2.1 mm in addition to previous optimize structure as shown in Fig. 17. In order to optimize the pitch value, the value has been increased by 0.1 mm as shown in Fig. 18. Since the energy transferred from one core to another with low evanescent field, it has been observed low sensitivity at longer distance for
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Fig. 15. Calculated wavelength shift of the transmission spectrum for symmetric-1, asymmetric and symmetric-2 design.
Fig. 18. Calculated sensitivity of the proposed PCF for various pitch values. Fig. 16. Calculated sensitivity of the proposed PCF for various asymmetric core values.
6. Conclusion small air hole diameter and large pitch value. Thus we proposed a novel compact temperature sensor based on asymmetric DECPCF with sensitivity of 42.99 nm/ C at 1.41 cm, which is much higher than that of any PCF sensor designed so far as compared with Table 2.
The main aim of this paper is to propose a highly sensitive compact temperature sensor using liquid filled PCF. Hence, we have proposed two types of DECPCFs having symmetric and asymmetric core radius that is filled up with liquid chloroform and their optical performances are studied theoretically by using numerical simulation. Typically, here we have discussed how the asymmetry can be designed by suitably changing the radius in one of the cores of DECPCF to enhance the sensitivity. The thermo-optic coefficient of chloroform liquid and silica is utilized to calculate the temperature sensitivity. We have analysed the sensitivity of temperature in an asymmetry DECPCF through the calculation of coupling length and transmission spectrum from the effective refractive index of the proposed PCF and compared the results with symmetric DECPCF.
Table 2 Comparison of earlier reported PCF temperature sensors with the here proposed chloroform filled asymmetric DECPCF.
Fig. 17. Calculated sensitivity of the proposed PCF for various cladding diameter of the air hole.
PCF materials
Length (cm)
Sensitivity nm/ C
Selectively filled Polarization PCF [7] alcohol filled PCF [9] Liquid and defect mode PCF [15] Liquid filled PCF [16] Polymer PCF [17] Plasmonic PCF with gold [19] Chloroform filled asymmetric DECPCF
11.7 6.1 10 0.7 23 0.1 1.41
2.58 6.6 1.85 24.75 1.595 2.15 42.99
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The proposed sensor optimized by asymmetric structure of the core, diameter and pitch value of the air hole. From this scheme, a novel compact temperature is proposed at 1.41 cm. Importantly, this sensitivity of temperature is hitherto record of 42.99 nm/ C using asymmetric DECPCF. The main advantage of our proposed PCF is easy to fabricate using the current fabrication techniques. Acknowledgement RVJ Raja acknowledges DST for providing financial assistantship through Fast track fellowship (SR/FTP/PS-096/2012) and CSIR: 03(1360)/16/EMR-II. K. P. thanks the DST, IFCPAR, NBHM, and CSIR, Government of India, for the financial support through major projects. References [1] J.C. Knight, Nature 424 (2003) 847. [2] G. Joshva Raj, R. Vasantha Jayakantha Raja, Philippe Grelu, R. Ganapathy, K. Porsezian, IEEE Phot. Technol. Lett. 28 (2016) 1209. [3] R. Vasantha Jayakantha Raja, K. Porsezian, J. Phot. Nanostructures - Fundam. Appl. 5 (2007) 171. [4] Z. Liu, C. Wu, M.V. Tse, Chao Lu, Hwa-Yaw Tam, Opt. Lett. 38 (2013) 1385. [5] H.Y. Choi, G. Mudhana, K.S. Park, U. Paek, B. Ha Lee, Opt. express 18 (2010) 141. [6] J. Xie, B. Xu, Y. Li, J. Kang, C. Shen, J. Wang, Y. Jin, H. Liu, K. Ni, X. Dong, C. Zhao,
S. Jin, Appl. Opt. 53 (2014) 4085. [7] Y. Cui, P.P. Shum, D.J.J. Hu, G. Wang, G. Humbert, X.Q. Dinh, IEEE Phot. J. 4 (2012) 1801. [8] C. Wu, H.Y. Fu, K.K. Qureshi, B. Guan, H.Y. Tam, Opt. Lett. 36 (2011) 412. [9] W. Qian, C. Zhao, S. He, X. Dong, S. Zhang, Z. Zhang, Sh. Jin, J. Guo, H. Wei, Opt. Lett. 36 (2011) 1548. [10] Y. Lu, M.T. Wang, C.J. Hao, Z.Q. Zhao, J.Q. Yao, IEEE Phot. J. 6 (2014) 6801307. [11] S. Qiu, Y. Chen, F. Xu, Y. Lu, Opt. Lett. 37 (2012) 683. [12] J. Hsu, C. Lee, P. Huang, C. Hung, P. Tai, IEEE Phot. Tech. Lett. 24 (2012) 1761. [13] C. Wua, Z. Fenga, B. Guan, H. Tamb, Proc. SPIE 9655 (2015) 965528. [14] Y. Yu, X. Li, X. Hong, Y. Deng, K. Song, Y. Geng, H. Wei, W. Tong, Opt. Lett. 18 (2010) 15383. [15] Q. Liu, S. Li, H. Chen, Z. Fan, J. Li, IEEE Phot. J. 7 (2015) 4500509. [16] W. Lin, B. Song, Y. Miao, H. Zhang, D. Yan, B. Liu, Y. Liu, Appl. Opt. 54 (2015) 1309. [17] K. Naeem, B. Hyeon Kim, B. Kim, Y. Chung, IEEE Sens. J. 15 (2015) 3998. [18] T. Srivastava, R. Das, R. Jha, Plasmonics 8 (2013) 515. [19] Q. Liu, S. Li, H. Chen, J. Li, Z. Fan, Appl. Phys. Exp. 8 (2015) 046701. [20] R. Vasantha Jayakantha Raja, K. Senthilnathan, K. Porsezian, K. Nakkeeran, IEEE J. Quant. Elect. 46 (2010) 1795. [21] R. Vasantha Jayakantha Raja, K. Porsezian, Shailendra K. Varshney, S. Sivabalan, Opt. Commun. 283 (2010) 5000. [22] A. Somac, J. Appl. Phys. 94 (2003) 6167. [23] K. Saitoh, M. Koshiba, IEEE J. Quan. Elec. 38 (2002) 927. [24] S. Kedenburg, T. Gissibl, T. Steinle, A. Steinmann, H. Giessen, Opt. Exp. 23 (2015) 8281. [25] K. Nielsen, D. Noordegraaf, T. Srensen, A. Bjarklev, T.P. Hansen1, J. Opt. A Pure Appl. Opt. 7 (2005) 13. [26] C.J.S. de Matos, C.M.B. Cordeiro, E.M. dos Santos, J.S.K. Ong, A. Bozolan, C.H.B. Cruz, Opt. Exp. 15 (2007) 11207. [27] Q. Liu, S. Li, H. Chen, J. Li, Z. Fan, Appl. Phys. Exp. 8 (2015) 046701.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Highly efficient compact temperature sensor using liquid infiltrated asymmetric dual elliptical core photonic crystal fiber N. Ayyanar a, R. Vasantha Jayakantha Raja b, *, D. Vigneswaran c, B. Lakshmi b, M. Sumathi a, K. Porsezian d a
Department of Electronics and Communication Engineering, Dhirajlal Gandhi College of Technology, Salem, 636 309, India Centre for Nonlinear Science and Engineering, School of Electrical and Electronics Engineering, SASTRA University, Thanjavur, 613401, India Department of Electronics and Communication Engineering, Sethu Institute of Technology, Kariapatti, 626 115, India d Department of Physics, Pondicherry University, Puducherry, 605 014, India b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 November 2016 Received in revised form 12 December 2016 Accepted 3 January 2017 Available online 25 January 2017
We propose a novel temperature sensor based on asymmetry in dual elliptical core photonic crystal fiber (DECPCF) structure featuring an enhanced sensitivity with wide detecting range over small distances. As we are interested in constructing compact temperature sensor, we put forth a novel design of asymmetric DECPCF where the core is infiltrated by chloroform. To accomplish the proposed aim, we consider the thermo-optic coefficient of chloroform and silica to anlayse the temperature dependent propagation characteristics of the proposed DEPCF. The unique property of temperature dependent effective refractive index has been exploited to tune coupling length and transmission spectrum, using finite element method. The subsequent calculation of transmission spectrum shows a temperature sensitivity of 42.99 nm/ C at 1.41 cm in the proposed asymmetric DECPCF. © 2017 Elsevier B.V. All rights reserved.
Keywords: Elliptical core photonic crystal fiber Asymmetric fiber Temperature sensor Optical liquid materials Optical switching
1. Introduction Optical fiber sensor has always been popular in the temperature sensor field, as they find indispensable advantages such as wide detecting range, electromagnetic interference (EMI), immunity and high sensitivity. Recently, photonic crystal fiber (PCF) sensors have attracted much attention of the researchers compared to the conventional fiber sensor for its design flexibility, easy fabrication and engineered optical properties such as an endlessly single mode, tailorable zero dispersion, enhanced birefringence and large effective mode area [1e3]. Numerous contributions have been made both theoretically and experimentally for achieving efficient temperature sensor using PCF with the variety of structures and methods. For instance, Liu et al. [4] reported an ultrahigh birefringence index-guiding PCF temperature sensor with the sensitivity of 11 pm/ C and 11.2 pm/ C for the fast and slow axis, respectively. In 2010, choi et al. proposed hollow core PCF for simultaneous measurement of refractive index and temperature which has the sensitivity of 8.9 nm/ C at low temperature and
* Corresponding author. E-mail address: [email protected] (R. Vasantha Jayakantha Raja). http://dx.doi.org/10.1016/j.optmat.2017.01.011 0925-3467/© 2017 Elsevier B.V. All rights reserved.
14.6 nm/ C at high temperature [5]. The temperature sensitivity of 3.102 nm/ C was achieved based on droplet like fiber circle at low cost [6]. Besides, several works have been demonstrated through various possibilities to improve the sensitivity and temperature sensing range of sensor by employing alternative materials of PCF fabrication, such as tellurite glasses, chalcogenide glasses and liquids, thus allowing efficient sensor in a fixed setup [7e14]. Along the lines of development of nonsilica PCF technology, there is a motivation to develop liquid core PCFs, due to large variation of refractive index on temperature as compared to silica core PCFs. For instance, Liu et al. [15] reported a PCF temperature sensor based on coupling between liquid core mode and defect mode with a sensitivity of 1.85 nm/ C, where the temperature changed from 20 C to 80 C. Lin et al. [16] reported a liquid filled PCF based multimodal interferometer with sensitivity of 24.757 nm/ C, temperature range of 19.5 Ce22.5 C. Naeem et al. [17] reported high sensitivity temperature sensor based on a selectively polymer filled two core PCF with sensitivity of 1.595 nm/ C, temperature range of 32 Ce38 C. A plasmonic temperature sensor based on a photonic crystal surface plasmon waveguide was obtained a sensitivity of 70 p.m./ C [18]. Plasmonic temperature sensor based on PCF coated
N. Ayyanar et al. / Optical Materials 64 (2017) 574e582
with nanoscale gold film with the sensitivity of 2.15 nm/ C [19] was reported. Even though several authors have reported the different techniques for temperature sensing using PCF, obtaining optical fiber temperature sensor with high sensitivity at the low distance over the wide spectrum window is the main theme of this work. To accomplish highly sensitive compact temperature sensor, the liquid which has high temperature sensitivity is the key requirement for the liquid filled PCF. Among the several liquids, chloroform emerges as an interesting candidate for the development of liquid PCFs [20e22], as their refractive indices are highly dependent on temperature. Also, the chloroform liquid has wide transmission window from visible to mid IR regime which will help to improve the detecting range of temperature. In the present work, at first, we design the dual elliptical core PCF (DECPCF) where the core is filled with chloroform in section 2. From the proposed structures, the essential optical properties, namely, effective refractive index and coupling characteristics are determined by employing the finite element method (FEM) [23] in section 3. In section 4, the transmission studies are carried out numerically for various temperature. The proposed structure is optimized by adjusting size of the asymmetric core, diameter and pitch of the air hole in section 5. From the transmission spectrum, the compact temperature sensor is constructed and its sensitivity is determined. 2. Modeling of DECPCF With an aim to investigate the performance of temperature sensor, we initially designed a twin core PCF with the air hole diameter d ¼ 2.2 mm and pitch constant L ¼ 3.1 mm. The core holes are filled by liquid chloroform and their inter core separation is C ¼ 2L. The experimental techniques to fabricate liquid core PCFs are already proposed, by use of capillary forces or pressure, and selective filling methods for linear and nonlinear pulse propagation [24e26]. The holes of cladding are filled up with air and the background material of cladding is made up of silica. The circular shape of cores are modified to elliptical to enhance the sensitivity. In order to explore the influence of the varying size of the elliptical core on temperature sensor in DECPCF, we put forth three types of chloroform filled PCF. In the first configuration, minor axis of two liquid cores (r1x and r2x) are increased by 50% from the size of air hole and major axis of two cores (r1y and r2y) are chosen as 1.1 times of minor axis. Hence, we designed a symmetric structure where
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both the minor and major axis of the elliptical cores having same size of r1x ¼ r2x ¼ 1.65 mm and r1y ¼ r2y ¼ 1.815 mm is called symmetric-1. Fig. 1 shows the cross section of the proposed DECPCF of design 1 with two symmetric elliptical cores covered by the triangular lattice of air holes. Even though several research have been carried out using twin core PCF, the symmetrical structures of cores exhibit less sensitivity for temperature. Hence, considering a DECPCF, we have tried to analyze how the asymmetry structures play an increasingly important role in the temperature sensor. Generally, the asymmetric dual core fiber can be achieved by three different ways namely, (i) varying the refractive index with constant core radius (ii) varying the core radius by keeping the constant refractive index and (iii) varying both the core radius as well as the refractive index. Since our aim is to get more sensitivity, we have designed an asymmetric structure in the design 2 where one of the core radius is varied in the y axis. Here, in the asymmetric structure, the major of the first core is adjusted from 1.1 times of minor axis to 1.2 times and hence r1y ¼ 1.815 mm is adjusted to r1y ¼ 1.98 mm. The reason for enhancement of sensitivity in the design 2 may be either asymmetry size of core or large core size in the y axis. To find the reason for having of this high sensitivity, in the design 3, both the core holes are replaced with large size in the y axis. The size of major axis of both the elliptical core is r1y ¼ 1.98 mm and r2y ¼ 1.98 mm. The fiber parameters of designs 1, 2 and 3 tabulated in Table 1. For chloroform liquid, variation of refractive index as a function of wavelength at a temperature of 20 C is given by Ref. [20].
n20 ðlÞ ¼ 1:431364 þ 5632:41 l2 2:0805 108 l4 6 þ1:2613 1013 l
(1)
where l represents wavelength of the propagating light in nm. n20 ðlÞ stands for the refractive index of chloroform liquid as a function of wavelength at a temperature of 20 C. From the obtained refractive indices by Eq. (1) at 20 C, the refractive indices for any other temperature is calculated by the formula,
nðl; TÞ ¼ n20 ðlÞ þ DT
dn ; dT
(2)
where nðl; TÞ is the refractive index of chloroform at a particular wavelength l for the desired temperature T. DT is the difference between the desired temperature and 20 C. dn/dt gives the rate of variation of refractive index as a function of temperature and takes the value of 7:91 104 K 1 for chloroform [22]. In cladding, we employ the Sellmeier-type equation for the refractive index of silica that includes the dependencies of wavelength and temperature as follows [27].
Table 1 The structure parameters of proposed symmetric and asymmetric DECPCF.
Fig. 1. Schematic diagram of DECPCF with geometrical parameters d ¼ 2.2 mm and pitch constant L ¼ 3.1 mm with C ¼ 2 L and that for r1x ¼ r2x ¼ 1.65 mm and r1y ¼ r2y ¼ 1.815 mm.
Pitch (L) Diameter (d) r1x r2x r1y r2y
Design 1
Design 2
Design 3
Symmetric-1
Asymmetry
Symmetric-2
3.1 mm 2.2 mm 1.65 1.65 1.815 1.815
3.1 mm 2.2 mm 1.65 1.65 1.98 1.815
3.1 mm 2.2 mm 1.65 1.65 1.98 1.98
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n2 ðl; TÞ ¼ 1:31552 þ 6:90754 106 T þ 2 0:788404 þ 23:5835 106 T l þ l2 0:0110199 þ 0:584758 106 T 2 0:91316 þ 0:548368 106 T l
(3)
l2 100
effective refractive indexes can be increased by choosing asymmetric core. As the effective refractive index values difference are high in the case of asymmetric DEPCF, it is clear that it affect the coupling length and transmission spectrum when compared with symmetry DECPCF. Coupling length equals to the minimum at which light is easily transfer from one core of the dual elliptical core to another one. The coupling length Lc of x and y-polarization is calculated by the formula as follows,
l
3. Coupling characteristics Here, we are interested in the study of coupling length, transmission spectra and sensitivity in the proposed structure by the way of calculating of effective refractive indices of the coupling modes. The coupling modes inside the dual core PCF can be calculated by two even modes and two odd modes which are called y y super modes. The four basic modes namely nxeven, neven , nxodd and nodd in the designed liquid filled DECPCF are calculated by using FEM and their field distribution are shown in Fig. 2, Fig. 3 and Fig. 4. The effective index difference between the supermodes of x-polariza tion and y-polarization can be calculated by Dneo ¼ nieven niodd , where i is x or y. The variation of effective index difference of super modes with respect to wavelength for x-polarization and y-polarization from 30 C to 34 C are shown in Fig. 5 to Fig. 7. The solid and dashed line denotes the effective index difference of supermodes for x and y-polarization, respectively. It has been observed from Figs. 5e7 that the effective index difference of super modes of asymmetric DECPCF decreases as wavelength increases, whereas the effective index difference of super modes increases in the case of symmetry DECPCF. The calculated refractive indices of symmetric-1 case at wavelength 1.55 mm are nxeven ¼ 1.40761, nxodd ¼ 1.407645, nyeven ¼ 1.4075958, and nyodd ¼ 1.407649 and their corresponding D n values are 3.45 105 and 5.34 105 for xpolarization and y-polarization respectively at 30 C. The symmetry structure of the DECPCF allows the overlapping of lower order modes of one core to the other and it is not confined well within the core as that of the core of asymmetric radius. But, in the asymmetric structure, the continuous energy exchange between the cores takes place at shorter distance as shown by Fig. 6. Thus, the difference of D n for asymmetric case is 3.57 104 for design 2 and symmetric2 case is 3.48 105 for design 3 in the x-polarization. It clearly shows that the symmetric structure results in less index difference when compared with asymmetric structure at various operating wavelengths for x and y-polarization. Thus, the index difference of
Lc ¼ 2 nieven niodd
(4)
Using Eq. (4), we have calculated the coupling length of symmetric and asymmetric DECPCF for temperature range from 30 C to 34 C. The temperature dependence of coupling length as a function of wavelength for x and y-polarization are plotted in Figs. 8e10. For a particular wavelength say 1.55 mm, the coupling length of design 1 of x-polarization is calculated as 22.42 mm and 22.1 mm for the design 3 at 30 C. It is found that, when the radius of the major axis is varied to asymmetric, the coupling length reduces to 2.17 mm which is 10 times smaller than that of symmetric PCF. As our prime aim is to design PCF for temperature sensor, the reduction of coupling length through asymmetric structure supports in designing a compact PCF sensor. Thus, the fast optical field transfer between the cores enables by introducing asymmetry which supports to achieve compact temperature sensor. 4. Transmission spectrum In order to find the temperature of the environment through PCF, we have calculated the intensity variation of pulse propagation. According to the mode coupling theory, the optical power transferred from one core to another core along the proposed PCF can be calculated by
Pout ðlÞ ¼ sin2
jneven nodd jpL
l
¼ sin2
Dneo pL l
(5)
where L is the length of the fiber. Thus, we can calculate the transferred optical power to the second core in the proposed fiber with a fixed length according to Eq. (5), when the light is injected into first fiber core of the proposed PCF. The transmittance can be calculated by
Tr ¼ 10log10
Pout Pin
Fig. 2. Electric field distribution of design 1 (symmetric-1). The arrow indicates the direction of electric field of even and odd mode for x polarization.
(6)
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Fig. 3. Electric field distribution of design 2 (asymmetric). The arrow indicates the direction of electric field of even and odd mode for x polarization.
Fig. 4. Electric field distribution of design 3 (symmetric-2). The arrow indicates the direction of electric field of even and odd mode for x polarization.
Fig. 5. Effective index difference of x and y-polarization as a function of wavelength for various temperatures for symmetric-1 design.
Here, Pin is assumed as maximum power of Pout . The transmission spectrum of the proposed PCF are calculated from Eq. (6). The length of the fiber for calculating transmission spectrum is considered as approximately ten times of the coupling length at 1.55 mm at 30 C. Hence, the length of the PCF has been considered as 16.7 cm, 1.54 cm and 16.6 cm for the design 1, design 2 and design 3 respectively for y polarization. Fig. 11 to Fig. 13 portrays the variation of transmittance as a function of wavelength when the temperature is varied from 30 C to 34 C for x- and y-polarization
of design 1, design 2 and design 3 respectively. It is observed that the dip of the transmittance gets blue shifted in symmetric structure DECPCF and red shifted in asymmetric structure DECPCF when the temperature is increased. From Fig. 11, we have observed a set of five dip wavelengths for x-polarization at 1584, 1565, 1546, 1524 and 1502 nm with their corresponding temperature being 30 C, 31 C, 32 C, 33 C and 34 C for symmetric-1. Thus, the dip wavelength blue shifts from 1584 nm to 1502 nm for symmetric-1 and shifts from 1582 nm to 1496 nm for symmetric-2 as
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Fig. 6. Effective index difference of x and y-polarization as a function of wavelength for various temperatures for asymmetric design.
Fig. 7. Effective index difference of x and y-polarization as a function of wavelength for various temperatures for symmetric-2 design.
Fig. 8. Coupling length as a function of wavelength for various temperatures for symmetric-1 design.
temperature increases from 30 C to 34 C. At the same time, the dip wavelengths are observed at 1420, 1462, 1503, 1544 and 1581 nm for the same temperature range and the total wavelength shift is calculated as 161 nm in asymmetric DECPCF as shown in Fig. 12. Fig. 14 depicts the position of dip wavelength of the transmittance spectrum for various temperature for symmetric and asymmetric DECPCF. The calculation shows the wavelength of the dip changes linearly with increasing temperature from 30 C to
34 C, which is used for temperature sensing applications. It can be clearly seen that the transmission dip wavelength moves toward longer wavelength with increasing temperature and the shift is high in our proposed asymmetric DECPCF than that in symmetric DECPCF. In general the sensitivity of the sensor can be measured through the shift of peak wavelength for variation of temperature. The sensitivity can be defined as
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Fig. 9. Coupling length as a function of wavelength for various temperatures for asymmetric design.
Fig. 10. Coupling length as a function of wavelength for various temperatures for symmetric-2 design.
Fig. 11. Calculated transmission of symmetric-1 design as a function of wavelength for various temperatures.
Dlp S¼ DT
(7)
where lp is the peak wavelength. Fig. 15 depicts the calculated wavelength shift of the transmission spectrum for each 1 C increase of temperature from 30 C in the proposed DECPCF. It is calculated that wavelength shift of 18.3 nm is obtained at 17.1 cm of symmetric DECPCF while it is 41.6 nm of 1.53 cm of asymmetric DECPCF by increasing the temperature 1 C for x-polarization. It has been calculated from Fig. 18 that the sensitivity of the design 1,
design 2 and design 3 are 18.3 nm/ C, 41.6 nm/ C and 18.7 nm/ C respectively for x-polarization and 18.7 nm/ C, 40.2 nm/ C and 18.6 nm/ C respectively for y-polarization. The sensitivity can also be increased by the further optimization of our proposed DECPCF structure for temperature sensing.
5. Optimization Here, our ultimate aim is to achieve maximum sensitivity at the short distance by optimizing asymmetry core radius, diameter and
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Fig. 12. Calculated transmission of asymmetric design as a function of wavelength for various temperatures.
Fig. 13. Calculated transmission of symmetric-2 design as a function of wavelength for various temperatures.
Fig. 14. Calculated dip wavelength of transmission spectrum as a function of temperature for symmetric-1, asymmetric and symmetric-2 design.
pitch of the air hole in the cladding. To achieve maximum wavelength shift for changing temperature, we first optimize the asymmetry design in DECPCF. To check the sensitivity in an asymmetric structure, the major axis of the first elliptical core (r1y) is adjusted by increasing and decreasing with respect to earlier structure. Thus the major axis is adjusted from 1.15 to 1.25 times of minor axis and hence we have chosen two additional structures of r1y ¼ 1.8975 mm and 2.0625 mm. From Fig. 16, it is calculated that the maximum sensitivity 42.99 nm/ C at 1.41 cm and 41.09 nm/ C at 1.43 cm are achieved for x and y polarization respectively when the
major axis is maximum. At the same time, we have not observed any significant changes when the major axis is increased beyond the 2.0625 mm. As the air filling factor increased for large air hole diameter and lower pitch value in the cladding, the mode doesn't confine well into the core. So that the diameter of the air hole is considered to 2 mm and 2.1 mm in addition to previous optimize structure as shown in Fig. 17. In order to optimize the pitch value, the value has been increased by 0.1 mm as shown in Fig. 18. Since the energy transferred from one core to another with low evanescent field, it has been observed low sensitivity at longer distance for
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Fig. 15. Calculated wavelength shift of the transmission spectrum for symmetric-1, asymmetric and symmetric-2 design.
Fig. 18. Calculated sensitivity of the proposed PCF for various pitch values. Fig. 16. Calculated sensitivity of the proposed PCF for various asymmetric core values.
6. Conclusion small air hole diameter and large pitch value. Thus we proposed a novel compact temperature sensor based on asymmetric DECPCF with sensitivity of 42.99 nm/ C at 1.41 cm, which is much higher than that of any PCF sensor designed so far as compared with Table 2.
The main aim of this paper is to propose a highly sensitive compact temperature sensor using liquid filled PCF. Hence, we have proposed two types of DECPCFs having symmetric and asymmetric core radius that is filled up with liquid chloroform and their optical performances are studied theoretically by using numerical simulation. Typically, here we have discussed how the asymmetry can be designed by suitably changing the radius in one of the cores of DECPCF to enhance the sensitivity. The thermo-optic coefficient of chloroform liquid and silica is utilized to calculate the temperature sensitivity. We have analysed the sensitivity of temperature in an asymmetry DECPCF through the calculation of coupling length and transmission spectrum from the effective refractive index of the proposed PCF and compared the results with symmetric DECPCF.
Table 2 Comparison of earlier reported PCF temperature sensors with the here proposed chloroform filled asymmetric DECPCF.
Fig. 17. Calculated sensitivity of the proposed PCF for various cladding diameter of the air hole.
PCF materials
Length (cm)
Sensitivity nm/ C
Selectively filled Polarization PCF [7] alcohol filled PCF [9] Liquid and defect mode PCF [15] Liquid filled PCF [16] Polymer PCF [17] Plasmonic PCF with gold [19] Chloroform filled asymmetric DECPCF
11.7 6.1 10 0.7 23 0.1 1.41
2.58 6.6 1.85 24.75 1.595 2.15 42.99
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The proposed sensor optimized by asymmetric structure of the core, diameter and pitch value of the air hole. From this scheme, a novel compact temperature is proposed at 1.41 cm. Importantly, this sensitivity of temperature is hitherto record of 42.99 nm/ C using asymmetric DECPCF. The main advantage of our proposed PCF is easy to fabricate using the current fabrication techniques. Acknowledgement RVJ Raja acknowledges DST for providing financial assistantship through Fast track fellowship (SR/FTP/PS-096/2012) and CSIR: 03(1360)/16/EMR-II. K. P. thanks the DST, IFCPAR, NBHM, and CSIR, Government of India, for the financial support through major projects. References [1] J.C. Knight, Nature 424 (2003) 847. [2] G. Joshva Raj, R. Vasantha Jayakantha Raja, Philippe Grelu, R. Ganapathy, K. Porsezian, IEEE Phot. Technol. Lett. 28 (2016) 1209. [3] R. Vasantha Jayakantha Raja, K. Porsezian, J. Phot. Nanostructures - Fundam. Appl. 5 (2007) 171. [4] Z. Liu, C. Wu, M.V. Tse, Chao Lu, Hwa-Yaw Tam, Opt. Lett. 38 (2013) 1385. [5] H.Y. Choi, G. Mudhana, K.S. Park, U. Paek, B. Ha Lee, Opt. express 18 (2010) 141. [6] J. Xie, B. Xu, Y. Li, J. Kang, C. Shen, J. Wang, Y. Jin, H. Liu, K. Ni, X. Dong, C. Zhao,
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