- Email: [email protected]
Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode and metal defect mode
Journal Pre-proof Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode and metal defect mode Zhenkai Fan, Zipeng Guo, Xiangzheng Kong, Zhuangyan Meng
PII: DOI: Reference:
S0030-4018(20)30002-X https://doi.org/10.1016/j.optcom.2020.125233 OPTICS 125233
To appear in:
Optics Communications
Received date : 10 October 2019 Revised date : 25 December 2019 Accepted date : 1 January 2020 Please cite this article as: Z. Fan, Z. Guo, X. Kong et al., Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode and metal defect mode, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125233. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Journal Pre-proof
*Manuscript Click here to view linked References
of
Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode
pro
and metal defect mode
Zhenkai Fan*,Zipeng Guo, Xiangzheng Kong, and Zhuangyan Meng
School of Information Science and Engineering, Hebei University of Science and Technology, Shijia zhuang 050018, China.
re-
E-mail: [email protected]
Abstract A high sensitivity wide range surface plasmon resonance (SPR) refractive index sensor based on photonic crystal fiber (PCF) has been proposed in this paper. The finite element method (FEM) is used to calculate and analyte the structure of PCF. The optical
urn al P
properties of the PCF are investigated by adjusting the refractive index of liquid analyte. By optimization, the fiber sensor achieves a high linearity refractive index sensitivity of 0.9979 and 1931.03 nm / RIU over a refractive index range from 1.350 to 1.460 and achieves a short PCF length of 1 mm. It is allowed the sensor to be small enough to be integrated into inspection equipment,and expected to gain important applications in the fields of biological monitoring, environmental monitoring, and chemical production.
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1. Introduction
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The waveguide characteristics and structural controllability of photonic crystal fibers (PCF)[1-2]have led to popularity of various PCF-based plasma devices. As the demand for higher performance sensing devices continues to increase, the SPR-PCF has received
pro
increasing attention. As the conditions under which surface plasmon resonance (SPR)[3-4] is excited the phase matching between fundamental mode and surface plasmon polariton (SPP) mode, the SPR is accompanied by significant loss peaks. For now, the structure of PCF is mainly filled with metal wires and coated metal films[5-6] for exciting the SPR. In practical applications in sensors, the metallic silver generally improves the sensor performance, but
re-
its chemical stability is poor. In actual development, a more stable metallic gold is usually selected to excite the SPR. As an ideal platform for plasma structures, the PCF has been enhanced in terms of versatility and performance and has achieved many application opportunities, especially for sensing applications. The SPR sensors offer great potential in
urn al P
areas such as chemical and biomedical sensing[7-9] due to its miniaturization, low sample requirements and excellent performance.
In recent years, as the fiber manufacturing technology and precision processing technology have been enhanced, and many new optical properties have been introduced into the PCFs. The advantages of miniaturization, versatility, and high sensitivity of the device are also realized on the SPR-PCF refractive index sensor. Xin Chen[10] et al. designed a D-type SPR-PCF sensor generated by coating the gold film on the open-loop channel for sensing refractive index. In the liquid detection environment, a low refractive index of 1.20-1.29 is achieved and the average sensitivity reaches 11055 nm / RIU. Tiesheng Wu[11] et al. proposed a D-type PCF sensor with different structural design. The gold film and analyte deposited on the polished surface of PCF. A high sensitivity of 21700 nm / RIU is obtained at the refractive index of 1.36. Compared with above designs, we use different type of PCF. and the liquid-core porous PCFs are relatively mature and simple to prepare. In
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addition, we have obtained a larger measurement range from 1.35 to 1.46. Thus, our high sensitivity of design sensor is more superior in some application areas that require higher measurement ranges.
2
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In this paper, we propose a SPR-PCF refractive index sensor, which can be used for wide
of
refractive index ranges measurement, and the finite element method (FEM)[12] is used to evaluate and analyze the structure of optical properties. A nanoscale gold film is coated in
pro
on one side of regular hexagon composed of second layer of holes to excite the SPR. The refractive index sensitive liquid material is implanted into the nearest second and third layers of the gold film coating aperture. The single metallic gold channel design effectively eliminates interference between adjacent channels and improves refractive index detecting range. In the simulation, we found that the refractive index sensitivity follows a linear fitting
RIU was obtained.
2. Theroy and design
re-
rule[13] in the measurement range. Through optimization, a high sensitivity of 1931.03 nm /
The PCF structure of the refractive index sensor is shown in Fig. 1(a). It has three layers
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of regular hexagonal air holes, the lattice constant is 0.8 μm, the outer diameter of hole having metal layer is 0.35 μm, the thickness of metal layer is 50 nm, the diameter of the conventional air hole is 0.3 μm. This design uses incomplete filling technology to selectively inject refractive index sensitive liquid materials into the second and third layers of air holes closest to gold-coated layer. Because the size of the filled air holes is on the order of microns, some traditional filling methods cannot be used. After the air holes that don’t need to be filled are manually glued with a UV-curable polymer[14], filling can solve the problem, but it also has insufficient accuracy and operation Complex. Fei Wang et al.[15] Used FIB (Focused Ion Beam) micromachining technology to mill the required air holes on the end face of PCF, and then welded a section of single-mode fiber on the end face of PCF. Only the required air holes were exposed to atmosphere, thereby using liquid penetration to complete the selectivity. This filling method is superior in our structural design. Fig. 1(b) shows peak loss that core mode resonantly coupled to metal defect mode, and reverse jump
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occurs simultaneously in the refractive index curve. Therefore, we can get the refractive index of measured object by the position of loss peak. Then we carried out this work and analyzed the sensitivity of phenomenon under different refractive index environments.
3
pro (b)
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(a)
Fig. 1.The cross-sections of the proposed SPR-PCF refractive index sensors of (a). (b) shows the loss and refractive index of the PCF dependance on the operation wavelength when the analyte refractive
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index na is 1.35 for the PCF.
The fiber is drawn from fused silica. In the simulation process, we use Semllmeier dispersion equation[16] to set scattering boundary conditions and finite element network. It is defined as:
n( ) 1
B2 B12 B2 2 2 2 3 C1 C2 C3 2
(1)
Where λ represents the wavelength of free space in μm, and other fitting constants have been given in [17]. In the preparation of fiber optic sensor, we can use drawing tower to draw the preform to produce the designed PCF structure. The metal film in pores can be synthesized into a nano-scale smooth gold coating by chemical vapor deposition (CVD)[18] orsilver mirror reaction (Tollens method)[19]. As we mentioned above, the refractive index can be obtained by the performance of loss peak. In addition, loss peak is also very important when performing a series of evaluations on sensor. The loss value can be obtained by the
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of
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following formula[20].
2 LC 8 . 6 86
4
Inme f[f
4
] 10
(2)
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The above formula shows the method of obtaining the limit loss value by the value
of
ofwavelength (λ) in μm and the imaginary part of effective refractive index (Im [neff]). A large amount of free electrons are present on the PCF coated metal surface. Their free
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movement generates plasma waves near the metal surface. When the propagation constant of incident light wave matches the propagation constant of plasma wave on metal surface, it causes free electron resonance in metal film. At this time, the gold nano sample absorbs a large amount of light energy and exhibits a strong resonance absorption peak. This corresponds to the loss peak in Fig. 3(a). In addition, gold nanoparticles have high sensitivity to dielectric constant. Since molecules adsorbed on the surface of gold
re-
nanoparticles cause changes in the surrounding dielectric constant, absorption or spectroscopy can be used to measure the adsorbed analyte molecules on the surface of metal nanoparticles.We know that Drude-Lorentz conductance model can explain the
using it.
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transmission properties of electrons in metals, so we define the dielectric function of gold
gold
D 2 L 2 2 j D j L
(3)
As one of the most successful metal models, the Drude model makes full use of free electron approximation and independent electronic approximationandKinetictheory. these parameters andexplanation have been provided to us in the literature [21] .
3. Analysis and discussion
In order to distinguish the different conduction modes of fiber, we analyzed the energy distribution of coupling mode at the wavelength of 1000-2000nm when the refractive index is 1.350, as shown in Figure 2 (wavelengths have been identified in the figure).Fig. 2(a) - Fig. 2(l) shows twelve fiber modes. As wavelength increases from 1000nm to 2000nm, electric field strength of core mode field from strong to weaker to strong.
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When core mode and metal defect mode are resonantly coupled, core mode field energy is largely transferred and forms resonant coupling mode, such as the electric field distribution of Fig. 2(d),which corresponds to loss peak. Through the previous discussion of formula, we
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of
found that peak loss is inextricably linked to the refractive index of analyte. Therefore, we
re-
pro
can obtain the analyte refractive index by the eigenvalues of resonant coupling mode.
Fig. 2.Twelve fiber modes when the phase change causes the electric field distribution to changeat a refractive index of 1.350.(d) Shows the electric field distribution of the resonant coupling mode
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Based on the above principle, we studied sensing performance of this structure in the refractive index range from 1.350 to 1.460. Fig.3 (a) shows the relationship between peak loss of the core mode and the operating wavelength when the analyte refractive index incareasing from 1.350 to 1.460. With the gradual increase of the refractive index, the resonance wavelength undergoes a significant red shift, and the loss peak gradually increases. Since phase matching is satisfied between the core mode and the surface plasmon mode, this SPR is obtained. The energy of core mode is transferred to the metal surface to form a surface plasmon mode. So the core mode loss has peaked. The increase of the refractive index of the liquid causes the phase matching conditions to change. The phase matching wavelength gradually increases, and the resonance wavelength also undergoes a significant red shift. Because the high refractive index liquid material has a strong ability to absorb light, so the core loss of the fiber gradually increases. Fig. 3 (b) shows the refractive index and peak wavelength of the linearly fitted analyte.In the test range,
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linear fitting result of this refractive index sensor is y=1931.03x-1364.44, the linear fit R2=0.9979, and average sensitivity is 1931.03 nm/RIU. By analyzing loss spectrum, we determined this PCF has good refractive index sensing performance over a wide 6
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measurement range, and we found that the lowest loss value was as high as 464084.9 dB/m in the test range of 1.350-1.460. Large losses would shorten the length of fiber, making sensor small enough to be integrated into device for inspection. Table. 1 shows loss peak
re-
pro
values and resonance wavelength values.
(a)
(b)
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Fig. 3 (a) The losses of the SPR-PCF refractive index sensor dependance on the operation wavelength when the analyte refractive index is from 1.35 to 1.46. (b) The resonance wavelength dependance on the analyte refractive index for the peak. The fitting formulas of the peaks are given. Table. 1. Resonance wavelength andpeakvalues for different analyte
1.350
1.360
1.370
1.380
1.390
1.395
1.400
1.405
1.410
1.415
1.420
1.430
1.440
1.450
1.460
1.24
1.26
1.28
1.30
1.32
1.33
1.34
1.35
1.36
1.37
1.38
1.40
1.42
1.44
1.46
53156.6
54082.3 55055.6
56090.9
58339.5
61076.7 64065.7
46408.4 46472.8 46772 48304 50520 51390.2 52264.1
Sensitivity is calculated as follows.
S ( )
peak na
[nm / RIU ]
(4)
Where peak represents the difference between resonant wavelengths of loss spectra of refractive indices of two consecutive analytes. The refractive index of the two analytes is subtracted to obtain na . Power loss is an important parameter and closely related to
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wavelength of light. As wavelength λ changes, the refraction and reflection of light in PCF changes, and SPR effect is accompanied by appearance and disappearance, which results in a change in optical power transmission[22].The optical power transmittance dependence on 7
67119
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the operable wavelength λ with refractive index increasing from 1.350 to 1.460 is shown
found that the length of the PCF is 1mm.
L ) 4.343
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Pout Pin exp(
of
by Fig. 4. Eq. 5 shows the calculation of output power[23], where L is the length of PCF. We
(5)
where Pout is the output power, Pin is the input power. L represents the length of the fiber, represents the limiting loss of the fiber. The calculation method of optical power transmittance[24] is as follows:
Pout ) Pin
(6)
re-
Tr 10 log10 (
As shown in Fig. 4, as the analyte refractive index changes from 1.350 to 1.460, the optical power transmission curve of this PCF shows a regular red shift of 212 nm. Furthermore, as the analyte refractive index changes from 1.350 to 1.460, the transmission
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of the dip wavelength[25]gradually decreases from -47 dB to -65 dB.
Fig. 4. Optical power transmittance dependence on the operable wavelength over a refractive index from 1.35 to 1.46.
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4.Conclusion
In this paper, we conducted a series of studies and evaluations on the optical
properties and performance of the designed sensors, and found that it have extremely wide
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measurement range and high sensitivity. A wide range of refractive index analysis solution was filled into the designed PCF, and the process of peak loss of resonance coupling was observed when the phase film and surface plasmon (SPP) modes were satisfied. The
pro
resonance loss peak pair was studied. Analyze the sensitivity of the liquid refractive index.Through simulation calculation, the sensor obtained an average refractive index of 1931.03 nm / RIU in wide range of 1.35-1.46, and the linear fit was 0.9979. In addition, the minimum loss value of the sensor is 46408.4dB/m, which can effectively shorten the PCF length. After calculation we found that the length of the PCF is 1mm. The performance level of this sensor is competitive compared to existing refractive index sensors. Better
Acknowledgments
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results can be obtained by improving the structure of the PCF
The work is supported by the Excellent Young Talents Project of Higher Education in
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Hebei Province (Grant No.BJ2018040), the Talent Introduction Project of Hebei University of Science and Technology, (Grant No. 1181324), and the College Students of Innovation and Entrepreneurship Project in Hebei Province (Grant No.201910082043 )
References
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[2] Dudley. J. M, and Taylor. J. R., Nature Photonics, 2009, 3(2), 85. [3] Azzam. S. I, Hameed. M. F. O, Shehata. R. E. A, Heikal. A. M, and Obayya. S. S., Optical and Quant- um Electronics, 2016, 48(2): 142. [4] Hasan. M. R, Akter. S, Rifat. A. A, Rana. S, Ahmed. K, Ahmed. R, Subbaraman. H, and Abbott. D., IEEE Sensors Journal, 2017, 18(1): 133-140. [5] Lu. Y, Hao. C. J, Wu. B. Q, Musideke. M, Duan. L. C, Wen. W. Q, and Yao. J. Q., Sensors,
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2013, 13(1), 956-965.
[6] Wang. Y, Huang. Q, Zhu. W, Yang. M, and Lewis. E., Optics Express, 2018, 26(2), 1910-1917.
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[7] Rifat. A. A, Mahdiraji. G. A, Sua. Y. M, Shee. Y. G, Ahmed. R, Chow. D. M, and Adikan. F. M., IEEE Photonics Technology Letters, 2015, 27(15): 1628-1631.
[8] Zu. P, Chan. C. C, Jin. Y, Gong. T, Zhang. Y, Chen. L. H, and Dong. X., IEEE Photonics
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Technology Letters, 2011, 23(13): 920-922.
[9] Malinin. A. V, Zanishevskaja. A. A, Tuchin. V. V, Skibina. Y. S, and Silokhin. I. Y., Biophotonics: Photonic Solutions for Better Health Care III. International Society for Optics and Photonics, 2012, 8427: 842746.
[10] Chen X, Xia L, Li C., IEEE Photonics Journal, 2018, 10(1): 1-9.
[11]Wu. T, Shao. Y, Wang. Y, Cao. S, Cao. W, Zhang. F, Liao. C, He. J, Huang. Y, Hou. M,
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and Wang. Y., Optics Express, 2017, 25(17): 20313-20322.
[12] Ahmed K, Morshed M. Sensing and Bio-Sensing Research, 2016, 7: 1-6. [13] Biancalana F, Tran T X, Stark S, et al. Emergence of geometrical optical nonlinearities in photonic crystal fiber nanowires., Physical Review Letters, 2010, 105(9): 093904.
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[14] D. K. C. Wu, B. T. Kuhlmey, B. J. Eggleton., Optics Letter,2009, 34(3): 322–324. [15] Wang. F, Yuan. W, Hansen. O, Bang. O., Optics Express, 2011,19(18): 17585-17590. [16] F Wang, Z Sun, C Liu, T sun, and P K Chu., Plasmonics, 2016, 12(6): 1961-1965. [17] Yu. X, Zhang. Y, Pan. S, Shum. P, Yan. M, Leviatan. Y, and Li. C., Journal of Optics, 2009, 12(1): 015005.
[18] Sazio. P. J, Amezcua-Correa. A, Finlayson. C. E, Hayes. J. R, Scheidemantel. T. J, Baril. N. F, Jackson. B. R, Won. D. J, Zhang. F, Margine. E. R, Crespi. V. H, Badding. J. V,and Gopalan. V., Science, 2006, 311(5767): 1583-1586. [19] Liu. B. H, Jiang. Y. X, Zhu. X. S, Tang. X. L, and Shi. Y. W., Optics Express, 2013, 21(26): 32349-32357.
[20] Akowuah. E. K, Gorman. T, Ademgil. H, Haxha. S, Robinson. G. K, Oliver. J. V., IEEE Journal of Quantum Electronics, 2012,48(11), 1403-1410. [21] Velázquez-González. J. S, Monzon-Hernandez. D, Martinez-Pinon. F, May-Arrioja. D.
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A, and Hernandez-Romano. I., IEEE Journal of Selected Topics in Quantum Electronics, 2016, 23(2): 126-131.
[22] Kang M. S., Butsch A., Russell P. S. J., Nature Photonics, 2011, 5(9): 549.
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[23] Giles. I. P, McNeill. S, Culshaw. B. Journal of Physics E: Scientific Instruments, 1985, 18(6), 502.
[24] Hoo. Y. L, Jin. W, Ho. H. L, Wang. D. N., IEEE Photonics Technology Letters, 2003,
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[25] Wong. G. K. L, Kang. M. S, Lee. H. W, Biancalana. F, Conti. C, Weiss. T, and Russell. P.
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S. J. , Science, 2012, 337(6093): 446-449.
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*Author Contributions Section
Author contributions selection Dear Editor：
of
The contribution of all authors has been clarified according to the attribution, and there is no conflict of interest
pro
Best！
Jo
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Zhenkai fan
PII: DOI: Reference:
S0030-4018(20)30002-X https://doi.org/10.1016/j.optcom.2020.125233 OPTICS 125233
To appear in:
Optics Communications
Received date : 10 October 2019 Revised date : 25 December 2019 Accepted date : 1 January 2020 Please cite this article as: Z. Fan, Z. Guo, X. Kong et al., Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode and metal defect mode, Optics Communications (2020), doi: https://doi.org/10.1016/j.optcom.2020.125233. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Journal Pre-proof
*Manuscript Click here to view linked References
of
Photonic crystal fiber based wide-range of refractive index sensor with phase matching between core mode
pro
and metal defect mode
Zhenkai Fan*,Zipeng Guo, Xiangzheng Kong, and Zhuangyan Meng
School of Information Science and Engineering, Hebei University of Science and Technology, Shijia zhuang 050018, China.
re-
E-mail: [email protected]
Abstract A high sensitivity wide range surface plasmon resonance (SPR) refractive index sensor based on photonic crystal fiber (PCF) has been proposed in this paper. The finite element method (FEM) is used to calculate and analyte the structure of PCF. The optical
urn al P
properties of the PCF are investigated by adjusting the refractive index of liquid analyte. By optimization, the fiber sensor achieves a high linearity refractive index sensitivity of 0.9979 and 1931.03 nm / RIU over a refractive index range from 1.350 to 1.460 and achieves a short PCF length of 1 mm. It is allowed the sensor to be small enough to be integrated into inspection equipment,and expected to gain important applications in the fields of biological monitoring, environmental monitoring, and chemical production.
Jo
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
1
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1. Introduction
of
The waveguide characteristics and structural controllability of photonic crystal fibers (PCF)[1-2]have led to popularity of various PCF-based plasma devices. As the demand for higher performance sensing devices continues to increase, the SPR-PCF has received
pro
increasing attention. As the conditions under which surface plasmon resonance (SPR)[3-4] is excited the phase matching between fundamental mode and surface plasmon polariton (SPP) mode, the SPR is accompanied by significant loss peaks. For now, the structure of PCF is mainly filled with metal wires and coated metal films[5-6] for exciting the SPR. In practical applications in sensors, the metallic silver generally improves the sensor performance, but
re-
its chemical stability is poor. In actual development, a more stable metallic gold is usually selected to excite the SPR. As an ideal platform for plasma structures, the PCF has been enhanced in terms of versatility and performance and has achieved many application opportunities, especially for sensing applications. The SPR sensors offer great potential in
urn al P
areas such as chemical and biomedical sensing[7-9] due to its miniaturization, low sample requirements and excellent performance.
In recent years, as the fiber manufacturing technology and precision processing technology have been enhanced, and many new optical properties have been introduced into the PCFs. The advantages of miniaturization, versatility, and high sensitivity of the device are also realized on the SPR-PCF refractive index sensor. Xin Chen[10] et al. designed a D-type SPR-PCF sensor generated by coating the gold film on the open-loop channel for sensing refractive index. In the liquid detection environment, a low refractive index of 1.20-1.29 is achieved and the average sensitivity reaches 11055 nm / RIU. Tiesheng Wu[11] et al. proposed a D-type PCF sensor with different structural design. The gold film and analyte deposited on the polished surface of PCF. A high sensitivity of 21700 nm / RIU is obtained at the refractive index of 1.36. Compared with above designs, we use different type of PCF. and the liquid-core porous PCFs are relatively mature and simple to prepare. In
Jo
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
addition, we have obtained a larger measurement range from 1.35 to 1.46. Thus, our high sensitivity of design sensor is more superior in some application areas that require higher measurement ranges.
2
Journal Pre-proof
In this paper, we propose a SPR-PCF refractive index sensor, which can be used for wide
of
refractive index ranges measurement, and the finite element method (FEM)[12] is used to evaluate and analyze the structure of optical properties. A nanoscale gold film is coated in
pro
on one side of regular hexagon composed of second layer of holes to excite the SPR. The refractive index sensitive liquid material is implanted into the nearest second and third layers of the gold film coating aperture. The single metallic gold channel design effectively eliminates interference between adjacent channels and improves refractive index detecting range. In the simulation, we found that the refractive index sensitivity follows a linear fitting
RIU was obtained.
2. Theroy and design
re-
rule[13] in the measurement range. Through optimization, a high sensitivity of 1931.03 nm /
The PCF structure of the refractive index sensor is shown in Fig. 1(a). It has three layers
urn al P
of regular hexagonal air holes, the lattice constant is 0.8 μm, the outer diameter of hole having metal layer is 0.35 μm, the thickness of metal layer is 50 nm, the diameter of the conventional air hole is 0.3 μm. This design uses incomplete filling technology to selectively inject refractive index sensitive liquid materials into the second and third layers of air holes closest to gold-coated layer. Because the size of the filled air holes is on the order of microns, some traditional filling methods cannot be used. After the air holes that don’t need to be filled are manually glued with a UV-curable polymer[14], filling can solve the problem, but it also has insufficient accuracy and operation Complex. Fei Wang et al.[15] Used FIB (Focused Ion Beam) micromachining technology to mill the required air holes on the end face of PCF, and then welded a section of single-mode fiber on the end face of PCF. Only the required air holes were exposed to atmosphere, thereby using liquid penetration to complete the selectivity. This filling method is superior in our structural design. Fig. 1(b) shows peak loss that core mode resonantly coupled to metal defect mode, and reverse jump
Jo
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65
occurs simultaneously in the refractive index curve. Therefore, we can get the refractive index of measured object by the position of loss peak. Then we carried out this work and analyzed the sensitivity of phenomenon under different refractive index environments.
3
pro (b)
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(a)
Fig. 1.The cross-sections of the proposed SPR-PCF refractive index sensors of (a). (b) shows the loss and refractive index of the PCF dependance on the operation wavelength when the analyte refractive
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index na is 1.35 for the PCF.
The fiber is drawn from fused silica. In the simulation process, we use Semllmeier dispersion equation[16] to set scattering boundary conditions and finite element network. It is defined as:
n( ) 1
B2 B12 B2 2 2 2 3 C1 C2 C3 2
(1)
Where λ represents the wavelength of free space in μm, and other fitting constants have been given in [17]. In the preparation of fiber optic sensor, we can use drawing tower to draw the preform to produce the designed PCF structure. The metal film in pores can be synthesized into a nano-scale smooth gold coating by chemical vapor deposition (CVD)[18] orsilver mirror reaction (Tollens method)[19]. As we mentioned above, the refractive index can be obtained by the performance of loss peak. In addition, loss peak is also very important when performing a series of evaluations on sensor. The loss value can be obtained by the
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following formula[20].
2 LC 8 . 6 86
4
Inme f[f
4
] 10
(2)
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The above formula shows the method of obtaining the limit loss value by the value
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ofwavelength (λ) in μm and the imaginary part of effective refractive index (Im [neff]). A large amount of free electrons are present on the PCF coated metal surface. Their free
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movement generates plasma waves near the metal surface. When the propagation constant of incident light wave matches the propagation constant of plasma wave on metal surface, it causes free electron resonance in metal film. At this time, the gold nano sample absorbs a large amount of light energy and exhibits a strong resonance absorption peak. This corresponds to the loss peak in Fig. 3(a). In addition, gold nanoparticles have high sensitivity to dielectric constant. Since molecules adsorbed on the surface of gold
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nanoparticles cause changes in the surrounding dielectric constant, absorption or spectroscopy can be used to measure the adsorbed analyte molecules on the surface of metal nanoparticles.We know that Drude-Lorentz conductance model can explain the
using it.
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transmission properties of electrons in metals, so we define the dielectric function of gold
gold
D 2 L 2 2 j D j L
(3)
As one of the most successful metal models, the Drude model makes full use of free electron approximation and independent electronic approximationandKinetictheory. these parameters andexplanation have been provided to us in the literature [21] .
3. Analysis and discussion
In order to distinguish the different conduction modes of fiber, we analyzed the energy distribution of coupling mode at the wavelength of 1000-2000nm when the refractive index is 1.350, as shown in Figure 2 (wavelengths have been identified in the figure).Fig. 2(a) - Fig. 2(l) shows twelve fiber modes. As wavelength increases from 1000nm to 2000nm, electric field strength of core mode field from strong to weaker to strong.
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When core mode and metal defect mode are resonantly coupled, core mode field energy is largely transferred and forms resonant coupling mode, such as the electric field distribution of Fig. 2(d),which corresponds to loss peak. Through the previous discussion of formula, we
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found that peak loss is inextricably linked to the refractive index of analyte. Therefore, we
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can obtain the analyte refractive index by the eigenvalues of resonant coupling mode.
Fig. 2.Twelve fiber modes when the phase change causes the electric field distribution to changeat a refractive index of 1.350.(d) Shows the electric field distribution of the resonant coupling mode
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Based on the above principle, we studied sensing performance of this structure in the refractive index range from 1.350 to 1.460. Fig.3 (a) shows the relationship between peak loss of the core mode and the operating wavelength when the analyte refractive index incareasing from 1.350 to 1.460. With the gradual increase of the refractive index, the resonance wavelength undergoes a significant red shift, and the loss peak gradually increases. Since phase matching is satisfied between the core mode and the surface plasmon mode, this SPR is obtained. The energy of core mode is transferred to the metal surface to form a surface plasmon mode. So the core mode loss has peaked. The increase of the refractive index of the liquid causes the phase matching conditions to change. The phase matching wavelength gradually increases, and the resonance wavelength also undergoes a significant red shift. Because the high refractive index liquid material has a strong ability to absorb light, so the core loss of the fiber gradually increases. Fig. 3 (b) shows the refractive index and peak wavelength of the linearly fitted analyte.In the test range,
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linear fitting result of this refractive index sensor is y=1931.03x-1364.44, the linear fit R2=0.9979, and average sensitivity is 1931.03 nm/RIU. By analyzing loss spectrum, we determined this PCF has good refractive index sensing performance over a wide 6
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measurement range, and we found that the lowest loss value was as high as 464084.9 dB/m in the test range of 1.350-1.460. Large losses would shorten the length of fiber, making sensor small enough to be integrated into device for inspection. Table. 1 shows loss peak
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values and resonance wavelength values.
(a)
(b)
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Fig. 3 (a) The losses of the SPR-PCF refractive index sensor dependance on the operation wavelength when the analyte refractive index is from 1.35 to 1.46. (b) The resonance wavelength dependance on the analyte refractive index for the peak. The fitting formulas of the peaks are given. Table. 1. Resonance wavelength andpeakvalues for different analyte
1.350
1.360
1.370
1.380
1.390
1.395
1.400
1.405
1.410
1.415
1.420
1.430
1.440
1.450
1.460
1.24
1.26
1.28
1.30
1.32
1.33
1.34
1.35
1.36
1.37
1.38
1.40
1.42
1.44
1.46
53156.6
54082.3 55055.6
56090.9
58339.5
61076.7 64065.7
46408.4 46472.8 46772 48304 50520 51390.2 52264.1
Sensitivity is calculated as follows.
S ( )
peak na
[nm / RIU ]
(4)
Where peak represents the difference between resonant wavelengths of loss spectra of refractive indices of two consecutive analytes. The refractive index of the two analytes is subtracted to obtain na . Power loss is an important parameter and closely related to
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wavelength of light. As wavelength λ changes, the refraction and reflection of light in PCF changes, and SPR effect is accompanied by appearance and disappearance, which results in a change in optical power transmission[22].The optical power transmittance dependence on 7
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the operable wavelength λ with refractive index increasing from 1.350 to 1.460 is shown
found that the length of the PCF is 1mm.
L ) 4.343
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Pout Pin exp(
of
by Fig. 4. Eq. 5 shows the calculation of output power[23], where L is the length of PCF. We
(5)
where Pout is the output power, Pin is the input power. L represents the length of the fiber, represents the limiting loss of the fiber. The calculation method of optical power transmittance[24] is as follows:
Pout ) Pin
(6)
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Tr 10 log10 (
As shown in Fig. 4, as the analyte refractive index changes from 1.350 to 1.460, the optical power transmission curve of this PCF shows a regular red shift of 212 nm. Furthermore, as the analyte refractive index changes from 1.350 to 1.460, the transmission
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of the dip wavelength[25]gradually decreases from -47 dB to -65 dB.
Fig. 4. Optical power transmittance dependence on the operable wavelength over a refractive index from 1.35 to 1.46.
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4.Conclusion
In this paper, we conducted a series of studies and evaluations on the optical
properties and performance of the designed sensors, and found that it have extremely wide
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measurement range and high sensitivity. A wide range of refractive index analysis solution was filled into the designed PCF, and the process of peak loss of resonance coupling was observed when the phase film and surface plasmon (SPP) modes were satisfied. The
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resonance loss peak pair was studied. Analyze the sensitivity of the liquid refractive index.Through simulation calculation, the sensor obtained an average refractive index of 1931.03 nm / RIU in wide range of 1.35-1.46, and the linear fit was 0.9979. In addition, the minimum loss value of the sensor is 46408.4dB/m, which can effectively shorten the PCF length. After calculation we found that the length of the PCF is 1mm. The performance level of this sensor is competitive compared to existing refractive index sensors. Better
Acknowledgments
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results can be obtained by improving the structure of the PCF
The work is supported by the Excellent Young Talents Project of Higher Education in
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Hebei Province (Grant No.BJ2018040), the Talent Introduction Project of Hebei University of Science and Technology, (Grant No. 1181324), and the College Students of Innovation and Entrepreneurship Project in Hebei Province (Grant No.201910082043 )
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*Author Contributions Section
Author contributions selection Dear Editor：
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The contribution of all authors has been clarified according to the attribution, and there is no conflict of interest
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Zhenkai fan