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Wavelength tunable TFBG based microwave sensor using surface plasmon resonance
The Egyptian Journal of Remote Sensing and Space Sciences (2016) 19, 1–6
H O S T E D BY
National Authority for Remote Sensing and Space Sciences
The Egyptian Journal of Remote Sensing and Space Sciences www.elsevier.com/locate/ejrs www.sciencedirect.com
RESEARCH PAPER
Wavelength tunable TFBG based microwave sensor using surface plasmon resonance Imran Khan *, Md. Mostafizur Rahman Department of Electronics & Communication Engineering, Khulna University of Engineering & Technology, Khulna 9203, Bangladesh Received 6 December 2013; accepted 30 November 2015 Available online 31 December 2015
KEYWORDS Microwave sensor; Surface plasmon resonance; Wavelength tuning; TFBG
Abstract A noble idea is used to propose a wavelength tunable tilted Fiber Bragg Gratings based microwave sensor using surface plasmon resonance. The Bragg wavelength shifts linearly about 6 pm/°C, due to the temperature change. This temperature variation occurs because of the microwave–metal interaction at cladding layer of the optical fiber based proposed sensor. The wavelength tuning can be achieved by using tilted Fiber Bragg Gratings. To use the wavelength sources of 850 nm, 1310 nm and 1550 nm, tiled Fiber Bragg Gratings with grating periods of 293.65 nm, 452.56 nm and 535.47 nm respectively are required in the same fiber with certain distances among them. Ó 2015 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
1. Introduction Optical fiber (OF) can be used as a sensor when it is specially designed. Design of an optical fiber for sensor application is done in such a way, so that a short portion of the fiber’s refractive index differs from the usual fiber core and cladding refractive index (Udd, 1991; Sharma et al., 2007; Marazuela and Moreno-Bondi, 2002). One of the techniques is to introduce Fiber Bragg Gratings (FBGs) in the fiber core. Fiber Bragg Grating is a periodic wavelength scale variation of refractive index, inscribed in the segment of the Fiber core. Bragg grating reflects the light at a particular wavelength which satisfies * Corresponding author at: Dept. of Electrical and Electronic Engineering, Jessore University of Science & Technology, Jessore 7408, Bangladesh. Tel.: +880 1717933007. E-mail address: [email protected] (I. Khan). Peer review under responsibility of National Authority for Remote Sensing and Space Sciences.
Bragg condition. This reflection in a grating occurs as coupling between forward and backward propagation modes at a certain wavelength. The coupling coefficient of the modes is maximal when a special condition (Bragg condition) between wave vectors of light and vector number of the grating is satisfied (Hill and Meltz, 1997): m kB ¼ 2 neff K
ð1Þ
where kB is the wavelength of light called Bragg wavelength, K is grating period, neff is effective refractive index of the core and m is the diffraction order. For a single FBG, theoretically there exists infinite number of Bragg wavelengths. It can be clearly seen from (1), as for different values of m, i.e. diffraction order of Bragg wavelength are different. These Bragg wavelengths are separated from each other by quite a large spectral range, so on practice, only one (first or sometimes second) Bragg resonance wavelength is being used. For instance, when the first Bragg wavelength of the grating (m = 1) is 1550 nm, then the second one is twice
http://dx.doi.org/10.1016/j.ejrs.2015.11.002 1110-9823 Ó 2015 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2 less, i.e. 750 nm, while the spectral range of sources used for Fibers usually doesn’t exceed 100 nm. Additional Bragg peaks can occur if the modulation of the refractive index in FBG is not sinusoidal (which is usually the case) even though most of the gratings inscribed in Fiber have nearly sinusoidal index modulation. It is possible to make the FBG in the core of the fiber with some tilting angle. This type of FBG is known as Tilted Fiber Bragg Grating (TFBG) Shevchenko and Albert, 2007. Here, the variation of the refractive index occurs at an angle to the optical axis. The reflected wavelength and bandwidth of the TFBG is dependent on the tilting angle. If a FBG’s period is varied, such as a linear variation in the grating period results chirp. This new type of FBG is known as Chirped Fiber Bragg Gratings (CFBGs). This CFBG has special characteristic, the reflected wavelength changes with respect to the change in grating period. This unique feature is used for many applications (Li et al., 2009). If the tilting is applied in case of CFBG, then Tilted Chirped Fiber Bragg Gratings will be created. This paper outlines the use of this TCFBG concept with surface plasmon resonance (SPR) to detect microwaves (MW). Now a days SPR based optical fiber is vastly used for sensing applications (Sharma et al., 2009; Slavik et al., 2001; Piliarik et al., 2003; Monzon-Hernandez et al., 2004; Homola, 2003; Willets and Van Duyne, 2007; Shao et al., 2010; Iga et al., 2005; Sharma and Gupta, 2006; Spackova´ et al., 2009). 2. Surface plasmons A conductor, such as metal has a lot of free electrons. The electrons inside the metal can be considered as plasma particles. Equal numbers of positive charged ions from lattice are also present inside the conductor. So, the total charge density in the conductor is zero. When an external field is applied these electrons will start to move. The electrons will move toward the positive region and at the same time the positive ions will move opposite to the electrons. Due to this moving mechanism a longitudinal oscillation will be introduced in the conductor which is known as the plasma oscillation (Willets and Van Duyne, 2007). A conductor and a dielectric are required to support the surface plasma to oscillate at its interface. In general, a metal and dielectric interface is used to support the surface plasmon oscillation. Due to this oscillation, a surface plasmon wave is generated. This surface plasmon is only TM polarized electromagnetic field because there is no solution of the Maxwell’s equation for TE polarized case. This wave is decayed exponentially in the metal. Surface Plasmon Wave (SPW) is characterized by: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi x eM eD b¼ ð2Þ c eM þ eD where, b is the propagation constant, x is the angular frequency, c is the speed of light in vacuum, eM and eD are the permittivities for metal and dielectric respectively. It can be seen from (2) that the property of the SPW is dependent on the property of the two materials i.e. the metal and dielectric media. To create the surface plasmon oscillation it is necessary to excite the electrons in the conductor by impinging EM field (light) on the surface. The electrical per-
I. Khan, M.M. Rahman mittivity for the conductor (metal) is negative and the electrical permittivity for the dielectric is positive. In the dielectric medium the propagation constant (maximum) can be written as: b¼
x pffiffiffiffi es c
ð3Þ
where, es is the permittivity of SPW. The propagation constant for surface plasmon wave is higher than the propagation constant of light in the dielectric medium. As a result, it is not possible excite the surface plasmon with normal light; thus it is required to use the light with extra momentum or energy with same polarization state as the surface plasmon wave. Moreover, the propagation constant should be matched with the surface plasmon wave. 2.1. SPR based TFBG sensors Based on the SPR principle, recently TFBG is used as a sensor (Shevchenko and Albert, 2007). For this sensor application the cladding of the fiber was removed and a metal film was deposited (either gold or silver), so that the surface plasmon can exist at the interface of the metal–dielectric as reported in Willets and Van Duyne (2007). Here, the guided mode can couple to cladding mode as the Bragg grating is tilted with a certain angle. As a result it can excite the surface plasmon waves. The phase matching condition is fulfilled for the surface plasmon waves by proper tilting of the Fiber Bragg Grating (FBG). 3. Microwave metal interaction The depth of penetration of MW in metal is in the order of nanometers; normally MW frequency is reflected from the bulk metal surface. So there is almost no effect of the MW for a bulk metal. But MW has a great impact on thin film of nanometer (nm) size. MW can penetrate the metal up to few nanometer range. As MW is an electromagnetic wave, when it propagates inside the metal film there will be eddy current creation which results in heating inside the metal film. Generally, a thin metal film is used at the cladding layer of the optical fiber so that a metal–dielectric (cladding) interface is created to support the surface plasmon wave for SPR based optical fiber sensor applications. Thickness of the metal film should be in nanometer range. So, microwave can interact easily with this metal film. For this type of sensor applications, gold, silver or sometimes metal alloy are used. Like other electromagnetic waves MW has also two components – H field and E field. The heat response of H field component is much more comparing with the E field heating. One over four times less power is required for heating with MW of H field than the E field as reported in Yoshikawa (2010). 4. TFBG based microwave sensor For the TFBG based sensor the wavelength source was fixed (Shevchenko and Albert, 2007). There was no flexibility to use different sources for the same sensor. In this study, a concept of SPR based OF sensor to detect MW with flexible reference wavelength source is proposed. At the same time the concept of Tilted Chirped Fiber Bragg Grating (TCFBG) is
Wavelength tunable TFBG based microwave sensor using surface plasmon resonance also used to get this flexibility. The flowchart of the proposed sensor’s sensing scheme is shown in Fig. 1. More than one TFBG will be used in the fiber with different grating periods (i.e. TCFBG) to get the flexibility. The tilting will be done so that the reflected mode becomes the cladding mode. Thus it excites the surface plasmon wave. As an example if it is required to use 850 nm, 1310 nm and 1550 nm as the reference source wavelengths then three TFBGs need to be used in the same fiber with grating periods of 293.65 nm, 452.56 nm and 535.47 nm respectively. This calculation is done using (1) considering effective refractive index of 1.44732 (Florian and Johannes, 2009) for SMF-28 optical fiber. The proposed sensor is shown in Fig. 2. To get the effect of heating (temperature variation) of CFBG, a laboratory experiment, was conducted as it was done for the FBG in Khan (2012).
3
4.1. Temperature effect on CFBG A common experiment was conducted on CFBG to know the exact temperature effect on it. Fig. 3 shows the experimental setup. In (1) two parameters depend on external condition change, i.e. change of temperature. These parameters are effective index of the core (neff ) and the period of the grating (K). When temperature changes the effective index is changing due to thermo-optic effect, while the period changes due to thermal expansion of the glass. As a result of temperature variation the effective index is changing by Dneff and the period of the grating by DK, which will result in an overall Bragg wavelength change DkB . So the Bragg condition will take the following form: kB þ DkB ¼ 2 ðneff þ Dneff Þ ðK þ DKÞ
ð4Þ
After simplifying the expression it will result in: kB þ DkB ¼ 2ðneff K þ neff DK þ K Dneff þ Dneff DKÞ
ð5Þ
The last term at the right hand side of (5) can be neglected as it is multiplication of two small quantities. Also taking into account (1) i.e. unperturbed Bragg condition, it will be the formula for the shift of Bragg wavelength:
Reference Wavelength Tuner of the Sensor
DkB ¼ 2ðneff DK þ K Dneff Þ
MW Frequency Impinging on the Detector/Sensor
Deep Penetraon or Low Penetraon
Low Penetraon into the Metal
Deep Penetraon into the Metal
Small Eddy Current Creaon
Huge Eddy Current Creaon
Heat Increased
Heat Increased
ð6Þ
A current source was used in the lab to heat the panel where the CFBG was attached. The current was increased from 0 A to 0.8 A which results in temperature increase of the panel from 27.7 °C to 54.6 °C respectively. Due to the temperature change there is a related wavelength shift which is listed in Table 1. This result is plotted in Fig. 4. In Fig. 4, linear fitting performed by MATLAB is presented together with experimentally obtained curve of Bragg wavelength dependence on temperature. The sensitivity according to this fitting is 7.44 pm/ °C. If more attention is paid to the experimental curve, it can be seen that in the beginning, the measurements weren’t performed appropriately, as in this region the dependence is not linear. Additionally a well linear region was observed starting from temperature 42 to 55 °C. The results of sensitivity was calculated only for that region and found as 8.9 pm/ °C. From the above result it is clear that the temperature response for the CFBG is linear same as the FBG for a specific wavelength. This result can be compared with that of (Khan, 2012). From the experimental result it can be seen that there is an average 6 pm wavelength shift due to 1 °C temperature change. 4.2. Calculation of error
Temp. > Ref. Temp.
Temp. >> Ref. Temp.
Wavelength Shi > Ref. Wavelength
Wavelength Shi >> Ref. Wavelength
Low MW Frequency Detected
High MW Frequency Detected
Figure 1
Flowchart of new sensor scheme.
For the calculation of the error let us consider the measurements of Sl. No. 12 and 18 (see Table 2). The absolute error of the temperature is 0.1 °C and for wavelength 0.001, so the sensitivity is: ðk18 Dkerr Þ ðk12 Dkerr Þ ðT18 DTerr Þ ðT12 DTerr Þ ðk18 k12 Þ Dkdif err 0:117 þ DSerr ¼ 0:0089 þ DSerr ¼ ¼ 13:1 ðT18 T12 Þ DTdif err
S¼
ð7Þ where, the errors from the difference can be calculated from absolute errors by:
4
I. Khan, M.M. Rahman
Figure 2
TFBG based proposed MW sensor.
Figure 3
Experimental setup.
Table 1
Experimental data for CFBG.
Sl. No.
Current (A)
Temperature (°C)
Wavelength (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0.00 0.04 0.16 0.20 0.24 0.28 0.32 0.36 0.40 0.44 0.48 0.52 0.56 0.60 0.64 0.78 0.76 0.80
27.7 28.4 30.7 31.6 32.5 33.4 34.5 35.7 37.0 38.5 40.1 41.5 43.2 44.7 46.8 48.8 52.6 54.6
1583.310 1583.312 1583.313 1583.318 1583.322 1583.327 1583.334 1583.342 1583.351 1583.361 1583.374 1583.385 1583.399 1583.414 1583.433 1583.450 1583.486 1583.502
Dkdif
DTdif
err
err
The error coming from the ratio can be calculated in the following way: DSerr
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s 2 2 Dkdif err DTdif err ¼S þ Dk DT ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s 2 0:0014 0:14 2 ¼ 0:0089 þð Þ 0:117 13:1
ð10Þ
¼ 0:0089 0:016 ¼ 0:00014nm= C DSerr ¼ 0:14pm= C
ð11Þ
So the sensitivity of the sensor with the calculated error is: ð8:9 0:14Þpm= C
ð12Þ
5. Efficient design consideration
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ðDkerr Þ2 þ ðDkerr Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2ðDkerr Þ2 ¼ 2ð0:001Þ2 ¼ 0:0014
ð8Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ðDTerr Þ2 þ ðDTerr Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2ðDTerr Þ2 ¼ 2ð0:1Þ2 ¼ 0:14
ð9Þ
Fig. 5 shows a chirped grating with length Lg and chirped Bandwidth (BW) Dkchirp. This BW is related as (Kashyap, 1999): Dkchirp ¼ 2neff ðKlong Kshort Þ ¼ 2neff DKchirp
ð13Þ
The phase matching condition for the grating is defined by (1). The number of steps that will be in a chirped grating can be formulated as:
Wavelength tunable TFBG based microwave sensor using surface plasmon resonance
5
65
Experiment
Linear Fitting
60
Temperature (°C)
55 50 45 40 35 30 25 1583.3
1583.35
1583.4
1583.45
1583.5
1583.55
Wavelength (nm)
Figure 4
Table 2
Bragg wavelength dependence on temperature and linear fitting.
the axis of the fiber is involved so the modified grating period will be:
Experimental data for error calculation.
Sl. No.
Temperature (°C)
Wavelength (nm)
12 18
41.5 54.6
1583.385 1583.502
Figure 5
N 2neff Dkchirp ¼ Lg pk2Bragg
K cos h
ð16Þ
where, h is the tilting angle. Using (15) and (16) the final equation becomes as follows: K claddingi kiresonance ¼ ncore þ n ð17Þ eff eff cos h
Chirped grating.
ð14Þ
where kBragg is the central Bragg wavelength of the grating. Using Lg = 100 mm, Dkchirp = 0.75 nm & Bragg wavelength 1550 nm in Kashyap (1999) 200 sections were found with 2 steps/mm and then next 42 steps was used with 0.42 steps/ mm. It was reported that if the sections of the grating are varied from lower to higher or vice versa the reflectivity characteristics remain approximately the same. There are several conditions that need to be considered to get TCFBG. The coupling condition for this case is (Spackova´ et al., 2009): claddingi kiresonance ¼ ncore Kg ð15Þ eff þ neff where, kiresonance is the ith resonance of ith cladding mode, ncore eff , i
Kg ¼
are the effective indices of core and cladding modes ncladding eff respectively and Kg is the period of the FBG. As tilted FBG will be used so the angle consideration need to be involved in (15). Here, the tilt angle of grating plane with respect to
If three TFBGs are used in the same fiber then it will have three reflected wavelengths which can excite the surface plasmon (SP). This results in wavelength flexibility. It can be compared with the result of Spackova´ (2009) for the first FBG, the SP-resonance wavelength was found approximately at 0.77 lm but for the second FBG, with a different period the SPresonance wavelength was approximately at 0.81 lm. So, by using chirped FBG, i.e. FBG with a different period, the SP waves can be excited and resonance occurred with different wavelengths. The sensor characteristic also depends on the refractive index change, FBG length variation and width of the gold film as indicated by Sharma and Gupta (2006), Iga et al., 2005, (Suzuki et al., 2008). 6. Conclusion Tilted Fiber Bragg Grating based new MW sensor is proposed. MW detection possibility was analyzed based on surface plasmon resonance. Thickness of metal film and polarization dependency affects the SPR wave, so the design needs an optimization. MW range that can be detected by this sensor depends on the metal film layer thickness and the type of metal used for the generation of SPR. The analysis was done considering the SPR generation, polarization, Bragg wavelength shift, metal film thickness, corresponding temperature effects or thermo-optic effect. Factors those were not considered in this analysis: power or energy of the MW frequency, amplitude of the MW frequencies, and different bands of MW frequencies. It may be possible to have an effect of high power/energy MW frequency and low power/energy MW frequency on the sensor.
6 The manufacturing cost of this sensor will be the main limitation. On the contrary the proposed sensor will be very small so, integration is possible with other electronics. Conflict of interest The authors declare no conflict of interest. Acknowledgement Authors would like to thank T. Baghdasaryan, Y. Chowdhury and VUB for their support. References Florian, J., Johannes R., 2009. Determination of the Effective Refractive Index of Various Single Mode Fibres for Fibre Bragg Grating Sensor Applications. Proc. Sensor Conf., OPTO. Hill, K.O., Meltz, G., 1997. Fiber Bragg Grating technology fundamentals and overview. J. Lightwave Technol. 15 (8). Homola, Jirˇ i, 2003. Present and future of surface plasmon resonance biosensors. Anal. Bioanal. Chem. 377, 528–539. Iga, M., Sekib, A., Watanabe, K., 2005. Gold thickness dependence of SPR-based hetero-core structured optical fiber sensor. Sensor Actuators, B 106, 363–368. Kashyap, Raman, 1999. Fiber Bragg Gratings. Academic Press. Khan Imran. Optical Fiber Based Microwaves Sensor Using Surface Plasmon Resonance, Proceedings of International Conference on Informatics, Electronics & Vision, IEEE Xplore, May 2012, pp. 310–315. Li Chenglin, Chen Na, Chen Zhenyi, Wang Tingyun. Fully distributed chirped FBG sensor and application in laser-induced interstitial thermotherapy, Proc. of SPIE-OSA-IEEE Asia Commun. Photonics, SPIE 7634, 76340D, 2009. Marazuela, M.D., Moreno-Bondi, M.C., 2002. Fiber optic biosensoran overview. Anal. Bioanal. Chem. 372, 664–682.
I. Khan, M.M. Rahman Monzon-Hernandez, D., Villatoro, J., Talavera, D., Luna-Moreno, D., 2004. Optical fiber surface-plasmon resonance sensor with multiple resonance peaks. Appl. Optics 43, 1216–1220. Piliarik, M., Homola, J., Manikova, Z., Ctyroky, J., 2003. Surface plasmon resonance sensor based on a single mode polarization maintaining optical fiber. Sens. Actuators, B 90, 236–242. Shao, Li-Yang, Shevchenko, Y., Albert, J., 2010. Intrinsic temperature sensitivity of tilted fiber Bragg grating based surface plasmon resonance sensors. Opt. Express 18 (11). Sharma, Anuj K., Gupta, B.D., 2006. Fibre-optic sensor based on surface plasmon resonance with Ag–Au alloy nanoparticle films. Nanotechnology 17, 124–131. Sharma, Anuj K., Jha, Rajan, Gupta, B.D., 2007. Fiber-optic sensors based on surface plasmon resonance. a comprehensive review. In: IEEE Sensor J. 7 (8). Sharma, Anuj K., Jha, Rajan, Pattanaik, H.S., Mohr, G.J., 2009. Design considerations for surface plasmon resonance-based fiber-optic detection of human blood group. J. Biomed. Opt. 14 (6), 064041. Shevchenko, Yanina Y., Albert, Jacques, 2007. Plasmon resonances in gold-coated tilted fiber Bragg gratings. Opt. Lett. 32 (3). Slavik, R., Homola, J., Ctyroky, J., Brynda, E., 2001. Novel spectral fiber optic sensor based on surface plasmon resonance. Sens. Actuators, B 74, 106–111. Spackova´, Barbora, Piliarik, Marek, Kvasnicka, Pavel, Themistos, Christos, Rajarajan, Muttukrishnan, Homola, J., 2009. Novel concept of multi-channel fiber optic surface plasmon resonance sensor. Sensor Actuators B 139, 199–203. Suzuki, H., Sugimoto, M., Matsui, Y., Kondoh, J., 2008. Effects of gold film thickness on spectrum profile and sensitivity of a multimodeoptical-fiber SPR sensor. Sensor Actuators, B 132, 26–33. Udd, E., 1991. Fiber Optic Sensors: An Introduction for Engineerings and Scientists. John Wiley and Sons, New York. Willets, Katherine A., Van Duyne, Richard P., 2007. Localized surface plasmon resonance spectroscopy and sensing. Annu. Rev. Phys. Chem. 58, 267–297. Yoshikawa, N., 2010. Fundamentals and applications of microwave heating of metals. J. Microw. Power Electromagn. Energy. 44 (1), 4–13.
H O S T E D BY
National Authority for Remote Sensing and Space Sciences
The Egyptian Journal of Remote Sensing and Space Sciences www.elsevier.com/locate/ejrs www.sciencedirect.com
RESEARCH PAPER
Wavelength tunable TFBG based microwave sensor using surface plasmon resonance Imran Khan *, Md. Mostafizur Rahman Department of Electronics & Communication Engineering, Khulna University of Engineering & Technology, Khulna 9203, Bangladesh Received 6 December 2013; accepted 30 November 2015 Available online 31 December 2015
KEYWORDS Microwave sensor; Surface plasmon resonance; Wavelength tuning; TFBG
Abstract A noble idea is used to propose a wavelength tunable tilted Fiber Bragg Gratings based microwave sensor using surface plasmon resonance. The Bragg wavelength shifts linearly about 6 pm/°C, due to the temperature change. This temperature variation occurs because of the microwave–metal interaction at cladding layer of the optical fiber based proposed sensor. The wavelength tuning can be achieved by using tilted Fiber Bragg Gratings. To use the wavelength sources of 850 nm, 1310 nm and 1550 nm, tiled Fiber Bragg Gratings with grating periods of 293.65 nm, 452.56 nm and 535.47 nm respectively are required in the same fiber with certain distances among them. Ó 2015 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-ncnd/4.0/).
1. Introduction Optical fiber (OF) can be used as a sensor when it is specially designed. Design of an optical fiber for sensor application is done in such a way, so that a short portion of the fiber’s refractive index differs from the usual fiber core and cladding refractive index (Udd, 1991; Sharma et al., 2007; Marazuela and Moreno-Bondi, 2002). One of the techniques is to introduce Fiber Bragg Gratings (FBGs) in the fiber core. Fiber Bragg Grating is a periodic wavelength scale variation of refractive index, inscribed in the segment of the Fiber core. Bragg grating reflects the light at a particular wavelength which satisfies * Corresponding author at: Dept. of Electrical and Electronic Engineering, Jessore University of Science & Technology, Jessore 7408, Bangladesh. Tel.: +880 1717933007. E-mail address: [email protected] (I. Khan). Peer review under responsibility of National Authority for Remote Sensing and Space Sciences.
Bragg condition. This reflection in a grating occurs as coupling between forward and backward propagation modes at a certain wavelength. The coupling coefficient of the modes is maximal when a special condition (Bragg condition) between wave vectors of light and vector number of the grating is satisfied (Hill and Meltz, 1997): m kB ¼ 2 neff K
ð1Þ
where kB is the wavelength of light called Bragg wavelength, K is grating period, neff is effective refractive index of the core and m is the diffraction order. For a single FBG, theoretically there exists infinite number of Bragg wavelengths. It can be clearly seen from (1), as for different values of m, i.e. diffraction order of Bragg wavelength are different. These Bragg wavelengths are separated from each other by quite a large spectral range, so on practice, only one (first or sometimes second) Bragg resonance wavelength is being used. For instance, when the first Bragg wavelength of the grating (m = 1) is 1550 nm, then the second one is twice
http://dx.doi.org/10.1016/j.ejrs.2015.11.002 1110-9823 Ó 2015 National Authority for Remote Sensing and Space Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
2 less, i.e. 750 nm, while the spectral range of sources used for Fibers usually doesn’t exceed 100 nm. Additional Bragg peaks can occur if the modulation of the refractive index in FBG is not sinusoidal (which is usually the case) even though most of the gratings inscribed in Fiber have nearly sinusoidal index modulation. It is possible to make the FBG in the core of the fiber with some tilting angle. This type of FBG is known as Tilted Fiber Bragg Grating (TFBG) Shevchenko and Albert, 2007. Here, the variation of the refractive index occurs at an angle to the optical axis. The reflected wavelength and bandwidth of the TFBG is dependent on the tilting angle. If a FBG’s period is varied, such as a linear variation in the grating period results chirp. This new type of FBG is known as Chirped Fiber Bragg Gratings (CFBGs). This CFBG has special characteristic, the reflected wavelength changes with respect to the change in grating period. This unique feature is used for many applications (Li et al., 2009). If the tilting is applied in case of CFBG, then Tilted Chirped Fiber Bragg Gratings will be created. This paper outlines the use of this TCFBG concept with surface plasmon resonance (SPR) to detect microwaves (MW). Now a days SPR based optical fiber is vastly used for sensing applications (Sharma et al., 2009; Slavik et al., 2001; Piliarik et al., 2003; Monzon-Hernandez et al., 2004; Homola, 2003; Willets and Van Duyne, 2007; Shao et al., 2010; Iga et al., 2005; Sharma and Gupta, 2006; Spackova´ et al., 2009). 2. Surface plasmons A conductor, such as metal has a lot of free electrons. The electrons inside the metal can be considered as plasma particles. Equal numbers of positive charged ions from lattice are also present inside the conductor. So, the total charge density in the conductor is zero. When an external field is applied these electrons will start to move. The electrons will move toward the positive region and at the same time the positive ions will move opposite to the electrons. Due to this moving mechanism a longitudinal oscillation will be introduced in the conductor which is known as the plasma oscillation (Willets and Van Duyne, 2007). A conductor and a dielectric are required to support the surface plasma to oscillate at its interface. In general, a metal and dielectric interface is used to support the surface plasmon oscillation. Due to this oscillation, a surface plasmon wave is generated. This surface plasmon is only TM polarized electromagnetic field because there is no solution of the Maxwell’s equation for TE polarized case. This wave is decayed exponentially in the metal. Surface Plasmon Wave (SPW) is characterized by: rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi x eM eD b¼ ð2Þ c eM þ eD where, b is the propagation constant, x is the angular frequency, c is the speed of light in vacuum, eM and eD are the permittivities for metal and dielectric respectively. It can be seen from (2) that the property of the SPW is dependent on the property of the two materials i.e. the metal and dielectric media. To create the surface plasmon oscillation it is necessary to excite the electrons in the conductor by impinging EM field (light) on the surface. The electrical per-
I. Khan, M.M. Rahman mittivity for the conductor (metal) is negative and the electrical permittivity for the dielectric is positive. In the dielectric medium the propagation constant (maximum) can be written as: b¼
x pffiffiffiffi es c
ð3Þ
where, es is the permittivity of SPW. The propagation constant for surface plasmon wave is higher than the propagation constant of light in the dielectric medium. As a result, it is not possible excite the surface plasmon with normal light; thus it is required to use the light with extra momentum or energy with same polarization state as the surface plasmon wave. Moreover, the propagation constant should be matched with the surface plasmon wave. 2.1. SPR based TFBG sensors Based on the SPR principle, recently TFBG is used as a sensor (Shevchenko and Albert, 2007). For this sensor application the cladding of the fiber was removed and a metal film was deposited (either gold or silver), so that the surface plasmon can exist at the interface of the metal–dielectric as reported in Willets and Van Duyne (2007). Here, the guided mode can couple to cladding mode as the Bragg grating is tilted with a certain angle. As a result it can excite the surface plasmon waves. The phase matching condition is fulfilled for the surface plasmon waves by proper tilting of the Fiber Bragg Grating (FBG). 3. Microwave metal interaction The depth of penetration of MW in metal is in the order of nanometers; normally MW frequency is reflected from the bulk metal surface. So there is almost no effect of the MW for a bulk metal. But MW has a great impact on thin film of nanometer (nm) size. MW can penetrate the metal up to few nanometer range. As MW is an electromagnetic wave, when it propagates inside the metal film there will be eddy current creation which results in heating inside the metal film. Generally, a thin metal film is used at the cladding layer of the optical fiber so that a metal–dielectric (cladding) interface is created to support the surface plasmon wave for SPR based optical fiber sensor applications. Thickness of the metal film should be in nanometer range. So, microwave can interact easily with this metal film. For this type of sensor applications, gold, silver or sometimes metal alloy are used. Like other electromagnetic waves MW has also two components – H field and E field. The heat response of H field component is much more comparing with the E field heating. One over four times less power is required for heating with MW of H field than the E field as reported in Yoshikawa (2010). 4. TFBG based microwave sensor For the TFBG based sensor the wavelength source was fixed (Shevchenko and Albert, 2007). There was no flexibility to use different sources for the same sensor. In this study, a concept of SPR based OF sensor to detect MW with flexible reference wavelength source is proposed. At the same time the concept of Tilted Chirped Fiber Bragg Grating (TCFBG) is
Wavelength tunable TFBG based microwave sensor using surface plasmon resonance also used to get this flexibility. The flowchart of the proposed sensor’s sensing scheme is shown in Fig. 1. More than one TFBG will be used in the fiber with different grating periods (i.e. TCFBG) to get the flexibility. The tilting will be done so that the reflected mode becomes the cladding mode. Thus it excites the surface plasmon wave. As an example if it is required to use 850 nm, 1310 nm and 1550 nm as the reference source wavelengths then three TFBGs need to be used in the same fiber with grating periods of 293.65 nm, 452.56 nm and 535.47 nm respectively. This calculation is done using (1) considering effective refractive index of 1.44732 (Florian and Johannes, 2009) for SMF-28 optical fiber. The proposed sensor is shown in Fig. 2. To get the effect of heating (temperature variation) of CFBG, a laboratory experiment, was conducted as it was done for the FBG in Khan (2012).
3
4.1. Temperature effect on CFBG A common experiment was conducted on CFBG to know the exact temperature effect on it. Fig. 3 shows the experimental setup. In (1) two parameters depend on external condition change, i.e. change of temperature. These parameters are effective index of the core (neff ) and the period of the grating (K). When temperature changes the effective index is changing due to thermo-optic effect, while the period changes due to thermal expansion of the glass. As a result of temperature variation the effective index is changing by Dneff and the period of the grating by DK, which will result in an overall Bragg wavelength change DkB . So the Bragg condition will take the following form: kB þ DkB ¼ 2 ðneff þ Dneff Þ ðK þ DKÞ
ð4Þ
After simplifying the expression it will result in: kB þ DkB ¼ 2ðneff K þ neff DK þ K Dneff þ Dneff DKÞ
ð5Þ
The last term at the right hand side of (5) can be neglected as it is multiplication of two small quantities. Also taking into account (1) i.e. unperturbed Bragg condition, it will be the formula for the shift of Bragg wavelength:
Reference Wavelength Tuner of the Sensor
DkB ¼ 2ðneff DK þ K Dneff Þ
MW Frequency Impinging on the Detector/Sensor
Deep Penetraon or Low Penetraon
Low Penetraon into the Metal
Deep Penetraon into the Metal
Small Eddy Current Creaon
Huge Eddy Current Creaon
Heat Increased
Heat Increased
ð6Þ
A current source was used in the lab to heat the panel where the CFBG was attached. The current was increased from 0 A to 0.8 A which results in temperature increase of the panel from 27.7 °C to 54.6 °C respectively. Due to the temperature change there is a related wavelength shift which is listed in Table 1. This result is plotted in Fig. 4. In Fig. 4, linear fitting performed by MATLAB is presented together with experimentally obtained curve of Bragg wavelength dependence on temperature. The sensitivity according to this fitting is 7.44 pm/ °C. If more attention is paid to the experimental curve, it can be seen that in the beginning, the measurements weren’t performed appropriately, as in this region the dependence is not linear. Additionally a well linear region was observed starting from temperature 42 to 55 °C. The results of sensitivity was calculated only for that region and found as 8.9 pm/ °C. From the above result it is clear that the temperature response for the CFBG is linear same as the FBG for a specific wavelength. This result can be compared with that of (Khan, 2012). From the experimental result it can be seen that there is an average 6 pm wavelength shift due to 1 °C temperature change. 4.2. Calculation of error
Temp. > Ref. Temp.
Temp. >> Ref. Temp.
Wavelength Shi > Ref. Wavelength
Wavelength Shi >> Ref. Wavelength
Low MW Frequency Detected
High MW Frequency Detected
Figure 1
Flowchart of new sensor scheme.
For the calculation of the error let us consider the measurements of Sl. No. 12 and 18 (see Table 2). The absolute error of the temperature is 0.1 °C and for wavelength 0.001, so the sensitivity is: ðk18 Dkerr Þ ðk12 Dkerr Þ ðT18 DTerr Þ ðT12 DTerr Þ ðk18 k12 Þ Dkdif err 0:117 þ DSerr ¼ 0:0089 þ DSerr ¼ ¼ 13:1 ðT18 T12 Þ DTdif err
S¼
ð7Þ where, the errors from the difference can be calculated from absolute errors by:
4
I. Khan, M.M. Rahman
Figure 2
TFBG based proposed MW sensor.
Figure 3
Experimental setup.
Table 1
Experimental data for CFBG.
Sl. No.
Current (A)
Temperature (°C)
Wavelength (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0.00 0.04 0.16 0.20 0.24 0.28 0.32 0.36 0.40 0.44 0.48 0.52 0.56 0.60 0.64 0.78 0.76 0.80
27.7 28.4 30.7 31.6 32.5 33.4 34.5 35.7 37.0 38.5 40.1 41.5 43.2 44.7 46.8 48.8 52.6 54.6
1583.310 1583.312 1583.313 1583.318 1583.322 1583.327 1583.334 1583.342 1583.351 1583.361 1583.374 1583.385 1583.399 1583.414 1583.433 1583.450 1583.486 1583.502
Dkdif
DTdif
err
err
The error coming from the ratio can be calculated in the following way: DSerr
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s 2 2 Dkdif err DTdif err ¼S þ Dk DT ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s 2 0:0014 0:14 2 ¼ 0:0089 þð Þ 0:117 13:1
ð10Þ
¼ 0:0089 0:016 ¼ 0:00014nm= C DSerr ¼ 0:14pm= C
ð11Þ
So the sensitivity of the sensor with the calculated error is: ð8:9 0:14Þpm= C
ð12Þ
5. Efficient design consideration
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ðDkerr Þ2 þ ðDkerr Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2ðDkerr Þ2 ¼ 2ð0:001Þ2 ¼ 0:0014
ð8Þ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ ðDTerr Þ2 þ ðDTerr Þ2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2ðDTerr Þ2 ¼ 2ð0:1Þ2 ¼ 0:14
ð9Þ
Fig. 5 shows a chirped grating with length Lg and chirped Bandwidth (BW) Dkchirp. This BW is related as (Kashyap, 1999): Dkchirp ¼ 2neff ðKlong Kshort Þ ¼ 2neff DKchirp
ð13Þ
The phase matching condition for the grating is defined by (1). The number of steps that will be in a chirped grating can be formulated as:
Wavelength tunable TFBG based microwave sensor using surface plasmon resonance
5
65
Experiment
Linear Fitting
60
Temperature (°C)
55 50 45 40 35 30 25 1583.3
1583.35
1583.4
1583.45
1583.5
1583.55
Wavelength (nm)
Figure 4
Table 2
Bragg wavelength dependence on temperature and linear fitting.
the axis of the fiber is involved so the modified grating period will be:
Experimental data for error calculation.
Sl. No.
Temperature (°C)
Wavelength (nm)
12 18
41.5 54.6
1583.385 1583.502
Figure 5
N 2neff Dkchirp ¼ Lg pk2Bragg
K cos h
ð16Þ
where, h is the tilting angle. Using (15) and (16) the final equation becomes as follows: K claddingi kiresonance ¼ ncore þ n ð17Þ eff eff cos h
Chirped grating.
ð14Þ
where kBragg is the central Bragg wavelength of the grating. Using Lg = 100 mm, Dkchirp = 0.75 nm & Bragg wavelength 1550 nm in Kashyap (1999) 200 sections were found with 2 steps/mm and then next 42 steps was used with 0.42 steps/ mm. It was reported that if the sections of the grating are varied from lower to higher or vice versa the reflectivity characteristics remain approximately the same. There are several conditions that need to be considered to get TCFBG. The coupling condition for this case is (Spackova´ et al., 2009): claddingi kiresonance ¼ ncore Kg ð15Þ eff þ neff where, kiresonance is the ith resonance of ith cladding mode, ncore eff , i
Kg ¼
are the effective indices of core and cladding modes ncladding eff respectively and Kg is the period of the FBG. As tilted FBG will be used so the angle consideration need to be involved in (15). Here, the tilt angle of grating plane with respect to
If three TFBGs are used in the same fiber then it will have three reflected wavelengths which can excite the surface plasmon (SP). This results in wavelength flexibility. It can be compared with the result of Spackova´ (2009) for the first FBG, the SP-resonance wavelength was found approximately at 0.77 lm but for the second FBG, with a different period the SPresonance wavelength was approximately at 0.81 lm. So, by using chirped FBG, i.e. FBG with a different period, the SP waves can be excited and resonance occurred with different wavelengths. The sensor characteristic also depends on the refractive index change, FBG length variation and width of the gold film as indicated by Sharma and Gupta (2006), Iga et al., 2005, (Suzuki et al., 2008). 6. Conclusion Tilted Fiber Bragg Grating based new MW sensor is proposed. MW detection possibility was analyzed based on surface plasmon resonance. Thickness of metal film and polarization dependency affects the SPR wave, so the design needs an optimization. MW range that can be detected by this sensor depends on the metal film layer thickness and the type of metal used for the generation of SPR. The analysis was done considering the SPR generation, polarization, Bragg wavelength shift, metal film thickness, corresponding temperature effects or thermo-optic effect. Factors those were not considered in this analysis: power or energy of the MW frequency, amplitude of the MW frequencies, and different bands of MW frequencies. It may be possible to have an effect of high power/energy MW frequency and low power/energy MW frequency on the sensor.
6 The manufacturing cost of this sensor will be the main limitation. On the contrary the proposed sensor will be very small so, integration is possible with other electronics. Conflict of interest The authors declare no conflict of interest. Acknowledgement Authors would like to thank T. Baghdasaryan, Y. Chowdhury and VUB for their support. References Florian, J., Johannes R., 2009. Determination of the Effective Refractive Index of Various Single Mode Fibres for Fibre Bragg Grating Sensor Applications. Proc. Sensor Conf., OPTO. Hill, K.O., Meltz, G., 1997. Fiber Bragg Grating technology fundamentals and overview. J. Lightwave Technol. 15 (8). Homola, Jirˇ i, 2003. Present and future of surface plasmon resonance biosensors. Anal. Bioanal. Chem. 377, 528–539. Iga, M., Sekib, A., Watanabe, K., 2005. Gold thickness dependence of SPR-based hetero-core structured optical fiber sensor. Sensor Actuators, B 106, 363–368. Kashyap, Raman, 1999. Fiber Bragg Gratings. Academic Press. Khan Imran. Optical Fiber Based Microwaves Sensor Using Surface Plasmon Resonance, Proceedings of International Conference on Informatics, Electronics & Vision, IEEE Xplore, May 2012, pp. 310–315. Li Chenglin, Chen Na, Chen Zhenyi, Wang Tingyun. Fully distributed chirped FBG sensor and application in laser-induced interstitial thermotherapy, Proc. of SPIE-OSA-IEEE Asia Commun. Photonics, SPIE 7634, 76340D, 2009. Marazuela, M.D., Moreno-Bondi, M.C., 2002. Fiber optic biosensoran overview. Anal. Bioanal. Chem. 372, 664–682.
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