- Email: [email protected]
Refractive index sensor based on all-fiber multimode interference
Optik 124 (2013) 1845–1848
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Optik journal homepage: www.elsevier.de/ijleo
Refractive index sensor based on all-fiber multimode interference Jianfeng Wang, Yongxing Jin ∗ , Yu Zhao, Xinyong Dong Institute of Optoelectronic Technology, China Jiliang University, Xueyuan Street, Xiasha Higher Education Zone, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 30 December 2011 Accepted 25 May 2012
Keywords: Fiber optic sensors SMS fiber structure Optical fiber interference Refractive index measurement
a b s t r a c t This paper reports the work on the development of refractive index sensors using the multimode interference technology in an optical fiber. The sensor consists of a short section of multimode fiber (MMF), from which the cladding around the core is removed, inserted between two single-mode fibers (SMF). The proposed refractive index sensor is theoretically and experimentally investigated in terms of its ability to shift the re-imaging resonant wavelength as a function of refractive index. It shows that the proposed sensor has a measurement resolution of 5.3 × 10−5 refractive index unit (RIU) for the changes of refractive indices from 1.336 to 1.372. The proposed sensor has a great potential for biological and chemical applications. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction The measurement of refractive index based on all-optical-fiber technology is attractive and promising for their unique advantages, such as immunity to electromagnetic interferences, compact size, and fast response. Therefore, several types of refractive index sensors based on fiber are widely provided. Recently, refractometer based on optical fiber Bragg grating (FBG) [1–3], long-period grating (LPG) [4–7], tilted fiber Bragg grating (TFBG) [8,9], and photonic crystal fiber (PCF) [10,11] have been widely investigated. The principle of LPG refractive index sensor relies on the changes of the effective indices of the cladding modes. The corresponding resonant wavelengths will shift in the transmission spectrum when the external refractive index around the LPG is changed. For a PCF refractive index sensor, the proposed sensing mechanism is based on the detection of the wavelength shift of the photonic band edge with the variation of the ambient refractive index. In the recent years, the multimode interference occurring in singlemode–multimode–singlemode fiber structure has been utilized in the design and fabrication of devices such as bandpass filter [12], temperature sensor [13,14], wavelength tunable fiber lens [15], and displacement sensors [16]. In this paper, the refractive index dependence of the shift of re-imaging resonant wavelength of the proposed sensor is theoretically and experimentally investigated. The proposed sensor consists of a short section of multimode fiber (MMF), from which the cladding around the core is removed, inserted between two singlemode fibers (SMF). The basis for this paper is based on the re-imaging resonant wavelength shifts for
measuring the refractive index change around the multimode fiber core section. 2. Theoretical background The proposed SMS fiber structure consists of input and output single-mode fibers (SMFs) and a sandwiched section of multimode fiber (MMF) core with a specific length L, as show in Fig. 1. The multimode fiber core section sandwiched between two single-mode fibers and has a step-index profile. The single-mode and multimode fibers are assumed to be aligned along the same axis. Moreover, there is on offset between the single-mode and multimode fibers at the two interfaces. Due to the circular symmetric characteristic of fundamental mode of the single-mode fiber, the input light is assumed to have a field distribution of E(r,0). When the light launches into the MMF core, the input field can be decomposed by the eigenmodes (LPnm ) of the MMF. Due to the circular symmetric of input field and an ideal alignment assumed above, only the LPnm modes can be excited [15]. Donate the field profile of LP0m as Fm (r), and neglect the small amount of radiation from the multimode fiber, we have [17] E(r, 0) =
0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.05.042
cm Fm (r)
(1)
m=1
where cm is the excitation coefficient of each mode and it can be calculated by the overlap integral between E(r,0) and Fm (r)
∞ E(r, 0)Fm (r)rdr cm = 0∞ 0
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Jin).
M
Fm (r)Fm (r)rdr
(2)
The excited mode number of the LP0m multimode fiber M ≈ V/M (V = {2/}a n2c0 − n2cl , where a is the radius of the MMF core, n2c0
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J. Wang et al. / Optik 124 (2013) 1845–1848
Input SMF
MMF core
Output SMF Z
L Fig. 1. Schematic configuration of the single-mode–multimode–single-mode fiber structure.
and n2cl is refractive index for the core and cladding of the multimode fiber, respectively and is the wavelength in the free-space). The interference between different modes occurs while the beam propagates along the MMF section. The phase difference between the LP0m and the LP0n (m, n are positive integers) cladding modes can be expressed as [13–15]: ˚m,n =
2(nm − nneff )L eff
=
2nm,n L eff
(3)
where L is the length of the MMF core, nm,n is the effective eff refractive index difference between the LP0m and the LP0n , and is the free-space wavelength. From Eq. (1), the wavelength spacing between two adjacent minima (the free space range) can be expressed as: ≈
2
Fig. 2. Amplitude distribution of the propagation field within the singlemode–multimode–single-mode fiber structure.
0.4
(4)
0.3
It can be seen that the free space range increases as the length of the MMF core decreases. At the same time, the change of refractive index around the MMF region, will also change the effective index difference between the LP0m and the LP0n of the cladding modes. This effect will induce changes in the coupling conditions between the core and cladding modes, which further brings about a shift of the interference fringes. In order to carry out the amplitude of the calculated field along the multimode fiber, a beam propagation method is employed to simulate light propagation within the SMS fiber structure. As a numerical example, a standard single-mode fiber (SMF28) is chosen as the single-mode fiber section and the respective index for the core and cladding are 1.4504 and 1.4447, respectively. The radius of SMF core is 4.1 m and the wavelength of the input light is 1550 nm. The parameters of the multimode fiber chosen are: refractive index of the core is 1.4667 and the core radius is 52.5 m. The corresponding amplitude distribution of the propagation field is presented in Fig. 2. The length of the MMF core section is about 20 mm. An eigenmode for the input SMF at a wavelength of 1550 nm is used as the input field for multimode fiber section and the input light intensity is unitary. The light spreads and converges while propagating along the multimode fiber and it is re-imaged with a cycle about 7.4 mm. The amplitude of the calculated field along the multimode fiber is cycle and symmetry. As shown in Fig. 2, the light intensity from the output SMF is 0.1346 comparing relatively with the unitary input light intensity. As a simulation example, the fiber parameters chosen are the same as those in the above Transmission spectra of a SMS structure with different surrounding refractive index is shown in numerical example. The measurable refractive index of surrounding liquid is supposed to be less than that of the MMF core. In this example, the length of the MMF core is 13 mm and the radius of the MMF core is 90 m, a desirable measurement range is chosen from 1.336 to 1.376. Transmission spectra of a SMS structure with different surrounding refractive index is shown in Fig. 3. The intensity of input light is also unitary. The monotonic red shift of the waveforms can be obtained analogously in the range of refractive index of outside environment from 1.336 to 1.376. Therefore, the effect can be used
0.2
nm,n L eff
1.336 1.346 1.356 1.366 1.376
0.1
0 1.5
1.52
1.54
1.56
1.58
1.6
Fig. 3. Transmission spectra of a SMS structure with different surrounding refractive index.
to indicate the refractive index of the substance surrounding the MMF core by measuring the light intensity from the output SMF. 3. Experimental results and discussion The proposed fiber refractive index sensor is shown in Fig. 1. A section of length L of MMF core segment is inserted between two standard single-mode fibers (SMF-28). The MMF core segment was fabricated with multimode step-index fiber with a core diameter of 105 m and a cladding diameter of 125 m by using chemical etching. The MMF was etched firstly with a 40% hydrofluoric (HF) acid solution for 5 min to a fiber diameter below 105 m, and then the acidic solution is replaced with 10% to slow down etching speed for avoiding the damage on the fiber Ge-doped core. The diameter of the multimode fiber core was around 90 m after the etching process. A section of SMF was spliced to the MMF core firstly on one end, and then the MMF core with a length L was cleaved in the other end. Finally, another section of SMF was spliced to the cleaved MMF core. The fabrication process was simple, and the sensor could be fabricated by using normal fiber splicing machine and conventional tools. To measure the transmission spectra of the proposed sensor, the light from amplified spontaneous emission broadband source (L + C band ASE source) was launched into the proposed sensor, and the light from the sensor was measured by using an AQ6370 optical spectrum analyzer (OSA). All the experiments were carried out in a controlled environment, where the effect of temperature variation
0 -10
1847
0
L = 20 mm
-5
L = 40 mm
-20 -30 1530
1550
1570
1590
1610
Wavelength (nm) Fig. 4. Transmission spectra for different length of MMF core.
Transmission (dB)
Transmission (dB)
J. Wang et al. / Optik 124 (2013) 1845–1848
n =1
-10
1.336 1.341
-15
1.346
-20
1.351
-25
1.362
1.357 1.367
-30 -35 1530
1.372
1540
1550
1560
1570
1580
1590
1600
1610
Wavelength (nm)
n=1
-10
Fig. 6. Transmission spectra of a sensor with different surrounding liquids, the interaction length L was 40 mm.
1.336 1.341
-15
1.351
-25
1.362
1.357 1.367
-30 -35 1530
1558
1.346
-20
1.372
1540
1550
1560
1570
1580
1590
1600
1610
Wavelength (nm) Fig. 5. Transmission spectra of a sensor with different surrounding liquids, the interaction length L was 20 mm.
Re-imaging resonant wavelength (nm)
Transmission (dB)
0 -5
1556 1554 1552 1550 1548 1.33
1.34
1.35
1.36
1.37
1.38
Refractive Index
can be ignored. To maintain the fiber straight and stationary during the experiments, the fiber sensor was fixed on a microscope slide. The sensing sections were cleaned with anhydrous ethyl alcohol, and then dried in air for each measurement, until the transmission spectra returned to its initial state which the surrounding medium around the MMF region was air. The measurement liquid sample used was salt solution providing a range of refractive index from 1.336 to 1.372. The transmission spectra for different lengths of MMF core (L = 20 mm and 40 mm) when the surrounding medium is air are shown in Fig. 4. Interference patterns were clearly obtained from the OSA. The dips in the transmission spectra were due to the re-imaging based on the multimode interference effect [16,18]. When the length of MMF core was 20 mm, one spectral interference fringe 1571.7 nm was observed. The resonant wavelength was suppressed by 19.5 dB and the 3 dB bandwidths is 12.8 nm. When the length of MMF core increased to 40 mm, the observable spectral interference fringes in two distinct spectral regions 1539.3 nm and 1582.1 nm were observed. The resonant wavelengths were suppressed by 20 dB and 25 dB and the 3 dB bandwidths of 11.8 nm and 5.2 nm were shown at 1539.3 nm and 1582.1 nm, respectively. The phenomenon can be explained as follows, when the length of MMF core section was short, the phase differences between their guided modes were small as shown in Eq. (3). The possible interference patterns by the interactions between the modes of the MMF should have very large free spectral range and therefore the less interference fringe signal within the measured wavelength range can be obtained from Eq. (4). Furthermore, the coupling coefficients of all excited modes of MMF depends on the wavelength and coupling length, the transmission loss of the structure is a function of the wavelength [19]. The refractive sensor based on transmission loss has been theoretically studied [20]. However, it requires a very accurate control of the length of MMF core to obtain the optimal measurement sensitivity. Therefore, the fabrication process is difficult. So the refractive index (RI) measurement based on the measurement of the resonant wavelength shift is reported and discussed in this paper. The transmission spectra of the proposed sensor (L = 20 mm) for various surrounding mediums are shown in Fig. 5. The measured re-imaging resonant wavelength at 1571.7 nm where the sensor was exposed in air exhibited a total red shift of 5.72 nm when
Fig. 7. Responses of the wavelength of a sensor to surrounding medium with different re-imaging resonant wavelength.
the surrounding RI was gradually increased from 1.336 to 1.372. Variations of less than 1 dB were observed in the dip loss for various surrounding mediums. The experimental results obtained are consistent with the simulation results obtained earlier. The transmission spectra of the proposed sensor (L = 40 mm) for various surrounding mediums are shown in Fig. 6. The measured reimaging resonant wavelength at 1539.33 nm in air exhibited a total red shift 6.76 nm when the surrounding RI was gradually increased from 1.336 to 1.372. It can be seen that the sensor is more sensitive as the length of the MMF core increases. At the same time, the measured re-imaging resonant wavelength at 1582.08 nm which the sensor was exposed in air which is due to the lower-order modes interference [13], exhibited a total red shift 5.31 nm when the surrounding RI was gradually increased from 1.336 to 1.372. Its resonant wavelength shift is smaller than that of the resonant wavelength 1539.33 nm. Therefore, the sensor is more sensitive due to the higher-order modes interference. The response of the wavelength of the proposed sensor to surrounding medium is shown in Fig. 7 when the re-imaging resonant wavelength is 1539.33 nm. Refractive index sensitivity in an order of 5.41 × 10−5 refractive index unit (RIU) is most likely to be obtained assuming the resolution of OSA 10 pm. Most fiber refractometer such as TFBG and LPG [9] have small sensitivities of about an order of 10−3 RIU for surrounding refractive index values less than 1.4, this refractive index region is very important for biomedical and chemical sensor application. So the proposed sensor shows great potential for chemical and biological application. 4. Conclusion A simple sensor design for the refractive index measurement has been presented and demonstrated theoretically and experimentally. The sensor is composed by inserting a length L section of MMF core by normal fusion splicing between two sections of SMFs. The transmission spectra of the different length (L = 20 mm and 40 mm) of proposed sensor for various surrounding RI mediums are measured. The results show that the re-imaging resonant wavelength exhibited a red shift with the increase of the
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J. Wang et al. / Optik 124 (2013) 1845–1848
surrounding RI on the MMF sensing region. The length of the MMF core is longer, and the refractive measurement is more sensitive. At the same time, the proposed sensor is also more sensitive to the change of refractive index on the MMF sensing region due to the higher modes interference. The detection sensitivity of refractive index of 5.41 × 10−5 RIU is achieved experimentally. Because of its high measurement sensitivity to the refractive index values less than 1.4, the proposed sensor is good application prospect for chemical and biological sensing. Acknowledgments This work was partially supported by the National Basic Research Program of China (973 Program) under grant no 2010CB327804, Science & Technology projects of Zhejiang (2009C11049). References [1] W. Liang, Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber Bragg grating refractive index sensors, Appl. Phys. Lett. 86 (2005) 151122. [2] X. Shu, B.A. Gwandu, Y. Liu, L. Zhang, I. Bennion, Sampled fiber Bragg grating for simultaneous refractive-index and temperature measurement, Opt. Lett. 26 (2001) 774–776. [3] A.N. Chryssis, S.M. Lee, S.B. Lee, S.S. Saini, M. Dagenais, High sensitivity evanescent field fiber Bragg grating sensor, IEEE Photon. Technol. Lett. 17 (2005) 1253–1255. [4] J.H. Chong, P. Shum, H. Haryono, A. Yohana, M.K. Rao, C. Lu, Y.N. Zhu, Measurement of refractive index sensitivity using long-period grating refractive refractometer, Opt. Commun. 229 (2004) 65–69. [5] B.H. Lee, Y. Liu, S.B. Lee, S.S. Choi, J.N. Jang, Displacements of the resonant peaks of a long-period fiber grating induced by a change of ambient refractive index, Opt. Lett. 22 (1997) 1769–1771.
[6] H.J. Patrick, A.D. Kersey, F. Bucholtz, Analysis of the response of long period fiber gratings to external index of refraction, J. Lightw. Technol. 16 (1998) 1606–1612. [7] V. Bhatia, Applications of long-period gratings to single and multi-parameter sensing, Opt. Express 4 (1999) 457–466. [8] T. Guo, H.Y. Tam, P. Krug, J. Alert, Reflective tilted fiber Bragg grating refractometer based on strong cladding to core recoupling, Opt. Express 17 (2009) 5736–5742. [9] G. Laffont, P. Ferdinand, Tilt short-period fiber-Bragg-grating-induced coupling to cladding modes for accurate refractometry, Meas. Sci. Technol. 12 (2001) 675–770. [10] J. Sun, C.C. Chan, Photonic bandgap fiber for refractive index measurement, Sens. Actuators B 128 (2007) 46–50. [11] X. Yu, P. Shum, G.B. Ren, N.Q. Ngo, Photonic crystal fibers with high index infiltrations for refractive index sensing, Opt. Commun. 281 (2008) 4555–4559. [12] W.S. Mohammed, P.W.E. Smith, All-fiber multimode interference bandpass filter, Opt. Lett. 31 (2006) 2547–2549. [13] E.B. Li, X.L. Wang, C. Zhang, Fiber-optic temperature sensor based on interference of selective higher-order modes, Appl. Phys. Lett. 89 (2006) 091119. [14] L.V. Nguyen, D. Hwang, S. Moon, D.S. Moon, Y. Chung, High temperature fiber sensor with high sensitivity based on core diameter mismatch, Opt. Express 16 (2008) 11369–11375. [15] W.S. Mohammed, A. Mehta, E.G. Johnson, Wavelength tunable fiber lens based on multimode interference, J. Lightw. Technol. 22 (2004) 469–477. [16] A. Mehta, W. Mohammed, E.G. Johnson, Multimode interference-based fiber-optic displacement sensor, IEEE Photon. Technol. Lett. 15 (2003) 1129–1131. [17] Qian Wang, Gerald Farrell, Wei Yan, Investigation on singlemode–multimode–single-mode fiber structure, J. Lightw. Technol. 26 (2008) 512–519. [18] Y. Jung, S. Kim, D. Lee, K. Oh, Compact three segmented multimode fiber modal interferometer for high sensitivity refractive-index measurement, Meas. Sci. Technol. 17 (2006) 1129–1133. [19] A. Kumar, R.K. Varshney, A.C. Siny, S. Pratha, Transmission characteristics of SMS fiber optic sensor structures, Opt. Commun. 219 (2003) 215–219. [20] Q. Wang, G. Farrell, All-fiber multimode-interference-based refractometer sensor: proposal and design, Opt. Lett. 31 (2006) 317–320.
Contents lists available at SciVerse ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Refractive index sensor based on all-fiber multimode interference Jianfeng Wang, Yongxing Jin ∗ , Yu Zhao, Xinyong Dong Institute of Optoelectronic Technology, China Jiliang University, Xueyuan Street, Xiasha Higher Education Zone, Hangzhou 310018, China
a r t i c l e
i n f o
Article history: Received 30 December 2011 Accepted 25 May 2012
Keywords: Fiber optic sensors SMS fiber structure Optical fiber interference Refractive index measurement
a b s t r a c t This paper reports the work on the development of refractive index sensors using the multimode interference technology in an optical fiber. The sensor consists of a short section of multimode fiber (MMF), from which the cladding around the core is removed, inserted between two single-mode fibers (SMF). The proposed refractive index sensor is theoretically and experimentally investigated in terms of its ability to shift the re-imaging resonant wavelength as a function of refractive index. It shows that the proposed sensor has a measurement resolution of 5.3 × 10−5 refractive index unit (RIU) for the changes of refractive indices from 1.336 to 1.372. The proposed sensor has a great potential for biological and chemical applications. © 2012 Elsevier GmbH. All rights reserved.
1. Introduction The measurement of refractive index based on all-optical-fiber technology is attractive and promising for their unique advantages, such as immunity to electromagnetic interferences, compact size, and fast response. Therefore, several types of refractive index sensors based on fiber are widely provided. Recently, refractometer based on optical fiber Bragg grating (FBG) [1–3], long-period grating (LPG) [4–7], tilted fiber Bragg grating (TFBG) [8,9], and photonic crystal fiber (PCF) [10,11] have been widely investigated. The principle of LPG refractive index sensor relies on the changes of the effective indices of the cladding modes. The corresponding resonant wavelengths will shift in the transmission spectrum when the external refractive index around the LPG is changed. For a PCF refractive index sensor, the proposed sensing mechanism is based on the detection of the wavelength shift of the photonic band edge with the variation of the ambient refractive index. In the recent years, the multimode interference occurring in singlemode–multimode–singlemode fiber structure has been utilized in the design and fabrication of devices such as bandpass filter [12], temperature sensor [13,14], wavelength tunable fiber lens [15], and displacement sensors [16]. In this paper, the refractive index dependence of the shift of re-imaging resonant wavelength of the proposed sensor is theoretically and experimentally investigated. The proposed sensor consists of a short section of multimode fiber (MMF), from which the cladding around the core is removed, inserted between two singlemode fibers (SMF). The basis for this paper is based on the re-imaging resonant wavelength shifts for
measuring the refractive index change around the multimode fiber core section. 2. Theoretical background The proposed SMS fiber structure consists of input and output single-mode fibers (SMFs) and a sandwiched section of multimode fiber (MMF) core with a specific length L, as show in Fig. 1. The multimode fiber core section sandwiched between two single-mode fibers and has a step-index profile. The single-mode and multimode fibers are assumed to be aligned along the same axis. Moreover, there is on offset between the single-mode and multimode fibers at the two interfaces. Due to the circular symmetric characteristic of fundamental mode of the single-mode fiber, the input light is assumed to have a field distribution of E(r,0). When the light launches into the MMF core, the input field can be decomposed by the eigenmodes (LPnm ) of the MMF. Due to the circular symmetric of input field and an ideal alignment assumed above, only the LPnm modes can be excited [15]. Donate the field profile of LP0m as Fm (r), and neglect the small amount of radiation from the multimode fiber, we have [17] E(r, 0) =
0030-4026/$ – see front matter © 2012 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2012.05.042
cm Fm (r)
(1)
m=1
where cm is the excitation coefficient of each mode and it can be calculated by the overlap integral between E(r,0) and Fm (r)
∞ E(r, 0)Fm (r)rdr cm = 0∞ 0
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Jin).
M
Fm (r)Fm (r)rdr
(2)
The excited mode number of the LP0m multimode fiber M ≈ V/M (V = {2/}a n2c0 − n2cl , where a is the radius of the MMF core, n2c0
1846
J. Wang et al. / Optik 124 (2013) 1845–1848
Input SMF
MMF core
Output SMF Z
L Fig. 1. Schematic configuration of the single-mode–multimode–single-mode fiber structure.
and n2cl is refractive index for the core and cladding of the multimode fiber, respectively and is the wavelength in the free-space). The interference between different modes occurs while the beam propagates along the MMF section. The phase difference between the LP0m and the LP0n (m, n are positive integers) cladding modes can be expressed as [13–15]: ˚m,n =
2(nm − nneff )L eff
=
2nm,n L eff
(3)
where L is the length of the MMF core, nm,n is the effective eff refractive index difference between the LP0m and the LP0n , and is the free-space wavelength. From Eq. (1), the wavelength spacing between two adjacent minima (the free space range) can be expressed as: ≈
2
Fig. 2. Amplitude distribution of the propagation field within the singlemode–multimode–single-mode fiber structure.
0.4
(4)
0.3
It can be seen that the free space range increases as the length of the MMF core decreases. At the same time, the change of refractive index around the MMF region, will also change the effective index difference between the LP0m and the LP0n of the cladding modes. This effect will induce changes in the coupling conditions between the core and cladding modes, which further brings about a shift of the interference fringes. In order to carry out the amplitude of the calculated field along the multimode fiber, a beam propagation method is employed to simulate light propagation within the SMS fiber structure. As a numerical example, a standard single-mode fiber (SMF28) is chosen as the single-mode fiber section and the respective index for the core and cladding are 1.4504 and 1.4447, respectively. The radius of SMF core is 4.1 m and the wavelength of the input light is 1550 nm. The parameters of the multimode fiber chosen are: refractive index of the core is 1.4667 and the core radius is 52.5 m. The corresponding amplitude distribution of the propagation field is presented in Fig. 2. The length of the MMF core section is about 20 mm. An eigenmode for the input SMF at a wavelength of 1550 nm is used as the input field for multimode fiber section and the input light intensity is unitary. The light spreads and converges while propagating along the multimode fiber and it is re-imaged with a cycle about 7.4 mm. The amplitude of the calculated field along the multimode fiber is cycle and symmetry. As shown in Fig. 2, the light intensity from the output SMF is 0.1346 comparing relatively with the unitary input light intensity. As a simulation example, the fiber parameters chosen are the same as those in the above Transmission spectra of a SMS structure with different surrounding refractive index is shown in numerical example. The measurable refractive index of surrounding liquid is supposed to be less than that of the MMF core. In this example, the length of the MMF core is 13 mm and the radius of the MMF core is 90 m, a desirable measurement range is chosen from 1.336 to 1.376. Transmission spectra of a SMS structure with different surrounding refractive index is shown in Fig. 3. The intensity of input light is also unitary. The monotonic red shift of the waveforms can be obtained analogously in the range of refractive index of outside environment from 1.336 to 1.376. Therefore, the effect can be used
0.2
nm,n L eff
1.336 1.346 1.356 1.366 1.376
0.1
0 1.5
1.52
1.54
1.56
1.58
1.6
Fig. 3. Transmission spectra of a SMS structure with different surrounding refractive index.
to indicate the refractive index of the substance surrounding the MMF core by measuring the light intensity from the output SMF. 3. Experimental results and discussion The proposed fiber refractive index sensor is shown in Fig. 1. A section of length L of MMF core segment is inserted between two standard single-mode fibers (SMF-28). The MMF core segment was fabricated with multimode step-index fiber with a core diameter of 105 m and a cladding diameter of 125 m by using chemical etching. The MMF was etched firstly with a 40% hydrofluoric (HF) acid solution for 5 min to a fiber diameter below 105 m, and then the acidic solution is replaced with 10% to slow down etching speed for avoiding the damage on the fiber Ge-doped core. The diameter of the multimode fiber core was around 90 m after the etching process. A section of SMF was spliced to the MMF core firstly on one end, and then the MMF core with a length L was cleaved in the other end. Finally, another section of SMF was spliced to the cleaved MMF core. The fabrication process was simple, and the sensor could be fabricated by using normal fiber splicing machine and conventional tools. To measure the transmission spectra of the proposed sensor, the light from amplified spontaneous emission broadband source (L + C band ASE source) was launched into the proposed sensor, and the light from the sensor was measured by using an AQ6370 optical spectrum analyzer (OSA). All the experiments were carried out in a controlled environment, where the effect of temperature variation
0 -10
1847
0
L = 20 mm
-5
L = 40 mm
-20 -30 1530
1550
1570
1590
1610
Wavelength (nm) Fig. 4. Transmission spectra for different length of MMF core.
Transmission (dB)
Transmission (dB)
J. Wang et al. / Optik 124 (2013) 1845–1848
n =1
-10
1.336 1.341
-15
1.346
-20
1.351
-25
1.362
1.357 1.367
-30 -35 1530
1.372
1540
1550
1560
1570
1580
1590
1600
1610
Wavelength (nm)
n=1
-10
Fig. 6. Transmission spectra of a sensor with different surrounding liquids, the interaction length L was 40 mm.
1.336 1.341
-15
1.351
-25
1.362
1.357 1.367
-30 -35 1530
1558
1.346
-20
1.372
1540
1550
1560
1570
1580
1590
1600
1610
Wavelength (nm) Fig. 5. Transmission spectra of a sensor with different surrounding liquids, the interaction length L was 20 mm.
Re-imaging resonant wavelength (nm)
Transmission (dB)
0 -5
1556 1554 1552 1550 1548 1.33
1.34
1.35
1.36
1.37
1.38
Refractive Index
can be ignored. To maintain the fiber straight and stationary during the experiments, the fiber sensor was fixed on a microscope slide. The sensing sections were cleaned with anhydrous ethyl alcohol, and then dried in air for each measurement, until the transmission spectra returned to its initial state which the surrounding medium around the MMF region was air. The measurement liquid sample used was salt solution providing a range of refractive index from 1.336 to 1.372. The transmission spectra for different lengths of MMF core (L = 20 mm and 40 mm) when the surrounding medium is air are shown in Fig. 4. Interference patterns were clearly obtained from the OSA. The dips in the transmission spectra were due to the re-imaging based on the multimode interference effect [16,18]. When the length of MMF core was 20 mm, one spectral interference fringe 1571.7 nm was observed. The resonant wavelength was suppressed by 19.5 dB and the 3 dB bandwidths is 12.8 nm. When the length of MMF core increased to 40 mm, the observable spectral interference fringes in two distinct spectral regions 1539.3 nm and 1582.1 nm were observed. The resonant wavelengths were suppressed by 20 dB and 25 dB and the 3 dB bandwidths of 11.8 nm and 5.2 nm were shown at 1539.3 nm and 1582.1 nm, respectively. The phenomenon can be explained as follows, when the length of MMF core section was short, the phase differences between their guided modes were small as shown in Eq. (3). The possible interference patterns by the interactions between the modes of the MMF should have very large free spectral range and therefore the less interference fringe signal within the measured wavelength range can be obtained from Eq. (4). Furthermore, the coupling coefficients of all excited modes of MMF depends on the wavelength and coupling length, the transmission loss of the structure is a function of the wavelength [19]. The refractive sensor based on transmission loss has been theoretically studied [20]. However, it requires a very accurate control of the length of MMF core to obtain the optimal measurement sensitivity. Therefore, the fabrication process is difficult. So the refractive index (RI) measurement based on the measurement of the resonant wavelength shift is reported and discussed in this paper. The transmission spectra of the proposed sensor (L = 20 mm) for various surrounding mediums are shown in Fig. 5. The measured re-imaging resonant wavelength at 1571.7 nm where the sensor was exposed in air exhibited a total red shift of 5.72 nm when
Fig. 7. Responses of the wavelength of a sensor to surrounding medium with different re-imaging resonant wavelength.
the surrounding RI was gradually increased from 1.336 to 1.372. Variations of less than 1 dB were observed in the dip loss for various surrounding mediums. The experimental results obtained are consistent with the simulation results obtained earlier. The transmission spectra of the proposed sensor (L = 40 mm) for various surrounding mediums are shown in Fig. 6. The measured reimaging resonant wavelength at 1539.33 nm in air exhibited a total red shift 6.76 nm when the surrounding RI was gradually increased from 1.336 to 1.372. It can be seen that the sensor is more sensitive as the length of the MMF core increases. At the same time, the measured re-imaging resonant wavelength at 1582.08 nm which the sensor was exposed in air which is due to the lower-order modes interference [13], exhibited a total red shift 5.31 nm when the surrounding RI was gradually increased from 1.336 to 1.372. Its resonant wavelength shift is smaller than that of the resonant wavelength 1539.33 nm. Therefore, the sensor is more sensitive due to the higher-order modes interference. The response of the wavelength of the proposed sensor to surrounding medium is shown in Fig. 7 when the re-imaging resonant wavelength is 1539.33 nm. Refractive index sensitivity in an order of 5.41 × 10−5 refractive index unit (RIU) is most likely to be obtained assuming the resolution of OSA 10 pm. Most fiber refractometer such as TFBG and LPG [9] have small sensitivities of about an order of 10−3 RIU for surrounding refractive index values less than 1.4, this refractive index region is very important for biomedical and chemical sensor application. So the proposed sensor shows great potential for chemical and biological application. 4. Conclusion A simple sensor design for the refractive index measurement has been presented and demonstrated theoretically and experimentally. The sensor is composed by inserting a length L section of MMF core by normal fusion splicing between two sections of SMFs. The transmission spectra of the different length (L = 20 mm and 40 mm) of proposed sensor for various surrounding RI mediums are measured. The results show that the re-imaging resonant wavelength exhibited a red shift with the increase of the
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surrounding RI on the MMF sensing region. The length of the MMF core is longer, and the refractive measurement is more sensitive. At the same time, the proposed sensor is also more sensitive to the change of refractive index on the MMF sensing region due to the higher modes interference. The detection sensitivity of refractive index of 5.41 × 10−5 RIU is achieved experimentally. Because of its high measurement sensitivity to the refractive index values less than 1.4, the proposed sensor is good application prospect for chemical and biological sensing. Acknowledgments This work was partially supported by the National Basic Research Program of China (973 Program) under grant no 2010CB327804, Science & Technology projects of Zhejiang (2009C11049). References [1] W. Liang, Y. Huang, Y. Xu, R.K. Lee, A. Yariv, Highly sensitive fiber Bragg grating refractive index sensors, Appl. Phys. Lett. 86 (2005) 151122. [2] X. Shu, B.A. Gwandu, Y. Liu, L. Zhang, I. Bennion, Sampled fiber Bragg grating for simultaneous refractive-index and temperature measurement, Opt. Lett. 26 (2001) 774–776. [3] A.N. Chryssis, S.M. Lee, S.B. Lee, S.S. Saini, M. Dagenais, High sensitivity evanescent field fiber Bragg grating sensor, IEEE Photon. Technol. Lett. 17 (2005) 1253–1255. [4] J.H. Chong, P. Shum, H. Haryono, A. Yohana, M.K. Rao, C. Lu, Y.N. Zhu, Measurement of refractive index sensitivity using long-period grating refractive refractometer, Opt. Commun. 229 (2004) 65–69. [5] B.H. Lee, Y. Liu, S.B. Lee, S.S. Choi, J.N. Jang, Displacements of the resonant peaks of a long-period fiber grating induced by a change of ambient refractive index, Opt. Lett. 22 (1997) 1769–1771.
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