Optical Fiber Technology xxx (2013) xxx–xxx

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Optica l Fiber Techno logy

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Design and analysis of a refractive index sensor based on dual-core large-mode-area fiber

Koppole Kamakshi a,⇑, Vipul Rastogi a, Ajeet Kumar b

a Department of Physics, Indian Institute of Technology Roorkee, Roorkee, India b Department of Applied Physics, Delhi Technological University, New Delhi, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 September 2012 Revised 15 February 2013 Available online xxxx

Keywords:Refractive index sensor Large-mode-area fiberLeakage loss Dual-core fiberResonant coupling

1068-5200/$ - see front matter � 2013 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.yofte.2013.03.007

⇑ Corresponding author.E-mail address: kamakshikoppole@gmail.com (K. K

Please cite this article in press as: K. Kamaks hi eTech nol. (2013), http://dx.doi.org/1 0.1016/j.yof t

We present a novel co-axial dual core large-mode-area (LMA) fiber design for refractive index sensing. Ina dual-core fiber there is resonant coupling between the two cores, which is strongl y affected by the refractive index (RI) of the outermost region. The transmittance of the fiber, therefore, varies sharply with the refractive index of surrounding medium. This characteristic of the proposed structure has been uti- lized to design a RI sensor. We have analyzed the structure by using the transfer matrix method. Our numerical results show that the proposed sensor is highly sensitive with the resolution of 2.0 � 10�6

around nex = 1.44376. Effect of design parameters on sensitivity of the proposed sensor has also been investigated.

� 2013 Elsevier Inc. All rights reserved.

1. Introductio n

Refractiv e index (RI) sensing is crucial for various industrial applications such as food processin g, chemical and medical diag- nostics. Over the past decades many fiber optic RI sensors have been exploited, aiming to measure the surrounding refractive in- dex [1–12]. The sensing mechanism s utilized in these sensors in- clude Fabry Perot interferometry [1,2], the index-depen dent Bragg waveleng th shift of a fiber Bragg grating (FBG) or long-period fiber grating (LPFG) [3–5] and variation of transmittan ce due tocore-diameter mismatch at splices [9]. Interferom eter-based sen- sors usually consist of two beams; one which serves as a sensing arm and while the other beam used as a reference arm. Sensing of the external RI in this type of sensors can be done by combinin gthe two beams to generate an interference pattern. The main prob- lem with interfero meter based sensors is the complexi ty as they often require a mechanism to split the incoming light into two arms. In the case of fiber gratings (both FBG and LPFG), sensitivity is measured from the shifts of the transmission/reflectance spectra due to the influence of the external RI on the coupling conditions ofthe fiber gratings. Since LPFG couples light from the core mode tothe cladding modes, it is highly sensitive when compare with the FBG-based RI sensor. Negative aspect of the fiber grating sensors is they are expensive because of the stringent grating fabrication processes and wavelength interrogation. Most of these sensors have showed the maximum performanc e in terms of index varia-

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amakshi).

t al., Desig n and analysis of a ree.2013.0 3.007

tion Dn of the order of 10�5. Recently, we have proposed a low-cost core diameter mismatch sensor designed in a single single mode fi-ber [10]. It measures the RI values below that of fused silica with the resolution of about 10�4.

Here, as an alternativ e to the previously presented refractome- ters, we present a novel fiber optic RI sensor based on a co-axial dual-core LMA fiber. To achieve an efficient LMA design we have chosen the design paramete rs in such a way that only the LP01

mode of the structure survives and higher order modes are stripped off. Variation in leakage loss of LP01 mode of the fiber with surroundi ng RI has been utilized in designing the sensor. LMA helps in good coupling of light into the fiber and effective single mode operation helps in achieving sufficiently high sensitivity.We have numerically investigated the effect of design parameters on the response of the sensor. We show the maximum resolution of the sensor to be about 2.0 � 10�6.

2. LMA fiber structure

The schematic of the proposed refractive index sensor set-up isshown in Fig. 1a. It uses a dual-core fiber having refractive-ind exprofile shown in Fig. 1b. The dual core fiber consists of two cores:the inner core of width ain and the outer core of width aout, two de- pressed cladding regions: inner cladding of thickness bin and outer cladding of thickness bout. Both the cores have refractive index n1

that correspond s to the RI of pure silica. The inner and outer clad- ding regions of the fiber have refractive index n2 and n3 respec-tively, which can be attained by using fluorine doping into pure silica using modified chemical vapor deposition (MCVD) or plasma

fracti ve index sen sor base d on dual-c ore large-mod e-area fiber, Opt. Fibe r

Fig. 1. (a) Schematic diagram of proposed refractive index sensor, (b) refractive index profile of a co-axial dual core LMA fiber, and (c) refractive index profile of an etched co- axial dual core LMA fiber.

2 K. Kamakshi et al. / Optical Fiber Technology xxx (2013) xxx–xxx

activated chemical vapor deposition (PCVD) techniques [13]. Here,we define the relative index difference between the inner core and inner cladding as D ¼ n2

1�n22

2n21

. The outer most high-index region, n1

beyond outer cladding make all the modes leaky. Thus, all the modes of the structure suffer from finite leakage loss. Higher order modes can be efficiently stripped off by the suitable choice of de- sign paramete rs. The sensing region is formed by etching the mid- dle portion of the dual-core fiber. The fiber is etched up to outer clad and some portion of the outer clad is also etched to expose it to the liquid to be sensed. The refractive-index profile of the etched region is shown in Fig. 1c. We have analyzed the proposed structure using the transfer matrix method (TMM) to calculate the leakage loss of the fundamenta l mode [14]. In TMM an arbitrary refractive index profile is divided into a large number of homoge- neous layers by using the staircase approximat ion. The relation- ship between the field coefficients in the layers can be derived by applying the boundary condition s at the interface of two con- secutive layers, which was given by a 2 � 2 matrix. The field coef- ficients of the innermost and the outermost layer of the profile can then be connected by simply multiplyi ng the transfer matrices ofall the intermedi ate layers. By applying suitable boundary condi-

Fig. 2. Spectral variation of leakage loss of the LP01 mode.

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tions in the innermost and outermost layers, a complex eigenvalue equation for propagation constant (b) is formed. Then the leakage loss of a mode can be calculated from the imaginary part of the propagat ion constant (bi) by using the following relation [15]:

Leakage loss ¼ 8:868k0ImðneffÞ ð1Þ

We have then calculated the correspondi ng transmittan ce in3 cm length of the fiber.

3. Numeric al results and discussion

The cladded fiber is a five layer structure with the parameters D = 0.006, ain = 14 lm, bin = 3 lm, aout = 10 lm, bout = 26 lm, and n3 = 1.4435. A fiber with these design parameters introduce s678 dB/m leakage losses to the LP11 mode and leaks out all the higher order modes in 3 cm of propagation length, however, the fi-ber also introduces a nominal loss of 0.01 dB to LP01 mode during this propagation length. Typical length of the cladded fiber before the sensing region is few tens of cm, which is sufficient to strip-off higher order modes and the fiber in the sensing region works assingle-m ode fiber.

The width of outer cladding in the sensing region has been ta- ken bout = 3.5 lm unless stated otherwise. We have carried out the numerical simulations on the performance analysis of the pro- posed design by calculating the leakage loss of the fundamenta lmode. Spectral variation of leakage loss of the fundamenta l mode for three different values of nex is shown in Fig. 2. The leakage loss curves shown in Fig. 2 are in fact the tails of resonance peaks and show that the resonance between the two cores is highly sensitive to nex. Since the higher value of nex makes the outer core more lea- ky, one can see that the resonant waveleng th shifts towards longer waveleng th side as the value nex decreases and lower value of nex

shows small spectral variation of leakage loss.To investigate the RI sensing characteristics of the fiber we have

studied the variation of leakage loss at k = 1550 nm as a function ofRI of the last layer is shown in Fig. 3. When we increase nex it taps more power from the inner core and the leakage loss of LP01 modeincreases. A sharp increase in leakage loss of the mode for nex > 1.44378 is due to the resonant leakage of power from inner

fracti ve index sen sor base d on dual-c ore large-mod e-area fiber, Opt. Fibe r

Fig. 3. Variation of leakage loss of fundamental mode with nex.Fig. 5. Transmittance versus the refractive index of the external medium (nex) inthe range nex = 1.4436 to nex = 1.4439 with k = 1550 nm for different values of bin.

K. Kamakshi et al. / Optical Fiber Technology xxx (2013) xxx–xxx 3

core to the outer core. This makes the device highly sensitive to the surrounding RI. Transmittan ce of proposed RI sensor as a function of RI of the last layer for three different lengths is shown in Fig. 4.The leakage loss of the structure is such that with 3 cm sensing length the transmittan ce varies from 100% to 20% in the sensing range of RI. A smaller length would reduce this variation to a smal- ler span as shown in Fig. 4 and 3 cm is also a practicall y suitable length of the sensing region for realization of the sensor. A longer length would bring down the transmittance to a much smaller va- lue and would require more sensitive detectors while using the source of moderate power. It is also clear that the response ofthe sensor is nonlinear and it has different sensitivit ies in different regions. The resolution of the sensor, which we define as Dn vari-ation for 1% change in transmittance is 1.01 � 10�5 around the RI1.44368. The resolution improves to 2.0 � 10�6 aroundnex = 1.44376. Such a resolution correspond s to the one obtained in surface plasmon based sensor [16].

We have also studied the effect of design parameters on the sensitivity of the proposed sensor. Fig. 5 shows the effect of inner clad width (bin) on the variation of transmittan ce with nex. From the figure it is obvious that the sensitivity changes slightly with bin. For example, for 2 lm increment in bin from bin = 3 lm, the transmittan ce varies from 90% to 64% in the sensing range 1.44382–1.44386, while 2 lm decrement in bin gives the transmit- tance variation from 90% to 19% in the range 1.4436–1.44375.However, one can notice the change in high sensitivity RI range.On increasing bin the coupling between the two cores becomes nar- rower. This is because the resonance peak becomes sharper when the separation between the two cores increases [17].

Fig. 4. Sensor response to the RI of external medium for different sensing lengths.

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We have also studied the impact of outer core width (aout) and outer clad width (bout) on the sensitivity of the sensor and the re- sults are summarized in Figs. 6 and 7. It is clear from the figuresthat the outer cladding thickness and outer core width do not have significant effect on the sensitivity of the sensor. The variation ofthe transmittance of the fiber changes from 91% to 41% in the range 1.44380–1.44388 for aout = 12 lm, while aout = 8 lm results inchange of transmittan ce of the fiber from 91% to 19% in the range 1.44366–1.44374 as shown in Fig. 6. Fig. 7 corresponds to the var- iation of the transmittan ce as a function of nex for the changes inbout. The high sensitivity RI range is 1.44373–1.44380 for 1 lmincremen t in bout from bout = 3.5 lm and 1.44376–1.44385 for 1 lm decrement in bout. It can be clearly seen that even if the sen- sitivity does not vary significantly, the high sensitivity sensing range shifts with bout and aout. This shifting is due to the change in resonance wavelength.

We have then worked out the tolerances with respect to n2 andn3 on the sensitivity of the proposed sensor. Our results show that a small change of ±5 � 10�4 in the values of n2 does not cause any significant effect on the sensitivit y of the sensor as this change isquite small as compared to the index difference between n1 andn2. However, a variation of ±2 � 10�4 in n3 shifts the resonance waveleng th significantly and hence the range of highly sensitive region shifts as shown in Fig. 8. The RI range of the proposed sensor corresponds to the chemical substances like W25240 – Diesel Clean-Up (RI = 1.4436 @ 20 �C) which can be exploited as diesel fuel additive and sulfuryl chloride which is a source of chlorine.

We have also designed the DCRLF based RI sensor for the range 1.4439–1.4444 which corresponds to RI of Cassava seed oil (1.444)

Fig. 6. Transmittance versus the refractive index of the external medium (nex) inthe range nex = 1.4436 to nex = 1.4439 with k = 1550 nm for different values of aout.

fracti ve index sen sor base d on dual-c ore large-mod e-area fiber, Opt. Fibe r

Fig. 7. Transmittance versus the refractive index of the external medium (nex) inthe range nex = 1.44372 to nex = 1.44386 with k = 1550 nm for different values ofbout.

Fig. 8. Effect of outer cladding index on transmittance.

4 K. Kamakshi et al. / Optical Fiber Technology xxx (2013) xxx–xxx

[18]. The various design paramete rs are D = 0.006, ain = 14 lm,bin = 1.5 lm, aout = 6.5 lm, bout = 2.5 lm, n3 = 1.4425 and l = 3 cm.The transmittan ce of the fiber is plotted in Fig. 9. We can see the variation in transmittan ce is from 86% to 36% and the maximum resolution of the sensor is 6.93 � 10�6.

In practical implementati on the sensing region is kept straight in an enclosure filled with the test liquid but the cladded fiber be- fore and after the sensing region may undergo bending. However ,

Fig. 9. Transmittance versus the refractive index of the external medium (nex) inthe range nex = 1.4439 to nex = 1.4444 with k = 1550 nm.

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such bends will have large bending radii. We have analyzed the bending performanc e of the proposed fiber using the method de- scribed in [19], which is a suitable method for calculating the bend loss of the multilayer, fiber such as the one proposed here. Bending loss ab (dB/km) of the fiber is given by [19]

ab ¼ 1:09ffiffiffiffiffiffiffiffiffiffiffiffiffi

paRW3

rSðV ;WÞ exp

�4RW3D

3V2a

" #ð2Þ

SðV ;WÞ ¼ a2

K20ðWÞ

Z 1

0

E2ðqÞE2ðaÞ

qdq

" #�1

; W ¼ b1=2V ð3Þ

where E(q) represen ts the radial field distribution in the straight fi-ber, a denotes the fiber core radius and R is the bending radius.Other paramete rs have their usual meaning. Eq. (2) represe nts anapproxi mate pure bend loss formula applicabl e to arbitrary refrac- tive-in dex profile fibers and neglects small corrections due to fielddeformat ion at the curved fiber. Such an approxi mation is justifiedin view of high sensitivity of bend loss with radius of curvature [19].Our results show bending loss less than 0.1 dB/m at bending radii larger than 20 cm.

4. Conclusi ons

We present a novel co-axial dual core LMA fiber based high sen- sitivity refractive index sensor. We have investigated the perfor- mance of the sensor with respect to the design parameters.Proposed sensor is highly sensitive in the range 1.44375–1.44382where the transmittan ce of the fiber varies from 90% to 25%. Weexpect that the structure would have ample potential in chemical sensor applications .

Acknowled gments

One of the authors (K. Kamaksh i) acknowled ges the financialsupport provided by Indian Institute of Technology , Roorkee, Min- istry of Human Resources and Development (MHRD) Governmen tof India. This work has been partially supported by the UK–IndiaEducation and Research Initiative (UKIERI) major award on ‘‘Appli- cation specific microstructur ed optical fibers’’. We also acknowl- edge help of our student Seema in carrying out some of the calculatio ns.

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fracti ve index sen sor base d on dual-c ore large-mod e-area fiber, Opt. Fibe r