SIR-Central Glass & Ceramic Research Institute, Council of Scientific and Industrial Research (CSIR), 196, Raja S.C. Mullick Road, Kolkata 700032, India
r t i c l e i n f o
rticle history:eceived 11 May 2012eceived in revised form 2 January 2013ccepted 2 January 2013vailable online xxx
eywords:iber optic sensorvanescent field interactiontched fiber
a b s t r a c t
Optical fiber loop resonator (FLR) and fiber loop ring down spectroscopy (FLRDS) are two recent tech-niques used for chemical and gas sensing application based on evanescent field interaction. In chemicalsensing applications, a short region of the fiber core is exposed to the external environment, so thatevanescent field can interact with the chemical. The evanescent field access region in single mode fiberhas been developed practically by etching a small portion of the cladding and it is used as sensor elementin the fiber loop cavity resonator.
In this study, a fiber loop cavity resonator has been theoretically modeled and the shift of cavityresonances due to change of the refractive index of the external medium is presented. The theoreti-cal predictions are compared with experimental results. It is observed that when the sample is applied to
iber ring loop cavityesonatorbsorption spectroscopyhemical sensor
the evanescent field interaction block (EAB), the cavity resonances remained symmetric while the widthof the signal is increased as expected from finesse degradation.
In the second step of this paper, modeling on the optimization of the parameters of EAB is carried outfor the sensor length (1–1.5 mm) and diameter (less than 20 �m) of the etched optical fiber. Modelingparameters are similar with practical realization. Finally, EAB as a sensor element for chemical sensingis used with a 419-ppm cobalt nitrate solution and isopropanol.
. Introduction
An optical fiber loop resonator (FLR) with high finesse can beabricated using a short or long length single mode fiber in a closedath. This closed path is called a fiber loop or fiber ring which
ncludes a low loss cavity [1]. A fiber loop is a powerful optical cav-ty used in many sensing applications [2–6], optical filtering [7–9],ing laser [10–13], fiber gyroscopes [14,15], etc. A direct couplingber loop resonator (FLR) using a directional coupler with smalloupling coefficient is demonstrated in [16]. Recently the taperedber and etched single mode fiber have been used as evanescenteld interaction regions in sensing applications [2,6]. This sensingomponent can be expressed as an evanescent field access blockEAB). In a recent paper (2010), some experimental results of chem-cal effects on a small length (2 m) fiber loop [17,18] are reported.t was observed that the use of evanescent field interaction withhe chemical on a small part of the fiber in the fiber loop cavityesulted in degradation of the finesse value of the cavity resonator.
s a result, a reasonable amount of wavelength shift occurs. Conse-uently it was also observed that the width of the cavity resonanceeak is increasing. These behaviors of the phenomena in fiber
loop are investigated [17,18]. In 2006 the fabrication technique ofbiconical fiber taper sensor was presented [19]. In another exper-iment (2010), the chemical etching technique for fabrication of an“etched fiber-sensor element” [20] is reported. Then the evanes-cent field interaction with some chemicals was investigated andthe optical power variation was clearly observed [20,21]. Similarly,analysis and fabrication of “tapered fiber-sensor element” werealso presented with application as a biochemical sensor [22]. Thedevelopment of the above sensor-elements represents the primarycomponent as an EAB which can be used in the fiber loop cavity.Some relevant and FLR application schemes commonly used in lit-erature are tabulated in Table 1 and it is compared with the presentwork.
This paper, presents a new modeling and theoretical investiga-tion of the resonances of an optical fiber loop cavity resonator. Itpresents the behavioral predictions of the resonance peak due tothe change of the RI of the external medium (with chemical) as wellas the effect without chemicals. It also compares the author’s designwith the experimental realization on chemical effects conductedpreviously [17,18].
In a second approach, the paper further describes new model-
ing on the optimization of evanescent field interaction region orEAB in terms of its length and diameter. From the theoretical opti-mization, the desired parameters are obtained for final fabricationof the EAB. Finally, the fabricated EAB (using the chemically etched
[1] All-fiber ring resonator. It acts like ascanning Fabry-Perot interferometer
Resonator fiber length is 3 m, single mode fiber(core diameter 4 �m). Two rotatable fiberloops acting as quarter wave plates are used tocounter any birefringence
Circulating intensity ratio is 25. Established a model up to finessevalue of 80. Theoretical analysis has been performed to improvefinesse value of the short loop resonator
[2] All tapered-fiber ring resonator ispresented for evanescent wave absorptionbased cavity ring down spectroscopy
All tapered-fiber is made with single modefiber (SMF-28). Overall length of tapered fiberis 28.0 mm and waste diameter of tapered fiberis 10.6 �m
Detected 1-octyne with minimum detection concentration is0.049% in non interacting solvent
[3] Sensing scheme is integrated optics basedring-resonator (using SixNy/SiO2
waveguides) for protein detection
Waveguide width is 1.8 �mSeparation between ring and straight sectionsis 1.6 �mRefractive indices of the waveguide materialswere 1.467 for SiO2 and 1.884 for SixNy
Minimum detectable concentration is 0.3 nM of avidin (protein) ina buffered saline solution. Detection limit is 6.8 ng/ml. Ambienttemperature control and uniform surface coating will enhanceperformance
[4] High-birefringence fiber loop mirror(HiBi-FLM) based on a Fiber SagnacInterferometer scheme. HiBi-FLM astemperature and strain sensor isdemonstrated
Single mode fiber is usedBeat length of high birefringent fiber (HBF) is3.1 mm (at 1550 nm)Length of HBF is 127 mm (for strain), 720 mm(for temperature)
Measurement Temperature sensitivity is 33.9 nW/◦CDuring strain measurement, peak wavelength tuning coefficientobtained is 2.8791 nm/mm in terms of displacement on acantilever beam made with organic glass
[5] Review of relevant experiments based onoptical ring resonator for chemical andbiochemical sensing during the period1985–2012
Different schemes are used in practicalexperiments with SMF for several analytes
Following analytes have been detected in different experiments.Such as ethanol, sodium chloride, ethylene glycol, DNA, protein,cancer biomarkers CA15-3, HER2/ECD, and CEA, viral particles(M13), Cells, VOCs (alkanes, alchohols, etc.)
[6] Fiber loop ring down evanescent field(FLRD-EF) sensing scheme for detection ofDNA and bacteria
Single mode fiber (SMF-28) is usedTotal loop length is 20 m, length of sensor headis 24 cm
Bulk RI based detection of three DNAs, label free detection of DNAand detection of bacteria are demonstrated.
[8] Analysis of an optical filter constructedusing simple ring resonator, electro-opticaldirectional coupler and fiber amplifier
Theoretical model is predicted for ringresonator application
Performance of optical filter as a function of coupling coefficient ofelectro-optical directional coupler and gain of amplifier areanalyzed theoretically
[9] Analysis of optical filter using optical ringresonator
Theoretical model is predicted for ringresonator application. Analysis oftransmission, group delay time and quadraticdispersion properties of an optical filter builtwith ring resonator are done
Expressions to compute Q factor, finesse, complex amplitude ofcirculating wave in ring, and insertion loss are predicted.
[17,18] All-fiber ring resonator. The practicalbehaviors of the phenomena in fiber loop areinvestigated
The resonator is built using SMF-28 fiber.Length is about 2.5 m and used a variable-ratiofiber coupler (1–99%) to inject near-infraredradiation into the cavity. A fiberevanescent-field access block (EAB) allowedthe (evanescent) cavity-field to interact withthe external environment
It is observed that the use of evanescent field interaction with thechemical on a small part of the fiber in the fiber loop cavityresulting degradation of finesse value. As a result a reasonableamount of wavelength shift occurs. Consequently it is alsoobserved that the width of the cavity resonance peak is increasingCavity transmission signals in different cases are obtained with asample (glycerol diluted at 99.5% with D2O)
Present paper(a) Fiber loop cavity resonator scheme forchemical sensing is theoretically modeled.The theoretical predictions are comparedwith practical result of [17,18] above(b) Modeling on optimization of parameters
(a) Length of fiber loop considered is 2.15 m,with fiber data of SMF-28(b) Effective length of evanescent absorptionblock (EAB) fabricated is 1 mm and packagedon silica block
(a) The nature of this theoretical model (Figs. 1 and 2 in presentwork) is same as practically demonstrated in [17,18]. It confirmedthe right way of further experiments(b) EAB is tested successfully for chemical sensing andincorporated in the fiber loop experiments. The EAB is also testedusing CO(NO3)2 solution and with isopropanol
ti
2
tsqcSo
r
F
of evanescent field interaction region (EAB)is performed
echnique) is tested for chemical sensing using cobalt nitrate andsopropanol.
. Principle of operation of an optical fiber loop cavity
The working principle of an optical fiber loop cavity is similar tohat of a Fabry-Perot cavity, where two mirrors are separated by aet distance and one of them is partially reflecting. When the fre-uency (wavelength) of the light source is continuously varying, atertain frequencies, constructive interference occurs in the cavity.imilarly destructive interference occurs to the light escaping outf the cavity.
These are the resonant frequencies of the cavity and are sepa-ated by the free spectral range (FSR). FSR is expressed as
SR = c
2nd(1)
where c is the velocity of light in free space, n is the refractive indexof the medium in the cavity and d is the separation between themirrors of the cavity. The round trip length of the optical powerin the cavity is 2d. Hence (at these resonant frequencies) energy isbuilt up in the cavity. The light output escaping from port 2 (Fig. 1)and circulating intensities entering in port 3 show sharp resonantpeaks.
In the case of optical fiber loop ring down spectroscopy, thechemical sensing is achieved by measuring the ring down time(RDT) and in fiber loop cavity resonator, this is done by measuringthe shift of the cavity resonances.
A fiber loop cavity can be fabricated by forming a section of a sin-gle mode fiber (SMF) and a low loss directional coupler. The output
intensity of the fiber loop cavity is the light intensity coming out viaport 2 of the coupler and circulating intensity is the light intensityentering into port 3. The corresponding electric fields (E2 and E3)are shown in Fig. 1.
162 C.L. Linslal et al. / Sensors and Actuators A 194 (2013) 160– 168
enSc
ˇ
k
w(caie
ˇ
waiao∣∣∣
k)1/
∣∣∣ )e−2˛
(1 −
alT(efoC
Fig. 1. Configuration of a passive optical fiber loop resonator.
The directional coupler couples a small fraction of the opticalnergy into the cavity (ring). At resonance, electric field compo-ents entering port 3 from ports 1 and 4 constructively interfere.imilarly destructive interference occurs when the electric fieldomponents enters in port 2.
The resonant conditions are [16],
01L = 2p� (2)
r = 1 − (1 − �0)(1 − a)e−2˛L (3)
here ˇ01 is the propagation constant of the fundamental modeLP01), L is the length of the loop, p is an integer, kr is the resonantoupling coefficient, �0 is the fractional coupler loss, ̨ is the fiberttenuation coefficient, and a constitutes other losses in the cav-ty including splicing losses. ˇ01 is the solution of the eigenvaluequation of a weakly guiding fiber and can be expressed as
01 = n01ω
c(4)
here n01 is the effective index of the LP01 mode, ω is the opticalngular frequency and c is the velocity of light in free space. ˇ01 Ls the total phase shift experienced by the electromagnetic wavefter one complete rotation in the fiber loop. The circulating andutput intensities of a fiber loop cavity can be expressed as
The plot of output and circulating intensities of a fiber loop cavitys a function of frequency is shown in Fig. 2. The output and circu-
ating intensities show sharp peaks at resonant frequencies (Fig. 2).he values are considered for calculation in the fiber loop cavityFig. 3) with low splitting coupler are four splicing losses 0.01 dBach, power loss of EAB (a parameter) is 0.05 dB (having l = 1 mm),ractional coupler loss (�0) is 0. 05 dB and the fiber attenuation (˛)f 2 m length fiber loop is 0.001 dB as per data sheet of SMF-28.alculated FSR of the cavity is 102.86 MHz and the finesse is 50.
2e−˛L cos ˇ01L(5)
L)
a)1/2(1 − k)1/2e−˛L cos ˇ01L
](6)
Fig. 2. (a) Output intensity, and (b) circulating intensity as a function of frequencyof a fiber loop cavity of length 2 m, ˛L = 0.001 dB, �0 = 0.05 dB, and a = 0.05 dB. FSR ofthe cavity is 102.86 MHz and finesse of the cavity is 50.
3. Fiber loop cavity as chemical sensor
In chemical sensing applications, a short region of the fiber coreof length l is exposed to the external environment, so that theevanescent field can interact with the chemical [17]. This sensingregion is expressed as an evanescent field access block (EAB). Aschematic of a fiber loop cavity for chemical sensing is shown inFig. 3.
3.1. Evanescent field access block (EAB)
An EAB is a short region in an optical fiber, which is used toexpose the evanescent field to the surrounding medium (chem-ical). In the experiment and theoretical analysis, a single mode
fiber (SMF28) is used to fabricate the EAB. The core and claddingdiameters of the fiber are 8 �m and 125 �m respectively. The fielddistribution of modes LP01 and LP02 are calculated theoretically(in Section 4.2) after reducing the cladding diameter to 16 �m by
etching.
Initially a single mode fiber (SMF-28) is considered for chemicaletching. After etching process, the core diameter is unaltered andfinal diameter of the cladding is 16 �m as mentioned in Table 2.
C.L. Linslal et al. / Sensors and Actuators A 194 (2013) 160– 168 163
F
Tteccpfio(iTc
3s
tNe
�
Tlpc∣∣∣
k)1/
∣∣∣ )(1 −(1 −
(csls
Table 2optimized length with etched fiber diameter for different refractive indices of exter-nal medium.
ig. 3. A fiber loop resonator with EAB as a chemical sensor inserted in the loop.
hus the important parameters of this element are the diameter ofhe cladding (d), length of the etching zone (l), i.e. exposed area toxternal medium, porosity or smoothness on the fiber surface afterhemical etching and optical power loss occurred due to reducedladding diameter. These parameters are directly involved for theropagation of electric field from one end to the other end of theber core. The optical power loss of this zone in dry condition (with-ut chemical) is taking significant contribution (a parameter in Eq.3)) when linking/spliced in the fiber loop. The optical power losss measured at the output end during on-line fabrication (etching).otal field loss of the fiber loop cavity is the addition of fractionaloupler loss (�0), fiber attenuation (˛) and the splicing losses.
.2. Theoretical realization of fiber loop cavity with chemicalensor element
In the EAB, the propagation constant of the LP01 mode is changedo ˇ′
01 due to the refractive index (RI) of the external environment.ow, the total phase shift (�) after one complete rotation can bexpressed as
= ˇ01(L − l) + ˇ′01l (7)
he resonant frequencies will shift due to this phase change. Theoss due to the chemical interaction will cause a reduction in theeak of the cavity resonances. The circulating and output intensitiesan be modified using Eq. (7) and can be expressed as
The EAB has an overlay of matching index buffer solutionRI = 1.45). The cavity resonances before and after applying the
hemical is shown in Fig. 4. In this figure, the solid line repre-ents cavity resonance considering only the buffer solution, dashedine represents cavity resonances considering chemical and bufferolution.
2e−˛L cos(ˇ01(L − l) + ˇ′01l)
(8)
a)e−2˛L)
a)1/2(1 − k)1/2e−˛L cos(ˇ01(L − l) + ˇ′01l)
](9)
Fig. 4. Cavity resonances before and after applying the chemical sample on the EABinserted in the loop.
If the RI of the external medium (chemical and buffer solu-tion) is greater than that of buffer solution, the cavity resonancesare shifted toward the left. Similarly, when the RI of the externalmedium is less than that of buffer solution, the cavity resonancesare shifted toward the right. This is shown in Fig. 5. The shift of thecavity resonances with an EAB of length 1 mm is shown in Fig. 5a.Fig. 5b shows cavity resonances with an EAB of length 1.5 mm. Byincreasing the length of the EAB, the cavity resonances in Fig. 5bexhibits a greater shift than that of Fig. 5a. The shift of the cav-ity resonances with different RIs of external medium, for l = 1 mm
and l = 1.5 mm are presented in Table 3. The analysis with l = 1 mmgauge length and diameter d = 16 �m, a frequency shift between
10.315 MHz and 0 MHz is observed for each increment of RI ofchemical in the order of 0.001 up to the RI of silica cladding (1.45).Similarly, when l = 1.5 mm and d = 16 �m, a frequency shift between
164 C.L. Linslal et al. / Sensors and Actuators A 194 (2013) 160– 168
Fs
1cp
3s
lnietccWr
TS
Fig. 6. Experimental realization of cavity transmission signals before and after
ig. 5. Cavity resonances of a fiber loop before and after applying the chemicalample of differing RI for an EAB of length (a) l = 1 mm, and (b) l = 1.5 mm.
5.427 MHz and 0 MHz is observed for each increment of RI ofhemical in the order of 0.001. The theoretical predictions are com-ared with the experimental result (Fig. 6).
.3. Experimental realization of fiber loop cavity with chemicalensor element
In 2010, Gagliardi et al., reported experimental results for a fiberoop cavity resonator for chemical sensing [17]. The cavity reso-ances before and after applying the chemical sample is shown
n Fig. 6. The length of the fiber loop was 2.15 m. The EAB had anffective length of ≈1 mm and was devised to have a 2.6% residualransmission with overlay of matching index (1.467). A solutionontaining glycerol diluted by 5% D2O was used as the sample. The
avity response was stored before and after applying the sample.
hen the sample was applied to the EAB, the width of the cavityesonances was increased due to finesse degradation.
able 3hift of the cavity resonances with different refractive indices of external medium.
applying the chemical sample [17,18] [Figure curtsey: http://www.mdpi.com/1424-8220/10/3/1823/#cite.].
The x axis (time) is directly related to the laser frequency scan-ning and the detector output voltage (y axis) is directly related tothe optical power falling on the detector. The nature of the curve forpractical measurement (Fig. 6), is similar to that of theoretical anal-ysis in Fig. 4 in present work. During practical experiment glyceroldiluted by 5% D2O was used, but in theoretical simulation the RI ofbuffer solution is considered as the RI of glass fiber (1.45). From boththe curve, it is observed that when the sample was applied to theEAB, the cavity resonances remained symmetric while the width ofthe signal was increased as expected from finesse degradation. Thewavelength shift is also observed after applying the chemicals.
4. Optimization of EAB parameters
An etched (reduced cladded) or tapered fiber can be used as anEAB. For a high finesse cavity, the losses introduced by the fiberloop and the EAB should be minimum. The loss in the EAB dependson the length and diameter of the fiber, which is the main part ofthe EAB, as well as the coupling of the LP01 mode and LP0m (m > 1)cladding modes. The different losses such as splicing loss, and cou-pling loss are considered in the analysis (in Section 2). The aim ofthe design is to minimize the mode field coupling loss in the EAB.Working of the sensor is based on the shift in the cavity resonancesof the fiber loop due to the chemical surrounding of the EAB. Thepresent experiment has been conducted in the laboratory in stableroom temperature. Thus the effect of the environmental tempera-ture on the shift of the cavity resonances is negligibly small, whenconsidering the refractive index (RI) of the chemical.
4.1. Etched fiber as an EAB
An etched fiber can be fabricated by partial removal of thecladding layer by using a chemical such as hydro fluoric (HF) acid[20]. The HF acid reacts with the silica cladding which is dissolved inthe acid. During etching, the cladding diameter is gradually reducedwhile the core diameter is unaltered.
4.2. Theoretical realization of the parameters of EAB
A schematic of an etched fiber is shown in Fig. 7. If the refractiveindex of the external medium is less than that of the cladding, theetched section of the fiber acts as a three layer fiber. A three layer
waveguide can support both core (LP01) and cladding modes (LP0m).The n01 and n0m are the effective indices of LP01 and LP0m modesrespectively. The refractive index profile is shown in Fig. 8.
C.L. Linslal et al. / Sensors and Actuators A 194 (2013) 160– 168 165
Fig. 7. A schematic of an etched fiber.
p
w
U
W
W
J
p
q
r
s
wnktIoa
tl
l
wmotm
akbeFevd
Fig. 9. Modal field distributions and cross sections of (a) LP01, and (b) LP02 modes
Table 2 shows the optimized length (theoretical) of an etched fiberwith fiber diameter for three RI value of the external medium.
Fig. 8. Refractive index profile of a three layer waveguide (theoretical).
The eigenvalue equation of a three layer waveguide, whose RIrofile as shown in Fig. 8, can be expressed as [22]
U1W3
�1JKpl − U1W2
W3�0Jrl = W2W2
U1�1Kql − W2
2U1W3�0
Sl (10)
here
1 = �0(k20n2
co − ˇ2)1/2
(11)
2 = �0(ˇ2 − k20n2
cl)1/2
(12)
3 = �1(ˇ2 − k20n2
ex)1/2
(13)
= J′l(U1)
U1Jl(U1)and K = K ′
l(W3)
W3Kl(W3)(14)
l = Il(W2R)Kl(W2) − Il(W2)Kl(W2R) (15)
l = Il(W2R)K ′l (W2) − I′l(W2)Kl(W2R) (16)
l = I′l(W2R)Kl(W2) − Il(W2)K ′l (W2R) (17)
l = I′l(W2R)K ′l (W2) − I′l(W2)K ′
l (W2R) (18)
here �0, �1 are the core and cladding radii respectively, nco, ncl,ex are the RIs of core, cladding and external medium respectively,0 = 2�/�; where � is the free space wavelength, ̌ is the propaga-ion constant, Jl and Yl are Bessel functions of first and second kinds,l and Kl are modified Bessel functions of first and second kinds ofrder l respectively, J′
l, Y ′
l, I′
land K ′
lare the corresponding derivatives
nd R is the cladding to core radius ratio (�1/�0).In an etched fiber, the power in the LP01 mode is mainly coupled
o the LP02 cladding mode, for efficient recoupling of this power theength of the etched fiber should be [19];
= 2b�
ˇ01 − ˇ02(19)
here ˇ01 and ˇ02 are the propagation constants of LP01 and LP02odes and b is an integer. The normalized modal field distributions
f LP01 and LP02 modes of an etched fiber having a cladding diame-er of 16 �m, core RI = 1.46125, cladding RI = 1.45625 and external
edium RI = 1.45, at a wavelength of 1.55 �m is shown in Fig. 9.The effective index (n01 or n02) is directly related to the prop-
gation constant (ˇ01 or ˇ02) by the relation n01 = ˇ01/k0, where0 is the free space wave number. The propagation constant cane calculated by solving the eigenvalue equation [22]. The plot offfective indices of LP01 and LP02 with fiber diameter is shown in
ig. 10. From the figure, it can be seen that n01 is less affected bytching unless the fiber diameter is below 20 �m, but the n02 isarying strongly with etching especially in the range of 0–50 �miameter.
of an etched fiber having a cladding diameter of 16 �m, core RI = 1.46125, claddingRI = 1.45625 and external medium RI = 1.45, at a wavelength of 1.55 �m.
Using Eq. (19), the length of an etched fiber can be optimized,by considering LP01 and LP02 modes in which maximum powertransfer takes place, as a function of fiber diameter. Fig. 11 showsthe optimized length of an etched fiber with fiber diameter (b = 5),when the external medium RI = 1.45 and wavelength 1.55 �m.
Fig. 10. Effective indices of LP01 and LP02 modes of an etched fiber.
166 C.L. Linslal et al. / Sensors and Actuators A 194 (2013) 160– 168
4
lencatc31cfaopddo
sot
5
ci
Fig. 11. Optimized length of an etched fiber with fiber diameter.
.3. Experimental set up for chemical etching technique
When an optical fiber is etched, the evanescent wave of guidedight interacts with the external medium. In 2010, Gangopadhyayt al., demonstrated the experimental results of the etching tech-ique and used it as chemical sensor [20]. A schematic of an onlinehemical etching set up is shown in Fig. 12. This set up consists of
white light source, a chemical chamber for etching, and a detec-or. The overall set up is computer controlled. The detector stationomprises two parts, (i) a silicon detector for the spectral region50–1100 nm and (ii) an InGaAs detector for the spectral range000–1700 nm. The chemical chamber contains HF solution (16 Moncentration). An optical fiber (SMF 28) of length 2 m was usedor the experiment. After removing the outer jacket of the fiber for
length ≈1 mm, it was dipped into the HF solution. The two endsf the fiber were connected to the source and detector, for onlineower monitoring. Since the acid evaporates with a constant rateuring the etching process, the fiber length exposed to the acidecreases linearly with respect to the etching time. A photographf an etched fiber is shown in Fig. 13.
Although the schematic realization of an etched fiber (Fig. 7)howing step etching of fiber cladding, but experimentallybserved that there is also a tapered section at the both end ofhe fiber (Fig. 13).
. EAB (sensor element) packaging
Direct application of the unclad sensor region (EAB) into thehemical would require precision packaging as a low diameter fibers fragile and unstable.
Fig. 12. Experimental set up
Fig. 13. Photograph of etched fiber after packaging on a quartz substrate with silicaepoxy. A small window is open to expose the chemical sample on the etched region(EAB) of the fiber [20].
After etching of fiber with desired diameter and length, thesensing zone is placed on a semi-tubular quartz substrate. It is hav-ing 5 mm diameter and 35 mm length. Both the ends of etched fiberare glued with the silica epoxy, so that the fiber should be straightand fixed robustly. A special epoxy is made/developed in the chem-ical laboratory. The composition of the chemical (Resin:Hardener)is in 1:2 ratio. Finally this adhesive is mixed with SiO2 powder in1:10 ratio. This silica paste is made by stirring properly in vacuum. Itis now used to fix the ends of the etched fiber inside semi-tubularquartz substrate. Heating arrangement for proper curing is doneand around 45 ◦C temperature is applied at the both ends. Thewhole packaging work has been done using on-line power mon-itoring. The power fluctuation due to perturbation and curing ofepoxy is continuously recorded with detector. It assured that thesensor element is alive up to the end of packaging.
In this way, the fiber sensor element can be protected in thesemi-tubular quartz substrate; this is equally applicable to fibers ofsmall gauge length with any cladding size, less than 20 �m (Fig. 13).The final encapsulation of the sensor element or EAB is in the sub-strate and this is shown in Fig. 14. The external pressure cannotreach the sensor element. The small opening in the substrate canbe used to expose the unclad fiber surface to the chemical.
6. Application of etched sensor for chemical sensing
The sensor fiber analysis and attenuation measurement set upis shown in Fig. 12. In the experiment Bentham spectrometer
for fiber etching [20].
C.L. Linslal et al. / Sensors and Actuators A 194 (2013) 160– 168 167
Fi
(olsstt1pB
vdawci2ic
r(Rpc
Fp(crt
ig. 14. Photograph of sensor encapsulation on a quartz substrate a small windows open to expose chemical sample on the etched region (EAB) of the fiber.
Bentham TMc300) is used for the fiber spectral loss measurementf sensor element (EAB). The instrument consists of a broad-bandight source with monochromater by which the scan of wholepectrum (350–1700 nm) can be done. Both the side of the sen-or element is keeping 1 m long fiber. One side is connected withhe source and other end is connected with a detector. Two detec-ors are used, one for the range 350–1100 nm and other for IR region000–1700 nm (InGaAs). The detectors are connected with the datarocessor and amplifier. PC is used to display the output scan usingentham software package.
Initial launch power has been scanned for the whole region ofisible and IR. An attenuation spectrum has been recorded afterropping the chemical on EAB. Fig. 15 is the attenuation spectrafter etching of the fiber in visible region, (a) red (lower plot) isithout chemical sample (dry chamber), (b) (upper plot) using a
hemical of (Co(NO3)2) solution of 419 ppm. The upper plots hav-ng five numbers were captured with (Co(NO3)2) solution in every
min time intervals. Fig. 16 is the attenuation spectra after etch-ng of the fiber in the IR region, (a) without chemical sample (dryhamber), (b) with isopropanol and (c) with water.
When the measurement conducted without chemical, the sur-oundings of etched fiber is covered with air as three layer structure
core, silica cladding and air cladding). After applying chemical, theI of external cladding (cobalt nitrate) is greater than air. Initiallyower leakage will increase and then some power will start to re-ouple in the propagation as predicted. In Fig. 15, after observing a
ig. 15. Attenuation spectra after etching of the fiber in visible region, (a) red (lowerlot) is without chemical sample (dry chamber), (b) (upper plot) with chemicalCo(NO3)2) solution. Upper plots (c1–c5) having five numbers were captured usinghemical (Co(NO3)2) solution in every 2 min time intervals. (For interpretation of theeferences to color in this figure legend, the reader is referred to the web version ofhis article.)
Fig. 16. Attenuation spectra after etching of the fiber into IR region, (a) withoutchemical sample (dry chamber), (b) with isopropanol and (c) with plain water.
large initial shift in the spectrum at visible range there is a powerfall near 1100–1150 nm. Hence it is observed that two differentwaveguides phenomenon taking place before and after using anexternal cladding (chemical). The power variation is due to the re-coupling of modes at the sensing region and it depends on the RI ofthe chemical. Hence, the sensor has been designed and it is workingin principle. This EAB can be used in the fiber loop as discussed inSection 3.
7. Conclusion
A theoretical model for the fiber loop cavity resonator has beenrealized in this paper for the application of an optical fiber chemi-cal sensor. In the sensor application of the fiber loop, the primaryand essential part is the evanescent field interaction region or EABthat is the etched sensor element. A detailed study of the evanes-cent field interaction region for prediction of diameter and lengthhas been carried out. The optimization of the EAB is also carriedout to achieve low power loss. From this theoretical analysis, thedesired dimensions were identified. Thereafter, the experimen-tal fabrication of evanescent field access sensor element has beenimplemented by etching a small portion of single-mode fiber withdifferent gauge lengths (1 mm, 1.5 mm, etc.). During etching thecore diameter was unaltered.
The analysis of frequency shift in the cavity resonances of a fiberloop cavity with different RIs of external medium has been pre-sented. It is observed that the cavity resonances shift toward the leftwhen the RI of external medium is greater than the RI of the buffersolution and toward the right when the RI of external medium isless than the RI of the buffer solution.
The evanescent field access region has been developed byetching a small portion of optical fiber which is suitable for theapplication in the fiber loop cavity. The packaging of the sameetched sensor element has been achieved using a quartz substratewith a suitable glass epoxy. The testing of these EAB has been car-ried out for the functionality of the sensor element with a 419-ppmcobalt nitrate solution and isopropanol. The power fluctuations onthe spectra of cobalt nitrate were also observed in the wavelengthscans.
Finally, a fiber-optic chemical sensor has been presented. The
theoretical design parameters of the fiber loop and the sensor ele-ment as identified in this work are very useful for further researchand suitable for chemical sensing, whose absorption wavelengthis in the wide bands (visible to near IR). An evanescent sensor
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lement or EAB is fabricated and packaged (Fig. 14). A small win-ow is present to expose chemical sample on the etched region ofhe fiber. The EAB can be implemented in a fiber loop with a pre-ision fiber-optic detection scheme such as fiber loop cavity ringown spectroscopy.
cknowledgments
The authors would like to acknowledge the support of theirector, CGCRI, Scientists and staff members of Fibre Optics andhotonics Division, CGCRI, Kolkata. The research is partly fundedy CGCRI in-house project OLP0301. The authors would also likeo acknowledge Prof. V.P. Mahadevan Pillai, Head, Department ofptoelectronics; University of Kerala for his invaluable support
o research students. The authors would also like to thank Mr.ineeth S., student of CUSAT for his help during resubmission of
he manuscript.
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Biographies
C.L. Linslal was born in Kerala, India in 1986. He received Bachelor degree in Elec-tronics and Communication Engineering in 2008 and Master degree in Photonicsin 2011 both from University of Kerala respectively. He did his research work forM.Tech. thesis in CSIR-Central Glass & Ceramic Research Institute (CGCRI) under theguidance of Dr. Tarun Kumar Gangopadhyay, Principal Scientist in Fibre Optics andPhotonics Division. The research work presented in this paper is carried out in CGCRI.Currently he is the Ph.D. student in International School of Photonics (ISP), CochinUniversity of Science & Technology (CUSAT). His research interests are fiber-opticsensors, fiber loop cavity resonator, fiber lasers, etc.
P.M. Syam Mohan was born in Kerala, India in 1986. He received his M.Sc. degreein Electronics from Mahatma Gandhi University, Kerala in 2008 and obtained hisM.Tech. degree in Optoelectronics and Optical Communication Engineering fromthe University of Kerala in 2011. He did his research work for M.Tech. thesis inCSIR-Central Glass & Ceramic Research Institute (CGCRI) under the guidance of Dr.Tarun Kumar Gangopadhyay, Principal Scientist in Fibre Optics and Photonics Divi-sion. The research work presented in this paper is carried out in CGCRI. Currently heis working as Optics Engineer at Vinvish Technologies Pvt. Ltd. Techno Park, India.Presently he is working on CSIR-NMITLI project “Design and development of all-fiber supercontinuum light source” in Collaboration with CSIR-CGCRI, Kolkata. Hiscurrent research interests are Photodynamic Therapy (PDT) Laser for cancer treat-ment, Reflectance spectroscopy based confocal microscope, Fiber lasers and fibersensor.
Arindam Halder was born at Kolkata in 1987. He received his B.Sc. degree inChemistry with Honours in 2007 and M.Sc. degree in Chemistry in 2009 both fromUniversity of Calcutta, India. Currently he is doing his Ph.D. work at CSIR-CentralGlass & Ceramic Research Institute, Kolkata, India as a Senior Research Fellow (CSIR).At present he is engaged for development of fiber laser at NIR region. His cur-rent research interests are spectral analysis of the developed fibers, fabrication ofspecialty optical fibers, spectroscopy and fiber sensors.
Tarun Kumar Gangopadhyay is working as Principal Scientist in Fibre Optics andPhotonics Division, of CSIR-Central Glass & Ceramic Research Institute (CGCRI),Kolkata, India. He graduated Bachelor of Electrical Engineering (B.E.E.) with first classin 1989 and Master of Electrical Engineering (M.E.E.) with first class in 1991, bothfrom the Jadavpur University, Calcutta, India. He graduated Ph.D. in December 2005in the field of fiber-optic sensor from the University of Sydney, Australia. His Ph.D.thesis title is: ‘Measurement of vibration using optical fiber sensors’. Dr. Gangopad-hyay is involved in R&D work of optical fiber sensors, Fiber Bragg Grating sensors(FBG) for smart structures, FBG sensors for Electrical Power line application andfiber-optic components such as bi-directional coupler, WDM coupler in CGCRI. Hiscurrent research interests are development of Bio-medical sensors, Fiber Fabry PerotSensor, fiber optic sensor for chemical detection, fiber ring loop resonator for cavityring down spectroscopy and packaging of fiber-optic sensor, etc. He has authoredseveral SCI journal papers (18 Nos.), international conference papers (23 Nos.) andthree patents. He has done some theoretical research work with the University ofKent, Canterbury, UK. He has done research work in SINTEF, NTNU, Norway, relatedto FBG sensor for On-line temperature monitoring of high voltage (400 kV) overheadtransmission lines for dynamic load regulation. Finally the setup has been installedon the Power Grid transmission in India for continuous three years measurement.
He has some research work as Visiting Scientist at CNR-Istituto Nazionale di OtticaApplicata (INO), Naples, Italy, on fiber-loop resonator, cavity ring down spectroscopyand absorption spectra of molecular gas/liquid species. He is a Senior Member ofOptical Society of America (No. 951188). He is also acting as manuscript reviewerin many SCI journals.
