Characteristics Analysis of Chemical Concentration Sensor Based on Three-layer FBG

doi:10.4236/opj.2013.32B063

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Optics and Photonics Journal, 2013, 3, 268-271

doi:10.4236/opj.2013.32B063 Published Online June 2013 (http://www.scirp.org/journal/opj)

Characteristics Analysis of Chemical Concentration Sensor

Based on Three-layer FBG

Zhaoxia Wu, Xinyan Yu, Erdan Gu, Zhi Kong, Wenchao Li

College of Control Engineering, Northeastern University at Qinhuangdao, Qinhuangdao Hebei 066004, China

Email: [email protected]

Received 2013

ABSTRACT

The three-layer structure model of the fiber Bragg grating is proposed in this paper. And through experimental study

and reasoning calculation to verify the three layer structure of the optical waveguide model, so the sensitivity character-

istic of the Bragg wavelength to the refractive index of chemical solution is obtained. And the relationships between the

concentration and the shift of Bragg wavelength of sucrose, ethanol, and Nacl solution are achieved. Finally the shifts of

the Bragg wavelength with the external concentration are obtained by experiments. A kind of simple structure, small

size, good stability, high sensitivity chemical concentration sensor is obtained.

Keywords: Fiber Bragg grating(FBG); Three-layer Structure; Refractive Index; Concentration of Solution; Chemical

Sensor

1. Introduction

The concentration of solution is considered as one of the

integrated indicators of medium solution. The concentra-

tion measurement and control of solution is widely used

on papermaking, chemical, sugar production, food indus-

try, environmental monitoring, etc. It is the important

technical means guarantee product quality and improve

the quality of products. Fiber Bragg grating (FBG) has

many virtues such as resistant to electromagnetic inter-

ference, high insulating, corrosion resistance small bulk,

low weight, high sensitivity, absolute code of wavelength,

compare with fiber, easy to reuse into network, etc. The

liquid concentration sensor takes FBG as its sensitive

element has positive research significance and potential

application prospect [1, 2]. The fiber Bragg grating sen-

sor plays an important role in the measurement of liquid

substance. It can distinguish the content of measured

substance by measuring wavelength change of reflection

center [3]. Therefore, improve the sensitivity of the fiber

Bragg grating on the refractive index of the surrounding

medium in the existing technical is the key [4]. At pre-

sent there are mainly two kinds of fiber grating sensors

used for measuring the refractive index change, fiber Bragg

grating (FBG) and long period fiber grating (LPFG) [5].

As a result of the coupling based on the core mode and

cladding mode, the external refractive index change will

have a greater impact on fiber cladding mode transmis-

sion properties; therefore the long period fiber grating

has a larger sensitivity. But the long period fiber grating

being very sensitive to optical fiber bending and laying

state, the high order cladding mode transmission proper-

ties being unstable, and its multi-Peak Value, large

bandwidth and the working mode of using the transmis-

sion spectrum only to detect limit its measuring precision

and multiplexing capability[6]. Currently, the refractive

index sensor based on fiber Bragg grating has been re-

ported [7, 8], but there are no in-depth theoretical analy-

sis, numerical calculations and experiment contrast re-

sults. This paper focuses on the FBG chemical sensor’s

theoretical analysis and establishes the three-layer struc-

ture model, while the three solutions of different concen-

trations were experimentally, and realize the high sensi-

tivity measurement of chemical solution concentration.

The sensor doesn’t need coating intermediate sensitive

material, can measure various chemical solution concen-

trations. The sensor has the advantages of simple struc-

ture, high precision, low cost, eases of reuse, can realize

on-line measurement and so on. Also it has broad appli-

cation prospects.

2. Theoretical Mode

Based on the fiber grating coupled mode theory and the

phase matching condition, the wavelength



of FBG

is:

eff





 (1)

where, neff is the effective refractive index of core region

of the fiber, Λ is the grating period.

Z. X. WU ET AL. 269

As the cladding thickness decreases (usually a few

microns), evanescent waves project through the cladding

layer. Under the effect of the evanescent wave, the effec-

tive refractive index of fibers and solution index have a

dependency relation [9]. The refractive index of solution

changes with the change of the concentration of solution.

Bragg wavelength changes with the change of the effec-

tive refractive index. So the response to the concentration

of chemical solution can realize by measuring the Bragg

wavelength to reflect the change of the concentration of

chemical solution.

First, discussed the changes of the external refractive

index SRI ranged from 1.32 riu to 1.46 riu and its effect

on FBG effective refractive index: suppose, λ is the op-

erating wavelength of LP01, neff is the refractive index of

LP01. And it is shown in Figure 1. The LP01 model’s

characteristic equation can be expressed as:

 

 

 

00 0

11 11

'/ '

''/''

Ju KvcKv Iv

Kv KvJuIvc

IvcKv IvKvc























(2)



2eff

uann



 (3)



2eff RI

vbnS



 (4)



'2 eff

vbnn



 (5)

cab (6)

where, a is the fiber radius, b is the cladding radius after

etching, n1 and n2 is respectively the core and the clad-

ding refractive index, J is the Bessel function of the first

kind, I is the variant Bessel function of the first kind, K is

the variant Bessel function of the second kind.

The reflectivity and transmissivity expression of the

Fiber Bragg grating are deduced through coupled mode

equation and boundary condition of Fibber Bragg grating,

in the phase matching condition, corresponding to the

maximum reflectivity and transmissivity:



max tanh tanh

RSL

















(7)

abr

core

cladding

S urrounding

medium

Figure1. Optical waveguide model of three-layer structure.



max coshTar SL (8)

Where







, k is the coupling coefficient,

Δβ is phase-mismatching condition.

So, the reflectance spectrum between Bragg reflection

wavelength and change of SRI offset size is obtained

through the reflective index change of solution. It can be

more clearly viewed how the external solutions influence

the Fiber Bragg grating center wavelength.

3. The Simulation Results

For the specific fiber grating, solutions meeting equation

(1) - (6) can be found if given the SRI range. And the

solutions are just the FBG switch resonant wavelength of

corrosion layer and the effective refractive index of neff.

The simulation experiment use the optical fiber type of

SMF-28TM which is used more commonly in communi-

cation, some specific parameters: n1 = 1.46810, n2 = 1.46281,

a = 4.15 μm. Set the main structure parameters of corro-

sion FBG: the grating period: Λ = 528.93nm; corrosion

zone diameters are respectively: DTH = 9.3 μm, 10.3

μm, 12.3 μm, 14.3 μm, 16.3 μm; the range of SRI: 1.32 -

1.46, then assuming that temperature is a constant. For a

given optical parameters, solve the characteristic equa-

tion (2) when the wavelength



= 1550 nm, and then get

the effective refractive index of neff. As shown in Figure

2 uniform corrosion of FBG in different cladding radius,

there is a nonlinear relationship between the Fiber Bragg

grating effective refractive index and changes of the ex-

ternal solution refractive index SRI.

The refractive index spectral shows that the sensitivity

of corrosion area grating is associated with SRI directly.

There exists a red—shift compared to the Bragg wave-

length, when the external solution refractive index in-

creases. Due to its fundamental guide model has a better

restriction in this fiber core range, and resulting in a

slight evanescent wave, when SRI is about 1.34, the ex-

ternal refractive index has little effect on wavelength

offset. When the external refractive index is close to the

1.321.34 1.36 1.381.4 1.42 1.441.46

1.463

1.4635

1.464

1.4645

1.465

1.4655

eff

b=4.65m

b=5.15m

b=6.15m

b=7.15m

b=8.15m

b=62.5m

Figure 2. Different cladding radius effective index of fun-

damental mode with cladding refractive index changing.

Z. X. WU ET AL.

270

cladding refractive index (SRI = 1.46), the offset reaches

a maximum, at the same time the sensor reaches the

maximum sensitivity; secondly, from (a) - (d) we can

know that the smaller the thickness of the cladding layer,

the higher the sensitivity of the sensor. So the relation-

ship between the FBG wavelength and the change of the

solution refractive index is obtained.(Figure 3)

15481548.4 1548.8 1549.2 1549.61550

0.2

0.4

0.6

0.8



/nm

P/Reflectivity

SRI=1.32



=1548.10

SRI=1.34



=1548.14

SRI=1.36



=1548.20

SRI=1.38



=1548.28

SRI=1.4



=1548.39

SRI=1.42



=1548.55

SRI=1.44



=1548.84

SRI=1.46



=1549.71

b=4.65um

(a) b=4.65μm, Δλ=1.587nm

1548.2 1548.615491549.4 1549.8 1550.2

0.2

0.4

0.6

0.8



/n m

P/Re fl ectivi ty

SRI=1.32

SRI=1.34

SRI=1.36

SRI=1.38

SRI=1.4

SRI=1.42

SRI=1.44

SRI=1.46

b=5.15um

(b) b = 5.15 μm, Δλ = 1.05786 nm

1549.2 1549.4 1549.6 1549.8 1550 1550.2

0.2

0.4

0.6

0.8



/nm

P/Reflectivity

SRI=1.32

SRI=1.34

SRI=1.36

SRI=1.38

SRI=1.4

SRI=1.42

SRI=1.44

SRI=1.46

b=6.15um

1549.8 1549.85 1549.9 1549.95 15501550.05 1550.1 1550.15 1550.2

0.2

0.4

0.6

0.8



/n m

P /Refl e ctivi ty

SRI=1.32

SRI=1.34

SRI=1.36

SRI=1.38

SRI=1.4

SRI=1.42

SRI=1.44

SRI=1.46

b=8.15um

(d) b = 8.15 μm, Δλ = 0.10579 nm

Figure 3. The refractive index spectrum of FBG in different

cladding radius.

4. Experimental Analysis

The measurement experiment structure diagram of the

concentration sensor based on Fiber Bragg grating as

follows: (Figure 4)

In order to study the sensitivity of the sensor, we select

sucrose, Nacl, ethanol three solution for experiment, analy-

sis the effects of different solutions at different concen-

trations (C less than 80%) to the fiber grating wavelength,

eventually complete the high precision measurement of

the chemical concentration sensor based on Fiber Bragg

grating.

The experimental data of three kinds of solution con-

centration C and refractive index can be measured by

WAY-type Abbe refract meter, and combine the partial

least squares, the linear expressions of the three kinds of

solution are established. The expressions are as follows:

Sucrose n = 1.3285 + 0.00184c

Nacl n = 1.3331 + 0.00185c

Ethanol n = 1.3404 + 0.00033c

where, C is the solution quality percentage concentration,

n is the refractive index. So the diagrams of the three

kinds of solution concentration and refractive index are

obtained. And it is shown in Figure 5. It can be seen

from the figure, there are no significant differences be-

tween the values calculated and the true value of the re-

fractive index of the three kinds of solution.

So when we take a fixed cladding radius, the diagram

of the wavelength of FBG with the change of concentra-

tion of external solution obtained, through the relation-

ship between external solution refractive index SRI and

the effective refractive index of Fiber Bragg grating neff,

Figure 4. The experimental structure diagram.

Figure 5. The correlation between concentration and refractive

index for different solutions.

Z. X. WU ET AL.

271

Figure 6. Relationshio between wavelength and sucrose

solution concentration.

the relationship between the effective refractive index neff

and resonant wavelength λB, and the relationship between

concentration of solution c and its refractive index SRI.

Figure 6 shows that, when measuring the 0 - 80% con-

centration alcohol solution, the center wavelength of fi-

ber grating changes a little with the change of the con-

centration of solution, and there has an approximate lin-

ear relation; when measuring Nacl and sucrose solution,

the change of wavelength with solution concentration

changing is obviously and show monotonically increas-

ing nonlinear relation. The wavelength of the fiber grat-

ing sensor changes obviously when the concentration of

Nacl changes. Therefore through the simulation we can

know that the sensitivity of the sensor is the largest and

the maximum sensitivity of the sensor is up to 185.59

pm/c%.

Therefore the sensitivity of the sensor is the largest,

from simulation we know that the maximum sensitivity

of the sensor is up to 185.59 pm/c%.

4. Conclusions

The paper combines the optical waveguide theory, fiber

grating coupling theory and evanescent field and partial

least squares method theory. Though characteristics

analysis and experimental of three-layer structure of the

Fiber Bragg grating sensor accomplished. An experiment

on the external solution concentration (c = 80%) is com-

pleted with the chemical sensors based on Fiber Bragg

grating, the spectrum and sensitivity of different concen-

tration of solution obtained. The sensor can be used to

measure the liquid environment chemical or biological

compositional changes, and can realize multi-point de-

tection at the same time, thus possessing good applica-

tion prospect in the filed of medical, pharmacy, petro-

chemical industry, catering trade, etc.

5. Acknowledgements

This work is supported by the Fundamental Research

Funds for The Northeastern University at Qinhuangdao.

(XNK201308).

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