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A novel fiber Bragg grating sensors multiplexing technique

Gao Hongwei *, Li Hongmin, Liu Bo, Zhang Hao, Luo Jianhua,Cao Ye, Yuan Shuzhong, Zhang Weigang, Kai Guiyun, Dong Xiaoyi

Institute of Modern Optics, Nankai University, Tianjin 300071, China

Received 25 October 2004; received in revised form 15 March 2005; accepted 15 March 2005

Abstract

We bring forward a novel FBG sensor multiplexing technique based on the combination of wavelength- and spatial-

division multiplexing technique. Furthermore, the data acquisition parameter of the multiplexing sensing FBG system is

defined. With this technique, the FBG sensors system not only avoid frequently switching and improve the average

response speed, but also are able to enact data acquisition rule according to the data acquisition factor that is composed

with priority (PRI), the delay time and the wavelength shift of FBG sensors. Therefore, the smart FBG sensors

multiplexing system is more rational and smarter than the conventional counterpart. The simulate result and the exper-

iment work demonstrated that the average delay time of smart system is less than conventional ones and immune to thescale size.

2005 Elsevier B.V. All rights reserved.

PACS: 42.65.Sf

Keywords: Fiber Bragg gating; Wavelength-division multiplexing; Spatial-division multiplexing; The information factor; Smart

1. Introduction

One of the attractive advantages of an FBG sen-

sor system is the multiplexing capability [1,2].Many multiplexing schemes have been proposed

based on techniques including time [3,4], space [5]

and wavelength-division multiplexing [2,6]. For

applications requiring a larger multiplexing gain,

a combination of the WDM, TDM and SDM tech-

niques may be used [7–10]. Among these tech-niques, the WDM/SDM technology has several

advantages, such as simple structure, credibility

and feasibility, etc. But there are still some

disadvantages.

In this paper, we put forward a novel combina-

tion of WDM and SDM technology. The multi-

plexing technique only scan and demodulate

0030-4018/$ - see front matter 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.optcom.2005.03.027

* Corresponding author. Tel.: +86 222 350 9849; fax: +86 222

350 8770.

E-mail address: [email protected] (H. Gao).

Optics Communications 251 (2005) 361–366

www.elsevier.com/locate/optcom

mailto:[email protected]

mailto:[email protected]



FBG sensors whose peak wavelength is shifting.

That avoids the optical switch frequently switch-

ing, prolongs the life of optical switch and shortens

the delay time. Moreover, We bring forward thedata acquisition parameter concept that makes

the data acquisition rule is more rational. Com-

pared with the conventional WDM/SDM system,

the smart FBG sensor system is more flexible

and smarter that is required in some large scale

or complex systems.

2. The conventional WDM/SDM technique

A primary advantage of using FBG for distrib-uted sensing is that a great amount of sensors may

be interrogated along a single fiber. WDM pro-

vides for tens sensors per fiber, but SDM can mul-

tiply this number several times by re-using of the

spectrum of the source. With combination of

WDM and SDM illustrated in Fig. 1, light from

a super fluorescent source (pumped Erbium doped

fiber) propagates through a 3 dB coupler and an

optical switch (OS) to enter the sensing FBG ar-

rays. These arrays consist of sets of FBG sensors

whose peak wavelengths are different. Accord-

ingly, the reflection light from FBG successively

returns to the detector. To re-using the super fluo-

rescent source and detector, the conventional data

acquisition rule selects different channels by aim-

less frequently switching the optical switch. In

order to locate sensor positions, the multi-

wavelength signals that are affected by FBG array

are demodulated into single ones by an adjustable

narrow filter. Analyzing the electric signals that

come from optical detector, we can obtain the

information of the probing area.

3. The smart fiber Bragg grating sensors system

The conventional WDM/SDM technology has

several advantages, such as simple structure, cred-

ibility and feasibility, etc. However, some disad-

vantages still remain:

(1) According to the conventional data acquisi-

tion rule, the optical switch aimless cycle

switching between different channels.

Whether the wavelength of FBG sensorsshifts or not, demodulation part has to scan

and demodulate every channel, which makes

the delay time longer. In order to shorten the

delay time, the optical switch has to be

switched as frequently as possible. Then,

with the limit switching number, the lifetime

of optical switcher is short. Therefore, the con-

ventional data acquisition rule is irrational.

(2) Whether important or not, whether how long

the delay time is, whether how much the

wavelength shifts, every optical channels

has the same priority to be demodulated.

The conventional data acquisition rule can-

not meet the smart demand of some large

scale or complex systems.

As shown in Fig. 2, a long period fiber grating

(LPG), a 3 dB coupler and a photodetector are

placed in the front of every fiber channel. As the

wavelength shifts, the output of photodetector

Broad Band

Source 3dB

Optical

Swithcer

Channel 2FBG 21

FBG1n

FBGn1 FBGnnChannel n

FBG2n

Channel 1FBG11

ComputerPhoto

Detector

Filter

Fig. 1. Scheme of the conventional WDM/SDM multiplexing FBG sensing system.

362 H. Gao et al. / Optics Communications 251 (2005) 361–366



would change because of the convolution effect of FBG and LPG [11]. According to the output of

photodetector, the computer judges which FBG

sensing channel detects the environmental fluctua-

tion. While wavelength shifting, the demodulation

part only demodulates the channels whose wave-

lengths are shifting. Without aimless frequently

switching, the average response speed of demodu-

lation part is improved and the life of optical

switch is prolonged. As wavelengths of two or

more channels shift, computer records when and

which channels are shifting to avoid the informa-tion loss Then, according to the data acquisition

parameter that we bring forward, demodulation

part will switch to and demodulate those channels

in turn. Therefore, compared with the conven-

tional SDM/WDM system, the smart FBG sensing

system is more rational.

As wavelengths of different channels may shift

simultaneously, the switching of optical switch is

a competition between different channels, causing

data acquisition conflict. To avoid conflicts and

improve data acquisition rule, we bring forward

‘‘data acquisition parameter’’ concept that consist

of priority, the delay time and the wavelength

shift. With the LPG linear demodulation edge fil-

ter technique, we attain the wavelength shift [11]:

I iðkÞ ¼Xn

k ¼1

Z 1

1

Rik ðk k0Þ H iðk0Þ dk0; ð1Þ

where I i (k) is the optical signal power of i th optical

channel, H i (k) is the LPGs transmission of i th

channel, Rik (k) is the optical power density of

k th FBG in i th optical channel. So, we can acquirethe wavelength shift by detecting I i (k). Thus the

data acquisition parameter C i can be written as:

C i ¼ ni ðk 1 T i þ k 2 I iÞ; ð2Þ

where ni is the priority of i th optical channel, T i is

the delay time, I i is optical power that obtain from

Eq. (1). k 1 and k 2 are the weights of the delay time

and wavelength shift, respectively. With the data

acquisition parameter C i , we can establish the data

acquisition rule that is composed of priority, the

delay time and wavelength shift information. In

addition, we can determine the proportion of three

factors: ni , k 1 and k 2, respectively. Therefore, the

data acquisition rule is more rational and smarter

than the conventional one.

In order to illustrate the technique described

above, we set ni = 1, k 1 = 1 and k 2 = 0. That is to

say, we establish the data acquisition rule simply

by the delay time. The sensing information is as-

sumed as random signal from Poisson distribution.

The demodulate time of the sensing system is

deterministic, while the conventional one is sto-

chastic. According to Queuing theory [12], theconventional system is a M/M/1 queue system

(the input and demodulate processes are both

Poisson processes) and the smart sensing system

is a M/D/1 queue system (the input processes Pois-

son processes and the demodulate process is deter-

ministic). Then, the delay time of the conventional

multiplexing technology and the smart multiplex-

ing technology can be defined as:

W con ¼ k l=2; ð3Þ

LPG4LPG2Optical

Circulator

2

2Optical

CirculatorOptical

Circulator

Channel 2FBG 21

FBG1n

3

3

3

1

1

FBG 41 FBG4nChannel4n

FBG2n

Channel 1FBG11

Broad Band

source

Computer

1*4 Optical

coupler

F-PP h o t o

d e t e c t or

Opitcal

Switcher

P D4P D1 P D2

3dB

3dB

3dB

21

LPG1

Fig. 2. The smart fiber Bragg grating sensors system.

H. Gao et al. / Optics Communications 251 (2005) 361–366 363



W sm ¼ q

l k

k qk

lð1 qk Þ

0:5; ð4Þ

where W con is the delay time of the conventionalFBG sensing system. W sm is the delay time of

the smart ones. k is the scale size of FBG sensing

array. q = k/l is the service density of the system,

k is the average signal rate and l is the average

demodulate rate.

4. Experiment

As shown in Fig. 3, the demodulation part

is composed of fiber Fabry–Perot (MICRON

OPTICS) whose finesse is 750, an A/D data acquisi-

tion card (AD-Link DAQ2010) with sample fre-

quency of 1 MHz, a computer (Evoc 810) which

is used to demodulate sensing information and

generate pulse drive voltage signal for PZT. There

is one FBG strain sensor on every PZT. The sam-

ple frequency of A/D card is 1 MHz and the

switching time of optical switcher is 0.05 s. Thus,

the service time of scanning and demodulating

one optical channel is 0.15 s. In order to compare

the theoretical values and the experimental results,

we choose three parameters for the analyticalmodel, described by Eqs. (3) and (4). These param-

eters are the average delay time of sensing system

when the scale size is decided, the average delay

time of sensing system when the average signal rate

is decided.

4.1. The average delay time vs. the average signal

rate

Our first experiment is to test the average delaytime vs. the average signal rate. The scale size is

four and the sensing signal rate controlled by com-

puter is from Poisson distribution. The computer

records the time when pulse signals are generated

and demodulated by smart and conventional ways,

respectively. Then, the average delay time together

with the average signal rate are recorded at every

0.5 step. The experiment results are plotted in

Fig. 4, where the x-axis is the average signal rate

in Hz, and the y-axis is the average delay time of

the FBG sensing system in ms. By comparing

experiment results (triangle line and star line) with

the theoretical results (dotted line and solid line)

within the same signal rate change, it can be seen

that they show the same behaviour. The theoreti-

cal values show a good fitted result to the experi-

mental data. The delay time of the smart system

is less than the conventional ones before the aver-

age signal rate reaches six. With the limit of the

demodulate rate, the sensing information would

be lost when the average signal rate comes up to

larger than six. Thus, our experimental results

show good agreement with the theoretical model.The difference in the average delay time is due to

the fact that the pulse signal is random and exper-

iment time (200 s/point) cannot be infinite as the-

ory assumed, hence there would be of a tiny

amount. In our experiment, the sampling time is

LPG4LPG2Optical

Circulator

2

2OpticalCirculator

Optical

Circulator

3

3

3

1

1

Broad Band

source

Computer

1*4 Opticalcoupler

F-P

P h o t o

d e t e c t or

OpitcalSwitcher

PD4PD 1 PD 2

3dB

3dB

3dB

2

1

LPG1

PZT1

FBG1

PZT2

FBG2

PZT4

FBG4

Fig. 3. The experiment setup of the smart FBG sensing system.




200 s for every data point. With more sampling

time, the better fitness between theoretical andexperimental curves would be achieved.

4.2. The average delay time vs. the scale size

As described above, our sensor is theoretically

immune to the scale size of sensing array. To ver-

ify this, further experiments are conducted to

investigate the delay time under the different

scale size and the average signal rate k = 2 Hz.

With the array size increase, the sampling time

of every signal point is 200 s. The results are

shown in Fig. 5. It can be seen that when thescale size changed from 2 to 10, the delay time

of smart system (approximately 20 ms) remained

fairly stable. While, with the array size increases,

the average delay time of conventional ones is

prolonged. It is several times larger than that of

the smart ones. As well as the first experiment,

the difference between theoretical and experimen-

tal results come from the sampling time (200 s/

point) cannot be infinite as theoretical assumed.

It is apparent that, the larger size of sensor array,

Fig. 4. Experimental plots between the average delay time and the average signal rate when the scale size is four.

Fig. 5. Experimental plots between the average delay time and the scale size when the average signal rate is 2 Hz.

H. Gao et al. / Optics Communications 251 (2005) 361–366 365



the more advantageous the smart multiplexing

technology would have.

5. Conclusion

With the WDM/SDM technique, we bring for-

ward a novel FBG sensors multiplexing technique

and the data acquisition parameter of multiplexing

sensing FBG system is defined. The theoretical

simulate and the experiment work indicate that

this technique has a few advantages: more rational

and smarter data acquisition rule, the shorter de-

lay time, the longer optical switch lifetime, avoid-

ance of frequently switching and the average

delay time immune to the scale size. Therefore, this

technique is able to meet the smart requirement of

some complex system and large scale distributed

system.

Compared with the traditional WDM/SDM

technology, there are also several disadvantages

in this sensor system. For example, the structure

is more complex and expensive. The LPG also

exhibits sensitivity to temperature and pressure

which may cause unintended demodulation of

adjacent channels. This can be overcome with a

thermally stabilized package. So, the demodulationpart might demodulate and scan the wrong optical

channels whose wavelengths are not shifting. With

the development of optical–electrical industry, the

price of optical–electric devices will fall down. To

solve the stability problem of LPG, there are some

encapsulation technologies [13].

Acknowledgements

Supported by the National High Technol-

ogy Program of China under Grant No.2002AA313110, the Tianjin Important Scientific

and Technological Project under Grant No.

043182011.

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