Fiber Optic Sensor Configurations

R. A. Perez-Herrera and M. Lopez-Amo

Department of Electric and Electronic Engineering, Universidad Publica de Navarra,

Campus Arrosadía s/n E-31006 Pamplona, Spain

Keywords: Fiber-Optic Sensor Multiplexing, Fiber-Optic Networks, Fiber Laser, Remote Sensing.

Abstract: The main goal of this work is to provide a brief overview of the fiber optic sensor multiplexing

configurations and techniques as well as some recent advances and trends of the most important fiber optic

sensor configurations. Distributed and point sensors are explained and a number of high performance

networks are shown. Finally, the concept of robust fiber optic sensor is presented as well as the main

records of distributed and remote sensing fiber optics topologies.

1 INTRODUCTION

The advantages of optical fiber sensor networks are

well known and have been widely analyzed in the

research literature on the subject. Fiber sensors

appear to be very attractive in some areas where they

offer innovative capabilities. At the same time, the

use of nonlinear effects and amplification in sensing

systems has attracted much interest in the last

decade.

Technologies for fiber-optics went through a

major growth period during the years 1994 to 2000.

This growth came about due to the junction of

several market technologies and drivers. Initially the

key drivers of the demand for bandwidth were data

traffic and the Internet. The second was the advent

of the optical amplifier, which attended the role in

optical networks that the transistor had played in the

electronics revolution. The optical amplifier was the

crucial issue because of the fact that it allowed the

simultaneous amplification of a number of channels,

as opposed to electronic regenerators that operated

channel by channel. A third technology was

wavelength-division-multiplexing (WDM), which

made a single strand of fiber act as many virtual

fibers.

WDM has permitted the capability of fibers to be

increased by more than two orders of magnitude

over the past few years, providing plenty of

bandwidth in telecommunications to fuel the growth

of data traffic and the Internet. The association with

the optical amplifiers allowed the revolution related

to WDM in optical networks.

Raman amplification had a slow start, but then

experienced a wide distribution with increasing

performance needs of optical networks. Any

deployment concerns about discrete or distributed

Raman amplification have been outweighed by the

performance improvements permitted with Raman

amplification. For example, distributed Raman

amplification improves noise performance and

decreases nonlinear penalties in WDM networks, in

this manner improving the two main restrictions in

dispersion-compensated, optically amplified

systems.

All these techniques, initially developed for

telecommunication systems, are now also used in

sensor networks. In this way, nonlinearities will be

used to provide amplification in different WDM

networks, improving the overall system

performance. The main concepts associated to these

technologies will be described next, as well as a

state of the art and future trends of modern

multiplexing optical fiber sensor networks.

2 MULTIPLEXING TECHNIQUES

IN SENSOR NETWORKS

Multiplexing is the simultaneous transmission of

two or more information channels along a common

path. A fiber sensor system includes three main parts

or subsystems: the sensing elements or transducers,

the optical fiber channel and the optoelectronic unit

(López- Higuera, 1998). Because the last subsystem

140

Perez-Herrera R. and Lopez-Amo M..

Fiber Optic Sensor Conﬁgurations.

DOI: 10.5220/0005431301400146

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (OSENS-2015), pages 140-146

ISBN: 978-989-758-092-5

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

uses to be the most expensive one, when it is

possible to multiplex a high number of sensing

points in the same network using common

optoelectronic unit, the cost per sensing element

decreases. Systems employing multiplexed arrays of

fiber Bragg Gratings (FBGs) have performed

successfully in numerous field trials and applications

involving a wide diversity of structures.

The growth of the technology and components

used in the optoelectronics units and multiplexing

networks has been helped by the fast growing of the

fiberoptic telecommunication technology. High

performance tunable lasers, optical amplifiers,

couplers, optical switches, filters and detectors are

available for sensors multiplexing due to the major

market that supposes telecommunications. Likewise,

certain multiplexing techniques, as WDM comes

from the telecommunications side. However, other

multiplexing techniques have been developed or

adapted specifically for fiber-optic sensors

multiplexing (Kersey, 1991).

The sensor networks can be designed by means

of only passive fibers (without utilizing optical gain)

or introducing optical amplification in some key

parts of the networks and hence passive or active

networks can be developed (Dandridge, 2002),

(Abad, 2002).

A variety of multiplexing techniques based on

different modulation formats have been developed,

each one with their own benefits for a particular

application. The modulation formats generally fall

into one of the following categories: Wavelength

Division Multiplexing, Time Division Multiplexing,

Frequency Division Multiplexing, Coherence

Multiplexing and Polarization Division

Multiplexing. However hybrid approaches

(simultaneous utilization of several modulation

formats inside the same network) have been also

considered. When a single fiber is devoted to each

sensor, we also use the term Spatial Division

Multiplexing as another multiplexing technique.

Figure 1 illustrates some of these multiplexing

modulation formats.

To efficiently design the most appropriate sensor

network, it is necessary to take into account different

aspects: the modulation and coding format of the

optical signal, the network topology, the inclusion or

not of optical amplification technologies, the

decoding method for the received signal and the type

or types of sensors multiplexed in the same network.

Finally, it must be also considered the economic

conditions, which would eventually determine the

most appropriate network. The choice of a different

multiplexing technique depends on the requirements

of the sensor network. The relative importance of

parameters such as cost, noise, bandwidth, and

flexibility constitute the basis for making the right

selection.

Figure 1: Multiplexing modulation formats: (a)

Wavelength Division Multiplexing (WDM); (b) Time

Division Multiplexing (TDM); (c) Frequency Division

Multiplexing (FDM); (d) Coherence Multiplexing (CM)

and (e) Polarization Division Multiplexing (PDM) (Lopez-

Amo, 2011).

A first subdivision between optical fiber sensor

networks will be attending on the type of the

multiplexed sensors. They could be a simple o

hybrid networks, terms used for the networks where

only one or more than one type of multiplexed

sensors in that order. Secondly, another subdivision

that must be done in order to clearly define a

network is based on the type of sensors used, as can

be seen in Figure 2. They can be transmissive

networks (based on transmissive sensors) or

reflective networks (where the reflected signals in

each sensor are used).

Figure 2: Serial multiplexed transmissive sensor network

(a) or reflective sensor network (b).

Finally, as can be seen in Figure 3, it is common

to differentiate among point and distributed sensors.

On one hand, the multiplexing of point sensor is

-1

Δλ

m-1

Sm-1

Δλ

Sm-1

Δλ

(a)

(e)

(c)

(d)

(b)

Sm-1

FiberOpticSensorConfigurations

141

based on processing the value of the measurand

corresponding to each separate sensing element. On

the other hand, in the distributed sensors only one

sensing element is needed and the objective of the

signal processing is to recuperate the measurand as a

function of position along the sensing section.

Figure 3: Point multiplexed and distributed sensors.

There are diverse topologies for locating sensors

in a network. They can be divided into five basic

configurations, shown in figure 4 (where each one in

turn can be of a transmissive or reflective type):

serial, dual bus or ladder, star, tree or mesh

topologies (Lopez-Amo, 2011).

Figure 4: Multiplexing topologies for fiber optic sensors:

bus, ladder, star, tree and mesh.

3 DISTRIBUTED AND POINT

SENSORS

A great variety of physical and chemical parameters

can be measured by using point sensors. However,

here are hundreds of applications in structural health

monitoring (SHM) were the key parameters to be

measured are temperature and occasionally strain

and vibration. In those cases, distributed sensing is

an appropriate choice when many measurement

points are needed along a serial or linear

multiplexing topology. It is worth noticing the

difference between distributed and multi-point

sensing. Distributed refers to the ability to

simultaneously detect scale and location of a

measurand anywhere along a continuous length of

sensing fiber. Nevertheless, multi-point sensing

refers that the measurement is done at specific

locations with point sensors (Perez-Herrera, 2013).

Distributed measurements are also based on non-

linear scattering effects to evaluate a number of

different parameters. Rayleigh scattering comes

from the interaction between the light with refractive

index fluctuations in the fiber core that appear in

spatial scales much shorter than the light

wavelength. Because of that, Optical Time Domain

Reflectometry (OTDR) was developed initially as a

network diagnostic tool for optical fiber

telecommunication systems. Different means of

measuring this delay time leads to other time domain

techniques, like Optical Frequency Domain

Reflectometry (OFDR) or polarization domain

reflectometry (POTDR) (Jones, 1988).

Stimulated Raman scattering (SRS), for example,

is generated by the interaction of the propagating

light with molecular vibrations in the medium. On

the other hand, stimulated Brillouin scattering (SBS)

involves acoustic phonons. In this respect, both

scattering processes involve three-waves in which

the incident (pump) light is converted into (Stokes)

light of longer wavelength with an unavoidable

excitation of a molecular vibration (SRS) or an

acoustic phonon (SBS). However, there are a

number of significant differences between SBS and

SRS that lead to markedly diverse systems.

Raman backscatter systems have found real

applications niches such as monitoring tunnels and

have been also commercially presented from several

decades ago. These sensors are mostly simple to

install and provide temperature resolutions of the

order of 0.1 to 1 ºC within resolution lengths of

order of one meter over interrogation lengths

extending to tens of kilometers (Culshaw, 2004).

Brillouin scatter in contrast is usually used in the

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frequency domain. The peak offset frequency for

Brillouin scattering is measured and is a unique

function of the acoustic velocity (and therefore

temperature and strain) in the optical fiber (Perez-

Herrera, 2013).

Figure 5: state of the art of long-range BOTDA sensors.

Figure 6: state of the art of high-resolution BOTDA

sensors.

One of the main drawbacks of this technique is

that the measurement range of these systems has a

trade-off between the measurement range and the

spatial resolution (Fernandez-Vallejo, 2012).

Consequently, present research in BOTDA sensors

has two different hot topics: long-range BOTDA

sensors, which are able to perform measurements in

tens of kilometers with meter resolutions, or high-

resolution sensors with centimeters spatial

resolution, but for relatively short-distances fibers.

Figures 5 and 6 summarize some of the advances in

long-range and high-resolution BOTDA sensors

respectively

4 HIGH PERFORMANCE

NETWORKS

The maximum measurement distance of fiber sensor

systems comes out to be a practical issue due to the

loss and noise and induced by the attenuation along

the fiber and the Rayleigh scattering respectively. A

number of diverse proposals have been

experimentally carried out in order to increase the

measuring distance of the fiber sensors.

For example, a 75-km long distance FBG sensor

system was experimentally demonstrated by (Fu,

2008). Though, every 25 km amplification was

needed which made the system more complex. An

ultra-long distance fiber Bragg grating sensor system

able to evaluate FBGs located 120 km far from

receiver position without using amplification was

demonstrated by (Saitoh, 2007). One year later, a

230 km FBTG sensor system using a high-speed

swept-wave- length light source using EDF

amplification was verified (Saitoh, 2008). (Leandro,

2011) presented and demonstrated a technique for

remote sensing of FBGs beyond 150 km combining

Brillouin, Raman, and erbium gain in a linear cavity

fiber laser. A long distance fiber laser system

composed by a random fiber laser with a reach of

200km able to multiple 11 optical fiber sensors

based on FBGs was proposed and experimentally

demonstrated by (Fernandez-Vallejo, 2013).

Figure 7: state of the art of the advances in remote sensing

systems for optical fiber sensors.

In most cases, complicated setups are employed

to reduce the noise effect in order to achieve longer

distance measurements. Nevertheless, (Fernandez-

Vallejo, 2011) proposed and experimentally

demonstrated a simple configuration able to detect

four multiplexed sensors located 250 km away

thanks to Raman amplification. In addition to this, a

FiberOpticSensorConfigurations

143

253 km ultra-long remote displacement sensor

system based on a fiber loop mirror and an OTDR

without using amplification was demonstrated by

(Bravo, 2011). Figure 7 illustrates some of the

advances in remote sensing systems for optical fiber

sensors over the last years.

5 ROBUST FIBER OPTIC

SYSTEMS

As it was previously pointed out, optical fiber sensor

networks provide sensing solutions for almost all

kind of applications and situations: from large

natural environments to large scale structures, such

as bridges and other civil constructions (Majumder,

2008).They are attracting an increasing interest

owing to their wide range of potential industrial

application in strategic sectors such as energy,

security (Fernandez-Vallejo, 2012), defense, or

transportation. Nevertheless, the constant operation

of the sensor network after accidental or malicious

harm is of increasing importance while the structure

being monitored is of high cost (power transmission

lines or oil pipelines); human protection is at risk

(nuclear plants or chemical storage locations) or

perimeter security is a concern (banks or airports)

(Li, 2004).

Four groups of protection to allow service to be

restored after a failure have been defined:

“dedicated” or “shared” protection, each of these has

sub-categories called “path” and “line” protection.

As a result, these four categories are dedicated line,

dedicated path, shared line and shared path (Pérez-

Herrera, 2014). The main difference between

dedicated and shared protection, derives from the

method the sensor unit is connected to the network.

Shared protection usually uses a switch although

dedicated protection employs an optical coupler.

Path protection and line protection differ in the

form of protection. In line protection, the sensors are

protected by the nearest switches to the failure, and

such switches are located in the sensor network itself

and not in the transmission/receiver node

(Ramamurthy, 2003). So, if a failure happens, the

network can reconfigure the path. Instead, in path

protection, each sensor is protected individually by

the switch positioned in the transmission or receiver

node which readdresses the information in the event

of a failure in the network.

Albeit most of optical sensor networks are based

on linear topologies, it is worth highlighting that

bus, star and ring topologies have been employed in

multipoint sensing systems too to overcome system

failure causing from breakpoints in the sensing

system. However, the sensing area is always

restricted in one dimension and not all types of

breakpoints can be restored. In order to solve this

disadvantage, a number of mesh sensing systems to

support more comprehensive sensing regions have

been recently proposed and experimentally

confirmed (Wu, 2010), (Peng, 2012). In these

topologies, the symmetric scheme warrants that the

proposed sensing system can be accessed from any

point.

Several optical sensor network based on optical

add-drop multiplexer (OADM) devices with a bus

configuration have been recently carried out (Bravo,

2013), (Rota-Rodrigo, 2013). These devices make it

possible to increase the amount of sensors in bus

networks in comparison with those that have optical

couplers and each sensor can be associated with a

different wavelength directly offered by each

OADM. In addition to this, these devices allow

networks to be developed for recovering operation

after failures and it performs self-diagnosis, that is,

the identification of the failed constituent(s) from the

patterns of surviving end-to-end connections at its

operating wavelengths (Perez-Herrera, 2012).

Thanks to this configuration it is possible to

coordinate self-diagnosis with protection switching

so as to reduce the momentary service interruption.

6 CONCLUSIONS

This work has reviewed the key fiber optic sensor

multiplexing main configurations and techniques as

well as some recent advances and trends in this field

of research. A summary of several high performance

networks has been presented. Finally, the current

trends in robust fiber optic sensor configuration have

been shown as well as the main records of

distributed and remote sensing fiber optics

topologies.

ACKNOWLEDGEMENTS

Financial support from the Spanish Comisión

Interministerial de Ciencia y Tecnología within

project TEC2013-47264-C2-2-R and FEDER funds

is acknowledged.

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