basics of optical fiber sensors

7/30/2019 Basics of Optical Fiber Sensors

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Development of a Pulsed Laser for Hybrid Optical FiberSensors for Simultaneous Distributed Temperature andDynamic Point Measurements

Table of Contents

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1. Introduction to Optical Fiber Sensors...............................................................................................11.1 Fiber-Optic Sensing Basics.............................................................................................................1

1.2 Performance Parameters of Fiber-optic Sensors.............................................................................3

1. Introduction to Optical Fiber Sensors

1.1 Fiber-Optic Sensing Basics

The telecommunication industry has been significantly changed by recent advances in fiber optic

technology [1]. It was possible to carry gigabytes of information at the speed of light, and there were

improvements and cost reductions in optoelectronic components. The emergence of fiber optic sensors(FOS) began when designers tried to combine the product outgrowths of fiber optic telecommunication

with optoelectronic devices.

Besides, as the loss in fibers was greatly reduced and the sensitivity of detection of losses increasedmaking it possible to sense changes in phase, intensity and wavelength from outside perturbations on

the fiber itself and this marked the birth of fiber optic sensing. The mechanisms that fibers are made tobe immune against in telecommunication systems are now made to be the ones that fibers should be

sensitive to for sensing applications.

Compared to their electronics counter parts, FOS have a number of advantages which make them moresuitable for use in many sectors. These include immunity to electromagnetic interference, insulation

against electric current, robustness and more resistance to harsh environments, high sensitivity. There

are also other important features like chemical passivity, wide operating temperature range,multiplexing capabilities to form sensing networks, light weight and easy integration into a variety of

structures.

Thus, they have potential applications in many strategic sectors such as real-time structural health

monitoring, concrete monitoring during settling or cracks in bridges and buildings. They are also used

in post seismic damage evaluation in buildings and heritage structures, leakage and distributedtemperature monitoring in dams,crack opening analysis and joints monitor in tunnels.

Fiber-optic sensors integrated in to buildings and structures called what are know as smart structures

and they are one of the main drivers of further development in the area. They provide designers with


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important information as to the amount of the strain to which different parts of the structure are subject

to.

FOS have also applications in biochemical sensing where they are used to measure physicalparameters,such as temperature and pressure. They are also used to shape sensing systems to determine

the precise position and shape of medical tools and robotic arms used during minimally invasive

surgery. The intrinsic physical characteristics of optical fibers make them extremely attractive tobiomedical sensing because their use will not interfere with conventional electronics found in medical

equipments and their immunity to electromagnetic waves makes them ideal for real time use during

diagnostic imaging with medical systems like Magnetic Resonance Imaging (MRI), ComputerTomography (CT) ,Positron Emission Tomography (PET ), etc...([2]).

An optical fiber sensing system consists of four basic components, namely: a light source, optical

fiber, transducer and a detector ([3]). The transducer modulates some parameter of the optical system

that can be measured such as intensity, wavelength, polarization and phase.

fiber

detector

Fig. 1: A basic FOS scheme

The physical parameter to be measured is introduced in terms of acoustic wave, flow, temperature,

electric filed, magnetic field, electric current, gas radiation, etc. Then the transducing occurs along thefiber in the form of electro-optic effect, thermal expansion, Raman scattering, Brillouin scattering,

Flourescense. This results in a change in the characteristics of the optical signal received at the detector.

There are many categories of FOS based on the part of the sensor used for classification [4].

They can be classified as intensity, phase, frequency or polarization sensors based on the modulationand demodulation process. Phase or frequency sensors are also called interferometric sensors because

interferometric techniques are needed to detect these parametes which also implies coherent hetrodyne

detection. Coherent sensors are more complex in design than Intensity (incoherent) sensors.

Based on their application, FOS can be classified as physical sensors (measurement of temperature,

stress, etc...): chemical sensors, (measurement of PH content, spectroscopic studies), bio-medical

sensors, etc...

FOS can also be classified as intrinsic or extrinsic, based on how the modulation takes place. In

intrinsic sensors, the modulation takes place directly in the fiber while in extrinsic sensors themodulation is performed by an external transducer.

Another main classification is into discrete (point) and distributed sensors. In distributed sensing,

sensor

source


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scattering phenomena in the fiber such as Rayleigh, Raman and Brillouin scattering are used to

determine the measurand (temperature, strain) and the fiber is used to continuously determine the value

of the parameter along the entire sensing span. In discrete (point) sensing, however, the measurement

of parameters is performed at predetermined points along the fiber where Fiber Brag Gratings (FBGs)are located.

Discrete sensors can also be multiplexed so that an array of FBG is placed and interrogated using ashared source and detector. A combination of point and distributed sensors gives rise to what are known

as hybrid distributed/point sensors which are used to simultaneously measure such parameters as

temperature and pressure both over the entire fiber or at discrete points along its span. Fig. 2 gives thebasic block diagram of these two types of FOS.

(a)

(b)

Fig. 1: Basic block diagrams of (a) a multiplexed discrete and (b) a distributed FOS, system.

1.2 Performance Parameters of Fiber-optic Sensors

There are many parameters that characterize a particular fiber optic sensing system and that are used to

compare the performance of various implementations, the main ones of which are given below.

Spatial Resolution

It is the measure of the smallest separation of distance between two points which the sensor can

identify any sensible change in spatial variation of the meausurand (strain, temperature) to be detected.

For a distributed fiber optic sensor, it is often defined either as the minimum distance over which the


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system is able to indicate the value of the measurand within the specified uncertainty (measuring

spatial resolution) or as the minimum distance that generates results which are within 10% of the

measurand transition amplitude (detection spatial resolution).

SensitivityThis is the change of a measured variable produced by a unit change of the measured value of the

environment. It is given by %/oC for temperature or Mhz/ micros-strain for strain.

Sensing Range

It is the fiber length over which meausremtns can be performed within stated sensing performance.

Measurand Resolution

The measurand resolution is smallest difference in the measured parameter which the sensor can

detect. It is defined as the estimated standard deviation of a specified series of measurements given as afunction of the distance for a particular spatial resolution and acquisition time.

Dynamic Range

The ration of the largest power and smallest value of the measured field that could be observed by the

sensing scheme, with a required accuracy.

Measurement time

This is the time the system need to obtain results within a set of particular sensing performances

Accuracy

The accuracy in measuring an environmental parameter is the measure of nearness of the measured

value with the sensor with the actual value.

Cross-sensitivity

It indicates how much the measurement is influenced by other environmental parameters different from

the actual measurand, that affect the mechanism used for sensing.

References

[1]. Fidanboylu, K.,Efendiolu, H. S.,Fatih University, Istanbul, Turkey, Fatih University, Istanbul,

Fiber Optic Sensors and their Applications-Keynote Address, International Advanced Technologies

Symposium (IATS09), May 13-15, 2009, Karabuk, Turkey

[2]. http://www.laserfocusworld.com/articles/2011/01/medical-applications-of-fiber-optics-optical-

fiber-sees-growth-as-medical-sensors.html

[3] http://www3.ntu.edu.sg/mae/research/programmes/sensors/sensors/fos/fosselva.html

[4].http://www.micronoptics.com/uploads/library/documents/Micron%20Optics%20Optical

%20Sensing%20Guide.pdf
http://www.laserfocusworld.com/articles/2011/01/medical-applications-of-fiber-optics-optical-http://www.laserfocusworld.com/articles/2011/01/medical-applications-of-fiber-optics-optical-fiber-sees-growth-as-medical-sensors.htmlhttp://www3.ntu.edu.sg/mae/research/programmes/sensors/sensors/fos/fosselva.htmlhttp://www.micronoptics.com/uploads/library/documents/Micron%20Optics%20Opticalhttp://www.micronoptics.com/uploads/library/documents/Micron%20Optics%20Optical%20Sensing%20Guide.pdfhttp://www3.ntu.edu.sg/mae/research/programmes/sensors/sensors/fos/fosselva.htmlhttp://www.micronoptics.com/uploads/library/documents/Micron%20Optics%20Opticalhttp://www.micronoptics.com/uploads/library/documents/Micron%20Optics%20Optical%20Sensing%20Guide.pdfhttp://www.laserfocusworld.com/articles/2011/01/medical-applications-of-fiber-optics-optical-http://www.laserfocusworld.com/articles/2011/01/medical-applications-of-fiber-optics-optical-fiber-sees-growth-as-medical-sensors.html

basics of optical fiber sensors

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