16082_Fiber Optics Sensors

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Sensors and Transducers-4

4/29/2012 1Jyotirmoy Pathak



Fiber Optics Sensors

• Optical Fiber

• Fiber Optic Sensors

• Measured Parameters

• Main Advantages

• Basic Components of FOS Setup

• Applications

•

Classification of FOS• Fiber Optic Strain Gages

• Products




Optical Fiber

• a filament of

transparentdielectric material,glass or plastic

•

usually cylindricalin shape

• a guidance systemfor light

Optical fiber is :




SNELL’S LAW:

n1 sin J = n2 sin J

where n is refractive index

Optical Fiber

Guidance is achieved throughmultiple reflections at the fiber walls.

Core, transparent dielectric material,surrounded by another dielectric

material with a lower refractive indexcalled cladding. (n1 >n2)

In practice, there is a third protectivelayer called jacket.

n1 n2

J1

J2




Ray Transmission through an Optical

Fiber

Critical angle of reflection (sin Jc = n2 /n1)




Fiber Optic Sensors

Basic Components:

• source of light

• a length of sensing

(and transmission) fiber

• a photo-detector

•demodulator

• processing and display optics

• required electronics




Fiber Optic Sensors

optical changes

refractive index

mode conversion

geometrical changes

size

shape

amplitude (intensity) phase

frequency polarization

optical radiation change

External Perturbation




Measured Parameters

• Light intensity

• displacement (position)

• pressure

• temperature

• strain (rotation and displacement)• flow

• magnetic and electrical fields

• chemical compositions

• velocity, acceleration and vibration

• force and stress




Main Advantages

Non-electric (immune to

electromagnetic and

radio-frequency

interference)

withstand high

temperature and harsh

environments (corrosion)

High shock survivability(explosion or extreme

vibration)

high accuracy and

sensitivity

light weight and small sizehigh capacity and signal

purity

multiplexing capacity

Can be easily interfacedwith data communication

systems




Basic Components of FOS Setup




Applications

• Real-time monitoring of civil engineering structures.

• Structural monitoring of aircraft, both in-flight and on-ground

• Instrumentation of robots used on board in the International

Space Station

• Testing and analysis of solid rocket motors

• Smart structures instrumentation

• Fiber Aerospace guidance and control

• Industrial control

• Damage localization in civil, mechanical, and aerospace

structures• Embedment in concrete structures




Classification of FOS

A. Based on application areas:

• physical sensors (measurement of temperature, stress, etc)

•

chemical sensors (measurement of pH content, gas analysis,spectroscopic studies, etc.)

• biomedical sensors (measurement of blood flow , glucose content, etc.)





B. Based on modulation and demodulation

process:

• phase-modulated sensors

– compare the phase of light in a sensing fiber to a

reference fiber in a device called interferometer.

– Light is not required to exit the fiber at the

sensor (no optical loss)

– more complex in design

– better sensitivity and resolution





Example:

Mach-Zehnder Interferometric sensor





B. Based on modulation and demodulation

process:

• intensity-modulated sensors

– Light is required to exit the fiber at the sensor

(optical loss)

– simpler in design

– more economical

– widespread in application





B. Based on modulation and demodulation process:

• spectrally-modulated sensors

– measures the changes in the wavelength of the light due to theenvironmental effects.





C. Based on sensing characteristics of fibers

• extrinsic sensors

– a coating or a device at the fiber tip performs the measurement.





C. Based on sensing characteristics of fibers

• intrinsic sensors

– fiber itself performs the measurement.




Fiber Optic Strain Sensors

A. Intensity Modulated Strain Gages

• Reflective sensors

– One bundle is used to transmit the light to a reflecting target

– Other collects the reflected light and transmits to a detector

– Any movement of the target will effect the intensity of the

reflected light.




– Plain reflective displacement sensors have a limited dynamic range of about 0.2 in.

– Can be improved by a lens system to 5 in.

– sensitive to the orientation and contamination of the reflectivesurface






A. Intensity Modulated Strain Gages

• Micro-bend Sensors

– If a fiber is bent, a portion of the trapped light is lost through thewall.





B. Phase Modulated Strain Gages

• Fabry-Perot Interferometers (FPI)

– light source is conveyed via an optical fiber to two mirrors(reflectors).

– When the displacement between the mirrors has changed due tostrain, optical spectrum changes

– absolute distance between the mirrors gives the strain.





– Extremely sensitive

– provides point-sensing capability

– excellent mechanical properties

– output is easy to process

– difficult to make rugged enough for harsh construction env.

(embedding in concrete)




Product: EFO Embedded Strain Gage

FISO Technologies• 70 mm long sensor

• can be embedded in concrete

• intrinsic Fabry-Perot strain gage is

bonded in a very small hole in the center

of the steel body.

• can be cast directly into the wet mix

• can be encapsulated into a concrete

briquette, then cast into wet concrete

• can be placed into a pre-drilled hole and

grouted.

• Diameters are 3mm and 30 mm.

• range, +/- 1000, 1500micro strain

•resolution <0.01% full scale

•Temperature range, -55 oC to 85oC




Product: Embeddable EFPI Strain Gage

Luna Innovations Inc.• 2-10 mm length, 350 micrometer outer diameter

•sensitivity, +/- 5000 micro strain

•resolution <1

•Temperature range, -100 oC to 350oC

•accuracy, 1%

•Measurement cycle, 100kHz




Product:Wide Sensing Fiber Optic Cable

SunX-Ramco Inc.•11 mm wide sensing area

•long sensing distance

•freely cuttable fiber cable

•2 m. lenght




Ultrasonic Sensor

Ultrasonic sensors areused for positionmeasurements

Sound waves emitted are

in the range of 2-13 MHz

Sound Navigation AndRanging (SONAR)

Radio Dection AndRanging (RADAR) – ELECTROMAGNETICWAVES !!

15° - 20°




Photo Multiplier





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16082_Fiber Optics Sensors