3. Introduction
• Optical sensor is a transducer which converts any form of
a signal to an optical signal in the measurable form.
• Optical fibers: strands of glass that transmit light over long
distances (wire in electrical systems)
• Light: transmitted by continuous internal reflections in
optical fibers (like electron in electrical systems).
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4. Optical fiber sensor system
Block Diagram of Optical Fiber Sensor System
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Optical Tx
Optical fibers
&
Actuators
Optical Rx
Control system
Data Acquisition
And health
Assessment
5. What Does F.O.S. Look Like? (Cont’d)
Strain Gage
Embeddable Strain Gage
Pressure Transducer
Displacement Transducer
Temperature Transducer
6. What Does F.O.S. Look Like? (Cont’d)
Various Fiber Optic CensorsVarious Fiber Optic CensorsFiber Optic Shape TapeFiber Optic Shape Tape
7. OFS Applications
Measurement of physical properties such as strain,
displacement, temperature, pressure, velocity, and
acceleration in structures of any shape or size
Monitoring the physical health of structures in real time
Damage detection
Used in multifunctional structures, in which a combination
of smart materials, actuators and sensors work together to
produce specific action
“Any environmental effect that can be conceived of can be
converted to an optical signal to be interpreted,” Eric Udd,
Fiber Optic Censors, John Wiley & Sons, Inc., 1991, p.3
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8. Fiber Optic Sensor Capabilities
• Rotation, acceleration
• Electric and magnetic fields
• Temperature and pressure
• Acoustics and vibration
• Strain, humidity, and viscosity
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9. 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
10. Advantages of F.O.S
• Lightweight / nonobtrusive
• Passive / low power
• EMI resistant
• High sensitivity and bandwidth
• Environmental ruggedness
• Complementary to telecom / optoelectronics
• Flexible
• Non-flammable 10
11. Disadvantage of fiber optic over
copper wire cable
• Optical fiber is more expensive per meter than
copper
• Optical fiber can not be join together as easily as
copper cable. It requires training and expensive
splicing and measurement equipment.
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13. Classification Of Fiber Optics Sensor Systems
The above classification of fiber optics sensor system can be explained in
following manner:
A.Based on Sensor location
i. Intrinsic
ii. Extrinsic
B.Based on operating Principle.
i.Based on intensity
ii. Based on phase
iii. Based on frequency
iv. Based on polarization
C. According to application
i. Physical sensor
ii. Chemical sensor
iii. Bio-medical sensor
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14. Based on Sensor location
• INTRINSIC or ACTIVE SENSORS
• Physical parameter to be sensed acts directly on
the fibre to produce changes in the transmission
characteristics
• Eg. Pressure and liquid level sensors
• EXTRINSIC or PASSIVE SENSORS
• Separate sensing element is used and the optical
fibre is a waveguide.
• Eg. Displacement and laser Doppler velocimeter
sensors
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19. Fiber Optic
Sensor
Principles
The general structure of an
optical fiber sensor system is
shown in Figure aside. It consists
of an optical source (Laser, LED,
Laser diode etc.), optical fiber,
sensing or modulator element
(which transduces the measurand
to an optical signal), an optical
detector and processing
electronics (oscilloscope, optical
spectrum analyzer etc.).
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20. Classification of FOS
Based on application areas:
• physical sensors (measurement of temperature, strech,
etc)
• chemical sensors (measurement of pH content, gas
analysis, spectroscopic studies, etc.)
• biomedical sensors (measurement of blood flow ,
glucose content, etc.)
22. There are three types of fiber optic cable commonly used
Single Mode
Step-index Multimode fiber
Plastic optic fiber
Fiber media
Optical fibers are the actual media that guides the light
24. SNELL’S LAW:
n1 sin ϑ = n2 sin ϑ
where n is refractive index
Optical Fiber
• Guidance is achieved through multiple
reflections at the fiber walls.
• Core, transparent dielectric material,
surrounded by another dielectric
material with a lower refractive index
called cladding. (n1 >n2)
• In practice, there is a third protective
layer called jacket.
n1
n2
ϑ1
ϑ2
29. Detectors
•Detector is the receiving end of a fiber optic link.
There are two kinds of Detectors
1. PIN (Positive Intrinsic Negative)
2. APD (Avalanche photo diodes)
PIN
APD
30. Idea of Modulation
• When sending information by an optical
fiber, the information must be encoded or
transformed somehow into information that
capable of being transmitted through a
fiber. The signal needs to be modulated.
There are two types of modulation Analog
and digital.
31. 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.
32. –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
reflective surface
Fiber Optic Strain Sensors
33. Fiber Optic Strain Sensors
A. Intensity Modulated Strain Gages
• Micro-bend Sensors
– If a fiber is bent, a portion of the trapped light is
lost through the wall.
34. Fiber Optic Strain Sensors
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 to strain, optical spectrum changes
– absolute distance between the mirrors gives the
strain.
35. Fiber Optic Strain Sensors
–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)
36. CONCLUSION
There are inherent advantages of fiber optic sensors
which include their ability to be light in weight, very
compact and small in size. Easy to launch light, low
ISI, resistance to electromagnetic interference, high
sensitivity, wide bandwidth and environmental
ruggedness make them widely used in different fields.
All these mentioned characteristics make best use of
optical fiber as sensor and the networks which are
made up of optical fiber are very advantageous in
industry for long time investment areas of media
access control, security and privacy.
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Single mode: Diameter of 8.3 to 10 microns., fairly narrow diameter
-It will propagate typically 1310 or 1550nm
-higher transmission rate (up to 50 times more distance than multimode)
-Cost more than multimode.
Step-index Multimode fiber
-made of glass fibers.
-diameter in the 50 to 100 micron range
-multiple paths of light can cause signal distortion at the receiving end, result in unclear or incomplete data transmission.
Plastic optic fiber
-POF is strong and very difficult to bend.
-POF is not suitable for long-distance transmission
-POF transmits very little infrared light
-It can used for cold lighting or lighting displays of artwork
When light enters the area between 2 difference materials it will produce two different indexes of refraction. The light will either entirely reflected or a portion of it will be refracted depending on the angle. If the light can be kept at an angle where it is entirely reflected, it will become trapped inside and transmitted along the fiber.
-A light sources, there are two types used today are LED (light emitting diode) and ILD (injection laser diode)
-ILD have a higher output potential and coupling efficiency, they are suited for long distance transmissions.
-LED have a lower bandwidth, or information capacity than ILD
There are two types of detectors are positive intrinsic negative(PIN) and avalanche photo diodes (APD).
A PIN can be operational with voltage as low as 5v
The APD require a large bias between 100v to 300v
