Transcript Fiber Optic Sensors David Gunther Applied Optics 10 March 2005 How they work • Fiber optic sensors measure properties of their environment. • They can measure anything which changes.

Fiber Optic Sensors
David Gunther
Applied Optics
10 March 2005
How they work
• Fiber optic sensors
measure properties of
their environment.
• They can measure
anything which
changes the way light
travels through the
fiber, or alters the
light’s properties.
What can they measure?
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Temperature
Pressure
Strain
Displacement
Acceleration
Flow rate
Vibration
Chemical concentrations
Electrical and Magnetic Fields
Rotation rate
Methods of Measurement
• Amplitude- and Intensity-Based Sensors
• Frequency- and Wavelength-Varying
Sensors
• Polarization and Phase-Modulating FiberOptic Sensing
Intensity-Based Sensors
• Detect changes in
light intensity,
correlate change to
change in pressure,
or temperature, etc.
• These systems have a poor ability to screen
noise, leading to lower precision, but are
simple and inexpensive systems.
Frequency- and WavelengthVarying Sensors
• Map changes in frequency or wavelength to
parameter of interest.
• Low alteration of signal outside of sensing area.
• Wavelength measurement is very sensitive; not
strongly affected by light loss in connections, or
source intensity fluctuations.
Wavelength-Varying Sensors…
• Fiber Bragg Grating: most common type.
– Characteristic reflected light wavelength dependent on
grating spacing. Stress applied to fiber changes spacing.
Polarization and Phase-Modulating
Fiber-Optic Sensing
• Polarization modulation sensors are the most
complicated and delicate instruments.
• Use Faraday Effect to measure magnetic fields, by
measuring polarization rotation.
• Polarization unintentionally altered by other processes:
bending, stretching, and twisting causes problems.
Advantages of Fiber Sensors
Many sensing applications could use mechanical or
electronic sensors, but fiber sensors have advantages
for some applications.
• Explosive environments- no electricity to start fires
– (ie. pressure sensors in rocket fuel tanks).
• Corrosive environments- silica fiber is chemically
resistant, unlike copper.
• Hot environments- many of these sensors work above
750 F. (400 C) The sensor can be cast into aluminum
metal.
• Remote sensing- signal detection/processing can be
done miles from active part of fiber sensor.
• Small size! 125um fiber same thickness as 36 AWG wire.
Industrial Use: Oil Wells
• Oil well temperature monitoring: measures
temperature at 1 meter intervals along 10 km
deep well pipes, with 0.1 C accuracy. With just
a single fiber!
Fiber-Optic Gyroscope
• Uses phase difference
between two beams
circling a fiber loop in
opposite directions to
very precisely
measure rotation rate.
– Precision better than
0.1 deg./hr readily
done: can measure
Earth’s rotation rate.
• Fiber gyros can be
made extremely
durable: they
withstand being fired
in mortar shells.
I-FOG Sensitivity examples
• The sensitivity can be scaled by changing the
loop area:
• A high-sensitivity I-FOG with
– 10 cm diameter loop, with 1 km fiber length, and 1550
nm light provided an Wp of 133°/s, and an Wµ of
0.15°/h respectively.
• A medium-sensitivity I-FOG with L, D, and l of
– 3 cm diameter loop, with 200 m fiber length, 850 nm
light provided an Omega pi of 1220°/s, and an
Omegaµ of 1.4°/h respectively.
Summary
There are several characteristics of optical fibers
that allow them to be used for sensors. These
include micro bending, interferometric effects,
refractive index change, polarization change,
fiber length change, fiber diffraction grating
effects, and the Sagnac effect (light traveling in
opposite directions around a loop used to sense
rotation).