1. David Gunther Applied Optics 10 March 2005
Fiber Optic Sensors
David Gunther
Applied Optics
10 March 2005

2. 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.

3. What can they measure? Temperature Pressure Strain Displacement
Acceleration
Flow rate
Vibration
Chemical concentrations
Electrical and Magnetic Fields
Rotation rate

4. Methods of Measurement
Amplitude- and Intensity-Based Sensors
Frequency- and Wavelength-Varying Sensors
Polarization and Phase-Modulating Fiber-Optic Sensing
Methods of Measurement

5. 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.

6. Frequency- and Wavelength-Varying 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.

7. Wavelength-Varying Sensors…
Fiber Bragg Grating: most common type.
Characteristic reflected light wavelength dependent on grating spacing. Stress applied to fiber changes spacing.

8. 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.

9. 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.
Advantages of Fiber Sensors

10. 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!

11. 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-Optic Gyroscope
Fiber gyros can be made extremely durable: they withstand being fired in mortar shells.

12. 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.
I-FOG Sensitivity examples

13. 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).
Summary