Fiber Optic Sensors

David GuntherApplied 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?

• 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 Fiber-Optic 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 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.

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).