Fiber Optic Sensor TechnologyMiao YuAssistant Professor Sensors and Actuators Laboratory Department of Mechanical Engineering University of Maryland, USA

IMAC XXVI, Orlando, FL

Feb 6, 2008

IntroductionFiber optic sensor systemsInput X source modulator Sensor Demodulator receiver Sensor output V0

Why fiber optic sensors? Harsh environment capability: intensive EMI, high temperature, chemical corrosion, high pressure, high voltage Light weight and small size Excellent performance: high sensitivity and large bandwidth Long range operation Multiplexed or distributed measurements

IntroductionCurrent applicationsMechanical measurements: force, pressure, strain/stress, displacement, temperature, acceleration, vibration, acoustics Electrical and magnetic measurements Chemical and biological sensing

IntroductionFiber Optic Sensor ClassificationsSensing region: Intrinsic vs. Extrinsic Intrinsic fiber optic sensor has a sensing region within the fiber and light never goes out of the fiber. In extrinsic sensors, light has to leave the fiber and reach the sensing region outside and then comes back to the fiber. Optical modulation mechanism Intensity modulated Phase modulated Wavelength modulated Polarization modulated

Intensity Modulated Fiber Optic Sensors

Intensity Modulated Fiber Optic SensorsBasic structuresReflection type Source: broadband Fiber: multimode is better Pout L Used as distance or pressure sensors Transmission type Similar to a movable reflector Used as strain or distance sensors Pin Pout L Pout PinMoving mirror fiber

L

Limitation: no reference signal so it suffers from light source intensity fluctuation and fiber loss varies

Intensity Modulated Fiber Optic SensorsBasic structuresDifferential intensity sensors Sensing arm

L1

Pout1

Pout2 Reference arm

L2

Take care of the power fluctuation issue by using differential intensity signal from sensing arm and reference arm Pout1 -Pout2 L1-L2

Intensity Modulated Fiber Optic SensorsOther intensity based sensorsMicrobending sensor

Intensity Modulated Fiber Optic SensorsOther intensity based sensorsIntensity based force sensor

Intensity Modulated Fiber Optic Sensors Properties of intensity based sensorsVersatile Simple design and easy signal interpretation Usually suffer from intensity fluctuations and low sensitivity

Phase Modulated Fiber Optic Sensors

Phase Modulated Fiber Optic SensorsFiber Interferometers as sensors

Output of the sensor

I = A + B cos = k0OPD = 2

nL2

=

2

2

OPD +

2

nL +

L n

In an interferometer based sensor,

, L, n

Phase Modulated Fiber Optic SensorsConventional fiber optic interferometers Two beam interferometer: Michelson interferometerDetector LD 2x2 Coupler

Mach-Zehnder interferometer

LD Coupler

Coupler Detector

Multi-beam interferometer Fabry-Perot interferometerLD 2x2 Coupler

Coherence light source:

Ring interferometer

Detector LD Coupler Detector

lc>>OPD

Properties: high sensitivity compared to intensity based sensors

Phase Modulated Fiber Optic SensorsLow coherence fiber optic interferometer: use low coherence light sourceL ight S ource 2x2 C oupler Ls S ensor interferom eter

index-m atched gel Lr

D etector

Advantages: high sensitivity, large dynamic range, noise resistance

Phase Modulated Fiber Optic SensorsTwo Beam Fiber Interferometers: single mode fibers and coherence light source

Michelson InterferometerLD 2x2 CouplerL1 R1 R2

Detector

L2

R1 R2 I0 I= ( R1 + R2 )[1 + 2 cos[2k ( L1 L2 )]] 4 R1 + R2fiber polarization states remain unchanged OPD>Lc, (Ls-Lr)