Fiber-Optic Accelerometer Fiber-Optic Accelerometer Using Wavefront-Splitting Using Wavefront-Splitting

InterferometryInterferometry

Hsien-Chi Yeh & Shulian ZhangHsien-Chi Yeh & Shulian Zhang

July 14, 2006July 14, 2006

OutlineOutline Working PrincipleWorking Principle Design of AccelerometerDesign of Accelerometer Preliminary ResultsPreliminary Results Conclusion & DiscussionConclusion & Discussion

• Gaussian beam emitting from single-mode fiber• optical (fiber) axis // specimen surface• L >> h, grazing incidence angle (85o-89o)

high reflectance

Fiber-Optic Equivalent of Lloyd’s Mirror

EE11

EE22

PROPAGATION OF A GAUSSIAN BEAM

)]2

1(exp[])(

exp[),,(2

22

220

222000

1 L

yxikL

Lw

yxz

L

zELyxE

]}2

)2(1[exp{}

])2([exp{),,(),,(

2

22

220

222000

2 L

yhxikL

Lw

yhxz

L

zEpnrLyxE

)

2(tan

221

yh

Lx

h : gap distance between the fiber axis and the sample surfacer : reflection coefficient that is a function of incident angle , refractive index of sample surface n, and polarization of incident light p

I(x,y,L) = (c/2)(E1+E2)(E1+E2)* ''),','(),,( dydxLzyxILzyxP

s-polarization light, observation plane at z = 7mm(A) = 650nm, core diameter = 4m, n = 3.85+0.016i (bare silicon surface)(B) = 1550nm, core diameter = 10m, n = 3.47 (bare silicon surface)

h = 100 m h = 200 m h = 300 m

(A)

(B)

INTERFERENCE FRINGESOBTAINED AT VARIOUS GAP DISTANCES

100 120 140 160 180 2000

50

100

150

200

250

300

350

gap distance between the sensor and the sample surface (um)

optic

al o

utp

ut (

pW)

p-polarizations-polarization

=1550 nm, n=3.472 (bare silicon surface)

SENSOR OUTPUT vs. GAP DISTANCE

quadrature point

Design of Design of Accelerometer Accelerometer

Displacement-Type AccelerometerDisplacement-Type Accelerometer

cantilever beam (spring) connecting base and cantilever beam (spring) connecting base and

proof massproof mass

proof mass mounted at the mid span of springproof mass mounted at the mid span of spring

200

220 )(

1

)(

)()(

xa

ia

xT

mafkxxbxm ext

Setup of Sensing HeadSetup of Sensing Head

Fiber Mounting Fiber Mounting

PlatePlate

Aluminum used: Cost effective, fast machiningAluminum used: Cost effective, fast machining

Proof Mass & SpringProof Mass & Spring

Design Nat. freq. ~ 10 Design Nat. freq. ~ 10 HzHz

Spring: low stiffness – Spring: low stiffness – OHP TransparencyOHP Transparency

Mass: high density Mass: high density

- stainless steel - stainless steel

Base length, c < 1cm as Base length, c < 1cm as separating distance of 1 separating distance of 1 cm is needed for good cm is needed for good interference patternsinterference patterns

Preliminary Tests Preliminary Tests

Resonance frequencyResonance frequency Seismic test on optical tableSeismic test on optical table Seismic test on workbench Seismic test on workbench

System SetupSystem Setup Sensing head placed on an anti-vibration tableSensing head placed on an anti-vibration table Using PZT to obtain the best operating Using PZT to obtain the best operating

position --quadrature pointposition --quadrature point

Sensing headSensing head

Pig-tailedPig-tailed

diode laserdiode laser

Photo-detector &Photo-detector &

amplifieramplifier

Power supplyPower supply

for adjustingfor adjusting

initial positioninitial position

Exp. 1: Natural Frequency of Exp. 1: Natural Frequency of SystemSystem

ff00 ~ 12.5 Hz ~ 12.5 Hz

Exp. 2: Noise Level of Exp. 2: Noise Level of AccelerometerAccelerometer

Noise level of sensorNoise level of sensor

output signal: 7 mVoutput signal: 7 mV

7 mV 7 mV 750 nm/V (conversion factor) ~ 5 nm 750 nm/V (conversion factor) ~ 5 nm

Noise level of acceleration:Noise level of acceleration:

5 nm 5 nm (12.5 Hz (12.5 Hz 2 2))22 ~ 3 ~ 3 gg

Exp. 3: Seismic Test: Building Exp. 3: Seismic Test: Building VibrationVibration

Sensor placed on normal workbenchSensor placed on normal workbench

Vibration Frequency of Building : 2.5 Vibration Frequency of Building : 2.5 HzHz

1.6 secPeriod of Oscillation, T = 0.4 sec

4 cycles

1Frequency of Oscillation = 2.5 Hz

0.4

Discussion & ConclusionDiscussion & Conclusion

Prototype of fiber-optic accelerometer based Prototype of fiber-optic accelerometer based on wavefront-splitting interferometryon wavefront-splitting interferometry

Operating range: DC-24 HzOperating range: DC-24 Hz Resolution: < 3 Resolution: < 3 gg Further improvementsFurther improvements

Finding suitable material for spring to achieve Finding suitable material for spring to achieve lower resonance frequency and optimal damping lower resonance frequency and optimal damping coefficientcoefficient

Reducing the size of sensing head to obtain larger Reducing the size of sensing head to obtain larger signal-to-noise ratiosignal-to-noise ratio

Closed-loop feedback control to achieve null-Closed-loop feedback control to achieve null-sensing capabilitysensing capability

Thank YouThank Youfor your attentions!for your attentions!