Fiber-Optic Accelerometer Using Wavefront-Splitting InterferometryHsien-Chi Yeh & Shulian Zhang

July 14, 2006

OutlineWorking PrincipleDesign of AccelerometerPreliminary ResultsConclusion & Discussion

Gaussian beam emitting from single-mode fiberoptical (fiber) axis // specimen surfaceL >> h, grazing incidence angle (85o-89o) high reflectanceFiber-Optic Equivalent of Lloyds MirrorE1E2

PROPAGATION OF A GAUSSIAN BEAMh : 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 pI(x,y,L) = (c/2)(E1+E2)(E1+E2)*

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 mh = 200 mh = 300 m(A)(B)INTERFERENCE FRINGESOBTAINED AT VARIOUS GAP DISTANCES

=1550 nm, n=3.472 (bare silicon surface)SENSOR OUTPUT vs. GAP DISTANCEquadrature point

Design of Accelerometer

Displacement-Type Accelerometer cantilever beam (spring) connecting base and proof mass proof mass mounted at the mid span of spring

Setup of Sensing Head

Fiber Mounting Plate

Aluminum used: Cost effective, fast machining

Proof Mass & Spring

Design Nat. freq. ~ 10 HzSpring: low stiffness OHP TransparencyMass: high density - stainless steel Base length, c < 1cm as separating distance of 1 cm is needed for good interference patterns

Preliminary Tests Resonance frequencySeismic test on optical tableSeismic test on workbench

System SetupSensing head placed on an anti-vibration tableUsing PZT to obtain the best operating position --quadrature pointSensing headPig-taileddiode laserPhoto-detector &amplifierPower supplyfor adjustinginitial position

Exp. 1: Natural Frequency of Systemf0 ~ 12.5 Hz

Exp. 2: Noise Level of AccelerometerNoise level of sensoroutput signal: 7 mV7 mV 750 nm/V (conversion factor) ~ 5 nm

Noise level of acceleration:5 nm (12.5 Hz 2)2 ~ 3 g

Exp. 3: Seismic Test: Building Vibration

Sensor placed on normal workbench

Vibration Frequency of Building : 2.5 Hz

Discussion & ConclusionPrototype of fiber-optic accelerometer based on wavefront-splitting interferometryOperating range: DC-24 HzResolution: < 3 gFurther improvementsFinding suitable material for spring to achieve lower resonance frequency and optimal damping coefficientReducing the size of sensing head to obtain larger signal-to-noise ratioClosed-loop feedback control to achieve null-sensing capability

Thank Youfor your attentions!