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Fiber-Optic Accelerometer Using Wavefront-Splitting Interferometry

Fiber-Optic Accelerometer Using Wavefront-Splitting Interferometry. Hsien-Chi Yeh & Shulian Zhang July 14, 2006. Outline. Working Principle Design of Accelerometer Preliminary Results Conclusion & Discussion. Fiber-Optic Equivalent of Lloyd’s Mirror. E 1. E 2.

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Fiber-Optic Accelerometer Using Wavefront-Splitting Interferometry

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  1. Fiber-Optic Accelerometer Using Wavefront-Splitting Interferometry Hsien-Chi Yeh & Shulian Zhang July 14, 2006

  2. Outline • Working Principle • Design of Accelerometer • Preliminary Results • Conclusion & Discussion

  3. Fiber-Optic Equivalent of Lloyd’s Mirror E1 E2 • Gaussian beam emitting from single-mode fiber • optical (fiber) axis // specimen surface • L >> h, grazing incidence angle (85o-89o)  high reflectance

  4. PROPAGATION OF A GAUSSIAN BEAM h : gap distance between the fiber axis and the sample surface r : 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)*

  5. INTERFERENCE FRINGES OBTAINED AT VARIOUS GAP DISTANCES s-polarization light, observation plane at z = 7mm (A)  = 650nm, core diameter = 4m, n = 3.85+0.016i (bare silicon surface) (B)  = 1550nm, core diameter = 10m, n = 3.47 (bare silicon surface) (A) h = 100 m h = 200 m h = 300 m (B)

  6. SENSOR OUTPUT vs. GAP DISTANCE =1550 nm, n=3.472 (bare silicon surface) quadrature point

  7. Design of Accelerometer

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

  9. Setup of Sensing Head

  10. Fiber Mounting Plate • Aluminum used: Cost effective, fast machining

  11. Proof Mass & Spring • Design Nat. freq. ~ 10 Hz • Spring: low stiffness – OHP Transparency • Mass: high density - stainless steel • Base length, c < 1cm as separating distance of 1 cm is needed for good interference patterns

  12. Preliminary Tests Resonance frequency Seismic test on optical table Seismic test on workbench

  13. System Setup • Sensing head placed on an anti-vibration table • Using PZT to obtain the best operating position --quadrature point Photo-detector & amplifier Sensing head Power supply for adjusting initial position Pig-tailed diode laser

  14. Exp. 1: Natural Frequency of System f0 ~ 12.5 Hz

  15. Exp. 2: Noise Level of Accelerometer Noise level of sensor output signal: 7 mV 7 mV  750 nm/V (conversion factor) ~ 5 nm Noise level of acceleration: 5 nm  (12.5 Hz  2)2 ~ 3 g

  16. Exp. 3: Seismic Test: Building Vibration Sensor placed on normal workbench

  17. Vibration Frequency of Building : 2.5 Hz

  18. Discussion & Conclusion • Prototype of fiber-optic accelerometer based on wavefront-splitting interferometry • Operating range: DC-24 Hz • Resolution: < 3 g • Further improvements • Finding suitable material for spring to achieve lower resonance frequency and optimal damping coefficient • Reducing the size of sensing head to obtain larger signal-to-noise ratio • Closed-loop feedback control to achieve null-sensing capability

  19. Thank You for your attentions!

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