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. Gaussian beam emitting from single-mode fiber optical (fiber) axis // specimen surface L >> h, grazing incidence angle (85 o -89 o )  high reflectance Fiber-Optic Equivalent of Lloyd’s Mirror E1E1E1E1 E2E2E2E2

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)(E 1 +E 2 )(E 1 +E 2 )*

5. 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) h = 100  mh = 200  mh = 300  m (A) (B) INTERFERENCE FRINGES OBTAINED AT VARIOUS GAP DISTANCES

6. =1550 nm, n=3.472 (bare silicon surface) SENSOR OUTPUT vs. GAP DISTANCE 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 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 - 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 Sensing head Pig-tailed diode laser Photo-detector & amplifier Power supply for adjusting initial position

14. Exp. 1: Natural Frequency of System f 0 ~ 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!