Optical Interferometry and Industrial Interferometers - a Tutorial -

1. Optical Interferometry and Industrial Interferometers- a Tutorial - Friedemann Mohr Pforzheim University of Applied Sciences

2. Outline 1 Physical Basics and components 2 Interferometry for path measurement 3 Laser vibrometry for vibration measurement

3. The photoelectric conversion process

4. Detecting a light wave

5. Components I: Lenses, beam transformation The tele-scope Gaussian beam trans-formation

6. Components II: Mirrors and retroreflectors

7. Components III: Lossless beamsplitters

8. Components IV: Lossless beam-splitters and their technical realisation

9. Components V: Lossy beamsplitters

10. Polarisation I:Polarisation ellipse and Jones vector

11. Polarisation II:Propagation and Jones matrix Polariser Retarder, here with specific retardation of Df=p/2(quarterwave plate)

12. Polarising beam splitters (PBS) left: crystal type (Wollaston prism) right: thin film type

13. Some characteristic polarisation states top: linear || x, linear || ybottom: linear, +45°, linear -45° top: circular, RH, circular, LH bottom: elliptical, RH elliptical, LH

14. He-Ne laser for interfero-metry I a tube designb gain curvec mode schemed real modes due to b and c

15. He-Ne laser for interfero-metry II NBS neutral beam splitterPOL polariserPBS polarising beam splitterD1, D2 detectorsC control circuit Stability improvement:~ 10-6 ~10-9 Coherence length: km‘s

16. 1 Physical Basics and components 2 Interferometry for path measurement 3 Laser vibrometry for vibration measurement

17. The historical Michelson-Morley experiment I Aim: Proving the existence of the ether

18. The historical Michelson-Morley experiment II Approach: Verification of Doppler effect on speed of light using high-resolution phase measurement

19. 1 Laser I 0 2 f1 I f2 1 1 D1 2 I 2 D2 Mach-Zehnderinterfero-meter I

20. Mach-ZehnderInterfero-meter

21. MZI arrangement for path measurement Problem: No directional information !!

22. Mach-Zehnder interferometer with directional sensitivity I Superposition of 2 orthogonally polarised waves yields not only output intensity but polarisation ellipse. Polarisation el-lipse carries two informations: Shape andelevation angle. Or: Phase difference and direction.

23. Mach-Zehnder interferometer with directional sensitivity II

24. Scalar case, from above: Calculating interference taking into account polarisation Vectorial case, requires Jones calculus:

25. Processing of directional signals I

26. Processing of directional signals II

27. Environmental factors Path is measured in multiples (or fractions) of wavelengthProblem: Wavelength is dependent on environmental factors:- temperature, J- atmospheric pressure, p- humidity factor of air, F- gas content of air, GSolutions:- measure all parameters (J, ...), calculate n, compensate arithmetically- measure n directly

28. MZI for tooling machine calibration

29. MZI based mask positioningin semiconductor industry

30. 1 Physical Basics and components 2 Interferometry for path measurement 3 Laser vibrometry for vibration measurement

31. Heterodyne interferometer for laser vibrometry

32. Acoustooptic modulator (Bragg cell)

33. Doppler shift

34. Heterodyne Interferometer: Calculation of Interference

35. Operating range of heterodyne vibrometer • Operating range depends on Bragg frequency • typically, fB = 40 MHz • |v|=10 m/s corr. to |fD| = 32MHz

36. Vibrometer block diagram / velocity decoder block

37. Vibrometer block diagram / fringe counter block

38. Vibrometer head Polytec design II

39. Operating range diagram of a laser vibrometer

40. Measurement example #1 for single point mode top: loudspeaker drive signalcenter: velocity decoder outputbottom: displacement decoder output

41. Measurement example #2 HD drive dynamic measurements: With stationary disk the R/W head touches the disk. With rotating disk the head is flying over the disk (hydrodynamic lubrication) Lowest possible flight height gives best storage density. Optimum is h=0. Vibrometer serves for measuring / optimizing resonance characteristics of flight control system by courtesy of Polytec GmbH

42. Valve position measurement in automotive industry using fiberoptic vibro-meters Using two fiber heads, differential velocity measurement between two points is possible. Application example #3 by courtesy of Polytec GmbH

43. More vibrometer application aspects • Measure body sound contactlessly and with high precision • Avoid mass loading of DUT • Acquire many data points in short time • Measure from points otherwise difficult accessible • Be widely independent from material properties • Measure from smooth, hot, minute, intricate structures • Measure high-frequency vibrations • Measure from large distances…..

44. Laser vibrometer in scanning mode

45. Fiber sensor coil deformation under forced vibration

46. Scanning vibrometer measurement example

47. Summary 1 Physical basics and components 2 Interferometry for path measurement:Operating concepts and applications 3 Laser vibrometry for vibration measurement: Operating concepts and applications

48. Vibrometers - More Applications • Medicine – Ear drum, hearing functions, heart • Zoologie – Elephants, insekts, spider webs • Household – Washing machines, vacuum cleaners, shavers • Entertainment – Loudspeakers • Military – Guns, mines • Civil engineering – Buildings, bridges • ..........and much more by courtesy of Polytec GmbH

49. Processing of directional signals III