6/13/2007Raphael Bouskila/Shaz Taslimi 1 The Laser Microphone Raphael Bouskila Shaz Taslimi

Raphael Bouskila/Shaz Taslimi 2 6/13/2007 Introduction Basic principle: use reflections from a window pane to detect the vibrations of the window  Sound in the room causes the window to vibrate  Displacement can be efficiently detected with laser interferometry

Raphael Bouskila/Shaz Taslimi 3 6/13/2007 Introduction Block diagram: InterferometerPhotodetector Signal processing WindowSpeaker Modulated laser beam Time-varying fringe pattern Electrical signal (small & noisy) Electrical signal (clean) Acoustic signal

Raphael Bouskila/Shaz Taslimi 4 6/13/2007 Interferometry theory Andrew G. Kirk, ECSE-423 lecture slides

Raphael Bouskila/Shaz Taslimi 5 6/13/2007 Interferometry theory Detector Reference Mirror Mirror Laser Beam Splitter Michelson interferometer

Raphael Bouskila/Shaz Taslimi 6 6/13/2007 Interferometry theory Dual-beam interferometer Detector 45º Mirror Mirror Laser Beam Splitter

Raphael Bouskila/Shaz Taslimi 7 6/13/2007 Interferometry theory Fringes move across the detector at the frequency of the glass vibrations ω Detector

Raphael Bouskila/Shaz Taslimi 8 6/13/2007 Interferometry theory Photodetector device: photodiode in series with biasing battery  Converts time-varying light signal from interferometer directly into electrical signal with same frequency

Raphael Bouskila/Shaz Taslimi 9 6/13/2007 Optics: Implementation

Raphael Bouskila/Shaz Taslimi 10 6/13/2007 Optics: Implementation Frequency response of optics  Note: input/output relationship is not precise due to inefficient power coupling in the audio channel

Raphael Bouskila/Shaz Taslimi 11 6/13/2007 Electronics: Theory Multi-stage amplifying bandpass filter  Transresistance amplifier  High-pass & low-pass filters  Baseband voltage amplifier  Class A output stage

Raphael Bouskila/Shaz Taslimi 12 6/13/2007 Electronics: Theory Transresistance amplifier  Photodiode: light-controlled current source  Current must be converted to voltage Figure modified from

Raphael Bouskila/Shaz Taslimi 13 6/13/2007 Electronics: Theory Bandpass filter  Desired signal range: Thresholds of hearing: ~20 Hz—20 KHz Practical range: 300—3400 Hz (telephone) Our design: 100 Hz—7.7 KHz

Raphael Bouskila/Shaz Taslimi 14 6/13/2007 Electronics: Theory A bandpass filter can be achieved with a cascaded high-pass and low-pass filter: High-pass filterLow-pass filter

Raphael Bouskila/Shaz Taslimi 15 6/13/2007 Electronics: Theory Baseband voltage amplifier  Inverting op-amp configuration  Designed for 100 V/V passband gain when combined with the filter Anas A. Hamoui, ECSE-434 lecture slides

Raphael Bouskila/Shaz Taslimi 16 6/13/2007 Electronics: Theory Class A output stage  “Emitter follower”— provides current buffering to drive 8 Ω speaker without loss of gain Anas A. Hamoui, ECSE-434 lecture slides

Raphael Bouskila/Shaz Taslimi 17 6/13/2007 Electronics: Theory Design:

Raphael Bouskila/Shaz Taslimi 18 6/13/2007 Electronics: Implementation

Raphael Bouskila/Shaz Taslimi 19 6/13/2007 Electronics: Implementation Frequency response of electronics TheoryImplementation

Raphael Bouskila/Shaz Taslimi 20 6/13/2007 Implementation issues Alignment of optics  Rotation of mirror by angle α  beam deflection by 2α Andrew G. Kirk, ECSE-423 lecture slides

Raphael Bouskila/Shaz Taslimi 21 6/13/2007 Implementation issues Ambient light noise  blackout tube  optical bandpass filter Internal reflection inside optical components  Anti-reflection coated components

Raphael Bouskila/Shaz Taslimi 22 6/13/2007 Implementation issues Glass resonances  Affected by: Glass dimensions:  Height: 40 cm  Width: 45 cm  Thickness: 2.2 mm Material properties:  Young’s modulus: 72 GPa  Poisson’s ratio: 0.24 Boundary conditions  C-C-C-C (fixed window) Resonant frequencyFrequency (kHz) 1 st nd rd th th th th th th th

Raphael Bouskila/Shaz Taslimi 23 6/13/2007 Results Full system transfer function

Raphael Bouskila/Shaz Taslimi 24 6/13/2007 Results

Raphael Bouskila/Shaz Taslimi 25 6/13/2007 Results Speech sample 1:  “Before I begin the lecture, I wish to apologize for something that is not my responsibility: but is the result of physicists all over the world and scientists, so called, have been measuring things in different units, and causing an enormous amount of complexity, so as a matter of fact, nearly a third of what you have to learn consists of different ways of measuring the same thing, and I apologize for it. It's like having money in francs, and pounds, and dollars and so on... with the advantage over money however is that the units, the ratios don't change, as time goes on. For example, in the measurement of energy, which is indicated up here, the unit we use here is the joule, and a watt is a joule per second. But there are a lot of other systems of measuring energy, depending on what it is. And I’ve listed three of them up at this thing for engineers […]” R.P. Feynman—Space & Time [excerpt] From the Feynman Lectures on Physics, at the California Institute of Technology, 1961 Original Recovered

Raphael Bouskila/Shaz Taslimi 26 6/13/2007 Future work possibilities Improve high-frequency cutoff of optics  Currently only 1 kHz Use an amplifier with a wider signal swing  TL084 op amp only has ±5 V rails Clipping distortion on loud signals Find a more representative window mount  True C-C-C-C boundary conditions should meet the theoretical predictions better Shrink the system and make it portable Use a different laser  higher power  greater microphone range  longer coherence length  less sensitive to manufacturing flaws  invisible beam  better for surveillance

Raphael Bouskila/Shaz Taslimi 27 6/13/2007 Acknowledgments Supervisor  Prof. Andrew G. Kirk Photonics lab manager  Josh Schwartz Technical help & advice:  Chris Rolston  Prof. Martin Rochette  Prof. Meyer Nahon  Prof. Anas Hamoui