laser microphone

6/13/2007 Raphael Bouskila/Shaz Taslimi 1

The Laser Microphone

Raphael Bouskila

Shaz Taslimi

Raphael Bouskila/Shaz Taslimi 26/13/2007

Introduction

Basic principle: use reflections from a window pane to detect the vibrations of the windowSound in the room causes the window to

vibrateDisplacement can be efficiently detected with

laser interferometry


Introduction

Block diagram:

Interferometer Photodetector

Signal processing

Window

Speaker

Modulated laser beam

Time-varying fringe pattern

Electrical signal (small & noisy)

Electrical signal (clean)

Acoustic signal

Acoustic signal


Interferometry theory

Andrew G. Kirk, ECSE-423 lecture slides



Detector

Reference Mirror

Mirror

LaserBeam Splitter

Michelson interferometer



Dual-beam interferometer

Detector

45º Mirror

MirrorLaser Beam Splitter


Interferometry theory Fringes move across the detector at the frequency of the

glass vibrations

ω

Detector

http://hep0.physics.lsa.umich.edu/dan/physics_340.htm


Interferometry theory Photodetector device: photodiode in series with biasing

battery Converts time-varying light signal from interferometer directly

into electrical signal with same frequency

http://engphys.mcmaster.ca/undergraduate/outlines/3f04/LAB3F3%202%20new1.pdf


Optics: Implementation


Optics: Implementation

Frequency response of optics

Note: input/output relationship is not precise due to inefficient power coupling in the audio channel

G

0

0.002

0.004

0.006

0.008

0.01

0.012

1 10 100 1000 10000


Electronics: Theory

Multi-stage amplifying bandpass filter Transresistance amplifier High-pass & low-pass filters Baseband voltage amplifier Class A output stage


Electronics: Theory

Transresistance amplifier Photodiode: light-controlled current source Current must be converted to voltage

Figure modified from http://www.national.com/onlineseminar/2004/photodiode/PhotodiodeAmplifers.pdf


Electronics: Theory

Bandpass filterDesired signal range:

Thresholds of hearing: ~20 Hz—20 KHz Practical range: 300—3400 Hz (telephone) Our design: 100 Hz—7.7 KHz


Electronics: Theory

A bandpass filter can be achieved with a cascaded high-pass and low-pass filter:

0.047 uF

470 Ohm

Vin Vout

1.6 kOhm

1 uF

Vin Vout

High-pass filter Low-pass filter


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


Electronics: Theory

Class A output stage “Emitter follower”—

provides current buffering to drive 8Ω speaker without loss of gain

Anas A. Hamoui, ECSE-434 lecture slides


Electronics: Theory

Design:

R5

1.6k

Q1Q2222A

R1

432.5

vOUT

0

CCC2

22u

U1

+

-

V+V-

OUT

1

2

34

5

0VCC

R2

47k

C2

22u

0

0

CEE2

22u

C1

.047u

-VEE

RB11.4k

I1

0Adc106uAac

C3

1u

CEE1

22u

0

VCC

0

CC

22u

0

R4

1MEG RL

8.2

R6

100

00VCC

R3

470

U2

+

-

V+V-

OUT

1

2

34

5

CCC1

22u

0

VCC

-VEE

0


Electronics: Implementation


Electronics: Implementation

Frequency response of electronics

Theory ImplementationFrequency response

0

200

400

600

800

1000

1200

1 10 100 1000 10000 100000

Frequency


Implementation issues

Alignment of optics Rotation of mirror by angle α beam deflection by 2α

Andrew G. Kirk, ECSE-423 lecture slides



Ambient light noise blackout tube optical bandpass filter

Internal reflection inside optical components Anti-reflection coated

components



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 frequency Frequency (kHz)

1st 2.313071114

2nd 3.547454712

3rd 5.636196473

4th 5.680437701

5th 6.830563356

6th 8.525134879

7th 8.792285816

8th 10.77197321

9th 11.56663755

10th 11.8945664


Results

-1

0

1

2

3

4

5

6

7

8

9

10 100 1000 10000

Frequency (Hz)

Full system transfer function


Results


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


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


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