Re: 'How could i transmit a sound wave by a laser ?'

Greetings:

By your question about the transmission of a sound wave using a laser beam 
I believe that you want to place sound information such as music or 
speaking on a laser beam  and transmit the laser beam  some distance to a 
point where the sound is recovered from the laser beam  for recording or 
listening. 

Lasers generate electromagnetic waves  which are all part of the 
electromagnetic spectrum which includes  microwaves and radio waves. All 
electromagnetic waves travel at the speed
of light; however, radio waves are greater than a meter in length, 
microwaves are centimeters in length and visible light waves are a fraction 
of a micrometer in length. 

The process of putting information on an electromagnetic, CARRIER wave, is 
called MODULATION. The process of recovering information from a carrier 
wave is called DEMODULATION. The entire process has more recently been 
simplified to MODEM (as a computer MODEM which modulates and demodulates 
computer generated information for transmission over telephone carrier 
systems). 

Modulating a laser optical carrier frequency can be performed in two ways, 
1) DIRECT MODULATION of the electrical current operating the laser 
transmitter or 2) EXTERNAL MODULATION in which the laser beam is modulated 
by a second optical device after the beam leaves the laser. 

                     DIRECT MODULATION
     __________      ______________      __________
     I         I     I            I      I        I  LASER
     I POWER/  I---->I ELECTRONIC I----->I LASER  I------>
     I BATTERY I     I MODULATOR  I      I        I  BEAM
     I_________I     I____________I      I________I


                     EXTERNAL MODULATION
     __________      _________           ____________
     I         I     I        I  LASER   I           I  LASER
     I POWER/  I---->I LASER  I--------->I OPTICAL   I------>
     I BATTERY I     I        I  BEAM    I MODULATOR I  BEAM
     I_________I     I________I          I___________I


There are many techniques  to modulate and demodulate  electromagnetic 
waves and the same techniques that are used at radio and microwave 
wavelengths can also be used at laser wavelengths. Some of the common forms 
are  amplitude modulation (AM),  frequency modulation (FM), and pulse code 
modulation (PCM). Computer MODEMS use FREQUENCY SHIFT KEYING (FSK) to 
interconnect with the Internet and each other. FSK, FM and phase shift 
keying (PSK) are used in satellite communication systems.

I do not know what level of complexity that you are interested in; however, 
most of the optical MODEM techniques can be fabricated by an  electronics 
engineer and/or technicians skilled in fiber optic communications. There 
are also a number of simple kits on the market to make a simple AM optical 
communication system. Edmund Scientific Co sells  fiber optic 
communications kits for transmitting voice signals and these can be used 
for transmission through the air by leaving out the fiber and replacing it 
with a pair of telescope mirrors (Assembled kit number A38299 or 
Unassembled Kit number A38300). The kits use light emitting diodes (LED) 
instead of laser diodes; however, Edmund Scientific also sells modulated 
laser diodes (Part number A53162 with 0.95 milliwatts of optical power or 
A53163, with 4.2 milliwatts of optical power). Edmund Scientific can be 
found at the following URL:

http://www.edsci.com

Amplitude modulation (AM) is the least complex technique to modulate sound 
signals on a laser beam. A simple photo diode detector and audio amplifier 
can be used to demodulate the AM optical signals. In an even more simple 
system I have used an aluminized mylar diaphragm made from a old balloon 
placed on an open ended coffee as a modulator. By reflecting the laser beam 
off the center of the reflecting diaphragm, and by talking into the open 
end of the coffee can, the vibrating diaphragm modulates the laser beam. I 
have also glued a small mirror to the center of a radio speaker and 
transmitted radio programs over a laser beam using the same technique.

For a simple AM optical receiver I have used  photodetectors wired to the 
volume control of  portable transistor radios as a simple optical receiver. 
Many years ago I wrote an article about these experiments 
in the Scientific American magazine which is available in most libraries in 
the USA. I do not know if the libraries in France have back issues of this 
magazine.The transistor receiver was also used to monitor sunlight off 
humming bird's wings (to measure the wing flapping rate) and for several 
other optical and electrical experiments.

A. E. Popa, "Transistor Radio Modified for Experiments",
 The Amateur Scientist, Scientific American, Janurary. 1973, p. 116.

Best regards,  your Mad Scientist
Adrian Popa