The Laser Spy System is considered by many to be the Holy Grail of high tech spy devices because it can give the user the ability to listen in on conversations that take place in a distant building without having to install a bug or transmitter at the location. The Laser Spy System was said to be invented in the Soviet Union by Leon Theremin in the late 1940s.
Using a non-laser based infrared light source, Theremin's system could detect sound from a nearby window by picking up the faint vibrations on the glass surface. The KGB later used this device to spy on the British, French and US embassies in Moscow. It is also interesting to note that Leon Theremin invented the world's first electronic instrument, a wand operated synthesizer named "The Theremin" after him.
The Laser Spy System goes by several names such as the Laser Microphone, Laser Listener, Laser Bug, Window Bounce Listener and a few similar names. The Laser Spy certainly works well under ideal conditions, but it has many strengths and weaknesses that will be discussed in this plan. Building your own Laser Spy is by far the best way to experiment with this technology as you can adjust the design to suit your needs, rather than forking over hundreds or thousands of dollars for an assembled kit that will likely be far inferior to one that you can build yourself.
Many of the kits I have seen for sale over the Internet not only use dated technology, but they incorrectly state that the system uses a modulated laser beam to convert window vibrations into sound, which is simply not the case. Let's put the mysteries to rest once and for all and build a working Laser Spy System from the ground up and explore the functionality of each subsystem that makes a working unit.
We will be starting with an ultra basic proof of concept test system that will show you how the Laser Spy converts vibration into sound and how careful alignment of both the laser and receiver are required for optimal performance. Ironically, the most basic configuration may prove to be the most useful, and the $20 you spend in parts could create a system that works as well (or better) than some of the ones that are for sale on the internet for thousands of dollars. As you will find out, the key to spying with a laser beam is in the alignment and reception of the beam, not some magical black box full of fancy filters and optical components.