Alignment

• In any kind of communications link, the "carrier" needs to be sent from one spot to another. For light beam communications, this means the light beam itself is generated at one end and has to be bright enough and lined up well enough to be detected at the other end. So any light beam communication project needs a way to point the beam of light, get it across the intervening distance and detect it at the other end. Lasers are easy to point and brighter than most other light sources, so they are the light generation method of choice for most communication systems. There are many different types of light detectors, but photodiodes have the best combination of speed, sensitivity and cost; so they are the detector of choice.
  
  

Modulation

• Setting up the optical link --- the light beam from one end to the other --- is only the start. To transmit information, the signal must be modulated. That is, the steady signal needs to be replaced with a signal that changes with respect to some input. For a laser communications link between your kitchen and the treehouse in the backyard, it's probably easiest to hook a microphone to a pulse-width modulation circuit. The laser sends a chain of pulses. As the voltage from the microphone changes, the width of the individual pulses changes. On the detector end, the receiver senses the pulses and converts them back to a voltage to drive a speaker.
  
  

Going the Distance

• Lasers are scattered, absorbed and bent as they travel through the air --- the further your beam goes, the more you have to worry.Kim Steele/Photodisc/Getty Images
  
  Obviously, to send your laser beam through a longer distance requires more optical power. But it also requires a couple other things that are not quite as obvious. For example, if you just point a small laser beam out the window, it spreads out quite rapidly. If you put it through an optical beam expander to make the beam larger, the beam will spread out much more slowly, leaving more power to reach the detector at the other end. In addition to weakening the beam, the air also distorts the shape of the beam. If you want to transmit high data rates over long distances, you need some kind of an adaptive optics system to bring the beam back to its clean shape.
  
  

Fiber Optics

• The difficulties mentioned in the previous section are one reason for using fiber optics. To make a fiber optics communications system, you still have a laser at one end and a photodetector at the other, but instead of going through free space, you send the light into a plastic or glass optical fiber. You need to match the wavelength of the laser to the wavelength at which the fiber absorbs the least light. There are kits and information available on the Internet to help you get started on your own project.