Introduction
Laser communications systems are wireless connections through the atmosphere. They work similarly to fiber optic links, except the beam is transmitted through free space. While the transmitter and receiver must require line-of-sight conditions, they have the benefit of eliminating the need for broadcast rights and buried cables. Laser communications systems can be easily deployed since they are inexpensive, small, low power and do not require any radio interference studies. The carrier used for the transmission signal is typically generated by a laser diode. Two parallel beams are needed, one for transmission and one for reception. Due to budget restrictions, the system implemented in this project is only one way.
Laser communications have been a hot topic lately, as solutions for how to satisfy ever increasing bandwidth needs are in high demand. Some have suggested that bandwidth could be distributed in neighborhoods by putting laser communication systems on top of homes and pointing them towards a common transceiver with a fast link to the Internet. With possible transmit speeds of up to a gigabit per second, this is an exciting area. Other applications for this technology include temporary connectivity needs (e.g. sporting events, disaster scenes, or conventions), or space based communications.
High Level Design
The system can send arbitrary data at 27.7kbps baud. The system illustrated above is set up to send voice data. The microphone input is conditioned (amplified and biased) so that the full eight bit range of the analogue to digital converter is utilized. A fair amount of time was spent optimizing the amplifier for the mic input so as to minimize popping but maximize quality.
Once the digital signal is obtained by the ADC, the MCU passes the signal to the UART. The UART sets a transmit pin high or low according to the serial protocol. Some conditioning is applied to this signal as well in order to ensure constant current to the laser.
On the receiver side, the signal is read by a photo transistor. The signal coming off the transistor is put through a comparator to generate appropriate high and low signals. The UART reads these signals and generates a byte acording to the serial protocol. This byte is applied to a port, sent through a digital to analogue converter, and applied to a speaker.
For more detail: Laser Communications System
