OpComms System I

System I was the OpComms group's first attempt at an optical communication system. It used an LED matrix for transmission and a homemade receiver circuit to receive the signal.

LED beacon

The group’s first transmitter design featured nine individual red LED emitters mounted in a square matrix, each independently focused by a small Fresnel lens [1]. The inherent risk in this system was lack of collimation [2] between sources, which would lead to excessive beam divergence, and distribution of power over a large area. The team managed to mitigate this issue with precise machining tolerances that maintained the lenses at fairly accurate right angles to the intended direction of transmission. The resulting transmitter acted as a very bright red spotlight, easily visible at long distances. With this transmitter, the group was able to achieve an optical link across a distance of approximately 500 meters. This alignment was done entirely manually, with the use of tripods for stabilization. The beacon was also used in later 10km tests to provide a visual identifier to locate the receive team in the dark. This often involved having a team member manually pulse the LEDs on and off. There have been proposals to build some electronics to automatically pulse at some specified rate, but nothing has yet been built.

Note: This setup is currently partially broken. Several of the lenses have fallen off and two LEDs were burned out in the course of testing. These issues could be quickly fixed with replacement LEDs and some glue.

Custom Receiver Circuit

Light from the LED beacon transmitter was received by a custom-built photodiode circuit on a breadboard. This consisted of a raw photodiode whose output was connected to a single transimpedance amplifier [3], which both filtered out high-frequency oscillations (like those expected from scintillation) and amplified the strength of the signal received. The design of this circuit was derived from the Analog Devices Photodiode Design Tool [4]. Uncertainty over how to proceed with further stages of amplification and filtering led to the purchase of the Thorlabs amplified photodiode system used in System II.

Rear Projection Television Receiver

A brief attempt was made to use the lens from a rear projection television to collect light and focus it onto the receiver. The screen of a rear projection television is a Fresnel lens [5] that focuses light from a projector in the rear (essentially a single point) into a beam the size of the screen with little divergence, such that the TV’s image is visible to a person sitting in front of it. The team investigated the possibility of using this lens in reverse, focusing all of the light incident on the screen into a single point in the rear where a receiver circuit would be placed. In order to confirm the focusing effect occurred as expected, the TV (with the projector removed) was taken outside on a sunny day and aimed at the Sun. Later inspection showed that sunlight was concentrated, not onto the projector’s former location, but instead onto a (now partially melted) section of the TV’s plastic housing. This test illustrated the difficulty of moving and aiming a structure the size of the television, a fairly serious constraint for portable test equipment. In addition, it was not clear that the team could successfully focus this particular lens into the point required. However, this type of lens - a Fresnel lens - was later used very successfully in System III.