Long Range Laser Communication

This project was born out of the purchase of a handheld green laser. A friend and I sought to find out how far away this laser could be seen.

To make fine adjustments in aiming the laser a 2 part mount was made. The first part controlled the altitude angle of the laser. It was made out of a door hinge which had earlier served as a barn door tracker with little success. A nut was glued to the lower half of the hinge where a bolt was threader through it. Turning the bolt caused it to thread into the nut pushing up on the hinge to enable very fine altitude adjustments. The second part of the mount enabled fine azimuth adjustments. The mechanism was salvaged from an orange juice squeezer. In the orange juice squeezer the rotation from a motor was decreased in speed and increased in torque by a series of gears. We used the same gears to make a small adjustment at the motor, by hand, into a very tiny adjustment at the laser end.

First Generation Geared Laser Mount

Laser Transmitting a 30mi Path

Laser on Geared Mount

We used this mount on a 4.2mi , 12.0mi 15.7mi , 16.8mi , 25.1mi , 27.8mi , and 31.3mi paths. At 31 miles the laser was able to cast a shadow at the other end. It was very hard to estimate the diameter of the spot because of slanted, non uniform nature of the ground, but it was many meters in diameter.

Laser From 4mi Away

Spot Cast From a Laser 15mi Away

Laser From 25mi

Laser on a 50mi Path as Seen From the Transmitter

Laser Receiver Box

I became interested in modulating the laser in order to achieve digital one way optical communication. This was achieved by creating a battery eliminator to replace the 2 AAA batteries that normally powered the laser. The push button on the laser was held down with a rubber band. The laser was controlled by switching the power to the battery eliminator. For simplicity I choose to modulate with Morse code. An Arduino micro controller using the Morse EnDecoder Library controlled the power to the laser with a mosfet. Plain text was typed into a computer serial monitor which was connected over usb to the micro controller. It then encoded the text into morse and modulated the power to the laser. The laser was still mounted onto the geared mount.

On the receiving end a Fresnel Lens (actually a plastic magnifying sheet) was mounted in a box. At the focal point of the lens was a photo resistor. The level of light at the photo resistor was detected by another Arduino micro controller. A potentiometer was used to set the optical squelch. The potentiometer adjusted the level of light of what was considered a logic high. With the laser off the potentiometer was adjusted so the light was detected as logic low, as indicated with an led. When the laser was on the increased light would cause the photo resistor to indicate a logic high. This logic level was sent through the same Morse EnDecoder Library and displayed on a computer connected over usb.

The optical communication box was successfully run across a record breaking 15ft of my living room. On a 51.7mi path the laser was clearly visible but the optical communication system was unable to differentiate between laser on and laser off. This was probably because of poor construction of the box which let the photo resistor flap around in the wind, out of the focal point. As well as light pollution in the path especially the Washington State Penitentiary.

With an improved construction of the box a 1.2mi path was used to test. Initially the laser was to bright and the photo resistor was unable to recover in time from each pulse. Adjusting the optical squelch worked, but as the battery powering the laser discharged and the laser got dimmer the optical squelch had to be reset. Additionally a new audible s-meter was created which increased the frequency of a tone proportional to the intensity of light detected. It was very useful in aligning the laser and detector. The entire test was done at 5wpm morse code. A photo diode circuit is being created to allow for faster modulation.

In testing a light to frequency converter was usable in a lab up to 20wpm.

Laser Transmitter Electronics Case
Laser Transmitter Electronics

Optical Box Under Testing
My Optics Lab

Using a small solar cell as the detector allowed for audible frequencies to be heard by plugging a pair of headphones directly into the solar cell.

Solar Cell as Laser Detector with Headphones

By feeding extremely amplified audio (about 40v PTP) into a mosfet which switched the 3.3v for the laser I was able to achieve. Voice, and music, transmission through the laser. The solar cell detector is plugged into an amplifier which is connected to the speaker.

This audio link can be utilized to send data using amateur radio protocols and software that are traditionally used to communicate over radio links. I used fldigi to send one way PSK and RTTY messages through the laser link. With only the solar cell, no lens, BPSK-125 and RTTY-45 was used successfully over a 0.6mi path. At this distance the detector had to be carefully positioned for it to work. At 1.0mi we were unable to get communication.

With the solar cell at the focal point of the fresnel lens we were able to achieve a voice conservation as well as RTTY and PSK text at 1.0mi and 2.1mi. Additionally MFSK images were tested successfully on the 2mi path.

Improved Audio Electronics

I’m doing additional work on the driving and receiving electronics to increase audio fidelity and reception range. The input audio wave will be converted into a PWM waveform before being fed into the laser. A preliminary test with an opamp comparator based PWM generator connected to a function generator showed that this produces a much higher quality signal.

First PWM Generator

Laser Microphone

I reflected the laser off a surface adjacent to a speaker. This reflected light was detected by the photo cell detector. The laser was unmodulated. This turned the system into a laser microphone, and the resulting audio decoded was intelligible, though filled with noise.

Reflector for Laser Microphone

Here are some slides with notes for a presentation on this project.