The European Microwave News
European Laser Communications Page

Editors - Simon Lewis GM4PLM and Jim Hatton GM4RJX



About Laser Communications What is a Laser? How Do I Build A Laser Transceiver?
Jim GM4RJX's Laser Activities
Biasing Laser Diodes (K3PGP) Laser Diode Specifications (K3PGP) A Laser Pen Transmitter (K3PGP) A Matching Laser Pen Receiver (K3PGP)

Welcome To The European Laser Communications Page

These pages form part of the European Microwave News web pages, please feel free to browse around the rest of the site after you have finished here! The EMN is a free service dedicated to microwave radio above 1 GHz and to any mode, be it SSB, CW, ATV or Data.

Although at the highest end of the microwave spectrum, these pages cover many aspects of lasers and the communications experiments being made with them

 

 
 

What are 'Laser Communications'?

Simply, the use of lasers as the communications medium, rather than using traditional radio links. Specialised transmitter and receiver circuitry is required to modulate the laser beam and to recover the modulation at the receiver. For many years, a number of US amateurs have been using lasers for communications purposes and commerical laser links are readily available. However, most of these experiments and commerical links operate over a few hundred metres. In recent years these path lengths have been extended and K3PGP has recently managed to hold a QSO over a 26 KM path using clodu scatter! With current technology and more importantly, cost of laser diode modules, these path lengths could be extended. Indeed, K3PGP is trying to complete the worlds first amateur moonbounce laser QSO!

Welcome to the amazing world of laser communciations!

 

 
 
 
 

What is a Laser?

Laser is an acronym for:

Light
Amplification, by
Stimulated
Emission of
Radiation
 
 

A laser usually consists of an active medium that amplifies light, combined with mirrors and lenses that bounce the light back and forward until it finally escapes from the laser as a beam.

Laser beams have a set of specific characteristics

There are many different types of laser medium, some of the more popular types are:

How Do I Build A Laser Transceiver?

A complete transceiver in its basic form can be seen below, The transmitter consists of a simple laser diode module, this can be either a dedicated module or a unit from a laser pointer. This is 'modulated' by a small tone generator unit, which generates an 800 Hz square wave. The laser beam is then cycled on and off by the 800 Hz square wave, you can see this by sweeping it quickly across a wall and seeing the broken beam.

The receiver is based around a phototransistor amplified by a small opamp. The recovered audio is fed to a speaker and audio amplifier. The receiver module receives the modulated beam and recovers the 800 Hz modulation and amplifys this into an audio tone.





Jim GM4RJX's Laser Activities

Some practical applications using basic laser equipment!

 

A real life laser QSO!

The brightness of a laser, even over distance, has got to be seen to be believed! This image taken by Jim using his digital camera shows the laser of Mark GM4ISM being seen at night over a 50 KM path. The skyline is that of Glasgow and Mark was located at the Blackhill transmitter site (halfway between Glasgow/Edinburgh). The laser is a bright dot on the horizon and having seen it over a similar path, the only way I can describe it, is like a car brake light! It's that bright! And it does really stand out from the general brightness of other objects at night. Jim 'illuminated' first!, using the navwarn lights of the tower as aiming point. Two metres was used a talkback. Once jim had aligned his laser, Mark used his laser light as an aiming point, as Jim was on a darkened hillside to the North of Glasgow. A two way QSO followed. The plan next is to find a longer path that is line of sight and dark, preferably with two visual aiming points! Certainly it would have been possible to extend the path to twice the 50KM - the only problem is finding suitable, and accesible sites!

 

 
 


 

The transmitter side of the laser unit uses a small, and very low power laser pointer module. These are removed from standard laser pointers! They have an output of only 2-5 mW! A typical unit is shown here. The laser is the small brass coloured block at the right of the unit. It has a small collimating lens in front of it (for focusing). The laser is normally supplied by by a simple battery supply, but in our applications the switch and battery are bypassed and the voltage feeding the laser is modulated at 800 Hz by a small switching supply. This gives an 800 Hz tone in the receiver speaker.


In a darkened room, the persistence of human vision allows you to see the on-off ratio of the 800 Hz pulses! The on-off switching can be seen in the image above.

 

The laser mounting is all important if you want to be able to point it in the right direction and keep it there. Minute movements at one end of the link will set the laser beam off by metres at the other end, over even short links. Longer paths are impossible if the laser is not mounted well. Jim's laser is built like the proverbial brick outhouse! It is mounted on large steel conduit and uses two slow motion motors to aim the laser in both vertical and horizontal azimuths. He also uses a motor controller circuit to slow the motors down even more! Consequently his aiming is very easy even over long paths. This picture also shows the additional gadgets fixed to his tripod mounted equipment, this includes imaging equipment to see over long paths (the visual version of a preamp!) and an infra-red, low light camera to see IR wavelength lasers. Jim is planning some QRO experiments with a 35 mW IR laser. Basic tests (in the shack) so far show that it is capable of burning paper at close range!


More Updates Later!

If you would like more information on laser communications or would like to arrange a sked please email Jim GM4RJX directly.



This page updated 11 November 1999