My Laser transmitter seen slightly off axis of the beam
After more than a decade trying to generate interest in laser communications experiments in England, 2002 finally saw the spark ignite. We are now exploring what can be achieved in sending signals from point to point by way of a Laser beam in free space. This is a vastly different challenge from using fibre optic cables to link two points.
Now in late 2003 we have a small and highly enthusiastic group of Radio Amateurs exploring this mode of communication.
This site exists to provide a point of reference for the UK activity and to draw together articles and links so that the newcomer can make a start into this fascinating mode, and the group can document progress.
The initial focus of activity has been to learn the techniques required to make a successful contact. Then distance has been increased. Initial belief was that MCW was the mode that would succeed. We used a tone that was keyed on and off. This worked so well that voice has also been used. The propagation conditions have given many surprises and much work remains to be done.
Areas for experimentation continue to grow including considerations of using a sub-carrier, like the European experiments with 25 kHz Laser SSB, (Single Side Band) or cloud and dust scatter. Watch this space!
The form that the equipment should take is also developing with both mirror reflectors and lens systems for receive aerials and nearly all equipment now employing rifle sights for alignment. Small precise adjustments in both azimuth (left and right) and elevation (up and down) are required - how do we do this successfully? Making initial contact is also a problem and much time has been devoted to solving this.
Add to these issues the need to look for still cold air and then to point the equipment with great accuracy and stability, and the challenge can be seen. A piece of practical equipment in use today produces a beam of light that expands at 400 micro radians. In plain English this is a beam of light 14 metres in diameter when seen over the distance of Dover to Calais across the English Channel.
Communicating by light is not new. For centuries the lighting of a beacon to alert surrounding areas to imminent danger was employed. The famous Elizabethan beacon chain sent word of the Spanish Armada from Cornwall to London. The limitation to the beacon chain was that it just signalled one message, Danger - Now.
The heliograph is an ancient device and using a mirror to reflect light from hill to hill worked well. With many codes devised to give the ability to send messages. Dull days ended the link.
With Morse code and electric lamps came the Aldis lamp. Here a set of Venetian blind like a hand trigger operated shutters and the flashes of light could be seen many miles away on a clear night. The Navy made much use of such signalling in the 20th Century.
Radio Amateurs are experimenters with communication by electromagnetic radiation, and light can be seen as a very short wavelength radiation. Light communication has always fascinated man and many designs for optical communications devices originated to take the Aldis lamp concept further.
The disadvantage of lamps that produce light by heating a wire to very high temperatures is the relatively slow rate at which their brightness can be made to alter, or (to be technical) modulated. Fluorescent lamps faired little better and could not be concentrated into a point source so that an optical system could be used to focus a beam onto a distant location.
In the 1960s the LASER was born. The special attributes of Laser light are that it is concentrated in a very narrow beam and all the waves are in step with one another (coherent).
All activities need guiding principles and the American Radio Relay League (ARRL) the American Radio Amateurs organisation established a few ideals to guide the amateur experimentation into optical communications. The main requirement was that any system of optical communication had to use at lease one stage of electronic processing of the received signal. It had to be a harder task than earlier methods and produce a longer range. The light used had to be coherent Laser light - Why? - Because they said so. It also made the process much more fun than using powerful searchlights as transmitters.
On a serious note, the encouragement of experimentation aspect was to turn minds onto the serious challenge of pointing a very narrow beam at a distant location. This can be seen as an extension of microwave communications where the main challenges are to know the precise frequency of operation and be able to point an invisible beam with high accuracy in three dimensions. As laser beams concentrate energy into very narrow beams, much narrower than radio dishes can produce, NASA is experimenting with Laser communications for keeping in touch with distant spacecraft.
In the vacuum of space light travels immense distances unless something gets in its way. On the earth our dense atmosphere weakens any light beam with absorption by dust and the moisture in the air. To point a pencil beam of light from hill to hill in real atmospheric conditions and to be able to receive Morse, speech or even television over the beam is a challenge to the spirit.
It is to rise to that challenge and to discover what is possible is what Amateur Laser Communications is all about.
Many Lasers are inefficient, that is they need a lot of power fed in to get a small amount of power out. Other lasers are very powerful and too dangerous to use freely in the atmosphere. Most are just too expensive or not available to the amateur experimenter.
Two types of Laser do fall into the realm of the amateur experimenter. The gas Helium/Neon Laser and the semiconductor Laser diode.
The He/Ne gas laser typically operates at a few thousandths of a Watt output and can be thought of as a special type of neon light. At the low powers available it is safe to use in experiments but safety is an imperative with all experiments carried out by licensed Radio Amateurs. The licence examination has safety as a major feature of the syllabus. The He/Ne Laser, like the fluorescent light, can only be directly modulated to a very limited extent. Too much of a change in the voltage on the tube causes the light to go out - not good. So ingenious methods have to be found - cue for a set of experiments. Professional modulating crystals do exist but are rarely seen in experimenter circles, as they are both exotic and expensive.
The other possible candidate for an experimenter's Laser is the semiconductor Laser diode. This can be thought of as similar to the familiar light emitting diode (LED) seen on the front of almost all consumer electronics. The Laser diode has a minute lasing cavity of about 1,000,000th of a metre and the problem exists of extracting the heat produced fast enough. The Laser diode lives on the very edge of death. One tiny spike of electricity that causes it to produce just too much light will cause instant extinguishing of the light - forever. Again a nice challenge.
Finally the electronic devices that take in light and convert it to electricity are not without their problems. They are either very expensive or just too precious to use or cheap and too slow and insensitive.
Updated 31 DEC 2003