LIGHT IS A LEGITIMATE AMATEUR BAND

The U.S. Code of Federal Regulations, Title 47, Part 97, Subpart C - Technical Standards, 97.61 Authorized emissions, lists the frequencies and emissions allocated to the Amateur Radio Service. The last entry in this table begins with: "Above 300.000 [GHz]." Above 300 GHz, among other things, contains light. Therefore, according to the FCC, light is an Amateur Radio band. Of course, all of the other FCC rules apply here too, Amateur Radio communications are defined to be between Amateur Radio licensees, you have to ID, etc.

In spite of this entry in Part 97 some Amateurs are resistant to the notion that they can use light in their hobby. I ran into opposition from the American Radio Relay League when I submitted lightwave contacts for ARRL VHF contests beginning in June 1979. After several unexplained rejections I took my cause to the ARRL Contest Advisory Committee. In mid 1980 I sent them several suggestions for rules. They adopted the most stringent set of rules submitted:

"Above 300 GHz, contacts are permitted for contest credit only between licensed amateurs using coherent radiation on transmit (e.g., laser) and employing at least one stage of electronic detection on receive."

"While no minimum distance is specified for contacts, equipment should be capable of real communications (i.e., able to communicate over at least 1 km)."

This is not a problem at light frequencies, my very first laser contacts were over a 24 km (15 mile) path!

The League also now awards VHF UHF Century Club certificates for laser communication. Five grid squares are required.

It should go without saying that all equipment used for contest points or records should be Amateur owned, if not also Amateur built. In comparison, there would be no technical challenge or feat in borrowing a NASA tracking dish to make a 432 MHz moonbounce contact.

In summary, the FCC says light is a legitimate Amateur Radio Service band. The ARRL requires lasers for transmitters and opto-electronic receivers (no "eyeball" receivers!)

PRACTICAL AND AFFORDABLE

Before investigating the details of the equipment and techniques used for lightwave communication it should be stressed that laser transmitters and receivers are not super- exotic, super-expensive devices. Such equipment is within the reach of most any Amateur. An excellent receiver capable of receiving signals from many tens of miles away can be built from new parts for under $100. A laser transmitter can be built for about the same amount. I used inexpensive home- built equipment for setting the current 57.7 mile Helium Neon laser Amateur Radio DX record. VHF contesters are passing up a great opportunity for QSO points and multipliers by not taking advantage of "Above 300.000 [GHz]." Experimenters are also missing out on a band where it is still possible to easily set a new world record or to develop a novel piece of equipment.

LASER TRANSMITTERS

In this paper I will not go into details such as what light is, how lasers work, etc. However interesting they are, these topics would take considerable space to discuss. I'll try to get right to the point as to what is needed to build a practical and affordable Amateur Radio laser transmitter.

Step one is to select a laser to use. Some factors to be considered in choosing a laser are: cost (they range from a few dollars to tens of thousands of dollars), wavelength (visible is very strongly preferred as you will find out when you try to aim one!), and output mode (continuous wave or high repetition rate is needed in order to be modulated with information). There are many dozens of kinds of lasers. Most fit into four main groups: gas, solid state, semiconductor, and liquid. None of the solid state lasers (Ruby and Neodymium YAG are examples) are affordable or realistically modulatable. The same is true for liquid lasers. Of the gas lasers only the Helium Neon is practical. The next most common gas lasers, Argon and Helium Cadmium, are usually well over $1000 even on the surplus market and thus not suitable for Amateur Radio use. A Helium Neon laser and power supply can be had for under $100 on the surplus market. For a long time semiconductor (diode) lasers were either very expensive or invisible infrared. They have been getting cheaper, and more visible, month by month. In June of 1994 new visible diode laser pointer pens began shipping for $50 in single quantities. This marks a significant breakthrough in laser price. While they do not offer the high beam quality as Helium Neon lasers do, they have other features that help make up for this shortcoming. Both the Helium Neon gas laser and the Diode laser used in pen pointers emit CW beams that are visible red.

HELIUM NEON LASERS

The Helium Neon laser has been the most common of all lasers, but is quickly being replaced by Diode lasers. We see "HeNes" in supermarket bar code scanners, video disk players, surveying instruments, and science classrooms. There are two main drawbacks to the HeNe. The high voltage power supply is one. Fortunately, the current demand is small, in the low milliampere range. The second drawback is that Helium Neon lasers are not very easy to modulate.

The "lasing" takes place in a "plasma" tube. Near the two ends of the tube are the electrodes A high voltage of a few thousand volts DC at a few milliamps is applied to the electrodes. The plasma tube is filled with a 7 part Helium, 1 part Neon mixture. The common off-the-shelf lasers have power outputs rated from 0.5 mW to 10 mW. Higher power units have been made but the HeNe laser is one that there is a limit to how much power can be generated. Just pumping more voltage through it does not yield much more power. This is not the kind of laser capable of burning holes through things. Actually a few milliwatts is plenty of power for most applications. The wavelength of most HeNes is 632.8 nanometers, bright red. Beam diameters are typically 1 millimeter. Beam divergences are typically one to two milliradians which is extremely tight. Power requirements range from 900 to 2500 volts DC with currents 4 to 7 milliamps. A starting pulse requirement of 7 to 10 KV is typical. Plasma tube sizes are usually 6 to 15 inches long, 1" to 1 1/2" diameter. Polarized light and non-polarized models are available. Polarized units are probably more desirable as polarized light is needed for many kinds of modulators. Polarization can be achieved by placing a polarizing filter in front of a non-polarized laser, but a significant amount of power will be lost in the process.

The HeNe laser is known as the "light bulb" of the laser industry. Luckily, the laser that is one of the most suitable for Amateur Radio use is also one of the most common on the surplus market. Actually, the plasma tubes, not complete lasers, are what is usually found. A power supply and packaging are left to the buyer. Typically available surplus units are plasma tubes of 0.5 to 5 milliwatts visible red output. Because Helium can slowly diffuse through glass, Helium Neon lasers have a shelf life. Units found on the surplus market may be brand new but just older than the manufacturer wants to sell. It may pay to buy a higher power tube (e.g., 3 to 5 milliwatts) to allow for more power output degradation. The higher power is advantageous for communications and other applications such as Holography.

Plasma tube prices are usually well under $100. Prices as low as $15 have been seen. It may not be wise to pay much over $100 for a plasma tube as a new one from the manufacturer probably wouldn't cost much more. Don't buy from anyone who won't guarantee that the laser functions. If the plasma tube is dead there is nothing you can do to fix it.

The only other major item needed is a power supply. This of course can be homebrewed. There are some very good commercial modules that run off of 12 volts DC and supply the starting pulses as well as the high voltage running current. If the power supply is to be purchased rather than built, make certain that it will supply the power required by your plasma tube. The larger plasma tubes may need more power than the smaller standard supplies can deliver. An undersized power supply will not just result in a dimmer beam, it will not likely work at all. Expect a commercial HeNe power supply to cost $50, give-or-take, usually more than the laser itself.

The two main surplus sources of HeNe and other lasers are:

Both have catalogs of surplus lasers and optics. Others can be found in the pages of Nuts & Volts Magazine.

MODULATING HELIUM NEON LASERS

The topic of modulating a HeNe reveals their biggest drawback. An obvious way to modulate the generation of laser light would be to modulate the laser power supply. In the case of the Helium Neon laser this would mean modulating the nominal 2KV DC power supply. Unfortunately, this approach does not work as well as we would like. The HeNe laser is built around an electrical gas discharge, sort of a glorified Neon sign. The reader may be familiar with the operation of the common Neon pilot light such as the NE-51. It has a threshold, beneath which it doesn't light at all. If much over the rated voltage is applied, the result will soon be a ruined lamp. Such is life with the HeNe laser. It doesn't like its power supply voltage or current to vary much. Modulation can be achieved via the power supply but the percentage of modulation will not be high. Commercially available modulated HeNe lasers, which are specifically made to be modulated, offer only 15% modulation. This is not meant to discourage the experimenter from modulating a laser in this fashion. Power supply modulation does have advantages, such as the fact that it does not distort the laser beam shape. This is very important for long distance atmospheric communications where the extremely narrow collimated beam is necessary.