MODULATION IN THE BEAM PATH

Modulation in the beam path avoids the aforementioned power supply problems. 100% modulation is achievable. There are a variety of objects that can be used as modulators.

A "chopper" usually takes the form of a disk with notches or holes cut into it. The disk is spun by a small electric motor so that a beam of light is rapidly chopped up by the passing openings. While this may seem like another crude approach, such devices are serious laser tools and can often be seen advertised in the laser trade magazines. A chopper certainly gives 100% modulation. The beam is full on or full off. The beam is not distorted or deflected. With a reasonable motor speed and with enough holes or slots in the disk, the beam will be interrupted at an audio rate. The audio modulated beam then can be interrupted to produce type A2 modulation, modulated CW. The Morse code interruption of the beam can be via a solenoid activated by a telegraph key, simply by hand, or by turning the power supply on and off. I have used this simple chopper method of modulation for all of my contacts. My first choppers used disks made from three inch diameter tin can lids. Two dozen slots were cut in the perimeter with a tin snips. The center of the lid was soldered to a toy electric motor shaft. The assembly was clamped to the side of the laser housing and powered from a nine volt transistor radio battery. The resulting audio tone modulated beam was interrupted by hand to achieve the final Morse code modulation. This crude scheme worked quite well and results in a very loud signals. Better and simpler choppers can be made from 12 volt DC equipment cooling fans that are now commonly available.

There are a several other beam-path modulation schemes, each with their own problems: LCD displays (slow response), large laser printer modulators (expensive), mirrors glued to speakers (poor frequency response), Pockels cells (expensive), etc. Due to space constraints and the drawbacks these devices have, they will not be discussed further.

DIODE LASERS

After a long wait, affordable and practical Diode lasers have finally arrived. As mentioned at the start of this paper, visible red continuous wave battery powered Diode lasers in the form of "pen pointers" have just hit the market for $50. Until recently, visible Diode lasers were quite expensive. The inexpensive Diode lasers were invisible infrared only. The beam from a Diode laser is not as narrow as most others lasers and requires shaping by added lenses to make it travel a long distance without dispersing. The quality of the lens- focused beam is not as good as that of a Helium Neon laser. The spot produced by today's inexpensive Diode lasers is not homogeneous, it contains bright and dark spots and is not perfectly round. The beam quality nonetheless appears adequate for our uses. No doubt, beam quality will continue to improve in future generations of Diode lasers. Two sources of the new $50 Diode laser pen pointers are:

Metrologic is the manufacturer of the device sold by Edmund.

Loose Diode lasers can be purchased and combined with a power supply if desired. A bare Diode laser will put out an unsatisfactory broad beam. A Diode laser "module" with an integral lens is a better way to go. Sharp manufactures a chip specifically for driving loose Diode lasers, the IR3C01. A drawback is that it requires both positive 5VDC and negative 12VDC. It has a TTL input line for controlling the laser. There is no input for amplitude modulating the laser, though. The IR3C01 can be purchased for $1.99 from:

Unicorn Electronics
10010 Canoga Avenue, Unit B-8
Chatsworth CA 91311
800-824-3432

Another drawback to dealing with loose Diode lasers - ones without integral lenses and power supplies - is that they are notoriously easy to burn out. The adjustment of the laser current is very critical. These devices have an operating current that varies from device to device. Setting this current requires monitoring the laser output power. Monitoring the output power requires a laser power meter, an expensive instrument, not available to most Amateurs. Another problem is that loose Diode lasers are known to be easily damaged by static electricity.

I decided that for the time being the best route to go is to buy one of the new $50 pen pointers. With a pen pointer the beam-forming lens is already in place and adjusted. There is also a power supply that operates the laser at the correct current and presumably provides static electricity protection. The entire lens, laser, and power supply is in a module about 1.5" long and 0.5" in diameter. The remaining problem is to see if some sort of modulation can be achieved through experimentation.