|
|
This paper was presented at 1994 Microwave Update in Estes Park Colorado.
Steve J. Noll, WA6EJO
1288 Winford Avenue
Ventura CA 93004
Contents
Light is a legitimate Amateur Radio band as defined in Part 97 of the Code of Federal Regulations. The American Radio Relay League has a contest rule that specifically allows lightwave contacts. Lasers are required for transmitters and electronic detection is required in the receivers. While Helium Neon lasers have long been under $100 on the surplus market, new diode pen-pointer lasers have just hit the $50 level making for small and inexpensive transmitters. A voice modulated Diode laser transmitter circuit using a $50 pen-pointer laser is shown here. Lightwave receivers are simple devices. This paper shows a photodiode-based receiver that is both inexpensive and performance competitive with a photomultiplier tube. Also discussed are transmitter and receiver construction, and operating techniques based on actual experience.
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."
The ARRL VHF contest rules also state:
"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!)
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.
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.
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:
|
MWK Industries |
Meredith Instruments |
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.
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.
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:
|
Edmund Scientific |
Metrologic Instruments |
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.
I have designed a voice modulated Diode laser transmitter using the Metrologic $50 pen-pointer laser. It seems to work well, although I don't claim it to be optimized. I spent only a day on this circuit, versus weeks on fine-tuning the photodiode receiver circuit presented here.
I first envisioned pulling a trick on the Diode laser. These modern Diode lasers contain not only a laser, but also a photodiode. The purpose of the photodiode is to monitor the output of the laser and provide the power supply with feedback as to how bright the laser is. This feedback circuit stabilizes the laser output over temperature changes, which they are quite sensitive to. I thought that if I shined a bright, modulated, LED into the laser's lens I could fool the feedback photodiode into modulating the laser. It didn't work at all. In fact, it worked so entirely and completely not at all that I suspect that Metrologic is really not using the feedback photodiode, even though their data sheet claims that. So much for tricks.
The trick that I thought wouldn't work, did. That is, modulating the power supply. I figured that if the Diode laser has such a critical operating point, and that it is easily damaged by static electricity, that the pen-pointer's power supply would be somewhat sophisticated and resistant to power source (battery) changes. It isn't. In fact it acts like the sophisticated laser power supply is maybe just a resistor. Varying the power supply current varies the laser's brightness. How simple! By the way, the pen-pointer in question uses two AAA batteries for power. Note that other models of pen pointers may have more sophisticated power supplies that would prevent them from working in the circuit I have designed. This circuit has been tested only on the Metrologic ML211 (Edmund G52,442) laser pen-pointer.
MIC is an electret condenser microphone element. It is Radio Shack catalog number 270-090, $1.79. The supply voltage can range from 2 to 10 VDC, but the microphone is optimized for 4.5V. (It's kind of noisy at 9V.) The two 10K resistors divide the 9V battery down to 4.5V.
IC1 is an LM324 quad op amp. Only one section is used. I'm sure that a number of other op amps would suffice. The 324 is good as it was designed for single supply operation and it draws little current.
The 1K and 100K resistors connected to op amp pin 2 set the gain. You may want to vary the 100K value for different microphones.
The 100K pot sets the voltage that the circuit "idles" at. Monitor TP with a DC voltmeter while adjusting the pot. I found 3.25 VDC +/- 0.05 VDC to be optimum. Mis-adjustment will result in a distorted voice signal.
Switch S1 has two positions. When it is connected to 9 Volts it is in the "Align" position. This will turn the laser on full brightness. When switched to the pot the switch is in the "Voice" position. Here the laser is biased for voice transmission. It will not be as bright because it is never fully on.
The transistor is needed because the op amp will not source or sink quite enough current alone to power the laser.
The string of four diodes is very important. They act like a 3 Volt Zener diode to limit the voltage to the laser. I don't know how much more than three volts the laser can take, and I don't want to spend another $50 to find out. I used 1N4007 diodes for the string. Others will work, including real Zener diodes. Make sure to check that the circuit limits the voltage to three volts before connecting the laser. You can test this by flipping S1 to the "Align" position while measuring the DC voltage where the laser would connect.
The power source for the transmitter is a 9 Volt transistor battery, but other voltages could be used too. It draws about 70 mA at 9 Volts. A 12 Volt Gel Cell or NiCad would give more amp-hours (life) than a 9 Volt battery, and would be rechargeable to boot. I wouldn't expect that any circuit changes would have to be made.
The Laser is again from a Metrologic ML211 pen-pointer, 3 Volt supply, 675 nm output wavelength, 2 milliwatt output power, 1.7 milliradian divergence. (Also known as Edmund Scientific p/n G52,442.) The Diode laser module simple pulls out of the pen-pointer housing. (There goes your 90 day warranty!) The pocket clip is positive, the coiled spring battery contact is negative.
A laser receiver consists of an antenna, a detector, an amplifier, and a speaker - the same as a simple radio receiver. In this case the antenna is a lens or mirror. The detector is a photodetector that converts light into electricity. The amplifier boosts the signal enough to drive a speaker. There may also be filters; optical and/or audio.
As with radio receivers, the antenna of a light communications receiver is the best area to make system improvements. "The bigger the better" applies here also. The purpose of the light antenna is to gather as much light from the desired source as possible, concentrate this light onto the detector, and reject light coming from unwanted sources. Unwanted sources are the sun, moon, street lights, etc. Lenses and curved mirrors are our main tools. Such elements need only be mounted in an appropriate tube or holder to make a complete antenna system. Obviously what we need is something like a telescope. Unlike a telescope, however, we needn't bother trying to obtain an actual detailed image of the light source. We need merely to gather as much light from the source as possible. Also, adjustable focus in not needed. A light communications antenna then can be much simpler and cheaper than a telescope.
The conventional lens is the most common light receiver antenna candidate. Because of weight and cost, four inches diameter is about the largest size inexpensive glass lenses appear in. Mirrors take over from there. Color corrected lenses (achromats) are not needed for our application because we are dealing with only one wavelength of light. One source of large diameter lenses are the magnifying lenses typically sold in stationary stores. These lenses may be made of plastic instead of glass and will require a little more care in handling to prevent scratches. Watch for used Luxo-type magnifying work lights that have a very large lens surrounded by a fluorescent bulb. Edmund Scientific sells a folding stand magnifier that contains a 4.3" diameter glass lens with a 8.5" focal length. It is their model G38,599 priced at $9.50. The lens can be popped out of the plastic stand and will make an excellent optical "antenna."
Fresnel lenses (pronounced fray-nel) are flat lenses, usually less that 1/8 inch thick. They come in round, square, and rectangular shapes and are usually made of plastic. One side is flat while concentric ridges are molded onto the other side. Fresnel lenses won't form much of an image, but they will gather a very large amount of light for their cost and weight. I have used a 10 1/4 inch diameter Fresnel lens mounted in a metal duct pipe to serve as an antenna for a portable 931 photomultiplier receiver. Edmund Scientific is a good source of Fresnel lenses.
Parabolic and spherical curved mirrors make excellent light antennas as witnessed by their heavy use in astronomy. Swap meets and garage sales may be sources for old reflector telescopes that use such mirrors. A small telescope with a photodetector mounted in place of the eyepiece would make a great light communications receiver, but a large Fresnel lens would probably gather much more light.
If only night time communication is anticipated, then optical bandpass filtering can be dispensed with. Daylight communication is a different story. The sun is quite bright, to say the least, and can swamp the photodetector. A very narrow bandpass filter is what we need. Luckily, Dielectric filters, sometimes called Interference filters, fit the bill. Edmund Scientific sells dielectric filters for several popular laser wavelengths for about $40 each. The specifications for their 632.8 nanometer Helium Neon laser filter state a half power bandwidth of 10 nanometers which is only 1.6 percent! The drawback is that the filter insertion loss is 50%, which isn't too bad considering the bandwidth. The filter should be placed just in front of the detector.
As it is with lasers, there are a large number of different photodetectors available. There are photodiodes, phototubes, photomultiplier tubes, phototransistors, photo field effect transistors, photovoltaic cells (solar cells), photoresistive cells, avalanche photo diodes, photodarlington transistors, even photo traveling wave tubes! We won't waste time exploring in detail some of the exotic kinds. The devices available with good performance at a reasonable cost are our main concern.
Next to cost, there are three other important parameters of concern. 1) Spectral response. Does the device function well at the wavelength of light we are dealing with? Presumably we will be mainly concerned with the red output of either a Diode or Helium Neon laser. 2) Sensitivity. Somewhat tied to spectral response at a given wavelength. The light sensitivities among the different kinds of sensors vary by at least a factor of 1000. 3) Speed. High speed is mainly a concern with short pulse width applications such as receiving light generated by a pulsed laser. Speed is not as much of a concern with audio or MCW modulated continuous output lasers.
Photovoltaic cells, or solar cells as they are commonly known, are not considered to be especially good for communications work. This is not what they are optimized for. They are fairly inexpensive but their speed is of question as their large area makes for large capacitance. Their "capture area" may very large, but this is not an advantage for situations where lenses or mirrors are used to gather the light. The spectral response is good for the portion of the spectrum that we are interested in, but this also means they are easily overloaded by sunlight.
Photoresistors have been around for a long time. Several kinds are available at very low cost. While the spectral response of the Cadmium Sulfide (CdS) photocell is very good at red light, they are considered far too slow for detecting modulated light.
In the category of phototransistors, there are bipolar phototransistors, photodarlington transistors, and photo Field Effect transistors (photo FET). Phototransistors are more sensitive than photodiodes as they have gain. This gain is achieved at the price of speed and dynamic range.
Bipolar phototransistors are inexpensive and have response times of 2 to 20 microseconds and good red sensitivity. They can be saturated by too much light and care should be taken to prevent this by optical shielding and filtering.
The photodarlington transistor contains a light sensitive transistor as in the bipolar phototransistor and an additional transistor that amplifies the output of the first one. These devices are very light sensitive. Minimum sensitivities are in the area of 6 to 24 mA/mW/cm2 for tungsten light. Speed is correspondingly slower, in the 10 to 100 microsecond range. As with phototransistors, photodarlingtons may have a base lead which is usually left unused. The Motorola MRD360 photodarlington is one of the best units with a minimum sensitivity of 24 mA/mW/cm2, a typical rise time of 15 microseconds, and a typical fall time of 65 microseconds. This device sells for under $3.
Common photodiodes are inexpensive, fast, and with good spectral response. They do not have the internal gain that photomultiplier tubes or avalanche photodiodes do, but much of that can be made up by use of a properly designed amplifier. Excellent photodiodes are available for $2 to $5 from sources like Newark and Digi-Key. Photodiodes generate a current proportional to the light striking them. Note that it is a current, not a voltage. The voltage generated by a photodiode is not linear with the light input. A current-to- voltage converter known as a transimpedance amplifier is used to convert the current output of a photodiode to a voltage. The current generated by a photodiode ranges from picoamps to milliamps. A transimpedance amplifier is as simple as one op amp with one feedback resistor. Not only does a current-to- voltage conversion take place in a transimpedance amplifier (e.g.: 1 milliamp in results in one millivolt out), but amplification occurs too. The amplification of the conversion is determined by the feedback resistor value: voltage out = current in times the feedback resistor value. Thus, an extremely tiny 1 nanoamp input (1 x 10-9 Amp) times a 100 megohm feedback resistance (1 x 108) results in 0.1 volt out - plenty to drive a speaker amplifier.
Until recently, I have done all of my Amateur Radio laser work, including record DX shots, with photomultiplier receivers. The Helium Cadmium laser record was also set with PMT receivers. I wish now to concentrate my efforts on photodiode receivers for several reasons: 1) I have learned how to design photodiode receivers that perform extremely well. 2) Photomultiplier tubes have drawbacks such as a high voltage power supply requirement, poor red sensitivity, and the cost of new tubes. 3) I feel that some Amateurs might be turned off by the drawbacks of PMTs, despite their superior performance. 4) I like to make equipment that is readily reproducible by others - and this is easier to do with simple devices like photodiodes than it is with PMTs. Nevertheless, because of their superiority in the area of low-noise high- gain, and their successful history of use in Amateur Radio laser work, photomultipliers deserve some coverage here.
Photomultiplier tubes are large and fragile, they require high voltage power supplies, they cost more than most of the common solid state detectors, they can be hard to find, and their spectral response is usually not good at red. Why use them? They have gain in the millions. Sensitivities typically range from thousands to tens of thousands A/W. The gain is developed inside of the tube and without excessive noise. Rise times of 3 nanoseconds are typical. PMTs can detect extremely low levels of light and are linear over wide ranges of light intensity. The old and popular 931A PMT is linear within 3% over a change in light intensity of 107. The Avalanche diode is the closest solid state equivalent, but it is even more costly, temperature sensitive, and requires a highly regulated high voltage power supply. If Avalanche photodiodes ever become low priced items then they may have the advantage over the PMT. But for now, the "hottest" optical front end is the PMT. Common PMTs look similar to a glass envelope octal base radio tube. They typically have eleven or more base pins depending on the number of stages. There is no filament. Light enters a side or end window and strikes a Photocathode. It is coated with chemicals that determine the portion of the spectrum that the tube will function best at. For every 10 to 10,000 photons that strike the Photocathode, one electron is released from it. This electron is attracted to a nearby element that is energized about 200 volts higher than the Photocathode. This element is called a Dynode. The striking electron dislodges two or more electrons from the Dynode. This process continues for several more Dynodes, one after another, each Dynode biased about 100 volts above the last. This process results in gains to 108. The electrons emitted from the last Dynode are collected by an Anode that serves as the output terminal. Most often a simple resistor voltage divider circuit is used to power the tube. Negative potential is applied to the Photocathode. A resistor connects the Photocathode to the first Dynode. Each succeeding Dynode is connected to the last by a resistor. The positive voltage is applied to the end of the string. The resistors connecting the Dynodes are typically 10K to 500K ohms. The resistor between the Photocathode and the first Dynode is usually twice that value. The voltage applied to the divider is on the order of 750 to 1500 volts with current consumption around 1 mA. The 931A and the 1P21 are two of the more common PMTs. These are nine stage tubes as they contain nine Dynodes. They require an eleven pin socket. Light enters the side of the tube to strike a Photocathode with a surface area of about 1/3 inch. The peak sensitivity is at 400 nm and is unfortunately less that 10% at 633 nm (HeNe laser). At 1000 volts supply voltage, the current amplification for the 931A is 800,000. The Allied Electronics 1994 catalog lists the 931A at $63. I have purchased many used tubes at swap meets for $1 each.
Most all lightwave receiver designs I have seen in electronic hobbyist books and Amateur Radio publications are truly pathetic. The best range I've seen claimed is a couple thousand feet, more often just a few hundred or even tens of feet. In my opinion anything under several miles does not merit the time of a Radio Amateur worth his or her salt. Bear in mind that for the ARRL VUCC laser award you have to do five two-way QSOs, one of those all the way across a grid square - something like 60 miles or more! A well designed receiver is essential. Photomultiplier receivers are well capable of this performance, but I believe that a properly designed photodiode receiver will also do. The photodiode receiver design I am presenting here works as well or slightly better than the photomultiplier receivers I used to set the current 57.7 mile HeNe laser DX record. This highly optimized design when bench tested side-by-side with one of my PMT receivers worked better. (See figure 2.)
Pd, the photodiode, is a EG&G Vactec VTP1188S. It is available as stock number 95F9029 from Newark Electronics for $2.36 (good photodetectors don't have to be expensive!) Its plastic package also serves as a lens. The package is about 0.3" in diameter and 0.3" tall. The typical sensitivity is 0.55 A/W (Amps output for Watts of light input). This does not mean that the device will put out 0.55 Amps under a 1 Watt light bulb - photodiodes don't generate that much current. 0.55 microamps for 1 microwatt is more realistic. The 0.55 A/W figure is typical for any silicon photodiode. The sensitivity of silicon photodiodes does not vary much from some standard figures due to the laws of physics. The sensitive area of the actual VTP1188S photodiode chip is 11mm2. Much smaller area photodiodes are available and result in higher speeds due to less capacitance. But the smaller the active area size is, the more critical the optics are. You have to get as much signal to efficiently illuminate the photodiode as possible, which would be more difficult if the chip is very tiny. Much larger area photodiodes are also available. They also offer us no advantage. They cost a lot more, don't put out any more Amps/Watt, and generate more noise. The VTP1188S is by no means the only photodiode that would work for our application, but it is an excellent performer, readily available, and inexpensive.
Rl, the load resistor, serves mainly to give capacitor Cc a discharge path to ground. Its value is not critical, but best performance was found at 50 megohms, plus or minus 10 megohms. This is a much higher resistance than most Amateurs, or even electronic professionals are used to dealing with. You will not find a resistor value above 10 megohms from any of the normal parts sources, such as Newark or Digi-Key. Specialty resistor manufacturers such as Victoreen make the high resistance, low capacitance resistors commonly used in photodiode transimpedance amplifiers. Values are available in the Gigohms! Unfortunately, these resistors are expensive ($5 to $15) and high minimum orders ($100) are required. Fortunately, the circuit shown here is not too critical and a string of 10 megohm resistors will work just fine. Cheap 10 megohm resistors are available from Digi-Key as their part number 10ME-ND, five for 28 cents, or 100 for $4.60. Strings of these resistors should be used for Rl and Rf. I have tested this circuit with both Victoreen resistors and strings of the Digi-Key resistors and saw no difference in the resulting signal-to-noise ratio.
Cc, the coupling capacitor, serves to let AC signals from the photodiode through to the amplifier while blocking DC signals. This is a very important feature of our circuit. Capacitive coupling is very unusual for transimpedance photodiode amplifiers - they are normally DC coupled. After working for over 2 1/2 years for one of the top photodiode manufacturers I had never seen this done! If you don't capacitively couple, the DC signals generated by sunlight, moonlight, incandescent light, etc., will totally overload the amplifier. The value of Cc is not at all critical. 0.1 uF works fine, but 0.01 uF and 1.0 uF work fine also. It should be a reasonably good capacitor, that is, not leaky.
Rf, the feedback resistance, is another high value resistor. 50 megohms in the form of a string of 10 megohm resistors works fine. Again, this value is not critical. This is a very important part of a transimpedance amplifier. The output voltage of the amplifier is determined by multiplying the input current times the value of this feedback resistor. This amplifier will put out 0.5 Volts (a very large signal) with an input of only 10 nanoamps (10-8 Amp)! The photodiode will generate 10 nanoamps with a light signal of only 18 nanowatts (0.000000018 Watt)! The "power" of a transimpedance amplifier is incredible.
Cf is a very small capacitor, in the order of 1 to 10 picofarads. Without it a phenomenon known as "gain peaking" will occur. At some relatively high frequency the gain of the amplifier will peak above what it is supposed to be as determined by the feedback resistor. Cf also is used to reduce the normal amplifier gain at high frequencies. Gain outside of the desired bandwidth, which is 300 to 3000 Hertz, the standard communications audio bandwidth, just contributes to unwanted noise. This capacitor can be adjusted for the best sounding signal when receiving a very weak signal. A better way to adjust it is to connect a sensitive AC voltmeter to the output of the amplifier. Make a signal source by powering a LED from a audio signal generator set to 1000 or 2000 Hertz. Shine the signal into the photodiode and adjust the physical arrangement and the signal generator output voltage to get a very weak signal ( a few mV). Turn the signal generator on and off while adjusting the capacitor for the largest ratio of signal-plus-noise (signal generator on) to noise (signal generator off.)
IC1 is an op amp. The "input noise current" rating of this op amp must be very low for this kind of circuit. Fortunately, there are a number of good op amps that aren't too expensive. I began by testing nine op amps ranging in price from $1 to $20. They included the Burr-Brown OPA627 and OPA111 - premier op amps for photodiode transimpedance amplifiers, the PMI OP07, the Analog Devices AD743, Linear Technology's LT1028, LT1037, and LT1055, the new National LMC6001, and even the lowly 741. The input noise current for some of the better op amps tested was rated under one femtoamp, that's 0.000000000000001 Amp! This rating is important as the current from the photodiode is going to be amplified 50 million times by our circuit. In some tests op amp A did better than op amp B, in other tests just the opposite. A transimpedance amplifier circuit is deceptively simple, there actually are dozens of nuances and a lot of interplay. The testing was done by measuring the signal- plus-noise to noise ratio in a 300 to 3000 Hertz bandwidth while receiving a weak signal from a 1 KHz square wave modulated red LED. The results in a nutshell: the OP07, OPA111, OPA627, AD743, LT1055, and LMC6001 all performed well. The losers were the LT1028 and LT1037 - presumably their extremely high bandwidths contributed to the noise. The 741 did surprisingly well, but there are much better choices for just a few dollars more. I have settled on the AD743JN because of its excellent performance and its price and availability ($5.53 from Newark). The Burr-Brown parts are top performers but are very hard to get and are expensive. An excellent alternate to the AD743JN would be the LT1055CN8 at $3.04 from Digi-Key.
When designing a circuit for absolute lowest noise even the op amp supply voltage needs to be taken into consideration. The high quality op amps we are dealing with are normally spec'd by the manufacturer for operation at +/- 15 Volts. Voltage causes current, current causes heating, heat causes noise. I measured lower noise at +/- 9 Volts, which is also convenient for battery operation.
The above covers all of the critical components in the laser receiver with the exception of circuit layout and shielding. Those used to dealing with RF and microwaves may think they have seen it all, and that there can't be much to a circuit that just amplifies audio. Nothing could be farther from the truth when you are dealing with gains in the tens of millions and currents in the picoamps! It is important to use a printed circuit board with plenty of ground plane. Keep the input well isolated from the output. Keep leads short. Keep the board and components clean and de-fluxed else picoamps will leak across the insulation. This much gain at audio frequencies will result in 60 Hertz pickup, so shielding is needed. I built the transimpedance amp alone on a 4" diameter round circuit board with the photodiode in the center. On the back of the circuit board I soldered a 3.4" diameter Coffee-mate(r) jar steel lid. I did the same on the front of the circuit board using a lid with a 0.5" hole in the center for the photodiode to look through. The finished round board fits into a 4" diameter plastic pipe along with a 4" diameter lens. The filter and speaker amp are on a separate round circuit board.
The filter is a TOKO THB111A 300-3000 Hertz active bandpass filter. It is available from Digi-Key for $21.39 (their part number TK5425-ND). It is 30 dB down at 100 Hertz and 7.9 KHz. Its main purpose is to attenuate 60 and 120 Hz optical interference street lights, room lights, etc. It also attenuates non-optical pickup of 60 Hz electric fields. And it also limits the upper bandwidth to reduce noise, which would be heard as a hiss in the speaker.
IC2 is the renowned National LM386 audio power amplifier. The capacitor across pins 1 and 8 sets the chip to its maximum gain of 200. This chip is available in three versions. The LM386-1 is optimized for 6 Volt operation at which it will put out 325 mW of audio power into an 8 ohm speaker. The LM386-3 is optimized for 9 Volt operation at which it will put out 700 mW. The LM386-4 is for 16 Volt supplies and will drive a 32 ohm speaker to 1 Watt. The -1 and -3 versions can be run as high as 15 Volts. Only the -4 requires the 0.05 uF capacitor and 10 ohm resistor on pin 5. These components can be left off circuits using the -1 and -3 chips. I strongly recommend not running the LM386 off of the same 9 Volt battery used to power the transimpedance amp. The LM386 draws significant current which may disturb the incredibly sensitive photodiode amplifier. It would be ideal to use a LM386-3 and power it from a separate 12 Volt NiCad or Gel Cell. Alternatively, a separate 9V alkaline transistor radio battery would do.
The Edmund Scientific folding stand magnifier lens is ideal for the receiver antenna (their model G38,599 at $9.50). The 4.3" diameter glass lens with a 8.5" focal length can be popped out of the plastic stand. Its diameter is perfect for mounting inside of a length of 4" ABS "DWV" pipe. This pipe can be purchased from a hardware store. It is black ABS plastic with a 4" inside diameter and a 4.5" outside diameter. The lens can be pinned between the end of the pipe and a plastic pipe coupling, also available from the hardware store. The same mounting method is used for the detector/transimpedance amp circuit board and the filter/speaker amp circuit board. The speaker and on-off switch can be mounted on a matching pipe cap. The entire laser receiver, including batteries, will fit in a neat and inexpensive 4.5" diameter by 2 foot long housing. A second pipe cap can be used to cover the lens end of the receiver during storage and transport. (See figure 3.)
A way to fasten the round pipe to a tripod is needed. The receiver can be mounted to a metal plate with a 1/4-20 threaded hole in it. 1/4-20 is the thread on a camera tripod screw. "Minerallac Straps" can be used to mount the pipe to the metal plate. Minerallac straps are "U" shaped metal clamps that snap around pipe and allow it to be fastened down by a single bolt. Electrical supply houses or very well stocked building supply stores will have these. Two straps can be used to fasten the receiver to the metal plate.
Obviously, the atmosphere is a good medium for transmitting visible light. Atmospheric transmittance can be considered excellent from 500 nanometers to about 950 nanometers. Meteorologically speaking, on an "exceptionally clear" day, visibility is considered to be 50 to 150 kilometers. It is not quite as simple to predict just how far you can communicate on a beam of light. There is a formula though:
Range = meters.
Po = The power output of the laser in watts.
Ar = The area of the receiver lens or mirror in square meters.
To = The transmissivity of the receiver optics.
Ta = The transmissivity of the atmosphere.
Pt = The threshold power of the receiver system in watts.
D = The laser beam divergence in radians.
Lets try some practical figures.
Po = 0.002 watt (2 milliwatt), typical for a small HeNe or Diode pen pointer laser.
Ar = 0.008 meter (4 inch diameter lens).
To = 0.84 (84%), no filters, two typical 8% reflections off of the lens surfaces.
Ta = 0.9 (considered a clear day).
Pt = 5 x 10-12. Approximate Noise Equivalent Power of the EG&G Vactec
VTP1188S photodiode in a 300-3000 Hz bandwidth.
D = 0.0015, (1.5 milliradians) for a typical Helium Neon laser or Diode laser pen pointer.
Range = 1,017 kilometers, or 633 miles!
Is this figure realistic? Not taken into account is the noise that the receiver circuitry adds to the signal. If we make Pt ten times worse we get a range of 328 kilometers, or 204 miles. Probably more realistic, but still fantastic. Atmospheric distortion and the problem of finding two points this far apart and still line-of-sight would likely be the real limiting factors.
Note the importance in this range formula of keeping the laser beam divergence small. Halving the divergence doubles the range. Small divergence is a quality to look for in a laser. Note too the importance of antenna area. A 2" diameter lens gives half the range than a 4" diameter lens.
I was a member of the Los Padres Microwave Group in the late 70's and early 80's. This was a West Coast VHF/UHF/Microwave contest group founded by W6OAL, WB9KMO, and myself. My job was to provide and operate the microwave-and-higher contest gear. After two years of one-way testing we completed a two way HeNe laser QSO with the K6MEP contest group in the June 1979 ARRL VHF QSO Party. We were located on Frasier Mountain, Ventura County, Santa Barbara section. The K6MEP group was on Reyes Peak, 15 miles away in the same county and section. The equipment at K6MEP was a 3 mW Spectra Physics HeNe laser owned by W6OAL. I modified the laser by adding a chopper modulator and a tripod mount. I built the receiver around a 931A PMT with a 3" lens. The gear the Los Padres group used was a 4 mW chopper modulated HeNe laser I built around a surplus plasma tube. The receiver was a 931A PMT with a 10 inch Fresnel lens. The choppers were made from small DC motors and tin can lids. The chop rate provided 1 KHz modulation. The resulting audio modulated beams were Morse code CW modulated by interrupting by hand. This proved to be one of the more difficult aspects of the contact as the hand interruption had to be made in inverse Morse code. It took a little getting used to! By far the most difficult problem was aiming the lasers. First, finding the other party is difficult at 15 miles. Second, keeping the laser aimed is equally hard. With a one milliradian beam divergence, discounting any path distortion, the beam at 15 miles away would be 79 feet in diameter. Moving the laser by just one degree would move the 79 foot diameter beam at the far end by a whopping 1382 feet! Contact was established by slowly sweeping the lasers back and forth across the suspected target point. Two meter liaison was essential. The person at the receiving end would signal on 2M when he saw a flash of red light. This was not sufficient to stop the beam in time but it did give the people at the transmitter end a good idea of where to sweep at a slower rate. Finally the beam would be somewhat steady at the receiving end. At night it was very bright red. Tests during the day were also successful, but the beam did not appear as bright and looked somewhat pink due to the competing sunlight. All equipment was mounted on standard SLR camera-type tripods. Heavier tripods, such as used for video cameras, would be highly desirable. Laser two way contacts were achieved by the Los Padres Microwave group with the same equipment a half dozen times since the 1979 contact, although it wasn't until the June 1982 ARRL VHF contest that the ARRL would grant us contest credit.
Since the original late 70's - early 80's shots, I have built several generations of equipment. Some important lessons learned were incorporated into improved equipment:
1) The support for the laser must be extremely stable. I've abandoned camera tripods and made my own heavy tripods.
Here's the problem... If you are using a laser with a typical 1.5 milliradian beam divergence, then the beam will spread out at a rate of 1.5 feet for every thousand feet of travel. Therefore, 50 miles away the beam is about 400 feet in diameter. If the laser or tripod twists by only 0.1 degree, the beam at the receiving end will move 460 feet - totally missing the target! If the laser is mounted on a baseplate a foot long, moving one end of that plate only 0.0017" will have the same effect. Camera tripods aren't up to this kind of stability.
I have made tripods with 3/4-inch steel pipe legs. The legs are spread out at a wider angle than a camera tripod to give more stability. The legs converge at a heavy aluminum plate, 1-foot square by 5/8-inch thick. The legs screw into 3/4- inch pipe threaded holes in the plate. This works.
2) There must be some very fine adjustment screws for precise aiming of the laser. A tripod pan-and-tilt head is out of the question. (The effective beamwidth of the laser receiver is much wider than that of the laser so a regular camera tripod would suffice for it.)
In early Helium Neon laser transmitters I mounted one end of the laser on what was effectively a swivel. The other end had a 10-32 screw for elevation adjustment and another for azimuth adjustment. With the arrangement I had, one turn of a 32 thread-per-inch screw would move the beam 2.6 beam diameters at the receive end. This is quite acceptable. In later lasers I used micrometers. These usually run at 40 threads-per-inch, not tremendously finer than a plain screw, but they are very well made and turn very smoothly. Used micrometers show up a swap meets and garage sales, but even new economy models aren't too expensive.
3) Aligning the laser, or, "initial signal acquisition", is the single biggest problem. An extremely bright light, like a handheld spotlight, is needed. Better yet - a Xenon strobe light. This is used in conjunction with radio liaison and rifle scopes mounted on the laser.
You can't understand how hard it is to get the laser lined up initially so the other end can see it. I have done most of my shots to a mountain, like Mount Pinos, probably the tallest in Southern California (over 8800'). You would figure that if you go out a few dozen miles from that mountain and look at it that you would see this nice prominent peak to point at. No way. All you see is a hazy outline of ridges, none more prominent than the other.
A compass helps, but it doesn't get you close enough. You can try to go by landmarks. On one memorable laser shot we were convinced the city lights we were seeing were from Bakersfield, near where we expected to see the laser coming from. But the lights were really from Lancaster, 60-degrees off target! How could one be so far off? When standing on a mountain, in the dark, directions become confused. This is where you need to use and trust a compass.
Some suggest using surveying equipment and sighting off of the North Star. Great if you can afford it. Such gear probably costs more than the lasers and receivers.
What works is first pointing the lasers as well as you can by compass bearings. Now you should know where too look within 10-degrees. The other end shines headlights or a handheld spotlight in your direction. This will work for a few dozen miles. For longer distances you need a Xenon strobe light. I mounted a strobe in an 18-inch diameter spun aluminum parabolic reflector to make a killer light beacon.
Once some light from the other end is seen you can center the cross-hairs of the rifle scope mounted on the laser on it. You need to have the rifle scope mounted firmly to the laser and aligned to the beam. Now the laser can be turned on. It's very unlikely that the laser will be seen yet at the other end. This is where the real work begins. You move the laser back and forth, up and down, very slightly with the fine adjustment screws. Eventually the other end will see a flash of light as the beam sweeps by them. At that point they yell into the liaison radio to stop. Again, it's unlikely that they will see the light, but you will be very close. A little more fine adjustment will hit the target.
![[....just a fancy line....]](../../images/line22.gif){kind=small}