Working With Lasers

For the uninitiated, the thought of working with lasers means wearing dark tinted goggles and heavy lead-lined gloves while sitting in a concrete, air-conditioned bungalow. Behind a six-inch glass partition are several lab assistants, complete with white coats, clipboards, and solemn faces. Giant computers and monitoring equipment adorn the laser laboratory, soaking up enough electricity to light up Las Vegas. Is that a strain of Hollywood B- movie music in the background? Any moment now a mad scientist will come out and begin the final phase of his quest for world power.

The movies have done a considerable job selling a false and overly dramatic view of lasers (witness the James Bond classic “Goldfinger”). On the contrary, the kinds of lasers available to the electronics hobbyist are so low in power that protective measures are unnecessary. The light radiation emitted by a helium-neon laser isn’t even strong enough to be felt on skin.

Of course, precautions must still be taken, but for the most part, experimenting with lasers can be done in the comfort of the family living room, under normal temperatures, and with no more electrical power than the current from a set of flashlight batteries.

This doesn’t mean hobby lasers are completely harmless. As with all electrical devices, some dangers exist, and it’s vitally important that you understand these dangers and know how to avoid them. In this section, you’ll learn about what you need to know to competently work with lasers, the basics of laser safety, and how to protect yourself and others from accidental injury.

Mechanical Background

The majority of us are far more comfortable with the mechanical side of hobby laser building than the electronic side. It’s far easier to see how a motor and lever work than to see how a laser tachometer operates. Whether or not you are comfortable with mechanical design, you don't need to possess a worldly knowledge of mechanical theory. Still, you should be comfortable with mechanical and electro-mechanical components such as motors and solenoids.

The Workshop Aptitude

To be a successful laser hobbyist, you must be comfortable with working with your hands and thinking problems through from start to finish. You should know how to use common shop tools and have some basic familiarity in working with wood, lightweight metals, and plastic.

BASIC SKILLS

What skills do you need as a laser experimenter? Certainly, if you are already well- versed in electronics and mechanical design, you are on your way to becoming a laser experimenter extraordinaire. But an intimate knowledge of neither electronics nor mechanical design is absolutely necessary.

All you really need to start yourself in the right direction as a laser experimenter is a basic familiarity with electronic theory and mechanics. The rest you can learn as you go. If you feel that you are lacking in either beginning electronics or mechanics, pick up a guide or two on these subjects online or at the library. See Section B for a selected list of suggested further reading.

Electronics Background

Study analog and digital electronic theory, and learn the function of resistors, capacitors, transistors, and other common electronic components. Your mastery of the subject does not need to be extensive but just enough so that you can build and troubleshoot electronic circuits for your laser systems. You’ll start out with simple circuits and a minimum of parts and go from there. As your skills increase, you’ll be able to design your own circuits from scratch, or at the very least, customize existing circuits to match your needs.

Schematic diagrams are a kind of recipe for electronic circuits. The designs in this guide, as well as most any guide that deals with electronics, are in schematic form. If you don’t know already, you owe it to yourself to learn how to read a schematic diagram. There are really only a dozen or so common schematic symbols, and memorizing them takes just one evening of concentrated study. A number of books have been written on how to read schematic diagrams (see Section B).

Sophisticated laser systems use computers for process control. If you wish to experiment with these control circuits, you need to have at least some awareness of how computers operate. Although an in-depth knowledge of computers and programming isn't required, you should have rudimentary knowledge of computers and the way computers manipulate data.

Mechanical Background

The majority of us are far more comfortable with the mechanical side of hobby laser building than the electronic side. It’s far easier to see how a motor and lever work than to see how a laser tachometer operates. Whether or not you are comfortable with mechanical design, you don't need to possess a worldly knowledge of mechanical theory. Still, you should be comfortable with mechanical and electro-mechanical components such as motors and solenoids.

The Workshop Aptitude

To be a successful laser hobbyist, you must be comfortable with working with your hands and thinking problems through from start to finish. You should know how to use common shop tools and have some basic familiarity in working with wood, lightweight metals, and plastic.

LASER SAFETY

Lasers that are sold and used commercially are subject to compliance with a strict set of laws enforced by the Center for Devices and Radiological Health (CDRH, formerly the Bureau of Radiological Health, or BRH). The CDRH, a department of the Food and Drug Administration, serves a similar purpose as the Federal Communications Commission: to ensure that products comply with recognized standards and that the dangers of laser radiation are kept to a minimum.

For regulatory purposes, the CDRH has divided lasers into six groups, or classes. The classification of lasers depends on their power output (in joules or watts), their emission duration, and their wavelength. The classification is then affixed as a sticker to the laser, such as the one shown in ill. 2-1.

* Class I applies to devices that have emissions in the ultraviolet, visible, and infrared regions of the electromagnetic spectrum, below which biological hazards have not been established (in other words, generally “harmless”). A helium-neon laser (at 632.8 nm), operating at less than a few microwatts, would be considered a Class I device.

ill. 2-1. This Class III helium-neon laser sticker is placed near the output of the laser and warns of possible danger to direct exposure to beam. The warning sticker is required of all commercially manufactured lasers, regardless of power output.

* Class ha applies to products whose light output does not exceed Class I limits for emission duration of 1,000 seconds or less but are not intended for direct viewing. One example of a class ha device is a hand-held bar-code scanner. Emission of Class Ha lasers is confined to wavelengths between 400 and 710 nm (visible spectrum).

* Class II devices are identical to Class I lasers, except the permissible wavelengths are confined to visible light between 400 and 710 nm and the beam may not be on or motionless for a period greater than ¼ second. Power output can be as high as 1 milli-watt.

* Class IIIa lasers comprise a large group of visible light devices (400 nm to 710 nm) with power outputs less than 5 milli-watts. Class IIIa lasers consist of most helium- neon lasers (the old BRH standards classified these He-Ne’s as Class IIIb).

* Class IIIb applies to devices that emit ultraviolet, visible, and infrared spectra. Class IIIb lasers can have power outputs ranging from 5 to 500 milliwatts in the visible spectrum; less if the beam is invisible. A low-output argon laser (100 mW for example) used in small light shows would be considered a Class IIIb product.

* Class IV represents the “laser brutes.” These devices exceed the limits of the other classes. The CDRH classifies a laser as a Class N device if it produces light radiation (visible or invisible) that's dangerous to eyes and flesh whether or not the beam is direct, reflected, or diffused. CO2, ruby, Nd:glass, and multi-watt gas lasers (such as 3- to 5-watt argons) fall into the Class N umbrella.

Much of the laser components available on the surplus market don't comply with CDRH standards, although a few do. If they are in compliance, you will see a sticker, such as the one in the figure, affixed somewhere on the device. Note that individual components may not in themselves comply with the CDRH regulations, even though the device from which they were taken meets the standards.

Most of the CDRH regulations pertain to such simple functions as:

* Placing the proper sticker(s) on the device.

* Covering the power supply during operation.

* Providing a power supply interlock that shuts the supply off if the case is opened.

* Inserting a key switch in the power supply mains to prevent unauthorized use.

* Adding a cover or slide to the output mirror to prevent accidental exposure to the beam when the laser is operating.

* Providing adequate instructions for the user. In all cases, user information and basic service information are required.

Several of the projects in this guide show you how to meet these compliance standards. As an experimenter working alone, however, the CDRH regulations don't strictly apply to you. As long as there is no danger of exposing others to harmful radiation or high voltages, you may do as you wish. It’s up to you to decide if you want to add the safety features to your own, personal laser projects.

If you plan on developing laser systems for use by others or for use in public places (such as laser light shows), you need to be sure that you comply with all the pertinent regulations. Otherwise, you might be subject to fines. There isn't enough room in this guide to reprint the full text of regulations, but you can obtain a copy of the CDRH laser requirements (Part 1040 of Food and Drug Administration regulations) by writing to the bureau (there may be a publication or distribution charge—check first). Their address is provided in Section A. Be sure to state the section and topic of your inquiry. If you plan on producing a light show, you must file a variance and indicate the type of equipment and safeguards you plan on using.

LASER LIGHT RADIATION

Lasers emit electromagnetic radiation, usually either visible light or infrared. The level of “radiation” is generally quite small in hobby lasers and has about the same effect on external bodily tissues as sunning yourself with the living room lamp.

Skin is fairly resilient, even to exposure to several tens or hundreds of watts of la ser energy. But the eye is much more susceptible to damage, and it's the effects of laser light on the retina that's the greatest concern. Even as little as 20 to 50 milli-watts of focused visible or infrared radiation can cause temporary and perhaps permanent blindness.

The longer the exposure to the radiation and the more focused the beam, the greater the chance that the laser will cause a lesion on the surface of the retina. Retinal lesions can heal, but many leave blind spots. In the worst case, with a laser that puts out a minimum of 100 to 200 milli-watts (such as an argon laser used in light shows), a constant exposure, of a few seconds or more on the optic nerve of the retina can cause total blindness.

Retinal damage when using hobby lasers—those with outputs of less than 5 or 10 milli-watts—is rare. In fact, there have been only a handful of reported accidents involving lasers in the several decades since they became available, and many of these have involved electrocution by the high-voltage power supply, not exposure to the laser beam.

One reason laser radiation accidents are rare is the same reason that astronomers don’t go blind looking through their telescopes. They know it’s foolish to stare at the sun through even a low-power telescope, but if they did, the pain in their eyes would signal a potentially dangerous situation to the brain, and the eyelids would instinctively clamp shut.

Obviously, solar study through telescopes or staring down the shaft of a laser beam are not hobbies you would normally indulge in, but a potential danger still lies in accidental exposure. The effects of exposure of bright lights on the eyes are cumulative. Making a habit of carelessly shining a laser into your eyes only increases the chance of eye problems later on in life.

The Dangers of Focused Laser Light

Laser light is the most dangerous to the eyes when it's focused into a sharp point. The beam of the typical helium-neon laser, as it exits the tube, is about 0.8 millimeter in diameter. A simple lens system can focus the beam to a much smaller diameter of about 10 micrometers (10 millionths of a meter). This is equivalent to one hundredth of a millimeter, so the beam has been reduced by a factor of about 100. The output of the laser has not increased, but the energy is focused onto a much smaller area.

Laser Goggles

The welder uses tinted dark goggles to block the bright light emitted from the torch and melting metal. A pair of laser goggles can help prevent much of the light produced by a laser from reaching your eyes, even if the beam is inadvertently directed toward you.

Laser goggles, such as the ones in ill. 2-2, are not an absolute necessity when experimenting with low power He-Ne and semiconductor lasers, though you will want a pair if your experiments often cause the beam to strike your eyes. Even with goggles, you should avoid direct exposure to the laser light. Some of the light can penetrate the goggles, and depending on the power output of the laser, can still be dangerous.

In all cases, you should use goggles when working with Class IV high-power lasers, such as ruby, Nd:glass and CO Laser goggles are manufactured to restrict light at only a certain light wavelength, such as 332 nm (neon-nitrogen), 694.3 nm (ruby), and 840 nm (gallium arsenide). You must specify the wavelength or laser when ordering. Be aware that laser goggles are expensive; a new pair can cost over $100. A few laser goggles are designed to block a series of wavelengths and can be used effectively with many types of lasers.

ill. 2-2. Laser safely goggles.

The Invisible Ray

Infrared lasers pose a different threat, in that the light they produce cannot be readily seen. Don’t be fooled by the false sense of security that just because you can’t see the beam that it'sn’t there—and won’t harm your eyes. You might not know you are staring down a beam delivering 10 to 40 watts of pulsed infrared radiation until your vision becomes blurry and you see dark spots.

Most infrared lasers, particularly the semiconductor variety, emit near-infrared radiation. Most laser diodes emit light in the region between 780 and 904 nm. If you look directly at the laser you might see a faint red glow. Don't look at this light for an extended period, particularly if the beam has been collimated or focused. The visible glow might be dim, but the actual light output of the laser could be quite high.

A pulsed laser diode can easily deliver 5 to 10 watts of energy; many produce up to 50 watts of pulsed energy. Though the beam is pulsed, the repetition rate is very fast and appears as a steady stream. You can be assured that continual observation of a focused, 10-watt beam will cause at least some damage to the eye.

HIGH-VOLTAGE ELECTROCUTION

The latest semiconductor lasers operate from low-voltage dc power packs. A number of projects later in this guide show how to construct semiconductor laser diode systems using flashlight batteries. These laser systems pose no threat of shock.

On the other hand, all gas lasers, including the popular helium-neon variety, require high-voltage power supplies. These power supplies boost the mains voltage from 12 volts dc or 117 volts ac to several thousand volts. The typical 12-volt-dc laser power supply produces up to 3,000 volts dc, an extraordinary amount when you consider that the electric chair puts out only 2,000 volts!

Some laser experimenters tend to disregard the high voltages. They assume that although the voltage is high, the current level is low. They rely on the old maxim “It’s the volts that jolts but its the mills that kills.” This is true but limited thinking. The current demand of the typical helium-neon laser is low, between 3 and 7 milli-watts. At normal dc levels (5 or 12 volts), this current isn't enough to be felt. But at the 1.2 to 3 kV level of standard laser operating voltages, even a 7-milliwatt jolt can, depending on the circumstances, kill you.

Laser power supplies should be properly shielded and insulated. Avoid operating a power supply in the open, and always cover exposed high-voltage parts. Insulating material such as high-voltage putty (available at TV repair shops and electronic stores) restricts arcing and provides a relatively shock-proof layer. It’s easy to forget that several thousand volts are coursing through a wire, and you might inadvertently touch it or brush across it. Even a direct contact isn't necessary. A 3 kV arc can jump a quarter of an inch or more, and , like lightning, discharge on the nearest object. If that object happens to be your fingers or elbow, you will receive a painful shock.

Admittedly, most shocks from laser power supplies won’t kill you, nor will they burn your skin. But don’t underestimate the power of low-current high voltages. Your body’s protective mechanism automatically reacts to the jolt. If you touch a live wire with your hand, your body quickly contracts to prevent further shock. If the jolt is large enough, you may be knocked backward onto the ground. Should you be holding the laser tube at the time, you might drop and shatter it, which adds the risk of cutting yourself on top of it all. Sound like the voice of experience? It is.

Phantom Current

Most laser power supplies use high-voltage capacitors at the output stage. Like all capacitors, these can retain current even after the power supply has been turned off. Though many laser supplies use bleeder resistors that drain the capacitors after power has been removed, not all do. Play it safe and assume that the output of any power supply—plugged-in or not—is potentially hot.

When working with the laser, make sure the power supply is off, then temporarily short the leads of the power supply together, or simply touch the positive terminal of the supply to ground (see ill. 2-3). That will discharge any remaining current, making the power supply safe to work on. Likewise, the laser tube behaves like a Leyden jar, which is a type of capacitor. It, too, can retain a current after power has been removed. Drain the remaining charge by shorting the terminals or leads together.

High Voltage Precautions

Take these simple steps to avoid unnecessary shocks:

* Don't apply power to the laser until all the high-voltage wires and components are shielded or insulated.

* Don't work near a laser that has high-voltage components exposed.

* Provide an interlock switch to protect against accidental electrocution when servicing the laser.

* Discharge the tube and power supply to avoid shock after power has been removed.

* Affix a “Danger—High Voltage” sticker to the power supply and tube to warn others that high voltages are present.

* Install a power key switch to prevent unauthorized use.

* Never leave the laser or power supply unattended. A laser isn't a toy, and children shouldn't be allowed to use one without your supervision. For maximum security, lock the laser in a desk or filing cabinet to prevent unauthorized use.

ill. 2-3. For safety’s sake, always discharge the static remaining in a gas laser by shorting the terminals with a jumper cable (be sure power is removed first!). If you can’t reach both terminals, short the anode to ground.

AC Electrocution

Some laser supplies and laser projects operate from the 117-volt ac house current. Observe the same precautions when working with ac circuits. A live ac wire can, and does, kill! Exercise caution whenever working with ac circuits and observe all safety precautions. If you are new to electronics and aren’t sure what these precautions are, refer to Section B for a list of books that can help you broaden your knowledge.

You can greatly minimize the hazards of working with ac circuits by following these basic guidelines:

* Always keep ac circuits covered.

* Keep ac circuits physically separate from low-voltage dc circuits.

* All ac power supplies should have fuse protection on the incoming hot line. The fuse should be adequately rated for the circuit but should allow a fail-safe margin in case of short circuit.

* When troubleshooting ac circuitry, keep one hand in your pocket at all times. Use the other hand to manipulate the voltmeter or oscilloscope probe. Avoid the situation where one hand touches ground and the other a live circuit. The ac flows from one hand to the other, through your heart.

* When possible, place ac circuits in an insulated Bakelite or plastic chassis. Avoid the use of metal chassis.

* Double- and triple-check your work before applying power. If you can, have some one else inspect your handiwork before you switch the circuit on for the first time.

* Periodically inspect ac circuits for worn, broken, or loose wires and components, and make any repairs necessary.

PROJECTS SAFETY

A number of the projects in this guide require work with potentially hazardous materials, including glass, razor blades, and wood and metal tools. Glass and mirror- cutting should be done only with the proper tools and safety precautions. Wear eye protection whenever you are working with glass. If you can, wear gloves when handling cut glass and mirrors. Finish the edges with a burnisher, flame, or piece of masking tape.

Razor blades provide a well-defined and accurate edge and are helpful in a number of optical experiments. You need new, unused blades; nicks and blemishes on the blade surface can impair your results. Obviously, you must exercise extreme care when handling razor blades.

One good way to handle razor blades is to coat the edges of the blade with a soft wax or masking tape. When you are ready to position and use the blade, peel off the wax or tape. When cutting with razor blades, wear eye protection and gloves. For maximum safety, place the blade behind a large piece of clear plastic. If any pieces break off during cutting, the plastic will protect your body.

An optional experiment in Section 22 involves the use of liquid nitrogen, a cryogenic gas that has a temperature of minus 196 degrees Celsius. While liquid nitrogen isn't flammable and non-toxic, its extremely cold temperature can cause frost-bite burns. You should handle liquid nitrogen only in an approved container (a Thermos bottle with a hole drilled in the cap often works), waterproof gloves, and eye protection. Following the recommendations and handling precautions given in Section 22 for more details. When treated properly, liquid nitrogen is safe and actually fun to use.

Construction plans require the use of wood- and metal-working tools. You can use hand or power tools; either way, carefully follow all operating procedures and use the tools with caution. Most power tools provide some type of safety mechanisms so don’t defeat them! Thoroughly read the instruction manual that came with the tool. A number of good books have been written on wood and metal working and the tools involved. Check your library for available titles.

Common sense is the best shield against accidents, but common sense can’t be taught or written about in a book. It’s up to you to develop common sense and use it at all times. Never let down your guard. The laser projects in this book are provided for your education and enjoyment. Don’t ruin the fun of a wonderful hobby or vocation because you neglected a few safety measures.