Laser Radiation

Is the radiation that comes from lasers the same kind of radiation that comes from radioactive material?
No, its not. Lasers or "Light Amplification by Stimulated Emission of Radiation" do not use radioactive material to create their intense energy. In this case, the radiation refers to the emission of a photon, or electromagnetic radiation from the electron shell of the atom. That is, it is a device that produces and amplifies light.

That sounds pretty simple. How does a laser amplify light?}
First, you have to think about an atom and its components. We have provided a detailed description of the atom in a previous tutorial in this "Radioactivity Basics" section. It is a good idea for you to have read that discussion before working on this one.

I did that already.
Okay, so we'll move on. Lasers "excite" electrons into a elevated energy state; when the electron falls back to the ground state or normal state, the energy given off is in the form of heat or light. The light photons are used by the LASER to strike other atoms nearby and excite their electrons also. Now multiple electrons are falling back to a ground state at the same time. Once the process is started, the light is amplified and forms a beam.

I understand that once you start "lasing", you just step back and watch the fireworks. What do you use to start the process?
You have to start with the right material and an energy source. You choose a material that you can "pump up". The goal is to excite many electrons at the same time. The hard part is keeping the electrons excited until you are ready to start the lasing process. The engineers who design lasers actually call this component the pumping system. (Now we know where Hans and Franz got their material. Well, maybe not.) The energy source must be selected or tuned to release the exact energy of the photon released when the electron returns to the stable state.

Hold up. You're describing the same process that we call neon lights or neon signs. Are all neon signs lasers?
Well, let's just say they're a distant cousin. But they are an example of spontaneous emission. The process starts the same, you use a gas like neon. You charge electricity through the gas and the electrons are excited. When they fall to the stable energy level, they give off a photon of visible light, all with the same wavelength. These photons travel in all directions and occur at any time until the energy is released. The travel paths of the photons are incoherent. A laser starts the same way as a neon light but is capable of releasing the photons upon demand, in the same direction and creating an avalanche of other photon emissions or stimulated emission. Coherent light is created which has a lot more energy in a small bundle than incoherent light.

Who figured this out?
In 1917, Einstein postulated that a photon released from an excited atom could, upon interacting with a second, similarly excited atom, trigger the second atom into de-exciting itself with the release of another photon. The photon released by the second atom would be identical in frequency, energy, direction, and phase with the triggering photon, and the triggering photon would continue on its way, unchanged. Where there was one photon now there are two. These two photons could then proceed to trigger more through the process of stimulated emission. But Einstein didn't invent a laser. He simply figured out that the process was possible.

Who did invent the laser?
The first laser was assembled in 1954 by Charles H. Townes and Arthur L. Shawlow using ammonia as the lasing medium and microwaves as the source of energy. It is reported that they spent over two years of research in their laboratory. The called their device a MASER (Microwave Amplification by Stimulated Emission of Radiation). This team of scientists were awarded the Nobel Prize in 1964 for their work with lasers, both microwave and optical wavelengths. However, the laser beam they produced was invisible to the eye.

Who made the first visible laser?
The first successful visible laser was assembled by Dr. T. H. Maiman in 1960 who used a ruby crystal as the lasing material and flash lamp as the source of energy. The ruby crystal was shaped as a cylinder and the ends of the crystal were polished to be parallel within a few angstroms (that's a pretty small distance by anybody's ruler). One side was coated with silver to reflect all of the light that struck it and the other end was not as reflective so a portion of the light could escape the crystal and form a beam.

Are all lasers today made from ammonia gas or rubies?
No. We now know that many different materials will lase, especially substrates and mixtures as well as different physical forms. There are many examples of solid, gas, dye and semiconductors in practical use today. Solid lasers commonly use Neodymium ions mixed in garnet. The Neodymium lases easily, and the garnet is easy to manufacture.

What about gas lasers?
The most common gas lasers are a mixture of helium and neon which emit the characteristic red light. Lasers made from dyes are common, such as Rhodamine 6G. One characteristic of a laser using a dye as the lasing material is its ability to be tuned or adjusted to a particular wavelength. This gives the dye laser many more applications and choices of energy sources than other types of lasing materials.

Any others?
Yes. A semiconductor laser is another example. This material is a solid but it is also a sandwich of two materials, like Gallium Arsenide diode laser. Semiconductor lasers are not particularly powerful and are commonly constructed in an array so as to focus more energy.

So tell me again, how do I build a laser?
Well, you will need three basic parts . . . a lasing medium, a pumping system and an optical cavity. The lasing medium is the source of the electrons and is capable of keeping the electrons in an excited state until stimulated. The pumping system imparts the source of energy to raise the electrons to the excited state. The optical cavity helps to amplify the photons that are released and selects the photons that are traveling in the same direction, forming the beam.

You know, I've been reading a lot about laser safety these days, particularly associated with laser pointers. Can lasers really hurt you?
Lasers are found in many applications today from presentation pointers, scanners in grocery stores and used in surgery for delicate operations. The part of the body that is most likely to be injured is the eye where the retina or the cornea is burned. The hazard becomes more of a potential for a laser that operates outside of the visible light wavelength. That is, it is invisible to your eye. By the time you realize there is a problem, it will be too late and you eyes may be damaged. A mirror or shiny surface may be the culprit where the laser beam is reflected inadvertently and you are exposed as a result. While the skin may be exposed, the deposition of energy in the form of heat may not be as much of a problem to the skin as the eye. Obviously, the more powerful the laser, the more likely that exposure to the skin could be a problem.

So that game of playing tag with laser pointers isn't really a good idea then?
No. In fact, some countries are banning the sale of laser pointers to children for that very reason.

How do you know which lasers to avoid?
The American National Standards Institute established four classes for lasers. They are named Class one (1) through four (4). A Class 1 laser cannot create a human hazard even if the beam is collected by viewing equipment, like binoculars. No controls are required.

What about Class 2 lasers?
A Class 2 laser is considered to be a low power laser and could cause damage if the victim continuously stares into the beam. ANSI recommends that a caution label be affixed on the unit warning you not to stare into the beam.

And a Class 3 laser?
A Class 3 laser is a medium power laser and may cause damage before the blink reflex of your eye can protect you. It is not likely that there is enough power to damage the skin or create serious reflections. There are safety precautions recommended by the manufacturer, including the use of approved glasses to shield the beam in the event of an exposure. Labels, signs and warning devices should be employed.

I guess Class 4 lasers are the big ones, right?
They can be. A Class 4 laser or a high power laser can cause damage to the eye and/or skin even if reflected. These lasers have sufficient power to ignite combustible materials. The safety precautions are stringent. In fact, many Class 4 lasers are enclosed in a shielded compartment and may not be operated when the compartment while the compartment is breached.

Are there any federal regulations that pertain to the safe use of lasers?
Yes. The Occupational Safety and Health Administration, or OSHA, has regulations that apply to lasers. OSHA uses the recommendations established by the American Conference of Governmental Industrial Hygienists to limit exposure to laser light.

What are those recommendations?
The ACGIH rules limit the length of exposure and the intensity of exposure. There are different criteria, or Threshold Limit Values (TLVs) depending upon on the wavelength of the laser beam. The TLVs are set to minimize injury to the cornea and the retina and limit the amount of energy that strikes the surfaces. The potential damage is very dependant on the wavelength because the eye may focus the beam differently according to the color or wavelength. For that reason the standard for exposure vary with the wavelength.

What do you mean?
Let me give you an example. A carbon dioxide laser emits a wavelength of 1060 nanometers. You are allowed to be exposed to approximately 50 milliwatts of this laser energy for no more than 100 seconds in a day for a continuous wave laser. On the other hand, if you are using a helium-neon laser (red color) you can expect a wavelength of 632 nanometers. You are allowed an exposure of less than ten (10) milliwatts of this laser energy per square centimeter for no more than 100 seconds in a day for a continuos wave laser. Now, if you are using a laser with krypton or argon gas (e.g. wavelength of approximately 400 nanometers), your exposure should be less than 0.1 milliwatts per square centimeter for no more than 100 seconds in a day.

Is the ACGIH the only agency that supports these limits?
No. The ACGIH rules are also consistent with the ANSI Standard, Z-136.1, Safe Use of Lasers. Similar information is provide in the ANSI document.

What about long term exposures to lasers? Any chance of long term damage over time?
There is no indication that long term exposure to laser light is a problem. If exposures are minimized to protect the eye and skin there should be no long term effects. However, research studies are conducted routinely especially as lasers become more common place in industry and public areas. Stay tuned for more on this topic.

So what is the bottom line?
Lasers are highly useful sources of non-ionizing radiation that have broad applicability throughout industry. As time goes on, I am sure we will see these handy devices put to even greater and more amazing uses. But as time goes on, and as we learn more about the potential effects of exposure to laser light, we will also be refining our protective mechanisms and limits to be sure we minimize any risk associated with these highly beneficial tools.

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