Laser is an acronym for "Light Amplification by Stimulated Emis sion of Radiation." The actual material that produces the laser light is called the lasing medium. It may be either a solid, liquid or a gas. Most large light shows use gas lasers with either krypton gas or a mixture of argon and krypton gases. Helium neon gas lasers are common in smaller laser displays. The gas is usually contained in a long, thin cylindrical glass tube. The pressure and the concentration of the gas in side the tube must be just right or else the laser will not operate.
At one end of the tube is a totally reflecting mirror; at the other end is a partially transmitting mirror (that is, it allows a small portion of light to pass through). Like a fluorescent light bulb, the lasers in light shows will work only when an electric current passes through the tube containing the gases.
When the electrical energy enters the tube, it "excites" the atoms of the lasing medium. What actually happens is the electrons of an atom absorb the electrical energy by jumping to higher energy states. After a small fraction of a second, the electrons will "fall" back to their normal energy state. When they do so, the atom gives off the excess energy in the form of a "photon" or small packet of light radiation.
Depending upon the amount of energy absorbed, the atom will give off a phpton with a particular wavelength. This photon of light can trigger (or "stimulate") the release (or "emission") of similar photons from the other excited atoms in the lasing medium. The photons bounce back and forth between the two mirrors at the ends of the tube. As they bounce back and forth they continue to trigger more and more photons, building up to higher and higher intensity. The stimulated photons of light radiation are of the same wavelength and move in the same direction as the original photons. Some of this light will pass out through the partially transmitting mirror at the end of the tube. This is the laser beam with the unique characteristic described before: coherence.
You may be wondering how a laser beam that is supposed to be composed of light waves with the same wavelength can result in the numerous color effects that are frequently seen in light shows. The answer lies in the atomic structure of the lasing medium and in the fact that the atom can release more than one level of energy at the same time.
Each of the electron orbits of an atom is bound to the nucleus by a specific level of energy. Those close to the nucleus are bound progressively more tightly. Those further away from the nucleus are bound progressively less tightly. In order for an electron to jump to another energy state, it must have absorbed a specific amount of additional energy to "boost" it into that higher state. And likewise, when that electron falls back to its normal state, the atom wilt give off the same specific amount of energy. The wavelength or color of the photon given off is determined by this specific amount of energy.
The atomic structure of krypton gas is relatively complex. It has 36 electrons. So a krypton atom has many electrons at different energy states available to absorb and then emit photons of various wavelengths and therefore various colors. By incorporating a prism at the end of the laser, the photons of the particular wavelengths (or colors) can be separated into several rays of different colors.
Another point that should be clarified has to do with the units by which the output of a laser is measured. Although the term "watt" is used to describe both lasers and conventional electric devices like light bulbs, the term can refer to very different things. When you have a 100 watt light bulb, the wattage refers to the power input or the electricity required to make the bulb work.
The output from a 100 watt light bulb is about 15 watts. And again, the 15 watts of energy coming out of a light bulb is spread out in all directions. When you have a 1 watt laser, the wattage refers to the optical output of the laser. This 1 watt of optical radiation is traveling in a narrow, concentrated beam in one direction. At a distance of 100 feet, the light from the 1 watt laser can be about 1 million times more concentrated than from the light bulb!
It should also be pointed out that the physical size of a laser has little bearing on its power out put. There are many lasers that are physically small, but have more power output than some large ones. The only reliable way of telling the power output of a laser is to look at the label on the product, which should indicate its class and absolute maximum power output.
Source: Excerpted from Laser Light Show Safety: Who's Responsible? PDF June 1980; Revised May 1986