The word laser is an acronym for light amplification by stimulated emission of radiation. Laser light is different from other forms of light because light produced by a laser is in phase. To understand what it means for light to be in phase, look at the following figure.
Figure. Visible light is only a small part of the total electromagnetic wave spectrum.
You can see that each type of light has a definite wavelength. Light consisting of all the same wavelength is called coherent light, and is said to be in phase.
Light consisting of many different wavelengths is called incoherent light. Incoherent light is not in phase. An incandescent light consists of many different wavelengths and is incoherent light. See Figure 1.
Figure 1. Incandescent light is incoherent light, it consists of many different wavelengths.
Laser Construction
Look at Figure 2. This is a typical laser. It consists of a light source such as the strobe and a ruby cylinder. One side of the cylinder is totally reflective. The other side is almost totally reflective, but there is a small area that permits the laser light to leave the tube.
The strobe emits a light beam that is absorbed by the ruby laser tube. This is the stimulation portion of the laser. When the light enters the tube, it tries to leave the tube at the ends.
Since the ends of the tube have reflective material, the light reflects back and forth through the tube. This back and forth reflection causes additional photons to be released. This is the amplification portion of the laser.
Figure 2. The light emitted from the laser is all the same wavelength and is in phase.
The light resonates inside the tube, which is sometimes referred to as an optical resonator cavity. The resonating light forces the light inside the tube to resonate at the same frequency, thus becoming coherent light. The light inside the tube is in phase.
The light escapes through the small hole in the reflective material at one end of the tube. This escape is known as emission of radiation. When the light reaches the level to produce the stimulation of more photons, it is referred to as lasing. This is the basic operating principle behind the laser.
Types of Lasers
There are many different types of laser material. The optical resonator can be made from solids, liquids, or gases. A ruby laser is not produced with the gem that is found in mining operations. The ruby laser uses a manufactured ruby consisting of an aluminum oxide compound and chromium.
Ruby Laser
As discussed in the previous section, one end of this ruby is totally reflective and the other is almost totally reflective except for a small opening in the reflective material. The opening allows the laser light to leave the resonant cavity of the tube.
The light from a ruby laser is usually pulsed rather than continuous. A continuous beam from a ruby laser would cause a dangerously high heating of the tube.
Ruby lasers that operate as a constant beam use a cooling system such as liquid nitrogen. These lasers that require cooling are very powerful. They are used for applications such as welding and cutting hard materials such as ceramics, metal, or diamonds.
Gas Laser
Gas lasers are quite common today, Figure 3. With a gas laser, the resonant cavity is filled with a gas such as helium-neon, CO2 (carbon dioxide), or argon.
Electrical terminals are placed inside the tube and a high voltage is applied to the gas. The gas ionizes and produces light. This begins the laser action. The laser action releases additional photons that result in an even more powerful light.
Figure 3. Gas-filled laser.
Different gas lasers are used for a variety of tasks. Argon lasers are used in the medical field to remove tattoos and perform eye surgery. CO2 lasers are very powerful and are found in industrial environments. They are used for cutting and drilling metal. The CO2 laser produces a beam with an infrared wavelength that can produce a great deal of heat. Infrared light is not normally reflected, which makes it ideal for metal work.
The gas in the gas laser can be replaced easily by pumping a different gas into the resonant cavity. Different gases produce different colored beams. This is how many different colors are achieved in a laser light show.
Argon lasers are commonly used in light shows because of their blue-green color spectrum. The helium-neon laser is the classical thin red beam laser you’ve seen commonly in movies and TV.
Another type of laser uses a liquid dye as its lasing medium. Many different colors of liquid dye can be pumped into the laser tube to produce different wavelengths of light.
Laser Applications
Lasers have many different applications. They are in industry, medicine, business, and entertainment.
Construction
The laser can be used for measuring distances and alignment. Laser systems are commonly used as surveying instruments.
When the laser technology is combined with a computer, the distances and angles can be recorded and stored. Calculations can be made by the computer, and a diagram or layout can be printed very easily and accurately.
The laser can be used as a construction level. The beam can be rotated to illuminate a line of light on a wall to assist installation of a drop ceiling or to determine the grade when removing or smoothing land.
Lasers have been used to align pipes in underground systems such as utility or storm drains. Here, the light is simply positioned in one section of pipe and a target is set inside each pipe as it is placed. The laser alignment system ensures the straightness and slope of the pipe.
Medicine
Lasers are also used in conjunction with fiber optics to perform many types of surgery. Three different fiber optic systems can be inserted into a patient.
One system provides regular low level light for vision. A second fiber is used for viewing inside the patient, like a camera. The third fiber is used to direct the laser beam to the targeted surgical area.
Lasers are also used for some types of angioplasty. Angioplasty is the surgical removal of plaque inside arteries. These techniques have saved countless lives and the pain of conventional, invasive surgery.
Supermarket checkout
Another commonly used laser known as a Nd:YAG (pronounced en-dee-yag). The Nd:YAG laser uses yttrium aluminum garnet, a solid, for the laser medium. You have seen these low power lasers many times at supermarket checkout counters.
Beneath a piece of glass in checkout counters, or in a hand-held gun, is a laser system used to identify product codes on labels. The familiar black parallel lines on labels are universal product codes (UPC) that can be digitized and transmitted to a computer easily. See Figure 4.
Figure 4. A typical checkout counter uses a laser to scan a bar code from a product label.
In this device, a laser is used to produce a narrow beam of light. The beam of light is shined on a rotating mirrored surface composed of many flat surfaces. The laser beam is reflected off the rotating mirrored surface causing it to flash at an accelerated rate across and through the glass top.
The light strikes the bar code and is either absorbed by the black bars or reflected back down to the receiver unit by the gaps between the bars.
The receiver unit is a light sensitive diode or transistor. The electrical impulse generated at the receiver is transmitted through wires to a computer where it is interpreted.
The bar code data identifies the item as a number. The number identifies the item and its price. It also enters the transaction into a computer data base to keep track of store inventory.
CD player
Another common application for a low-power laser, such as a laser diode, is the compact disc player (CD). Look at Figure 5.
The beam from a laser diode is directed to the CD through an optical lens. The laser beam is reflected off the CD to a mirror and then to a photo receiver. The photo receiver receives the beam and converts it to electrical impulses.
Figure 5. A typical CD player arrangement.
The CD itself contains many tiny pits smaller than a human hair. The quantity and length of the pits varies over the disc. This information represents the recorded sound pattern.
The laser light hitting the photo receiver varies in intensity depending on whether the beam is reflecting from a pit or reflecting from the flat area of the disc.
Changes in intensity (moving from a pit to a flat area or vice versa) represent 1s. Lack of change over a period of time represents a 0. This light intensity data is converted to an electrical signal. These digital electrical impulses are converted to analog signals, amplified, and then sent on to a speaker to be converted to sound.
Laser Classification
Lasers are classified by their maximum possible output during normal operation. Wattage is not the only consideration for rating power of a laser. Beam width is also of prime consideration. Laser classifications are as follows.
Class I
These beams are fairly safe to work with. These devices include the lasers in pointers, checkout counters, and disc players.
Class II
Visible laser beams only. These will cause damage if the beam strikes the eye for over 1/4 second.
Class III
These beams will cause eye damage regardless of the length of exposure time.
Class IV
Exposure to these beams will not only severely damage the eye, but skin damage can also occur. These laser beams can cause combustible materials to ignite. Extreme caution must be used.
Eye protection must be worn when working with Class II or higher lasers.
Summary
1. Laser is an acronym for light amplification by stimulated emission of radiation.
2. Laser light is a coherent light.
3. Lasers are used in industry, medicine, business, and entertainment.
4. Eye protection must be worn when working with Class II or higher lasers.
