How Everything Works
How Everything Works How Everything Works

Page 2 of 3 (21 Questions and Answers)

642. How do lasers work?
Lasers use systems with excess energy to amplify light. These systems, typically atoms or atom-like structures in solids, are in excited states—they have more than their minimum amounts of energy. An excited system can get rid of its excess energy in many different ways, but certain systems tend to emit the excess energy as photons—particles of light. While an excited system will emit a photon spontaneously if you wait long enough, it can also duplicate a passing photon if that passing photon has the proper characteristics. Most importantly, the excited system must be naturally capable of emitting the passing photon spontaneously—the passing photon's wavelength and travel path must be such that the excited system is able to duplicate it.

This duplication effect makes it possible to amplify light. When a single photon passes by a number of identical excited systems, those systems may duplicate the photon many times so that many identical photons emerge. This phenomenon is the basis for laser amplifiers. When one of the photons emitted spontaneously by the excited systems is deliberately sent back and forth through those systems with the help of mirrors, the laser amplifier becomes a laser oscillator—it both initiates and amplifies the light. The light that ultimately emerges from the laser oscillator or amplifier differs from normal light because the laser light consists of many identical photons. They all have identical wavelengths (colors) and follow identical paths through space. They also exhibit dramatic wave effects, particularly interference.

677. I have heard that diode lasers won't work in ring laser gyroscopes because these lasers are not single frequency. If this is true, will a prism or a diffraction grating isolate one of the frequencies? - M
While most diode lasers operate at several frequencies simultaneously, it's possible to make lasers that function at only one frequency. In fact, such "single mode" diode lasers are extremely stable light sources and the basis for much current research in optical science. For example, the recent observations of Bose condensation in vapors of alkali metal atoms were made with the help of single mode diode lasers.

The phrase "single mode" refers to a single longitudinal wave that travels back and forth through the device while it is operating. This single wave has one frequency and one wavelength. It is selected from other possible waves through the use of interference effects. For the wave to be stable inside the laser cavity (the laser is bounded at each end by a mirror, thus forming an optical cavity), the cavity's length must be an integer or half integer multiple of the light's wavelength. While that criterion alone will allow several possible waves to form, coupling a second cavity to this laser cavity further restricts the wave so that only a single wave can operate inside the laser. The diode laser will then have only a single mode of operation and will emit a single frequency of light.

725. How does a discotheque laser work and how could I build one? — JPD, Spiennes, Belgium
If you are referring to a system that displays illuminated line drawings on a wall that move with the music, then building one is easy. You need a small isolated speaker—just the electronic device, not a whole speaker unit—that you can connect to the music amplifier. Place an elastic membrane over that speaker—a stretched sheet of thin rubber from a latex glove should work well. Then glue a tiny, front-surface mirror to that rubber membrane, choosing a point that is about midway between the middle of the speaker and its edge. A front surface mirror is one that is shiny on its top, so that light doesn't have to go through glass before reflecting. A broken fragment of mirror, about 3 mm on a side, should work. Finally, shine the beam of a laser pointer onto the mirror and begin to play music through the speaker. The mirror will move with the music and the reflected laser beam will form pretty patterns on the wall.

776. Could you see a laser beam in outer space since it can't reflect off of anything? — RM, Rochester, NY
No. The reason that you can see a very intense laser beam as it passes through the air is that light can scatter off of dust particles and air molecules. When it does, some of the laser light is sent toward your eyes and you see the light coming toward you from the laser beam's path. But if there is no air in the path of the laser beam, the light will travel without scattering and you won't see the path at all.

955. How do you make lasers that burn?
Lasers use excited atoms or atom-like systems to amplify light. Putting mirrors around such excited atoms or atom-like systems allows them to amplify their own light until the laser is emitted vast numbers of identical light particles or "photons." To burn something with laser light, there must be a great many excited atoms or atom-like systems and they must be very efficient at amplifying light. Probably the easiest to build powerful laser is a carbon dioxide laser. This laser uses an electric discharge in a mixture of nitrogen and carbon dioxide gas to produce excited carbon dioxide molecules. These molecules amplify infrared light at a wavelength of 10.2 microns extremely efficiently, so that a laser consuming about 1000 watts of electric power can emit approximately 100 watts of infrared light. That's enough power to burn things very quickly. Even more powerful carbon dioxide lasers are used in industry to cut and machine metals, including thick steel plates. But while they are surprisingly simple to build and operate, given the right components, carbon dioxide lasers require dangerous high voltage power supplies. There were many physics graduate students electrocuted in the 1960's while tinkering with homemade carbon dioxide lasers.

1064. How can you make a hologram? — JM, Kettering, OH
The classic technique for making a hologram begins with splitting the light from a laser into two parts. Part of the laser light is used to illuminate a scene while the other part is used to illuminate a piece of film placed in front of the scene. Actually, the film is exposed to light from two sources: (1) the second part of the laser beam and (2) a portion of the first part of the laser beam that the objects reflect toward the film. Lights from these two sources don't simply add when they reach the film; they interfere with one another. Laser light is unusual in that it is coherent light—a giant wave consisting of numerous identical particles of light. When the wave from the laser and the wave reflected from the objects meet at the film, they interfere. When the crest of one wave joins the crest of the other wave, the two waves form an extra large crest—constructive interference. But when the crest of one wave joins the trough of the other wave, the two waves cancel and produce essentially nothing—destructive interference. Because of this interference, the film ends up recording not only the intensity information that we associate with normal photography; it also records phase information that is an important aspect of waves. This phase information indicates where crests and troughs in the wave occurred. Because the hologram contains both kinds of information, it allows a viewer to see things that they would not see in a simple photograph.

To make a hologram, you should take a laser and split its light into two unequal portions with the help of a laser beam-splitter (or even a glass slide). The laser should operate at only a single wavelength, so that its light is highly coherent, and it should have a coherence length much longer than any distance in the scene—two requirements that are met by most common continuous-wave lasers, including laser pointers and basic helium-neon lasers. Send the stronger portion of the laser beam through a diverging lens and allow it to illuminate a scene that is otherwise in complete darkness. Light reflected from this scene should reach the film holder in which the hologram will be made. Send the weaker portion of the laser beam through another diverging lens and allow it to illuminate the film holder from the scene side. For best results, the light reflected from the scene on the film holder should be about as bright as light from this second beam.

Now place fine-grained black and white film in the film holder. Be sure that the film is sensitive to the laser light—some black and white films aren't sensitive to red light. Allow light to strike the film for long enough to expose it. Finally, develop the film and observe the developed film while it's illuminated from behind with laser light that has been spread out by a diverging lens. You should see the original scene as a three-dimensional image.

Unfortunately, there is one detail I've omitted until now. To make sure that the phase information is properly recorded, you must be sure that nothing moves by even a fraction of a wavelength of laser light during the entire exposure period. That's a very demanding requirement. Vibrations are everywhere and they will spoil the hologram. If you want this technique to work, you'll have to isolate everything—the laser, the optics, the scene, and the film—from vibrations. In a laboratory, this vibration isolation is done by floating a massive optics table on a cushion of air. All of the objects involved in making the hologram are rigidly attached to this table so that they can't move. As an alternative, you can put all the objects for the hologram on as rigid and massive a surface as you can find and support that surface on a thick layer of foam rubber. Make the holograms at night when there is little traffic of any sort around and be sure that nothing is jiggling about nearby that might shake the floor even a little bit. If you're careful, you ought to be able to create a hologram with such an arrangement.

1106. How do the display lasers used in sporting events work? I think it has something to do with mirrors.
They do use mirrors. When you bounce a laser beam from a mirror, any small change in the mirror's orientation can cause a large change in the beam's final destination. Simple laser light shows bounce lasers from low-mass mirrors that are mounted on elastic membranes. As those membranes are driven into motion by sound waves, the mirrors tip and turn and the laser beams move around in beautiful patterns on a distant screen or wall. In laser light shows that produce specific shapes and images, the mirrors that steer the laser beams are driven by high-speed electromagnetic mechanisms that can change a mirror's angle dramatically in thousandths of a second. With several of this electromagnetically controlled mirrors working together and guided by a computer, the beam can be steered to draw complicated shapes on a screen or other surface.

1123. What type of laser is in a laser printer? — DFC, Asheville, NC
A laser printer uses a single diode laser that's scanned across the surface of the photoconductor drum by a rapidly turning, multifaceted mirror. These diode lasers are very similar the ones used in laser pointers or supermarket barcode readers. The multifaceted mirrors are typically octagonal prisms that are aluminized to make them highly reflecting and spun by a motor. The laser beam bounces off the spinning mirror and its reflection sweeps across the photoconductor. Modulating the current supplying power to the diode laser causes its brightness to fluctuate so that it writes information on the surface of the photoconductor.

1156. I heard of a laser induced fluorescence instrument that is used in aiding cancer diagnosis. Could you tell me how this instrument works?
You are probably referring to a device developed at the BC Cancer Research Center in Vancouver, British Columbia and now available commercially from Xillix Technologies. A scientist from that research center gave me the following description of their technique.

The instrument is based on the discovery that most tissues when illuminated by blue or UV light emit a natural fluorescence spectral signature known as autofluorescence. This fluorescence signature is the sum of the emission of the various biochemical fluorphores present in the tissue. If the tissue chemical or physical structure changes, then the spectral signature changes. By exploiting differences in the spectral signature between cancerous and healthy tissue one can create an imaging device that can "see" the difference in the color of the autofluorescence of the tissue and detect changes that may indicate the presence of cancer. The sensors used to see the low levels of fluorescence light employ similar technology to military night vision devices. Once areas of change are located and confirmed by analysis of a biopsy sample treatment can begin. This technique is primarily useful for early stage cancers that are not visually apparent to a physician.

1467. I work in a company shop that uses a 600-watt laser with a wavelength of 1064 nm. How safe is this machine? What is the radiation hazard, if any? I've noticed that my eyes feel strange after working with it for 4-5 hours. It also has an uncomfortable smell. — EC
The laser you're using is a neodymium-YAG laser. It uses a crystal of YAG (yttrium aluminum garnet), a synthetic gem that was once sold as an imitation diamond, that has been treated with neodymium atoms to give it a purple color. When placed in a laser cavity and exposed to intense visible light, this crystal gives off the infrared light you describe. You can't see this light but, at up to 600 watts, it is actually incredibly bright. You don't want to look at it or even at its reflection from a surface that you're machining. That's because the lens of your eye focuses it onto your retina and even though your retina won't see any light, it will experience the heat. It's possible to injure your eyes by looking at this light, particularly if you catch a direct reflection of the laser beam in your eye.

In all likelihood, the manufacturer of this unit has shielded all the light so that none of it reaches your eyes. If that's not the case, you should wear laser safety glasses that block 1064 nm light. But it's also possible that the irritation you're experiencing is coming from the burned material that you are machining. Better ventilation should help. High voltage power supplies, which may be present in the laser, could also produce ozone. Ozone has a spicy fresh smell, like the smell after a lightning storm, and it is quite irritating to eyes and nose.
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