Despite the teachings of Austin Powers, lasers aren’t just designed to be attached to sharks’ heads. Electricians use them too. David Herres explains.

Electricians have occasion to work with electrons every day, measuring and directing their flow and harnessing their energy for the benefit of end users. In the course of this work, most of us ponder the nature of these elementary particles and speculate on their origin.

Ultimately, an electron is unknowable. It has a negative charge and minute mass. We can describe its properties and behaviour under fixed circumstances, but how or why it came to be remains a mystery.

What we do know is that it orbits an atomic nucleus, not in a flat plane like planets orbiting the sun, but in three-dimensional shells. And sometimes, they are knocked out of orbit, travelling through comparatively vast spaces until they collide with another body.

Based upon what is known about electrons, humans have built numerous devices to capture their energy and perform useful work for us. The development of the laser is a remarkable twentieth-century success story involving researchers throughout the world whose efforts finally came together to make first a working model and finally mass-produced devices that are now in almost every home.

‘LASER’ is actually an acronym that stands for ‘Light Amplification by Stimulated Emission of Radiation’. The key word is stimulated. Emission of radiation can be either stimulated or spontaneous. If it is spontaneous, the light will not be coherent and you won’t have a laser.

If a solid body is heated sufficiently, it will emit a broad spectrum of radiation including visible light. This light is a mix of colours and the waveforms do not lie on the same plane but vibrate every which way. If a portion of this light is accurately focused by means of a lens or concave mirror to make a tight beam as in a spotlight, the beam of light will spread, making a cone that progressively widens the farther from the source, even if travelling in a vacuum. The reason for this is not any inaccuracy in grinding of the lens or mirror, or impurity in the medium through which the light travels. The spreading out of what was intended as parallel rays is due to the nature of the light itself. Since the light waves lie in different planes, they interfere with one another in a process known as diffraction.

It is possible to organise all the light waves at the source so that the waveforms are in phase, of the same frequency and lie in the same plane. Then, the beam of light will not spread and consequently its intensity will not diminish as it travels in the direction it is aimed.

The device that generates such a beam of light is the laser. Using it, we here on earth can make a small spot of light on the moon. Such a narrow beam could never be created solely by a system of lenses and mirrors no matter how accurately ground.

Laser-generated light is coherent, monochromatic and collimated. To create this light, the laser, with an external power supply, makes use of the quantum processes of absorption and stimulated emission. In an atom, the orbiting electrons are at a higher energy state when they occupy a shell that is farther from the nucleus, and their energy state is lower when they occupy a shell that is nearer to the nucleus. Unlike the planets that orbit the sun, these atomic shells are discrete distances from the nucleus, not just anywhere. Accordingly, the energy levels are specific values.

Electrons can spontaneously move from the higher to the lower state. If they do so, they emit a particle of light, known as a photon. If a photon from outside collides with an electron that is in the lower energy state, closer to the nucleus, that electron will absorb the photon and jump to a higher energy state, farther from the nucleus. But it won’t stay there very long. It will quickly emit a photon and drop back to the lower energy state.

The electron alternates between the two energy states, each time absorbing or emitting a photon depending on which way it is going. The ordinary state of affairs is for the electron to be in the lower energy state.

For laser action to occur, there has to be a condition known as population inversion. This is the key concept in laser technology. What it means is that electrons are induced to move to the higher energy level, where they are available to drop back to the lower energy level. At that moment, a photon is emitted. This event will be either spontaneous or stimulated.

For population inversion to take place, there has to be a continuous injection of energy. In a laser, this is accomplished by the pump.

Various materials can serve as the laser medium. A very basic device is the helium-neon laser. It works because of the fact that helium can be excited to a level that is close to neon. When the helium and neon atoms collide, the electrons in the neon jump to the higher energy level, moving to a shell that is more distant from the nucleus. The helium is the pumping gas and the neon is the lasing medium.

Another type of laser (there are many) is the carbon dioxide laser. Nitrogen is the pumping gas and carbon dioxide is the lasing medium. This type of laser can put out 10kW continuously and much higher amounts in short pulses. Due to its high power, it is used to cut and weld metals. Other types of lasers are the argon-ion laser, the ruby laser, the neodymium yttrium-aluminium garnet laser, and laser diodes, constructed by joining two layers of doped gallium arsenide. Dye lasers are tuneable, which means they can operate at multiple frequencies.

A typical laser consists of an optical cavity, which is a cylindrical drum-like container that holds the laser or gain medium. At either end of this cavity is a highly polished mirror. As the optical medium, gas or solid, is excited by energy from the pump, the light that is generated inside the cavity bounces back and forth between the two mirrors. During each cycle, the light is further amplified, all the while coherent.

The back mirror is 100% reflective. The front mirror is 99% reflective. One percent of the light is transmitted through the front mirror, or output coupler, to the outside world, and that constitutes the laser beam. That doesn’t sound like much, but considering the speed of light through the medium, it adds up fast. The two mirrors must be aligned very accurately to produce a good beam. The pump constantly replenishes the energy in the laser cavity. The pump can process either an electrical charge or light pulses that strike it from an outside source. Powerful lasers are driven by nuclear reactors. Small laser pointers can run off a single AAA cell.

Computer hard drives, CD and DVD players, a computer mouse, carpenter’s transit, bar code readers, laser printers and many other types of electronic equipment incorporate laser technology.

You may be called upon to diagnose and repair one of these items. If so, remember that even a low-powered laser can cause blindness. Never look directly into the beam or let it fall on your eye. Higher powered machines can cause serious injury and elaborate precautions are necessary to prevent contact. Never disable an interlock switch or operate it with guards removed.

Small units are sealed and cannot be repaired, but it is often necessary to replace a laser component. It is extremely heat sensitive, so when soldering leads, extensive heat sinking is essential.