Laser: social medium avant la lettre

Lasers are everywhere around us, and are indispensable in our daily lives in applications such as metal cutting and welding, in DVD players and laser printers, to send signals through glass fibers, or to sinter particles to each other in 3D printing manufacturing processes. And where would science fiction movies like Star Wars be without the laser beam …

Mirrors and lenses to deflect or reflect laser beams inside the laser printer

The word laser is an abbreviation for light amplification by stimulated emission of radiation. Therefore, ‘laser’ is a phenomenon, and not a device as such, although in everyday speech ‘the laser’ indeed sometimes refers to the laser-generating device or the resulting laser beam. Lasers already exist for about fifty years, and because of their mutual stimulation you can actually consider them as one of the oldest social media.
Where an incandescent lamp emits light in all directions and with all colours of the rainbow, a laser does something completely different: it emits light in one direction and with one wavelength, resulting in an intense beam of light. How is it possible that a laser points the ‘noses’ of all light particles into the same direction?

The ‘laser-as-a-device’ consists of an amplifying medium – usually a solid material or a gas – between two mirrors that has an amplifying effect on the energy of an external source. The supply of external energy is achieved, for example, by irradiating the amplifying medium with light from (xenon) flash lamps, light from other lasers or by passing an electrical current through the amplifying medium – in laser terms known as the ‘pump’. If you add enough energy to an atom of this amplifying medium, then the atom ends up in an excited state, in which an electron has moved from the ground state to a higher energy state. However, the electron doesn’t feel comfortable here, and prefers to return to its original ground state. The energy difference between the higher state and the ground state is released during this spontaneous return in the form of a light particle or heat. So far nothing new. But what is happening now? If you irradiate such an atom having an electron in the higher energy state with a light particle with the same energy as the aforementioned energy difference, then this light particle stimulates the electron to fall back to the ground state, where a light particle is released again – in addition to the original light particle that continues to irradiate. In other words, one light particle results in two light particles, each with the same energy – and thus also the wavelength of light – that also radiate into the same direction. For good laser operation, it is essential to have many atoms in the excited state – so a good ‘pump’ is important. Then this process can maintain itself: two light particles lead to four light particles, which in turn lead to eight light particles, and then sixteen, thirty-two, and so on. The parallel mirrors on both sides of the amplifying medium provide an ‘extra amplification’ by returning the light particles back into the amplifying medium. If you make one of the two mirrors semi-transparent, you can let the laser light come out of the device and you have the light source available. Of course you can also choose to remove one of the mirrors lightning fast, but in practice that is impossible to do …

Because of this chain reaction, one light particle ultimately stimulates the emission of countless light particles, all with the same direction and the same wavelength, all in line with each other – a very intense, narrow beam of light. When the chain reaction ends after one ‘pump’ operation, we call it a pulsed laser – where ‘pumping’ again creates a new pulse. When using laser pulses with a high power (around 1 kW) and an ultra short duration (femtoseconds 10-15 sec to picoseconds 10-12 sec) for materials cutting or welding, the advantage is that unwanted damage such as piece deformation, thermal degradation or change in materials properties is restricted to a minimum.
With a continuous laser, the power of the laser light emitted remains constant, for example because an electrical current is constantly being driven through the amplifying medium, resulting in new atoms being excited all the time. The power of a laser varies between several milliwatts – for example in laser pointers – and several kilowatts – in the case of welding and cutting.
Diode lasers are used for applications such as the control of light signals through glass fibres, in laser pointers or in DVDs – in fact the most commonly used laser. A diode laser is nothing more than an LED (light emitting diode) that produces laser light. In a diode laser, a semiconductor material is the amplifying medium. To make this work, you have to send a much higher current through the diode – dozens of milliamps – than you would do with a normal LED. If you are sending a lower current through the diode laser, it works like a normal LED; there are too few atoms in the excited state to be able to work as a laser. In solid state lasers, the amplifying medium is usually a solid oxide crystal that is doped with a rare earth, such as neodymium-doped yttrium aluminium garnet (Nd: YAG or Nd: Y3Al5O12).

In addition to solid materials, gases can also form the basis for a laser. In the case of a CO2 laser, a mixture of carbon dioxide, nitrogen and helium is brought into an excited state by rushing a continuous electric current through the gas – somewhat similar to a fluorescent tube – which subsequently emits stimulated light particles. All these light particles, collectively called the laser beam, have the same wavelength in the infrared range of 10.6 um for the CO2 laser, and all radiate into the same direction. This gives a very intense beam of radiation with several kilowatts of continuous power. In addition to cutting and drilling materials (even on a microscale) you can also weld metals together with such a powerful CO2 laser. Although solid materials are not the basis of this laser, a CO2 laser is indeed suitable for machining or processing of solid materials. Other applications are within medicine, such as removing tumours with the laser.