Lasers do not only play a pivotal role in science fiction stories, but also in our everyday lives. Today, lasers are found in many applications, from consumer electronics down to medical procedures. Some of us would even go as far as saying that the absence of lasers would certainly compromise the life styles we have become accustomed to. As already mentioned in the title page, the word LASER is actually an acronym for Light Amplification by Stimulated Emission of Radiation. In order to gain some insight into the workings of a laser, we have to
dwell a little bit on the components which make up a laser. We will also need to venture a little bit into the physics side a little, but this is covered in the subsequent links: The Atomic Model, the Electromagnetic Spectrum and
Laser Light Generation.
In its most simplistic form, a laser consists of three components:
The “Pump” is the energy source of a laser. As will be described in the Laser Light Generation page, laser light will only develop, if a population inversion of ground-state atoms to “excited-state” atoms can be achieved in the Gain Medium. In order to achieve this population inversion, energy in a suitable form needs to be supplied to the system. Different types of energy delivery systems are available, and each is unique to an individual type of laser. Excitation by light is shown in the illustration above and is referred to as optical pumping. Excitation by electricity is another option and is usually found in gas and semiconductor lasers. Finally, the energy derived by chemical reactions is also used as an energy source.
Within the context of laser physics, the Gain Medium (aka optical medium or active laser medium) is the material and part of a laser in which the optical gain (light amplification) occurs. As already described, this gain is usually generated by a process of stimulated emission. The gain medium is a required element of a laser to compensate for the losses occurring in the resonator cavity (below). The gain medium utilizes the energy from the pumping source to add energy to the amplified light. There are a variety of different gain media available for lasers. The type of gain medium also defines the type of laser. Examples of such gain media include:
- Crystals, such as ruby or sapphire
- Crystals which are “doped” with rare earth ions, such as Nd:YAG (neodymium-doped yttrium aluminum garnet) lasers or Yb:YAG (ytterbium-doped yttrium aluminum garnet) lasers
- Glasses, such as Yb:glass (silicate glasses doped with laser-active ions, in this case ytterbium)
- Ceramics which are usually also doped with rare-earth ions
- Laser dyes, which are usually suspended in a liquid form and usually used in dye lasers
- Gases or gas mixtures, which are usually pumped with electrical discharges. Examples are CO2 lasers or Excimer lasers.
- Semiconductors such as GaAs, GaAlAs, or InGaAs, which are typically pumped with electrical currents
The optical amplification in the gain medium of a laser is the product of stimulated emission, where the re-circulating light within the resonator cavity induces transitions of laser-active ions from an “excited” state to a lower energy state, during which a photon is emitted.
Every laser requires a Resonator Cavity in which the laser beam can recirculate, passing through the gain medium several times, thus providing feedback and amplification to the laser light. The resonator cavity typically encompasses the gain medium, however, which is often the case in semiconductor lasers, the resonator cavity is part of the gain medium, where a full-mirrored and a half-mirrored optical coating is applied to opposite ends of the gain medium (in the illustration above, the left mirror is the full mirror, whereas the right mirror is the half-mirror with the exit hole for the laser light).
The light inside the resonator cavity will reflect via the mirrors multiple times. During this reflection process, constructive interference will select for only certain frequencies or wavelengths, whereas destructive interference will annihilate other wavelengths. The more often a beam is reflected through the optical cavity, the more stable its frequency pattern will be. Furthermore, the probability of stimulated emission to take place is proportional to the time the beam spends inside the gain medium. The partially reflecting mirror through which the laser beam eventually exits (aka - the optical coupler) is usually designed to have only a 1-20% pass-through rate. This way the reflection rate and beam quality can be controlled very precisely. Most resonator cavities consist of two facing mirrors. Depending on whether the mirrors are plane or spherical and the combination in which they are arranged, several types of optical resonators have been identified.
These are the basic components of a laser. We will elaborate on each of these in more detail on their individual pages.
