Facts about laser

A laser device is an optical cavity which focuses its' beams of light into high intensity. The beams' wavelength and what lasing medium's being used determines its qualities. The lasing medium is what names the laser, such as carbondioxide laser.

While processing material it's important with a parallel beam and that the laser emit a beam with a specific wavelength. Laser beams can be focused to tiny focal spots. To achieve a lager spot the beam can be defocused. This enables processes such as welding, cutting and marking. This also makes a small thermal impact on the material which results in none or very tiny deformations.

Equipment for linking the beam can be used to reach areas which is difficult to access. In difference from plasma jet or electro spinning jet, laser beams doesn't get affected by electrical or magnetic fields. Mirrors or optic fibre cables are often used to link the beam depending on the wavelength of the light.

The laser medium/wavelength and the effect and size of the focal spot needs to be taken in consideration while choosing lasers and optics.

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A laser is an optical cavity. Lasers radiates within the areas of ultra violet laser, visible light and infrared laser. A specific laser type is often limited to a certain wavelength directed by the lasing medium. The lasing medium is a material with properties allowing it to amplify light or radiation from a set wavelength. (pic.1). The capacity of amplify light refers to the first two letters in the acronym of the word laser - Light Amplification.

By placing two parallel mirrors on each side of the lasing medium makes the light bounce back and forth. The light that passes through the lasing medium is being amplified. One of the two mirrors, the output coupler is partially transparent. The output laser beam is emitted through this mirror. The direction of the beam is determined by the mirrors. (pic. 2)

Almost all atoms, molecules and ions have the ability to amplify light. The atoms, molecules or ions just needs to get in the right "condition". You need to take a closer look on processes involved while optic radiation interact with matter to be able to decide the condition. From now on we will only mention atoms, but with the same reasoning for ions and molecules.  

A photon can be absorbed while optic radiation hits a collection of atoms due to a higher energy state of one of the atoms. The atom is "lifted" to a higher energy state (a photon is the lowest energy state of light). The difference in the two energy states needs to be equal to the energy state of the photon for the absorption to take place. The energy of the photons depend on the wavelength of the light. Therefor absorption can only take place on a certain wavelength (pic. 3)

An atom in the higher level of energy can spontaneously decay to a lower lying level, releasing the difference in energy between the two states as a photon. This is called emission or florescence. (pic. 4)

The most interesting process, from a laser point of view, is a process called stimulated emission. If a photon with the right level of energy hits an atom in the excited state, the atom can be stimulated to emit a new photon with the same wavelength, phase and direction (polarisation) as the original photon (pic. 5). The stimulated emission has been given the acronym of the word laser its last three letters - Stimulated Emission of Radiation.

Einstein established this relationship. He also showed the equal probability for absorption and stimulated emission. To achieve light amplification the number of atoms in one excited state need to exceed the number of atoms in the lower energy-state (so called population inversion). It is an unnatural phenomenon but can be maintained with various tricks.  Therefor all laser construction is about raising atoms to a high energy-state to be able to use the stimulated emission to create amplified light.

Pumping

Usually there is always more atoms in the lower energy-state than in the excited state. To achieve more atoms in the excited state than the lower energy-state is done by using atoms in several states of energy. This is called population inversion. By supplying energy to the atoms, some of them will raise into an excited state. The majority of the atoms are still on the ground state and have therefor not achieved population inversion between level 4 (the upper laser level) and the basic level. Thereafter the atom decay to level 3. Population inversion will be obtained since there are no added atoms to level 2. It is pretty easy to obtain population inversion if the life-span is long on level 3 and short on level 2. To maintain the inversion new atoms constantly needs to be excited to level 4. This process is called pumping atoms to a higher level (pic. 6).

Supply energy

There are two common methods to supply energy to the atoms. The first method is by electrical glow discharge in gas, where the gas molecules becomes the lasing medium. The second method is to pump the lasing medium with a flash-lamp or a lamp with a continuous wave.

The most common lasers for processing, neodymium- and carbon monoxide lasers, are both examples of pumping methods. The CO₂ laser is an electrical glow discharge laser and the neodymium laser is an optically pumped laser (solid-state), pumped for example by a flash-lamp or diodes. In the neodymium laser it is the ion Nd3+ that is laser active. The ion's doped in a solid material such as YAG (yttrium aluminium garnet) or glass.