...
The simplest type of an optical resonator or optical cavity is consist of two parallel mirrors which are located around the gain medium to supply reaction of the light. These mirrors have an optical coverage that specifies their reflective features. One of the mirrors is an excellent reflector while the other is partially reflective which is called the output coupler. The latter mirror produces the laser beam by allowing some of the light to leave the cavity. The light that is produced by impulsive emission from the medium is reflected back to the medium by the mirrors. In the medium the light may be amplified by stimulated emission. Before leaving the cavity the light may reflect from mirrors and pass over the gain medium many times.
Figure 1. Laser components
More complex lasers have four or more mirrors to form the cavity. Other optical devices, such as spinning mirrors, modulators, filters, and absorbers, may be placed within the optical resonator to produce a variety of effects on the laser output, such as altering the wavelength of operation or the production of pulses of laser light. [3]
5.3 How a Laser works
Electrons are at specific energy levels . We can describe this energy levels as rings or orbits around a nucleus. Electrons that are in outer orbits have higher energy levels in compared with the electrons that are in inner rings. By injecting the energy, electrons can go to a higher energy levels. When an electron goes from a higher energy level to a lower energy level it releases its excess energy as a light. The color and wavelength of this emitted light is depended on the amount of energy that is released. Based on the lasing material that is used certain wavelengths of light are absorbed to excite or energize electrons and when the electrons fall back to their initial level specific wavelengths are emitted.
For example in a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity lamp is spiraled around the ruby cylinder to provide a flash of white light that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. Upon returning to their normal state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal.
Figure 2. components of the first ruby laser
The quartz flash tube emits an intense burst of light causing by high voltage electricity which excites some of the atoms in the ruby crystal to higher energy levels. Some of the atoms at a specific energy level emit photons (particles of light). At first the photons are emitted in all directions. Photons from one atom stimulate emission of photons from other atoms and the light intensity is rapidly amplified.Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification.The photons leave through the partially silvered mirror at one end. This is laser light. [4]
...
[7] Thenakedscientists.com, (2012). Stronger lasers with mirrors? - The Naked Scientists. [online] Available at: http://www.thenakedscientists.com/HTML/questions/qotw/question/3542/ [Accessed 5 Nov. 2014].
Figures
[1] Laser components, copied from: http://www.globalspec.com/learnmore/optical_components_optics/lasers/carbon_dioxide_lasers
[2] Components of the first ruby laser, copied from: http://www.laserfest.org/lasers/how/ruby.cfm