LASER a brief introduction

http://www.dmphotonics.com/

LASER means a technique of “Light Amplification by Stimulated Emission of Radiation,” was invented in 1960. In fact consist on active medium, that when stimulated by some specific energy source (photon energy) emits LASER light, which consists on the first photon plus the emitted photon, as showed on figure 1.

[http://u20science.org/akshivb/2013/04/17/laser/ - 17/04/2013]

LASERs are a coherent, monochromatic and directionality light source. The coherence is a property that indicates the ability of the waves to interfere with each other. Two waves that are coherent can be combined to produce an unmoving distribution of constructive and destructive interference (a visible interference pattern) depending on the relative phase of the waves at their meeting point. Waves that are incoherent, when combined, produce rapidly moving areas of constructive and destructive interference and therefore do not produce a visible interference pattern [1 - http://www.worldoflasers.com/laserproperties.htm].

 LASERs are said as monochromatic light source because they generate light on a very narrow band spectrum of wavelength, and in this case the peak of intensity is on one specific wavelength that makes them as a monochromatic light source.

Directionality is related with the multiple reflections inside of optical cavity that also produce a well-collimated beam, because only photons traveling parallel to the cavity walls will be reflected from both mirrors. If the light is the slightest bit off axis, it will be lost from the beam [1].

Active medium to LASER can be gas, liquid or solid. This medium is localized on the cavity center and is placed on the first stage of the laser system the oscillator, this is the place where the active medium receives energy to make a population inversion and is knew as a amplification medium. When the active medium has a good population inversion it emits a LASER light, which the wavelength depends of medium properties and of wavelength seed (utilized as an energy source to make a population inversion on active medium).

Here I will show as work a LASER oscillator to a state solid LASER, which means that we have as active medium a crystal. In fact LASER cavity consist on a crystal on the center of cavity, a set of mirrors and an energy source, as show the figure 2.

As we can see on figure 2, oscillator has a set of two mirrors, where one mirror has high reflection, and the other has a partial transmit, which means that some part of the amplified light can be emitted by the oscillator when it gets a specific energy, and before get this energy the beam is trapped on the cavity and will be how many tours inside of oscillator as necessary to get the energy to could be emitted. This specific energy is correlated with the longitudinal mode of the cavity.

To a cavity with l length some a specific set of frequencies could satisfy boundary conditions on mirrors, on the other hand, it means that just some frequencies we will have a constructive interference each time that a beam did a complete turn inside of the cavity.  These specific frequencies are cavity longitudinal modes:

C is light velocity, n_eff is effective refractive index inside of the cavity and l is cavity length.

Furthermore longitudinal modes, the frequencies that will be emitted by optical cavity need to be inside of the gain curve that has, in general, a big bandwidth. By the equation [1] is possible to find the frequencies permitted spacing:        

It is possible to estimate longitudinal modes number N of a LASER by the relationship between the gain curve bandwidth and frequencies permitted spacing:      

In this way the frequencies that will be emitted need satisfy the gain curve and the equation [1]. On the next post about lasers I will explain more about lasers system.

Kelly Cristine Camargo, PhD