http://www.dmphotonics.com/LASER means a technique of Light Amplificationby Stimulated Emission of Radiation, was invented in 1960. In fact consist onactive medium, that when stimulated by some specific energy source (photonenergy) emits LASER light, which consists on the first photon plus the emittedphoton, as showed on figure 1.[http://u20science.org/akshivb/2013/04/17/laser/- 17/04/2013]LASERs are a coherent, monochromatic anddirectionality light source. The coherence is a property that indicates theability of the waves to interfere with each other. Two waves that are coherentcan be combined to produce an unmoving distribution of constructive anddestructive interference (a visible interference pattern) depending on therelative phase of the waves at their meeting point. Waves that are incoherent,when combined, produce rapidly moving areas of constructive and destructiveinterference 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 avery narrow band spectrum of wavelength, and in this case the peak of intensityis on one specific wavelength that makes them as a monochromatic light source.Directionality is related with the multiplereflections inside of optical cavity that also produce a well-collimated beam,because only photons traveling parallel to the cavity walls will be reflectedfrom both mirrors. If the light is the slightest bit off axis, it will be lostfrom the beam [1].Active medium to LASER can be gas, liquid orsolid. This medium is localized on the cavity center and is placed on the firststage of the laser system the oscillator, this is the place where the activemedium receives energy to make a population inversion and is knew as aamplification medium. When the active medium has a good population inversion itemits a LASER light, which the wavelength depends of medium properties and ofwavelength seed (utilized as an energy source to make a population inversion onactive medium).Here I will show as work a LASER oscillator toa state solid LASER, which means that we have as active medium a crystal. Infact LASER cavity consist on a crystal on the center of cavity, a set of mirrorsand an energy source, as show the figure 2.As we can see on figure 2, oscillator has a setof two mirrors, where one mirror has high reflection, and the other has apartial transmit, which means that some part of the amplified light can beemitted by the oscillator when it gets a specific energy, and before get thisenergy the beam is trapped on the cavity and will be how many tours inside ofoscillator as necessary to get the energy to could be emitted. This specificenergy is correlated with the longitudinal mode of the cavity.To a cavity with l length some a specific set offrequencies could satisfy boundary conditions on mirrors, on the other hand, itmeans that just some frequencies we will have a constructive interference eachtime that a beam did a complete turn inside of the cavity. These specific frequencies are cavitylongitudinal modes:C is light velocity, neff iseffective refractive index inside of the cavity and l is cavitylength.Furthermore longitudinal modes, the frequenciesthat will be emitted by optical cavity need to be inside of the gain curve thathas, in general, a big bandwidth. By the equation