The actual process that occurs during the production of a beam of laser light is known as lasing process. The action which generates the laser light takes place within few nanoseconds. This process occurs so rapidly that it appears to be instantaneous now we discuss step by step the excitation and stimulation process in the laser.
Before excitation the shape of active medium is as
In this figure 0 = excited atom . = unexcited atom
Here virtually all of the atoms in active medium are atoms in any material will be at an excited energy state whenever the material is at temperature above the absolute zero.
When energy is pumped into the active medium by the excitation mechanism then much of it is observed in the active medium and most of unexcited atoms go in excited state as shown
Excitation by pumping light
In step 2 some of energy pumped is stored in electrons within the excited atom now this energy is released immediately as electrons spontaneously drop to lower or ground state. Some of the photons released by the spontaneous emission leave the active medium through the sides, while other photons which have been released by the spontaneous emission act as triggers to produce stimulated emission. This occurs when the released photon passes near an atom, which is still in the excited state.
Step 3 is as shown as
Fluorescence and start of stimulated emission
Before step 4 it is important to note that the output mirror must be in perfect alignment with the axis of the active medium, otherwise the reflected beam will be off axis and will walk out of the active medium.
When the mirror is aligned properly, then reflected light serves to trigger more stimulated emission within the active medium, resulting in an amplification of the reflected light. The photons released by stimulated emission are coherent, monochromatic and traveling in the same direction. That is why avalanche continues as the wave of energy proceeds through the active medium. The high reflectance (HR) mirrors returns the beam to the active medium with little or no loss, shown as
Growth of stimulated emission along the axis of medium
When the avalanche of photons quickly increases as it travels through the active medium then increasing wave of energy exist in the amplifying medium and strikes the output mirror. This mirror allows a predetermined percentage of light to exit as the useful laser beam. The remainder of the light is reflected back in the active medium of the laser. This is shown as
Continued amplification by stimulated emission and output from laser
In this way the beam of laser is produced but remember that all the steps occurs in this process simultaneously and thus the light output through the output mirror is continuous process rather than a series of burst.
As long as energy is pumped into the laser and the atoms remains in an excited state then above process will be continue.