Distributed feedback lasers, amplified resonators

Lasers have three primary components (Fig. 4) 1) an active medium that amplifies incident electromotive waves 2) an energy pump that selectively pumps energy into the active medium to populate selected levels and to achieve population inversion and 3) an optical resonator, or cavity, composed of two opposite mirrors a set distance apart that store part of induced emission concentrated in a few resonator modes. A population inversion must be produced in the laser medium, deviating from the Boltzman distribution thus, the induced emission rate exceeds the absorption rate, and an electromotive wave passing through the active medium is amplified rather than attenuated. The optical resonator causes selective feedback of radiation emitted from the excited species in the active medium. Above a pump threshold, feedback converts the laser ampler to an oscillator, resulting in emission in several modes. 

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