文档介绍：Brief introduction of laser
By Jiang Wei and Zeng Xiangze
Physics College Shandong University
Principal of laser light
The interaction between electrons consists of three basic process:spontaneous emission, stimulated absorption and stimulated emission.
spontaneous emission is the process in which excited atoms can experience spontaneous transition from high energy level to low energy level. Simultaneously, certain photon is emitted .
E2-E1=hμ where μ is the frequency of the photon
stimulated absorption:
When there is an atom at its low energy level and an photon with energy ing inside, the atom is likely to absorb the photon and transfer to the high energy level if hμ=E2-E1
The transition does not happen spontaneously but occurs due to the photon stimulating
E=C12u(μ21) N1hμ12dtdv
where C12 is Einstein Coeffient
stimulated emission
When there is an atom at its high energy level and an photon with energy ing inside, the atom is likely to transfer to the low energy level and emit a photon if hμ=E2- photon is same as the source photon in frequency, emitting direction, phase and polarization.
E=B21u(μ21) N2hμ21dtdv
where B21 is Einstein Coeffient
Einstein has proved that B21 is equal to C12.
For an atomic system,stimulated absorption and emission both happen if there exists optical signal with energy density u(μ21).
From equation 1 and 2, we know:
s