12. T he operation of the Laser Light: Coherent, polarized The stimulating and emitted photons have the same: frequency phase direction
13. Two level system absorption Spontaneous emission Stimulated emission h h h E 1 E 2 E 1 E 2 h =E 2 -E 1
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15. Einstein’s coefficients Probability of stimulated absorption R 1-2 R 1-2 = ( ) B 1-2 Probability of stimulated and spontaneous emission : R 2-1 = ( ) B 2-1 + A 2-1 assumption: n 1 atoms of energy 1 and n 2 atoms of energy 2 are in thermal equilibrium at temperature T with the radiation of spectral density ( ): n 1 R 1-2 = n 2 R 2-1 n 1 ( ) B 1-2 = n 2 ( ( ) B 2-1 + A 2-1 ) E 1 E 2
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19. How to realize the population inversion? Thermal excitation: Optically, electrically . impossible. The system has to be „pumped” E 1 E 2
24. E 1 E 3 E 2 szybkie przejścia akcja laserowa - optical pumping - occupation of E3 of a short life time, 10-8s. It is a band, the metastable and ground states are narrow : - electrons are collected on E2: population inversion - stimulated emission (one photon emitted spontaneously starts the stimulated radiation ) - Beam of photons moves normally to the mirrors – standing wave. The laser operation