Sample Lab Report October 29Th 2012 (B) Homework: (1) the Emission of Two Photons in a Pair Is Obviously from Stimulated Emission

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Sample Lab Report October 29th 2012 (b) Homework: (1) The emission of two photons in a pair is obviously from stimulated emission. The fact that a single photon emitted towards a strange direction is called spontaneous emission. This is caused by an electron in an excited state decaying to a lower energy state which is not occupied. In a spontaneous emission situation, there is no particular phase or direction of emission associated with the photon. One can decrease the lifetime of the energy to create more spontaneous emission. Otherwise most emitted photons are from stimulated emission. (2) The number of red paired photons increases within the tube. That is because the light from the second lamp will cause electron excitation from ground state to the 2nd excited state (level 3) of the atom. Since the lifetime of level 3 is very small and the lifetime of level 2 is large, the excited electrons tend to decay to the 1st excited state and stay there. This provides sources for the stimulated emission. One spontaneous emitted photon will cause another stimulated emission. Because the reflectivity of the mirrors is very high, the photons cannot transmit through them. They will accumulate within the tube. (d) Homework: (1) A stable laser beam is obtained. In this situation, the system is in a balanced state that stimulated emission is created by absorbing the energy from the lamp while the energy is also released from the system in the form of laser beam. (2) If the mirror reflectivity is very high (>~90%), the tube will explode. One can reduce this reflectivity so that the internal power will decrease. Or if the lamp is tuned off, the internal power will stop increasing but will not decrease though. (3) If one wants to create a laser, he/she needs enough occupation on the 1st excited state (level 2) for stimulated emission. Therefore three levels are needed. The decay from level 3 to level 2 is a huge source for this occupation. The minimization of the level 3 lifetime is for increasing the decaying rate so that more electrons will stay on level 2. .

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