(a) Both levels 2 and 3 are raised by same fixed amount with respect to level 1. All probabilities remaining same.

Question

Recall our laser set-up from the class with energy levels 1, 2 and 3. Level 2 is the highest and Level 3 has energy in between levels 1 and 2. Level 2 is very short lived and spontaneously goes down to level 3 with much more probability than going to level 1. Level 3 on the other hand is long lived and obviously electron there can only go to level 1 by spontaneous emission. Photons of energy E₁₂ = E₂ − E₁ are used to pump electrons and achieve population inversion between level 1 and 3. 

For each of the following changes, describe what would change in output laser characteristics in terms of its frequency and power and what changes would be required in the light that is being used for pumping in terms of its frequency and power. 

(a) Both levels 2 and 3 are raised by same fixed amount with respect to level 1. All probabilities remaining same. 

(b) Level 2 remains same while level 3 is brought closer to the level 1, all probabilities remaining same.

Answer 1

(a) The photons of energy E₂ − E₁ = E₁₂ would be required to achieve population inversion. As difference increased, thus pumping photon frequency and hence power would be increased. Also as E₃₁ = E₃ − E₁ is increased, the output laser photons frequency and hence power is increased.

(b) The frequency and power of input light remains the same as E₁₂ is unchanged. The frequency and hence power of output laser light is decreased.