Lasers

Question 1

Spontaneous emission

Answer 1

When an electron is in an excited state is is said to be unstable, the electron can jump down to a less excited state by randomly emitting a photon of specific energy.

Question 2

Stimulated emission

Answer 2

When a incident photon of same energy and frequency of the electrons unstable state is released, causing the electron to jump down energy level and release a photon along with the incident photon

Question 3

Similarities of incoming photon and stimulated photon

Answer 3

Same frequency,phase,direction and polarisation

Question 4

Laser meaning

Answer 4

Light amplification by stimulated emission of radiation

Question 5

Normal state

Answer 5

N2<N1 (thermal equillibrium)

Question 6

Laser state

Answer 6

N2>N1 (population inversion)

Question 7

Pumping

Answer 7

Process of feeding energy into a system to create a population inversion where N2>N1

Question 8

Best population distribution for level 2

Answer 8

N2=N1 as 50:50 chance that it will either go up or down a energy level

Question 9

Metastable energy level

Answer 9

A meta stable energy level means that electrons in this level can stay there for a long time,before decaying to ground state

Question 10

Metastable level for level 3

Answer 10

N2

Question 11

Notes for level 3

Answer 11

Pumping still has to occur between N1 and N3 but it happens at a different frequency to the laser transition
Pumping must exceed stimulated emission
It is still difficult tot raise over half the electrons from ground state N1 to N3 and hence N2 even with a meta stable level.

Question 12

Radiationless meaning

Answer 12

doesn’t produce an optical photon

Question 13

Advantage of level 4

Answer 13

N2 is practically empty of electrons compared to metastable level N3. This means achieving a population inversion where N3>N2 is much easier

Question 14

Notes for level 4

Answer 14

N2 must have electrons that quickly decay to the ground state so that N2 is small, otherwise this could upset population inversion
Pumping still takes place at a different frequency to the laser transition
The efficiency of the laser depends upon the power in and the power out
Power in=depends upon the energy required to continuously p[ump the laser to produce the population inversion
Power out=depends upon how much energy is wasted in the non radiative transitions Ra and RB the larger the transitions the less energy is available for laser transition.