Laser Action

[logearav]

Revered Members,

An electron in ground state makes its way to an excited state upon absorption of photon of energy, equivalent to energy difference between ground state and excited state, and after some time, it decays by emitting the photon and returns to the ground state. This is called spontaneous decay.

For Laser action, population inversion and stimulated emission should occur.

Now let me explain the scenario

1) An atom(electron) in the energy level E1 absorbs a photon and goes to a state of higher energy say E3

2) While decaying from E3 to E1, it reaches a metastable state E2. Now, due to longevity of the stay in E2 than in E3, we can achieve population inversion in E2

My question is ,

1) Do we supply a photon to the electron which stays in the meta stable state E2, so as to trigger stimulated emission? I mean, when the electron comes to E2, do we supply photon again externally or the photons come due to the spontaneous emission by other electrons which arrive to E2?

2) If yes, what is the energy of the supplied photon. Is it E3 - E1 or E2 - E1?

[swansont]

1) Do we supply a photon to the electron which stays in the meta stable state E2, so as to trigger stimulated emission? I mean, when the electron comes to E2, do we supply photon again externally or the photons come due to the spontaneous emission by other electrons which arrive to E2?

2) If yes, what is the energy of the supplied photon. Is it E3 - E1 or E2 - E1?[/font]

 

The lasing action starts because of spontaneous emission, but you amplify with the stimulated emission, usually by having multiple passes of photons — that's why you have mirrors. The photon energy giving the stimulated emission is that of the laser transition: E2-E1. It has to have the same energy.

[logearav]

thanks swansont.

My doubt is an electron which comes to metastable state has already has one photon absorbed in it which was supplied when the electron was in ground state. My question is, do we supply one more photon to this electron which has one absorbed photon already? I mean where the stimulating photon comes from? Do we supply externally?

[swansont]

thanks swansont.

My doubt is an electron which comes to metastable state has already has one photon absorbed in it which was supplied when the electron was in ground state. My question is, do we supply one more photon to this electron which has one absorbed photon already? I mean where the stimulating photon comes from? Do we supply externally?

 

You can, but generally it comes from spontaneous emission.

[logearav]

Thanks again. Now the stimulating photon travel along the axis of mirror, it gets reflected from one side of the mirror and gets incident on other atoms and the lasing action starts. In what way, the reflection from the mirror helps in amplifying the laser signal?

[swansont]

Thanks again. Now the stimulating photon travel along the axis of mirror, it gets reflected from one side of the mirror and gets incident on other atoms and the lasing action starts. In what way, the reflection from the mirror helps in amplifying the laser signal?

 

Each incident photon stimulates the emission of another photon. This will happen at some rate as you go through the gain medium giving you a gain of G, and you will get that gain with each reflection. You couple some fraction of the light out, of course, so you eventually reach steady-state.

[logearav]

What do you mean by Gain medium,sir?

[swansont]

What do you mean by Gain medium,sir?

 

It's the region where the lasing occurs. Where you get the stimulated emission — that's what amplifies the light, i.e. you get more photons out than you put in.