## energy conservation in laser

kindly help me understanding energy conservation in laser
 Perhaps you could give a bit more detail about what it is that you don't understand? Energy is conserved in a laser, as everywhere.
 Energy is put into some kind of gain medium by an external source (electrical discharge, pump laser, etc.). The energy being put into that gain medium drives atoms to a higher electrical state. This energy is released in the form of photons, much like a light bulb. The difference here is that a small seed beam "stimulates" the release of these photons. Energy is conserved. The energy exiting the laser will always be less than what is applied.

## energy conservation in laser

thanx for the reply
 let me explain in detail what i really want to know,correct me if i m wrong ,how two photons are of exactly same energy because incident photon at metastable state is carrying energy equal to difference of energies between lower level and higher energy level when it undergo inelastic collision results into two photon which emitted in between two level which are meta stable level and lower state level,so energy incident is greater no doubt it is spent inelastically but after this process emitted photon should have energy equal to difference of energies of metastable state and lower level,but on the contrast it emitt incident energy.
 The electron is already in the high energy state when the first photon strikes it. All that the photon does is persuade the electron to emit its energy as a coherent photon. The energy comes from whatever process excited the electron in the first place - an electrical discharge, pump laser or whatever. It's loosely analogous to knocking something out of a tree by throwing a ball at it. Both items will land with energy mgh (total 2mgh), even though you only put in enough energy to get one ball up there (mgh). The other mgh was provided earlier. Does that make sense?
 i did'nt get it ,how electron is already in excited state????can u provide any mathematical pathway apart from A,B coeff. for energy
 The electron is already in the excited state because it was initially pumped with some energy to get it to this stage. This can be done by the methods indicated by user Ibix. This excited state is very short lived, so a so called 'population inversion' is required to be set up to produce a state with a high enough lifetime for laser action to occur.
 The Wikipedia article on the Helium-Neon laser has an energy level diagram and a summary of the operation of this laser, which is accurate as far as I can certify from a fifteen-year-ago course on laser physics. An electrical discharge into a tube filled with Helium and Neon excites the Helium atoms into a high energy state. That's your energy input. The Helium transfers the energy to the Neon by collisions, leaving the Helium back in the ground state and the Neon in a meta-stable excited state. When hit by a photon, the Neon also decays back to the ground, emitting a laser photon. The Wikipedia article has more detail including a proper pathway (I left out a few steps) and energy differences; the above is a quick summary. Other lasers work by different pathways, but always there is an energy input creating a population of electrons in a meta-stable excited state. All that the laser photons do is knock electrons out of the meta-stable state, generating another photon as the excited state decays to ground. If you turn off the energy input, you quickly run out of excited electrons and the laser stops. I'm not quite sure what you mean by a mathematical pathway in this context; happy to help if you give more detail of what you want.