# Coherence in stimulated emission

Laser light shows the unique property of coherence. This is due to the stimulated emission process of photons. But why is it that stimulated emission leads to coherence?

• Because the stimulated photon is coherent with the stimulating photon, a process that can continue through the gain medium. – Jon Custer Jun 5 '20 at 13:30
• Exactly ,in the gain medium when the stimulated photon passes another excited atom , it emits a photon in the same direction which is coherent with the latter. My question is why does this happen? – Optimistic Electron Jun 5 '20 at 13:42
• @JonCuster Isn't your comment equivalent to "It is coherent because it's coherent"? I mean, the fact that the stimulated photon is coherent with the stimulating photon is stipulated in the question, no? – garyp Jun 5 '20 at 14:56

In this link stimulated emission is explained in the third page:

Note that the energy of the incoming photon doing the stimulation is $$E_2-E_1$$ the two energy levels . This means the first photon passes and a second one comes out, which when conditions are appropriate can lead to the lazing phenomenon.

This is with pictures and words, but coherence is a mathematical phenomenon. It means that one has solved the quantum mechanical problem and wavefunctions exist which have a fixed phase between them, the definition of coherence:

In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency, and the same waveform. Coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference.