Rami Arieli: "The Laser Adventure" Chapter 2.7 page 2
Properties of Laser Radiation
The photon which is emitted in the stimulated emission process is identical to the incoming photon.

They both have:
 
1. Identical wavelengths (and thus, frequencies) - Monochromaticity.
 
2. Identical directions in space - Directionality.
 
3. Identical phase - Coherence.

These are the properties of laser radiation.

Remember that two photons with the same wavelength (frequency) have the same energy:

E = hn = hc/l

The incoming photon does not change at all as a result of the stimulated emission process.

As a result of the stimulated emission process, we have two identical photons created from one photon and one excited state. Thus we have amplification in the sense that the number of photons has increased.

This is the process that was explained in the introduction:

Light Amplification by Stimulated emission of Radiation = LASER

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