How Does Stimulated Emission Enable Laser Coherence and Gain?

Stimulated emission is the core process of “Light Amplification by Stimulated Emission of Radiation” (LASER). It is a quantum “cloning” process that creates an army of identical photons, which is the source of both gain and coherence.

Enabling Gain (Amplification)

A medium with a population inversion ($N_2>N_1$) is called a “gain medium.”

1. When a photon passes through this medium, it can be absorbed by an atom in state 1, or it can stimulate an atom in state 2 to emit.
2. Because $N_2>N_1$, the probability of stimulated emission exceeds that of absorption.
3. In stimulated emission, one incident (“trigger”) photon enters, and two photons (the trigger plus the new one) leave.
4. Those photons stimulate more atoms, doubling the count each step → exponential amplification.

This positive feedback produces optical gain for the light signal.

Enabling Coherence

The “cloning” in stimulated emission preserves the stimulating photon’s quantum properties, yielding laser light that is:

• Temporally coherent (same phase): The emitted wave is phase-locked to the stimulating wave, so the beam builds into a single, stable waveform.
• Spatially coherent (same direction): The new photon is emitted in the same propagation direction, producing a highly directional (collimated) beam.
• Monochromatic (same frequency): The frequency is set by the level spacing: $h\nu =E_2-E_1$, so photons share the same wavelength.