Polarization of a Laser | Physics Explained

Question:

The photons in a laser tend to be “self-reinforcing”: if some of the photons are in one state, the other photons tend to be as well. This explains the relatively narrow range of wavelengths in laser light. Can this same idea also explain why laser light is highly polarized? If so, how?

Answer:

Understanding Polarization in Lasers

The key mechanism behind the polarization of laser light is stimulated emission, which “favors” the replication of photons in a specific mode—including their polarization state.

1. Initial Emission: Random Polarizations

When a laser’s gain medium is first excited, it emits photons through spontaneous emission. These photons typically have a random distribution of polarizations.

If the laser cavity is completely symmetric with no preference, the resulting emission might be only partially polarized.

2. Role of Stimulated Emission

As soon as a few photons with a specific polarization emerge—whether due to chance or slight asymmetries in the cavity—these photons stimulate more emissions in the same polarization state.

    The rate of stimulated emission is proportional to the number of photons already in that mode. So, more photons in one polarization → more likely that additional photons match it.
  

3. Self-Reinforcing Polarization

This creates a feedback loop: one polarization mode gets slightly ahead, and then quickly dominates. Over time, this builds up a beam of light that is highly polarized.

The favored polarization grows exponentially.
Other polarizations are comparatively suppressed.

4. Cavity and Medium Effects

While stimulated emission provides the self-reinforcing mechanism, external factors also play a role in determining final polarization:

Optical cavity design: Some laser cavities have polarization-selective mirrors or elements that amplify only certain polarizations.
Gain medium anisotropies: The atomic or molecular structure of the medium might naturally favor a particular polarization.
Mode competition: Losses might be higher for other polarizations, further enhancing the dominant one.

5. Summary of Polarization in Lasers

    Just like how lasers produce a narrow wavelength through mode amplification, they also amplify one polarization state. Stimulated emission “locks in” the polarization by reproducing photons in the dominant polarization mode.
  

Conclusion:

The high degree of polarization observed in laser beams arises due to the same stimulated emission principle that narrows their wavelength. Once a polarization mode gains a lead, it becomes self-reinforcing. Combined with the laser cavity design and gain medium properties, this leads to a beam that is highly polarized—often in a single, well-defined direction.

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