The primary function of mirrors in an optical resonant cavity is to create a feedback loop that amplifies light energy. The all-reflecting mirror bounces nearly 100% of photons back into the active medium to trigger further emission. The partial-reflecting mirror traps most photons to sustain this reaction but allows approximately 15% of high-energy photons to escape, forming the actual laser beam.
Core Insight: The resonant cavity acts as an energy trap. By bouncing light back and forth between these two specific mirrors, the system forces photons to travel through the active medium repeatedly. This process, known as stimulated emission amplification, transforms weak light into a concentrated beam with high directionality and energy density.
The Mechanics of Optical Feedback
The Role of the All-Reflecting Mirror
This mirror serves as the "back wall" of the laser cavity. Its coating is designed to reflect nearly all photons back into the active medium.
By preventing light from escaping the rear of the device, it ensures that photon energy remains within the cavity to stimulate further amplification.
The Role of the Partial-Reflecting Mirror
This component acts as the output coupler or gatekeeper. While it reflects the majority of light back into the cavity to maintain resonance, it is engineered to be slightly transparent.
It allows approximately 15% of high-energy photons to penetrate the mirror and exit the cavity. This escaping percentage constitutes the working laser beam used for treatment.
Generating the Clinical Beam
The interaction between these two mirrors is what generates the specific properties required for dermatology.
Because the light is forced to bounce back and forth before exiting, the resulting beam achieves high directionality (the light travels in a straight line).
Furthermore, this containment builds up high energy density, ensuring the beam is powerful enough for clinical interaction with tissue.
The Critical Balance of Reflectivity
Maintaining Amplification
The relationship between the two mirrors represents a precise trade-off between energy storage and energy output.
If the partial-reflecting mirror were to allow too much light to escape (significantly more than 15%), the cavity would lose photons faster than it could generate them. This would kill the "resonance," preventing the laser from firing.
Beam Quality vs. Output Power
Conversely, if the partial mirror reflected too much light back, the energy inside the cavity might become too intense without producing a useful beam.
The 15% transmission rate is a calculated threshold. It ensures the internal reaction is self-sustaining while providing sufficient external power for medical procedures.
Understanding Laser Performance
- If your primary focus is Clinical Efficacy: Recognize that the beam's high energy density is a direct result of the mirrors trapping light long enough to achieve saturation before releasing it.
- If your primary focus is Technical Troubleshooting: Understand that any damage to the partial-reflecting mirror's coating will alter that critical 15% transmission rate, immediately degrading beam power and directionality.
The precise collaboration between the all-reflecting and partial-reflecting mirrors is the fundamental mechanism that turns raw energy into a controlled dermatological tool.
Summary Table:
| Mirror Type | Reflectivity Rate | Primary Function | Clinical Benefit |
|---|---|---|---|
| All-Reflecting | ~100% | Acts as a back wall; bounces all photons into active medium | Ensures maximum energy retention and stimulation |
| Partial-Reflecting | ~85% (15% Output) | Output coupler; allows controlled escape of high-energy photons | Forms the actual treatment beam with high directionality |
| The Cavity System | Variable | Creates a feedback loop for stimulated emission | Produces high energy density for clinical efficacy |
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References
- María Isabel Arredondo, Julieth Herrera. Láser en dermatología. DOI: 10.29176/2590843x.275
This article is also based on technical information from Belislaser Knowledge Base .
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