What is a Folded Resonator?

A folded resonator is a type of resonator construction in which the interior optical path is bent with the help of mirrors. Such an arrangement helps in the compact packaging of a long laser cavity. Using concave mirrors for folding the optical path induces a focal power and avoids the formation of unwanted divergent elements.

The most effective design of a folded resonator is created using curved mirrors, as this allows the formation of much optical longer cavities. Optical cavities, or resonators, are important parts of a laser. They provide the gain medium and feedback of the laser light. The light trapped in the resonator cavity gets reflected multiple times in order to produce standing waves at certain resonating frequencies.

A folded resonator arrangement is usually made up of more than two mirrors. Typically a pair of curved mirrors and a pair of plane mirrors (quasi-collimated) are used in order to produce a folded cavity. Apart from mirrors, some intracavity elements are also placed in the resonator in order to eliminate astigmatism.

References

• Folded Resonator – dual polarization competition laser displacement sensor - Elsevier
• A low cost RF oscillator incorporating a folded parallel coupled resonator - Academia
• The Physics and Technology of Laser Resonators – Dennis Hall