F - P Cavity vs Coaxial Spherical Cavity

In optical resonator design, the loss mechanisms and stability of different cavity types critically affect laser performance. This article examines the geometric loss and stability differences between a plane (Fabry–Pérot) cavity and a coaxial spherical cavity, explaining why the latter is more commonly used in practical laser systems.

1. Main Resonator Loss Mechanisms

Optical intensity in a resonator decays for several reasons. The five common loss categories are:

1. Mirror transmission loss: Energy lost as light transmits through the cavity mirrors.

2. Gain-medium absorption and scattering loss: Energy absorbed by or scattered within the active medium.

3. Geometric loss: From a geometric-optics perspective, rays that stray beyond the cavity aperture escape.

4. Diffraction loss: Energy leaking due to diffraction at finite apertures or imperfect boundaries.

5. Accessory loss: Additional losses introduced by elements like beam tubes or cooling components.

Among these, geometric loss most directly reflects a cavity’s stability, as it determines whether light remains confined over many round trips.

2. Geometric Loss and Instability in a Plane (F–P) Cavity

Cavity Structure

A plane cavity (Fabry–Pérot cavity) consists of two flat mirrors facing each other, with light reflecting back and forth between them.

Extremely Stringent Geometric Conditions

• The only perfectly confined rays are those exactly perpendicular to the mirror surfaces.

• Any slight angular deviation will cause rays to exit the cavity after a finite number of reflections.

Parameter Sensitivity: Resonator Instability

• If the two mirrors are even slightly misaligned, all non-normal rays will quickly escape.

• A sudden rise in geometric loss due to parameter drift is called resonator instability, leading to a rapid drop in output power—or even quenching of laser oscillation.

Thus, a plane cavity demands exceptional manufacturing and alignment precision, making stable, reliable laser output difficult to maintain in real-world applications.

3. Advantages of a Coaxial Spherical Cavity

Cavity Structure

A coaxial spherical cavity uses two (or one flat and one) spherical mirrors sharing a common optical axis. The curved surfaces create a natural trapping region around the axis.

Relaxed Stability Conditions

• Paraxial rays (those making small angles with the optical axis) are naturally confined.

• Geometrically, all near-axis rays undergo many round trips without escaping.

• Manufacturing and alignment tolerances are much looser, reducing the risk of instability.

Manufacturing and Application

• Spherical mirror fabrication is well established and cost-effective.

• Coaxial spherical cavities are the structure of choice in most solid-state, gas, and fiber lasers.

4.Practical Recommendations

• For small-scale, low-power, or laboratory test lasers—where alignment precision can be assured—a plane cavity may simplify the setup.

• For industrial, high-power, or long-term stable operation lasers, a coaxial spherical cavity is strongly recommended to ensure optimal beam quality and consistent output.

5. Conclusion

• The plane (Fabry–Pérot) cavity offers the simplest architecture but is extremely sensitive to alignment, leading to high geometric loss and potential resonator instability.

• The coaxial spherical cavity provides a much wider stability region, effectively confines paraxial rays, and benefits from mature manufacturing techniques, making it the preferred choice for most laser systems.

By understanding these geometric-loss and stability differences, designers can weigh structural complexity against performance requirements to select the most suitable resonator type for their application.