Contents
Source: MDPI
Understanding Unstable Optical Resonators
An optical resonator can be classified as stable or unstable based on its design. In a stable resonator, a ray injected into the system will stay within it for many round trips, while in an unstable resonator, the ray will eventually be ejected. The ABCD matrix algorithm is commonly used to determine the stability of a resonator and the required changes to transition between stable and unstable regimes.
Wave Optics and Resonator Modes
A deeper analysis of resonator properties involves wave optics and the study of resonator modes. Unstable resonators exhibit unique characteristics such as significant diffraction losses, which increase with higher mode orders. The mode profiles in unstable resonators can be complex, often showing ring structures that require numerical methods for accurate calculation.
Output Coupling in Unstable Resonators
Unstable laser resonators are designed to utilize diffraction losses as the useful laser output. Output coupling can be achieved through various methods, such as using ordinary laser mirrors with extended field distributions or scraper mirrors to extract light from the intracavity beam. Variable reflectivity mirrors are also employed to improve beam quality by avoiding ring structures in the output beam profile.
Advantages and Limitations
While most laser resonators are stable, unstable resonators offer advantages in generating high-power laser beams with relatively high quality. They are particularly beneficial in gain media with high gain levels, such as flashlamp-pumped or diode-pumped lasers. However, analyzing and optimizing unstable resonators can be more challenging compared to stable resonators, requiring numerical beam propagation and a deep understanding of optical concepts.
Conclusion
Unstable optical resonators play a significant role in high-power laser systems, offering advantages in certain applications where high output power and beam quality are crucial. Despite their more complex nature, unstable resonators are valuable in specific laser technologies, providing a unique solution for achieving high optical power with acceptable beam quality.
Source: Fosco Connect
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