Annular (HSURIA) resonators: some experimental studies including polarization effects.

Area of Science:

• Optics and Photonics
• Laser Physics
• Resonator Design

Background:

• Development of advanced laser systems requires specialized resonator designs for efficient beam generation.
• Annular resonators offer unique properties for high-power laser applications.
• The half-symmetric unstable resonator with internal axicon (HSURIA) is a novel design for creating annular laser modes.

Purpose of the Study:

• To establish a repetitively pulsed carbon dioxide (CO2) laser facility for testing annular resonators.
• To evaluate the performance of the HSURIA design, specifically comparing configurations with conical and flat end mirrors.
• To assess beam quality and polarization characteristics of the HSURIA.

Main Methods:

• A large-aperture CO2 laser facility was constructed, providing uniform gain over an 18-cm outer diameter and 10-cm inner diameter annular region.
• The HSURIA was tested with equivalent Fresnel numbers up to 4.5, incorporating a W-axicon mirror for beam compaction.
• Two HSURIA configurations were analyzed: one with a conical end mirror and another with a flat end mirror.

Main Results:

• The conical end mirror configuration produced a higher-order azimuthal mode with tangential polarization and an on-axis null, attributed to polarization scrambling.
• Replacing the conical mirror with a flat end mirror resulted in an l=0 mode with a central far-field spot and uniform polarization.
• Beam quality, measured by the n(2) ratio, achieved values as low as 1.2 with the flat end mirror configuration. A 20-microrad tilt caused a twofold degradation in n(2).

Conclusions:

• The HSURIA design with a flat end mirror effectively generates a high-quality, unpolarized annular laser mode.
• The conical end mirror introduces polarization scrambling, leading to undesirable higher-order modes.
• The HSURIA demonstrates good beam quality and sensitivity to mirror alignment, crucial for practical applications.