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

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Abstract
A repetitively pulsed CO2 laser facility was developed for testing annular resonators. The large-aperture device exhibits generally uniform gain over an annular region of 18-cm o.d. and 10-cm i.d. The half-symmetric unstable resonator with internal axicon (HSURIA) was tested at equivalent Fresnel numbers up to 4.5. This resonator design incorporates a W-axicon mirror beam compactor that transforms a cylindrical-mode region into an annular-mode region. Two HSURIA configurations were evaluated: (a) with a conical end mirror and (b) with a flat end mirror in the annular leg. With the conical end mirror, the aligned resonator produced a predominantly higher-order azimuthal mode with an on-axis null in the far field. The output was strongly linearly polarized with the electric-field vector tangential to the optic axis in both the near and far fields. The higher-order tangentially polarized mode appears to be the result of a geometric polarization scrambling effect caused by the conical end mirror. The boundary conditions for the conical or W-axicon mirrors imply that the radial electric field has a 180° phase shift on reflection, whereas the tangential component is unchanged. Thus, a tangentially polarized mode is self-reproducing, but a linearly polarized mode is not. To eliminate the polarization scrambling effect in the HSURIA, the conical end mirror was replaced with a flat end mirror. The HSURIA with a flat end mirror produced a central spot in the far field that indicated an l = 0 mode with no spatial variations in polarization. Beam quality was measured in terms of the ratio n2 of the theoretical (geometric-mode) power transmitted through an aperture of the central lobe diameter to the observed power; n2 values as low as 1.2 were obtained. The variation of beam quality with tilt of the flat end mirror indicated a factor of 2 degradation in n2 for a 20-μrad tilt, which is in good agreement with theory.

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