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

Abstract

A repetitively pulsed CO(2) 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 cylindricalmode 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 conitions for the conical or W-axicon mirrors imply that the radial electric field has a 180 degrees 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 n(2) of the theoretical (geometric-mode) power transmitted through an aperture of the central lobe diameter to the observed power; n(2) 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 n(2) for a 20-microrad tilt, which is in good agreement with theory.

Spontaneously implemented spatial coherence in vertical-cavity surface-emitting laser dot array

“…Bending the aperture to form not an oblong but a ring and gaining a dimension-like half-sliced donut creates an annular cavity with a virtually circular-dot far-field pattern and a significantly narrow emission angle 21 . Such a cavity has been investigated with a meter-wide bulky setup and gives Bessel beams with a radial, or sometimes azimuthal, polarization.…”

High power tube solid-state laser with zigzag propagation of pump and laser beam

“…Let us first consider implementation of a Half-Symmetric Unstable Resonator with Internal Axicons (HSURIA) 16 with a laser module and cryogenic cooling system (Fig. 3).…”

High energy laser optics manufacturing: a preliminary study

“…3 -9 ' 10 Recently, Chodzko et al have ' shown experimentally that surface properties of D-T conical or waxicon reflectors can produce signifi cant polarization effects within an annular gain (HSURIA) resonator. 24 They measured single sur face reflectivities of azimuthal and radial polariza tions and found differences of about 3% for an inci dent wavelength of 10.6 ftm. Because the rays strike six diamond-turned surfaces per round trip in the resonator, the reflectivity difference (R s > Rp) provides a strong selector for the azimuthally polarized mode.…”