Off-axis unstable laser resonator: operation

Diffraction patterns formed by an off-axis paraboloid surface

We derive a general, closed-form expression for the diffraction patterns including the aberrations that are due to off-axis alignment and positioning, of a paraboloid mirror. The diffraction patterns obtained in the focal plane of an off-axis paraboloidal mirror suffer modifications by the aberrations that are inherent in these surfaces: astigmatism and coma. Different magnifications in two perpendicular spatial directions indicate astigmatism. The Airy function, which affects a single spatial coordinate, describes the coma aberration. We identified the coma by the increased number of intensity zeros within adjacent lobules.

Experimental evaluation of an unstable ring resonator with 90 degrees beam rotation: HiQ experimental results

The first experimental results on a confocal unstable resonator with 90 degrees beam rotation (UR90 or HiQ) are presented. A beam quality of 1.06 was observed, and it varied by a maximum of 0.08 over a resonator misalignment range of 1 mrad. The outcoupled beam's polarization was circular. Reverse mode suppression was achieved on UR90 with a suppressed vs unsuppressed ratio of 115:1. Measurable reverse mode suppression was obtained with as little as 2% of the full reverse mode strength. Most of these results are compared with those for a conventional ring resonator.

Unstable ring resonator with a compact output beam: description and experimental evaluation

A novel type of unstable resonator is described, and its operation with a CO(2) laser is discussed. The laser mode is rectangular in cross section, and the field is inverted about a line that makes a 45 degrees angle with the edge of the beam during a round-trip pass. Beam inversion, low magnification, and reasonably low sensitivity to misalignment allow efficient power extraction from low-gain lasing media while also outcoupling a solid rectangular beam. A beam quality of 1.1 was observed. The beam quality varied from 1.1 to 1.6 during misalignment of the rear rooftop mirror by ~0.06 mrad. A further misalignment of ~0.03 mrad stopped the lasing.

Unstable-stable resonators with toroidal mirrors

A resonator with toroidal mirrors is described. This resonator behaves like an off-axis unstable confocal resonator in one transverse dimension and like an on-axis concave-convex stable resonator in the other orthogonal dimension. Some experimental results are reported for a fast flow high power cw-CO(2) laser whose transverse cross section is restricted in the stable-resonator direction. These cavities allow an output laser beam with a fully illuminated cross section which is well suited for focusing. Moreover, the fraction of the available laser power which may be concentrated in the central lobe of the focal plane intensity distribution is 2.5-4.5 times higher than other unstable resonators with similarly restricted modal volumes. Finally the alignment requirements are examined.

Modes of empty off-axis unstable resonators with rectangular mirrors

The effect of Fresnel edge diffraction on mode loss and far-field beam quality is investigated for empty offaxis unstable resonators with rectangular mirrors. For the cases investigated, mode-loss separation and beam quality of the lowest-loss mode are maximized nonconcurrently when the optical axis intersects the small convex mirror near one of its corners rather than its center. Although these properties indicate that the near-corner off-axis resonator can achieve superior beam quality to that achieved by an on-axis resonator, it is also shown that a large instability in beam quality can occur near the operating point which gives maximum mode-loss separation and superior beam quality.

Extra-cavity feedback into unstable resonators

Unstable resonators constructed of totally reflecting optics are particularly sensitive to extra-cavity feedback. This is demonstrated experimentally by reflecting the attenuated output of an injection mode-locked TEA CO(2) laser, fitted with a confocal unstable resonator, back into the laser resonator. Even after attenuation by ~10(6), significant perturbation ( greater, similar10%) could be observed in the temporal characteristics of the output train. A theory of extra-cavity feedback in the geometric limit is presented.