Optical EngineeringMode properties and characteristics of negative branch unstable ring resonators
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Laboratory-scale negative-branch unstable ring resonators were designed to simulate large Fresnel number, large-mode cross section positive-branch high-power ring lasers. These laboratory-scale lasers can be built having small-mode cross sections, long lengths, and large Fresnel numbers by incorporating imaging systems within the resonators. Large variations of Fresnel numbers can also be obtained by simple positioning of the output aperture. To verify that the diffractive modes of the high-power lasers are accu rately si mulated, ba re cavity mode discri mi nation properties of these ring resonator designs were calculated by various 3-D diffractive models. Modeling was completed for resonators with equivalent Fresnel numbers up to 1 1 and with equivalent Fresnel numbers of 10 and above using a fast Fourier transform (FFT) and a virtualsource code, respectively. Comparison of the FFT and the virtual source code results, at moderate Fresnel numbers, indicated good agreement between the different modeling techniques. Good mode discrimination was observed at half-integral equivalent Fresnel numbers, whereas poor mode discrimination was observed at integral equivalent Fresnel numbers. These results show that less computational intensive codes, such as the virtual source code, can quickly and cheaply model a resonator having a wide range of Fresnel numbers. These results also show that the equivalent Fresnel number of a near-imaging negative-branch ring resonator is a reasonably valid parameter for determining resonator diffractive phase and irradiance profiles even for resonators having large Fresnel numbers. The modeling results show that the modes of positive- and negative-branch resonators, having the same Fresnel number, behave the same to within a complex conjugate.