Article

Annular converging wave resonator: new insights

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Abstract

A simple theory describing the annular converging wave resonator has been developed. The theory explains results obtained in two recent experiments. Configurations of the converging wave resonator that could be used to extract power from a large-volume annular gain region in a single mode with nonzero azimuthal order are also discussed.

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... This wave is getting focused with every round trip inside the cavity, but when the diffraction limit is reached, the formerly convergent wave becomes divergent again. The additional reflector is either covering the complete output area and has a partial reflectivity coating [8][9][10][11], as, e.g., shown in Fig. 1(a), or it has a high reflectivity coating and is covering only a part of the output area [9,12], as, e.g., shown in Fig. 1(b). The overall performances of these resonator setups were not too promising. ...
... A negative-branch setup is chosen here because the fabrication of the output coupler is easier than for the positive-branch setup. A negative-branch setup was also applied in earlier publications, where an external mirror was used in combination with an unstable resonator to extract the energy from an annular gain medium [12,14]. In this paper the URROC is evaluated numerically and tested experimentally, using a wavelength of 1.315 μm. ...
Article
Full-text available
The properties of a laser beam coupled out of a standard unstable laser resonator are heavily dependent on the chosen resonator magnification. A higher magnification results in a higher output coupling and a better beam quality. But in some configurations, an unstable resonator with a low output coupling in combination with a good beam quality is desirable. In order to reduce the output coupling for a particular resonator, magnification fractions of the outcoupled radiation are reflected back into the cavity. In the confocal case, the output mirror consists of a spherical inner section with a high reflectivity and a flat outer section with a partial reflectivity coating. With the application of the unstable resonator with reduced output coupling (URROC), magnification and output coupling can be adjusted independently from each other and it is possible to get a good beam quality and a high power extraction for lasers with a large low gain medium. The feasibility of this resonator design is examined numerically and experimentally with the help of a chemical oxygen iodine laser.
Conference Paper
The application of standard unstable resonators does not allow for an independent adjustment of the resonator magnification and the output coupling. Either you get a high magnification together with a high output coupling, or vice versa. Certain laser types, like e. g. thin-disc lasers or chemical oxygen iodine lasers, permit only quite low optimum output couplings. The corresponding low resonator magnification is equal to a poor beam quality. In order to apply unstable resonators with a high magnification also to low gain media an additional mirror surface retroreflects a part of the out coupled radiation back into the cavity. The output coupling is reduced efficiently, whereas the resonator magnification stays high. Accordingly low gain media can be operated with high power extraction in combination with a good beam quality. Numerical and experimental investigations are shown. The experiments are performed with a chemical oxygen iodine laser operating at a wavelength of 1.315 mu m and demonstrate the feasibility of this resonator design.
Chapter
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Chapter
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Article
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Article
The application of standard unstable resonators does not allow for an independent adjustment of the resonator magnification and the output coupling. Either you get a high magnification together with a high output coupling, or vice versa. Certain laser types, like e.g. thin-disc lasers or chemical oxygen iodine lasers, permit only quite low optimum output couplings. The corresponding low resonator magnification is equal to a poor beam quality. In order to apply unstable resonators with a high magnification also to low gain media an additional mirror surface retroreflects a part of the out coupled radiation back into the cavity. The output coupling is reduced efficiently, whereas the resonator magnification stays high. Accordingly low gain media can be operated with high power extraction in combination with a good beam quality. Numerical and experimental investigations are shown. The experiments are performed with a chemical oxygen iodine laser operating at a wavelength of 1.315 μm and demonstrate the feasibility of this resonator design.
Article
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Article
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Article
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Article
Stable-unstable resonators have proved suitable for the extraction of a high-quality beam from a gain area that consists of a rectangular slab. Such gain areas have two substantially different transverse dimensions, and the resonators are stable in the small dimension while unstable in the larger one. Using off-axis unstable resonators avoids a central beam obscuration and improves beam quality. The adaptation of stable-unstable resonators to annular gain areas is described in this paper. The resulting resonators are stable in the radial direction and unstable in the azimuthal direction. Different unstable resonators, wound to match the annular geometry, are presented. The resonator modes are calculated numerically using a 3D-diffraction code that considers gain and misalignment. Resonator design parameters are obtained from a geometrical description of the resonator. Experimental results from a diffusion-cooled CO<sub>2</sub> laser confirm theoretical predictions and show that the resonators are capable of extracting beams that are nearly diffraction-limited with high efficiency from an annular gain medium. Output powers of 2 kW have been obtained from a gain length of 1.8 m
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A technique is described for improving the propagation characteristics of high order laser beam modes. A thin phase plate inserted in the beam forces all portions of the wave front to oscillate in phase and yields a substantial improvement in the far field intensity distribution. Depending on the mode order, the peak intensity at a distant target may be readily increased by more than a factor of ten.
Article
A new type of resonator that generates an annular geometric mode by use of spherical mirrors has been developed. The four-element cavity consists of an external confocal unstable resonator with a double-sided 45 degrees coupling mirror and a flat feedback mirror. The flat feedback mirror is placed on the plane wave side of the confocal cavity to form an annular mode between the feedback mirror and the coupling mirror. A plane annular wave (matched to an annular gain medium) is fed back into the unstable resonator that alternately converges and diverges to produce a diverging annular output beam. Experiments were performed on a cw HF laser. Observations of mode patterns on the flat feedback mirror and the convex mirror and of far-field beam quality were made. Far-field measurements indicated near-diffraction-limited beam quality for a peak on-axis intensity mode on the convex mirror. A nearly uniform annular mode was observed on the flat feedback mirror.
Article
The properties of a new class of laser resonators are investigated theoretically and experimentally. In these lasers the radiation propagates longitudinally in an annular amplifying medium, and useful low loss modes can be obtained even when the axial region is obscured. Alignment characteristics and far field patterns are discussed, and experiments have been conducted using coaxial double-discharge CO(2) devices.
An experimental study of unstable resonators for annular mode volume lasers," presented at 1977 Conference on Laser Engineering and Applications, paper 10.3; experiment also reported in Tech. Rep., Perkin-ElmerToric resonator experimental study-final report-dye laser head
  • A W Kreuzer
  • Mccullough
Kreuzer, and A. W. McCullough, "An experimental study of unstable resonators for annular mode volume lasers," presented at 1977 Conference on Laser Engineering and Applications, paper 10.3; experiment also reported in Tech. Rep., Perkin-Elmer, "Toric resonator experimental study-final report-dye laser head," to be published as AFWL-TR-76-327 (Air Force Weapons Laboratory, Kirt-land AFB, N.M.).
Advanced unstable resonators," presented at the Sixth Winter Colloquium on Quantum Electronics
  • E A Sziklas
E. A. Sziklas, "Advanced unstable resonators," presented at the Sixth Winter Colloquium on Quantum Electronics, Steamboat Springs, Colo., Feb. 1976.