Transverse-mode selection techniques for different resonators have in common that they generally restrict the area of the gain medium to discriminate against higher-order modes. For stable spherical resonators with g <1, the selection techniques (e. g. including a restrictive aperture in the cavity or matching the TEM₀₀ mode size to the pump size) rely on the fact that the higher-order modes cover a larger cross-section area than the fundamental mode. Restricting the active area to the size of the TEM₀₀ mode generally introduces only small diffraction losses for the fundamental mode but substantially limits achievable output powers.

In plane and near-plane resonators, the area of the gain medium filled by the lowest mode is not smaller than that of the higher-order modes at any Fresnel number (or a cross-section size). Consequently, the higher-order modes are non-competitive, and only a small number of low-order modes are involved in oscillation [1]. In most cases, however, it is a very high sensitivity to the cavity distortions, rather than a mode competition, which dominates in determining the output-beam angular divergence. Restricting the active area decreases the influence of cavity distortions but introduces high absolute diffraction losses for all transverse modes, including low-order ones. In order to achieve TEM₀₀ oscillation, one has to restrict the Fresnel number, typically, in the range 1 to 1.5. This makes plane (or near plane) resonators highly inefficient.

In this work, we demonstrate a new concept of transverse mode selection in plane or near-plane resonators, which makes them competitive with spherical resonators. The concept is based on using image rotation (IR), which essentially decreases the sensitivity to cavity distortions and increases the azimuthal homogeneity of both the gain and refractive index of the laser medium. Consequently, the characteristics of the resonator become closer to those of an ideal empty resonator in which, as we mentioned above, the higher-order modes are non-competitive. Furthermore, the difference of resonance frequencies of transverse modes becomes as an important factor for the transverse mode selection as their differential diffraction losses. All these features make it possible to achieve both single-frequency- and TEM₀₀ -mode operation simultaneously by using a single-frequency mode selection technique such as injection seeding of a single-frequency radiation into an actively Q-switched resonator, or passive Q-switching.

Injection seeding of the radiation with a spectral linewidth narrower than the transverse mode frequency spacing provides a condition for the transverse mode with a lowest diffraction loss (i. e. the TEM₀₀-mode) to reach threshold first, saturate the gain medium and prevent development of any other transverse modes. Whereas the seeding leads to decreasing of the pulse buildup time, passive Q-switching increases it, which results in "natural selection" of the longitudinal modes [2]. We show that the natural selection also contribute to transverse mode selection in the resonators with image rotation (IR).

A layout of a 20-Hz, Q-switched, flashlamp-pumped, Nd:YLF oscillator with a 5x100-mm rod is shown in Fig. 1. As a seeding source we used a 20-mW cw, single-frequency, diode-pumped 1053-nm Nd:YLF laser. We use the pulse-build-up-time-reduction (PBUTR) technique to frequency lock a longitudinal mode of the ring laser to a longitudinal mode of the seed laser.

Near-field beam profiles of the ring resonator with no IR (N=7), with IR and seeding (N=7), and with IR and seeding (N=3) are displayed in Fig. 2 a, b, and c, respectively. The oscilloscope traces of Q-switched pulses in seeded and unseeded operation are shown in Fig. 3.

By comparing the output pulse energies versus misalignment angles, we show that the resonator with IR is less sensitive to the misalignment than the resonator without IR. We also introduce a ratio between Fresnel number and a rotation angle, which defines the "efficiency of mixing" for different transverse modes. The sensitivity of the resonator with/ and without IR that is illustrated by the graphs in Fig. 3. Applying the semigeometrical approach developed in [3] we also consider a misalignment sensitivity of the RIR resonator.

Finally, using the single transverse-mode frequency selection technique developed here we were able to achieve TEM₀₀, single-longitudinal mode operation in the resonator with a Fresnel number N=3. The maximum output pulse energy was 75 mJ while operating the laser within the range of repetition rates 5 - 30 Hz. Similar results were obtained using a LiF:F₂^- passive Q-switch.

Fig. 2. Near-field profiles of ring resonator.

Fig. 3. Seeded and unseeded operation of the Nd:YLF laser.

Fig. 4. Output pulse enrgies as a function of misalignment angles.