Objective Solid-state lasers have attracted extensive attention because of their advantages of high efficiency and high beam quality. Solid-state tube lasers with zigzag beam paths (SSZTLs) combine the outstanding characteristics of rod and slab lasers, such as compact structure and light weight, and overcome the shortcoming of insufficient thermal load capacity, providing a potential approach to realize high-power lasers. These advantageous characteristics have resulted in some attractive applications in the fields of military defense and industrial manufacturing. However, the beam quality of SSZTLs is extremely sensitive to alignment and fabrication errors of the gain medium owing to their unique tubular structure. To improve the beam quality of SSZTLs, we propose a novel self-correction method to eliminate annular off-axis aberrations of the laser beam in SSZTLs.
Methods Taking neodymium-doped yttrium aluminum garnet (Nd∶YAG) two-stage tube laser amplifiers as an example, a novel beam-correction method based on a right-angled conical reflector was proposed for the self-correction of annular off-axis aberrations in high-power two-stage thin-walled tube laser amplifiers. A right-angled conical reflector makes the laser beam propagate through two conjugate beam paths successively, which compensates the off-axis aberrations. First, based on a numerical simulation, the validity of this method to correct fabrication and alignment errors of a single tube was verified. Then, the correction effect of this method on matching errors of two-stage tubes was further analyzed. Finally, the influence of the structural parameters of the right-angled conical reflector on the aberration correction effect was discussed and the wavefront aberration caused by alignment and fabrication errors of the right-angled conical reflector was quantitatively analyzed using the annular Zernike polynomial decomposition theory.
Results and Discussions The proposed self-correction method can effectively correct alignment and fabrication errors induced by a single tube medium. For tube media with concentricity, parallelism, and alignment errors, the output beam quality of SSZTLs with planar mirrors degrades rapidly and almost linearly with an increase in these errors. The output beam quality after correction is significantly improved, and the energy concentration of the focal spot becomes better (Fig. 3). In addition, for alignment error, although the focal spot distribution is well focused after correction, its off-axis tendency is not corrected. When the alignment error is large, the focal spot distribution shows an obvious off-axis tendency, but the ideal focal spot distribution can still be obtained. The self-correction method can only correct the off-axis aberration induced by two-stage tubes and contributes little to the correction of input beam inclination angles [Figs. 3(c6)--(c8) and Fig. 4]. To further illustrate beam quality degradation characteristics, laser beam aberrations were analyzed using the annular Zernike polynomial decomposition theory of wavefront aberration. As shown in Fig. 5, irrespective of the type of error considered, the main aberrations are tilt and coma and the peak valley (PV) of off-axis aberrations decreases by about two orders of magnitude compared with those in two-stage tubes without self-correction (Fig. 5). In addition, the proposed method can effectively correct the matching errors of two-stage tubes. When concentricity and parallelism matching errors exist in the two-stage tubes, the PV of the wavefront distortion decreases dramatically (Tables 1 and 2).
Since achieving perfect assembly of the right?angled conical reflector and two?stage tubes in actual corrections is difficult, analyzing the key parameters of the right?angled conical reflector and their effects is necessary. Taking the sensitive concentricity error as an example, the correction effects of a non?ideal right?angled conical reflector were analyzed. When the right?angled conical reflector has translation, rotation, and taper errors tolerance for the translation and rotation errors can reach tens of microns and milliradians, respectively, while that for taper error is only microradians. Therefore, the proposed method has high tolerance for alignment errors and is highly sensitive to fabrication errors (Fig. 8). To further illustrate the effects of alignment and fabrication errors of the right?angled conical reflector on wavefront phase, laser beam aberrations were analyzed using the annular Zernike polynomials decomposition theory. Results show that the main components of translation error are tilt and coma, rotation error mainly induces astigmatism, and taper error primarily results in defocus (Fig. 9).
Conclusions In this paper, we proposed a self-correction method for the annular aberrations of high-power two-stage tube laser amplifiers. The simulation results validate the self-correction method in correcting alignment and fabrication errors of a single tube as well as matching errors of two-stage tubes. The proposed method can significantly eliminate the distortion wavefront caused by concentricity and parallelism errors of a single tube and the matching errors of two-stage tubes. However, the proposed method fails to compensate for the tilt aberration caused by alignment error. In conclusion, the proposed method not only dramatically improves the beam quality of two-stage tube laser amplifiers but also expands the range of incident angles for the input beam. Moreover, the right-angled conical reflector used in this work has a large tolerance for alignment errors, which is beneficial for engineering applications. In practical applications, to ensure the availability of the self-correction method, controlling fabrication errors of the right-angled conical reflector is necessary.
