Shack-Hartmann wavefront sensors calculate the position of focal spot in each sub-aperture from intensity distributions, the noises of the detector itself would have a certain impact on the detecting accuracy and would lead to inaccurate wavefront detections using conventional centroiding method. It has been demonstrated that the correlation algorithms with template matching is able to improve the accuracy. In this paper, several correlation algorithms such as absolute difference function, absolute difference function-squared, square difference function, cross-correlation function and normalized cross-correlation are compared at different signal-to-noise ratios. To further improve the accuracy, interpolation algorithms including equiangular line fitting, parabola interpolation, gauss interpolation and least square method are brought in, which turns out that least square method could minimize the detecting error. Besides, simulations within single aperture and full aperture both illustrate that cross-correlation function is most robust but needs more calculations, so is least square method. Moreover, although absolute difference function would be inaccurate at low signal-to-noise ratios, it still can obtain high detecting accuracy at high signal-to-noise ratios and it minimizes the calculations.
Fresnel zone lens (FZL) telescope is attracting increasing attention owing to its small volume and light weight. However, depending on the fabrication method of FZL, the linewidth and etch depth of FZL may deviate from the set value. It must be considered that the performance of the FZL is influenced by aberrations generated during manufacture. We simulate the effect of fabrication errors on optical performance of FZL and find that the linewidth error of FZL structure is the main cause of image degradation. In addition, we provide a method for correcting aberrations of FZL telescope with adaptive optics system (AOS). This method is verified by the experimental system. The results show that the image resolution is successfully improved after AO correction. The full-width, half-maximum value of a far-field image is improved from 0.065 to 0.038λ. The peak value of image energy after correction has increased by 4.23 times.
Since the phase diversity (PD) wavefront sensor has the advantages of simple structure and high light energy utilization, it is one of most attractive wavefront detection tools. The accuracy of retrieval wavefront depends on the precision of the detective intensity distribution of the CCD camera. However, limited by manufacture craft, the noise data are inevitably recorded, so CCD has only good performance in fixed dynamic ranges. In this paper, we propose a simple modified phase diversity wavefront sensor based on the altered exposure time of camera to improve the dynamic ranges of CCD. The two images are taken under normal exposure time and saturated exposure time of CCD, and then they are stitched to form a perfect image including accurate high space frequency and low space frequency information. Under same signal to noise ratio a comparison between the improved phase diversity wavefront sensor and the traditional phase diversity wavefront sensor is made by using simulation. The results show that this method can significantly enhance the retrieval wavefront accuracy.
The slab laser is a promising architecture to achieve high beam quality and high power. By propagating the laser beams in zigzag geometries, the temperature gradient in the gain medium can be well averaged, and the beam quality in this direction can be excellent. However, the temperature gradient in the non-zigzag direction is not compensated, resulting in aberrations in this direction which lead to poorer beam quality. Among the overall aberrations, the main contributors are two low-order aberrations: astigmatism and defocus. These aberrations will magnify beam divergence angle and degrade beam quality. If the beam divergence angles in both directions are almost zero, the astigmatism and defocus are well corrected. Besides, the output beams of slab lasers are generally in a rectangular aperture with high aspect ratio (normally 1:10), which need to be reshaped into square in many applications. In this paper, a new method is proposed to correct low-order aberrations and reshape the beams of slab lasers. Three lenses are adapted, one is a spherical lens and the others are cylindrical lenses. These lenses work as a beam shaping system, which converts the beam from rectangular into square and the low-order aberrations are compensated simultaneously. Two wavefront sensors are used to detect input and output beam parameters. The initial size of the beam is 4mm×20mm, and peak to valley (PV) value of the wavefront is several tens of microns. Simulation results show that after correction, the dimension becomes 40mm×40mm, and peak to valley (PV) value of the wavefront is less than 1microns.
Slab geometry is a promising architecture for power scaling of solid-state lasers. By propagating the laser beams along zigzag path in the gain medium, the thermal effects can be well compensated. However, in the non-zigzag direction, the thermal effects are not compensated. Among the overall aberrations in the slab lasers, the major contributors are two low-order aberrations: astigmatism and defocus, which can range up to over 100 microns (peak to valley), leading to detracted beam quality. Another problem with slab lasers is that the output beams are generally in a rectangular aperture with high aspect ratio (normally 1:10), where square beams are favorable for many applications. In order to solve these problems, we propose an automatic low-order aberration compensation system. This system is composed of three lenses fixed on a motorized rail, one is a spherical lens and the others are cylindrical lenses. Astigmatism and defocus can be compensated by merely adjusting the distances between the lenses. Two wave-front sensors are employed in this compensation system, one is used for detecting the initial parameters of the beams, and the other one is used for detecting the remaining aberrations after correction. The adjustments of the three lenses are directly calculated based on beam parameters using ray tracing method. The initial size of the beam is 3.2mm by 26mm, and peak to valley(PV) value of the wave-front is 33.07λ(λ=1064nm). After correction, the dimension becomes 40mm by 40mm, and peak to valley (PV) value of the wave-front is less than 2 microns.