The thermal induced effect errors including the surface distortion of heated mirror and micro-thermal turbulence fluctuation at the optical surface dramatically degrade the image quality of the telescope. To address the problem, we have proposed an air-knife system consisting of an annular flushing subsystem and a central sucking subsystem and reported its simulation analysis. This paper presents the detailed experimental performance of the air-knife thermal control system. The scaling experiment is conducted in a thermo-cycling experiment room with different environmental conditions, where the temperature fluctuations and wavefront perturbation of the scaling mirror can be accurately measured. It is shown from the experimental results that the approximately laminar forced air flow at the optical surface does blow away the turbulence fluctuation and not induce novel low order wavefront aberrations. Meanwhile, the air knife system contributes to the stability of the thermal boundary layer and enhances the convective heat exchange between mirror and air around. As a result, the air-knife system significantly decreases the surface-to-air temperature difference and improves the image quality with a thermal response. Furthermore, it is found that thermal control efficiency is less significant with the increase of the air intake flow or the decrease of the surface-to-air temperature difference. The scaling experiment results demonstrate the practicability of the air-knife thermal control system for large-aperture primary mirror.
The resolution of an optical imaging system is often limited by various phase aberrations. To get the joint estimation of an object and aberrations of an imaging system with perturbations, a defocused spatial diversity technology with aperture scanning was proposed and evaluated. This technology creates spatial diversity images by scanning an aperture in the defocused plane of the aberrated imaging system. Based on these diversity images, the stochastic parallel gradient descent algorithm was used to recover the phase aberration of the imaging system by adaptively optimizing the coefficients of Zernike polynomials. Then the near-diffraction-limited image of the object can be restored using a multiframe Winner–Helstrom filter. Numerical simulations performed with different phase distributions validated the technology. The technology proposed may be widely used in aberrated imaging systems for aberration detection and image restoration.
Limited by the size and weight of prism and optical assembling, Rotational Risley-prism-array system is a simple but effective way to realize high power and superior beam quality of deflecting laser output. In this paper, the propagation of the rotational Risley-prism-array-based Gaussian beam array in atmospheric turbulence is studied in detail. An analytical expression for the average intensity distribution at the receiving plane is derived based on nonparaxial ray tracing method and extended Huygens-Fresnel principle. Power in the diffraction-limited bucket is chosen to evaluate beam quality. The effect of deviation angle, propagation distance and intensity of turbulence on beam quality is studied in detail by quantitative simulation. It reveals that with the propagation distance increasing, the intensity distribution gradually evolves from multiple-petal-like shape into the pattern that contains one main-lobe in the center with multiple side-lobes in weak turbulence. The beam quality of rotational Risley-prism-array-based Gaussian beam array with lower deviation angle is better than its counterpart with higher deviation angle when propagating in weak and medium turbulent (i.e. Cn2 < 10-13m-2/3), the beam quality of higher deviation angle arrays degrades faster as the intensity of turbulence gets stronger. In the case of propagating in strong turbulence, the long propagation distance (i.e. z > 10km ) and deviation angle have no influence on beam quality.
Rotational Risley-prism-array system is an effective way to realize high power and high beam quality of deflecting laser output. In order to reveal the quality performance of deflecting beam, the beam compression in the direction of deflection and far field energy centrality of a hexagonal-distributed 7-Gaussian beam array based on rotational Risley-prism-array were studied in detail in this paper. The analytic formulae of the pointing position for the outgoing beam based on the prisms’ rotational angles are calculated by using nonparaxial ray tracing method. Then, the analytical expression for intensity propagation was derived based on the extended Huygens-Fresnel principle. From the irradiance distribution and PIB curve in the focal plane, the quantitatively simulation shows that the beam compression will be more significant as the deflecting angle of emergent increases. The energy centrality will decrease as the propagation distance increases, the fill factor decreases and the deviation angle increases. The mathematical model and calculation results can offer a reference for optical engineering application.
The sparse-optical-synthetic-aperture systems enlarge the aperture and increase the spatial resolution of telescope system via several sub-apertures distributed in specific way. The difficulty of its realization lies in detecting and correcting co-phase errors of the sub-apertures. This paper proposed the method of multi-spectral modulation detection of co-phasing errors for sparse-optical-synthetic-aperture systems. The method can detect the errors via phase modulation on a sub-aperture in the situation of different wavelengths. Firstly, this paper introduced the theory and implementation process of the method in detail. Then the paper analyzed the detection performance of the method and the influence of the sub-apertures structure on detection performance based on a three-sub-aperture system. These results show that the method can accurately detect the sub-apertures' co-phasing errors of the sparse-optical-synthetic-aperture systems. Compared with the current methods, the method proposed in this paper has many advantages, such as faster detection speed and wider detection range.