Atmospheric turbulence induces laser guide star (LGS) spot wandering in the sodium layer, which introduces trouble to adaptive optics systems. Experimental study of LGS spot wandering usually needs on-sky test. However, the on-sky test of LGS spot wandering is expensive and complicated. Since spatial light modulators (SLMs) are able to simulate atmospheric turbulence, we have designed and set up a SLMs based LGS simulator to study LGS spot wandering. This LGS simulator is prominent to build a bridge between theoretical study and on-sky test. Its performance is tested for vortex beam generated annular LGS which was proposed to reduce the LGS spot wandering in our former paper.
The excitation and emission properties of optical materials can be adjusted by nanostructures and to achieve high optical efficiency in the optically pump laser with short absorption length and high threshold pump power, we present and theoretically investigate a Yb-doped thin film on a 1D grating structure in this paper. High reflectivity at the pump and emission wavelength are realized simultaneously and in terms of the guided-mode resonance theory, the local field of high reflected light is enhanced which will increase the absorption of associated laser wavelength. we analyze parameters of the nanostructure in detail based on rigorous coupled-wave theory and an appropriate structure is decided. We set up a simple quasi-three-level model and demonstrate that this designed structure can effectively improve the optical efficiency of optically pump solid state laser.
We present an annular laser guide star (LGS) concept for large ground-based telescopes in this paper. The more stable annular LGS is generated by turbulence-resisted vortex beam. In the uplink, a vortex beam tends to wander more slightly than a Gaussian beam does in atmospheric turbulence. This may enable an annular LGS to wander more slightly than a traditional Gaussian beam generated LGS does, which would ease the burden of uplink tip-tilt mirror and benefit a dynamical closed-loop adaptive optics system. We conducted numerical simulation to validate the feasibility of this concept. And we have gotten 31% reduced variance of spot wandering of annular LGS. Besides, we set up a spatial light modulator based laser guide star simulator for beam propagation in turbulent atmosphere to experimentally test the annular LGS concept. Preliminary experimental results are given. To the best of our knowledge, it is the first time this concept is formulated.
The Gerschberg-Saxton (GS) algorithm is a classic algorithm for phase retrieval. It is usually based on FFT (fast Fourier
transform) and IFFT (inverse fast Fourier transform). We improve the GS algorithm based on the Wigner distribution.
Instead of FFT and IFFT, Wigner distribution is not only used in the propagation but also filtered to optimize the signal
during the iteration. The simulation results illustrate that the new method could effectively improve the efficiency and
accuracy of phase retrieval.