Bessel beam has the advantages of reducing scattering artefacts and increasing the quality of the image and penetration.
This paper proposed to generate a guide star by Bessel beam with vortex phase, and to use the beacon with special spot
structure to measure the atmosphere turbulence aberrations. With the matching algorithm of measured characteristic spot
in each subaperture, the detection accuracy of Hartmann wavefront sensor can be improved. Based on wave optics
theory, the modeling of Bessel beam guide star and wavefront sensing system was built. The laser guide star beacon
generated by Bessel beam with vortex phase and beacon echo wave measured by Hartmann sensor were both simulated.
Compared with the results measured by echo wave from Gauss beam generated guide star beacon, this novel method can
reduce the error of wavefront detection and increase the detection accuracy of Hartmann sensor.
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.
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.
Sodium laser guide star (LGS) is the key for the success of modern adaptive optics (AO) supported large ground based telescopes, however, for many field applications, Sodium LGS’s brightness is still a limited factor. Large amounts of theoretical efforts have been paid to optimize Sodium LGS exciting parameters, that is, to fully discover potential of harsh environment surrounding mesospheric extreme thin sodium atoms under resonant excitation, whether quantum or Monte Carlo based. But till to now, only limited proposals are demonstrated with on-sky test due to the high cost and engineering complexities. To bridge the gap between theoretical modeling and on-sky test, we built a magnetic field controllable sodium cell based lab-bench, which includes a small scale sum-frequency single mode 589nm laser, with added amplitude, polarization, and phase modulators. We could perform quantitative resonant fluorescence study under single, multi-frequency, side-band optical re-pumping exciting with different polarization, also we could perform optical field modulation to study Larmor precession which is considered as one of devils of Sodium LGS, and we have the ability to generate beams contain orbital angular moment. Our preliminary sodium cell based optical re-pumping experiments have shown excellent consistence with Bloch equation predicted results, other experimental results will also be presented in the report, and these results will give a direct support that sodium cell based lab-bench study could help a Sodium LGS scientists a lot before their on-sky test.