摘要:
信标技术历经了自然信标、瑞利信标、钠信标以及信标阵列的发展过程仍无法实现真正意义上的全天空覆盖,而多色信标是大口径地基自适应望远镜实现100％天空覆盖率的有效手段之一.但是现有的多色信标的仿真模型都存在着一定的不足,特别是对一级激发中间层钠原子形成多色信标的仿真模型无法准确计算出多色信标的回波光子数.本文提出了基于布洛赫方程的仿真模型,对一级激发中间层钠原子形成多色信标回波光子数进行数值仿真,充分考虑多普勒展宽、进出作用场、碰撞以及反冲作用的影响,并采用回光效率描述回波光子数的多少.仿真结果表明,存在最优的激光器线宽使得回光效率达到最大,若采用功率密度为10 W/m2的激光器激发中间钠层,最优线宽为18 MHz,最大330 nm回光效率为0.907 Photons/[s·sr·atom·(W/m2)].
Abstract:
Sodium laser guide star (LGS) becomes an essential part in modern astronomical adaptive optics system, especially for the next generation extremely large ground based telescope. The LGS technology has experienced the developmental stages as natural guide star, Rayleigh LGS, sodium LGS and constellation of LGS. The sky coverage is still limited in that the LGS cannot not be used to detect the tip/tilt aberrations. While the polychromatic laser guide star (PLGS) is one of the most effective ways to enlarge the sky coverage to 100％. Previous simulation models are insufficient for the accurate calculation of the return flux, especially for the simulation model of PLGS which is generated by one-photon excitation of mesospheric sodium atoms. The simulation model based on Bloch equations proposed in this paper can be used to compute the return flux of one-photon excited PLGS precisely. Doppler broadening, beam atom exchanging, collisions and recoil are taken into account in the model. The return flux is validated by the return efficiency. The simulation results indicate that with one-photon excitation of sodium atoms, a return efficiency of 330 nm is minimum compared with those of other wavelengths;the saturation power density will decrease with recoil increasing and increase with collision rate increasing; an optimal line-width exists up to maximum the photon return efficiency. In the best case, when the power density is 10 W/m2 at the sodium layer, the maximum return efficiency at 330 nm is 0.907 photons/s/sr/atom/(W/m2) with an optimal laser line-width of 18 MHz.
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