Technology of microsecond-pulse sodium laser guide stars asterism (Invited)

Sodium laser guide stars (LGS), known as artificial stars, can be used to detect and correct wavefront aberrations induced by atmospheric turbulence, which can significantly improve the adaptive-optical telescope's imaging quality. Due to the limited corrected field of view of single LGS, multiple sodium LGS, created by exciting sodium atoms in the Earth's mesosphere via multiple yellow laser beams, is developed to yield high-resolution imaging in a much larger field-of-view, which has important applications in the fields of precision astronomical observation and space target detection. The successful implementation of microsecond-pulse sodium guidestars constellation via 100 W level pulsed sodium laser was reported, based on a small angle precise polarized combining and splitting technology. At Lijiang Observatory, four-ways~20 W/beam yellow laser beam with kHz repetition-rate and hundred-μs pulse width were projected up to the sky through one launching telescope, and generated a distinctive four-point grouping on a 40" field of view with variable configurations of linear, parallelogram, rhomboid and square. The spot size of each guide star was about 3.25" and the corresponding brightness was around 8 magnitude in V band. The sodium return signal could well avoid Rayleigh light interference by the pulse synchro controlling technology to deliver higher spatial resolution. This could serve as a technical reference for multi-conjugate correction systems on large-aperture astronomical telescopes.