Abstract
The brightness of the artificial beacon is one critical performance parameter for adaptive optics. Here, a 40-watt level narrow-linewidth microsecond pulsed yellow laser is produced at 589 nm with a high repetition frequency of 600 Hz and a pulse duration of 120 μs. An experiment to project the pulse beam up to the sky and measure the fluorescence photon returns of the Na atoms has been held on the 1.8-meter telescope in Lijiang observatory. During the sky test, a laser guide star (LGS) spot is firstly observed with Rayleigh scattering elimination by means of a gateable pulse format. And, the central wavelength of the laser could be accurately locked to be 589.1584 nm with a linewidth of ~0.34 GHz to match that of sodium-D2a line. Optical pumping with circularly polarized light has also been used to increase the brightness of sodium LGS. In order to maximize the return flux, sodium D2b repumping option is done by an electro-optic modulator with the optimum D2a-D2b frequency offset. As a result, a bright sodium LGS with the return flux of 1610 photons/cm2/s is achieved, corresponding to ~47 photons/cm2/s/W of emitted laser power, which represents a significant improvement in terms of brightness reported ever.
Highlights
The ground-based large-aperture telescopes, as a powerful tool for comprehensive understanding of the Universe, advanced rapidly with higher sensitivity and higher spatial resolution[1] and achieved significant observations[2]
These distortions can be practically eliminated in real time via a technology known as adaptive optics (AO), where AO uses the laser guide stars (LGS) as reference sources to probe atmospheric turbulence and provide feedback to deformable mirrors in order to compensate image blur effects induced by this turbulence[3,4,5]
The effectiveness of a sodium LGS-AO system depends on producing sufficiently bright guide star that the photon return from the laser beacons does not limit the resolution of the wavefront sensor, and this motivates the development of high-power sources tuned to sodium strong D2a absorption line
Summary
Sodium LGS takes advantage of the 32S1/2–32P3/2 electric dipole transition, known as the D2 line. The seed lasers employ three-mirror unidirectional ring cavities with the only difference being optical coatings It contains two reflectors, a polarizer, a nonlinear crystal, an etalon, two Nd:YAG. The nonlinear crystal is employed as frequency doubler to suppress relaxation oscillations[37], eliminating the saturation effect of sodium atoms absorption to improve the return flux efficiency. The 1064 nm oscillator is remotely angle-tuned with a PZT-controlled etalon to allow the SFG laser frequency to be set at the sodium D2a resonance at any time, using feedback from the wavelength control system. By controlling the rotation angle of the half-wave plate, one could change the emitted laser power afterward with still maintaining the vertical polarization direction as required by an electro-optic modulator (EOM). The distance between the optical axes of the launch telescope and the receive telescope is nearly 1.46 m, and this distance is enough to separate the Rayleigh scattering and the sodium LGS
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