基于波长330nm激光激发多色激光导星回波光子数的数值计算与探讨

Numerical calculation and discussion on return photons of polychromatic laser guide stars by a laser beam with 330nm wavelength

采用无模激光器发射波长为330 nm的激光激发多色激光导星,需要考虑脉冲激光重频率、激光带宽、激光初始光斑直径以及大气透过率对回波光子数的影响.通过数值模拟,计算了高斯光束的脉冲激光和连续激光激发多色激光导星在实际大气中后向辐射330 nm和2207 nm波长的回波光子数.数值计算结果表明,在垂直发射和接收的情况下,当到达大气中间层的激光能量为1 W时,连续激光能够获得更多的回波光子数,并且回波光子数几乎无起伏.对于脉冲激光,提高脉冲激光重频率达到50 kHz以上时,多色激光导星330 nm的回波光子数随脉冲重频率的增加趋于有限值;当大气能见度小于5 km且大气相干长度为12.8 cm时,大约需要34 W以上的激光发射能量,才能获得满足使用自然星全倾斜探测的330 nm回波光子数.对于连续激光,相同情况下,大约需要20 W以上的激光发射能量.

The properties of return photons of polychromatic laser guide stars excited by a modeless laser with 330 nm wavelength are investigated in this paper by numerical simulation. The repetition rate, linewidth, initial diameter of laser spot and atmospheric transmittance have great influences on the return photons at 330 nm and 2207 nm from polychromatic laser guide stars. First, the laser linewidth is optimized by solving the rate equations of interaction between laser and sodium atoms. We find that the 0.6 GHz linewidth for the continuous wave laser and the 1.0 GHz linewidth for the pulse laser are beneficial to obtaining the higher excited probability of sodium atoms. Based on the fitted relation between the excitation probability of sodium atoms and laser intensity, considering the random distributions of laser intensity at the mesosphere due to the influence of atmospheric turbulence, the return photons from polychromatic laser guide stars are numerically calculated. The results show that the return photons at 330 nm excited by the continuouswave laser are more than those excited by the pulse laser. And the return photons excited by continuous-wave laser almost do not fluctuate when laser power arriving at sodium layer is 1 W. Furthermore, effects of the repetition rate of pulse laser and the laser initial diameter on the return photons at 330 nm are studied. The two results are obtained as follows. The first result is that the increment of return photons at 330 nm will converge to a constant value when the repetition rate of pulse laser is over 50 k Hz. The second result is that the initial diameter of continuous wave laser has no effect on the return photons but the effect of pulse laser is more obvious. Particularly, the atmospheric transmittance is an important factor of influence because it causes a severe loss of light power at 330 nm wavelength. Under the conditions of 5 km atmospheric visibility and 12.8 cm atmospheric turbulence coherence length, the launched power of pulse laser with 50 ns duration should be more than 34 W for obtaining enough return photons required for the effective detection of atmospheric turbulence tip-tilt with the natural stars. But for the continuous-wave laser, the launched power should be more than 20 W. In the case of 10 km atmospheric visibility, if the same return photons at 330 nm are required,the launched power of pulse laser will also be more than that of the continuous-wave laser under the same conditions.Therefore, the continuous-wave laser has more advantages than the pulse laser in exciting the polychromatic laser guide stars. We hope that the above results will be beneficial to the further experimental research.