Progress on high power quasi-continuous-wave sodium beacon laser

Abstract

A 33W 589 nm sodium beacon laser with the linewidth of 0.4 GHz and the beam quality of M 2 = 1.2 is achieved by sumfrequency mixing 1064 nm and 1319 nm quasi-continuous-wave (QCW) diode-pumped Nd:YAG lasers with a LBO crystal. The repetition rate of the laser is 500 Hz with the pulse width of 120 μs. Compared with the continuous-wave (CW) sodium beacon laser, the QCW microsecond-pulse sodium beacon laser can provide a gatable pulse format to eliminate the interference of atmospheric Rayleigh scattering and reduce stretching phenomenon of the imaging spot for sodium guide star, so the better correction effect can be achieved by using the adaptive optics system with the microsecond-pulse laser. A microsecondpulse sodium beacon laser prototype is developed to generate a sodium laser guide star which is observed by the 1.8m telescope in Yunnan province. Keywords-sum-frequency generation; yellow laser; solid-stated laser; sodium beaconDiode-pumped solid-state yellow lasers have become interesting for many applications in medicine, biotechnology, display, communications, and remote sensing. Especially, high power and high beam quality and narrow linewidth yellow laser at the sodium D2 line of 589nm can be used to create an artificial guide star for adaptive optical telescope systems by exciting atomic sodium in the upper atmosphere (85 to 105km) [1]. It is very difficult to achieve high power solid-state yellow laser with directly frequency doubling in general because the fluorescence spectrum intensity of corresponding fundamental frequency for Nd or Yb laser is too weak. At present, there are two main methods to produce high average power and narrow linewidth solid-stated yellow laser. One is sum-frequency mixing the two lines of Nd:YAG lasers operating near 1064 and 1319 nm [2]. For example, output power of 50W is demonstrated with sum-frequency mixing of injection-locked Nd:YAG continuous-wave (CW) lasers [3]. The other is frequency doubling of 1178nm CW Raman fiber amplifiers and output power of 50W is achieved [4].In this letter, we have developed a high beam quality and narrow linewidth yellow beam at 589nm by extra-cavity sumfrequency mixing 1064 and 1319 nm radiations of quasicontinuous-wave (QCW) diode-pumped Nd:YAG lasers in a LBO crystal. The 1064 and 1319nm lasers are generated by means of a diode-pumped Nd:YAG master-oscillator poweramplifier (MOPA) system. Both 1064 and 1319 nm seed laser are ring master-oscillators with the gain media of Nd:YAG crystal. Both 1064 and 1319 nm seed lasers are amplified by two laser amplifiers. The beam width of the 1064 and 1319 nm lasers inside the nonlinear optical crystal of LBO is optimized. As a result, QCW average output power of 33 W with the linewidth of 0.4 GHz and the beam quality factor of M2 = 1.2 at 589nm is demonstrated. Compared with the continuouswave (CW) sodium beacon, the QCW microsecond-pulse sodium beacon can provide a gatable pulse format to eliminate the interference of atmospheric Rayleigh scattering and reduce stretching phenomenon of the imaging ...