The Mount Stromlo LGS facility includes two laser systems: a fiber-based sum-frequency laser designed and built by EOS Space Systems in Australia, and a Semiconductor Guidestar Laser designed and built by Aret´e Associates in the USA under contract with the Australian National University. The Beam Transfer Optics (BTO) enable either simultaneous or separate propagation of the two lasers to create a single LGS on the sky. This paper provides an overview of the Mount Stromlo LGS facility design, integration and testing of the two sodium guidestar lasers in the laboratory and on the EOS 1.8m telescope.
Researchers at the Starfire Optical Range have been researching laser beacons for AO for nearly 30 years. We developed four different sodium-wavelength lasers, all of which were based on diode-pumped, sum-frequency Nd:YAG oscillators. In 2016, we combined light from two commercial, 22 W, sodium-wavelength lasers to form a single beacon. These lasers, which use resonant-frequency doubling of light from a Raman fiber-amplifier, were built by MPB Communications and Toptica Photonics, under a contract from the European Southern Observatory. In 2019, we started to develop and procure a 75 W sodium-wavelength laser to enable better correction of turbulence in poor seeing. In conjunction with ESO, we have also started work to increase the return flux from sodium beacons by shifting or chirping the laser wavelength to compensate for recoil of optically pumped sodium atoms. Initial tests have demonstrated that chirping can increase beacon brightness by 10–20 percent.
We present our laboratory tests of a commercial near-infrared camera, the C-RED One, which was built by First Light Imaging. The camera uses a high-speed, low-noise, HgCdTe avalanche photo-diode (APD) array from Leonardo. We used the photon transfer technique to measure the gain and read noise. In early 2019, we identified the source of a fluctuating sensor bias, which limited the noise performance of the camera. We also measured the effect of the cryo-cooler on vibration of the array, which limited its performance as a tilt sensor. We sent the camera back to the FLI, where they installed a redesigned power-supply board to address the issue with the fluctuating sensor bias. FLI also installed passive damping to reduce the vibration of the sensor array. Here we present the performance characteristics of the camera before and after the repair of the camera by the manufacturer.
Scientists at the Starfire Optical Range have been researching mesospheric sodium beacons for adaptive optics since 1992. Since then, we have developed four different sodium-wavelength lasers, all of which were based on diode-pumped, sum-frequency Nd:YAG oscillators. In 2016 we combined light from two commercial sodium-wavelength lasers, which use resonant-frequency doubling of light from a Raman fiber-amplifier, to form a single beacon. We used standard polarization techniques to combine the two lasers and produce a single beam with both left-hand and right-hand circularly polarized light. To avoid competitively pumping sodium atoms of the same velocity class, we tuned each of the lasers slightly off of the peak of the Doppler-broadened sodium line. However, the return flux from this beacon was lower than we expected, even when we accounted for the effect of detuning. In this paper, we propose an explanation for this phenomenon and show results from computer modeling and on-sky tests to support our claim. We believe the sodium atoms, which were optically pumped by the shorter-wavelength laser between the D2a F=2, m=+2 and F=3, m=+3 states, were down-pumped by the longer-wavelength laser to the D2b F=2, m=+2 and F=1, m=+1 states. Thus, these atoms were no longer available for optical pumping, which reduced the efficiency of the lasers in producing return flux.
Addendum: We hope to show we can mitigate this effect by increasing re-pumping of the sodium D2b line on the shorter-wavelength laser.
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