A new generation of sodium guide star lasers will be deployed at the European Southern Observatory’s (ESO) Adaptive Optics Facility in Paranal Chile, home to the Very Large Telescope (VLT). The four lasers are due to be delivered by August 2014 and it is hoped that the first light on the sky will be seen in Paranal in early 2015.
The purpose of the laser system will be to resonantly excite the naturally occurring sodium atoms in the Earth’s mesosphere at an altitude of 90km, creating an ‘artificial star’ for the adaptive optics of the telescope. This 'star' allows the active mirror control system to correct for wavefront distortions caused by turbulence in the Earth’s atmosphere; therefore allowing much sharper images to be obtained.
The lasers were produced over the last three years by Toptica Photonics, which develop, produce and sell diode and ultrafast fibre lasers for scientific and industrial applications. Each laser system consists of a narrow band and tunable diode laser with 5mW output power at 1,178nm, a Raman fibre amplifier stage developed by MPB Communications of Canada, and a subsequent resonant frequency doubling stage. The output frequency is actively locked to a high resolution precision wavelength meter supplied by HighFinesse. The work was performed under a multi-year ESO development contract using the ESO fibre Raman amplifier (EFRA) license for the amplifier step.
The guide star laser overcomes the problems observed with previous laser approaches, which led to difficulties in operation due to the observatory environment (altitude of up to 4,200m, gravity-invariant operation, minimal heat dissipation and remote automated control). The laser head that will be directly installed onto the guide star launch telescope can be spatially separated by up to 27m from the more bulky pump-laser. It also features a patented integrated D2b sideband generation (repumper) scheme, increasing the return flux for the generation of much brighter laser guide stars than previously possible.
‘We are proud that we have already received follow-up orders from other telescope facilities,’ said Dr Wilhelm Kaenders, president of Toptica. ‘Although the technology path for the laser was clear to us very early on, it was the system engineering part that was most demanding.’