Currently, several major projects have set out to build ever-larger telescopes on the ground of our Earth. With these impressive devices, astronomers aim to further increase the image resolution, enabling detailed studies of the universe. The already operational Very Large Telescope (VLT) of the European Southern Observatory (ESO) in Chile, as well as the Keck Observatory in Hawaii, are just two examples that display the extensive efforts that are put into such projects. The billion-dollar activities that have been launched for the next generation of so-called extremely large telescopes with 30-m-class primary mirrors are going to take it to a new level, however.
In theory, the diffraction-limited resolution of an 8.2-m visible to near-infrared telescope such as the VLT is about 0.02 arcsec. However, local variations of the atmosphere’s refractive index lead to rapid wavefront distortions (“twinkling”) of the light propagating from the stars toward the telescope. This effect leads to image blurring comparable to the flickering above a hot tarmac road, limiting the achievable resolution (“seeing”) of an earthbound telescope. The main contributions stem from turbulent layers close to the ground as well as in the stratosphere below 50 km of altitude. Even at the best sites on Earth, such as the Chilean Atacama desert, this effect leads to a seeing-limited resolution of only about 0.4 arcsec.
This effect can be corrected using adaptive optics (AO) systems built into the telescope. Such a system employs a wavefront sensor to measure the atmosphere’s distortions to the light of a bright (natural) guide star and subsequently compensates the induced error in near real-time (kHz rates) with a deformable mirror.
Operating principle of an adaptive optics system
This removes most of the image-blurring effect of the atmosphere and significantly improves the telescope’s resolution. However, natural guide stars that are bright enough for an AO system cannot be found in every part of the sky, especially not with the small fields of view of modern telescopes. Therefore, AO systems need to create their own “artificial” stars to enable a reliable operation in every part of the sky.
Positioning a sodium laser guide star
In the 1980s, a method was proposed that makes use of the relatively high concentration of atomic sodium in an approximately 10 km thick layer located at the edge of the mesosphere 80–100 km above Earth's surface. Using a laser, an optical excitation of these atoms promises high photon return from a location far above the major portion of atmospheric fluctuations. Of course such an artificial “sodium laser guide star” can then be positioned in any section of the sky for optimal performance of the AO correction in the current field of view of the telescope.
The difficulty in creating such a guide star is to provide the required narrow-band high-power laser light at the exact sodium resonance frequency near 589 nm (D2a line) to generate enough photon return. Because there is no material available that has a direct laser transition at this wavelength, conversion schemes are necessary. In the late 1990s, dye lasers were used at some telescopes although they are inefficient and require flammable or even carcinogenic liquids, as well as frequent maintenance. The next generation of “guide star lasers” were solid-state lasers that used nonlinear crystals for frequency conversion. These lasers were bulky and very cumbersome to be operated in a big telescope, requiring clean room environments and daily attention. Apart from that, it turned out that not only the raw laser power was key for the high photon returns needed for fast AO control loops, but also the particular spectral laser format plays a major role.
Laser system with few infrastructural demands
In 2002, ESO started to collaborate with TOPTICA Photonics and MPB Communications to develop a dedicated off-the-shelf product that also supports easy integration and maintenance-free operation. The core components of such a system are a stable quantum-dot distributed feedback (DFB) diode as seed laser, a polarization-maintaining narrowband Raman fiber amplifier (patented ESO technology) and efficient frequency-doubling by resonant second-harmonic-generation. All optical components are integrated into a robust system with a maximum of user convenience and reliability for telescope operators. The “SodiumStar” laser system offers a high flexibility combined with minimal infrastructural demands, which is beneficial in particular for small, agile and remotely operated telescopes. A system software with health check capability. as well as an intuitive service GUI. enable intelligent control and status tracking.
Four SodiumStar guide star lasers were installed at the Unit Telescope 4 (UT4) of the Very Large Telescope (VLT) at ESO’s Paranal Observatory in early 2016. Credit ESO F. Kamphues
Using several units of such a laser system, the complete air volume above the telescope’s main mirror can be probed, executing so-called “atmospheric tomography”. In early 2016, four SodiumStar lasers were installed at one of the VLT telescopes in order to implement this advanced operation mode of AO (Fig. 2). With this setup, near-diffraction-limited images of the complete field of view of the telescope should be achievable.
This successful implementation is not only a major milestone for the astronomy community, it also acknowledges the joint effort of the involved industry partners. Consequently, the SodiumStar laser was recognized with the third prize of the 2016 Leibinger innovation award, which also acknowledges that the technologies developed have the capability to produce multi-Watt single-frequency and even tunable light in spectral regions not easily accessible otherwise, i.e., from 510 to 690 nm, and even 255 to 385 nm by adding another frequency conversion step.
Written by Tim Paasch-Colberg, Wilhelm Kaenders, Martin Enderlein, TOPTICA Photonics AG