Guide Star Set for VLT

A new solid-state laser aims to give giant Earth-bound telescopes better vision by reducing atmospheric distortion.

German manufacturer Toptica Photonics has developed a 22-W 589-nm source, which is tuned to the frequency needed to excite a layer of sodium atoms in the atmosphere to create an artificial guide star up to 2.5 times brighter than previously achieved.

The European Southern Observatory last week completed a three-month testing period using the new technology. Four of the lasers are set to be installed on the ESO’s Very Large Telescope in Chile by August, with the system planned to be in operation next year.

Guide star laser. Courtesy of Topica Photonics.

ESO said the new technology had “changed the landscape.”

“Five years ago, the options for obtaining high-power, reliable lasers in a compact format suitable for the requirements of the (VLT’s) Adaptive Optics Facility were very limited,” the organization said.

The new laser system, developed under a €5.2 million project awarded in 2010, is based around a narrowband, tunable infrared diode laser with 50-mW output power at 1178 nm. Together with a Raman fiber amplifier stage developed by project collaborator MPB Communications and a frequency-doubling stage, it produces a powerful 598-nm source.

That output frequency is actively locked to a high-resolution precision wavelength meter and fired into the sky directly above the telescope, exciting the naturally occurring layer of sodium atoms at an altitude of 90 km. The technology increases return flux dramatically, generating much brighter laser guide stars than was previously possible.

Eighty percent of the new laser’s power resonates with the D2a transition in sodium atoms, while two side bands spaced equally on either side of this main spectral line each contain 10 percent of the power. The higher-frequency side band resonates with the sodium D2b transition.

“This is a unique feature never routinely used so far in a major observing facility,” ESO said.

The laser head that will be directly installed onto the guide star launch telescope can be spatially separated by up to 27 meters from the more bulky pump laser.

The technology may have implications for ESO’s forthcoming Extremely Large Telescope, which requires multiple laser guide star units for its own adaptive optics system.

For more information, visit www.toptica.com.