High-power lasers probe the night sky

30 Jul 2002

Astronomers at the world's largest telescope facility are preparing to use high-power lasers to help gather the most accurate images of distant galaxies ever seen. Jacqueline Hewett reports on Keck's plans for the deployment of its laser guide star.

From Opto & Laser Europe July/August 2002

Next month, researchers at the 10 m Keck telescope in Hawaii will test a fully integrated adaptive optics (AO) system that promises to generate infrared images with four times the resolution of those from the Hubble Space Telescope. A key part of the system will be a so-called laser guide star (LGS). The LGS will be used to generate a bright spot of light high in the night sky, providing a reference point for astronomers.

"An LGS is an artificial star," explained Domenico Bonaccini, who heads an LGS research group based at the European Southern Observatory (ESO). "It is generated by shining a high-power laser into the upper atmosphere to create a 50 cm spot of light. AO systems then use this point as a reference to correct for the effects of the atmosphere on optical wavefronts in real time." Although such systems are relatively simple in principle, they involve the use of specialized laser systems and deformable optics, both of which have to be integrated into the telescope.

Opening up the skyDeanna Pennington, LGS project leader at Lawrence Livermore National Laboratories and system engineer at the Keck telescope, says that the advent of the LGS has opened up whole new areas of the sky to astronomers. "To use adaptive optics you need a very bright star to be close to the object you want to study, to act as a reference. Relying on these 'natural' guide stars limits observations to less than 1% of the sky, but an LGS enables you to reach some 60 to 70% of the sky," she explained.

There are two methods of generating an LGS. One is to use the Rayleigh scatter of the atmosphere at a height of 10-20 km; the other is to excite sodium atoms in the upper atmosphere. Keck has chosen the latter approach, using a powerful 589.2 nm laser to irradiate the sodium atoms in the mesosphere at a height of 90 km above sea level.

A resonant fluorescent process means that the irradiated sodium atoms create a glowing spot in the upper atmosphere. Scattered light travels back down through the atmosphere to the ground, where it is analysed by a wavefront sensor. The sensor determines what optical distortions have been induced by the atmosphere. Although this artificial star is not visible to the naked eye, it provides enough light for the AO system's wavefront sensor to interpret.

A high-speed computer calculates the corrections needed to enable the AO system to respond to the constantly changing turbulence of the atmosphere. Light collected by the telescope is then passed to a deformable mirror, which changes shape to precisely counteract the atmospheric distortions. This process generates the best possible astronomical images.

Unfortunately, there is a shortage of commercial 589.2 nm lasers with sufficient optical power to make an LGS. Astronomers have been left with no choice but to design their own lasers to do the job. At present only the 3 m Shane telescope at the Lick Observatory in California has an operational LGS that is being used by scientists for observations.

According to Pennington, Lick's pulsed laser system served as a prototype for Keck's system. "There really weren't any technologies out there that could create the power level and wavelength we needed in a continuous wave (CW) mode. We have made use of a high-power dye-laser system that was used for isotope separation here at Livermore," she explained.

Both the Lick and Keck laser systems are based on a proprietary dye. The dye-laser oscillator is pumped by a frequency-doubled Nd:YAG laser. Five other Nd:YAG lasers pump a preamplifier and an amplifier to achieve the required power levels. "With a pulsed beam you need 15-20 W of power to get enough sodium backscatter," said Pennington. "We run the Lick system at 13 kHz and the Keck device at 26 kHz."

Whereas the US-based research team is using a pulsed approach, the ESO group headed by Bonaccini has been collaborating with the Max Planck Institute for Extraterrestrial Physics in Germany to develop a 10 W CW dye laser. "We plan to have our first LGS by the end of 2003," Bonaccini told OLE.

This will require a laser with good optical quality. Bonaccini says that the output from the CW laser is diffraction-limited and has an M² value of better than 1.3. "We make a 50 cm diameter spot in the sky, which we want to keep as small as possible," he said. "If I were to double the diameter of the guide star, the power of the laser would have to increase by four."

Bonaccini's team is also building a prototype fibre Raman laser that will operate at 589.2 nm. "We think from our theory that we can achieve the goal of 10 W CW next year," he added. "We have applied for a patent on the laser system. We have found a way that gives narrowband [about 1 GHz] output." Bonaccini points out that although you can buy fibre Raman laser systems with output powers of up to 100 W, they are all broadband with linewidths of several nanometres.

Bonaccini says that his team can apply the same principle to generate any wavelength from the visible to the infrared. "We are now looking for a company to work with us in Europe and help us engineer the device," he told OLE. "There is a large market, not just in astronomy, for such a tool. We want to go from a breadboard to a fully engineered system. These lasers are very compact and do not require aligning."

Multiple starsIt is the compact nature of the fibre Raman laser that the ESO researchers are looking to exploit. One of the biggest limitations of the LGS technique is that light coming down on the mirror is in a cone shape, rather than a column shape, making it difficult to correct for the entire diameter of the telescope.

However, using multiple lasers to create a pattern of LGS areas in the sky - so-called multi-conjugate AO - would create a tomographic image and lead to a better correction. This improvement will open up the field of view for astronomers. By using a number of guide stars, a larger area of the sky can be corrected at any one time.

All of the designs currently on the table for new cutting-edge telescopes, such as the ESO's Overwhelmingly Large Telescope, include this technology. Bonaccini says that the Raman fibre laser is more rugged and compact, and is easier to operate and handle than its dye-laser counterpart. "When we want to upgrade the laser guide star facility from one LGS to five (in 2005 or 2006) it becomes prohibitive to use 5 x 10 W output dye lasers, or a single 50 W dye laser, on board the telescope," he explained.

In the meantime, researchers are eager to begin using the Keck LGS, especially those who are studying extremely faint, distant galaxies. Because light from distant galaxies is shifted to longer wavelengths by the expansion of the universe, AO systems are now set to provide us with valuable clues about galaxy formation.

LGS systems are opening up the sky to observers and no doubt spectacular images at unrivalled resolution will follow. With AO techniques outperforming Hubble in the near infrared, this method gives observers a very powerful tool for reaching previously unseen areas of space.

For more information
European Southern Observatory www.eso.org
Lick Observatory www.ucolick.org
Keck Observatory www2.keck.hawaii.edu

Adaptive optics: large-scale solutions

Imaging systems based on adaptive optics (AO) can compensate for optical distortion introduced by the medium between an object and its image. When a light beam passes through a uniform medium, its wavefronts travel together and remain in phase. In a non-uniform medium, however, parts of the beam travel at a different speed. This induces distortions in the optical wavefronts and thus in the image.

All AO systems work by determining the shape of this distorted wavefront and then correcting it. An adaptive optical element, typically a deformable mirror, is used in AO systems to restore the wavefronts by applying an opposite cancelling distortion.

A telescope's AO system uses light from a laser guide star or a bright natural star as a reference. This light is analysed by a wavefront sensor and commands are sent to actuators that change the shape of a deformable mirror to provide the required compensations. For the system to work properly, it must have time to respond to wavefront changes while they are still small; to use an AO system in the Earth's atmosphere, it is necessary to update the mirror's shape several hundred times a second.