Thought Leaders

Developing a New Guide Star Laser System for the Very Large Telescope

The Very Large Telescope (VLT) is the most productive ground-based telescope on the planet, outperformed only by the Hubble Telescope that orbits the earth. In order to make high resolution observations of the universe, ground based telescopes are dependent on guide star lasers (GSLs) which, until now, have been complicated to use and require constant maintenance.

AZoOptics spoke to Dr. Wilhelm Kaenders, Co-Founder and President of TOPTICA, and Dr. Martin Enderlein, systems engineer for TOPTICA's guide star laser, about the science behind their new SodiumStar system and the frontiers of astronomy that it will open up.

Why is the use of guide stars important for astrophysics?

WK: Most astrophysical observations rely on ground-based telescopes, which have the big disadvantage that light from astronomical objects has to go through the Earth’s atmosphere to reach them. Thus, the resolution of large optical telescopes on the ground is fundamentally limited.

Winds, varying local temperature and pressure profiles lead to blurred images, similar to what you can observe when looking along a hot tarmac road in summer.

Bright natural stars can be used as reference objects to measure and counteract these image distortions with so-called adaptive optics control systems. These bright stars are known as guide stars. By using this method, ground-based telescopes can reach even better resolutions than the Hubble space telescope, and they are ultimately limited only by the size of the primary mirror.

In the late 1980s people began discussing how to create artificial guide stars anywhere in the sky by exciting sodium atoms located in the atmosphere with lasers. This method takes advantage of a natural layer of atomic sodium between 90 and 100 km above ground, which is replenished by meteors.

If you use the right laser sources this sodium can be made fluoresce very brightly. However, the required laser technology was not ready at the time. We are now able to build the first efficient and reliable lasers that allow us to install such an artificial guide star on any telescope.

Guide Star Lasers provide a bright point in the sky that astronomers can use to correct the blurring of astronomical signals as they pass through the earth's atmosphere. G. Hüdepohl (atacamaphoto.com)/ESO

What are the advantages of using guide star lasers over natural guide stars?

ME: Firstly, the sky coverage is very limited when using only natural guide stars. A guide star has to have a certain brightness and there is not always a bright star close to a region that astronomers would like to observe.

On top of this, more advanced methods have been developed that use multiple guide stars in certain patterns. These asterisms can only be created artificially and allow even better image corrections for large fields of view. The upcoming generation of 30-m to 40-m extremely large telescopes, or ELTs, for instance, will be critically dependant on laser guide stars using six to eight separate lasers for creating such guide star constellations.

What astronomical systems and phenomena can guide star lasers help in observing?

WK: Only by using this technology can large state-of-the-art telescopes tap their full spatial resolution potential.

Hot topics in astronomy today include exoplanet discovery and characterization to answer a fundamental question of human mankind, whether or not we are alone in the universe.

Another quest is the resolution of very crowded star-forming regions or areas obscured by dust. One particular region of interest is the surroundings of the black hole at the center of our galaxy. Right now, there happen to be visible objects in close proximity to this black hole. A high resolution observation of these objects will allow Einstein’s theory of general relativity to be tested under extreme conditions.

For more details on this you can ask our astronomical collaborators from the European Southern Observatory (ESO).

GSL's will allow astronomers to observe the supermassive black hole at the center of our galaxy (artists impression). NASA, ESA, D. Coe, G. Bacon (STScI)

Why was there a need to develop a more advanced GSL system, such as TOPTICA’s SodiumStar system?

ME: There had been two generations of sodium lasers before the SodiumStar.

The first generation lasers used messy liquid dyes to convert the laser frequency to the sodium resonance in the yellow spectral region at 589 nm.

The second generation consisted of pairs of solid-state lasers which were combined in a process called sum-frequency generation. To reach the necessary power levels using these inefficient type of frequency conversion required high-power pump lasers with electrical power consumption of several tens of Kilowatts. Apart from that, both were technically extremely complex systems, which were hard to maintain on a daily basis even if stored in special cleanroom environments.

Also, the laser format has never been optimal for an efficient excitation of the atmospheric sodium. The key technology in the SodiumStar is a “re-pumper” which uses a second frequency component of the laser. This allows recovering sodium atoms that have decayed into ‘dark states’ - where they stop interacting with the main laser frequency. Using this re-pumper the guide star brightness can be increased by a factor of up to 4.

How are the SodiumStar lasers more reliable than the lasers used previously at the Very Large Telescope?

ME: The old VLT laser was a first-generation dye laser, which was relatively low-power and had to be maintained on a daily basis.

The SodiumStar combines the technologies of diode lasers, fiber lasers, and resonant second-harmonic generation. All of these are inherently more robust and power-efficient. They can be integrated directly into the telescope structures and move with it during observation, instead of being in a separate clean room facility.

Apart from that, ESO demanded a heavily formalized project management, which took care of a solid systems engineering and risk assessment. The result is the first fully engineered, turnkey sodium guide star laser system available as a commercial off-the-shelf product.

TOPTICA's SodiumStar GSL has the ability to be up to four times brighter than previous GSL systems. ESO/J. Girard (djulik.com)

What components are required to create the SodiumStar laser?

ME: The primary laser source is a robust and long-lived quantum-dot diode laser, which is actively frequency-stabilized by a high-resolution solid-state wavelength meter. In addition, its diode current can be directly modulated to create the re-pumper frequency component.

All of these are key TOPTICA technologies. What follows is a Raman Fiber Amplifier (RFA), which is supplied by our project partner MPB Communications, who have been enabled to manufacture this ESO-patented technology.

The final step is a resonant second-harmonic generation, which again is a key TOPTICA technology and is in use in most atomic physics and quantum optics labs around the world. Also, systems engineering and software integration are key strengths of TOPTICA, which have been brought to the next level during this project.

How do you expect the SodiumStar system to impact the field of astrophysics?

WK: Our current customers seem to be extremely satisfied and this news has rapidly got around in the astronomy community. As a consequence, the TOPTICA/MPB laser is the baseline technology for all three ELT projects. But as there is a lot of focus on these new telescopes, existing ones are struggling for funding.

Still, it seems all of the major 8- to 10-m telescopes will eventually be equipped with our lasers. It is also the only commercial GSL system on the market. So I think it would be no exaggeration to say it is going to be one of the essential enabling technologies for next-generation, ground-based optical astronomy.

Advances in GSL technology are going to enable astronomers to find out more about our universe. Image of the Helix Nebula. ESO

Do you expect the SodiumStar system to be used for other applications?

ME: There are two other applications where adaptive optics and laser guide stars might become important in the future.

One is the optical detection of space debris. There are millions of large and small pieces from old satellites and rockets orbiting Earth with extreme speeds. This is an increasing threat to working satellites and space travellers. The movie “Gravity”has brought this issue to public attention. For an optical detection of these fast moving objects, artificial guide stars are essential.

Other applications that are being discussed in the community are free-space optical communication with satellites, new laser-driven propulsion technologies for extragalactic spaceflight, or re-orbiting of small pieces of space debris. Here adaptive optics will be necessary to focus ground-based high-power lasers on their targets using a process called uplink correction.

Why do you think the team at the Very Large Telescope chose to work with TOPTICA to improve their GSL system?

WK: TOPTICA is well known in the physics community for its high-quality diode and fiber laser engineering.

We are also a financially sound company that has been steadily growing since our foundation in 1998. Together with our expertise in the key technologies for this laser system, this had convinced ESO at the time that we are a reliable partner for the development but also the many years to come for the operation of their new facilities.

The GLS system at the Very Large Telescope consists of 4 different lasers, allowing complex observations to be made. ESO/G. Hüdepohl

TOPTICA have just been awarded the third place Berthold Leibinger Innovationspreis for your work in Laser Physics. How tough was the competition?

WK: We were already very honoured to be selected as finalist among a large group of contenders.

At the final selection meeting it became clear what a tough competition we had gone into. Of course there is always a struggle between start-up technologies and established innovation, but we feel very honoured with the process, the extraordinary quality of the jurors, and the consideration that was given to the individual price winners.

Irrespective of the exact position, every one of the finalists deserves, and has gotten high respect from the Leibinger Innovation team.

Why do you think TOPTICA were awarded?

WK: TOPTICA has engaged with the ESO scientists years before the actual guide star tender process for these device started. As a scientifically-rooted company, although ESO came to us for specific solutions that we had as products, we could not resist to ask questions beyond the problem at hand and started to add own suggestions.

This kind of cooperative atmosphere was instrumental for the work to get started but also during the execution of it, including all three partners.

In my opinion the innovation of technology always takes a strong team effort. I personally can only try to represent the people that have contributed and that would deserve more limelight for this.

Where can our readers find out more about TOPTICA Photonics, the Very Large Telescope and the project you have completed together?

ME: A lot of information can be found on the TOPTICA and ESO webpages and dedicated press releases.

There is also a recent article in Laser Focus World from our group. Many more technical details can be found in our regular contributions to SPIE conferences on astronomical instrumentation over the last years. Last not least, talk to us, we are happy to share our insights.

Find out more about the science behind Guide Star Lasers

About Dr. Wilhelm Kaenders

After being infected by cold-atom physics as a PhD student at the Institute of Quantum Optics in Hannover, and being part of the technology of Prof. Hänsch's group at the Max Planck Institute in Garching, in 1998 Dr. Wilhelm Kaenders started a successful business activity with tunable diode laser technology.

Since then the company has grown to 220 employees and expanded its product range to frequency-converted lasers providing high power at exotic wavelengths, as well as pico- and femtosecond fiber lasers, frequency combs and terahertz systems.

Apart from his business activities, he has been involved in technical management of small and large R&D and product management teams and has set up and run multiple multi-layer European and German funded R&D projects.

He has published more than 20 articles in peer-reviewed journals, owns 2 patents and has served in multiple roles for OSA, SPIE, and DPG. He is a Fellow of OSA.

About Dr. Martin Enderlein

Since his PhD in experimental quantum optics from Max Planck Institute of Quantum Optics and the University of Freiburg in 2013, Dr. Martin Enderlein has worked for TOPTICA Photonics AG as lead engineer in the guide star laser team.

Apart from that, he has worked on several R&D projects and currently serves as project coordinator of a German joint research project investigating single-crystal diamond as a Raman-active medium for high-power lasers. He has published 9 articles in peer-reviewed journals.

Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.

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