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Wish upon a star

EuroPhotonics
Oct 2009
Jörg Schwartz, j.schwartz@europhotonics.com

MUNICH, Germany, and MONTREAL – When searching outer space for sharp images of remote stars, large-scale astronomical telescopes employ adaptive optics to actively compensate for atmospheric disturbances. However, these devices need control mechanisms in the form of “guide stars,” which can be generated by sending a laser beam into the atmosphere. The guide star, when observed with the same telescope optics as real stars, can yield information about the aberrations induced by atmospheric turbulence. Now new types of lasers are being developed for this job.

Although nowadays telescopes can be mounted on space stations, large terrestrial telescopes are still important tools for astrophysicists when it comes to researching the universe. The size of the optics matters, because large dimensions can help gather photons at extremely low light levels from remote galaxies.

Another benefit of large-size optics is improved resolution – at least in theory. Unlike space-mounted devices, however, Earth-based telescopes have to cope with atmospheric disturbances, even though most high-end observatories, like the European Southern Observatory (ESO) facilities in the Atacama Desert of Chile, have been built in regions with very clear air. When there are atmospheric disturbances, increasing telescope diameters to more than 25 cm does not improve resolution, explains Dr. Axel Friedenauer of Toptica Photonics, “unless adaptive optics are used.” This technology works so well that larger observation apertures allow a terrestrial telescope to achieve resolution better than that produced by space-based equipment.

To produce the control mechanism based on a reference to known distortions that is required for adaptive optics, the atmospheric sodium layer, at a height of 90 km and with a thickness of 10 km, is resonantly excited by powerful laser sources to generate strong resonance fluorescence using orange light (Na D2 line). The resonance spot in the sky is then observed with the same telescope optics used for the stars in the background. Precise information about the aberrations induced by the atmospheric turbulence on the artificial star allows a deformable mirror in the optical train of the telescope to be controlled such that it compensates for those aberrations.

Special laser needed

The laser needed to do this job must be resonant with sodium ions. In addition, it must offer a narrow linewidth, high power, excellent beam quality and stability, high reliability and turnkey operation, avoiding the need for a laser physicist to operate the device.

The approach developed by Munich, Germany-based Toptica Photonics, together with the Montreal company MPB Communications, combines fiber laser and Raman amplification technologies to generate more than 20 W at 589 nm in a TEM00 beam. It commercializes a fiber laser and Raman fiber amplifier concept designed by ESO scientists and presented at the CLEO Europe conference in June 2009.

“The design uses a master oscillator/ power amplifier configuration,” says co-author Friedenauer, with Toptica’s 100-kHz-linewidth diode laser serving as the seed for two Raman fiber amplifiers generating 1178-nm radiation. Raman amplification of a narrowband 1178-nm signal to high powers in a single-mode optical fiber is challenging because of an unwanted nonlinear process known as stimulated Brillouin scattering. It tends to limit the forward-traveling signal power by reflecting it backward. The Raman expertise of MPB Communications was used in developing fiber amplifiers with stimulated Brillouin scattering suppression, to allow amplification of linearly polarized narrowband 1178-nm signals to very high output powers. More than 20 W of linearly polarized continuous-wave light has been demonstrated at 1178 nm from an all-fiber polarization-maintaining Raman fiber amplifier with an emission linewidth narrower than 4 MHz.

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A laser guide star is generated by high-power laser radiation sent to the 90-km-high sodium layer in the atmosphere. Fluorescence emission generated there helps to compensate for optical aberrations caused by atmospheric turbulences.

For the complete guide star laser system, two such signals are coherently combined for power scalability. The combined beam is then injected into a resonantly enhanced frequency doubler using second-harmonic generation in a lithium-triborate crystal to output up to 25 W at 589 nm. This corresponds to an overall conversion efficiency of more than 80 percent.

“The reduction of complexity in comparison to former solutions for guide stars can now be combined with scalability,” said Toptica President Dr. Wilhelm Kaenders.

Until now, dye lasers and sum-frequency lasers have been used to generate laser guide stars. These devices can be cumbersome and high-maintenance. The goal of the Toptica/MPB Communications consortium bid is to replace these lasers with compact, low-maintenance narrowband and scalable high-power turnkey laser systems. Raman offers an advantage over other candidate technologies by providing a gain per length that is proportional to pump intensity, but without physical limits, as well as a technology that has been well established by telecom applications.


GLOSSARY
adaptive optics
Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
astronomy
The scientific observation of celestial radiation that has reached the vicinity of Earth, and the interpretation of these observations to determine the characteristics of the extraterrestrial bodies and phenomena that have emitted the radiation.
brillouin scattering
The nonlinear optical phenomenon of the spontaneous scattering of light in a medium by its interaction with sound waves passing through the medium. The scattering takes place on an atomic level.
fiber laser
A laser in which the lasing medium is an optical fiber doped with low levels of rare-earth halides to make it capable of amplifying light. Output is tunable over a broad range and can be broadband. Laser diodes can be used for pumping because of the fiber laser's low threshold power, eliminating the need for cooling.
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