Multiconjugate adaptive optics may prove more is more
A novel laser guide star and multiconjugate adaptive optics (AO) system may dramatically improve the image astronomers receive through the Gemini South Observatory (Cerro Pachon, Chile). AO systems minimize atmospherically induced image degradation in ground-based telescopes. An optical sensor detects wavefront errors in the light emitted by a bright point source (guide star) in the angular vicinity of the observation target. Feedback from the sensor adjusts a deformable mirror to cancel the wavefront distortion.
Because astronomical targets rarely have natural guide stars in their vicinity, astronomers use lasers to generate artificial guide stars. With a single guide star and deformable mirror, however, the atmospheric effects are compensated in only the immediate vicinity of the guide star. The proposed Gemini South system will incorporate five guide stars and three deformable mirrors to correct for atmospheric turbulence in three dimensions instead of the conventional two. Brent Ellerbroek, AO program manager at Gemini South Observatory, expects the system to increase the area of the sky in which an astronomer can achieve good image quality by a factor of 10.
The proposed multiconjugate adaptive-optics system for Gemini South includes five sodium-wavelength lasers and three deformable mirrors for a 3-D atmospheric turbulence reduction.
The Gemini guide-star system is based on generating resonant backscatter from sodium atoms using 589-nm output from five separate laser beams. Each of the three deformable mirrors will be optically conjugate to a different altitude in the atmosphere. "The system corrects turbulence on a one-to-one basis instead of an integrated basis," Ellerbroek says.
The Gemini South project is important for two reasons, says Claire Max of Lawrence Livermore National Laboratory (LLNL; Livermore, CA) and the Center for Adaptive Optics at the University of California (Santa Cruz, CA). Not only will it dramatically increase the field of view for astronomers, but it also stands as a test of the basic principles of multiconjugate AO. This second point is significant beyond Gemini, according to Max. The National Academy of Science is recommending that a 30-m ground-based telescope be built. Such a large-aperture telescope would require a multiconjugate AO system. Thus, the Gemini system is an important proof of concept. "We're all hoping this system works," Max says.
So far, everything has fallen into place except the lasers. Other researchers, including Max, have successfully demonstrated the sodium-layer guide-star technique using dye lasers (see oemagazine, February 2001, page 30). According to Ellerbroek, those systems won't work for Gemini South. "For 10 W beacons, dye sodium lasers are complex systems and are not yet fully reliable," says Ellerbroek. "We want five lasers instead of just one, and dye lasers are not practical because of their size and efficiency." Instead, the Gemini South design uses sum frequency mixing of 1064-nm and 1319-nm output from neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers in a nonlinear crystal to generate the 589-nm output required to excite the sodium.
Coherent Technologies (CTI; Boulder, CO), a Gemini vender, recently demonstrated 10 W of 1319-nm output from an Nd:YAG mode-locked laser with bandwidth and pulse duration matched to the sodium interaction. "This is a significant step toward a sum-frequency guide star, since the 1319-nm laser is widely regarded as the highest risk component," says Iain McKinnie, senior research scientist for CTI. The more conventional 1064-nm laser will be easier to build, he adds.
The CTI design uses compact, pulsed amplifiers for each laser rather than building higher-power oscillators. Even in thermally compensated resonators, beam quality and polarization degradation is a threat at 1319 nm for such high powers. After amplification, the laser output is split into five beams and mixed in separate nonlinear crystals. CTI is working with a crystal vender to use an emerging nonlinear material with 10 times the nonlinear gain of lithium triborate (LBO).
Meanwhile, researchers at Lite Cycles (Tucson, AZ) are taking a different tack to produce the five pairs of wavelengths. Their system will be based on a diode-pumped slab laser configuration, which they expect to generate more than 50 W of pulsed output. The configuration will be split into five beams before frequency mixing. "The pulse format is the so-called 'macro-micro' format, where there are thousands of mode-locked pulses (micro-pulses) within a larger macro-pulse envelope," says James Murray of Lite Cycles. The micro-pulse length is set so that the pulse bandwidth matches the sodium D2 bandwidth of approximately 1 GHz, and the macro-pulse length of 200 µs matches both the sodium-layer thickness and upper-state lifetime of Nd in the YAG host crystals. This pulsed approach eliminates Rayleigh backscatter noise altogether. The system is slated for demonstration in about six months.
Whether or not the final sodium laser guide-star system is ready, Ellerbroek expects Gemini South to be operational by 2005. "We have a conventional laser guide-star AO system buried inside our multiconjugate AO system, with the option of upgrading when the ultimate laser system is complete," he says. Assuming enough laser progress is made within the next year, Ellerbroek anticipates that the multiconjugate AO system will be up and running by the beginning of 2006.
