laser guide stars (original) (raw)
Acronym: LGS
Definition: small bright spots in the sky, generated with laser beams for use in astronomy with adaptive optics imaging
Alternative terms: artificial guide stars, sodium guide stars, laser beacons
methodsRelated: adaptive opticslaser applicationsRaman laserssolid-state lasersfiber lasersnonlinear frequency conversiontelescopes
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DOI: 10.61835/nys Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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Contents
What are Laser Guide Stars?
The quality and size of modern astronomical telescopes have been enormously increased; telescopes with mirror diameters of several meters and very high surface quality are used in many observatories. The image resolution of the best and largest of these telescopes is already no longer limited by the optics themselves, but by atmospheric distortions: the light from astronomical objects can travel over huge distances in space without significant distortions, but temperature and pressure variations associated with turbulences in the Earth's atmosphere can lead to significant distortions, even at favorable locations on mountains with a clear sky.
A straightforward solution to this problem is to use space-based telescopes. However, these cannot be as large as terrestrial telescopes, and are very expensive to build, launch, operate, and maintain. Therefore, the alternative solution of atmospheric correction is being increasingly adopted, which makes it possible to strongly reduce the effect of atmospheric distortions for Earth-based telescopes: the wavefront distortions caused by the atmosphere are compensated for with adaptive optics, based on e.g. deformable mirrors with many degrees of freedom. Such a system obviously requires exact information on the current atmospheric distortions. These can be measured by analyzing the wavefronts from a distant point-like object such as a star (called guide star), since without distortions this light would have essentially plane wavefronts.
For precise wavefront correction, the guide star has to be close (in terms of direction) to the object under investigation, and has to be sufficiently bright. Unfortunately, however, there is not always a suitable natural guide star in a sufficiently close direction. In this situation, an artificial guide star (or laser beacon), temporarily created by shining an intense laser beam into the atmosphere, can replace a natural star [1]. Some laser light then comes back to the telescope and can be analyzed e.g. with a Shack–Hartmann wavefront sensor. An improved scheme may even use multiple laser guide stars [10, 11].
The position of the artificial guide star may drift somewhat, but this can be corrected e.g. by comparing it with that of a natural star, which does not have to be particularly bright.
Types of Laser Guide Stars
Figure 1: The William Herschel Telescope at the Roque de Los Muchachos Observatory, La Palma, with a green laser beam as used for a Rayleigh laser guide star. Credit: Tibor Agocs.
The two dominant types of laser guide stars are the sodium beacon and the Rayleigh beacon. The principle of the sodium guide star is to tune the wavelength of the laser radiation to a narrow absorption resonance of sodium atoms at 589.2 nm. This causes sodium atoms, naturally occurring in the mesosphere at an altitude of around 90 km, to absorb laser light and subsequently to emit fluorescence at the same wavelength. This approach has the nice feature of obtaining fluorescence light essentially only from a narrow range of high altitudes. Its disadvantage is that the required orange/yellow laser source with a power of e.g. 10 W or even 50 W and a small linewidth is not easy to construct and accordingly expensive.
Sodium Beacons
Available technological options for sodium beacons include the following:
- A Raman laser based on a bulk crystal can be pumped with a frequency-doubled Q-switched neodymium-based solid-state laser.
- A 1178-nm Raman fiber laser (or Raman MOPA) may be pumped with an ytterbium-doped fiber laser, with subsequent frequency doubling e.g. in periodically poled KTP.
- There are sources based on sum frequency mixing of two laser sources (continuous-wave or pulsed), e.g. at 1064 and 1319 nm, or at 938 and 1583 nm.
- A pulsed dye laser may be made as a “modeless laser” (superluminescent source) for effectively exciting sodium atoms with different longitudinal velocities [7].
Rayleigh Beacons
In contrast, a Rayleigh guide star is based on Rayleigh scattering in the lower atmosphere. In order to use only the scattered light from the higher parts of the atmosphere (at roughly 30 km height), one uses a pulsed laser together with time-gating detection in the wavefront sensor. As the Rayleigh beacon is not based on a narrowband resonance, the chosen wavelength is not critical, except that it should be short because Rayleigh scattering is most efficient at short wavelengths. A common choice is that of a green laser source, such as a frequency-doubled solid-state laser, but a copper vapor laser (→ gas lasers) or an excimer laser can also be used. Such laser sources can be less complex than those of sodium guide stars, and at the same time more powerful, but the lower altitude of the backscattered light compromises the quality of the wavefront correction.
In many cases, laser guide star sources emit nanosecond pulses, rather than continuously. The pulsed format and the required small emission linewidth simplify the nonlinear frequency conversion in the laser source, and short pulses make time-gated detection possible.
Laser Guide Star Systems in Use or in Development
A couple of large astronomical observatories use adaptive optics with laser guide stars:
- the Lick Observatory of the University of California
- the Palomar Observatory of Caltech
- the Keck Observatory in Hawaii
- the William Herschel Telescope of the Isaac Newton Group in La Palma, Canary Islands (using a Rayleigh guide star)
- the Very Large Telescope of ESO, Gemini North, and the Multiple Mirror Observatory (MMTO) in Arizona
- the Gemini Observatory in Hawaii and Chile
- the Subaru Telescope on Manua Kea, Hawaii
- the Large Binocular Telescope in Arizona
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is a laser guide star?
A laser guide star, also called an artificial guide star or laser beacon, is an artificial light source created by directing a powerful laser beam into the atmosphere. It serves as a reference point for adaptive optics systems to correct atmospheric distortions in ground-based telescopes.
Why are laser guide stars necessary for large telescopes?
The image resolution of large ground-based telescopes is limited by atmospheric turbulence. Adaptive optics can correct these distortions but require a bright reference star near the object of interest. When a suitable natural guide star is unavailable, a laser guide star is created artificially.
What are the two main types of laser guide stars?
The two dominant types are the sodium beacon and the Rayleigh beacon. Sodium beacons excite sodium atoms in the mesosphere (at ~90 km), while Rayleigh beacons rely on Rayleigh scattering of light in the lower atmosphere (at ~30 km).
How does a sodium guide star work?
A sodium guide star is created by a laser tuned to the 589.2-nm absorption resonance of sodium atoms, which exist naturally in the mesosphere. These atoms absorb the laser light and then emit fluorescence, creating a point-like light source at a high altitude.
What is a Rayleigh guide star?
A Rayleigh guide star is based on Rayleigh scattering of laser light from molecules in the lower atmosphere. It uses a pulsed laser and time-gated detection to isolate the backscattered light from a specific altitude, typically around 30 km.
What kind of lasers are used to create guide stars?
Suppliers
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âš™ hardware
MPBC is part of the “GuideStar Alliance,” contributing the Raman fiber amplifier and fiber laser pumps to the most powerful fiber laser guide star systems used in astronomy. The Raman fiber amplifier technology was first developed and patented by the European Southern Observatory (ESO). MPBC licensed the technology, and have collaborated to develop reliable, maintenance-free, ruggedized systems installed at:
- ESO’s 4 Laser Guide Star Facility (4LGSF) in Paranal, Chile;
- KECK, Gemini North, and Subaru Observatories in Mauna Kea, Hawai’i;
- Gemini South in Cerro PachĂłn, Chile;
- Wendelstein I & II, Brannerburg, Germany;
- GTC, Canary Islands, Spain;
- ALASCA Program, Tenerife, Spain.
Since joining the GuideStar Alliance, MPBC has developed a narrow band diffraction-limited Raman fiber amplifier (RFA) at 1178 nm providing 130 W of output power. These RFA units also have the advantage of a remote pumping scheme, allowing the user to integrate the RFA up to 50 m away from the electronics cabinet, ideal where available space is a factor.
MPBC has an expanded single-frequency amplification portfolio, and are able to offer virtually any wavelength where silica-based fibers are transparent, supporting novel applications for the scientific and commercial research community.
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TOPTICA Projects offers high-power cw Guide Star lasers, delivering more than 20 W of optical output power, tunable around 589 nm (sodium resonance). These systems are currently in use at all major optical telescopes and provide an outstanding performance.
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VEXLUM offers VECSEL technology as a cost-effective alternative to traditional diode and fiber amplifier setups for sodium guide star lasers. For more information or to start a discussion, contact the VEXLUM team.
Bibliography
| [1] | R. Foy and A. Labeyrie, “Feasibility of adaptive telescope with laser probe”, Astron. Astrophys. 152, L29 (1985) |
|---|---|
| [2] | L. A. Thompson and C. S. Gardner, “Experiments on laser guide stars at Mauna Kea Observatory for adaptive imaging in astronomy”, Nature 328, 229 (1987); doi:10.1038/328229a0 |
| [3] | B. M. Welsh and C. S. Gardner, “Nonlinear resonant absorption effects on the design of resonance fluorescence lidars and laser guide stars”, Appl. Opt. 28 (19), 4141 (1989); doi:10.1364/AO.28.004141 |
| [4] | M. P. Jelonek et al., “Characterization of artificial guide stars generated in the mesospheric sodium layer with a sum-frequency laser”, J. Opt. Soc. Am. A 11 (2), 806 (1994); doi:10.1364/JOSAA.11.000806 |
| [5] | K. Avicola et al., “Sodium-layer laser-guide-star experimental results”, J. Opt. Soc. Am. A 11 (2), 825 (1994); doi:10.1364/JOSAA.11.000825 |
| [6] | C. E. Max et al., “Design, layout, and early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics”, J. Opt. Soc. Am. A 11 (2), 813 (1994); doi:10.1364/JOSAA.11.000813 |
| [7] | J.-P. Pique and S. Farinotti, “Efficient modeless laser for a mesospheric sodium laser guide star”, J. Opt. Soc. Am. B 20 (10), 2093 (2003); doi:10.1364/JOSAB.20.002093 |
| [8] | J. C. Bienfang et al., “20 W of continuous-wave sodium D2 resonance radiation from sum-frequency generation with injection-locked lasers”, Opt. Lett. 28 (22), 2219 (2003) doi:10.1364/OL.28.002219 |
| [9] | F. Marc et al., “Effects of laser beam propagation and saturation on the spatial shape of sodium laser guide stars”, Opt. Express 17 (7), 4920 (2009); doi:10.1364/OE.17.004920 |
| [10] | F. Rigaut et al., “Gemini multiconjugate adaptive optics system review - I. Design, trade-offs and integration”, Mon. Not. R. Astron. Soc. 437 (3), 2361 (2014) |
| [11] | B. Neichel et al., “Gemini multiconjugate adaptive optics system review — II. Commissioning, operation and overall performance”, Mon. Not. R. Astron. Soc. 440 (2), 1002 (2014); doi:10.1093/mnras/stu403 |
| [12] | T. J. Kane et al., “Laser remote magnetometry using mesospheric sodium”, J. Geophys. Res.: Space Physics 123 (8), 6171 (2018); doi:10.1029/2018JA025178 |
| [13] | Y. Lu et al., “208 W all-solid-state sodium guide star laser operated at modulated-longitudinal mode”, Opt. Express 27 (15), 20282 (2019); doi:10.1364/OE.27.020282 |
| [14] | X. Yang et al., “Diamond sodium guide star laser”, Opt. Lett. 45 (7), 1898 (2020); doi:10.1364/OL.387879 |
| [15] | P. Ma et al., “Kilowatt-level ytterbium-Raman fiber amplifier with a narrow-linewidth and near-diffraction-limited beam quality”, Opt. Lett. 45 (7), 1974 (2020); doi:10.1364/OL.387151 |
| [16] | H.-Y. Li et al., “Numerical study on the influence of the linewidth of a QCW pulsed sodium laser on the brightness of a guide star”, Opt. Express 29 (24), 40397 (2021); doi:10.1364/OE.443293 |
| [17] | Keck Observatory in Hawaii, http://www.keckobservatory.org/ |
| [18] | Lick Observatory of the University of California, http://mthamilton.ucolick.org/ |
| [19] | Palomar Observatory of Caltech, http://www.astro.caltech.edu/palomar/ |
| [20] | Isaac Newton Group of Telescopes of La Palma, http://www.ing.iac.es/ |
(Suggest additional literature!)
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