Using a Guide Star Laser as an Astronomical Point of Reference for Increased Image Resolution

Table of Contents

Introduction
Modern Astronomy
Adaptive Optics
Guide Stars
The SodiumStar System
Conclusion

Introduction

Next-generation telescopes use adaptive optics to remove the atmosphere-induced wavefront distortion which gives distorted images. Laser Guide Stars are laser-excited sodium atoms that are in the Earth’s upper atmosphere. LGSs are used as a reference object for adaptive optics.

The SodiumStar available from TOPTICA is a small, reliable, turn-key guide star laser system at 589nm. It has a linewidth below 5MHz with 20W output power (Figure 1).

Figure 1. TOPTICA’s SodiumStar: 20 Watt at 589nm.

Modern Astronomy

Over the past few years the size of telescopes used have been steadily increasing to allow the observation of astronomical objects. In order to improve the collection efficiency and the optical resolution, multi- telescope arrays and a huge primary mirror were developed.

A number of telescopes featuring more than 8m-diameter mirrors have been constructed, for instance, the 10m Keck Telescope in Hawaii or the extremely large telescope of the European Southern Observatory in Chile (Figure 2).

Figure 2. In operation: Guide Star laser at the Very Large Telescope on Cerro Paranal, Chile.

Earth’s atmospheric distortions offset the telescopes high resolution. For example, with different layers of temperature that display different refraction indices act as tiny lenses and blur the wavefront produced by the astronomical object. As a result, images become distorted when they are obtained with ground-based telescopes.

These distortions, however, can be prevented by deploying the telescope over the Earth’s atmosphere, i.e.in space, like with the the Hubble Space Telescope. However, the mirrors’ large weight and high costs makes it difficult to transport the telescopes into space.

Adaptive Optics

Adaptive optics is used to rectify optical aberrations in astronomy and also applied in retinal imaging systems. With respect to ground-based telescopes, adaptive optics can be used to produce a plane wave, and therefore to acquire diffraction-limited images.

After being released by a star and emitted through space waves are blurred by the atmosphere. The inward-bound wavefront is directed towards a deformable mirror, and then, studied using a wavefront sensor such as a Shack-Hartmann sensor.

To acquire a flat wavefront in real-time, a continuous feedback signal is emitted by the wavefront sensor to adapt the deformable mirror curvature. Using this method a point- like object can act as a point in the ensuing image, providing diffraction-limited images.

Guide Stars

Point-like sources in the angle of view serve as reference objects in the adaptive optics technique. As a result, Natural Guide Stars (NGS), which are bright stars of point-like appearance are used. In order to ensure that the NGS appear point-like in the image, the telescope optics is adapted continuously.

Distortion of the wavefront for neighboring objects is also rectified this way. However, it is quite rare for stars to qualify as NGS and this restricts this method to only small regions of the sky. This issue can be overcome with laser-based methods (laser guide stars, LGS). With the aid of these techniques, artificial point-like objects can be produced in the night sky.

The mesosphere is located 90 to 110km above the surface of the Earth and contains sodium atoms. Either dye lasers or a sum-frequency combination of solid state lasers can be used to produce sodium laser guide stars. However, these approaches are largely restricted in terms of optical output power, and also require extensive maintenance.

A highly intense laser, which is resonant with the sodium atoms, can be used to excite the sodium atoms in the mesosphere. It is also essential to use a continuous wave laser of less than 5MHz linewidth at 589nm along with the fine-tuning capacity to match the sodium’s hyperfine structure. An adequate flux of fluorescence photons at the telescope can be produced with 20W power.

The SodiumStar System

Such requirements can be met by using TOPTICA’s SodiumStar laser system, which was developed in collaboration with Canadian- based company, MPB Communications. The SodiumStar integrates a resonant frequency-conversion to 589nm, a narrow-band Raman fiber amplifier approach, and a spectrally narrow diode laser at 1178nm (DL DFB) used as seed laser.

Raman amplification is known to be built on the nonlinear interaction between a broad-band pump laser and a seed laser inside an optical fiber. Inelastic Raman scattering of a higher-frequency pump photon is caused by a seed photon of low frequency, and another photon at seed laser frequency is produced.

The surplus energy to the medium is then transmitted by a phonon. Whilst this method helps in realizing high output powers, it results in a broadened linewidth. ESO developed a new method to acquire narrow-band Raman amplification, with a linewidth restricted by the seed laser linewidth. TOPTICA Photonics has licensed and developed this technique.

Conclusion

Featuring less than 5MHz linewidth at 589nm and 30GHz tunability, the SodiumStar is a strong and robust industrial laser system that can be remotely operated and withstand adverse environments at high altitudes. At present, the SodiumStars have been implemented as a seed laser in the DL RFA SHG pro built on a DL 100/ pro design, and includes a mode-hop free tuning range of 20GHz, 2W output power, and less than 1MHz linewidth (Figure 3).

Figure 3. Scalability of the output power offers solutions for other high power.

Back in 2013, four SodiumStars were integrated into the large telescope at Paranal, Chile, and they were built directly within the existing telescope as part of power scalable approach. The same approach can be applied to laser atom cooling, laser image projectors, Sodium LIDAR, super resolution microscopy, medical therapy, laser TV and other high power continuous-wave visible laser applications.

This information has been sourced, reviewed and adapted from materials provided by Toptica Photonics.

For more information on this source, please visit Toptica Photonics.

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