Manfred Hager

The ESO transportable LGS Unit for measurements of the LGS photon return and other experiments

Sodium laser guide stars (LGS) are used, or planned to be used, as single or multiple artificial beacons for Adaptive Optics in many present or future large and extremely large telescopes projects. In our opinion, several aspects of the LGS have not been studied systematically and thoroughly enough in the past to ensure optimal system designs. ESO has designed and built, with support from industry, an experimental transportable laser guide star unit, composed of a compact laser based on the ESO narrow-band Raman Fiber Amplifier patented technology, attached to a 30cm launch telescope. Besides field tests of the new laser technology, the purpose of the transportable unit is to conduct field experiments related to LGS and LGS-AO, useful for the optimization of future LGS-AO systems. Among the proposed ones are the validation of ESO LGS return flux simulations as a function of CW and pulsed laser properties, the feasibility of line-of-sight sodium profile measurements via partial CW laser modulation and tests of AO operation with elongated LGS in the EELT geometry configuration. After a description of the WLGSU and its main capabilities, results on the WLGSU commissioning and LGS return flux measurements are presented.

RFA-based 589-nm guide star lasers for ESO VLT: a paradigm shift in performance, operational simplicity, reliability, and maintenance

Large telescopes equipped with adaptive optics require 20-25W CW 589-nm sources with emission linewidths of ~5 MHz. These Guide Star (GS) lasers should also be highly reliable and simple to operate and maintain for many years at the top of a mountain facility. Under contract from ESO, industrial partners TOPTICA and MPBC are nearing completion of the development of GS lasers for the ESO VLT, with delivery of the first of four units scheduled for December 2012. We report on the design and performance of the fully-engineered Pre-Production Unit (PPU), including system reliability/availability analysis, the successfully-concluded qualification testing, long-term component and system level tests and long-term maintenance and support planning. The chosen approach is based on ESOs patented narrow-band Raman Fiber Amplifier (EFRA) technology. A master oscillator signal from a linearly-polarized TOPTICA 20-mW, 1178-nm CW diode laser, with stabilized emission frequency and controllable linewidth up to a few MHz, is amplified in an MPBC polarization-maintaining (PM) RFA pumped by a high-power 1120-nm PM fiber laser. With efficient stimulated Brillouin scattering suppression, an unprecedented 40W of narrow-band RFA output has been obtained. This is then mode-matched into a resonant-cavity doubler with a free-spectral-range matching the sodium D2a to D2b separation, allowing simultaneous generation of an additional frequency component (D2b line) to re-pump the sodium atom electronic population. With this technique, the return flux can be increased without having to resort to electro-optical modulators and without the risk of introducing optical wave front distortions. The demonstrated output powers with doubling efficiencies >80% at 589 nm easily exceed the 20W design goal and require less than 700 W of electrical power. In summary, the fiber-based guide star lasers provide excellent beam quality and are modular, turn-key, maintenance-free, reliable, efficient, and ruggedized devices whose compactness allows installation directly onto the launch telescope structure.

PM fiber lasers at 589nm: a 20W transportable laser system for LGS return flux studies

In this paper we present the rationale and design of a compact, transportable, modular Laser Guide Star Unit, comprising a 589nm laser mounted on a 300mm launch telescope, to be used in future experiments probing the mesospheric sodium properties and to validate existing LGS return flux simulations. The 20W CW 589nm Laser is based on the ESO developed concept of 589nm lasers based on Raman Fiber Amplifiers, refined and assembled together with industry. It has the same laser architecture as the laser which will be used for the VLT Adaptive Optics Facility. We have added to the 20W CW laser system the capabilities of changing output polarization, D2b emission levels, power level, linewidth and to operate as pulsed laser with amplitude modulation. We focus in this paper on the laser description and test results.

20 W at 589 nm via frequency doubling of coherently beam combined 2-MHz 1178-nm CW signals amplified in Raman PM fiber amplifiers

We report the recent breakthroughs of our research activities carried out in the framework of the ESO R&D programs to develop 589-nm CW sources for laser guide stars[1] (LGS). Large telescopes equipped with adaptive optics require 20–25 W 589-nm CW light sources with an emission linewidth ≪ 50 MHz. Towards this goal, we have been working [2.3] on the development of high-power narrowband 1178-nm Raman fiber lasers with subsequent frequency doubling to 589 nm. Fiber lasers are an asset and probably the best laser type in remote and difficult operation sites like astronomical observatories. Typically, they are compact, maintenance-free, turn-key, ruggedized devices. Moreover their output beam quality is excellent, an important requirement for LGS. The lasers which we are aiming at are part of Laser Guide Star Facilities, in which the laser beam is projected at 90 km at the Earths Mesosphere, producing LGS by excitation and optical pumping of mesospheric sodium atoms.

Identification of maritime target objects from high resolution TerraSAR-X data using SAR simulation

In general, interpretation of signatures from synthetic aperture radar (SAR) data is a challenging task even for the expert image analyst. For the most part, this is caused by radar specific imaging effects, e.g. layover, multi-path propagation or speckle noise. Specifically for the application in maritime security, ship signatures exhibit additional defocusing effects due to the ship’s movement even when they are anchored. Focusing on object recognition, the detection of target signatures can be done with a pretty good chance of success, but the identification is often impossible. To assist image analysts in their recognition tasks, a SAR simulation tool has been developed recently. It is very simple to operate, by simulating available 3D model data of ships and test the resulting simulated signatures with their real counterpart from SAR images. This is a very robust way to identify larger vessels out of current one meter resolution space borne SAR data. Nevertheless, for smaller vessels this can be still very challenging, because the resolution is too coarse. Recently, TerraSAR-X initiated a new staring spotlight imaging mode that enhances cross-range resolution significantly and therefore also improves the chance for the identification of smaller vessels. This paper demonstrates the capabilities of the developed simulation tool in assisted target recognition specifically on ship signatures. The improvement of recognition performance will be studied by comparing results for TerraSAR-X sliding spotlight mode and staring spotlight mode data.