Laser Clean Room

The upgrade of the LGSF to 4 LGSF takes full advantage of the existing Laser Clean Room. As much as possible, the electronics cabinets are in the LCR, the interlock panel and the fibre laser sources are in the LCR, and most of the heating/cooling is confined to this space. The easy access to LCR helps in servicing and maintenance, and the numerous safety issues becomes more manageable.

The existing LCR was already dimensioned to host multiple lasers, in the LGSF project. Moreover, with fibre lasers the power consumption is much reduced. Hence very little modifications are required to the Laser Clean Room for the upgrade to 4LGSF.

Figure 1: The Laser Clean Room is a class 10000 volume, built under the UT4 Nasmyth A platform (opposite side of NACO). It hosts three electronics cabinets, one optical table to be used for 4LGSF maintenance, electric power, cooling liquid distribution and general services. There are connection points for cooling, LAN, compressed air, and safety measures such as a fire extinguishing system based on Inergen gas, limited access system for Class IV laser environments and anti-Earthquake passive measures. Small changes only are requested for the 4LGSF upgrade.

 

Lasers

The baseline lasers are 1178nm fibre Raman lasers, which are frequency doubled to 589nm. The fibre delivers 20+W CW at 1178nm, 1GHz linewidth. The frequency doubling is done via a single pass on PPSLT, a non linear crystal. The fibre laser is an on-going development at ESO, together with the companies IPF Technology Ltd, UK, Toptica (D) and the Russian branch of the company Volius. ESO has so far reached 2.9W CW at 589nm in its lab, aiming to reach full power in the second half of 2006.

The fibre laser system is contained in one 19 inch module; five of these units will be placed in the electronic cabinet next to the LCR entrance door. The polarization maintaining single mode fibre will reach directly the Launch telescopes, using a fireproof fibre cable and going through the altitude cable wrap up to the UT4 centerpiece. At each launch telescope, a small box contains the frequency doubling PPSLT crystal, its temperature controller and the frequency feedback control sensor. The frequency doubling unit is located at the Launch Telescope.

 

Launch Telescopes

The 1178nm fibre laser and the frequency doubling units are part of the industrial laser delivery.

The four Launch Telescopes located on the centrepiece have demanding requirements. The projected laser beam quality has to be diffraction limited to guarantee the minimum LGS angular size, which imposes constraints on the optical train and calls for the maximum simplification possible. Care has to be applied for optics working at high power densities. This requirement coupled with the flexibility to point the LGS at 0, 60 or 330 arcsec off-axis, as required by MUSE and Hawk-I, has driven the choice of the Launch Telescope optics toward a refractive, single lens f/5 design, with the fibre laser output at its focal plane. We have explored also reflective designs, and came to the conclusion that a refractive solution is preferable. The first Eigenfrequency of the LTS has to be >60 Hz, in order to avoid LGS wandering and unwanted jitters. This imposes strict choices on the mechanical support structure of the LTS, which is in CFRP to ensure stiffness and reduce weight.

Figure 2: The four Launch telescopes of the 4LGSF baseline design are mounted on the UT4 centerpiece.The singlet large BK7 lenses are supported by a carbon fibre cone-shaped shell, which grants high rigidity and stability. The fibre laser output goes directly at the focal plane of the f/5, 500mm diameter lens.

Above the Launch telescope, there is a movable shutter curtain to protect the lens when it is not in use, and a long baffle (see Fig 3.4 below) to avoid as much as possible scattering light in the telescope environment. This is also to avoid that the telescope volume becomes at Class IV laser volume.

 

LGS Diagnostic System

In operation, there can be the diagnostic system in or not. A motorized flipper mirror can optionally send the output beam to a Coherent LM-45 calibrated power-meter, to measure the output beam power.

Figure 3:  Operation mode with the Beam Diagnostic System on. 1% of the intensity feeds the diagnostic path. If the beam is deflected to the Bolometer by the flipper mirror, the system can be diagnosed in daytime as well.