SodiumStar 20/2 High Power cw Tunable Guide Star

Laser Guide Star Adaptive Facilities Atmospheric Monitoring Laser Cooling SodiumStar 20/2 High Power cw Tunable Guide Star Laser

Existing and upcoming next generation current technology. Total weight of 700 kg, The frequency doubling technique employs optical telescopes require highly reliable warm-up times of less than 30 minutes, a newly-developed doubly-resonant SHG: 589 nm high-power laser sources with power consumption of about 700 W (@ BOL), · highly efficient and robust excellent beam quality – so-called Guide integrated optical sideband generation · fast control electronics

Star – for the implementation of (D2b repumping) and a novel remote pum- · based on state-of-the-art optomechanics . Under contract from the ping option along with many other features · active beam control options. European Southern Observatory (ESO) and provide next generation telescope designers the W.M. Keck Observatory, industrial part- with unprecedented degrees of freedom. The system architecture avoids wave- ners TOPTICA Photonics and MPB Com- front-critical optical modulators such as munications have jointly developed, pro- The SodiumStar represents a paradigm shift EOMs or AOMs and guarantees excellent duced and field-tested such a Guide Star in operational simplicity. This eliminates the beam quality. Laser for direct integration into the ESO VLT need for labor-intensive daily alignment tu- telescope structure and established the ne-ups and lengthy system warm-up times. new standard for this emerging field. The complete laser unit consists of line-re- placeable units (LRUs) which can be swap- Operational Principle ped on-site for service and repair. The main- A master oscillator signal from a linearly po- tenance is reduced to minimize downtime larized 1178 nm cw diode laser is stabilized and manpower requirements at the obser- to a high resolution (10 MHz) wavelength vatories. For future needs, the design offers meter. Its output is efficiently amplified in a the possibility of scaling the output power polarization-maintaining (PM) Raman fiber to even higher levels while preserving the amplifier (RFA) based on ESO’s patented excellent beam quality. technology. The spectrally narrow output is mode-matched to an efficient resonant Direct Integration into the Telescope Remote Pumping: The inclusion of the frequency doubling cavity delivering more Structure control electronics in the SHG module, in than 20 W of optical output at 589 nm. In contrast to the currently used dye or combination with the flexibility of the RFA solid-state laser systems which rely on a / fiber pump laser architecture, makes stationary laser clean room at the observa- “remote pumping” possible. This novel Semiconductor Second RFA Master Harmonic tory facility, the SodiumStar can be directly technique allows spatial separation bet- Oscillator Generation integrated into the telescope structure ween the compact Laser Head and the

Wavelength Fiber Laser with its gravity-varying condition. As part Electronics Cabinet of 32 m and therefo- Measurement Pump Module of ESO‘s 4LGSF, four of these lasers re enables highly flexible telescope inte- are remotely operated via an Ethernet gration. The compact Laser Head can be Schematic of Laser Unit architecture compri- sing a semiconductor master oscillator serving connection. located for example in close proximity to as seed source for a PM Raman fiber ampli- the laser launch telescope while the Elec- fier which is pumped by a polarized fiber laser. The emission wavelength is stabilized via a The distributed feedback based seed laser tronics Cabinet containing the pump di- highly accurate wavelength meter. Subsequent technology guarantees: odes, drive electronics and power supplies to amplification, the high-power laser radiation · intrinsic single-mode operation (heat sources), can be installed in a more is converted to the yellow spectral region within the second-harmonic generation mo- · precise wavelength stabilization convenient location well separated from the dule (SHGM). Frequency sidebands for optical · fast wavelength toggling for measuring launch telescope. repumping are generated via current modulation of the seed laser. the Rayleigh scattering background. The high-power Raman fiber amplification The engineering of a deployable laser Modular Design and Highest Availabili- has its known strong points: system based on ESO‘s original concept ty for Astronomical Observation · surface-to-volume ratio of fiber lasers has resulted in a ruggedized, compact and Utmost care was taken to design and test allows favorable power scalability modular solution, with focus on reliability the laser unit for remote operation in the · no need for optical alignment due to and serviceability. Options of support for harsh environments encountered on the top all-fiber approach integration on-site, periodic inspections for of a mountain, driving thermal, vibrational · absence of linewidth broadening during preventive maintenance as well as full ser- and even earthquake stability to the limits of amplification. vice support are offered. Fully Engineered Ready for Integration

SodiumStar 20/2 laser head with RFA (left) and SHGM (right) on a water-cooled cold plate.

SodiumStar 20/2 electronics cabinet.

Fiber Laser Power Supply Schematic of the second harmonic generation module (SHGM). Fiber Laser Pump Module

Wavelength Measurement Module

Programmable 3 meters Logic Controller Module Power Entry Module

Main / Seed Laser Module 27 meters Hydraulic Module

Remote pumping option: The laser head can be separated up Interior view of SodiumStar 20/2 electronics cabinet. to 32 meters from the electronics cabinet. Emitted spectrum of the laser recorded via a scanning FPI with Fast detuning of emission frequency by 5 GHz within 2.5 s for sidebands for repumping (Na D2b). measuring the Rayleigh scattering background.

Emission wavelength during climate chamber test ±40 MHz spec, Long-term power measurement showing a mean value of 22 W with ambient temperature over 80 hours. peak-to-peak variation of 2.1 %.

Pointing and lateral shift of the laser output beam during a tilting test Shack-Hartmann wavefront measurement of the output beam of the laser head carried out at full operational power level of 22 W of with rms wavefront error of 23 nm. output power at 589 nm. Specifications and Key Features

Optical Specifications Laser output power ≥ 20 W Key Features Laser wavelength* 589.159 nm · Robust and fully engineered system Wavelength tunability ± 6 pm · Direct integration into telescope Frequency stability ± 40 MHz structure Laser linewidth ~ 5 MHz · Gravity invariant operation Power stability peak-to-peak < 10 % · All polarization-maintaining fiber Intensity noise rms < 3 % approach Optical beam diameter 4σ 3 ± 0.1 mm · Excellent polarization properties Beam ellipticity < 5 % · High wall-plug efficiency Beam quality (rms wavefront error) < 50 nm · Minimized cooling requirements Beam pointing rms < 160 µrad · State-of-the-art optomechanics Beam shift rms < 100 µm · Active beam control implemented Polarization extinction ratio > 100:1 · Remote pumping scheme (optional) Linear polarization stability < 2° Repumper amplitude 0 .. 15 % Cooling System Specifications Coolant temperature 15 °C Temperature stability ± 0.75 K Coolant flow rate ~ 5l/min General Specifications Electrical power consumption < 1 kW Operating temperature range -10 .. 25 °C Operating altitude (above sea level) < 4300 m Dimensions laser head (including insulation cover) 44 x 93 x 73 cm³ (H x W x D) Weigth laser head 80 kg Dimensions electronics cabinet 173 x 91 x 93 cm³ (H x W x D) Weigth electronics cabinet 600 kg

*other wavelength / beam diameter on request

TOPTICA Projects GmbH and MPB Communications, Inc. are licensees of the ESO Fiber Raman Amplifier Technology (EFRA) developed and transferred by ESO (www.eso.org). Mechanical Drawings 295

705 90 Laser Beam Exit Point 215 505 1730

910 930

TOPTICA Projects GmbH INVISIBLE LASER RADIATION AVOID EYE OR SKIN EXPOSURE TO Lochhamer Schlag 19 DIRECT OR SCATTERED RADIATION CLASS 4 LASER PRODUCT D-82166 Graefelfing/Munich (DIN EN 60825-1:2014) Germany Phone: +49 89 85837-500 Fax: +49 89 85837-510 [email protected] www.toptica.com

BR-SodiumStar 20/2-2018-06