Laser Development for Sodium Laser Guide Stars at ESO

Telescopes and Instrumentation

Domenico Bonaccini Calia1 and SINFONI instruments (Bonaccini et 10 :1. These requirements on the laser Yan Feng1 al., 2003), and in 2013 the installation and the launch equipment are stringent. Wolfgang Hackenberg1 of a further four LGS is planned as part For routine operation at astronomical Ronald Holzlöhner1 of the Adaptive Optics Facility (AOF; observatories, the laser should also be Luke Taylor1 Arsenault et al., 2006) project. Future ex rugged, turn-key, remotely operated Steffan Lewis1 tremely large telescopes such as the from the control room, and require little European Extremely Large Telescope maintenance. These laser characteristics (E-ELT) will also require multiple laser have been specified for the AOF, but 1 ESO guide stars (currently 6–8 are envisaged), they are also relevant to the E-ELT base- and it is essential to provide reliable, line requirements. It must be mentioned compact, low-maintenance laser sources that special formats of pulsed lasers A breakthrough in the development at a reasonable cost to meet the needs of may become useful in the coming years of sodium laser guide star technology these telescopes. to reduce or eliminate the effects of at ESO was made in 2009. The laser spot elongation (Beckers, 1992; Beletic research and development programme To produce sufficiently bright guide et al., 2005) in large aperture telescopes has led to the implementation of a stars, lasers at 589 nm with powers of and to determine the rapidly varying narrowband Raman fibre laser emitting around 20 W Continuous Wave (CW) sodium profile precisely, but have not yet at the wavelength of the sodium lines and extremely good beam quality are been pursued. at 589 nm with demonstrated power needed. The brightness of the guide star beyond 50 W. Fibre lasers are rugged depends, amongst other things, on Dye lasers provided the first generation and reliable, making them promising the detailed atomic physics of the sodium of 589-nm lasers to the astronomical candidates for use in the next genera- layer. As this has not been well mod- community. They were the only possible tion of laser guide star systems, such elled, extensive design simulations of the choice at the time when Keck and ESO as the Adaptive Optics Facility planned mesospheric sodium return flux have decided to build their laser guide star for installation on VLT UT4 in 2013. been undertaken (Milonni et al., 1998; facilities. One model of a 589-nm dye Drummond et al., 2004; Holzlöhner et al., laser was built by the Lawrence Livermore 2010). For the AOF multiple laser guide National Laboratory for the Lick and Introduction star facility, this has resulted in specific Keck Observatories; a different dye laser requirements on the optical characteristics model was made for ESO by the Max- Laser guide stars (LGS) can be used of the laser, as summarised in Table 1. Planck-Institut für extraterrestrische as reference beacons for adaptive optics Physik in Garching (Rabien et al, 2003), as (AO) and significantly enlarge the sky Firstly, to facilitate efficient optical pump- part of the LGSF project (Bonaccini coverage of AO on optical telescopes. ing and achieve small LGS sizes, the et al., 2003). Although extremely useful for Sodium LGS are obtained by illuminating emitted laser-beam wavefront error has pioneering LGS–AO techniques and the natural layer of atomic sodium in to be better than 70 nm root mean for conducting the first LGS–AO observa- the mesosphere at 80–100 km altitude square (rms), with a goal of 25 nm rms. tions, this class of laser has the draw- using a wavelength of 589 nm (the sodium Secondly, a highly polarised output back that it requires high maintenance

D2 lines) and causing it to fluoresce. is needed to produce circular polarisation and preparation time before an observing In this way, an artificial “star” can be pro- and perform optical pumping of the night, which, at astronomical observato- duced that is a useful alternative to a mesospheric sodium atoms, which further ries, creates manpower loads with a natural guide star where none exists at enhances the resonant backscatter considerable footprint on the observatory that sky location. AO uses the laser guide signal. Circular polarisation is obtained operation. These considerations make dye stars as reference sources to probe by, for example, inserting a quarter-wave lasers possibly undesirable candidates atmospheric turbulence and provide feed- plate in the launch telescope system. for multiple laser guide star systems. Fur- back to deformable mirrors in order to Finally, re-pumping of the sodium atoms thermore, dye lasers are limited to stable compensate image blur effects induced is extremely advantageous (Kibblewhite, gravity vector installations. by this turbulence. Sodium LGS produce 2008) for the LGS return flux and is less focus anisoplanatism (cone effect) achieved by emitting two identical laser When the first conceptual design of the than Rayleigh LGS and they can probe lines at the centres of the D2a and the AOF multiple laser guide star facility the entire extent of the atmosphere D2b sodium lines, with an intensity ratio of was conceived at the end of 2005, off-the- (Ageorges & Dainty, 2000). Parameter Value Several large telescopes are equipped Format CW (continuous wave) Table 1. Laser optical characteristics Wavelength 589 nm with AO and LGS facilities, and future specified for the VLT Adaptive Optics Extremely Large Telescopes will require Power (laser device/in air) 20 W/16 W Facility. LGS–AO for some operational modes. Linewidth < 5 MHz 1 D re-pumping denotes blue-shift- ESO installed its first laser guide star Polarisation Linear, Pol. ratio > 100 :1 2b ing a fraction of the D line laser Wavefront error (rms) < 70 nm (< 25 nm goal) 2a on the Very Large Telescope (VLT) Unit power by 1.71 GHz in order to boost 1 Telescope 4 (UT4; Yepun) for the NACO Sodium D2b re-pumping ratio 12 % the sodium fluorescence efficiency.

12 The Messenger 139 – March 2010 shelf solid-state lasers at 589 nm, with Figure 1. VLT UT4 the characteristics listed above, did not (Yepun) shown with the LGSF laser beam exist and ESO therefore launched an propagated at Paranal. internal research and development (R&D) The Galactic Centre programme to support the goal of Courtesy of S. Seip is visible over the dome. achieving second generation laser char- The photograph, taken in 2007, is a 5-s expo- acteristics: turn-key, compact, solid-state sure during full Moon. CW lasers at 589 nm to be used rou- tinely, requiring limited maintenance, and sufficiently ruggedised to be mounted next to the laser launch telescopes on the altitude structure of the telescope.

This R&D programme, which reached a successful conclusion at the end of 2009, has resulted in the demonstration of progressively increased laser output power in the last two years, reaching up to 50.9 W CW output at 589 nm and a measured linewidth of 2.3 MHz. During the course of this development, we have capitalised on a significant industry trend towards increasing use of high power fibre lasers in multiple industry segments, while developing a unique and innovative narrowband Raman fibre amplifier technology as a solution to those laser problems that are specific to guide stars and therefore could not readily be solved by recourse to the commercial sector. In the final year of the programme, we have made this technology available to industry and an industrial consortium has been able to independently demonstrate a 20 W class 589-nm laser based on the ESO narrowband Raman amplifier technology. the technology readiness levels of differ- We decided to explore the technology ent laser technologies was performed of lasers at 1178 nm, to be frequency Laser technology background at the beginning of the R&D phase, visit- doubled to 589 nm in nonlinear crystals ing different institutes and several laser using Second Harmonic Generation With the operation and experience of the companies in Europe and in the US, and (SHG), shown schematically in Figure 2. ESO LGSF, it has become clear that new studying the vast laser literature. We studied ytterbium fibre lasers care- laser sources had to be developed for the next generation instruments or telescopes, that meet stringent requirements concerning reliability, compactness, and "NMSQNK$KDBSQNMHBR /NVDQ@MC"NNKHMF turn-key operation at astronomical sites. When we started the R&D activity, sum- frequency solid-state lasers combining 1064-nm and 1319-nm lasers were being pursued independently by the US Air .OSHB@K%HAQD Force and by the Gemini project together +@RDQML with Coherent Technologies as the industrial partner. In both cases these are Figure 2. The laser scheme is shown. A fibre laser solid-state lasers with free-space optics ML5HRHAKD and bulk optical tables full of components at 1178 nm feeds a compact second harmonic +HFGS&DMDQ@SHNM generation unit based on nonlinear crystals, which that must remain aligned during tele- converts two photons at 1178 nm into one photon scope operation. A broad exploration of at 589 nm.

The Messenger 139 – March 2010 13 Telescopes and Instrumentation Bonaccini Calia D. et al., Laser Development for Sodium Laser Guide Stars at ESO

fully, but it seemed at that time very we further narrowed down the technology fication with powers of about 40 W CW unlikely that they can ever lase directly at to Raman fibre lasers, because existing at 1178 nm. As an illustration, the power 1178 nm, while suppressing the ampli- rare-earth doped fibre lasers have low density in a 5-micron core fibre at fied spontaneous emission at shorter gain at 1178 nm. In contrast to rare-earth 40 W exceeds 2 × 108 W/cm2, giving rise wavelengths, in particular to 1064 nm. doped fibre amplifiers, such as the well- to nonlinear effects due to the interaction VECSELs (Vertical External Cavity Sur- known EDFAs (erbium-doped fibre ampli- of the electromagnetic radiation with face Emitting Laser, also called Optically fiers), Raman fibre amplifiers take ad the glass. Today, other laser technologies Pumped Semiconductor Lasers) were vantage of a nonlinear conversion process such as the photonic crystal ytterbium considered and discussed with Coherent in the fibre which converts “pump energy” lasers/amplifiers, VECSELs, and bismuth- Inc. in Santa Clara, but their technology from the laser at short wavelengths to doped fibre lasers have risen in tech readiness was low for our application. the signal wavelength via optical phonons, nology readiness level to become poten- We therefore further explored the boom- rather than via atomic transitions. As tial laser sources at 1178 nm, both CW ing fibre laser technology for the following shown below, however, we had initially to or pulsed. Furthermore, new fibre lasers/ reasons: overcome several technological problems. amplifiers allow novel sum-frequency – fibre lasers are alignment-free, being photon combinations to reach 589 nm, long waveguides with the photons con- Broadband Raman fibre amplifiers (RFA) using fibre lasers with rare earth dopants fined to the fibre core; are extensively used in the telecom such as thulium or neodymium; however, – their output optical beam quality is munications market. Our challenge was these developments have yet to be fully diffraction-limited in single mode fibres to achieve narrowband fibre Raman ampli- demonstrated. at the powers of interest to us; – the heat is distributed along the fibre volume, hence there are no overheating locations in the fibre laser creating strain, Nonlinear optical effects in optical fibres SRS lifetime, or beam optical quality issues Stimulated Raman scattering is the non at the powers of interest to us, contrary Sufficient optical intensities can momen linear effect at the core of our RFA technol- to other known solid-state lasers such tarily modify the optical properties of ogy, producing a frequency shift, in this as VECSELs or waveguide amplifiers; a medium so that its behaviour depends case of the 1120-nm photons of the pump – there is a robust industrial base for nonlinearly on light power. This circum- fibre laser to 1178 nm. SRS is a combi stance can be expressed mathematically nation of the Raman inelastic scattering fibre lasers, and they are commercially by expanding the susceptibility χ, which process with stimulated emission, which available (at other wavelengths than describes the dependence of the optical amplifies the optical signal with low noise 1178 nm) with powers even higher than polarisation of a medium on the electric and distributed amplification along the required; field, in a Taylor series. The second-order fibre. The pump photons undergo inelastic – fibre lasers can be rack-mounted and term χ(2) is responsible for second har- scattering with the glass molecules of located further away from the com- monic generation and sum-frequency gen- the fibre core, exciting vibration states and pact 589-nm SHG unit using the fibre eration (SFG) that are used for frequency creating “optical phonons”, which divert laser output as relay. Thus it is pos- conversion in materials such as lithium part of the photon energy so that the pump sible to create compact laser heads, tri-borate (LBO). However, the silica glass, photons at 1120 nm are shifted to longer where the 589-nm light is produced, of which optical fibres are made, obeys wavelengths, known as the Stokes shift. that are mounted directly on board the a structural centre symmetry, implying that The extent of the wavelength shift and the (2) (3) laser launch telescope; χ vanishes, and thus χ is the first non- efficiency of the Raman process at a given (3) – fibre lasers are simple and contain zero nonlinear expansion term (χ is a light intensity are related to the material very few components, hence are gener- complex third-order tensor; Boyd, 2003). composition and the index profile of the In the particle picture, (3) effects describe fibre core. ally more reliable and also intrinsically χ the interaction of four different photons. cheaper than other lasers. SBS Nonlinear effects due to χ(3) can be con- Stimulated Brillouin scattering limits the Fibre laser technology has made very ceptually divided into parametric effects, output power of the RFA, depending on its fast progress in recent years, with Raman including the Kerr nonlinearity (index emitted linewidth. It arises from the inter fibre lasers used in the telecom indus- variation of the glass due to high electric action of photons with acoustic phonons try, and ytterbium lasers used in the fields that can induce self-focusing and generated in the fibre core. In a simplified material processing and the car industry, self-phase modulation), four-wave mixing model of SBS via the electrostrictive ef- among others. This development has (FWM) and non-parametric processes fect, a travelling acoustic wave is created led to the availability of in-fibre compo- in which light energy is exchanged with the that carries forward a periodic variation nents such as fibre Bragg gratings (FBG, glass, such as stimulated Raman scatter- of refractive index in the fibre core, produc- equivalent to free-space mirrors placed ing (SRS) and stimulated Brillouin scattering, in effect, a long optical grating that inside the fibre), fibre couplers (equivalent ing (SBS). While we exploit SRS in Raman reflects part of the signal back towards the to free-space beam splitters), and wave- amplification, SBS and FWM are unwanted seed laser (see Figure 3). The grating effects. Brief explanations of these effects, modulation is amplified progressively to length division multiplexers (WDM, equiv- important for the LGS development, are gether with the Raman signal. The onset of alent to free-space dichroics). In the presented. SBS with increasing power is very sudden, broad technological class of fibre lasers,

14 The Messenger 139 – March 2010 Technical approach and its threshold depends on the fibre FWM length and core material, the fibre acoustic Four-wave mixing, or self-modulation, is We have pursued 1178-nm narrowband waveguiding properties, the laser wave- induced by the Kerr effect in the interaction Raman fibre amplifier technology length, the optical power and, importantly, between the photons at different frequen- (Bonaccini et al., 2006; Feng et al., 2008). the bandwidth of the radiation. Linewidths cies and the medium. Repeated beating The RFA output radiation is frequency less than a few tens of MHz lead to low effects between the generated photons at doubled in a commercially available, com SBS thresholds: a standard RFA at 1178 nm different frequencies create strong line pact resonant cavity producing a 589-nm is limited to output powers of only 2–4 W! broadening. In optical amplifiers, this effect beam. Second harmonic generation Valuable experience was gained with leads to a mixing of the signal with optical mitigation of SBS during the LGSF project, noise and hence broadens the laser line. (SHG), or frequency doubling, is a para- where SBS can occur in the 27.5-metre Counter-propagating the pump laser limits metric nonlinear process by which, fibre relay to transfer the Parsec dye-laser FWM effects in the RFA, and that is the using suitable nonlinear crystals, two pho- beam from the optical bench to the launch solution adopted. FWM is a phase-sensitive tons are combined into one with twice telescope, and we developed special process and can be effectively suppressed the energy, or half the wavelength, of each photonics crystal fibres to suppress SBS by a phase mismatch between the photons of the fundamental frequency photons. (Hackenberg et al., 1999). In the RFAs at different wavelengths (Boyd, 2003). A schematic of the 589-nm laser is shown described in this article, we employ novel in Figure 4. All subsystems of the lasers ESO-proprietary SBS suppression tech- except the RFA are commercially available niques that push up the SBS threshold by off-the-shelf. A commercially available more than an order of magnitude. 1178-nm seed, frequency stabilised by a wavemeter with an absolute error of less than 10 MHz, feeds, via a single-mode fibre, an 1178-nm high power RFA, whose output is frequency doubled in a compact SHG resonant cavity containing an +@RDQ2HFM@K LBO nonlinear crystal. The 1178-nm narrowband RFA source has been the core of our research. 1DkDBSDC 2SNJDR+HFGSV@UD BNTRSHB6@UD Nonlinear effects such as Stimulated Brillouin scattering (SBS) and four-wave mixing (FWM) (see insert) were limiting the laser linewidth to tens of GHz for the Figure 3. Schematic illustration of the SBS effect the forward signal photons to create phonons, powers of interest, while we aimed at is shown. In the fibre core, the forward signal sending back and amplifying a fraction of the a few MHz laser linewidth. The invention is amplified via SRS. The travelling acoustic wave lower energy SBS “Stokes” photons, shifted in created by the interaction of the photons with wavelength to the red. The SBS at high powers achieved at ESO, and being patented, the acoustic phonons creates a periodic refrac- can be very effective and send back > 99 % of uses SBS suppression methods, pushing tive index pattern, which extracts energy from the forward signal. the SBS threshold power up by an order of magnitude. The FWM is limited by the properties of the fibres and by counter-propagating pump and radiation signals. As a consequence, very little line broadening is observed in the optical am %QDPTDMBX .OSHB@K6@UDLDSDQ plifier, even at full power. "NMSQNKKDQ l,'Y The high-power fibre components needed for the RFA at 1178 nm had to be pro +@RDQ"NMSQNKKDQ "@UHSX"NMSQNKKDQ gressively developed with the laser indus- 1DRNM@MS"@UHSX/G@RD+NBJHMF2HFM@K1% try, such as in-line wavelength division multiplexers, free-space isolators, in-fibre isolators, couplers and high-power 1120- 2DLHBNMCTBSNQ 2,%HAQD .OSHB@K%HAQD 2,%HAQD 1DRNM@MS2DBNMC nm polarisation-maintaining (PM) pumps. +@RDQ#HNCD 1@L@M LOKHjDQ '@QLNMHB MLL6 6 "@UHSX+!. To minimise the R&D risk, we have furthermore followed two paths in parallel: .TSOTSML an in-house development of RFA based 6 on fibres that do not maintain optical po larisation (non-PM); and, in parallel, via Figure 4. A schematic of the 589-nm contracts with industry, the development fibre laser and its control is shown. of RFA based on PM fibres. Both amplifier

The Messenger 139 – March 2010 15 Telescopes and Instrumentation Bonaccini Calia D. et al., Laser Development for Sodium Laser Guide Stars at ESO

.TSOTS!D@L 2HMFKD1% NTSOTS/NVDQ@MC2!2RHFM@K ML

6 MLK@RDQ !@MCVHCSG ,'Y

/TLO %HADQ+@RDQ ML

1DSTQMKHFGS6

.OSHB@K%HAQD ML/NVDQ6 /ML 2!2QDSTQM LOKHjDQ2ONNK 2DDC+@RDQ ML ML/TLOONVDQ6

Figure 5. The layout of the Raman fibre amplifier is Raman amplifier results Figure 6. The 1178-nm RFA output power as a func- shown. The output from a high power pump fibre tion of the pump power at 1120 nm is shown. The laser at 1121 nm is coupled via a fused glass optical maximum value obtained is 39 W. The onset of SBS fibre coupler into a fibre spool, where it amplifies We have progressively increased the (black curve, right axis) is seen as return light going the output of a low power seed laser at 1178 nm by achieved RFA power once the right meth- back toward the optically isolated seed source. The the nonlinear optical process of Stimulated Raman ods to overcome SBS were found, RFA optical conversion efficiency is 28 % and its Scattering. following the progressive availability of the wall-plug efficiency 5%. necessary fibre components. From 4 W technologies have different pros and output power at 1178 nm in November cons for the RFA and pose different risks. 2007, we moved to 39 W in August 2009, Today we can say that both approaches maintaining a linewidth below 1.5 MHz have been highly successful, meeting and an all-fibre system. In August 2009 record power and intensity output from a and exceeding their goal targets. PM RFA we had reached 39 W CW with a single narrowband RFA at spectral power densi- solutions are to be preferred because non-PM RFA system developed in-house, ties well in excess of those normally toler- they are simpler to implement and are together with a novel 150-W fibre laser able in non-SBS-suppressed systems. thus becoming commercially available. pump at 1120 nm (Feng et al., 2009); see Figure 6. Using an adaptation of the The spectral properties of the RFA Besides developing the RFA and inte technology developed at ESO, MPBC Inc. output are shown in Figure 7, which indi- grating them in the laser system at our in Canada had produced 44 W with a cates a very clean spectrum with more labs, we have explored the scalability single PM RFA by the end of 2009. These than 45 dB emission above amplified of power, successfully developing coher- RFAs give ample margin to reach 20 W spontaneous emission and a linewidth of ent beam combination (CBC) schemes. at 589 nm — the laser power specifi below 1.5 MHz, measured at 39 W. In the following sections we report the cation for the AOF with four LGS and the The 1178-nm laser beam is collimated and results obtained with the single RFA and E-ELT LGS system — with adequate mode-matched to a compact resonant with coherent beam combination. linewidth. These results represent both cavity (Figure 8). Frequency doubling

1% $LHRRHNM+HMD@SML 1% $LHRRHNM2ODBSQTL l

l Figure 7. RFA emission linewidth, measured %6', ,'Y with a scanning Fabry Perot (left, linear plot), (MSDMRHSXC! l and an Ando spectrum analyser (right, log plot)

(MSDMRHSX@QA TMHS are displayed. The right plot shows that there is a single laser emission line, no residual l pump signal at 1120 nm l l l 6@UDKDMFSGML present and no second %QDPTDMBXNEERDS,'Y order SRS Stokes.

16 The Messenger 139 – March 2010 Figure 8. Layout of the is performed by slightly modifying a com- ,R mercially available SHG unit. The SHG RFA and the 589-nm ,R SHG resonant cavity is is a very compact bow-tie cavity config shown. uration with a 30-mm long LBO nonlinear crystal (see Figure 10) and a control λ # λ system based on the Pound–Drever–Hall ML technique. LBO is well known to be ML # /TLO 1% (RNK@SNQ able to handle very high laser powers +DMR without lifetime issues. Optical conversion efficiencies up to 86 % have been achieved (Taylor et al., 2009), thanks both (RNK@SNQ to the diffraction-limited beam quality +!. of the single mode RFA (ensuring a good #NTAKHMF"@UHSX mode-matching capability) and the RFA low intensity/phase noise behaviour. 2DDC 28 W CW at 589 nm have been obtained , , /HDYN (Feng et al., 2009) with a single RFA and R R SHG in September 2009 (Figure 9 and 10), with excellent beam wavefront quality.

MLK@RDQONVDQ@MCBNMUDQRHNMDEjBHDMBX

ML/NVDQ6

"NMUDQRHNM$EjBHDMBX MLK@RDQONVDQ "NMUDQRHNM$EjBHDMBX ML/NVDQ6

Figure 9. Plots of laser output power at 589 nm (yel- Figure 10. Photograph of the compact bow-tie SHG low curve, left axis) and SHG efficiency (black curve, cavity with the 30-mm LBO crystal mounted in its right axis) are shown as a function of the 1178-nm temperature controlled oven (centre). The 1178-nm power entering the SHG cavity, obtained with the laser beam enters from the left side into the crystal; single RFA. the generated thin yellow beam is visible to the right, exiting the crystal.

%6', ,'Y

(MSDMRHSXKHM RB@KD @QA TMHSR Figure 11. Left: The output laser beam intensity profile taken at 589 nm in high power operation is shown. The wavefront error measured with an interferometer is 11 nm rms. Right: line l l l profile at high power, measured with %QDPTDMBXNEERDS,'Y the optical spectrum analyser.

The Messenger 139 – March 2010 17 Telescopes and Instrumentation Bonaccini Calia D. et al., Laser Development for Sodium Laser Guide Stars at ESO

The output beam quality has been meas- "!" ured at high power using a Phasics SID4 "NMSQNKKDQ interferometer. The wavefront error meas- /GNSNCHNCD ured over the 1/e2 diameter was below /G@RD 0.018 waves, or 11 nm rms (Figure 11), ,NCTK@SNQ , well below the 70 nm rms specification. 1@L@M 1DRNM@MS LOKHjDQ 2'& ML "@UHSX ,@RSDQ 2,%HAQD Power scaling .TSOTSML 2OKHSSDQ 1@L@M LOKHjDQ , During the development there was the risk that a single RFA would not reach suf ficient power levels. In order to mitigate Figure 12. Schematic layout of the power-scaling the CBC allows constructive interference to be this risk, we developed coherent beam using free-space coherent beam combination based kept in the 50/50 splitter M2, in the direction of the on non-polarising maintaining RFA is shown. A SHG cavity. combination (CBC) of 1178-nm laser servo loop controlling the phase of one RFA arm of beams. By coherently combining the output beams of different RFAs, the power can be scaled. We have demonstrated that two or more RFAs of equal power can be coherently combined at near-unity MLNTSOTS@MC2'&DEjBHDMBXVHSG BG@MMDK"!" Figure 13. Power-scaling of RFAs combination efficiency, in free space using two cascaded coherent beam combinations is shown. In this setup using bulk optics (see Figure 12) via the three non-PM RFAs have been com- colinear interference technique, or bined in sequence via free-space co directly in-fibre without free-space laser linear CBC, obtaining, after the optical beams. We employed a phase control isolator, power of more than 60 W at 1178 nm, with 93 % CBC efficiency. loop acting on a fibre stretcher on one of The CBC output at 1178 nm is used the RFAs and a 50/50 beam splitter. The /NVDQ to feed the SHG cavity. The 589 nm power obtained is shown by the yellow loop controls the piston term of the 2'&DEjBHDMBX wavefront phase in the fibres at a band- MLONVDQ6 2'&DEjBHDMBX curve, related to the left axis. The corresponding SHG conversion effi- width of several tens of kHz. A woofer/ ciency is shown by the black squares, tweeter technique was used, cascading given by the right axis. two different fibre stretchers, to com- bine ample phase range to cope with the large phase changes during the laser MLONVDQ6 warm-up with high bandwidth.

CBC has been demonstrated in-house both with bulk optics and, via research August 2009 we demonstrated more than Outlook contracts with industry, in-fibre (in-line), 60 W at 1178 nm, optically isolated and using 50/50 fibre splitter components polarised, from a three-arm free-space The 589-nm laser research and develop- (couplers). It has thus been demonstrated cascaded CBC system based on non-PM ment programme at ESO has made for the first time that CBC with narrow- RFAs (see Figure 13). This is the maxi- great progress to support the goal of im band RFAs is possible and indeed mum narrowband power at 1178 nm pro- plementing a second generation of laser extremely efficient. We note that this result duced so far via CBC. technology for multiple laser guide star is very encouraging and applicable to AO systems. We have designed, built, a wide realm of laser light combination. Thus we have demonstrated that reliable and demonstrated novel narrowband, RFA power-scaling is possible and can CW high-power Raman fibre amplifiers We have used CBC with stable output be efficiently achieved with CBC, even (RFAs) feeding compact 589-nm laser power and efficiencies from 93 % to with cascaded CBC systems. The CBC heads, attacking and solving some fairly > 97 % with both PM and non-PM RFAs loop controller electronics and the in-fibre fundamental laser technology issues. and with two and three CBC channels. phase actuators are commercially avail There has been steady progress in terms CBC with PM RFA uses in-fibre 50/50 able as off-the-shelf components. With the of laser output power in the past two couplers (in-fibre beam splitter). Since the three-way CBC at 1178 nm via SHG, years for both 1178-nm RFAs and lasers power scaling is done all in-fibre, the we have reached 50.9 W CW at 589 nm at 589 nm. It has been demonstrated that setup is extremely compact and there are (Taylor et al., 2010), with more than 85 % 589-nm lasers based on a novel RFA, no optics to align. With two-channel CBC peak conversion efficiency and a laser that suppresses stimulated Brillouin scat- systems we have consistently demon- linewidth below 2.3 MHz (see Figure 14). tering, can deliver the required power strated more than 30 W CW at 1178 nm, We have observed very stable perform- and spectral formats to meet the needs optically isolated and polarised. In ance of this system. of the next generation multiple laser guide

18 The Messenger 139 – March 2010 non-PM fibre setups, in different configu- community much more attractive options rations. The demonstrations have been for the supply and deployment of reliable, done both in-fibre and with colinear compact, next generation lasers for beams in free space, obtaining CBC effi- LGS–AO, suited for operation at astro- ciencies up to 97 %. The three-beam, nomical observatories. free-space, cascaded CBC produced a laser beam close to 60 W CW at 1178 nm. This laser beam has been mode-matched Acknowledgements to the SHG cavity, and we have dem We sincerely acknowledge Guy Monnet, who onstrated more than 85 % peak conver- as Division head very much encouraged and sup- sion efficiency and laser powers up to ported the start of the laser development, as well 50.9 W at 589 nm. This is the highest as Roberto Gilmozzi, Martin Cullum and Roberto laser power published so far for narrow- Tamai, all of whom fostered our continuing work. We also acknowledge the technical contributions to the band CW fibre lasers at 589 nm. From laser laboratory activities by Ivan Guidolin, Bernard a laser research perspective, a natural Buzzoni and Jean-Paul Kirchbauer. continuation of the team activities would be to investigate the feasibility of the References pulsed laser format for “LGS spot-track- ing”, to cover potential long-term advan- Ageorges, N. & Dainty, C. 2000, Laser Guide Star tages for adaptive optics systems. Adaptive Optics for Astronomy, Kluwer Agrawal, G. P. 2001, Non Linear Fibre Optics, Academic Press In the final year of the programme we Arsenault, R. et al. 2006, The Messenger, 123, 6 have made, and continue to make, these Beckers, J. M. 1992, Appl. Optics, 31, 6592 developments available to the laser Beletic, J. W. et al. 2005, Experimental Astronomy, industry, and an industrial consortium 19, 103 Bonaccini Calia, D. et al. 2003, Proc. SPIE, 4839, Figure 14. 50.9 W CW at 589 nm obtained from the has independently demonstrated a 20 W 381 three-way free-space CBC, as shown by the display. class 589-nm laser based on narrow- Bonaccini Calia, D. et al. 2006, Proc. SPIE, 6272, band RFA technology. Thanks to the in 627 herent wavelength flexibility of the Raman Boyd, W. R. 2003, Nonlinear Optics, Academic Press (2nd ed.) star facility. Moreover, the ESO Laser effect, high-power lasers may also Drummond, J. et al. 2004, PASP, 116, 278 Systems Department of the ESO Tech- become available at wavelengths inac- Feng, Y. et al. 2008, Optics Express, 16, 10927 nology Division has achieved a world cessible today, for different applications Feng, Y. et al. 2009, Optics Express, 17, 19021 record in terms of power output of nar- in astronomy and elsewhere, for example Feng, Y. et al. 2009, Optics Express, 17, 23678 Hackenberg, W. et al. 1999, The Messenger, 98, 14 rowband RFAs, inventing novel tech- in the life and geophysical sciences. Holzlöhner, R. et al. 2010, A&A, 510, A 20 niques to overcome the undesired non The lasers demonstrate power levels and Kibblewhite, E. 2008, Proc. SPIE, 7015, 70150M-1 linear effects in the RFA. beam parameters that meet or exceed Milonni, P. W. et al. 1998, JOSA A, 15, 217 the AOF requirements and are relevant to Rabien, S. et al. 2003, Proc. SPIE, 4839, 393 Taylor, L. R. et al. 2009, Optics Express, 17, 14687 We have further demonstrated laser the E-ELT baseline laser needs. The Taylor, L. R. et al. 2010, Optics Express, accepted power scalability via coherent beam research and development programme combination, using RFAs with PM and has given ESO and the astronomical

An example of adaptive optics in action is shown, but using natural guide stars. This near-infrared image of the dust-obscured Galactic Bulge globular cluster Terzan 5 was formed from J and K images obtained with the Multi-conjugate Adaptive Optics Demonstrator (MAD) instrument on the VLT. The field of view is 40 arcseconds. See ESO Press Release eso0945 for more details.

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