Multi-Watt 589-nm Na D 2 -line Generation via Frequency Doubling of a Raman Fibre Amplifier: A source for LGS-assisted AO.

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1 Multi-Watt 589-nm Na D 2 -line Generation via Frequency Doubling of a Raman Fibre Amplifier: A source for LGS-assisted AO L. Taylor, Y. Feng, D. Bonaccini Calia, W. Hackenberg European Southern Observatory, Karl-Schwarzschild-Str. 2, Garching b. München, Germany Abstract We develop a novel solid state fibre laser system, AFIRE, for the purposes of laser guidestar (LGS) assisted adaptive optics (AO), based on the second harmonic generation (SHG) from a highpower (P 1178 ~25W) CW narrowband ( ν < 3GHz) Raman fibre amplifier developed by IPF. We present what we believe to be the highest power, narrowband single-pass CW 589nm SHG result reported to date, P 589 ~ 4.2W from P 1178 ~ 19W (η VIS > 22%). We demonstrate our understanding of the arising absorption-induced thermal effects (namely, dephasing and degradation of the conversion), offer predictions towards higher powers and conversion levels, and show that our current results are essentially pump-power limited. We are confident of the scalability of both the IR and visible parts of our system, to these higher output powers and conversion efficiencies. Keywords: Second harmonic generation, SHG, Sodium Laser Guidestar, LGS, Periodically Poled Material, PPKTP, PPSLT 1- Background and Introduction The use of Laser Guide Stars for Adaptive Optics has started to be routinely employed to enlarge by ~8% the sky coverage of Adaptive Optics systems associated to Near Infrared instrumentation in modern telescopes. LGS based on the mesospheric sodium backscatter require lasers with linewidths ν < 2 GHz at 589nm, either CW, QCW or pulsed. The equivalent powers in air required for astronomical AO are 6-12 W for CW and QCW formats. As there are no solid state materials directly lasing at 589nm, non linear effects such as Sum Frequency Generation and Frequency Doubling (SHG) are used to convert the frequencies of supporting pump lasers. Sum Frequency schemes combining 164 and 1319 nm have been used successfully for freespace solid state systems, having either an external sum frequency cavity based on BBO [1] or a Periodically Poled Non Linear Crystal (PPNLC) [2]. Advances in Adaptive Optics II, edited by Brent L. Ellerbroek, Domenico Bonaccini Calia, Proc. of SPIE Vol. 6272, , (26) X/6/$15 doi: / Proc. of SPIE Vol

2 A sum frequency scheme combining 1583 and 938nm fibre lasers is under development in a collaborative effort between LLNL and ESO, using a PPNLC. Visible output powers around P 589 ~ 3W have so far been produced from this work [3]. A Raman conversion Fibre Laser at 1178nm is under development at ESO under the acronym of AFIRE, which uses a PPNLC to frequency double at 589nm (see dedicated article in these Proceedings) [4]. Also Direct conversion (532 to 589nm) Fibre Raman Lasers are being explored at ESO with support from PPARC [5]. ESO has the goal to use the AFIRE in the upgrade of the LGSF to 4LGSF foreseen by 211, as part of the Adaptive Optics Facility project [6] Experience has shown in the systems at Lick Observatory, Keck and ESO, that dye lasers are practicable 589nm sources. However they are difficult to operate and maintain in remote locations, and their use require substantial technical support. This is a major issue in telescopes instrumentation. Free space resonators with solid state components are the next step in delivering a stable laser system, and recently the SOR and the Gemini observatories have implemented them successfully. There is no doubt that in the long term fibre lasers, if available, should be used in the telescope environments for the LGS systems. There are numerous advantages besides their much lower cost, including turn-key operation, rack-mounted system, virtually maintenance free and alignment free operation, delivery of the diffraction limited laser beam at the Launch Telescope focal plane via the fibre laser itself. The PPNLC are special crystals which in single pass can produce the desired frequency conversion. Their performance at high power is crucial for the route of sum frequency or frequency doubling lasers and we are pursuing them both experimentally and theoretically. In the following we report our experience with several such crystals under test at the ESO laser lab, for frequency doubling of the 1178nm CW fibre Raman laser source from IPF Technologies. The work is ongoing at the time of this writing. 2- Theory We use a Periodically Poled Non Linear Crystal (PPNLC) for the frequency up-conversion of the IR pump lasers to visible wavelengths. The conversion process relies on the second-order nonlinear susceptibility of the material, following which the propagating (fundamental, or IR) wave induces a nonlinear polarisation term. This in turn allows the excitation of higher-energy waves, which can be enhanced under certain conditions. The periodic inversion of the nonlinear coefficient allows, by design, a strong enhancement of the selected conversion, relative to the conversion in the non-poled bulk material. We use a single-period grating, the periodicity of which is governed by the selected propagating wavelengths and desired operating temperature. The sample is thermally controlled to fine-tune the conversion Proc. of SPIE Vol

3 via the thermal dependence of the dispersion. We aim to minimise the phase mismatch between the propagating IR and visible beams at each domain boundary, in order to optimise the conversion. Starting from Maxwell s equations for the propagation of waves in a dielectric and loss-less medium, and using the planar wavefront approximation, we can establish a set of coupled amplitude equations for the propagating equations. These can be converted to a similar set of coupled intensity equations, which can be solved for the specific case of frequency doubling to yield, in the weak conversion regime: ( ) 2 Ishg := Γ 2 L 2 I sinc kl fundamental 2 2 (equation 1) The term k represents the phase mismatch between the propagating beams, integrated up until the output facet of the sample, of length L. In the case of strong conversion, we can no longer assume the incident IR beam is undepleted with propagation through the sample, and so we solve the same coupled intensity equations to obtain: ( ( )) 2 Ishg() z := I tanh Γ I z (equation 2) In both cases, we use the simplification: Γ 2 := ( ) 2 4 d ω 2 eff ( ) 2 c 3 ε n fundamental n shg (equation 3) The factor d eff is the nonlinear coefficient of the material; this coefficient directly determines the quantity of output visible power for given focussing conditions and incident pump powers. These equations imply that in the weak conversion regime, the conversion efficiency scales with the local intensity of the incident pump beam, and with the length over which we can maintain the intensity of the illumination. A trade-off between these two terms appears (as described by Boyd and Kleinman [7]), following which we can determine the optimum focussing conditions within the ideal sample, for best conversion. It follows from this analysis that these ideal conditions are satisfied when the confocal range is approximately one third of the sample length, specifically, 2z R ~ L sample /2.84. Assuming ideal phase matching and zero absorption, we can expect near-complete conversion to the visible, following equation Experimental We use a first generation narrowband Raman shifted 1178nm CW fibre amplifier as fundamental for the second harmonic conversion. This source was made by IPF, and is based on a master oscillator power amplifier (MOPA) configuration. It must be said that a second generation system with even narrower linewidth is being produced by the company Volius. Precise details regarding this source are given elsewhere in these proceedings [4]. Proc. of SPIE Vol

4 The beam quality at the output of the amplifier is M ~ 1.7. The output power and pointing stabilities are measured to be excellent, once thermal stabilisation of the system has occurred. A schematic of the SHG experimental set-up is given in figure 1. irpm I vispm half-wave coffimating clualter-wave PBS fibre output lensourl PP matenal P ellin-bro c a U collimating lens U focussing half-wave BR lens Figure 1: Schematic of the experimental set-up for the frequency up-conversion. A fibre collimator was provided with the 1178nm Raman amplifier system, but we use an additional 1mm collimating lens to further reduce the divergence of the 1178nm beam to less than.3mrad. The output polarisation is controlled using a quarter- and half-waveplate pair, and we are able to use the reflected part from a polarising beamsplitter cube for the conversion. This set-up has the advantage of being able to vary the useful power for the conversion, whilst maintaining the laser properties. A second folding mirror is used before again controlling the axis of polarisation using a halfwaveplate, adjusting the polarisation to the particular axis of the sample. We focus the beam through the selected periodically poled sample (PPKTP or PPMgOSLT in the cases studied here), collimate the output from the sample and spatially split the fundamental from the SHG output using a Brewster AOI prism. We are able to separately measure the overall output from the sample, or the power contained within either visible or IR output beams. All optics and samples are suitably AR coated, and we are typically able to vary the resultant focal radii over the range 2-4µm. We confirm (using the knife-edge scanning technique) that the propagation of the beam through focus matches our predictions (in air). 4- Results We study the conversion over a range of focussing conditions and in various samples. Pieces of different lengths are also considered, based on the results of Louchev et al. [8]. We use samples of both PPKTP and PPMgOSLT. Work is ongoing at the time of this writing and further optimisation of the conversion remains; current results are shown in figures 2 and 3. Proc. of SPIE Vol

5 Pvis (W) mm PPKTP data and fits Quadratic, ideal Pump depleted, ideal efficiency to vis (W) mm PPKTP data and fits Linear, ideal Pump depleted, ideal Figure 2: Conversion (power, efficiency) as function of incident pump power, 2mm PPKTP mm PPMgOSLT data and fits 2 2mm PPMgOSLT data and fits Pvis (W) efficiency to vis (W) Quadratic, ideal Pump depleted, ideal Linear, ideal Pump depleted, ideal Figure 3: Conversion (power, efficiency) as function of incident pump power, 2mm PPMgOSLT These results represent what we believe to be the highest reported CW single-pass 589nm SHG to date. We understand that the slight non-uniformity in the power curves, around P 589 ~ 3.25W, comes from a slight calibration discrepancy between our detectors (a different detector was used for the measurements, P 589 3W). We expect the true high-power values are in fact a little higher than those we have to report here. Sharma [9] claims the highest single-pass SHG to 59nm in a PPMgOLN sample until October 24 (1.52W 59nm, with 7.7% conversion and ±1.3% power instabilities over a 2hour period), and Georgiev et al. [1] achieve 3.3W from 23W 1178nm (broadband) 1178nm, also CW and with a source manufactured by IPG. Dawson et al. [11] demonstrate up to 2.7W CW 589nm SFG (from 18W combined 938nm and 1583nm powers), increasing this value to 3.5W under pulsed Proc. of SPIE Vol

6 conditions (19W combined IR powers). The values presented in this paper are at both higher powers and increased efficiencies, relative to other reported results. We observe a roll-off in the conversion at the higher power and conversion levels. This becomes evident, as we compare the data to the ideal quadratic conversion and (hyperbolic) tangential pump-depleted predictions plotted on the same graphs (weak- and strong-conversion regimes respectively, c.f. equations 1 and 2). These high-power predictions are based on fits to the low-power, low-conversion data (η VIS < 1%). It becomes obvious that the ideal quadratic dependence of the conversion breaks down as the pump depletion becomes significant (typically for η VIS > 5%), but that some additional effects force an additional roll-off in the conversion, past the lower predictions offered by the pump-depleted conversion curves (shown in black in figures 2 and 3, and for η VIS > 1%). We initially assumed a loss-less material, for the derivation of the governing equations. We know that the absorption of the samples is non-zero and increases typically with decreasing wavelength. This is illustrated in figure Absorbance Transmission [%] Absorbance Transmission [%] Wavelength [nm] Wavelength [nm] Figure 4: Absorption and transmission measurements for the PPKTP and PPMgOSLT samples (LHS and RHS respectively) The relative variations (near oscillatory behaviour of the transmission of the PPMgOSLT vs. the PPKTP) in figure 4 arise from the different AR coatings used on the samples. The measured throughput values are considered representative of the samples properties at the coating design wavelengths (589nm and 1178nm respectively), in the case of the PPMgOSLT. At other wavelengths, we accept that the spectral absorption curves represent a convolution of the material and coating properties. The absorption of these samples is of the order of a fraction of a percent at 1178nm, increasing to 2-5% in the visible. We suspect this absorption leads to a complex cascaded process forcing a degradation in the conversion levels at the higher intensities; the localised heating, conversion and power generation profiles are simultaneously linked. Work has been started on understanding this complex problem [8, 12]), and is continued within the framework of this research. Proc. of SPIE Vol

7 Toven for best Pvis (C) Oven temp for best conversion, PPKTP Toven for best Pvis (C) Oven temp for best conversion, PPMgOSLT Figure 5: Required thermal de-tuning of the oven controller to maintain best conversion, as a function of incident pump power A link between the thermal loading of the sample and the roll-off in the conversion becomes apparent, at the η VIS ~ 1% conversion level as shown in figures 2 and 3. This result is supported by figure 5, presenting the required thermal retuning of the oven controller as a function of incident pump power. The idea is further reinforced, considering a comparison with the thermal acceptance width of the sample under typical operating conditions, as shown in figure 6. norm Pvis (A.U.) Fit 2mm PPKTP, 19um focus temperature (degc) norm Pvis (A.U.) Fit 2mm PPMgOSLT, 31um focus temperature (C) Figure 6: Thermal tuning curves for the two samples, under different focussing conditions. Effectively, we see that for any re-tuning, T.5 FWHM of the thermal tuning curve, little roll-off of the conversion is observed. Only for greater values of T do we see a significant degradation of the conversion; this occurs typically at η VIS ~ 1%, or around P 1178 ~ 8W under these experimental conditions (equivalent T ~ 2 C for typically thermal acceptances of 3-4 C as shown in figures 5 and 6). For higher pump powers, we see an approximate linear increase in the required retuning T, consistent with the linear dependence of the heating on absorption as given by Innocenzi et al. [13], and an associated degradation in the conversion efficiency. The theoretical Proc. of SPIE Vol

8 modelling associated with the thermal dephasing and degradation of the conversion is currently ongoing. A direct consequence of this absorption, localised heating as a function of intensity, and thermal dephasing and degradation of the conversion, is a variation in the optimum focussing conditions away from those predicted for conversion in a loss-less medium, assuming planar wavefront propagation of the beams. Effectively, a further complication to the focussing and conversion trade-off considered by Boyd and Kleinman, involves the localised absorption of the sample, thermal conductivity and thermal acceptance of the sample for conversion. We are currently investigating this balance on a case-by-case basis, for a number of samples of PPKTP and PPMgOSLT. We are also investigating the temporal stability of the conversion under these conditions. 5- Discussion and Conclusions We recall the basic theory for second harmonic generation in the weak- and strongconversion regimes in ideal materials. We see that focusing the beam through the sample will increase the local IR pump intensity, in turn increasing the conversion efficiency at this point. A trade-off between a tight focus, and the length over which this tight focus can be maintained, is shown to lead to an ideal set of focussing conditions for best conversion (2z R ~ L sample /2.84). We investigate the SHG to 589nm under CW conditions in PPKTP and PPMgOSLT, and present our results. Figures 2 and 3 indicate that P 589 ~ 4.2W has been achieved under single-pass SHG from 1178nm. Typical conversion levels of the order of 2% have been easily achieved; we expect to be able to at least maintain, if not improve, these efficiencies with increasing pump power. Effectively, the conversion in the studied samples has not yet been fully optimised at the highest power levels. The absorption of the sample (both linear and non-linear) is shown to affect the previously established optimum focussing conditions for best conversion, via thermal dephasing. This is illustrated in particular through the heating of the sample, forcing the local temperature of the sample outside of the thermal acceptance bandwidth for conversion. A further trade-off appears here, suggesting that slightly non-ideal poling (and thus, a wider thermal acceptance bandwidth for conversion), might be beneficial for high-power, high efficiency, single pass CW SHG conversion. We expect a required relaxation of the ideal focussing conditions, mainly varying with the absorption and thermal conductivity of the sample. The results presented here are to our knowledge, the highest single-pass CW 589nm SHG power levels and efficiencies reported to date. Our vendor guarantees a stable linearly polarised 3W-class 1178nm (equally narrowband) Raman shifted fibre amplifier, within the shortest timescales. Using this source, and based on the above results, we expect to easily (and conservatively) achieve 5W CW narrowband 589nm radiation. Plans exist to polarisation combine two such sources, to result in a 1W 589nm CW source suitable for the purposes of LGS assisted Proc. of SPIE Vol

9 AO. In conclusion, we are confident that a 1W class CW 589nm narrowband source, satisfying the specifications of a source suitable for LGS assisted AO, can be achieved. References 1. Bienfang, Denman, Grime, Hillman, Moore and Telle, 2W of continuous wave sodium D2 resonance radiation from sum-frequency generation with injection locked lasers, Optics Letters, vol.28, no. 22, pp , Bamford, Sharpe, Cook, Tracy, Lopez, High-average-power visible generation in periodicallypoled nearly-stoichiometric Lithium Tantalate, 25 CLEO/QELS (Baltimore, MD) 3. Pennington, Beach, Dawson, Drobshoff, Payne, Bonaccini, Hackenberg and Taylor, Compact fibre laser for 589nm laser guide star generation, CLEO Europe, June D. Bonaccini Calia, W. Hackenberg, S. Chernikov, Y. Feng and L. Taylor: AFIRE: fiber Raman laser for laser guide star adaptive optics (in these proceedings) 5. Feng et al., Design of a narrow-band 589nm laser by direct Raman shift in single-mode fibre, (in these proceedings) 6. S. Stroebele, R. Arsenault, D. Bonaccini-Calia, R. D. Conzelmann, B. Delabre, R. Donaldson, M. Duchateau, E. Fedrigo, W. K. P. Hackenberg, N. Hubin, M. Le Louarn, S. Oberti, E. Vernet, S. Esposito, R. Stuik: The ESO Adaptive Optics Facility (in these proceedings) 7. Boyd and Kleinman, Parametric interaction of focused Gaussian light beams, J Appl Phys, 39, 8, 3597, Louchev et al., Thermal inhibition of high-power second-harmonic generation in periodically poled LiNbO 3 and LiTaO 3 crystals, Appl Phys Lett, 87, 13111, Sharma, 1.52W frequency-doubled fibre based continuous wave orange laser radiation at 59nm, Rev Laser Eng, 33, 2, 13, Georgiev et al., Watts-level frequency doubling of a narrow line linearly polarized Raman fibre laser to 589nm, Opt express, 13, 6772, Dawson et al., Multi-Watt 589nm fibre laser source, Proc SPIE 612, 6121F-1, Liao et al., Thermally induced dephasing in periodically poled KTP frequency-doubling crystals, JOSA B, 21, 12, Innocenzi et al., Thermal modeling of continuous-wave end-pumped solid-state lasers, Appl Phys Lett, 56, 7, 1831, 199 Proc. of SPIE Vol