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absorption cross section of Yb3+ doped yttrium lanthanum oxide (YLO) ceramic and its application in diode pumped amplifier. Saumyabrata Banerjee,1,* Joerg ...
Temperature dependent emission and absorption cross section of Yb3+ doped yttrium lanthanum oxide (YLO) ceramic and its application in diode pumped amplifier Saumyabrata Banerjee,1,* Joerg Koerner,2 Mathias Siebold,3 Qiuhong Yang,4 Klaus Ertel,1 Paul D. Mason1, P. Jonathan Phillips,1 Markus Loeser,3,6 Haojia Zhang,4 Shenzhou Lu,4 Joachim Hein,2 Ulrich Schramm,3 Malte C. Kaluza,2,5 and John L. Collier1 1 Central Laser Facility, STFC Rutherford Appleton Laboratory, Didcot, OX11 0QX, UK Institute of Optics and Quantum Electronics, Friedrich Schiller University of Jena, Germany 3 Helmholtz Center Dresden-Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany 4 School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China 5 Helmholtz-Institute Jena, Fröbelstieg 3, 07743 Jena, Germany 6 TU Dresden, 01062 Dresden, Germany *[email protected] 2

Abstract: Temperature dependent absorption and emission cross-sections of 5at% Yb3+ doped yttrium lanthanum oxide (Yb:YLO) ceramic between 80K and 300K are presented. In addition, we report on the first demonstration of ns pulse amplification in Yb:YLO ceramic. A pulse energy of 102mJ was extracted from a multi-pass amplifier setup. The amplification bandwidth at room temperature confirms the potential of Yb:YLO ceramic for broad bandwidth amplification at cryogenic temperatures. ©2013 Optical Society of America OCIS codes: (140.0140) Lasers and laser optics; (140.3280) Laser amplifiers; (140.3615) Lasers, ytterbium;

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Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A726

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1. Introduction Diode-Pumped Solid State Lasers (DPSSL) and amplifiers producing energetic, highrepetition-rate (~10 Hz) pulses are envisaged to play a major role in the future for high-energy laser-matter interaction applications such as inertial confinement fusion, particle acceleration and generation of pulsed x-rays. Yb-doped host materials are a leading candidate for highenergy DPSSL gain media, owing to their relatively long fluorescence lifetime, low quantum defect, and simple level structure (eliminating effects like exited state absorption). However, the quasi-three-level nature of Yb-doped materials limits their efficiency at room temperature, because of the reabsorption loss caused by the thermal population in the lower lasing level. Although it has been reported that the reabsorption is not a major concern in continuous wave (cw) and high-repetition rate (>>1kHz) systems [1], it plays a significant role in low repetition rate (~Hz) lasers and amplifiers due to the complete decay of the lower laser level population between subsequent pulses. Efficient high-power cw and ultrashort pulsed laser operation has been realized in Yb-doped materials, such as Yb:YAG [2],Yb:Gd2SiO5 [3,4], Yb-doped fibres [5], Yb-doped glass [6], and Yb-doped ceramics [7–11]. In these investigations, the performance of these Yb-doped materials has proven to be superior to their Nd-doped counterparts. Polycrystalline ceramics have emerged as a potential candidate for high-energy lasers [12] as they can be fabricated in large sizes, whilst retaining large damage thresholds and excellent thermal properties. In particular, ceramic Yb:YAG shows excellent thermal properties with high emission and absorption cross-sections resulting in a lower saturation fluence (~13 J/cm2 at 300 K). The efficiency of Yb-doped systems can be significantly improved by operating at cryogenic temperatures, eliminating reabsorption losses and relaxing the pumping requirements [13]. Moreover, cryogenic operation increases the emission and absorption cross-sections, resulting in a further reduction of the saturation

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Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A727

fluence (~2 J/cm2 at 175K for Yb:YAG) and enhancing the gain. However, a reduction in temperature reduces the gain bandwidth of Yb:YAG, limiting its ability to generate ultrashort pulses and hence its usefulness for high-energy, high-efficiency ultrashort pulse DPSSL amplifiers. Recently, alternative gain materials such as ceramic Yb-doped sesquioxides have gained considerable attention owing to their broader gain bandwidths as well as better thermal properties. Efficient high-power diode-pumped cw lasers and passively mode locked ultrashort pulsed lasers have been demonstrated with polycrystalline Yb:Y2O3 ceramics [14,15]. It has been shown that transparent ceramic Y2O3 can be fabricated at a relatively low sintering temperature of 1700 °C, which is about 700 °C lower than its melting point (2430 °C), by a nanocrystalline and non-press vacuum sintering technology [16]. The sintering temperature can be further decreased by adding La2O3 as a sintering aid in Y2O3 to form transparent ceramics of yttrium lanthanum oxide (Y1−xLax)2O3 (YLO) doped with Yb [17]. Qiang Hao et al. [18] have demonstrated room temperature diode-pumped tuneable cw laser operation of Yb3+ doped ceramic Yb:(Y1−xLax)2O3, x = 0.1. A smooth spectral tuning curve from 1018 nm to 1086 nm was reported, when pumped by diodes emitting at 940 nm, with broad spectral widths of up to 30 nm, showing its potential for application to ultrashort pulse amplification. In order to access the potential of ceramic YLO for efficient, high-energy operation at low temperatures, we have measured the temperature dependency of the absorption and emission cross-sections of a ceramic Yb:YLO sample and have performed ns pulse amplification experiments to determine the gain bandwidth at room temperature. In this paper, we present the absorption and emission cross-section of a 5% (all doping concentrations quoted are given in atomic %) doped Yb:YLO sample from 80 K to 300 K and compare the spectra with a 2% doped ceramic Yb:YAG sample. Furthermore, results from the first amplification experiments of ns pulses at room temperature for ceramic Yb:YLO are also presented. 2. Spectral measurements The chemical composition of the Yb:YLO ceramic sample supplied by the School of Materials Science and Engineering (SMSE), Shanghai University was (Yb0.05Y0.85La0.1)2O3, corresponding to 5% Yb doping. The sample was 11 mm in diameter and 3 mm thick. The emission and absorption spectra of this sample were measured using a Yokogawa ANDO A6315A/B optical spectrum analyzer. A fiber coupled white light source was used to measure the absorption spectra and a 9 W fiber coupled laser diode was used as an excitation source for measuring the fluorescence emission spectra as well as the fluorescence lifetime. The setup described in [19] was used to maximize the fluorescence signal near the zero phonon line, ensuring that the measurement volume was at the surface of the sample, thus minimizing the effect of reabsorption for acquiring the fluorescence measurements. The wavelength dependent absorption cross-sections σa (λ) were obtained by using Beer-Lambert’s law, and the emission cross-sections σe (λ) were determined by using both the reciprocity method (RM) as well as the Fuchtbauer-Ladenburg (FL) equation. Figure 1 shows example results for both approaches and the combined emission cross section along with the absorption cross section for 5% Yb:YLO at room temperature. Since no energy level data is currently available in the literature for Yb:YLO, we use values given in [20] for Yb:Y2O3 instead. As both materials are very similar, the energy levels should be in the same range, to give a sufficient approximation for the partition functions which are given in Table 1. The corresponding levels are [0, 414, 498, 874] cm−1 for the 2F7/2 manifold and [10243, 10651, 11045] cm−1 for the 2F5/2 manifold. A liquid nitrogen cryostat was used to cool the sample to temperatures of 80 K and the procedure described in [19] was used to measure and calculate the absorption and emission cross-section at different temperatures. Table 1 lists the partition functions at different temperatures. The refractive index needed for calculation with the FL formalism was approximated to 1.89 by a value for Y2O3 [20]. The refractive index is assumed to be constant for all temperatures, as the change in refractive index with temperature will be very small #190326 - $15.00 USD (C) 2013 OSA

Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A728

compared to the absolute value and so far have no significant influence on our calculations. The radiative lifetime also needed for FL-method is further used as free parameter to match the results from both calculation values. Figure 2 shows the absorption cross section σa for different temperatures between 80 K to 300 K. The inset shows the temperature dependent change in the bandwidth (FWHM) of the absorption peak at 952 nm. Note that the absorption bandwidth (FWHM) is about 4 nm even at cryogenic temperatures as low as 80 K, which significantly relaxes the bandwidth requirements of the pump diodes for use at these low temperatures. The temperature dependent emission cross-section for 5% doped Yb:YLO is shown in Fig. 3.

Fig. 1. Emission and absorption cross-section calculation for 5at% Yb:YLO at room temperature. Table 1. Partition functions at different temperatures Temp(K) 300 220 140 80

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ZU 2.343 2.160 2.034 2.002

ZL 2.488 2.217 2.041 2.001

ZL/ZU 1.062 1.026 1.003 1.000

Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A729

Fig. 2. Temperature dependent absorption cross-section for 5% doped Yb:YLO ceramic. The inset shows the variation of the absorption bandwidth (FWHM) at 952 nm as a function of temperature.

Fig. 3. Temperature dependent emission cross-section for 5% doped Yb:YLO ceramic. The inset shows the variation of the emission bandwidth (FWHM) at 1031 nm as a function of temperature.

The main emission lines near 1031 nm show broad bandwidth emission and an increase of the emission cross-section by a factor of 2.2 as the temperature is reduced from room temperature (300K) to 80 K. Note that the emission line near 1072 nm also shows broad bandwidth emission even at cryogenic temperatures, however, with only 33% of the crosssection of the 1031 nm peak. We found that in the temperature range 80 K ≤ T ≤ 300 K the temperature dependent emission cross-section for 5% doped Yb:YLO at 1031 nm can be

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Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A730

approximated by a polynomial fit expressed as the following function, where T is in K and σe is in cm2,

s e ( T ) = 4.29711 × 10−21 + 3.73712 × 10−22 * T – 3.43657 × 10−24 * T 2 + 1.14391 × 10−26 * T 3 – 1.33639 × 10−29 * T 4 In Fig. 4 cross sections for Yb:YLO measured at 80K are compared to cross sections for Yb:YAG at room temperature. In these cases both materials show similar features whith regard to bandwidth and peak emission cross section. Due to this, cryogenically cooled Yb:YLO could be used as a substitute material for room temperature Yb:YAG. The advantage for Yb:YLO in this case is that due to cryogenic cooling, reabsorption is not present, which offers highly efficient operation, while in contrast Yb:YAG cannot be used for the amplification of ultra-short pulses at cryogenic temperatures since the bandwidth is significantly reduced. Fluorescence lifetime measurements were performed by reimaging the output of the fiber collecting the light from the sample onto a photodiode with a 1000 nm long pass edge filter. The fluorescence signal was generated by exciting the sample with a fiber coupled laser diode emitting at 970 nm with 1 ms pulse duration and analyzed by fitting the falling edge of the signal to an exponential function. The measured value was (759 ± 20) µs, however, this was approximately 200 µs shorter than the best-fit value of 950 µs determined from the emission cross-section calculations by matching results of MC and FL method. This discrepancy between the measured and the estimated value of the florescence lifetime might be attributable to concentration quenching, interactions with impurities or both, which leads to a shorter lifetime in our sample.

Fig. 4. Absorption and emission cross-sections for 2at% Yb:YAG at 300 K and 5at% Yb:YLO at 80 K.

To assess the optical quality of the ceramic Yb:YLO sample, we illuminated the sample with a diverging visible laser diode (green) and recorded the image on a screen positioned 50 cm behind the sample. Figure 5(a) shows the reference image of the beam without any sample. Figure 5(b) shows the transmitted beam through the Yb:YLO sample and Fig. 5(c) shows the image obtained for a commercially available Yb:YAG ceramic (Konoshima, Japan) #190326 - $15.00 USD (C) 2013 OSA

Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A731

sample. The Yb:YLO sample shows signatures consistent with grain boundaries and scattering centers that were not evident in the Yb:YAG sample. Consequently, further improvements in the manufacturing procedure for ceramic Yb:YLO will be required to remove these inhomogeneities and improve the overall optical quality of the material.

Fig. 5. (a) Image of the reference screen with no sample inserted, (b) is image with Yb:YLO ceramic sample inserted (SMSE, China), (c) is image with commercially available Yb:YAG ceramic sample inserted (Konoshima, Japan) .

3. Amplification experiment

To evaluate the potential of Yb:YLO ceramic for broadband pulse amplification, we performed ns pulse amplification by utilizing a multi-pass amplifier setup seeded by a tuneable Yb:CaF2 oscillator delivering 1 mJ energy per pulse at 10 Hz operation. The oscillator was tuneable from 1023 nm to 1050 nm with a spectral bandwidth of 0.8 nm and pulse duration of 7 ns (FWHM). A schematic diagram of the amplification setup is shown in Fig. 6.

Fig. 6. Schematic diagram of the Yb:YLO amplification experiment.

The 5% Yb:YLO ceramic sample was held by an aluminum holder and was kept in air without any active cooling. The sample was coated for antireflection (AR) at 940 nm as well as for 1030 nm wavelengths and was pumped by a stack of 25 fast axis collimated diode laser bars (Jenoptik Laserdiode GmbH, Germany) with a peak output power of 4 kW and a central wavelength of 940 nm. The near field pump beam profile was imaged onto a single microlens array (SUSS MicroTec AG, Germany) for homogenization. The resulting pump beam profile is shown in Fig. 7(a). A dichroic mirror M1 coated for AR at 940 nm and highreflection (HR) at 1030 nm was used to separate the pump from the seed beam. An image relaying multi-pass architecture [21] was utilized to propagate the seed beam 12 times through the gain medium. Figure 7(b) shows the near field profile of the seed beam.

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Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A732

Fig. 7. (a) Pump profile, (b) seed beam profile, (c) transmitted seed beam profile with no amplification, and (d) amplified beam profile with full pump power.

We first evaluated the spectral gain bandwidth of the Yb:YLO sample by recording the 12-pass gain of the system for input wavelengths ranging from 1023 nm to 1050 nm. The oscillator output was reduced to 40 µJ to avoid damage of the sample. The sample was pumped for duration of 1.3 ms at a wavelength of 938 nm, 14 nm shorter than the absorption peak at 952 nm. Due to this mismatch, only 72% of the pump input was absorbed limiting the overall amplification efficiency. Figure 8(a) shows the evolution of gain with respect to the pump input wavelength. A gain bandwidth (FWHM) of 16 nm was recorded at 1.2 J of absorbed pump, which reduced to 8.2 nm at 2.5 J of absorbed pump. However, compared to a 2-pass amplifier setup for a 3% Yb:YAG sample [22], Yb:YLO shows 60% broader spectral gain bandwidth at room temperature under comparable conditions. This gives evidence that ceramic Yb:YLO at cryogenic temperatures should give a similar amplification bandwidth and gain as that of Yb:YAG at room temperatures, making it an attractive candidate for broad bandwidth amplification at cryogenic temperatures for efficient short pulse generation. Further amplification at the main emission peak (1031 nm) was performed using an increased seed input energy of 1 mJ. Figure 8(b) shows the dependence of the output energy as a function of the absorbed pump energy. An output energy of 102 mJ was extracted from the setup for 2.9 J of absorbed pump energy, corresponding to an internal conversion efficiency of 3.5%.

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Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A733

Fig. 8. (a) Evolution of gain with respect to the pump input. Seed input reduced to 40µJ. (b) Amplification results with respect to the absorbed pump energy, oscillator tuned to 1031nm with a seed input of 1mJ.

The measured spatial profile of the seed beam after 12 passes with no amplification is shown in Fig. 7(c) along with the amplified beam profile at full pump power in Fig. 7(d). The significant degradation in beam quality at full pump power is likely due to the spatial inhomogeneities observed in the sample as illustrated in Fig. 5, further limiting the amplification efficiency. This also highlights the need for further investigation into the thermal and thermo-optical properties (dn/dt) of the Yb:YLO material. During the amplification experiments, we observed the appearance of dark cloudy spots whilst pumping at full power for more than 100 shots, however, the spots disappeared after annealing the sample at 350°C for 5 hours in air. This suggests the possible presence of impurities causing formation of color centers in the Yb:YLO sample used in this experiment. Conclusion

In summary, we have presented the temperature dependent absorption and emission crosssection of 5% Yb3+ doped ceramic yttrium lanthanum oxide (Yb:YLO). The emission crosssection increases by a factor of 2.2 when reducing the temperature from 300 K to 80 K. In addition, we report the first demonstration of ns pulse amplification in Yb:YLO ceramic. An output energy of 102mJ was reached for an absorbed pump energy of 2.9 J at 1031 nm. The increased amplification bandwidth at room temperature (8.2 nm as compared to 3.2 nm for Yb:YAG) confirms the potential of Yb:YLO ceramic for broad bandwidth amplification of high-energy pulses at cryogenic temperatures. However, further improvement of the fabrication process is necessary to remove inhomogeneities and improve the overall optical quality of the material together with investigation into the thermal and thermo-optical properties (dn/dt) of the Yb:YLO material. Acknowledgments

This work was partly supported by the European Social Fund (ESF) through the Thuringian Ministry of Economy, Employment, and Technology (project number 2011 FGR 0122). Furthermore, the authors from the Friedrich Schiller University of Jena, Germany are grateful for the support by the BMBF (contract 03Z1H531).

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Received 9 May 2013; revised 18 Jun 2013; accepted 20 Jun 2013; published 28 Jun 2013 1 July 2013 | Vol. 21, No. S4 | DOI:10.1364/OE.21.00A726 | OPTICS EXPRESS A734