Klaus Ertel

We report on laser-induced damage behaviour in the nanosecond regime of anti-reflection coated, large aperture, ceramic Yb:YAG. Experiments were carried out to identify suitable coatings for use in large-aperture DiPOLE cryogenic laser amplifiers and involved the testing of 9 coating types from 7 different coating suppliers. Comparison between ex-situ S-on-1 and raster scan test results confirmed that, in the case of large aperture optics, the raster scan protocol provides a more accurate estimate of failure levels compared to the S-on-1 protocol. Significant variability in performance under raster scan testing was observed for different coating types, with some coatings failing under irradiation at 5 J/cm² fluence while others surviving irradiation at 15 J/cm². Comparison between raster scan results and operational experience in DiPOLE amplifiers indicates that raster-scan testing results can be used as a good indicator for suitability in operational use.

We report on the successful demonstration of second and third harmonic conversion of a high pulse energy, high average power 1030 nm diode pumped Yb-doped yttrium aluminum garnet (Yb:YAG) nanosecond pulsed laser in a large aperture lithium triborate (LBO) crystal. We demonstrated generation of 59.7 J at 10 Hz (597 W) at 515 nm (second harmonic) and of 65.0 J at 1 Hz (65 W) at 343 nm (third harmonic), with efficiencies of 66% and 68%, respectively. These results, to the best of our knowledge, represent the highest energy and power reported for frequency conversion to green and UV-A wavelengths.

In this paper, we present a model to predict thermal stress-induced birefringence in high energy, high repetition rate diode-pumped Yb:YAG lasers. The model calculates thermal depolarisation as a function of gain medium geometry, pump power, cooling parameters, and input polarisation state. We show that model predictions are in good agreement with experimental observations carried out on a DiPOLE 100 J, 10 Hz laser amplifier. We show that single-pass depolarisation strongly depends on input polarisation state and pumping parameters. In the absence of any depolarisation compensation scheme, depolarisation varies over a range between 5% and 40%. The strong dependence of thermal stress-induced depolarisation on input polarisation indicates that, in the case of multipass amplifiers, the use of waveplates after every pass can reduce depolarisation losses significantly. We expect that this study will assist in the design and optimisation of Yb:YAG lasers.

We report on frequency doubling of high-energy, high-repetition-rate ns pulses from a cryogenically gas cooled, multi-slab Yb:YAG laser system, using a type-I phase–matched lithium triborate (LBO) crystal. Pulse energy of 4.3 J was extracted at 515 nm for a fundamental input of 5.4 J at 10 Hz (54 W), corresponding to a conversion efficiency of 77%. However, during long-term operation, a significant reduction of efficiency (more than 25%) was observed owing to the phase mismatch arising due to the temperature-dependent refractive index change in the crystal. This forced frequent angle tuning of the crystal to recover the second-harmonic generation (SHG) energy. More than a five-fold improvement in energy stability of SHG was observed when the LBO crystal was mounted in an oven, and its temperature was controlled at 27°C. Stable frequency doubling with 0.8% rms energy variation was achieved at a higher input power of 74 W when the LBO temperature was controlled at 50°C.

We present measurements of the temperature dependence of the absorption cross-section of Yb:YAG at the zero phonon line (ZPL) near 969 nm. Experiments were carried out on a 1.08 mm thick, ceramic, 1.1 at-% Yb-doped YAG sample over a temperature range between 80 K and 300 K. Results show that the ZPL characteristics strongly depend on temperature. The absorption cross section increases from 0.8 · 10⁻²⁰ cm² to above 49 · 10⁻²⁰ cm² as temperature is decreased from 300 K to 80 K. The full-width at half maximum of the absorption line decreases with temperature, from 2.38 nm at 300 K to 0.05 nm at 80 K. The absorption peak shifts from 969.04 nm at 300 K to 968.39 nm at 80 K. To the best of our knowledge, this is the first time that the ZPL of Yb:YAG has been characterised with enough resolution at cryogenic temperatures and we expect that this data will assist in the design and optimisation of Yb:YAG lasers pumped on this absorption line.

We compare for the first time the influence of different Yb:YAG gain media on the performance of a large-area, high average–power laser system with an output energy of up to 6 J. Monocrystalline slabs grown by a new technique without central growth defect are compared with ceramics. Small signal gain, maximum output energy and thermal lensing are compared for ceramic slabs with co-sintered amplified spontaneous emission (ASE) absorber cladding, monocrystalline slab with and without optically bonded ASE absorber cladding, and surface structured monocrystalline slabs. We show that these large monocrystals with optically bonded absorber cladding have similar performance to cladded ceramics, so far the only material for high-energy Yb:YAG lasers.

Improving the efficiency and scaling the pulse rate of high energy Yb:YAG nanosecond pulsed diode pumped solid state laser (DPSSL) amplifiers is vital to unlock their potential for practical applications. Applications include industrial materials processing, sources for high density matter compression experiments, and as pump sources for PW-class amplifiers, which can themselves generate high-brightness secondary radiation and ion sources for remote imaging and medical applications.

We report on the experimental characterisation of the temperature dependence of the optical rotatory power of crystalline right-handed $\alpha$-quartz at 1030 nm wavelength. The temperature range covered in this study is between 77 K and 325 K. For the measurement we propagated light through a 13.11 mm thick quartz plate collinearly with the optic axis. The plate is anti-reflection coated and rotates the polarisation plane of 1030 nm light by 89.3 deg at room temperature, corresponding to a specific rotatory power of 6.8 deg/mm. When placed between parallel polarisers, the transmission through the system was 0.03% at room temperature and increased to 1% at 77 K, showing a measurable change in rotatory power. At 77 K, the angle of rotation imparted by the quartz plate is 85 deg, corresponding to a specific rotatory power of 6.5 deg/mm. To the best of our knowledge, this is the first time that the temperature dependence of optical activity of $\alpha$-quartz is reported for cryogenic temperatures in the infrared. We expect that the measurement results provided in this paper will assist in the design and characterisation of optical systems operating under cryogenic conditions.

Efficient frequency conversion of high energy, high repetition rate 1 µm laser radiation suitable for pumping OPCPA systems and Ti:Sapphire lasers is crucial for the production of ultra-short, high peak power pulses at high repetition rate. Applications include advanced imaging, novel medical treatments and applied and fundamental science. Two 100 J / 10 Hz nanosecond pulsed Yb:YAG lasers, based on “DiPOLE” cryogenic gas-cooled amplifier technology developed at the Central Laser Facility (CLF) [1], have been built. The first DiPOLE100 laser is installed at the HiLASE facility in 2017 [2]. The second is being commissioned at the CLF and will be delivered to the European X-ray Free Electron Laser (XFEL) facility in Germany as a UK funded contribution-in-kind for installation on the High Energy Density (HED) instrument. This system will achieve its full 1 kW design output at the end of 2018, and will deliver high repetition rate 10 GW pulses and frequency converted for to a wide ranging user programme of high-energy density matter experiments (HED). In this paper, we present results of second and third harmonic generation (SHG and THG) experiments using the CLF’s 10 J, 10 Hz DiPOLE laser to pump an LBO crystal. A SHG conversion efficiency of > 75% was achieved in a xx mm LBO crystal for a 1030 nm pulse energy of 8 J and duration 10 ns operating at 10 Hz , corresponding to 4.2 J (5.4 J at the fundamental) J pulses at 515 nm. Furthermore, we have achieved > 40% conversion to the third harmonic in a 6 mm thick LBO crystal, corresponding to 3 J (7 J at the fundamental) at 343 nm, with a 2 ns temporal pulse width. Finally, we compare the SHG performance of LBO and YCOB crystals [3]. We present setup of the 10 J and 100 J conversation at HiLASE and show preliminary results. We will also present initial results of the commissioning of D100-X of the second amplifier and frequency conversion, which will be deployed on both DiPOLE100 systems, where it will be used for damage testing of optics and laser shock peening at HiLASE [4,5], and dynamic shock compression experiments at XFEL [6]. These results will be compared to our calculations on the efficiency of SHG and THG for a number of different temporal pulse shapes and durations. Efficient frequency conversion of high energy, high repetition rate 1 µm laser radiation suitable for pumping OPCPA systems and Ti:Sapphire lasers is crucial for the production of ultra-short, high peak power pulses at high repetition rate. Applications include advanced imaging, novel medical treatments and applied and fundamental science. Two 100 J / 10 Hz nanosecond pulsed Yb:YAG lasers, based on “DiPOLE” cryogenic gas-cooled amplifier technology developed at the Central Laser Facility (CLF) [1], have been built. The first DiPOLE100 laser is installed at the HiLASE facility in 2017 [2]. The second is being commissioned at the CLF and will be delivered to the European X-ray Free Electron Laser (XFEL) facility in Germany as a UK funded contribution-in-kind for installation on the High Energy Density (HED) instrument. This system will achieve its full 1 kW design output at the end of 2018, and will deliver high repetition rate 10 GW pulses and frequency converted for to a wide ranging user programme of high-energy density matter experiments (HED). In this paper, we present results of second and third harmonic generation (SHG and THG) experiments using the CLF’s 10 J, 10 Hz DiPOLE laser to pump an LBO crystal. A SHG conversion efficiency of > 75% was achieved in a xx mm LBO crystal for a 1030 nm pulse energy of 8 J and duration 10 ns operating at 10 Hz , corresponding to 4.2 J (5.4 J at the fundamental) J pulses at 515 nm. Furthermore, we have achieved > 40% conversion to the third harmonic in a 6 mm thick LBO crystal, corresponding to 3 J (7 J at the fundamental) at 343 nm, with a 2 ns temporal pulse width. Finally, we compare the SHG performance of LBO and YCOB crystals [3]. We present setup of the 10 J and 100 J conversation at HiLASE and show preliminary results. We will also present initial results of the commissioning of D100-X of the second amplifier and frequency conversion, which will be deployed on both DiPOLE100 systems, where it will be used for damage testing of optics and laser shock peening at HiLASE [4,5], and dynamic shock compression experiments at XFEL [6]. These results will be compared to our calculations on the efficiency of SHG and THG for a number of different temporal pulse shapes and durations. 1. S. Banerjee et al,"100 J-level nanosecond pulsed diode pumped solid state laser," Opt. Lett. 41, 2089-2092 (2016). 2. P. Mason et al, "Kilowatt average power 100 J-level diode pumped solid state laser," Optica 4, 438-439 (2017). 3. P. J. Phillips et al, ’High energy, high repetition rate, second harmonic generation in large aperture DKDP, YCOB and LBO crystals.’ Optics Express 24(17), 19682-19694 (2016). 4. J. Nygaard, ‘Laser shock peening using DiPOLE laser system’, The AILU Laser User Magazine Issue 86, Autumn 2017. 5. The authors gratefully acknowledge funding for this work from H2020 Widespread Teaming action and the Czech Ministry of Science. 6. M. McMahon at al, ”Conceptual Design report- Dynamic Laser Compression Experiments at HED Instruments of European XFEL, “ XFEL.EU TR-2017-001, February, 2017