This PDF file contains the front matter associated with SPIE Proceedings Volume 11033, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Petawatt laser applications, such as laser plasma acceleration, EUV generation, neutron generation, and materials processing are average-power limited. However, the highest average-power petawatt-class laser to date has an average power of less than 1 kW. Scaling Petawatt-class lasers beyond 10 kW of average power requires a paradigm shift in laser design. To date, average power scaling has been accomplished by increasing the repetition rate of single-shot lasers, in which each shot represents a complete pump/extraction cycle. We propose an alternative scheme, multipulse extraction, in which the gain medium is pumped continuously and the upper state population is extracted over many pulses. This method has two primary benefits: First, because efficient extraction is not necessary in a single pulse, the extraction fluence (and hence the B-integral) can be much lower than in a single pulse design. Second, there isn’t a need to pump within a single inverse lifetime, and therefore less expensive, less complex, and more efficient CW pump sources can be used. Multipulse extraction requires that the gain material have an inverse lifetime significantly less than the desired repetition rate. The design and optimization two multipulse extraction amplifiers, a 10 kHz-100 fs-30J amplifier and a 200 Hz-240 fs-240 J amplifier, will be presented. These point designs have applications in laser plasma acceleration and neutron generation, respectively

Ultrashort pulses with TW to multiple PW peak power are mostly generated by state-of-the-art Ti:Sapphire- (Ti:Sa) based chirped pulse amplification (CPA) systems [1,2]. The main reasons of using Ti:Sa crystals are the extremely large gain bandwidth and exceptional physical properties of the gain medium [3], the availability of the high energy pulsed pumping sources. Additionally, carrier-envelope phase (CEP) stabilization schemes can be easily implemented for Ti:Sa laser systems up to TW peak power [4]. Polarization-encoded CPA (PE-CPA) is a recently developed technique, which holds promise to support a gain bandwidth sufficient for compressed pulses in the few-cycle regime with Ti:Sa-based amplification [5,6]. The performance of the PE-CPA technique is however affected by the spectral amplitude and phase variations along both gain cross-sections. The population inversion induced refractive index changes (RICs) were measured within the main emission band of Ti:Sa. By using spectrally resolved interferometry (SRI), spectral phase changes of amplified pulses were measured in a Jamin-type arrangement. The spectral phase shift induced by inversion for both π- and σ-polarized pulses was extracted at different pump fluence values. At room temperature, a zero-phase shift was found with a sign change at the peak of the gain spectrum, while for σ-polarized pulses no such behavior was observed in the investigated spectral range. By decreasing the temperature of the crystal to 30 K, similar behavior was found, however, the zero-phase crossing was found to be shifted to around 760 nm. The electronic RICs are explained by the polarizability difference between excited and unexcited Ti3+ ions in the crystal. Compression of PE-amplified pulses is currently under investigation by numerical simulations and will be experimentally tested with a spectral bandwidth corresponding to sub-10 fs pulse duration. Carrier-envelope phase (CEP) fluctuations were also investigated in a PE amplifier stage by using the SRI technique. Temperature and inversion instabilities were found to be the major sources of CEP noise caused by the amplification process, accounting for 60 mrad of CEP noise in a four-pass amplifier stage with 1.2 J/cm2 absorbed pump fluence. CEP stability of the PE amplification was compared to the conventional process, which showed a sub-10 mrad noise increase in case of the PE amplifier under similar operating conditions. However, the degradation of the CEP stability in the PE amplifier compared to the conventional stage was found to be below 10 mrad within the same operational conditions [7]. References 1. Z. Gan, L. Yu, S. Li, C. Wang, X. Liang, Y. Liu, W. Li, Z. Guo, Z. Fan, X. Yuan, L. Xu, Z. Liu, Y. Xu, J. Lu, H. Lu, D. Yin, Y. Leng, R. Li, and Z. Xu, Opt. Express 25(5), 5169-5178 (2017). 2. A. Golinelli, X. Chen, E. Gontier, B. Bussière, O. Tcherbakoff, M. Natile, P. d’Oliveira, P.-M. Paul, and J.-F. Hergott, Opt. Letters 42(12), 2326-2329 (2017). 3. P. F. Moulton, “Spectroscopic and laser caracteristics of Ti:Al2O3,” J. Opt. Soc. Am. B 3(1), 125-133 (1986). 4. F. Lücking, V. Crozatier, N. Forget, A. Assion, and F. Krausz, Opt. Lett. 39(13), 3884–3887 (2014). 5. M. Kalashnikov, H. Cao, K. Osvay, and V. Chvykov, Opt. Letters 41(1), 25-28 (2016). 6. H. Cao, M. Kalashnikov, K. Osvay, N. Khodakovskiy, R. S. Nagymihaly, and V. Chvykov, Laser Phys. Lett. 15 045003 (2018). 7. R. S. Nagymihaly, H. Cao, P. Jojart, M. Kalashnikov, A. Borzsonyi, V. Chvykov, R. Flender, M. Kovacs, and K. Osvay, J. Opt. Soc. Am. B 35(4), A1-A5 (2018).

Temporal shape of laser pulses generated by modern high peak power laser systems plays one of the key roles in modern laser-matter interactions. High intensity laser-matter experiments, especially using Petawatt lasers, require a pre-pulse free shape within the dynamic range well above 10^10 and in the time range up to few picoseconds before the main pulse. In CPA systems laser pulses experience numerous transformations during their amplification, starting from being short, then chirped, amplified, and re-compressed. Nonlinear interaction takes place during the pulse amplification, different techniques are being used to optimize the pulse bandwidth, etc. As a result, the laser pulse accumulates distortions of the spectral phase, also caused by impurities in the optical media, nonlinear effects that take place during the amplification and propagation. This leads to the degradation of its temporal profile and appearance of additional pre-/post pulses and pedestals. Aiming to identify the major issues and reasons of the pulse degradation, in addition to amplified spontaneous emission we analyzed the temporal profile of laser pulses generated by CPA-based Ti:Sapphire laser systems in the time range of few picoseconds around the major pulse. We have found that within this temporal interval, and 10^10 range of intensity the laser pulse has typically features that are coherent with the major pulse and those which are not. The later are associated with scattering on surfaces of optical elements and in optical media of the laser system. The impact of those features can be partly reduced by optimization of the laser system. The coherent with the major pulse features after nonlinear interaction during amplification generate temporal replicas in front and behind the major pulse. Those are hardly to be cancelled, since the gain Ti:Sapphire media itself generates a ragged coherent with the major pulse post-pedestal, that after nonlinear interaction and the pulse re-compression gets transferred to the leading front of the laser pulse. This feature limits the temporal contrast of Ti:Sapphire lasers, since it is unavoidable. A new method of the temporal contrast filtering based on rotation of polarization ellipse in noble gases combined with differentially pumped hollow core fiber (HCF) will be discussed also. The method can substitute a commonly used generation of crossed-polarized wave (XPW) in double-CPA laser systems, since it allows the conversion efficiency of > 50%, a smooth beam profile and high energy stability. It supports also a similar to XPW contrast improvement of 3-4 orders of magnitude and allows generating laser pulses with pulse duration in a sub 5-fs range. Contrary to a conventional HCF technique the new method supports a very smooth spectrum of converted pulses and hence a better temporal shape.

Crytur is a company with long tradition of growing and processing crystals for technical applications, with history reaching back to 1943. Recently we have developed Crystal Improved Growth (CRIG) method for production of large core-free single crystals of YAG. The diameter currently achieved is 140 mm (in case of undoped crystal), and the crystal weight is up to 10 kg. The method was used to grow un-doped YAG crystals, YAG:Ce crystals for large scintillating screens, and Yb:YAG and Nd:YAG for high power solid-state laser systems.

Large laser slabs were manufactured from Yb:YAG doped crystals for Diode-Pumped Solid State Laser (DPSSL) system Amos, which operates within Extreme Light Infrastructure in the Czech republic (ELI Beamlines). The dimension of the largest Yb:YAG laser slab produced is 120×120×8 mm, there is no visible stress under crossed polarizers and the wavefront distortion in the clear aperture region is smaller than λ/10 (λ=633 nm) in its Peak-to-Valley value. The edges of the slab are from diffusively bonded Cr:YAG cladding in order to suppress ASE (Amplified Spontaneous Emission).

In 2018 the performance of three sets of laser slabs (ø55x5 mm) with differently realized ASE suppression was characterized at cryogenic temperatures at HiLASE Centre in terms of small signal gain measurements as well as amplification test under 30 J pumping at 1 Hz and 10 Hz repetition rates. We provide data that show that the crystal slabs have comparable properties to the ceramic slabs (produced by Konoshima company, Japan) currently in use at HiLASE.

We present a novel approach to combine diode-pumped, moderately low-gain media with the advantages of an unstable cavity. To this end, we propose to utilize a spatially tailored gain profile in the active medium instead of using a graded reflectivity mirror to provide an eeffective shaping mechanism for the intra-cavity intensity distribution. The required gain profile can be easily generated with a state-of-the-art homogenized laser diode pump beam in an end-pumped configuration.

The inherent momentum of photons generates a force when a laser beam reflects from a mirror. Since this force is linearly related to the laser beam’s optical power, we can use this radiation pressure effect as an optical power meter for lasers. We have successfully demonstrated this approach as a high-accuracy CW laser power meter for powers ranging from 1 kW to 50 kW. Here we investigate the possibility of using the same experimental setup to measure the pulse energy in high energy laser pulses. Without access to a high energy pulsed laser, we perform preliminary tests with high energy pseudo pulses using a short (10 ms to 1 s) duration square wave output from a CW Yb-doped fiber laser for average powers from 2.5 kW up to 10 kW. We tested “pulse” energies from 50 J to 10 kJ. We demonstrate theoretically that integrating the measured force on a mirror when these pulses are reflected equates to the energy per pulse even for pulse durations shorter than the force sensor’s response time. Experimentally, we show a linear relationship between the launched pulse energy and its value measured with radiation pressure. Our measurement noise floor is on the order of 100 J per pulse and we find a preliminary discrepancy of 8 % between the radiation pressure measurement and the known pulse energy. Using a modified scale with an analog output, tested with magnetically emulated pulses brings this discrepancy down to approximately 1 %.

In high power laser system, the wavefront quality of large optic elements in the mid-frequency region plays a critical role in the system performance and safe operation. A simple and efficient measurement method for mid-frequency wavefront error is used, which employs the extended ptychographical iterative engine algorithm and has simple structure, low environment requirement and flexible adjustable frequency ranges. This method has been successfully implemented for the wedge focused lens to achieve accurate mid-frequency measurement. Further it can be extended to a wide range of large optical components, especially for which the wavefronts are not easy to be measured using interferometers.

Tunable and milli-Joule level pulsed Tm based laser, are demonstrated in this paper. The spectral bandwidth was narrowed down to 0.15 nm FWHM. For the Actively Q-switched Tm:YLF laser, we achieved 33 nm of tunability range between 1873 nm and 1906 nm, using a pair of YAG Etalons. Using the same tunability technic for the Passively Qswitched Tm:YAP laser, we achieved 11 nm of tunability between 1930-1941 nm. The Tm:YLF laser was actively Qswitched using an acousto-optic modulator, while achieving mJ level pulse energy along the whole tuning range at a repetition rate of 1 kHz. Up to 1.97 mJ of energy per pulse was achieved at a pulse duration of 37 ns at a wavelength of 1879 nm, corresponding to a peak-power of 53.2 kW and at a slope efficiency of 36 %. The Tm:YAP laser was Passively Q-switched using Cr:ZnS saturable absorber (SA) as modulator, achieving mJ level pulse energy along the whole tuning range. Up to 1.2 mJ of energy per pulse was achieved at a pulse duration of 24 ns at a wavelength of 1935 nm, corresponding to a peak-power of 50 kW. The combination of both high energy pulsed lasing and spectral tunability, while maintaining narrow bandwidth across the whole tunability range, enhances the laser abilities, which could enable new applications in the sensing, medical and material processing fields. Which in the case of the of the Passively Qswitched Tm laser has a major advantage in terms of foot print.

We present the absorption spectroscopy and continuous-wave laser operation of Tm:YLF at cryogenic temperatures. At 100 K, a maximum output power of 2.55 W corresponding to a maximum slope efficiency of 22.8% is obtained using 15% output coupling transmission. The output laser wavelength is centered at 1877 nm for Elc.

The free-running and passive Q-switched laser properties of Er,La:SrF₂-CaF₂ crystal, that is appropriate for generation at 2.74 μm, are presented. The sample of Er,La:SrF₂-CaF₂ (composition 4 wt.% of ErF₃, 12 wt.% of LaF₃, 77 wt.% of CaF₂, and 7 wt.% of SrF₂, thickness 8.2 mm) had plan-parallel polished faces without anti-reflection coatings. The excitation of Er,La:SrF₂-CaF₂ was carried out by a 975 nm laser diode radiation in pulsed (pulse duration 1 ms, repetition rate 10 Hz) and CW mode. Laser resonator was hemispherical, 10 mm or 45 mm in length with flat pumping mirror (HR @ 2.7 μm) and spherical output coupler (r = 50 mm, R = 97.5 % or R = 95 % @ 2.5 - 2.8 μm). For CW mode of operation of Er,La:SrF₂-CaF₂ the highest obtained slope efficiency was 20.2 %. The maximum output power 0.35 W was achieved in this case. With semiconductor saturable absorber (SESAM) in laser resonator the shortest pulse duration of 33.4 ns with repetition rate 50 kHz was obtained. The maximal pulse energy 5.4 μJ with peak power 122 W was reached for 44.2 ns pulse duration. Since the emitted laser wavelength 2.74 μm is relatively close to absorption peak of water located at 3 μm, one of the Er,La:SrF₂-CaF₂ laser possible usage should be in medicine and spectroscopy.

We report the first-of-its-kind compact and robust coherent source operating in mid-IR based on Fe:ZnSe chalcogenide gain medium optically pumped by Er:ZBLAN fiber laser. In the research, we study the CW operation of cryogenically cooled laser based on Fe:ZnSe single crystals with different doping level grown from the vapor phase on a single-crystal seed by using the concurrent-doping technology. The maximal output power achieved is 2.1 W with 59% slope efficiency with respect to absorbed pump power, which is close to the Stokes shift limit. Measured Fe:ZnSe output spectra indicate a significant influence of re-absorption on generation wavelength. For high doping levels and output powers, spectrum shifts to the red wing, which makes possible continuous tuning from 4012 to 4198 nm. As well, tunability of the laser in a wide range of temperature is investigated.

The spectroscopy properties and wavelength tunability of diode pumped laser based on Tm³⁺, Ho³⁺ co-doped (Gd_3-xGax)(Ga_5-y Al _y)O₁₂ mixed gadolinium-gallium-aluminium garnet (Tm, Ho:GGAG) single crystal were investigated. A fiber (core diameter 400 μ m, NA = 0.22) coupled laser diode was used for longitudinal pumping of Tm, Ho:GGAG crystals. The laser diode was operating in the pulsed regime (5 ms pulse length, 10 Hz repetition rate, 785 nm emission wavelength, maximum power amplitude 16.8 W). The 150 mm long hemi- spherical laser resonator consisted of a flat pumping mirror (HR @ 1.852.11 μm, HT @ 0.78 μm) and concave (r = 150 mm) output coupler with a reflectivity of ≈97% @1.8 2.10 m. Wavelength tuning was accomplished by using 1.5 mm thick SiO₂ birefringent plate. For the sample with 3.97 at.% Tm, 0.38 at.% Ho concentration, the measured maximal output power amplitude was 1.04 W at 2086 nm with a slope efficiency of 23.6 % and the tunability range 19862110 nm (124 nm). For the sample with 7.65 at.% Tm, 0.68 at .% Ho concentrations, the measured maximal output power amplitude was 2.30 W at 2090 nm with a slope efficiency of 26.5 % and the tunability range 20032116 nm (113 nm).

Laser amplifiers at high repetition rate are critical for many applications in the chemical, physical and biological sciences. A variety of laser sources from XUV to THz can be derived from Ti:Sapphire laser amplifiers at moderate to low conversion efficiencies. High repetition rate applications require NIR and IR sources based on optical parametric chirped pulse amplifier (OPCPA) to drive these sources, offsetting the conversion efficiency losses with an even higher average power beam to drive the frequency conversion processes. We use these technologies at next generation free-electron laser (FEL) facilities. The Linac Coherent Light Source (LCLS), LCLS-II upgrade, will provide sub-femtosecond and femtosecond X-ray pulses at 100 kHz, and later up to 1 MHz repetition rate. The higher repetition rate benefits pump-probe experiments for weakly scattering samples and serves a variety of experiments which require attenuation to avoid perturbation and damage of the sample by the X-ray probe. A millijoule R&D laser amplifier was developed to test experimental conditions for optical laser beam delivery at LCLS-II. The laser can be operated at two distinct wavelength ranges. At 800 nm center wavelength we use the second harmonic of an Yb:YAG amplifier system to pump an OPCPA in a BBO crystal. A second tunable version operates between 1.45-2 m center wavelength using the fundamental Yb:YAG beam to pump a KTA OPCPA with average output powers in excess of 100 W. Currently the amplifier is operated 24 hours, 7 days a week. It is based on a simple and robust design, which ensures long term stability with good output beam quality.

The usage of coherent radiation in the mid-infrared (mid-IR) wavelength range (2 – 8 μm) covers a wide spectrum of applications in many different fields. To satisfy the demand for a high-power, picosecond mid-IR source, we are developing an optical parametric system tunable between 1.45 and 3.5 μm in wavelength. The pumping of the system is provided by an in-house built Yb:YAG thin-disk laser, delivering 80 W of average power at 93 kHz pulse repetition rate, 1030 nm wavelength and ~1.3 ps pulse duration. The optical parametric system consists of a double-pass optical parametric generator (OPG) based on a periodically poled lithium niobate. By utilizing four periods of poling and temperature tuning, wavelength tunability range is from 1.45 to 3.5 μm. Maximum signal output power was around 85 mW at 1850 nm wavelength at 2 W pump power. Subsequent amplification of the signal generated in the OPG stage takes place in an optical parametric amplifier (OPA), which consists of a pair of walk-off compensating KTA crystals, pumped by up to 50 W average power. Maximum output signal and idler sum power after the OPA stage was 8.4 W. The wavelength tunability of the amplifier spans from 1.5 to 3.2 μm. Further increase in the tunability range as well as gapfree tuning is currently crystal mount-limited.

In recent years, considerable effort has been made to develop high-energy infrared (IR) femtosecond laser sources owing to their advantages for applications in ultrafast and strong-field laser science. In this paper, we show our work on developing TW-class mid-infrared (MIR) femtosecond laser pulses in the 1–4 μm region using a dual-chirped optical parametric amplification (DC-OPA) method, which solved the energy scaling difficulties in standard OPAs. Using a sub-joule class Ti:sapphire laser as a pump for the DC-OPA system, MIR femtosecond pulses with 100-mJ-class energy and a flexible wavelength tunability are confirmed. Efficient energy scaling of DC-OPA is examined experimentally. Moreover, we find different features of DC-OPA from conventional OPA and narrow spectral bandwidth laser pumped optical parametric chirped pulse amplification (OPCPA). Temporal chirps in DC-OPA play critical roles in optimizing efficiency and spectral bandwidth. In addition, bandwidth narrowing of amplified pulses in DC-OPA can be minimized by optimizing the chirps of the seed and pump pulses. DC-OPA can be universally employed for energy scaling of near-IR, MIR, and far-IR pulses, regardless of the type of nonlinear crystal, and is helpful for efficiently generating few-cycle, carrier-envelope phase (CEP)-stable IR pulses with TW-class peak power.

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

At present, Faraday isolators (FIs) working with high power laser radiation mainly use terbium gallium garnet (TGG) crystals as media for magneto-optical elements. High optical quality TGG crystals are commercially available, however it has several disadvantages: a large thermally induced lens and relatively high level of thermally induced depolarization. Therefore, at present, there are attempts to create a magnetoactive media that would replace TGG: the most promising media are TSAG, KTF and NTF crystals and also TAG ceramics. We propose to use cerium fluoride (CeF3) crystal, which is uniaxial, but superior to TGG in thermo-optical characteristics. CeF3 crystal is known for a long time, as found in nature in the composition of various minerals (fluorite, tisonite). More recently, it began to be used as a fast, high radiation hardness scintillator. Cerium Fluoride is a good scintillation crystal with high density and short decay time. Its high Verdet constant and paramagnetic nature of rotation have been known from the 30s of the 20th century. Another important advantage from the viewpoint of using this material in high-power laser systems is a possibility of producing large-aperture (up to ~10 cm) optical elements from CeF3. In this respect CeF3 surpasses most of the magneto-active crystal media. For example, the largest aperture of a TGG single crystal with quality fit for producing an FI is only 40 mm. The CeF3 crystal is transparent in a wide range of visible and near IR wavelengths (300-2500 nm) and has a high Verdet constant in this area whereas the TGG crystal becomes optically nontransparent starting from ~1.4 mm. At radiation wavelength of λ=1 µm, the Verdet constants of TGG and CeF3 are almost equal (37 and 36.7 rad/T/m respectively), which ensures an equal length of crystals, and they can be replaced one by one in FI. We measured the thermally induced depolarization and thermal lens in this crystal. When the radiation power approaches 1 kW, the dependence of thermally induced depolarization on laser power becomes quadratic and almost coincides to the analogous dependence for the TGG crystal with absorption coefficient of 1.5*10-3 cm-1. However the thermal lens induced in CeF3 has a 6.5 lower optical strength for the same laser beam parameters. We created a Faraday isolator on CeF3 using a magnetic system with a field strength of 2.8 kOe. The crystal length was 8 mm. Up to a power of 700 W, the degree of isolation was no worse than 30 dB; it was practically independent of laser power and was determined by the quality of the crystal. It should be noted that the uniaxial nature of the crystal imposes restrictions on the divergence of the laser beam, which should not exceed 8 mrad to maintain the degree of isolation. The mentioned advantages together with the feasibility of producing large-aperture elements allow us to conclude that the CeF3 crystal is promising for developing FIs for high-power laser systems.

In this work, a novel multi-layer approach of high band-gap birefringent columnar coatings was proposed and investigated. The growth of anisotropic columnar nano-structures with elliptical shape cross-section was initiated by self-shadowing effect, which was induced by placing the substrate at oblique angle during the deposition process. Amorphous silica was deposited in so-called serial bi-deposition manner to form anisotropic films with high thickness uniformity. The combination of birefringent nano-structured and isotropic layers allows to form zero-order wave-plates with desirable phase delay difference (as an example, λ/4). Low optical losses and high transparency (T~99%) are demonstrated while indicating the potential to withstand laser fluences of 40 J/cm² and 15 J/cm² in nanosecond regime at 355 nm and 266 nm wavelengths, respectively.

The thin disk (TD) technique has been used in laser oscillators and amplifiers for almost three decades to increase the average power of ultrafast laser systems [1]. Different host materials have been tested mostly with Yb-doping, which resulted in a wide range of pulse durations from 1 ps to sub-100 fs in combination with high average power, good stability and reliability [2]. An even more industrial approach, the thin-slab laser geometry, uses slab-shaped gain media instead of disks, where the cooling is performed from both large surfaces of the crystal [3]. A great advantage of Yb-based TD and thin-slab systems is that the laser crystal can be pumped by continous wave laser diodes due to the long active lifetime of the gain medium. However, the small gain bandwidth supports only sub-ps pulse duration, if high energy pulses are required. Ti:Sapphire (Ti:Sa), on the other hand, offers a gain bandwidth that supports sub-20 fs pulse duration in combination with peak powers up to the petawatt regime without using any post compression techniques [4]. Amplification with the TD geometry has been already demonstrated to be efficient based combined with Ti:Sa, but only in the multi-J regime with a repetition rate of 10 Hz [5]. Scaling the TD geometry to mJ-class pulses leads to serious thermal issues in the gain medium due to the decreased cooled surface to volume ratio. Recently, laser diodes operating around 450 nm were also found to be a good pumping source for Ti:Sa amplifiers with repetition rates in the 100 kHz regime [6], while laser operation in Ti:Sa pumped by a stack of LEDs was also demonstrated and stated to be scalable for high pulse energies [7]. For this reason, thin-slab type amplification is suggested based on Ti:Sa to obtain mJ-class, kW level average power pulses in combination with sub-20 fs pulse duration. Numerical simulations were performed on the amplification and thermal performance for different pulse energies, repetition rates and so average powers. The results revealed the limitations of the thermal performance in the thin-slab geometry. Water-cooled amplifiers were found to be able to provide pulses with 2 kW average power with reasonably low temperature gradient in the crystal. Room temperature cooling enabled 0.3 J output energy and extraction efficiency greater than 50% with a repetition rate exceeding 10 kHz. An increase of one order of magnitude in the repetition rate is possible when cryogenic cooling is considered with 77 K coolant temperature. Laser diode pumping was also investigated in combination with 1 MHz repetition rate of the amplified pulses, where the wall-plug efficiency of the amplifier system is expected to be increased significantly compared to conventional solid-state laser pumping. Experimental demonstration of the thin-slab type Ti:Sa amplification is under work with repetition rates ranging from 2 to 10 kHz in combination with a pump average power of 50 W. References 1. J. Speiser, J. Opt. Soc. Am. B, 26, 26-35 (2009). 2. F. Druon et al, IEEE Phot. Journ. 3(2), 268-273 (2011). 3. P. Russbueldt et al, IEEE J. of Sel. Top. in Q. El. 21(1), 3100117 (2015). 4. Sergei Kühn et al, J. Phys. B: At. Mol. Opt. Phys. 50 132002 (2017). 5. V. Chvykov et al, Opt. Lett. 41(13), 3017-3020 (2016). 6. S. Backus et al, Opt. Expr. 25, 3666-3674 (2017). 7. P. Pichon et al, Optica 5(10), 1236-1239 (2018).

The average power of diode-pumped fiber lasers have developed deep into the kW regime in the past years. However, stimulated Raman scattering (SRS) is still a major factor limiting the further power scaling. To date, many methods for SRS suppression have been proposed in fiber systems, such as the application of large-mode-area (LMA) fibers or enlarging the fiber mode area, spectrally selective fibers, or lumped spectral filters like long-period gratings (LPGs). The enlarging of fiber mode area must be combined with controlling numerical aperture (NA) for the operation of fundamental mode. Otherwise it will leads to a decreased transverse modal instability (TMI) threshold in fiber lasers, which also limits further power scaling. It is quite difficult to realize by today’s material and manufacturing technologies of fibers. The designing of spectrally selective fibers is usually very complex. Besides, it is also not easy to manufacture such fibers and it is still limited by the maximum fiber core size that can be employed. The working principle of lumped filters is similar to that of spectrally selective fibers, but it is much easier to design and fabricate such filters. LPGs have good filtering properties, but the filtering characters of LPGs is instable for their high sensitivities to the environment variable such as temperature, strain or humidity. Here, we have demonstrated the mitigation of SRS in kilowatt-level diode-pumped fiber amplifiers using a chirped and tilted fiber Bragg grating (CTFBG) for the first time. The CTFBG is designed and inscribed in LMA fibers, matching with the operating wavelength of the fiber amplifier. With the CTFBG being inserted between the seed laser and the amplifier stage, a SRS suppression ratio of ~10 dB is achieved in spectrum at the maximum output laser power of 2.35 kW, and there are no reduce in laser slope efficiency and degradation in beam quality. This work proves the feasibility and practicability of CTFBGs for SRS suppression in high-power fiber lasers, which is very useful for the further power scaling.

Aimed to maintain excellent beam quality, the influence of pointing deviation on the beam quality is theoretically studied in the dual-grating spectral beam combination (SBC). The incident light field of the fiber laser array with the pointing deviation is built by the transformation of coordinates, and the variation rule of the combined beam quality with random perturbations is discussed by the principle of beam diffraction and the statistical analysis. As a result, the degradation of beam quality for the pointing deviation is respectively 0.31(±0.13) and 3.06(±1.27) for the standard deviation of 0.1 mrad and 0.5 mrad, spreading as a Normal distribution. It can be concluded that the pointing deviation of laser array will destroy the condition of the SBC of the common aperture output, resulting in the continuous growth of the M² factor. These analyses provide a valid basis for setting up the experimental system of dual-grating SBC.

High-power lasers require tremendous power consumption, generate large heat loads in short time periods, and have challenging cooling requirements. A cooling system of phase change energy storage that reduced the volume and weight by many times was proposed. The system included a laser cooling circuit and a phase change cooling circuit. Designed a phase change cold storage heat exchanger, which was the closed heat exchanger that the cooling circuit consisted of multiple bundles of copper tubes and the phase change material was paraffin. Based on the experimental results, designed another open phase change energy storage cooling system with better performance. Comparisons of these two types of phase-changing heat exchangers showed that choosing water as phase-changing material can get more accurate temperature control.

A series of optical ceramics based on dysprosium oxide were produced and characterized. The influence of ceramics composition on optical transmission of the obtained samples was investigated. The temperature dependences of the Verdet constant of dysprosium based ceramics with various compositions: (Dy_0.7Y_0.25La_0.05 )₂O₃, (Dy_0.85 Y_0.1La_0.05)₂O₃ and (Dy_0.9Y_0.05La_0.05)₂O₃ in the temperature range 80 – 294 K and in the wavelength range 405 – 1064 nm were studied and their approximations for the temperature – wavelength range are obtained. Cryogenic cooling by the liquid nitrogen leads to increase studied ceramics’ Verdet constant value in ~3.46 times, relatively 294 K, that allow to use this material, in cryogenically cooled Faraday isolators for Tm³⁺ and Ho³⁺ lasers, and for lasers operating in the visible spectral region.

The interest in the development of coherent mid-infrared radiation sources is caused by its potential application in medicine, spectroscopy, laser remote sensing of the atmosphere, metrology, and in many other fields of interest. This study presents temperature dependence of spectral properties of Cr:ZnSe laser active medium in range of 78-380 K. The temperature influence on the absorption, fluorescence and oscillation spectra were investigated in detail. While heating the Cr:ZnSe crystal from 78K to 380 K, the absorption peak maximum has shifted for 65nm toward a shorter wavelength from 1813nm to 1748nm together with the absorption spectrum broadening from 262nm to 373nm and decreasing the absorption coefficient. The FWHM of the fluorescence spectrum was broadened from 280nm (2030-2310 nm) to 488nm (1896-2384 nm) when the temperature of active medium was increasing. Pulsed laser operation from Cr:ZnSe active medium longitudinally pumped by an Er:YLF laser at 1735nm was investigated. The temperature dependence of Cr:ZnSe laser output energy and oscillation spectrum were studied. The highest output energy was 3.84mJ at 78K which together with the FWHM pulse duration of μ200 s corresponds to the power of 19mW. The laser radiation emission was observed at three wavelength bands which intensity was changing during the increase of crystal temperature. However, the oscillation band around wavelength of ~2360nm occurred for all measured temperatures. As a result, by cooling the system, the wavelength of maximum absorption is being shifted to the longer wavelengths as well as the wavelength of maximum fluorescence spectrum intensity.

We present temperature influence (from 78 to 300,K) on tuning and laser properties of thulium doped solid solution CaF₂-SrF₂ crystal (Tm:CaF₂SrF₂). The sample was 8.5 mm thick Tm:CaF₂SrF₂ block cut and face polished parallel to growing axis without any AR coating. The composition of sample was 60 mol% CaF₂, 38 mol% SrF₂ doped with 2 mol% TmF₃. The sample was mounted in temperature controlled copper holder of the liquid nitrogen cryostat. The 148mm long semi-hemispherical laser resonator consisted of flat pumping mirror (HR @ 1.80 - 2.10 μ m, HT @ 0.78 μm) placed inside cryostat, and curved output coupler (r=150mm, R = 98:0% @ 2 μm) placed outside. For longitudinal pumping a fiber coupled laser diode was used. The diode was operating in the pulse mode (10 ms pulse length, 10 Hz repetition rate) at wavelength 763 nm. The 2mm thick MgF₂ birefringent filter was placed at Brewster angle inside the resonator for laser wavelength tuning. With decreasing temperature the output oscillation wavelength shifted to shorter wavelength and range of tunability decreased from 117nm at 300K to 89nm at 100K following the fluorescence spectrum narrowing. The overall tunability was from 1795nm at 79K to 1944nm at 300 K. The highest output pulse energy 6.8mJ with slope efficiency 10% was obtained at 1814nm for T = 78 K. The temperature of Tm:CaF₂SrF₂ was found to have significant influence on laser output wavelength and tunability.

The influence of pumping beam diameter on output of the room-temperature operated Q-switched longitudinally diode-pumped Yb:YAG microchip laser was investigated. The tested microchip laser was based on monolith crystal (diameter 3mm) which combines in one piece an active laser part (Yb:YAG crystal, 10 at.% Yb/Y, 3mm long) and saturable absorber (Cr:YAG crystal, 1.36mm long, initial transmission 90% @ 1031 nm). The microchip resonator consisted of dielectric mirrors directly deposited on the monolith surfaces. The pump mirror (HT for pump radiation, HR for generated radiation) was placed on the Yb:YAG part. The output coupler with reflection 55% for the generated wavelength was placed on the Cr³+-doped part. For longitudinal pumping, fibre coupled (core diameter 400 ¹m, NA= 0:22) laser diode was used. The diode was operating in pulsed regime (repetition rate 20 Hz, pulse length 3 ms, maximum pumping energy 95 mJ, wavelength 934 nm). Three various pumping optics offering pumping beam radius 0.20, 0.27, and 0.34mm were used. The wavelength of microchip laser emission was 1031 nm. The pumping beam radius did not signficantly influenced the pulse duration which was 1:5 § 0:3 ns (FWHM) in all three cases. The highest generated single Q-switched pulse energy (1.08 mJ) was obtained for pumping beam radius 0.27mm for maximum pumping. The corresponding peak power was 0.72MW.

We have developed a high-power single mode monolithic tandem-pump fiber source operating at 1018 nm by employing a low core/cladding diameter ratio of 20/400 μm ytterbium-doped fiber. A record output power of 472 W is obtained, with a slope efficiency of 49.4% and near diffraction limited beam quality factor of M2 = 1.17. Careful numerical simulations were performed in order to determine the most suitable laser parameters such as reflectivity of output coupling fiber Bragg grating, length of active fiber and ytterbium ion concentration regarding a co-pumped oscillator setup. The signal output power behavior indicates that there are no limitations for further power scaling, except for insufficient pump power. This is apparently the highest recorded output signal power, efficiency and beam quality factor in monolithic 1018 nm fiber lasers using 20/400 μm ytterbiumdoped fibers. This structure can also be utilized as a high brightness pump fiber source of core-pumping ytterbium-doped highpower fiber amplifiers in a monolithic setup.