**1. Introduction**

A rapid progress is recently observed in the field of compact extreme-ultraviolet (EUV) and X-ray sources with high brightness and small footprint enough to be installed in laborato‐ ries in educational and research institutions, manufacturing facilities, hospitals, and other suitable sites [1]. This may advance scientific and technical disciplines in practical applica‐ tions by complementing large-scale synchrotron radiation and free-electron laser sources. Applications span a wide range from biomedical, semiconductor, fundamental and applied

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research, environmental engineering to industrial nondestructive testing. Component technology progress is one of the key factors in these advancements of the compact EUV through hard-X-ray sources. These key elements are instrumentation, optics, detectors, data management and processing, and one of the most significant factors is the progress of high average power, short-pulse solid-state lasers.

Semiconductor industry has been struggling in the past two decades to establish a technolog‐ ical system of extreme-ultraviolet lithography as the ultimate scheme, and the establishment of reliable, high average power (>100 W) 13.5 nm source has been always the most critical challenge. The basic architecture is now realized as the LPP (laser-produced plasma) EUV source, in which the conditioning of the mist target from a liquid tin droplet is essential for higher conversion efficiency and perfect recovery of the injected tin atoms [2]. The mist formation is performed by a diverging shock wave inside the microdroplet, which is driven by an impulse generated by an irradiation of picosecond solid-state laser pulse of mJ level pulse energy. The system repetition rate is typically 100 kHz, and the laser average power is more than 100W. The size of the droplet is 10 μm in diameter, and the required laser beam quality and stability must meet the requirements.

Lasing was reported in the EUV spectrum region by efficient excitation of dense plasma columns at 100 Hz repetition rate using a tailored pump pulse profile of a 1 J picosecond cryogenic Yb:YAG laser [3]. The average power of the 1 J picosecond laser is 100 W. The tabletop soft-X-ray laser average power is 0.1 mW at *λ* = 13.9 nm and 20 μW at *λ* = 11.9 nm from transitions of Ni-like Ag and Ni-like Sn, respectively. Lasing on several other transitions with wavelengths between 10.9 and 14.7 nm was also reported. The efficient X-ray laser operation was realized by an optimized pump pulse design as a nanosecond prepulse followed by two picosecond pulses to create higher density plasma of Ni-like ions of higher temperature for higher gain in longer time and in larger space. The high average power of these compact soft X-ray lasers promises to enable various applications requiring high photon flux with coherence.

Laser Compton X-ray source has been established as a compact, high-brightness shortwavelength source. The basic principle is similar to an undulator emission, and a high-intensity laser field is used as the modulating electromagnetic field. The laser Compton X-ray source is demonstrated as a compact short-wavelength imaging approach combined with the phase contrast method of biosamples. Single-shot imaging is critical for many practical applications, and the required specification depends on the usable laser pulse with some threshold param‐ eters because all other component technologies are well matured. The optimization of the laser Compton hard X-ray source by single-shot base is already studied in detail [4, 5]. Experimental results well agreed with theoretical predictions. Highest peak brightness is obtained in the configuration of counterpropagating laser pulse and electron beam bunch, in the minimum focusing area before nonlinear threshold [6, 7]. A single-shot phase contrast bioimaging was demonstrated in the hard X-ray region [8]. The employed laser was a picosecond CO2 laser of 3 J pulse energy [9], but the laser system was not an easy and compact one for further broad applications in various laboratories and hospitals. The Extreme Light Infrastructure–Nuclear Physics (ELI–NP) facility will have a brilliant γ-beam of 104 photons/s/eV, ≤0.5% bandwidth, with Eγ < 19.5 MeV, which is obtained by the laser Compton method from an intense electron beam (Ee > 700 MeV) produced by a warm linac [10]. The main purpose is to provide an opportunity for the production of radioisotopes for medical research. The repetition rate is 100 Hz with a 1 J, picosecond Yb:YAG laser. A standard laser Compton X ray source is under construction as the STAR project at the University of Calabria (Italy) to generate monochro‐ matic tunable, ps-long, polarized X-ray beams, ranging from 20 to 140 keV. The X-rays will be devoted to experiments of material science, cultural heritage, advanced radiological imaging with microtomography capabilities [11]. An S-band RF gun produces electron bunches at 100 Hz, boosted up to 60 MeV by a 3 m long S-band cavity. It is critical to use a high-brightness linac of low emittance and high pointing stability to focus higher charge bunch to a smaller spot size down to 10 μm. The allowed spatial stability is a few μm. The research and devel‐ opment of the X-ray generation laser is the key technology for higher and stable X-ray generation. The Yb:YAG laser is ideal for a compact, high pulse energy picosecond pulse and should be synchronized to the RF system in less than picosecond time jitter.

research, environmental engineering to industrial nondestructive testing. Component technology progress is one of the key factors in these advancements of the compact EUV through hard-X-ray sources. These key elements are instrumentation, optics, detectors, data management and processing, and one of the most significant factors is the progress of high

Semiconductor industry has been struggling in the past two decades to establish a technolog‐ ical system of extreme-ultraviolet lithography as the ultimate scheme, and the establishment of reliable, high average power (>100 W) 13.5 nm source has been always the most critical challenge. The basic architecture is now realized as the LPP (laser-produced plasma) EUV source, in which the conditioning of the mist target from a liquid tin droplet is essential for higher conversion efficiency and perfect recovery of the injected tin atoms [2]. The mist formation is performed by a diverging shock wave inside the microdroplet, which is driven by an impulse generated by an irradiation of picosecond solid-state laser pulse of mJ level pulse energy. The system repetition rate is typically 100 kHz, and the laser average power is more than 100W. The size of the droplet is 10 μm in diameter, and the required laser beam

Lasing was reported in the EUV spectrum region by efficient excitation of dense plasma columns at 100 Hz repetition rate using a tailored pump pulse profile of a 1 J picosecond cryogenic Yb:YAG laser [3]. The average power of the 1 J picosecond laser is 100 W. The tabletop soft-X-ray laser average power is 0.1 mW at *λ* = 13.9 nm and 20 μW at *λ* = 11.9 nm from transitions of Ni-like Ag and Ni-like Sn, respectively. Lasing on several other transitions with wavelengths between 10.9 and 14.7 nm was also reported. The efficient X-ray laser operation was realized by an optimized pump pulse design as a nanosecond prepulse followed by two picosecond pulses to create higher density plasma of Ni-like ions of higher temperature for higher gain in longer time and in larger space. The high average power of these compact soft X-ray lasers promises to enable various applications requiring high photon flux with

Laser Compton X-ray source has been established as a compact, high-brightness shortwavelength source. The basic principle is similar to an undulator emission, and a high-intensity laser field is used as the modulating electromagnetic field. The laser Compton X-ray source is demonstrated as a compact short-wavelength imaging approach combined with the phase contrast method of biosamples. Single-shot imaging is critical for many practical applications, and the required specification depends on the usable laser pulse with some threshold param‐ eters because all other component technologies are well matured. The optimization of the laser Compton hard X-ray source by single-shot base is already studied in detail [4, 5]. Experimental results well agreed with theoretical predictions. Highest peak brightness is obtained in the configuration of counterpropagating laser pulse and electron beam bunch, in the minimum focusing area before nonlinear threshold [6, 7]. A single-shot phase contrast bioimaging was demonstrated in the hard X-ray region [8]. The employed laser was a picosecond CO2 laser of 3 J pulse energy [9], but the laser system was not an easy and compact one for further broad applications in various laboratories and hospitals. The Extreme Light Infrastructure–Nuclear

photons/s/eV, ≤0.5% bandwidth,

average power, short-pulse solid-state lasers.

102 High Energy and Short Pulse Lasers

quality and stability must meet the requirements.

Physics (ELI–NP) facility will have a brilliant γ-beam of 104

coherence.

Compact short-wavelength sources are emerging due to the progress of extreme ultraviolet lithography (EUVL) in semiconductor industry. The EUVL has been intensively developed in the field of various component technologies, for example, Mo/Si high reflectivity mirror at 13.5 nm wavelength, new types of resist of higher sensitivity at this wavelength, and plasma-based 100 W class stable EUV sources. Further increase of average power is expected for large-scale manufacturing to kW level and shorter wavelength to 6.7 nm where a higher reflectivity mirror seems available. The necessity to evaluate an alternative approach is recently proposed based on high repetition rate free electron laser (FEL), to avoid a risk of the source power limit by the plasma-based technology. The possibility is indicated to realize a high repetition rate (superconducting) FEL to generate a multiple kW 13.5 nm light. It is important to note that the present FEL pulses are characterized typically as 0.1 mJ pulse energy, 100 fs pulse duration, and 1 mm beam diameter, and generated in the SASE mode. The beam fluence is higher than the ablation threshold of typical resists, and the beam has a higher spatial coherence, which leads to speckle patterns. The beam is composed of many short spikes with high peak intensities [12]. Seeding an FEL with an external coherent source has been studied together with SASE operation to increase the brightness and pointing/energy stability compared to SASE mode. An efficient seeding method was established by using UV wavelength laser in which the seed laser modulates the electron beam into coherent bunching at the harmonics of the seed laser wavelength. The bunching is intensified in another undulator for coherent FEL action, and the method is named as high-gain higher harmonic generation (HGHG). A successful demonstration is reported from FERMI as a double stage-seeded FEL with a fresh bunch injection technique [13]. The fresh bunch scheme was demonstrated as the FEL radiation produced by one HGHG stage acts as an external seed for a second HGHG stage. A 10 Hz demonstration was reported in the EUV wavelength region. The development of higher repetition rate FEL requires new optical laser developments to meet the needs of laser-induced FEL seeding. Conventional copper accelerating cavities operate up to tens to hundreds of hertz, but superconducting (SC) cavities, allow a much higher repetition rate of up to few megahertz. FLASH at DESY has a maximum repetition rate of 1 MHz within a burst structure (electron bunch train) of 800 μs at 10 Hz. Future linear accelerator designs plan an SC linear accelerator capable of a continuous repetition rate of up to 1 MHz. This presents major challenges for the design and operation of laser-seeded FELs in both burst and continuous mode. At lower repetition rates, conventional Ti:Sapphire lasers are currently used for laser-induced FEL seeding at, for example, FERMI FEL-1. The future requirements of a tunable, high repetition rate laser with sufficient pulse energy can be met with optical parametric chirped-pulsed amplification (OPCPA). A tunable OPCPA is demonstrated at 112 W in burst mode. The center wavelength is located in the wavelength region of 720–900 nm. The repetition rate is 100 kHz and the pulse energy is 1.12 mJ with 30 fs pulse duration. The OPCPA pumping laser power limits the scalability of the OPCPA output, and it was demonstrated for a 6.7–13.7 kW (burst mode) thin-disk OPCPA-pump amplifier, increasing the possible OPCPA output power to many hundreds of watts. Furthermore, the third and fourth harmonic generation experiments are performed for the FEL seeding purpose [14].

Recent solid-state laser progress is closely related to the demands in the field of laser microa‐ blation in industry. Fiber laser is advancing in the high repetition rate, short-pulse operation mode in the subpicosecond pulse length. Significant progress has been made on the scaling of the performance of subpicosecond fiber laser systems in the past decade. The current limitation exists in the achievable peak power and average power of a linear amplifier. The maximum of the available average power in a single fiber laser is determined by the mode instabilities. Several hundred watts is the typical maximum power, depending on the properties of the fiber and other system parameters. The pulse energy is ultimately limited by the extractable energy of the fiber, nonlinear pulse distortions, and damage issues. Four coherently combined fiber amplifiers were reported as a single CPA system [15]. The average power was 530 W and combined pulse energy was 1.3 mJ. It is expected to realize higher system parameters from a beam combined fiber laser, especially in higher average power in pulsed mode. The beam quality was excellent and the beam combination efficiency was as high as 93%. It is expected that with the coherent combination concept and further progress in fiber laser technology, average powers in the range of 1 kW and pulse energy of 10 mJ are realistic parameters in the future. A 10 J, 10 kHz femtosecond laser system is under conceptual design by a coherent combination of 10,000 fibers as the extension of the coherent combining scheme for high repetition rate PW laser [16].

Another promising laser is the InnoSlab laser, which is a thin slab laser cooled from both surface and is reported as a Yb:YAG InnoSlab amplifier with femtosecond pulses of <3 mJ pulse energy with a repetition rate of 100 kHz. The chirped pulse amplification is essential to achieve high average power generation in the power amplifier stage. The laser system is consisted of a 10 mW seed laser with a pulse repetition rate of 100 kHz to MHz, and a preamplifier stage, and a high power InnoSlab amplifier which is followed by a grating pulse compressor. This laser system is ideal for OPCPA pumping and micromaterial processing [17]. The highest average power picosecond laser was reported from a thin-disk multipass laser amplifier, delivering 1.4 kW with pulses of 4.7 mJ pulse energy and duration of 8 ps at 300 kHz repetition rate [18]. The beam quality factor was better than M2 = 1.4. The experiments showed that the thin-disk multipass amplifier can scale pulse energy and average output power independently in the repetition rates between 300 and 800 kHz. Frequency doubling by means of an LBO crystal led to 820 W of average power at a wavelength of 515 nm with 1170 W of incident IR power which corresponded to a conversion efficiency of 70% and an SHG pulse energy of 2.7 mJ. By sum-frequency generation between the beams at 1030 and 515 nm in a second LBO crystal, an average UV power of 234 W (780 μJ of pulse energy) was generated at the wave‐ length of 343 nm with a conversion efficiency of 32%. The output powers in the green and UV spectral region are limited by thermal effects and the apertures of the crystals employed. Future work may try to use shorter seed pulses as well as to increase the output power by imple‐ menting a higher number of passes in the amplifier and the pump module and by increasing the pump power. For the higher harmonic generation, crystals with larger apertures and an improved temperature control is critical to further improve the performance.

capable of a continuous repetition rate of up to 1 MHz. This presents major challenges for the design and operation of laser-seeded FELs in both burst and continuous mode. At lower repetition rates, conventional Ti:Sapphire lasers are currently used for laser-induced FEL seeding at, for example, FERMI FEL-1. The future requirements of a tunable, high repetition rate laser with sufficient pulse energy can be met with optical parametric chirped-pulsed amplification (OPCPA). A tunable OPCPA is demonstrated at 112 W in burst mode. The center wavelength is located in the wavelength region of 720–900 nm. The repetition rate is 100 kHz and the pulse energy is 1.12 mJ with 30 fs pulse duration. The OPCPA pumping laser power limits the scalability of the OPCPA output, and it was demonstrated for a 6.7–13.7 kW (burst mode) thin-disk OPCPA-pump amplifier, increasing the possible OPCPA output power to many hundreds of watts. Furthermore, the third and fourth harmonic generation experiments

Recent solid-state laser progress is closely related to the demands in the field of laser microa‐ blation in industry. Fiber laser is advancing in the high repetition rate, short-pulse operation mode in the subpicosecond pulse length. Significant progress has been made on the scaling of the performance of subpicosecond fiber laser systems in the past decade. The current limitation exists in the achievable peak power and average power of a linear amplifier. The maximum of the available average power in a single fiber laser is determined by the mode instabilities. Several hundred watts is the typical maximum power, depending on the properties of the fiber and other system parameters. The pulse energy is ultimately limited by the extractable energy of the fiber, nonlinear pulse distortions, and damage issues. Four coherently combined fiber amplifiers were reported as a single CPA system [15]. The average power was 530 W and combined pulse energy was 1.3 mJ. It is expected to realize higher system parameters from a beam combined fiber laser, especially in higher average power in pulsed mode. The beam quality was excellent and the beam combination efficiency was as high as 93%. It is expected that with the coherent combination concept and further progress in fiber laser technology, average powers in the range of 1 kW and pulse energy of 10 mJ are realistic parameters in the future. A 10 J, 10 kHz femtosecond laser system is under conceptual design by a coherent combination of 10,000 fibers as the extension of the coherent combining scheme for high

Another promising laser is the InnoSlab laser, which is a thin slab laser cooled from both surface and is reported as a Yb:YAG InnoSlab amplifier with femtosecond pulses of <3 mJ pulse energy with a repetition rate of 100 kHz. The chirped pulse amplification is essential to achieve high average power generation in the power amplifier stage. The laser system is consisted of a 10 mW seed laser with a pulse repetition rate of 100 kHz to MHz, and a preamplifier stage, and a high power InnoSlab amplifier which is followed by a grating pulse compressor. This laser system is ideal for OPCPA pumping and micromaterial processing [17]. The highest average power picosecond laser was reported from a thin-disk multipass laser amplifier, delivering 1.4 kW with pulses of 4.7 mJ pulse energy and duration of 8 ps at 300 kHz repetition rate [18]. The beam quality factor was better than M2 = 1.4. The experiments showed that the thin-disk multipass amplifier can scale pulse energy and average output power independently in the repetition rates between 300 and 800 kHz. Frequency doubling by means of an LBO

are performed for the FEL seeding purpose [14].

104 High Energy and Short Pulse Lasers

repetition rate PW laser [16].

Carbon fiber-reinforced plastic (CFRP) is the most promising light material in aircraft or similar machines. CFRP was processed with the kW picosecond laser with 8 ps pulses and an average output power of up to 1.1 kW at a pulse repetition rate of 300 kHz with a maximum pulse energy of 3.7 mJ. Heat accumulation influences are studied for the processing quality in high average power operation [19]. The pulse overlapping and repetitive scans are studied for the heat accumulation effect in the report. The study indicates an estimation of optimized feed rates and maximum scan speeds. The kW picosecond thin disc laser demonstrated its applic‐ ability in the cutting application of a 2 mm CFRP with a high cutting speed of 0.9 m/min and smaller thermal damage less than 20 μm. These lasers, such as fiber, InnoSlab, and thin disc, have been proving solutions for high beam quality, short-pulse generation in the high average power regime in the past two decades. An alternative approach was reported by a cryogeni‐ cally cooled Yb:YAG by demonstration to have significant potential for efficient near-diffrac‐ tion-limited high average power lasers [20]. A single-pass amplifier was reported with 250 W output power, 54% optical-optical efficiency, M2= 1.1 and a power oscillator with 300 W output power demonstrated 64% optical-optical efficiency, and M2= 1.2. In each case, the laser systems were based on end-pumped laser rod gain modules cryogenically cooled in liquid nitrogen cryostats. The single-pass amplifier is a simple way, compared to fiber or thin disc, to boost the power of a laser oscillator. The output power in the experiments was limited only by the incident pump power. The cryogenically cooled, bulk Yb:YAG four-pass amplifier was operated at 100 kHz repetition rate [21]. The amplified optical pulses were 2.5 mJ pulse energy with <20 ps pulse length before compression and the spectrum for 3.6ps in transform limited duration. The measured power stability was less than 0.5% in half an hour full power opera‐ tion. A flat-top spatial profile was measured with near-diffraction-limited beam divergence. This compact amplifier is ideal for pumping of OPCPA. This chapter describes recent progress of high average power, picosecond thin-disc laser from the research and development of the HiLASE project during 2012–2015. HiLASE R&D laser center is a technological infrastructure in Dolní Břežany near Prague in the Czech Republic, which was founded in a close connection to the ELI activity. Major effort is to develop lasers for high-tech application, in which the shortwavelength generation is one of the dominant ones. HiLASE focuses on the development of kW-class thin-disk-based picosecond and subpicosecond lasers from mJ to sub-1-J pulse energy. Laser pulses are emitted at repetition rates from 1 to 100 kHz with prospective upgrade up to 1 MHz near fundamental wavelength. In order to cover the broadest application potential of the lasers, it was also initiated high-power harmonic frequency generation and high-power mid-IR picosecond system consisting of an OPG followed by double OPA systems (**Figure 1**).

**Figure 1.** Building of the HiLASE R&D Centre in Dolní Břežany, Czech Republic.
