The fluctuation–magnification effect on the peak intensity of a laser pulse caused by the nonlinear Kerr effect in the chirped volume Bragg grating (CVBG) compressor is investigated experimentally for a high-energy, thin-disk, chirped pulse amplification system. The nonlinear Kerr effect occurs at the blue end, and the accumulated nonlinear phase shift (B-integral) in the CVBG rises with the increase of laser pulse energy. Experiments show that small fluctuations in peak power of uncompressed pulses cause increasing of the temporal fluctuation and spatial fluctuation due to high Kerr-nonlinearity in the CVBG when B-integral is larger than π. Thus the initial fluctuation would be magnified by the CVBG compressor.
Thin-disk lasers are present in science and industry since the early 90s, yet not so many companies offer them commercially. This was a strong motivation for HiLASE to develop a versatile thin-disk laser platform that would be easily customized to user’s needs and provide a wide range of laser parameters, like output power up to 100 W, pulse energy up to 20 mJ, repetition rate 1-200 kHz, 1 ps pulse duration and wavelength range from Mid-IR to UV. During this presentation, we would like to introduce several customized systems used in different applications and their successful integration into industrial processes.
Shaping and compression of short laser pulses in the course of a second harmonic (2H) generation is a well verified technique in the case of Nd:glass laser radiation converted in KTP crystal cut for type II phase-matching. The laser pulse, few ps in duration, is divided into two orthogonally polarized pulses and one is delayed in respect to the other. Both of them enter the nonlinear optical crystal, in which they propagate with different group velocities. As they are approaching each other, short 2H pulse is generated at their temporal overlap. Shaping of the 2H pulse is achieved by changing the predelay and input pulse intensities. For current high power picosecond laser sources of choice, Yb:YAG lasers, borate crystals have to be used for efficient harmonic generation because of their convenient thermal properties and long lifetime. Although the requirement on group velocities is not fulfilled for the LBO and BBO crystals, numerical calculations have revealed conditions where pulse shaping and even significant pulse compression occur. Fundamental input pulse of 1.8 ps was compressed down to 0.8 ps (FWHM) by 2H generation in LBO crystal. In this work, details about the calculations and measurements of shaping of 2H picosecond Yb:YAG laser pulses in LBO and BBO crystals in dependence on input intensities and predelay will be presented. The shaped 2H pulse measurements performed by an autocorrelator agree well with the calculations, achieving almost 3-fold compression. For the most interesting cases, 2H pulses are characterized by FROG in detail.
In many fields of modern physics and industrial applications high-average power pulsed diode-pumped solid-state lasers are essential. Scaling of these lasers towards higher pulse energies is often limited by the onset of thermal effects which are determined by the average power. In this paper we would like to propose a way of increasing the pulse energies by operating the PERLA B laser system in 100 Hz burst mode with 1 ms burst duration and intra-burst repetition rate of 10 kHz. The CPA-based system incorporates fiber front-end, regenerative amplifier and the multipass amplifier followed by the booster amplifier and <2ps compressor.
In this paper, a practical model of a thin disk regenerative amplifier has been developed based on an analytical approach,
in which Drew A. Copeland [1] had evaluated the loss rate of the upper state laser level due to ASE and derived the analytical
expression of the effective life-time of the upper-state laser level by taking the Lorentzian stimulated emission line-shape
and total internal reflection into account. By adopting the analytical expression of effective life-time in the rate equations,
we have developed a less numerically intensive model for predicting and analyzing the performance of a thin disk
regenerative amplifier. Thanks to the model, optimized combination of various parameters can be obtained to avoid
saturation, period-doubling bifurcation or first pulse suppression prior to experiments. The effective life-time due to ASE
is also analyzed against various parameters. The simulated results fit well with experimental data. By fitting more
experimental results with numerical model, we can improve the parameters of the model, such as reflective factor which
is used to determine the weight of boundary reflection within the influence of ASE. This practical model will be used to
explore the scaling limits imposed by ASE of the thin disk regenerative amplifier being developed in HiLASE Centre.
The technology for extreme ultraviolet (EUV) lithography sources is maturing. Laser produced plasma (LPP) sources with usable power >100 W have been used in high-volume manufacturing (HVM) applications, and 250-W sources are expected to be introduced in HVM soon. However, a further increase of power and cleanness may benefit a powerful picosecond (ps) laser in the near-infrared and wavelength converted spectral region. The HiLASE Centre has been working in thin-disc laser technology and has demonstrated a 0.5-kW platform Perla-C based on a very compact Yb:YAG regenerative amplifier. 100-kHz ps operation has been achieved with a fundamental spatial mode and excellent long-term pointing and energy stability. It is reported on a thin-disc-based ps Yb:YAG solid-state laser technology platform Perla developed in the Czech Republic and the present performance of delivering >4 mJ, <2-ps pulses at a 100-kHz repetition rate with the potential to be upgraded to 1 kW of average power and 1-MHz pulse repetition rate. The ps laser extendibility is important for kW-class LPP sources and controlled free electron laser EUV sources in 10-kW power region.
Relations among absorption of pump beam, quantum defect and thermal load were investigated for pump wavelengths of
968 nm and near 940 nm in two independent, real-time measurement experiments complemented with thermal
distribution simulation. Saturation of absorption at 969 nm pumping for non-lasing operational regime, which affects
temperature rise and exists independently of the thin disk type, disk head construction, pump power and pump beam
diameter is reported. Disk temperature dependence of absorption, quantum defect and disk geometry and large difference
between absorption, disk temperature and o-o efficiency at both pump wavelengths are discussed.
High average power picosecond Yb:YAG thin-disk lasers are being developed at Hilase. A compact 1 mJ/100 kHz and 4 mJ/100 kHz zero-phonon-line-pumped regenerative amplifiers PERLA C with a CVBG compressor provide <2 ps long pulses in a nearly diffraction-limited beam. The output was successfully converted to 2nd and 4th harmonic frequency with high conversion efficiency. High energy, QCW-pumped beamline PERLA B is operated at 45mJ/1kHz in fundamental spatial mode and pulse length < 2ps. Its second stage amplifier is being assembled and 1.8 J was extracted. The latest development status of all thin-disk beamlines at the Hilase center is reported.
High average power picosecond lasers have become an import tool in many fields of science and industry. We report on progress in development of 100 kHz, 100 W picosecond Yb:YAG thin disk laser amplifier with fundamental spatial mode at the HiLASE laser center. More efficient direct pumping to an upper laser level has been employed in order to suppress thermal loading of the thin disk active medium and to increase system stability. We also carefully analyzed and described all benefits of this so called zero phonon line pumping (ZPL) for fundamental spatial mode cavity design and successfully increased extraction efficiency of the amplifier to > 28 %. A novel approach of high-power picosecond pulse compression using a space saving and easy-to-align chirped-volume Bragg grating (CVBG) with high dispersion and high net efficiency approaching 88 % allowed us to build a robust and highly compact pulse compressor. A 100 kHz train of sub-1-milijoule pulses compressed bellow 2 ps (FWHM) in almost diffraction limited Gaussian beam has been successfully generated from this highly compact (900 x 1200 mm) thin-disk-based Yb:YAG regenerative amplifier.
Even though thin-disk medium mounted on a diamond substrate is generally used for high average power operation, we
found that the pulsed pumping of the Yb:YAG thin-disk mounded on a copper-tungsten heatsink could improve both
optical-to-optical O-O efficiency and beam quality. We are expecting that the increase of O-O efficiency is caused by the
suppression of ASE. However, the mechanism of beam quality improvement is not clear. We developed a precise
measurement system of thin-disk deformations based on a Hartmann-Shack wavefront sensor. Investigating thin-disk
dynamics under pulsed pumping can help to greatly improve the mode matching and allow obtaining higher output
energy.
An overview of Czech national R&D project HiLASE (High average power pulsed LASEr) is presented. The HiLASE project aims at development of pulsed DPSSL for hi-tech industrial applications. HiLASE will be a user oriented facility with several laser systems with output parameters ranging from a few picosecond pulses with energy of 5 mJ to 0.5 J and repetition rate of 1-100 kHz (based on thin disk technology) to systems with 100 J output energy in nanosecond pulses with repetition rate of 10 Hz (based on multi-slab technology).
We are developing an Yb:YAG thin disk regenerative amplifier operating at 1 kHz repetition rate which should deliver output of 100 W of average power which corresponds to the pulse energy of 100 mJ. In order to achieve such high output energy, large size mode matching on a thin-disk is required to avoid optical damage but on the other hand, larger mode area is more susceptible to the influence of optical phase distortions (OPD’s) thus limits achievable pulse energy and beam quality. We developed a compact setup allowing precise measurement of the thin-disk deformations by implementation of a Hartmann-Shack wavefront sensor and a single mode probe laser diode. In comparison to the interferometric measurement methods, our approach brings a number of advantages like simplicity of alignment, compactness and robustness, at the same time keeping the high precision of measurement in a range of few nanometers.
We are developing 100-kHz picosecond Yb:YAG thin disk regenerative amplifier with 500-W average power for medical and industrial applications. Especially in case of the next generation of semiconductor lithography, high average power solid-state laser with picosecond pulse duration as pre pulse source is a key element to realize 1-kW EUV lithography source. We compared the output characteristics of CW laser operation pumped at 940-nm and 969-nm, and measured the surface temperature of thin disk. We found that the surface temperature of thin disk pumped at 960-nm was much lower than that pumped at 940-nm. We obtained 83-W output from thin disk regenerative amplifier at the repetition rate of 100-kHz pumped at 969-nm. The measured pulse duration was 1.9-ps.
We are developing a 100-mJ Yb:YAG thin-disk regenerative amplifier operating at 1-kHz repetition rate pumped at zero-phonon-line (968.825-nm1) and delivering 1-2 ps pulses for EUV plasma sources applicable in science and industry. Recently we achieved the output energy of nearly 50-mJ from a single laser-head cavity with good beam quality (M2<1.2) as well as stable beam-pointing (<4μrad). Applying pulsed pumping with the pulse duration shorter than the upper state lifetime of Yb:YAG helps to reduce the ASE and thermal loading of the thin-disk.
We are developing one joule energy and one picosecond pulse duration laser system at the repetition rate of 120 Hz
based on Yb-doped solid-state materials. The amplified output from the thin disk regenerative amplifier is amplified by
a cryogenically conduction cooled single slab amplifier. In this work, we also present a new measurement method of
a gain distribution insensitive to mode matching. One of the advantages of this method is a fact that it provides real
dimensions of the gain distribution. Knowledge about it allows one to find the optimum spatial mode matching to
maximize the output energy.
KEYWORDS: Finite element methods, Thermal modeling, 3D modeling, High power lasers, YAG lasers, Convection, Optimization (mathematics), Thermal effects, Analytical research
We analyzed the transient response characteristics of Yb:YAG thin disk in to clarify the experimentally obtained
advantages of pulsed pumping in 1-kHz repetition rate reported in ref. 2. We applied commercial 2D FEA software
which can calculate transient response of thermal effects. The temperature distributions of thin disk in both the CW
power of 125-W and the average power of pulsed 125-W have been calculated. Even the net heat power were same in
both CW and pulsed pumping, the temperature distribution was lower in pulsed pumping which can provide higher O-O
efficiency and smaller beam distortion. The time evolution of OPD in the pulsed pumping has been analyzed, too.
Development of high-power, picosecond laser sources is a desirable venture for both industry and research. Within the
Hilase project, we are conducting research on both 500-mJ, 1-kHz and 5-mJ, 100-kHz picosecond laser sources based on
the Yb:YAG thin-disk technology. We have developed a prototype thin-disk regenerative amplifier operating up to 10-
kHz repetition rate pumped by the 940-nm fiber-coupled laser diodes. We achieved 5-mJ pulse energy at 10-kHz
operation and 29.5-mJ at 1-kHz. Afterwards, we developed the high-energy regenerative amplifier operating at fixed
repetition rate of 1-kHz and the pulse energy was achieved up to 40-mJ. Simultaneously, we elaborated the highrepetition
rate regenerative amplifier operating at 100-kHz with pulse energy of 220-μJ. The amplified pulse was
compressed with the efficiency of 88% using chirped volume Bragg grating.
We are developing high-energy thin-disk regenerative amplifier operating at kilohertz repetition rate delivering 1-2 ps long pulses for science and industry applications. We have developed a prototype Yb:YAG thin disk regenerative amplifier which can operate up to 10-kHz repetition rate pumped by the 940-nm laser diode. The pulse energy was achieved to 5-mJ at 10-kHz operation and to 29.5-mJ at 1-kHz operation. We found that the pulsed pumping method can improve the optical-to-optical efficiency. We demonstrated the zero-phonon-line-pumped (968.825-nm1) regenerative amplifier at 1-kHz, and obtained the output energy of 32-mJ with nearly diffraction limited beam profile.
We are developing one kilohertz picosecond Yb:YAG thin disk regenerative amplifier with 500-W average power for medical and industrial applications. In case of high energy pulse amplification, a large area mode matching in gain media, which is drastically degenerated by the optical phase distortion, is required to avoid optical damage. We designed in-situ thin disk deformation measurement based on the combination of a precise wavefront sensor and a single mode probe beam. In contrast to a conventional interferometric measurement, this measurement is compact, easy-to-align, and is less affected by mechanical vibrations.
We present recent progress in development of a compact regenerative laser amplifier based on the Yb:YAG thin-disk
technology delivering up to 100 mJ picosecond pulses with a repetition rate of 1 kHz, designed for seeding a kW-class
multipass amplifier for industrial and scientific applications. System utilizes two laser heads pumped at zero-phonon line
(968.825 nm1) by stabilized high-power pump diodes operated in pulsed regime. The concept is based on the CPA
technique where seed pulses produced by fiber oscillator at 1030 nm are stretched and compressed by use of
transmission gratings.
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