In the 2015 review paper 'Petawatt Class Lasers Worldwide' a comprehensive overview of the current status of highpower facilities of >200 TW was presented. This was largely based on facility specifications, with some description of their uses, for instance in fundamental ultra-high-intensity interactions, secondary source generation, and inertial confinement fusion (ICF). With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification (CPA), which made these lasers possible, we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed. We are now in the era of multi-petawatt facilities coming online, with 100 PW lasers being proposed and even under construction. In addition to this there is a pull towards development of industrial and multidisciplinary applications, which demands much higher repetition rates, delivering high-average powers with higher efficiencies and the use of alternative wavelengths: mid-IR facilities. So apart from a comprehensive update of the current global status, we want to look at what technologies are to be deployed to get to these new regimes, and some of the critical issues facing their development.

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Petawatt and exawatt class lasers worldwide

The development of nonlinear optical (NLO) borate crystals for generation of visible and UV light is reviewed. We first discussed on the basic principles of laser frequency conversion. Then, we examine the trends in research on NLO crystals. The background and present status of NLO borate crystals are summarized. The main considerations are focused on the discussion of crystals like CsLiB6O10 (CLBO), GdxY1−xCa4O(BO3)3 (GdYCOB) and K2Al2B2O7 (KAB). Properties of related materials like β-BaB2O2 (BBO), LiB3O5 (LBO), KBe2BO3F2 (KBBF), Sr2Be2BO7 (SBBO), CsB3O5 (CBO), GdCa4O(BO3)3 (GdCOB) and YCa4O(BO3)3 (YCOB) are included for comparison. We aim to provide a complete view of developing a new NLO borate material for actual laser applications. This review covers various aspects including the search for new materials, the growth of bulk crystals, the characterization of crystal properties as well as the development of new techniques to overcome obstacles in actual laser application, namely, thermal dephasing and laser-induced damage. Finally, perspectives on NLO borate crystals and all-solid-state UV lasers are evaluated.

Recent development of nonlinear optical borate crystals: key materials for generation of visible and UV light

Petawatt solid-state lasers require meter-sized gratings to reach multiple-kilojoule energy levels without laser-induced damage. As an alternative to large single gratings, we demonstrate that smaller, coherently added (tiled) gratings can be used for subpicosecond-pulse compression. A Fourier-transform-limited, 650-fs chirped-pulse-amplified laser pulse is maintained by replacing a single compression grating with a tiled-grating assembly. Grating tiling provides a means to scale the energy and irradiance of short-pulse lasers.

Demonstration of coherent addition of multiple gratings for high-energy chirped-pulse-amplified lasers.

By use of KBe2BO3F2 (KBBF) crystal with a size of 10 mm×10 mm×1.2 mm and a special prism-coupling technique (PCT), fourth-harmonic generation of Ti:sapphire laser systems from 200 to 179.4 nm has been achieved. Moreover, with a Ti:sapphire laser with a 50-fs pulse duration and a 1-kHz repetition rate, conversion efficiency as high as 13% from 400 to 200 nm without any surface-loss correction has also been obtained. The data show that with the PCT a KBBF crystal can produce deep-UV coherent light with measurable power output.

Second-harmonic generation from a KBe(2) BO(3)F(2) crystal in the deep ultraviolet.

The laser challenges and state of the art in high-energy, solid-state petawatt lasers for fast ignition (FI) research are reviewed. A number of new laser systems are currently under construction or being planned that will facilitate proof-of-principle FI experiments. Recent technological advances in each of the major ultrafast laser subsystems are reported, including chirped-pulse generation and broadband amplification in the front end, high-energy amplification, and pulse compression with adaptive wavefront correction. Unique challenges related to operating high-energy chirped-pulse-amplification laser systems for FI, such as protection from target back reflections, are also addressed.

Laser Challenges for Fast Ignition

An array-grating compressor for high-power chirped-pulse amplification lasers

We present a detailed description of the pulse shaping of ultrashort laser pulses with nonlinear optical crystals by introducing a time delay between two fundamental pulses. We demonstrate that the frequency reconversion process from a second-harmonic pulse to fundamental pulses will play an important role in fundamental pulse compression and train pulse generation. A collinear type II potassium dihydrogen phosphate (KDP) crystal can be used to achieve complete pulse shaping for a 1053 nm wavelength. Under conditions of a 50 GW/cm2 fundamental intensity, a 1.0 ps fundamental pulse duration and a 0.7 ps delay time, the second-harmonic pulse can be compressed to 0.14 ps. In addition, the fundamental pulse can be compressed to 0.3 ps by introducing an intensity unbalance into the two fundamental pulses. Furthermore, the train pulse can be generated by increasing the fundamental intensity over 20 GW/cm2. A train pulse with three 0.1 ps individual pulses with a 2.5 THz frequency interval can be generated for a 100 GW/cm2 fundamental intensity. We also demonstrate that pulse shaping with a noncollinear type I BBO crystal can be achieved at wavelengths near 800 nm.

https://iopscience.iop.org/article/10.1143/JJAP.38.6351/pdf

Pulse Shaping of Ultrashort Laser Pulses with Nonlinear Optical Crystals

An improved multipass Michelson interferometer is implemented. This technique uses the fact that the wavelength of a laser diode varies in proportion to the diode's injection current. With this method the sensitivity augmentation is accomplished by inserting a beam splitter into one arm of the interferometer, resulting in multiple reflections between the end mirror and the beam splitter. In addition, the interference of laser beams reflected from two arms can be accomplished with unequal arms in the condition of a short coherence length. The sensitivity increase of interference fringes and the compensation of the short coherence length have been demonstrated in experiments.

Multipass Michelson interferometer with the use of a wavelength-modulated laser diode.

We present the theory behind and discuss experimentation on the spectral control of a laser diode with a Michelson external cavity. This structure is advantageous in terms of variable resonance mode spacing and the wavelength tuning range achieved by adjusting the path-length difference of the external cavity. This laser diode is easy to construct and uses mainly inexpensive components. In theory, we replace the end facet of the laser medium and the mirrors of the external cavity with an equivalent mirror, and therefore, resonance conditions of the laser diode with the external cavity can be analyzed. Through experiment, we investigated dependences of the bandwidth and wavelength tuning range of the laser diode on the path-length difference of the external cavity and the injection current. We achieved a 0.75 nm extension of the wavelength tuning range and a 1/6 reduction in the bandwidth.

Theory behind and Experimentation on the Spectral Control of a Laser Diode with a Michelson External Cavity

We discuss and evaluate properties of CsLiB6O10 (CLBO) and KBe2BO3F2 (KBBF) crystals for second-harmonic generation with ultrashort laser pulses. Basic properties that are determined by the first- and second-order dispersions of the refractive index are discussed, and numerical calculations, including the group-velocity mismatch are carried out. We propose a novel method that compensates for the group-velocity mismatch in order to further increase the conversion efficiency under the condition in which the duration of the second-harmonic pulse is less than that of the fundamental input pulse.

http://iopscience.iop.org/article/10.1143/JJAP.36.6353/pdf

Tiejun Zhang

Papers

An array-grating compressor for high-power chirped-pulse amplification lasers

Pulse Shaping of Ultrashort Laser Pulses with Nonlinear Optical Crystals

Multipass Michelson interferometer with the use of a wavelength-modulated laser diode.

Theory behind and Experimentation on the Spectral Control of a Laser Diode with a Michelson External Cavity

Properties of CsLiB6O10 and KBe2BO3F2 Crystals for Second-Harmonic Generation with Ultrashort Laser Pulses