**3. Pulse duration shortening**

Shanghai Institute of Optics and Fine Mechanics, Chinese 5 PW CPA: laser, 4-pass EDPamplifier [30]. Effective suppression of the parasitic lasing in the final booster amplifier was done using the EDP-technology combined with index-matching cladding technique and the precise control of the time delay between the input seed pulse of 35 J and pump pulses of 312 J. The output energy of 192.3 J from the final amplifier was corresponding to a pump con-

The design of the final EDP-amplifiers was recently developed for three pillars of the Extreme Light Infrastructure (ELI) project [33]. ELI is the ambitious pan-European laser research project. The major mission of the ELI facility is to make a wide range of cutting-edge ultrafast light sources available to the international scientific community. The first purpose of the facilities is to design, develop and build ultra-high-power lasers with focusable intensities and average powers reaching far beyond the existing laser systems. The secondary purpose is to contribute to the scientific and technological development toward generating 200 PW pulses, being the ultimate goal of the ELI project. PW-class lasers have been planned to build in the three pillars of ELI. 2 PW peak power, 10 Hz repetition rates and <20 fs pulse duration lasers will be part of the ELI-BEAMLINES and the ELI-ALPS, while the L4 of the ELI-BEAMLINES as well as the ELI-NP lasers aiming at 300 J/10 PW lasers. The roadmap for 200 PW laser facility was paved by the ELI consortium. This will increase the available laser power by at least one order of magnitude in its first three pillars, and on another more order of magnitude in its fourth ultra-high-intensity pillar. The laser power frontier was planned to be pushed into

The examples of final EDP-amplifiers design of 10s PW laser system are presented on the **Figures 8** and **9**. As the estimations demonstrate, the EDP-technology is able to significantly increase the output energy and intensity. There are calculated as optimal output parameters: the diameter of the pump area and crystal −19 and 20 cm, the pump energy −960 J, the input and the output energy −60 and 600 J. The losses with the EDP technology can be made under

The total losses in 4 cm thickness crystal of conventional amplifier is about 70%, and for 3 cm, it is −80% as seen from the **Figure 8a**, and significant gross begins from 30 and 20 ns of the pump, respectively. On the **Figure 8b**, dependences of losses for optimal extraction of the

For 3-pass, the delay between passes 1 and 2 is about 20 ns and 2 and 3–30 ns, whereas the scheme with delays between passes of the 4-pass amplifier (15, 20, 30 ns) is presented also in the **Figure 8**b. Taking in account the compressor transmission efficiency 70% [32], the compressed energy up to 400 J can be expected. Output peak power about 30 PW can be reached in one channel with pulse duration 10–15 fs. This amplifier could serve as a building block for ELI fourth pillar, and seven EDPCPA channels will be enough for approaching 200 PW.

As a final remark of this part, we can emphasize that there is a big gap between the reached now output energy of 200 J and potential possibilities of the EDP amplifiers of 600–800 J extracted energy with today's available Ti:Sa crystals, which should be filled in the closest future. Moreover, manufacturers are working now under the larger aperture Ti:Sa crystal and

3-pass and 4-pass EDP amplifier are demonstrated with the crystal thickness of 4 cm.

version efficiency of 62% to the output laser energy.

sub-exawatt regime by the establishment of ELI s fourth pillar.

5% (**Figure 8**).

74 High Power Laser Systems

### **3.1. Preserving and restoring pulse spectral bandwidth**

Here, we will concern another way of the peak power increasing, namely pulse shortening. It was discussed above that the gain narrowing and saturation limit the achievable pulse duration to about 30 fs [30, 31]. To achieve the 10–15 fs pulse duration designed for the Apollon system as well as for the 10 PW laser of the ELI [18], further scientific and technological efforts have to be made to avoid spectral narrowing in the power amplifiers.

The idea to use optical rotatory dispersion (ORD) (the angle of polarization rotation dependence on wavelength) spectral filter was suggested for conservation of bandwidth in low gain multipass Ti:Sa amplifiers, typically used for the intermediate and duty end amplifiers of multi-TW-PW class systems [34]. The spectral gain can be effectively re-shaped using difference of π- and σ-emission cross-sections of Ti:Sa crystal which will result in the keeping bandwidth of efficient amplification [13]. The overall amplification process can be kept lossless and efficient as the effective spectral gain is tuned, unlike to the spectral shaping of the pulse. This technique is especially advantageous for amplifiers where the energy extraction is of major importance in intermediate and duty end ones of ultrashort pulse large-scale laser systems.

was mounted next to the Ti:Sa crystal (both with thickness of 15 mm). Both crystals had orthogonally directed C-axis to compensate for the inherent temporal walk-off. The incident angles of amplified beams on the active medium were kept as small as possible to minimize uncompensated walk-off and the total length of the amplifier was set to 2.1 m. To eliminate the spectral interference fringes from the outgoing amplified spectrum, the sapphire crystal was mounted on a fine rotator stage and was adjusted. The compressed pulses were loosely focused into a 5 mm BK7 glass plate by a lens (focal length 3 m) with the most homogeneous part of the SPM signal filtered out by a pinhole located at the focus of the lens. Using this technique, the bandwidth of the seed pulse was broadened in self-phase modulation (SPM) stage and stretched in

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the bulk (**Figure 11**) to show the broadband amplification capability of the scheme.

reflections on surfaces of the uncoated ORD quartz.

**Figure 11.** Schematic of the experiment system. Spectra were measured at places p1–p4.

A multi-passing SF6 glass block (80 cm path) was used for stretching the broadband pulse with a near top-hat spectral profile and a bandwidth of 91 nm, the last one was then collimated by a lens (focal length 1.5 m). The seed pulse energy was about 35 μJ due to the energy losses at the pinhole, as well as two metallic mirrors used in the bulk material stretcher and

The polarization encoded amplification was tested for a high (~200) and low (~30) gain case to demonstrate the effectiveness of the PE-CPA method. The broadband seed was initially amplified without polarization encoding in order to form a baseline (**Figure 12a**) with a gain of about 200. The gain-narrowed feature with a resulting bandwidth of less than the half of the seed was demonstrated at the recorded spectrum. The output spectra closed to the input seed resulted in FWHM bandwidth of 82 nm using polarization encoded amplification. A dip was produced at the center due to the spectral edges of the spectrum that experienced a high gain. The following experiments demonstrated that spectral-dependent gain feature can even broaden the bandwidth depending on the initial shape of the seed spectrum and an amplifier saturation.

The optical rotatory dispersion (ORD) is applied to encode/decode the polarization state of the amplified spectrum in addition to using both π- and σ-emission cross-sections. Polarization vectors of spectral components are encoded before amplification using an ORD quartz crystal (right rotating quartz in **Figure 10b**) with a resulting distribution between π- and σ-crosssections (**Figure 10a**).

The emission cross-section of σ-polarized light is nearly 0.4 of the π-polarized light. The sides of the spectral band are aligned toward the π-axis, while the components around the central and most intense part of the spectrum are directed closer to the σ-axis. This allows shaping of the spectral gain so the gain narrowing can be substantially reduced or eliminated at all. The second quartz crystal (left rotating quartz in **Figure 10b**), with an opposite sign of ORD decodes the polarization state of spectral components after amplification. The whole polarization encoded spectral distribution fits between π- and σ-directions by using of two achromatic λ/2 plates. A temporal walk-off between the π- and σ-components due to birefringence in Ti:Sa can be compensated by the addition of an undoped, but orthogonally oriented sapphire of the same thickness.

The 10 Hz, mJ laser system of the Max Born Institute was using proof-of-principle experiment (**Figure 11**). The 20 fs pulses from oscillator were stretched to 120 ps and amplified in a 10-pass Ti:Sa amplifier to ~1 mJ. The pulses with 35 nm bandwidth were compressed using a diffraction grating compressor to their transform limited pulse duration of 28 fs.

The computer modeling of the PE-CPA amplifier resulting in the broadest bandwidth showed an optimum thickness of 17.5 mm of the quartz crystal (**Figure 10b**). A spectrally dependent model of amplification of strongly chirped pulses was used for modeling the PE-CPA amplification. It includes ORD encoding, saturation of amplification and decoding of polarization vectors. The polarization encoded amplifier was built in a 6-pass configuration with a Ti:Sa crystal of 15 mm thickness (**Figure 11**). It was pumped from both sides by the second harmonic beam of a Q-Switched Nd:YAG laser. The pump diameter was de-magnified from 6 mm to 2.5 mm by two lenses (focal lengths 1 m) to fit it with the diameter of the seed pulse. An undoped sapphire

**Figure 10.** Distribution of polarization directions of spectral components by a 17.4 nm ORD quartz (a); principle schematic of the polarization encoded amplifier (b).

was mounted next to the Ti:Sa crystal (both with thickness of 15 mm). Both crystals had orthogonally directed C-axis to compensate for the inherent temporal walk-off. The incident angles of amplified beams on the active medium were kept as small as possible to minimize uncompensated walk-off and the total length of the amplifier was set to 2.1 m. To eliminate the spectral interference fringes from the outgoing amplified spectrum, the sapphire crystal was mounted on a fine rotator stage and was adjusted. The compressed pulses were loosely focused into a 5 mm BK7 glass plate by a lens (focal length 3 m) with the most homogeneous part of the SPM signal filtered out by a pinhole located at the focus of the lens. Using this technique, the bandwidth of the seed pulse was broadened in self-phase modulation (SPM) stage and stretched in the bulk (**Figure 11**) to show the broadband amplification capability of the scheme.

π- and σ-emission cross-sections of Ti:Sa crystal which will result in the keeping bandwidth of efficient amplification [13]. The overall amplification process can be kept lossless and efficient as the effective spectral gain is tuned, unlike to the spectral shaping of the pulse. This technique is especially advantageous for amplifiers where the energy extraction is of major importance in

The optical rotatory dispersion (ORD) is applied to encode/decode the polarization state of the amplified spectrum in addition to using both π- and σ-emission cross-sections. Polarization vectors of spectral components are encoded before amplification using an ORD quartz crystal (right rotating quartz in **Figure 10b**) with a resulting distribution between π- and σ-cross-

The emission cross-section of σ-polarized light is nearly 0.4 of the π-polarized light. The sides of the spectral band are aligned toward the π-axis, while the components around the central and most intense part of the spectrum are directed closer to the σ-axis. This allows shaping of the spectral gain so the gain narrowing can be substantially reduced or eliminated at all. The second quartz crystal (left rotating quartz in **Figure 10b**), with an opposite sign of ORD decodes the polarization state of spectral components after amplification. The whole polarization encoded spectral distribution fits between π- and σ-directions by using of two achromatic λ/2 plates. A temporal walk-off between the π- and σ-components due to birefringence in Ti:Sa can be compensated by

the addition of an undoped, but orthogonally oriented sapphire of the same thickness.

diffraction grating compressor to their transform limited pulse duration of 28 fs.

The 10 Hz, mJ laser system of the Max Born Institute was using proof-of-principle experiment (**Figure 11**). The 20 fs pulses from oscillator were stretched to 120 ps and amplified in a 10-pass Ti:Sa amplifier to ~1 mJ. The pulses with 35 nm bandwidth were compressed using a

The computer modeling of the PE-CPA amplifier resulting in the broadest bandwidth showed an optimum thickness of 17.5 mm of the quartz crystal (**Figure 10b**). A spectrally dependent model of amplification of strongly chirped pulses was used for modeling the PE-CPA amplification. It includes ORD encoding, saturation of amplification and decoding of polarization vectors. The polarization encoded amplifier was built in a 6-pass configuration with a Ti:Sa crystal of 15 mm thickness (**Figure 11**). It was pumped from both sides by the second harmonic beam of a Q-Switched Nd:YAG laser. The pump diameter was de-magnified from 6 mm to 2.5 mm by two lenses (focal lengths 1 m) to fit it with the diameter of the seed pulse. An undoped sapphire

**Figure 10.** Distribution of polarization directions of spectral components by a 17.4 nm ORD quartz (a); principle

schematic of the polarization encoded amplifier (b).

intermediate and duty end ones of ultrashort pulse large-scale laser systems.

sections (**Figure 10a**).

76 High Power Laser Systems

A multi-passing SF6 glass block (80 cm path) was used for stretching the broadband pulse with a near top-hat spectral profile and a bandwidth of 91 nm, the last one was then collimated by a lens (focal length 1.5 m). The seed pulse energy was about 35 μJ due to the energy losses at the pinhole, as well as two metallic mirrors used in the bulk material stretcher and reflections on surfaces of the uncoated ORD quartz.

The polarization encoded amplification was tested for a high (~200) and low (~30) gain case to demonstrate the effectiveness of the PE-CPA method. The broadband seed was initially amplified without polarization encoding in order to form a baseline (**Figure 12a**) with a gain of about 200. The gain-narrowed feature with a resulting bandwidth of less than the half of the seed was demonstrated at the recorded spectrum. The output spectra closed to the input seed resulted in FWHM bandwidth of 82 nm using polarization encoded amplification. A dip was produced at the center due to the spectral edges of the spectrum that experienced a high gain. The following experiments demonstrated that spectral-dependent gain feature can even broaden the bandwidth depending on the initial shape of the seed spectrum and an amplifier saturation.

**Figure 11.** Schematic of the experiment system. Spectra were measured at places p1–p4.

Seed pulses (energy ~200 μJ) were sent directly to the amplifier, by-passing the compressor and SPM stages to investigate the low gain scenario. A pulse with a bandwidth of about 48 nm was achieved after polarization encoded amplification with a net gain of ~30. In contrast to the top-hat seed spectrum (**Figure 12a**), the amplified Gaussian spectrum is almost 40% broader compared to the seed due to the high-gain experienced by the spectral wings (**Figure 12b**).

It was also shown that an additional polarization rotation takes place during the pulse amplification, substantially changing the conditions of high-energy amplification and introducing the distortion into initial polarization encoding. Despite the additional polarization rotation occurring during amplification impeding the decoding process, the energy efficiency of the back conversion to the linear polarized state can be improved by increasing the thickness of the second quartz crystal. The computer modeling shows that an efficiency as high as 99% to convert energy to a linear polarized state for moderate gain values can be reached with the optimal thickness of the second quartz.

Good compressibility of spectrally shaped PE amplified pulses is highly important for the practical application of the PE-CPA method. For testing its compressibility, the pulse spectrum was broadened to ~72 nm during the PE amplification and then compressed down to compressor consisting of two diffraction gratings. The compressor reduced the bandwidth to ~ 60 nm. Single-shot second-order autocorrelator (AC) was used for pulse duration measurements. The width of the measured AC trace is 28.0 fs which corresponds to the pulse duration close to 18.5 fs.

High-energy polarization encoded Ti:Sa amplifiers are able to deliver an amplified bandwidth of 200 nm, which was predicted by detailed modeling, making it a promising technique for intermediate and final amplifiers of high field Ti:Sa CPA-laser systems. This technique may

**Figure 13.** PE-CPA amplifier (simulation): (a) thicknesses of encoding and decoding quartz plates are equal (17.4 mm); and (b) the second quartz plate is 35 mm thick. The seed pulse has a Gaussian spectrum with FWHN = 180 nm. Pump

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The multiple compression method based on spectral broadening using self-phase modulation (SPM) in the bulk of material with the further recompression of the chirped pulse is able to deliver even shorter pulse duration (potentially below 10 fs) without energy sacrificing. Previous attempts were not suited for the highest powers because it used SPM with combination of spatial filtering in near field [35, 36] in order to provide spatially uniform phase due to the axial uniform part of the Gaussian distributed intensity. These approaches were restricted

In Ref. [37], the next logical step was suggested to use super-Gaussian beam profile due to its higher uniformity. If a laser beam has spatially uniform intensity, SPM can be used in nearfield without spatial filtering and consequently eliminating constraints of the laser energy and transmission efficiency. In a proof-of-principal experiment, spectral broadening of 30 nm pulse spectra to ~80 nm was demonstrated. A flat-top beam profile of the HERCULES laser [31] at 30TW power level was used for SPM spectral broadening and compression of a fraction of this power from 30 to 14 fs by a prism-based compressor was fulfilled. The laser pulse with the

fused silica plate after compression by the standard grating compressor to 30 fs. Glass plates of 0.8, 2 and 2.5 cm thicknesses were used for SFM in the experiments. After that, the 100 times attenuated energy and reduced to 1 cm diameter beam was directed into the prism compressor. The spatially resolved spectrum of the pulse was measured and shows the deviation of the energy within 6% through most part of the beam aperture. It follows from **Figure 14a** that the

on the bulk of

pave the way to PW class Ti:Sa lasers with tens of Joule few cycle laser pulses.

energy about 1 J and 6 cm beam diameter was directed at intensity ~1 TW/cm<sup>2</sup>

**3.2. Multiple compression stages**

energy: 50 J, seed energy: 2 J, 4 passes.

to only mJ-level of the output energy.

The computer modeling shows that a 200 nm spectrum is achievable at a multi-joule level in a PE-CPA Ti:Sa amplifier with seed pulses broader than the experimental bandwidth. This output requires seed pulses with a smooth spectrum preferably with a Gaussian shape. The computer modeling of the output spectras of 4-pass PE-CPA amplifier seeded by a 2 J Gaussian-shaped pulse (FWHM of 180 nm) and pumped by 50 J is shown in **Figure 13**. The spectrum is broadened to 200 nm after amplification. Nearly, 30% depolarization losses (**Figure 13a**) can be compensated by doubling the thickness of the decoding quartz (**Figure 13b**).

**Figure 12.** (a) Amplified spectra with (solid line) and without (dash line) the PE technique under a high gain amplification; (b) spectra before (dash line) and after the PE amplifier (solid line) at a gain of 30.

**Figure 13.** PE-CPA amplifier (simulation): (a) thicknesses of encoding and decoding quartz plates are equal (17.4 mm); and (b) the second quartz plate is 35 mm thick. The seed pulse has a Gaussian spectrum with FWHN = 180 nm. Pump energy: 50 J, seed energy: 2 J, 4 passes.

High-energy polarization encoded Ti:Sa amplifiers are able to deliver an amplified bandwidth of 200 nm, which was predicted by detailed modeling, making it a promising technique for intermediate and final amplifiers of high field Ti:Sa CPA-laser systems. This technique may pave the way to PW class Ti:Sa lasers with tens of Joule few cycle laser pulses.

#### **3.2. Multiple compression stages**

Seed pulses (energy ~200 μJ) were sent directly to the amplifier, by-passing the compressor and SPM stages to investigate the low gain scenario. A pulse with a bandwidth of about 48 nm was achieved after polarization encoded amplification with a net gain of ~30. In contrast to the top-hat seed spectrum (**Figure 12a**), the amplified Gaussian spectrum is almost 40% broader compared to the seed due to the high-gain experienced by the spectral wings (**Figure 12b**).

It was also shown that an additional polarization rotation takes place during the pulse amplification, substantially changing the conditions of high-energy amplification and introducing the distortion into initial polarization encoding. Despite the additional polarization rotation occurring during amplification impeding the decoding process, the energy efficiency of the back conversion to the linear polarized state can be improved by increasing the thickness of the second quartz crystal. The computer modeling shows that an efficiency as high as 99% to convert energy to a linear polarized state for moderate gain values can be reached with the

Good compressibility of spectrally shaped PE amplified pulses is highly important for the practical application of the PE-CPA method. For testing its compressibility, the pulse spectrum was broadened to ~72 nm during the PE amplification and then compressed down to compressor consisting of two diffraction gratings. The compressor reduced the bandwidth to ~ 60 nm. Single-shot second-order autocorrelator (AC) was used for pulse duration measurements. The width of the measured AC trace is 28.0 fs which corresponds to the pulse duration

The computer modeling shows that a 200 nm spectrum is achievable at a multi-joule level in a PE-CPA Ti:Sa amplifier with seed pulses broader than the experimental bandwidth. This output requires seed pulses with a smooth spectrum preferably with a Gaussian shape. The computer modeling of the output spectras of 4-pass PE-CPA amplifier seeded by a 2 J Gaussian-shaped pulse (FWHM of 180 nm) and pumped by 50 J is shown in **Figure 13**. The spectrum is broadened to 200 nm after amplification. Nearly, 30% depolarization losses (**Figure 13a**) can be compensated by doubling the thickness of the decoding

**Figure 12.** (a) Amplified spectra with (solid line) and without (dash line) the PE technique under a high gain amplification;

(b) spectra before (dash line) and after the PE amplifier (solid line) at a gain of 30.

optimal thickness of the second quartz.

close to 18.5 fs.

78 High Power Laser Systems

quartz (**Figure 13b**).

The multiple compression method based on spectral broadening using self-phase modulation (SPM) in the bulk of material with the further recompression of the chirped pulse is able to deliver even shorter pulse duration (potentially below 10 fs) without energy sacrificing. Previous attempts were not suited for the highest powers because it used SPM with combination of spatial filtering in near field [35, 36] in order to provide spatially uniform phase due to the axial uniform part of the Gaussian distributed intensity. These approaches were restricted to only mJ-level of the output energy.

In Ref. [37], the next logical step was suggested to use super-Gaussian beam profile due to its higher uniformity. If a laser beam has spatially uniform intensity, SPM can be used in nearfield without spatial filtering and consequently eliminating constraints of the laser energy and transmission efficiency. In a proof-of-principal experiment, spectral broadening of 30 nm pulse spectra to ~80 nm was demonstrated. A flat-top beam profile of the HERCULES laser [31] at 30TW power level was used for SPM spectral broadening and compression of a fraction of this power from 30 to 14 fs by a prism-based compressor was fulfilled. The laser pulse with the energy about 1 J and 6 cm beam diameter was directed at intensity ~1 TW/cm<sup>2</sup> on the bulk of fused silica plate after compression by the standard grating compressor to 30 fs. Glass plates of 0.8, 2 and 2.5 cm thicknesses were used for SFM in the experiments. After that, the 100 times attenuated energy and reduced to 1 cm diameter beam was directed into the prism compressor. The spatially resolved spectrum of the pulse was measured and shows the deviation of the energy within 6% through most part of the beam aperture. It follows from **Figure 14a** that the initial FWHM of the space-integrated spectrum ~30 nm was broadened to ~80 nm with the 2 cm glass plate.

with a 3 mm glass plate and incident intensity of 8 TW/cm<sup>2</sup>

and much higher intensity (up to tens of TW/cm<sup>2</sup>

and 43 TW/cm<sup>2</sup>

Further development of this idea was done in Ref. [38], where SPM in thin film (within 1 mm)

significantly reduce the instability and white light generation discussed above and is capable to compress 25 fs large energy pulses as high as 1 kJ to the 1–2 fs level. In order to demonstrate the potential of thin-film-compressor, numerical simulation was done with the following initial beam parameters: central wave length 800 nm, pulse duration T = 27 fs, flattop transverse intensity distribution with diameter 160 mm and total energy 27 Joules. Thicknesses of the first and second thin film elements for SPM are 0.5 mm and 0.1 mm. The fundamental peak

As a conclusion of this part of chapter one can deduce, the discussed technologies of the pulse shortening are able to reduce the pulse duration to additional one order without energy sacrificing, which means the output peak power of a few hundred PW are possible from the single

As it was discussed in Introduction (Section 1.3), the applications of the ultra-high peak power laser pulses required also high repetition rate or the other words high average power laser systems. Limitation of the high peak power laser systems on the repetition rate can be overcome by TD-technology and hence can increase their average power. Making the active medium thin reduces the longitudinal gain, which can be compensated by increasing the concentration of active ions and/or using crystals with higher emission cross-section, and the most promising crystal with required characteristics for these amplifiers is Ti:Sa. However, the attempt to increase the longitudinal gain leads to a dramatic growth of the gain in the transverse direction of the active medium. Therefore, TASE and TPG are strongly dependent on the axial gain and the ratio of the pumped area diameter to the crystal thickness (aspect ratio). For thin Ti:Sa active media, these effects cause significant depletion of the inverted population, and hence

**4. High repetition rate of ultra-high peak power laser systems**

limit the extracted energy, the same way as it was detailed discussed in Section 2.

The EDP-technique was suggested to be applied for TD crystals [39] and successfully tested [41] mostly in powerful final stages of ultra-high peak power laser systems with hundreds of TWs to tens of PWs power, which are operating in the saturation regime. The overall gain of the amplifier can be kept as small as ~10–20, since the extraction of energy is a major importance. On the other hand, a large amplifier aperture is required for the high energy level. Therefore, power stages of the said Ti:Sa systems require a crystal size ranging from 5 to 20 cm, contrary to "conventional" TD amplifiers with the aperture of few millimeters where several tens of passes can be done [40]. The reasonable number of passes through the amplifier in this case, is restricted up to 6 due to geometrical complexity and available space. These two requirements form the lower boundary for the small signal gain of 3–5 per pass or the saturated one of 1.5–2.

, after the first and second stages with temporal recompression proce-

at pulse durations 6.4 fs and 2.1 fs correspondingly.

of the white light generation.

intensity is 4.7 TW/cm<sup>2</sup>

channel of CPA laser systems.

dure 16.6 TW/cm<sup>2</sup>

, and resulted in a very low level

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) was suggested. This technique is able to

New Generation of Ultra-High Peak and Average Power Laser Systems

The final compressor consisted of two prisms (glass SF10) separated by ~50 cm, after which the single-shot autocorrelator was used for pulse duration measurement. **Figure 14b** and **c** demonstrate the autocorrelation picture and trace lineout, respectively. The initial nearlytransform limited pulse, with the duration from the FWHM autocorrelation trace of ~30 fs was shown on **Figure 14b** and the solid blue curve in plot (c) inferred assuming Gaussian pulseshape. The pulse after the SPM used with the 2.5 cm glass plate and recompression is shown by autocorrelation (b) and the dashed pink curve in plot (c). The compressed pulse duration exhibits shortening by a factor of 2 leading to ~14 ±1 fs.

During these experiments, the instability from shot to shot of the output pulse shape, distribution of the significant part of the energy into wings (**Figure 14b** and **c**) and high level of the white light generation were observed. The distortion of the output pulse shape and its instability is a result of uncompensated third-order dispersion (TOD) of prism compressor that was used as second stage of compression. Besides, TOD coefficient in nonlinear Schrödinger equation of laser pulse propagation through transparent media is inversely proportional to the third power of the pulse duration and the pulse duration consequently varies inversely with incident peak intensity due to SPM. This is why even small fluctuation of the incident intensity leads to significant distortion of the shape of the compressed pulse. A solution of these two complications could be the replacement of the prism compressor with chirped mirrors with compensated TOD. Using about 10 bounces on the chirped mirrors and the glass plate at Brewster angle promises compressor throughput above 90%.

On the other hand, reducing the white light generation could also be possible since the main processes involved in this generation are efficient only when phase matching conditions are fulfilled, which could be destroyed by correctly choosing incident intensity and glass plate thickness. Evidence of such possibility can be found in Ref. [36], where the SPM was done

**Figure 14.** (a) Pulse spectrums: yellow curve (triangles): SPM with glass plate of 2 cm thickness, pink curve (squares): with plate of the 0.8 cm and blue curve (diamonds) represent the spectrum without the glass plate (no SPM); (b) autocorrelation of compressed pulse after SPM, (c) autocorrelation trace of the initial transform limited pulse (solid blue line) and compressed pulse after SPM (dashed pink line).

with a 3 mm glass plate and incident intensity of 8 TW/cm<sup>2</sup> , and resulted in a very low level of the white light generation.

initial FWHM of the space-integrated spectrum ~30 nm was broadened to ~80 nm with the 2 cm

The final compressor consisted of two prisms (glass SF10) separated by ~50 cm, after which the single-shot autocorrelator was used for pulse duration measurement. **Figure 14b** and **c** demonstrate the autocorrelation picture and trace lineout, respectively. The initial nearlytransform limited pulse, with the duration from the FWHM autocorrelation trace of ~30 fs was shown on **Figure 14b** and the solid blue curve in plot (c) inferred assuming Gaussian pulseshape. The pulse after the SPM used with the 2.5 cm glass plate and recompression is shown by autocorrelation (b) and the dashed pink curve in plot (c). The compressed pulse duration

During these experiments, the instability from shot to shot of the output pulse shape, distribution of the significant part of the energy into wings (**Figure 14b** and **c**) and high level of the white light generation were observed. The distortion of the output pulse shape and its instability is a result of uncompensated third-order dispersion (TOD) of prism compressor that was used as second stage of compression. Besides, TOD coefficient in nonlinear Schrödinger equation of laser pulse propagation through transparent media is inversely proportional to the third power of the pulse duration and the pulse duration consequently varies inversely with incident peak intensity due to SPM. This is why even small fluctuation of the incident intensity leads to significant distortion of the shape of the compressed pulse. A solution of these two complications could be the replacement of the prism compressor with chirped mirrors with compensated TOD. Using about 10 bounces on the chirped mirrors and the glass plate at Brewster angle promises compressor throughput

On the other hand, reducing the white light generation could also be possible since the main processes involved in this generation are efficient only when phase matching conditions are fulfilled, which could be destroyed by correctly choosing incident intensity and glass plate thickness. Evidence of such possibility can be found in Ref. [36], where the SPM was done

**Figure 14.** (a) Pulse spectrums: yellow curve (triangles): SPM with glass plate of 2 cm thickness, pink curve (squares): with plate of the 0.8 cm and blue curve (diamonds) represent the spectrum without the glass plate (no SPM); (b) autocorrelation of compressed pulse after SPM, (c) autocorrelation trace of the initial transform limited pulse (solid blue

line) and compressed pulse after SPM (dashed pink line).

exhibits shortening by a factor of 2 leading to ~14 ±1 fs.

glass plate.

80 High Power Laser Systems

above 90%.

Further development of this idea was done in Ref. [38], where SPM in thin film (within 1 mm) and much higher intensity (up to tens of TW/cm<sup>2</sup> ) was suggested. This technique is able to significantly reduce the instability and white light generation discussed above and is capable to compress 25 fs large energy pulses as high as 1 kJ to the 1–2 fs level. In order to demonstrate the potential of thin-film-compressor, numerical simulation was done with the following initial beam parameters: central wave length 800 nm, pulse duration T = 27 fs, flattop transverse intensity distribution with diameter 160 mm and total energy 27 Joules. Thicknesses of the first and second thin film elements for SPM are 0.5 mm and 0.1 mm. The fundamental peak intensity is 4.7 TW/cm<sup>2</sup> , after the first and second stages with temporal recompression procedure 16.6 TW/cm<sup>2</sup> and 43 TW/cm<sup>2</sup> at pulse durations 6.4 fs and 2.1 fs correspondingly.

As a conclusion of this part of chapter one can deduce, the discussed technologies of the pulse shortening are able to reduce the pulse duration to additional one order without energy sacrificing, which means the output peak power of a few hundred PW are possible from the single channel of CPA laser systems.
