**3. Hybrid systems with XeF(C‐A) amplifiers**

Among the gaseous active media with broad amplification band, XeF(C‐A) is the most widely studied. That is why this transition was the first to study the hybrid approach with the use of the broadband excimer molecules. Studies of the femtosecond pulse amplification on the XeF(C‐A) transition were pioneered by demonstration of 1 TW peak power due to the direct amplification of 250 fs seed pulses from a mode‐locked dye laser in the e‐beam‐driven active medium [15]. However, application of electron‐beam excitation to the XeF(C‐A) active medium faces serious difficulties associated with the above mentioned transient absorption and competition of B‐X and C‐A transitions due to strong mixing of closely lying *B* and *C* states of the XeF excimer by electrons. As it was discussed above, transient absorption results in the afterglow gain formation. Strong B‐C states mixing causes C‐state depopulation via ASE depletion of the B‐state which has two orders of magnitude higher stimulated emission cross section for the B‐X transition compared with that for the C‐A transition. Using five‐component mixture comprising F2, NF3, Xe, Kr, and Ar has circumvented this problem due to formation of Kr2F excimers strongly absorbing emission at the wavelength of the B‐X transition (353 nm). As discussed above, optical pumping of this active media is free of both of these shortcomings.

#### **3.1. Hybrid systems based on surface discharge‐driven XeF(C–A) amplifiers**

**2.3. Kr2F active medium**

1388 High Energy and Short Pulse Lasers

KrF(B) forms Kr2F(4 2

Emission band of Kr2F at 420 nm is spectrally shifted relative to emissions of XeF(C‐A) at 480 nm and Xe2Cl at 490 nm that enables twofold broadening of the amplification band (**Figure 1**)

The Kr2F excitation mechanism relies on the KrF2 photodissociation absorption in the VUV spectral range around 164 nm [42] to produce KrF(B) excimers. The utilization of KrF(B) in secondary processes is different, depending on the composition and pressure of the working mixture. For example, in low‐concentration Xe admixtures, exchange processes take place, resulting in XeF(B) formation with a yield close to 100% and laser action at 353 nm upon pumping by radiation from exploding wire [42]. Being mixed with Kr at a pressure of ∼1 bar,

Γ) excimers in three‐body recombination collisions [43].

It was found that the laser action in Kr2F\* also occurs if, instead of Kr, nitrogen is admixed to the working mixture. This observation was attributed to the formation of KrN2F\* four‐atomic excimers that produce Kr2F\* in exchange reactions with Kr atoms generated upon photochem‐ ical decomposition of KrF2 vapor by VUV pump radiation [43]. Note that, despite a complex Kr2F\* formation mechanism, which involves three stages of chemical transformations in mixtures with nitrogen (at one of the stages, products of photochemical processes react with each other), the Kr2F\* yield is rather high providing ∼70% of KrF(B) molecules to be trans‐ formed into Kr2F\* excimers [43]. Lasing in Kr2F at 450 nm was observed upon optical pumping of KrF2:N2 = 1:1500 Torr and KrF2:CF4:Kr = 1:300:1200 Torr gas mixtures by the VUV radiation

The most detailed results of experimental investigations of above considered broadband active

Among the gaseous active media with broad amplification band, XeF(C‐A) is the most widely studied. That is why this transition was the first to study the hybrid approach with the use of the broadband excimer molecules. Studies of the femtosecond pulse amplification on the XeF(C‐A) transition were pioneered by demonstration of 1 TW peak power due to the direct amplification of 250 fs seed pulses from a mode‐locked dye laser in the e‐beam‐driven active medium [15]. However, application of electron‐beam excitation to the XeF(C‐A) active medium faces serious difficulties associated with the above mentioned transient absorption and competition of B‐X and C‐A transitions due to strong mixing of closely lying *B* and *C* states of the XeF excimer by electrons. As it was discussed above, transient absorption results in the afterglow gain formation. Strong B‐C states mixing causes C‐state depopulation via ASE depletion of the B‐state which has two orders of magnitude higher stimulated emission cross section for the B‐X transition compared with that for the C‐A transition. Using five‐component mixture comprising F2, NF3, Xe, Kr, and Ar has circumvented this problem due to formation of Kr2F excimers strongly absorbing emission at the wavelength of the B‐X transition (353 nm). As discussed above, optical pumping of this active media is free of both of these shortcomings.

media and a complete bibliography on the works can be found in Refs. [3, 20, 45].

with the use of two different active media in an amplifier chain.

from an open discharge initiated by an exploding wire [44].

**3. Hybrid systems with XeF(C‐A) amplifiers**

Beginning with the proposal of femtosecond pulse amplification in the optically driven XeF(C‐ A) active medium [14], experimental studies in the field of the hybrid (solid/gas) technology started with the development of the XeF(C‐A) amplifier optically driven by the VUV radiation from large‐area multichannel surface discharges (see [5] and references cited therein). For this purpose, two versions of the photochemically driven XeF(C‐A) amplifier based on the surface discharge as an optical pump source have been built at the P.N. Lebedev Physical Institute (LPI, Moscow, Russia) and at the Lasers Plasmas and Photonic Processes (LP3) Laboratory (Marseille, France) (**Figure 3**). They differ from each other by an output aperture (3 × 11 and 5 × 18 cm<sup>2</sup> , respectively), discharge initiation technique, and pump energy. Pumping sources are based on the multichannel surface discharges initiated along the side walls of rectangular half a meter long dielectric chambers filled with a mixture of XeF2 vapor, argon, and nitrogen at 1 atm. The pumping scheme, in which two planar sources pump an active medium placed between them, provides spatially homogenous excitation of the medium. Moreover, rectan‐ gular aperture of the designed amplifiers offers a simple approach to the development of the multipass optical scheme for energy extraction from XeF(C‐A) active medium characterized by rather low values of small‐signal gain upon optical pumping. In a multipass scheme of the "wedge‐trap configuration," a seed pulse runs between two tilted intracell mirrors allowing for up to 45 double passes through the active medium to be realized.

**Figure 3.** Photographs of the XeF(C‐A) amplifiers built at (a) LP3 and (b) LPI. (c) Inside view of the amplifier cell with multichannel surface discharges fired along its side walls.

Operating performances of the XeF(C–A) amplifier, which were measured at the LP3 Labora‐ tory with the use of a hybrid Ti:sapphire/optical‐parametric‐amplifier front end system, demonstrated a total multipass gain factor of 102 , corresponding to a small‐signal gain of 2 ×  10−3 cm−1, with spectrally and spatially homogeneous amplification.

More details and a complete bibliography on the operating characteristics of the laser amplifier and pump sources relying on the multichannel surface discharge are summarized in [5].

#### **3.2. Hybrid systems based on XeF(C‐A) amplifiers pumped by the VUV radiation of e‐beam converter**

The promising results obtained in the course of the studies of the surface discharge pumped XeF(C‐A) amplifiers motivated the development of an alternative pump technique based on the conversion of e‐beam energy to the VUV radiation, which is expected to be more practical from the viewpoint of the XeF(C‐A) amplifier scaling. The main principle of laser action upon pumping by the e‐beam excited xenon emission was introduced for the first time in [31] to pump a XeF(B‐X) laser operating in the UV region. Later on, this technique was applied to pump a multijoule XeF(C‐A) laser in the visible [33].

To study this approach, two hybrid fs systems, THL‐30 with a design peak power of ∼10 TW and THL‐100 designed for 50–100 TW peak power, have been built at the LPI and Institute of High‐Current Electronics (IHCE, Tomsk, Russia), respectively. Both of these systems comprise Ti:sapphire front ends (Avesta Project Ltd), frequency doublers, prism pair stretcher, and power‐boosting XeF(C‐A) amplifiers driven by e‐beam‐to‐VUV‐flash converters made at the IHCE.

#### *3.2.1. THL‐30 hybrid system*

Photos of the front end and XeF(C‐A) amplifier incorporated into the THL‐30 hybrid system are presented in **Figure 4**. **Figure 5** shows the cross sectional schematic diagram of the XeF(C‐ A) amplifier. The e‐beam converter (2) of cylindrical form is filled with pure xenon at a pressure of 3 bars. Emission of Xe2\* is excited by four 120 cm long × 15 cm wide radially converging beams of electrons accelerated up to 450 keV in the vacuum diode (1) and injected into the converter through 40 μm Ti foils. The laser cell (3) consists of a 12 × 12 × 128 cm square cross‐ sectional tube with arrays of 10 rectangular CaF2 windows (12 × 12 cm) sealed on its side walls. The cell containing XeF2/N2 mixture at 0.25–1 bar is housed into the e‐beam converter along its axis. In this configuration, the laser cell is thus immersed into the xenon. The distance of 7.5 cm between the Ti foil and CaF2 windows is chosen to assure that the electrons issued from the e‐beams are stopped in the converter at the xenon operating pressure of 3 bars. The e‐beams excite the xenon over a 250 ns pulse to produce Xe<sup>2</sup> \* fluorescence at 172 nm with a ∼30% average fluorescence efficiency [33]. The Xe2\* radiation is transmitted through arrays of CaF2 windows into the laser cell to photodissociate XeF2. The e‐beam energy deposited into xenon

**Figure 4.** THL‐30: Photographs of the Ti:sapphire front end (left) and XeF(C‐A) amplifier (right).

is measured to be 2.5–3 kJ, which yields a small signal gain of (1.5–2.5) × 10-3 cm-1 in the laser cell [46, 47].

**Figure 5.** Cross‐sectional schematic diagram of the XeF(C‐A) amplifier: (1) vacuum diode, (2) e‐beam converter, and (3) photolytic laser cell.

Seed pulses with 50 fs duration and 5 mJ energy at 475 nm are produced in the solid‐state front end consisting of a Ti:sapphire oscillator operating at 950 nm, regenerative and multipass amplifiers, and KDP frequency doubler spectrally matching the front end to the boosting XeF(C‐A) amplifier. Nonlinear frequency upconversion also allows for temporal cleaning of seed pulses injected into the final gas amplifier.

Before the final amplification, the seed pulses are negatively chirped to 1 ps with the use of a prism‐pair arrangement. Besides avoiding nonlinear pulse distortion in the amplifier, seed pulse stretching is required to exceed the rotational reorientation time of the XeF(C) molecules, which is estimated to be about 0.8 ps [48, 49]. If the seed pulse is linearly polarized and shorter than the above value, saturation of only a portion of all excited XeF molecules is possible (because of random molecular orientation), thus limiting energy extraction from the amplifier. Down‐chirped pulses can be then recompressed due to the positive group velocity dispersion in bulk glass and/or chirped mirrors.

A multipass optical scheme for energy extraction from the active medium has a 3D "wedge‐ trap" configuration formed by two pairs of tilted intracell mirrors providing the displacement of a beam in two mutually orthogonal directions.

In test experiments, an output energy of 0.25 J has been extracted from the amplifier seeded with a 4 mJ pulse [46, 47], indicating that 5 TW peak power can be obtained in THL‐30. Presently, this system is mainly used for the development of key technologies purposed for the implementation in the THL‐100 system to be discussed below in more detail.

#### *3.2.2. THL‐100 hybrid system*

**3.2. Hybrid systems based on XeF(C‐A) amplifiers pumped by the VUV radiation of e‐beam**

The promising results obtained in the course of the studies of the surface discharge pumped XeF(C‐A) amplifiers motivated the development of an alternative pump technique based on the conversion of e‐beam energy to the VUV radiation, which is expected to be more practical from the viewpoint of the XeF(C‐A) amplifier scaling. The main principle of laser action upon pumping by the e‐beam excited xenon emission was introduced for the first time in [31] to pump a XeF(B‐X) laser operating in the UV region. Later on, this technique was applied to

To study this approach, two hybrid fs systems, THL‐30 with a design peak power of ∼10 TW and THL‐100 designed for 50–100 TW peak power, have been built at the LPI and Institute of High‐Current Electronics (IHCE, Tomsk, Russia), respectively. Both of these systems comprise Ti:sapphire front ends (Avesta Project Ltd), frequency doublers, prism pair stretcher, and power‐boosting XeF(C‐A) amplifiers driven by e‐beam‐to‐VUV‐flash converters made at the

Photos of the front end and XeF(C‐A) amplifier incorporated into the THL‐30 hybrid system are presented in **Figure 4**. **Figure 5** shows the cross sectional schematic diagram of the XeF(C‐ A) amplifier. The e‐beam converter (2) of cylindrical form is filled with pure xenon at a pressure of 3 bars. Emission of Xe2\* is excited by four 120 cm long × 15 cm wide radially converging beams of electrons accelerated up to 450 keV in the vacuum diode (1) and injected into the converter through 40 μm Ti foils. The laser cell (3) consists of a 12 × 12 × 128 cm square cross‐ sectional tube with arrays of 10 rectangular CaF2 windows (12 × 12 cm) sealed on its side walls. The cell containing XeF2/N2 mixture at 0.25–1 bar is housed into the e‐beam converter along its axis. In this configuration, the laser cell is thus immersed into the xenon. The distance of 7.5 cm between the Ti foil and CaF2 windows is chosen to assure that the electrons issued from the e‐beams are stopped in the converter at the xenon operating pressure of 3 bars. The e‐beams

average fluorescence efficiency [33]. The Xe2\* radiation is transmitted through arrays of CaF2 windows into the laser cell to photodissociate XeF2. The e‐beam energy deposited into xenon

**Figure 4.** THL‐30: Photographs of the Ti:sapphire front end (left) and XeF(C‐A) amplifier (right).

\*

fluorescence at 172 nm with a ∼30%

pump a multijoule XeF(C‐A) laser in the visible [33].

excite the xenon over a 250 ns pulse to produce Xe<sup>2</sup>

**converter**

14010 High Energy and Short Pulse Lasers

IHCE.

*3.2.1. THL‐30 hybrid system*

Architecture of THL‐100 laser is substantially similar to that described above for the THL‐30 hybrid system. Its optical scheme is shown in **Figure 6**.

**Figure 6.** Optical scheme of THL‐100 laser system. Ti:Sa front end—Start‐480M; SF1 and SF2—vacuum spatial filters; compressor—4‐cm‐thick fused silica plates at Brewster angle; D—260 μm diaphragm; concave mirror‐1—F = 7.5 m; concave mirror‐2—F = 12 m.

#### *3.2.2.1. Front end*

The front end (Start‐480M manufactured by "Avesta Project Ltd") shown in **Figure 7a** consists of a Ti:sapphire master oscillator pumped by a CW pump laser (Verdy‐8) at a wavelength of 532 nm, grating stretcher, regenerative and two multipass amplifiers pumped by repetitively pulsed lasers (SOLAR Laser Systems) at a wavelength of 532 nm, spatial filter after the final amplifier, grating compressor, and generator of the second harmonic. The output energy of 20 mJ is produced in a 50 fs pulse at the wavelength of the second harmonic (∼475 nm) [50]. The front end operates in the single pulse mode and with a repetition rate of 10 Hz. The output beam of 2.5 cm diameter is directed to the prism pair arrangement (**Figure 7b**) with negative group velocity dispersion, which allows a seed pulse to be stretched. It consists of a mirror telescope with a magnification M = 3, two fused silica prisms allowing a 75‐mm‐diameter beam to pass, and the mirrors for beam transportation between the prisms in the forward and backward directions. The maximum distance that can be realized between the prisms is 9.6 m, corresponding to the 2.4 ps pulse duration. After the prism pair, the laser beam is directed into the XeF(C‐A) amplifier.

**Figure 7.** (a) Femtosecond Ti:sapphire front end. (b) Prism pair arrangement.

#### *3.2.2.2. Photodissociation XeF(C‐A) amplifier*

**Figure 8** shows a photograph of the XeF(C‐A) amplifier. Its principle of operation and the design is similar to the above described XeF(C‐A) amplifier of THL‐30 laser system. However, it has its differences: the e‐beam converter is driven by six electron beams instead of four and its pump energy is four times higher. The amplifier includes two high‐voltage pulse generators operating at U0 = 90 kV or 95 kV charging voltage, a vacuum electron diode, gas chamber with a foil support structure, which filled with xenon, and laser cell with two mirror units for multipass amplification of the laser beam. The design and specifications of the XeF(C‐A) amplifier are described in detail in [47, 51–54].

**Figure 8.** General view of the XeF(C‐A) amplifier.

**Figure 6.** Optical scheme of THL‐100 laser system. Ti:Sa front end—Start‐480M; SF1 and SF2—vacuum spatial filters; compressor—4‐cm‐thick fused silica plates at Brewster angle; D—260 μm diaphragm; concave mirror‐1—F = 7.5 m;

The front end (Start‐480M manufactured by "Avesta Project Ltd") shown in **Figure 7a** consists of a Ti:sapphire master oscillator pumped by a CW pump laser (Verdy‐8) at a wavelength of 532 nm, grating stretcher, regenerative and two multipass amplifiers pumped by repetitively pulsed lasers (SOLAR Laser Systems) at a wavelength of 532 nm, spatial filter after the final amplifier, grating compressor, and generator of the second harmonic. The output energy of 20 mJ is produced in a 50 fs pulse at the wavelength of the second harmonic (∼475 nm) [50]. The front end operates in the single pulse mode and with a repetition rate of 10 Hz. The output beam of 2.5 cm diameter is directed to the prism pair arrangement (**Figure 7b**) with negative group velocity dispersion, which allows a seed pulse to be stretched. It consists of a mirror telescope with a magnification M = 3, two fused silica prisms allowing a 75‐mm‐diameter beam to pass, and the mirrors for beam transportation between the prisms in the forward and backward directions. The maximum distance that can be realized between the prisms is 9.6 m, corresponding to the 2.4 ps pulse duration. After the prism pair, the laser beam is directed into

concave mirror‐2—F = 12 m.

14212 High Energy and Short Pulse Lasers

the XeF(C‐A) amplifier.

**Figure 7.** (a) Femtosecond Ti:sapphire front end. (b) Prism pair arrangement.

*3.2.2.1. Front end*

An electron accelerator generates six 100 cm long × 15 cm wide electron beams with a maxi‐ mum energy of 550 keV (at U0 = 95 kV) at the total diode current of 250 kA in a 150 ns (FWHM) pulse. Bunches are injected into the chamber filled with xenon at a pressure of 3 bar. The electron beam energy is converted to the VUV radiation of Xe2\* excimers at a wavelength of 172 nm with an efficiency of 30–40%. Inside the gas chamber along its axis is the hexagon laser cell (**Figure 9a**), on the side faces of which there are a total of 54 windows made of CaF2. The windows with size of 12 × 12 cm and 2 cm thick are set in grooves on the rubber gasket (Viton) and sealed by means of clamping flanges. The windows are arranged opposite grates with foil, through which the electron beam is injected into the gas chamber. This provides the best geometric coupling of the cell with laser pump source. The output aperture of the laser cell has a diameter of 24 cm. Both ends of the laser cell are sealed with fused silica windows with a diameter of 30 cm. Inside the cell, there are two mirror units that provide multiple passage of the seed pulse through the active region. Each unit has 16 mirrors of different diameters arranged along its perimeter. The mirror reflection coefficient is 99.5–99.7%. The laser cell is filled with a gas mixture consisting of 0.25–0.5 bar high purity nitrogen and 0.1–0.4 Torr XeF2 vapor. **Figure 9b** shows an internal view of the laser chamber with the mirror unit at the rear end. To maximize the efficiency of conversion of the electron beam energy into the VUV radiation, the high purity (99.9997%) xenon is used and the e‐beam converter gas chamber is evacuated to a pressure of 10-4 Torr. As xenon purity decreases due to exposure of the electron beams to the structural elements of gas chamber, its purification is carried out by a filtering device "Sircal MP‐2000."

**Figure 9.** (a) Laser cell of the XeF(C‐A) amplifier. (b) Inside view of the laser cell with the mirror unit.

#### *3.2.2.3. Experimental techniques*

The active medium of the XeF(C‐A) amplifier is created under the action of the VUV radiation at a wavelength of 172 nm. XeF excimer molecules are formed in the B‐state via XeF2 photo‐ dissociation. The upper state of the XeF(C‐A) laser transition is formed as a result of the relaxation of the XeF(B) molecules in collisions with the molecules of the N2 buffer gas. Amplification of the seed pulses was carried out in a multipass optical scheme (33 passes). The laser beam entering into the XeF(C‐A) amplifier was made slightly divergent, so that during amplification, it was steadily increased in diameter from 20 mm (inlet) to 62 mm (the penulti‐ mate mirror), making a double round‐trip along the inside perimeter of the laser cell. The penultimate convex mirror directed the beam to a flat mirror 100 mm in diameter, located on the optical axis. After reflection from this mirror, the beam propagated along the optical axis of the cell reaching 120 mm in diameter at the amplifier output.

Small‐signal gain distribution over the laser chamber cross section was measured at four passes through the active medium of the XeF(C‐A) amplifier with the help of the Sapphire‐488 CW semiconductor laser (488 nm). In addition, the total small‐signal gain, G, was measured in the 33 pass amplification scheme. For measuring the power of amplified spontaneous emission (ASE) of the XeF(C‐A) amplifier, the output radiation without a seed pulse was focused by concave mirror with a focal length of 22 m on an aperture with diameter 1.1 mm, behind which a filter and calibrated photodiode were located.

Seed pulses of 50 fs duration from the front end were pre‐lengthened to 1–2.4 ps in the prism pair with negative group velocity dispersion. After amplification, the down‐chirped laser pulses were recompressed in a double pass of a collimated beam with a diameter of 20 cm through three 4‐cm‐thick fused silica plates at the Brewster angle. Energy losses in the compressor did not exceed 2%. To measure the pulse duration, the central part of the beam (with the help of two fused silica wedges and a spherical mirror with diameter of 90 mm and focal length of 12 m) was assigned to the autocorrelator ASF‐20–480 through an aperture with a diameter of 260 μm.

#### *3.2.2.4. Experimental results*

beams to the structural elements of gas chamber, its purification is carried out by a filtering

**Figure 9.** (a) Laser cell of the XeF(C‐A) amplifier. (b) Inside view of the laser cell with the mirror unit.

of the cell reaching 120 mm in diameter at the amplifier output.

a filter and calibrated photodiode were located.

a diameter of 260 μm.

The active medium of the XeF(C‐A) amplifier is created under the action of the VUV radiation at a wavelength of 172 nm. XeF excimer molecules are formed in the B‐state via XeF2 photo‐ dissociation. The upper state of the XeF(C‐A) laser transition is formed as a result of the relaxation of the XeF(B) molecules in collisions with the molecules of the N2 buffer gas. Amplification of the seed pulses was carried out in a multipass optical scheme (33 passes). The laser beam entering into the XeF(C‐A) amplifier was made slightly divergent, so that during amplification, it was steadily increased in diameter from 20 mm (inlet) to 62 mm (the penulti‐ mate mirror), making a double round‐trip along the inside perimeter of the laser cell. The penultimate convex mirror directed the beam to a flat mirror 100 mm in diameter, located on the optical axis. After reflection from this mirror, the beam propagated along the optical axis

Small‐signal gain distribution over the laser chamber cross section was measured at four passes through the active medium of the XeF(C‐A) amplifier with the help of the Sapphire‐488 CW semiconductor laser (488 nm). In addition, the total small‐signal gain, G, was measured in the 33 pass amplification scheme. For measuring the power of amplified spontaneous emission (ASE) of the XeF(C‐A) amplifier, the output radiation without a seed pulse was focused by concave mirror with a focal length of 22 m on an aperture with diameter 1.1 mm, behind which

Seed pulses of 50 fs duration from the front end were pre‐lengthened to 1–2.4 ps in the prism pair with negative group velocity dispersion. After amplification, the down‐chirped laser pulses were recompressed in a double pass of a collimated beam with a diameter of 20 cm through three 4‐cm‐thick fused silica plates at the Brewster angle. Energy losses in the compressor did not exceed 2%. To measure the pulse duration, the central part of the beam (with the help of two fused silica wedges and a spherical mirror with diameter of 90 mm and focal length of 12 m) was assigned to the autocorrelator ASF‐20–480 through an aperture with

device "Sircal MP‐2000."

14414 High Energy and Short Pulse Lasers

*3.2.2.3. Experimental techniques*

The gain characteristics of the active medium (viz. value and spatial distribution of the small‐ signal gain) depending on the initial composition of the working mixture were examined using 4‐passes amplification of the probe laser. The optimal composition of the gas mixture (0.25 Torr XeF2 and 190 Torr nitrogen) corresponding to the maximum value of the unsaturated total gain (G = (5–6) × 10<sup>3</sup> ) was found as a result of a compromise between the uniformity of the radial distribution of the small‐signal gain and its maximum lying in the peripheral region of the laser cell, where the main part of the beam trajectories is located. It should be noted that the heterogeneity of the small‐signal gain distribution over the chamber cross section had almost no effect on the uniformity of the beam intensity, as the diameters of the laser beam and laser cell differ by almost an order of magnitude.

In the test mode, when the maximal pump energy (at U0 = 95 kV) of the VUV radiation was 240 J, the energy of output radiation attained 1 J at amplification of a 1 ps seed pulse with an energy of 1.8 mJ. At the level of 0.7 J output energy, the pulse duration after the bulk fused silica compressor was measured with and without activation of the XeF(C‐A) amplifier. In both cases, the pulse width was measured to lie in the range of 50–60 fs (**Figure 10a**). This indicates that a peak power of 14 TW was reached at the output of the laser system [47].

**Figure 10.** (a) Autocorrelation function of the output pulse with an energy of 0.7 J (in the Sech<sup>2</sup> approximation). (b) The time behavior of the diode current (1) and total gain (2). U0 = 95 kV.

The ASE power from the XeF(C‐A) amplifier, which was measured in the angle of 0.5 mrad with the seed pulse blocked, turned out to be as low as 0.7 W. At the 14 TW peak power obtained in the above experiments, this ASE power corresponds to the temporal contrast ratio of 2 × 10<sup>13</sup>. Since the temporal contrast at the 1010 level is now routine to obtain in Ti:sapphire systems, and taking into account the nonlinear frequency doubling required to spectrally match the Ti:sapphire front end to the XeF(C‐A) boosting amplifier, the temporal contrast of the hybrid (solid/gas) systems seems to be determined only by the ASE of the XeF(C‐A) amplifier and is expected to reach 1012–10<sup>13</sup> at a peak power of about 100 TW.

For further improvements of the THL‐100 operating performances, the XeF(C‐A) amplifier pump system has been upgraded [51, 52] to increase the maximum VUV pump energy up to 300 J at U0 = 95 kV. As a result, the total small‐signal gain at 33 passes was enhanced up to (5– 6) × 10<sup>4</sup> . **Figure 10b** shows the temporal behavior of the diode current (1) and total small‐signal gain (2) at 33 passes through the active medium. **Figure 11** shows the dependence of the small‐ signal gain in arbitrary units vs the partial pressures of nitrogen and XeF2 vapor.

**Figure 11.** (a) Dependence of the small‐signal gain vs the nitrogen pressure, p(XeF2) = 0.25 Torr, 1—U0 = 90 kV, 2—U0 =  95 kV. (b) Dependence of the small‐signal gain vs the XeF2 pressure, p(N2) = 0.5 bar, U0 = 90 kV.

Amplification of a chirped pulse in the XeF(C‐A) amplifier was carried out with the use of the laser mixture containing 0.2 Torr XeF2 and 0.5 bar nitrogen at U0 = 95 kV. An output energy of 2.5 J was reached when a 2.4 ps seed pulse with an energy of about 1 mJ in a super‐Gaussian beam was injected into the XeF(C‐A) amplifier [51]. An autograph of the output laser beam on a photographic paper sheet is shown in **Figure 12**. The energy obtained in this experiment promises a peak power as high as ∼50 TW to be attained in the visible after pulse recompres‐ sion to the initial duration of 50 fs.

**Figure 12.** Imprint of the output laser beam with 2.5 J energy.
