**4.3. Measurement procedures**

The laser beam of front-end was amplified in the XeF(C-A) amplifier in a multipass optical scheme (33 passes) formed by 32 round mirrors increasing in diameter. The mirrors with a reflectivity of 99.7% were fixed along the perimeter of the intracell flanges of the laser cell. A divergent laser beam is injected into the laser cell, so that it expands in diameter from 2 cm at the inlet to 6 cm after 32 passes. When traveling between the mirrors, the beam makes two circular rounds in the laser cell. The penultimate convex mirror directs the beam to a flat 10 cmdiameter mirror located on the optical axis of the laser cell. A reflected beam propagates along the optical axis and is ejected from the laser cell with a diameter of 12 cm.

Input in the XeF(C-A) amplifier laser pulse was preliminary stretched up to 1 ps in a prism stretcher with negative group-velocity dispersion. The amplified negatively chirped laser pulse was expanded up to 20 cm and then one was collimated and compressed in three fused silica plates of 4 cm thickness at a Brewster angle on two passages. The energy loss in the compressor was about 2%. For measurement of the amplified pulse duration, the 90 mm-dia central part of the beam was attenuated by two wedges and focused by a 12-m focal length spherical mirror into a 0.25 mm-dia aperture placed in front of an ASF-20-480 single-shot autocorrelator. The sech2 function was used in autocorrelator to fit the temporal intensity profile.

The energies of the e-beams in the converter and VUV radiation in the laser chamber were measured by TPI-2-7 calorimeter. The output energy ща XeF(C-A) amplifier was measured with an OPHIR energy meter placed in the output beam, which was attenuated with a fused silica wedge and focused to a spot of 2.5 cm. The part of the beam passed through the wedge plate was used to record an image of laser beam on a photographic paper. The small signal gain of active medium was measured using a Sapphire-488 CW semiconductor laser after four passes of its probe beam through the active medium. This laser emits at 488 nm wavelength coinciding with the amplification band maximum of the XeF(C-A) transition.

### **4.4. Experimental results**

A high-purity xenon (99.9997%) was supplied to the gas chamber to provide maximum efficiency of the e-beam to VUV radiation conversion. The chamber preliminary evacuated to a pressure of 10−4 torr. During operation of the amplifier, the intensity of xenon VUV radiation gradually decreased due to outgassing from the stainless steel walls of the gas chamber, exposed to the electron beam. Xenon was circulated through a Sircal MP-2000 purifier to sup-

In our experiments, the mixture in the laser cell was replaced after each shot, because repeated

The laser beam of front-end was amplified in the XeF(C-A) amplifier in a multipass optical scheme (33 passes) formed by 32 round mirrors increasing in diameter. The mirrors with a reflectivity of 99.7% were fixed along the perimeter of the intracell flanges of the laser cell. A divergent laser beam is injected into the laser cell, so that it expands in diameter from 2 cm at the inlet to 6 cm after 32 passes. When traveling between the mirrors, the beam makes two circular rounds in the laser cell. The penultimate convex mirror directs the beam to a flat 10 cmdiameter mirror located on the optical axis of the laser cell. A reflected beam propagates along

Input in the XeF(C-A) amplifier laser pulse was preliminary stretched up to 1 ps in a prism stretcher with negative group-velocity dispersion. The amplified negatively chirped laser pulse was expanded up to 20 cm and then one was collimated and compressed in three fused silica plates of 4 cm thickness at a Brewster angle on two passages. The energy loss in the compressor was about 2%. For measurement of the amplified pulse duration, the 90 mm-dia central part of the beam was attenuated by two wedges and focused by a 12-m focal length spherical mirror into a 0.25 mm-dia aperture placed in front of an ASF-20-480 single-shot auto-

function was used in autocorrelator to fit the temporal intensity profile.

pumping of the mixture decreased the output energy by 20–30%.

the optical axis and is ejected from the laser cell with a diameter of 12 cm.

port the gas purity recovery.

**Figure 15.** Laser cell of the XeF(C-A) amplifier.

20 High Power Laser Systems

**4.3. Measurement procedures**

correlator. The sech2

First of all, the VUV radiation energy transmitted through the CaF<sup>2</sup> windows into the laser cell was measured. Its value was 240–260 J. In view of the quantum efficiency of laser transition and 100% quantum yield of XeF(C) state production, the integral value of the energy stored in the active medium is ~ 90 J. In actuality, the actual lifetime of the XeF(C) state, which is determined by radiative decay and quenching, is much shorter than the pump pulse width. This makes the maximum current value of the energy stored on the XeF(C-A) transition 10 times less than the integral value.

The time profile of the small-signal gain near the CaF<sup>2</sup> windows is shown in **Figure 16**. These results correlate well with those found in the experiments on femtosecond pulse amplification. As can be seen in the **Figure 16**, the maximum gain is 0.004 cm−1 and the FWHM of the amplified signal is ~ 200 ns. The amplification of picosecond pulses was performed within the time interval of 146 ns (33 passes) close to the gain profile maximum.

**Figure 16.** Time profile of the e-beam current in the diode and gain measured near the CaF<sup>2</sup> windows with the continuous laser at 488 nm for a XeF<sup>2</sup> vapor pressure of 0.25 torr.

The small-signal gain distribution over the active medium cross section, measured at different XeF<sup>2</sup> vapor pressures, is shown in **Figure 17**. These measurements show that decreasing the vapor pressure enhances the gain distribution uniformity, but at the same time, it greatly decreases the gain near the pump windows. Final optimization of the gas mixture composition was made by achieving the maximum output energy of the amplified picosecond pulse in the real multipass amplification scheme. The maximum output energy was obtained for a XeF<sup>2</sup> vapor pressure of 0.2–0.25 torr.

In our experiments, the seed pulse energy varied from 0.04 to 2 mJ.At an input energy of 0.04 mJ, the gain of active medium is far from saturation. The total gain factor was 2.5 × 10<sup>3</sup> by 100 mJ output energy. The total gain factor was reduced down to 5 × 10<sup>2</sup> under near-saturation conditions at a seed pulse energy of 2 mJ. In this case, the output energy was 1 J. The shot-to-shot fluctuations in the output pulse energy were within 10%. An imprint of the output laser beam on a photographic paper is shown in **Figure 18**.

**Figure 18.** Autograph of the output laser beam.

High-Power Laser Systems of UV and Visible Spectral Ranges

http://dx.doi.org/10.5772/intechopen.71455

23

**Figure 19.** Pulse duration of the laser beam with energy of 0.7 J.

The ASE power of the XeF(C-A) amplifier with seed pulse blocked was measured for an estimation of the contrast ratio. It was 1 W within an angle of 0.2 mrad, which is close to the output beam divergence angle. Thus, the temporal contrast ratio between the output laser peak and background ASE was ~ 10<sup>13</sup>. It means that the real contrast of output beam is determined by the seed pulse temporal contrast, which is usually ~ 10<sup>10</sup> at the second harmonic frequency [39].

After compression, the pulse duration was measured both with and without amplification in active medium. In both cases, the pulse duration was 50–60 fs inferred assuming sech<sup>2</sup> pulseshape. The output beam energy was 0.5–0.7 J in these experiments. The autocorrelation function for the compressed pulse at 0.7 J output energy, which corresponds to 50 fs pulse duration,

**Figure 17.** Small signal gain distribution from the window toward the center of the laser cell for different XeF<sup>2</sup> vapor pressures. The nitrogen pressure is 190 tоrr.

High-Power Laser Systems of UV and Visible Spectral Ranges http://dx.doi.org/10.5772/intechopen.71455 23

**Figure 18.** Autograph of the output laser beam.

The small-signal gain distribution over the active medium cross section, measured at differ-

the vapor pressure enhances the gain distribution uniformity, but at the same time, it greatly decreases the gain near the pump windows. Final optimization of the gas mixture composition was made by achieving the maximum output energy of the amplified picosecond pulse in the real multipass amplification scheme. The maximum output energy was obtained for a

In our experiments, the seed pulse energy varied from 0.04 to 2 mJ.At an input energy of 0.04 mJ,

tions at a seed pulse energy of 2 mJ. In this case, the output energy was 1 J. The shot-to-shot fluctuations in the output pulse energy were within 10%. An imprint of the output laser beam

The ASE power of the XeF(C-A) amplifier with seed pulse blocked was measured for an estimation of the contrast ratio. It was 1 W within an angle of 0.2 mrad, which is close to the output beam divergence angle. Thus, the temporal contrast ratio between the output laser peak and background ASE was ~ 10<sup>13</sup>. It means that the real contrast of output beam is determined by the seed pulse temporal contrast, which is usually ~ 10<sup>10</sup> at the second harmonic frequency [39]. After compression, the pulse duration was measured both with and without amplification in

active medium. In both cases, the pulse duration was 50–60 fs inferred assuming sech<sup>2</sup>

**Figure 17.** Small signal gain distribution from the window toward the center of the laser cell for different XeF<sup>2</sup>

shape. The output beam energy was 0.5–0.7 J in these experiments. The autocorrelation function for the compressed pulse at 0.7 J output energy, which corresponds to 50 fs pulse duration,

the gain of active medium is far from saturation. The total gain factor was 2.5 × 10<sup>3</sup>

output energy. The total gain factor was reduced down to 5 × 10<sup>2</sup>

vapor pressures, is shown in **Figure 17**. These measurements show that decreasing

by 100 mJ

pulse-

vapor

under near-saturation condi-

ent XeF<sup>2</sup>

22 High Power Laser Systems

XeF<sup>2</sup>

vapor pressure of 0.2–0.25 torr.

on a photographic paper is shown in **Figure 18**.

pressures. The nitrogen pressure is 190 tоrr.

**Figure 19.** Pulse duration of the laser beam with energy of 0.7 J.

is shown in **Figure 19**. This result gives the peak power ~ 14 TW. According to literature data, the highest power of femtosecond pulses in the visible spectrum was attained earlier upon nonlinear conversion to the second harmonic in a Ti:sapphire laser [39] and upon direct amplification of 250-fs pulses in an electron beam–excited XeF(C-A) amplifier [40]. These values (4 TW and 1 TW, correspondingly) are far below the power attained in the present work.

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High-Power Laser Systems of UV and Visible Spectral Ranges

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25

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