**2.2. Calculated parameters of the amplified radiation**

The short spontaneous lifetime (10–14 ns) of the upper laser level of excimer molecules and a high small signal gain g0 (0.05–0.1 cm−1) lead to a relatively high level of amplified spontaneous emission (ASE). The purpose of the calculations is to predict the output parameters of the laser system with allowance for the ASE in the optical schemes.

The output characteristics of the radiation are calculated for the preamplifier and the main amplifier where the ASE effect is maximal. All of the intensity distributions are presented for a time-constant pump power. **Figure 2** demonstrates the radiation intensity distribution with respect to the length of the preamplifier active medium for various intensities of the input signal. The calculations are performed for the following initial parameters of the active medium: g0 = 0.08 cm−1, α = 0.015 cm−1, τef = 2.5 ns, and σ = 4 × 10−16 cm2 , where α is absorption coefficient, τef is the efficient life time of XeCl\* molecule, and σ is the stimulated emission cross section The spontaneous lifetime of the XeCl\* molecules is τ<sup>s</sup> = 14 ns. Based on the results of the calculations and the input pulse duration, we may conclude that an input energy of about 10 mJ is sufficient for the saturation of the amplifier at two passes. In this case, the intensity of the amplified signal as about 2I<sup>s</sup> (Is is the saturation intensity). For input signals with an intensity of no less than 100 kW/cm<sup>2</sup> , it is expedient to employ single-pass amplification. In general, the ASE effect is weak provided that the input intensity is more than 2 kW/cm<sup>2</sup> at the given parameters and sizes of the active medium.

For the main amplifier with a lower pump power, the parameters of the active medium are as follows: *g*<sup>0</sup> = 0.065 cm−1, α = 0.0145 cm−1, τef = 3 ns, σ = 4 × 10−16 cm2 , and τ<sup>s</sup> = 14 ns [7]. **Figure 3** demonstrates the intensity distributions of the amplified radiation and ASE for the single-pass (a) and double-pass (b) configurations. It is seen that the ASE effect is significantly stronger than the effect in the preamplifier in spite of the comparable values of the product gL, where L is active medium length and g is the gain coefficient. For example, at a kilowatt level of the input signal, the ASE intensity at the output of the main preamplifier is close to the intensity of the amplified radiation, so that the latter cannot effectively make use of the population inversion. Note that the one-dimensional model slightly overestimates the signal-to-noise ratio (especially in the case of double-pass amplification). Therefore, the real ASE effect should be even stronger. Thus, the minimum intensity of the input signal of the main amplifier should be about 10 kW/cm<sup>2</sup> . At a pulse duration of 80 ns, the corresponding energy density is about 1 mJ/cm<sup>2</sup> . In this case, preference should be given to the double-pass configuration with regard to the energy of the amplified radiation.

To start the master complex and to lock it to the amplifiers, we employ a high-voltage pulsed oscillator. The switch on time of the master oscillator and the amplifier is controlled using the cable and artificial delay lines. When the radiation is amplified in the laser system, the following condition is satisfied: the maximum intensity of the input pulse coincides with the pump

To determine the wave-front distortions of the amplified beam on the optical elements, we calculate the optical path taking into account the positions of the elements and the surface finish. For the optical elements with a diameter of up to 100 mm, the wave-front distortion of the laser beam is no greater than λ/4. For a diameter D of greater than 100 mm, the distortion

To find the energy parameters of the radiation, we numerically simulate the amplification modes. For this purpose, we employ the one-dimensional model developed at HCEI based on the system of nonstationary equations for the concentrations of the excimer molecules and

The short spontaneous lifetime (10–14 ns) of the upper laser level of excimer molecules and a

ous emission (ASE). The purpose of the calculations is to predict the output parameters of the

The output characteristics of the radiation are calculated for the preamplifier and the main amplifier where the ASE effect is maximal. All of the intensity distributions are presented for a time-constant pump power. **Figure 2** demonstrates the radiation intensity distribution with respect to the length of the preamplifier active medium for various intensities of the input

(0.05–0.1 cm−1) lead to a relatively high level of amplified spontane-

power maximum in each of the amplifiers.

**Figure 1.** Photograph of the electric-discharge preamplifier.

**2.2. Calculated parameters of the amplified radiation**

laser system with allowance for the ASE in the optical schemes.

is no greater than λ (wave length).

the photon fluxes [7].

4 High Power Laser Systems

high small signal gain g0

In the calculations, we employ the nonstationary model and, therefore, observe the shape of the desired signal and the ASE pulse shape [7]. The pulses at the input and output of the

**Figure 2.** Intensity distributions of (1)–(4) the amplified signals and (5) the total ASE for (1)–(3) and (5) the double-pass and (4) single-pass amplification in the active medium of the preamplifier. The signal traveling from the right-hand side to the left-hand side is reflected by the mirror on the left-hand side. Curve *5* corresponding to the ASE is presented for the conditions of curve *1.*

active medium may substantially differ from each other. This difference depends on the difference between the radiation and pump pulse shapes, on the steepness of the pulse edges and the length of the active medium.

**2.3. Experimental results**

for the spatial filtering of the radiation.

mirrors and the loss resulting from the aperture matching.

100 cm (L is the optical path length in between the stages).

The experiments on the amplification of radiation in the laser systems are performed at a pulse duration of τ0.5 = 80 ns. The purpose of the experiments is to measure the energy characteristics of the amplifiers and to determine the wave-front distortions in the optical path. **Figure 4** shows the experimental optical scheme. In the case under consideration, the amplified beam diverges owing to the presence of the negative lens *5*. This makes it possible, on the one hand, to increase the output energy of the first stage and, on the other hand, to relatively easily match the apertures of the electric-discharge amplifiers. To match the beam size with the aperture of the output amplifier, we additionally employ telescope *6*, which also provides

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**Table 1** demonstrates the measured output radiation energies and divergences for each of the amplification stages. Here, *A* is the beam size at the amplifier output; *E*in and *E*out are the input and output energies, respectively; and Ed is the energy inside the angle Qd. The larger beam size in the preamplifier and amplifier corresponds to the presence of the telescope. The difference between the output and input energies of the stages is related to the loss at the turning

It is seen from **Table 1** that the radiation divergence at the output of the first stage is close to the diffraction-limited divergence. Nevertheless, a worsening of the discharge homogeneity

**Figure 4.** Optical scheme of the experiment: *1* and *4* totally reflecting mirrors, *2* pinhole with a diameter of 1.5 mm, *3* semitransparent mirror*, 5* negative lens with *F* = − 80 cm, *6* telescope consisting of lenses with focal lengths of 50 and

**Figure 3.** Intensity distributions of (solid lines *1–4*) the amplified signals and (dashed lines *1–4*) the total ASE for (a) one and (b) two passes in the active medium of the main amplifier.

## **2.3. Experimental results**

active medium may substantially differ from each other. This difference depends on the difference between the radiation and pump pulse shapes, on the steepness of the pulse edges

**Figure 3.** Intensity distributions of (solid lines *1–4*) the amplified signals and (dashed lines *1–4*) the total ASE for (a) one

and (b) two passes in the active medium of the main amplifier.

and the length of the active medium.

6 High Power Laser Systems

The experiments on the amplification of radiation in the laser systems are performed at a pulse duration of τ0.5 = 80 ns. The purpose of the experiments is to measure the energy characteristics of the amplifiers and to determine the wave-front distortions in the optical path. **Figure 4** shows the experimental optical scheme. In the case under consideration, the amplified beam diverges owing to the presence of the negative lens *5*. This makes it possible, on the one hand, to increase the output energy of the first stage and, on the other hand, to relatively easily match the apertures of the electric-discharge amplifiers. To match the beam size with the aperture of the output amplifier, we additionally employ telescope *6*, which also provides for the spatial filtering of the radiation.

**Table 1** demonstrates the measured output radiation energies and divergences for each of the amplification stages. Here, *A* is the beam size at the amplifier output; *E*in and *E*out are the input and output energies, respectively; and Ed is the energy inside the angle Qd. The larger beam size in the preamplifier and amplifier corresponds to the presence of the telescope. The difference between the output and input energies of the stages is related to the loss at the turning mirrors and the loss resulting from the aperture matching.

It is seen from **Table 1** that the radiation divergence at the output of the first stage is close to the diffraction-limited divergence. Nevertheless, a worsening of the discharge homogeneity

**Figure 4.** Optical scheme of the experiment: *1* and *4* totally reflecting mirrors, *2* pinhole with a diameter of 1.5 mm, *3* semitransparent mirror*, 5* negative lens with *F* = − 80 cm, *6* telescope consisting of lenses with focal lengths of 50 and 100 cm (L is the optical path length in between the stages).


**Table 1.** Parameters of the radiation amplified with an MELS-4 k laser system.

or an increase in the diameter of the beam amplified in it leads to a decrease in the energy concentrated inside the angle Qd to a level of no greater than 50%. Using single-pass amplification in the passive part of the active medium of the first stage, we additionally increase the output energy of the master oscillator by a factor of about 20.

For the second stage, an input energy of 3 mJ is sufficient for the saturation of the amplifier active medium at two passes. A minor increase in the divergence after the second amplification stage in comparison to the divergence after the first stage is related to the presence of the ASE. The active medium of the preamplifier does not contribute to the observed increase in the divergence that is related to the distortions in the remaining part of the optical path (air and optical elements).

> of the system. **Figure 5d** and **e** shows the densitograms of the focal spots for the new scheme. It is seen that the focal spot remains unbroken even at a beam diameter of 150 mm. The calculations show that a certain broadening in comparison to the diffraction size is related to the

> **Figure 5.** Densitograms of the focal spots for beam diameters of (a) 35, (b) and (c) 75, and (d) and (e) 150 mm: *1* original radiation, *2* amplified radiation, and *3* original radiation having passed through the amplifier a few seconds after its work.

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In the measurements of the densitograms, the time interval between the amplifier activations is no less than 30 min. At shorter pauses, the divergence of the original radiation increases even for a beam diameter of 35 mm (**Figure 5a**). Nevertheless, the distortions are not observed when the radiation of the master laser enters the working medium of the amplifier immedi-

The laser system consists of five excimer lasers (Photon-1–Photon-5), the synchronization and starting system, and the matching optical elements. In three lasers, the working mixture is excited by an electric discharge. In two lasers, electron-beam excitation is used. In the experiments, the parameters of the laser radiation are measured using conventional methods and devices. To measure the time and energy characteristics of the laser pulses, we employ an FEK-22 vacuum photodiode, TPI and IKT-2 N calorimeters, and an OPHIR calorimeter with

**Figure 6** shows the exterior view of the first laser (Photon-1) [34]. The electrodes of the discharge gap are mounted inside the steel discharge chamber of the laser, which has a diameter

spherical aberration in the telescope.

**3.1. Discharge lasers**

ately after the termination of the pump pulse.

**3. Laser system with an output aperture of 40 cm**

an L30A-EX head. The signals are detected with Tektronix oscilloscopes.

The atmospheric turbulent flows impede the measurements of the divergence at the output of the main amplifier. In particular, the position and structure of the focal spot are unstable when the radiation of the master oscillator passes through the optical system in the absence of amplification. The instability strongly depends on the presence of heat sources in the vicinity of the optical path and on the time interval after the operation of the amplifiers. This is the reason for the approximate value of the ratio Ed/Eout for a beam size of 10 × 12 cm. We may state that, in this case, nearly 50% of the energy is concentrated in an angle of 5 × 10−5 rad. This result is in agreement with the results of the alternative measurements in which the output radiation is focused by a lens with *F* = 1.5 m on the titanium foil with a thickness of 50 μm: a hole diameter of 100 μm corresponds to a divergence of about 6.5 × 10−5 rad.

To more thoroughly study the effect of the heterogeneities in the active medium of the main amplifier and the optical path, we amplify the radiation of the master oscillator at three beam diameters: 35, 75, and 150 mm, respectively. **Figure 5** demonstrates the intensity distributions of the original radiation (the amplifier is switched off) and the amplified radiation measured in the far-field region. The first three panels correspond to the single-pass amplification in the active medium. It is seen that, for beam diameters of 35 and 75 mm, the divergence of the original and amplified radiation is close to the diffraction-limited divergence Qd. At a beam diameter of 150 mm, the focal spot is broken into a few spots, so that the divergence is significantly higher than Qd.

The most probable reason for this lies in the fluctuations of the air density in the optical path, since, in the case under consideration, the distance between lens *5* and the focal spot is about 25 m. We change the optical scheme to decrease the possible effect of air. In the new scheme, the amplified radiation is expanded with a telescope in front of the amplifier, amplified, reflected by the mirror, amplified on the return pass, compressed by the same telescope, and detected. For detection, the reflection mirror was slightly misaligned relative to the optical axis High-Power Laser Systems of UV and Visible Spectral Ranges http://dx.doi.org/10.5772/intechopen.71455 9

**Figure 5.** Densitograms of the focal spots for beam diameters of (a) 35, (b) and (c) 75, and (d) and (e) 150 mm: *1* original radiation, *2* amplified radiation, and *3* original radiation having passed through the amplifier a few seconds after its work.

of the system. **Figure 5d** and **e** shows the densitograms of the focal spots for the new scheme. It is seen that the focal spot remains unbroken even at a beam diameter of 150 mm. The calculations show that a certain broadening in comparison to the diffraction size is related to the spherical aberration in the telescope.

In the measurements of the densitograms, the time interval between the amplifier activations is no less than 30 min. At shorter pauses, the divergence of the original radiation increases even for a beam diameter of 35 mm (**Figure 5a**). Nevertheless, the distortions are not observed when the radiation of the master laser enters the working medium of the amplifier immediately after the termination of the pump pulse.
