**2.1. Experimental method and equipment**

The MELS-4 k laser system consists of the master complex, preamplifier, and a UFL-100 M amplifier [7]. The master complex contains two electric-discharge XeCl lasers. One of them serves as the master oscillator. The combination of the two lasers makes it possible to vary the parameters of the output radiation. In particular, for single-pass lasing, the radiation energy is *E* = 15 mJ, the spectral line half width is Δν = 0.01 cm−1, and the pulse FWHM is 50 ns. In the case of injection locking, the parameters of the output beam are E = 100 mJ, Δν = 0.01 cm−1, and τ = 100 ns, while more than 50% of the radiation energy is concentrated inside the diffraction angle [31]. In the case of double-pass lasing with the phase conjugation, the parameters are *E* = 50 mJ, Δν = 0.01–0.4 cm−1, and τ = 30 ns, and the divergence is close to the diffractionlimited Qd [32]. When the pulse is compressed to a pulse duration of 1–2 ns upon stimulated Brillouin scattering, the parameters are *E* = 10 mJ, Δν = 0.01 cm−1, and Qd [33].

The preamplifier represents an electric-discharge lase with an active volume of 6x11x80 cm<sup>3</sup> (**Figure 1**). This laser consists of a metal housing that contains the dielectric laser chamber, capacitors with a total capacitance of 368 nF that are directly connected to the electrodes, and the x-ray source. A discharge gap and a storage capacitor (0.4 μF) are placed outside. The laser mixture Ne/Xe/HCl = 1000/10/1 is photo-ionized at a pressure of 2–4 atm. The storage capacitor is connected to the discharge gap, and 300 ns prior to the moment when the voltage across the electrodes reaches the maximum value, the x-ray source is switched on. The radiation of this source initiates the discharge. The x-ray radiation is injected through a stainless steel grid with a geometrical transparency of 50%. The doze inside the laser chamber is about 25 mR. The laser energy amounts to 6–10 J at a pulse duration of τ = 80–160 ns.

The active volume of the main amplifier is 25 × 25 × 100 cm<sup>3</sup> . The gas is excited by two electron beams [6]. The electron accelerators are placed at the top and bottom of the laser chamber, which has an internal volume of 360 l. In each accelerator, the vacuum diode and the high-voltage generator are placed in a single metal housing. The cathode of the vacuum diode is directly fixed on the last stage of the high-voltage generator. The maximum energy of the laser with a plane-parallel cavity is 210 J, and the pulse duration is τ = 250 ns. In the amplification mode, the windows of the laser chamber are tilted at an angle of 100 relatively to the optical axis.

signal. The calculations are performed for the following initial parameters of the active medium:

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

For the main amplifier with a lower pump power, the parameters of the active medium are as

onstrates 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 inten-

is the saturation intensity). For input signals with an intensity of no less than

, it is expedient to employ single-pass amplification. In general, the ASE effect is

, where α is absorption coefficient,

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

5

High-Power Laser Systems of UV and Visible Spectral Ranges

at the given parameters and sizes

, and τ<sup>s</sup> = 14 ns [7]. **Figure 3** dem-

. At a pulse duration of

. In this case, preference should be given

g0 = 0.08 cm−1, α = 0.015 cm−1, τef = 2.5 ns, and σ = 4 × 10−16 cm2

weak provided that the input intensity is more than 2 kW/cm<sup>2</sup>

follows: *g*<sup>0</sup> = 0.065 cm−1, α = 0.0145 cm−1, τef = 3 ns, σ = 4 × 10−16 cm2

sity of the input signal of the main amplifier should be about 10 kW/cm<sup>2</sup>

to the double-pass configuration with regard to the energy of the amplified radiation.

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.*

80 ns, the corresponding energy density is about 1 mJ/cm<sup>2</sup>

as about 2I<sup>s</sup>

100 kW/cm<sup>2</sup>

(Is

of the active medium.

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

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 power maximum in each of the amplifiers.

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 is no greater than λ (wave length).

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 photon fluxes [7].
