**4. Laser system THL-100**

**Lasers Operating mode Energy, J Pulse duration, ns Line width, cm−1 Divergence, μrad**

**Table 2** summarizes the experimental results for the laser system. The maximum energy (330 J) is obtained in the case of the single-pass amplification in the Photon-5 laser when the ASE flux and the absorption in the active medium are minimized. In the case of double pass amplification, both the absorption and the ASE contribution increase (the ASE intensity increases owing to the reflection from the rear mirror with *R* = 99%). This leads to a decrease

In the laser system, the Photon-2, Photon-3, Photon-4, and Photon-5 lasers start working as slave amplifiers. In this case, the windows of all of the laser chambers are tilted at angles providing for the absence of feedback. The Photon-1 laser serves as the master oscillator [34]. Its optical scheme makes it possible to generate a high-quality beam in a certain part of the active volume and to amplify this beam in the remaining part. To improve the spatial structure of the radiation, we employ two pinholes with a diameter of 2 mm. In this case, the Fresnel number is *N* ~ 2 for a cavity with a length of 1.5 m. The spectral selection is realized with an auto-collimation diffraction grating (1800 mm−1). The feedback in the cavity is maintained via the first diffraction order. To decrease the contribution of the noise component to the output radiation, we outcouple the laser beam through a semitransparent mirror with a reflectance of *R* = 30%. Then, the low-power high-quality radiation of the master laser is additionally amplified at two passes in the same active medium, so that the output beam diameter increases to 7 mm. The output pulse of the Photon-1 laser has an energy of 50 mJ, a duration of 250 ns, and a spectral line width of 0.9 cm−1. The divergence of the laser beam, which contains 80% of the energy (0.13 mrad), is greater than the diffraction-limited divergence by a factor of 1.2. This beam is expanded using a lens telescope with a magnification of *M* = 1.5 to match the beam diameter with the sizes of the active media of the Photon-2 and Photon-3 lasers. The beam is amplified at three passes in the Photon-2 laser and one pass in the Photon-3 laser. The output beam diameters of these lasers are 3 and 6 cm, respectively. For further matching of the beam diameter with the sizes of the active media of the Photon-4 and Photon-5 lasers, we employ a lens telescope with a fivefold magnification. After the beam expansion, the radiation is amplified at one pass in the active medium of the Photon-4 laser and at one

> – 0.9

> – 0.9

> – 0.9

– –

– – – 130

– 60

– 37

– –

– –

300 200–250

250 200–250

300 200–250

250 200–250

350 200–250

Photon-1 Laser

16 High Power Laser Systems

Photon-2 Laser

Photon-3 Laser

Photon-4 Laser

Photon-5 Laser

master oscillator

amplifier

amplifier

amplifier

amplifier

**Table 2.** Parameters of the radiation of the photon lasers.

1.5 0.05

in the energy of the amplified radiation to a level of 250 J.

or two passes in the active medium of the Photon-5 laser.

3.5 0.5

10 5

120 40

660 250, 330

### **4.1. Femtosecond pulse generator**

The femtosecond pulse generator (front-end) of laser system consists of a femtosecond Ti:sapphire master oscillator, regenerative and multipass amplifiers, a pulse stretcher and a pulse compressor, and generator of second harmonic. It operates at a pulse repetition rate of 10 Hz and in a single pulse mode. Output energy of front-end is up to 5 mJ at 475 nm. Transform-limited pulse duration is 50 fs.
