**1.1. Limitation on the output energy**

delay for each, finally guiding them consequently to the same direction and expanding such a way to the pulse duration. If in the stretcher, harmonics with longer wavelengths were directed along the shorter optical paths (positive or normal dispersion), then the compressor has to accomplish the reverse (negative or anomalous dispersion). Since the diffraction gratings are used both in the stretcher and the compressor, the functions of stretching and compression will be close to each other and the higher-order dispersion can be compensated

Simplified layout of the classical CPA laser system is presented in **Figure 3**. Here the short pulse from the mode-locked oscillator passing through the stretcher is expanded usually by 4–5 orders. Further, the pulse with low intensity is amplified in the chain of amplifiers by

Several different laser active medias were used for oscillators and amplifiers in the CPA systems, such as dye, Nd:YAG, Nd:Glass, and so on. The most preferable from them was Ti:Sapphire (Ti:Sa) crystal due to its very large bandwidth emission spectra (FWHM ~ 200 nm), very high thermal conductivity and mechanical hardness. This setup was able to achieve pulse durations as short as below 10 femtosecond, if the Ti:Sa oscillators [13] with a few fs mode-locked pulse and amplifiers are adopted. Besides the laser amplifiers [14], optical

Exploiting this technology, researchers developed laser systems with 100 s terawatt level out-

boundaries on the continued improvement of its parameters, the foremost of which is the

intensity (see **Figure 1**) but, as any other technology, CPA has some

several orders of magnitude, and is then compressed back to the short duration.

**Figure 3.** Simplified layout of the CPA laser system (from http://www.llnl.gov/str/Petawatt.html).

parametrical amplifier (OPA) has been presented in CPA schemas [15].

up to 4th order.

66 High Power Laser Systems

put power and 1019 W/cm<sup>2</sup>

limitation on the output energy.

As mentioned above, CPA laser systems have reached sub-petawatt output powers with the energy as low as several tens of Joules [16]. However, for that new generation of the ultra-high power lasers, the modest energy will no longer be enough, and the kJ-level has to be reached setting the next milestone at tens or even hundreds of petawatt [17, 18]. The modern ability to stretch pulse duration is restricted at the few orders by the existing grating technology, so further energy increase requires once again the enlargement of amplifier apertures due to the damage threshold. There are two candidates for the final amplifier of the ultra-high CPA systems: the optical parametrical amplifiers [15] and the laser amplifiers. Both of them possess their own advantages and shortcomings. OPA has several attractive properties as the gain bandwidths large enough to support light pulses as shorter as 10 fs; low amplifier heating due to "Optical cooling," which leads to the potential of a high-repetition-rate; large high-quality nonlinear amplifier crystals; high pulse contrast due to the absence of amplified spontaneous emission (ASE) outside the pump pulse duration. At the same time, OPA possesses low efficiency (below 20%); there are severe requirements on the pump beam quality, such as limitation on the spatial fluctuations in amplitude; short pump pulses (below 1 ns), in order to match with the stretched signal pulses for higher efficiency and their precise matching in space and time with a ps accuracy; and so on [15].

The laser amplifiers are free of most of these restrictions, but possess problems of their own, which include severe losses due to th disk shape of the crystals and very high transverse gain and so the high transverse ASE (TASE), and possible parasitic generation (TPG). TASE as well as TPG leads to significant depletion of the inverted population, and thus the stored energy. The ASE from the pumped Ti:Sa crystal before and after the threshold of the parasitic generation is demonstrated in **Figure 4** [16]. The absence of ASE from the significant part of crystal demonstrates the reduction of the population inversion and so loses the stored energy. Because of this, enlarging the aperture of the amplifier fails to be an unlimited means of obtaining more output energy under the constraints imposed by the damage threshold limit. Therefore, in this case, the main limitation that arises on the path toward ultra-high output power and intensity is the restriction on the pumping and extraction energy imposed by

**Figure 4.** ASE from the pumped Ti:Sapphire crystal before and after threshold of parasitic generation [16].

TASE and TPG within the booster amplifier volume. As a result, the suppression of parasitic generation is a very important task that has been solved for the next generation of the ultrahigh power laser systems. The technology allowed to solve this bottleneck problem will be discussed below in this chapter.

more than two orders higher when cryogenically cooled). But the attempt to increase the longitudinal gain in TD-amplifier by rising the concentration of active ions and/or using crystals with higher emission cross-section leads to dramatic increase of the gain in the transverse direction and consequently to the big losses and inability to store pump energy due to TASE. Technology

New Generation of Ultra-High Peak and Average Power Laser Systems

http://dx.doi.org/10.5772/intechopen.70720

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As it was mentioned above, the main limitation that arises on the path toward ultra-high output power and intensity of the CPA laser systems is the restriction on the pumping and extraction energy imposed by TASE and TPG within the booster and final large aperture

The reflectivity reduction of the side wall of the gain crystals by grinding, sandblasting and/ or coating with an index-matched absorptive polymer or liquid layer in the laser amplifiers is the conventional procedure used to prevent parasitic generation (TPG) [24]. However, the difficulty to find the exact index matching within existing absorbers still restricts the diameter of the pump area to 6–8 cm, corresponding to an extracted energy to around 30 J from Ti:Sa [16]. The amplifier apertures enlarging, or to further pump fluence increasing has led to severe parasitic generation and has failed to increase extracted energy. The additional restriction on storing and extracting energy by TASE in larger gain apertures was demonstrated [25]. TASE necessarily increases with the aperture size, limiting the maximum stored energy that is why this restriction is even stronger than parasitic lasing because the threshold for the latter can be increased due to development of the new index matching materials for absorbers. The uniform luminescence on the left picture of **Figure 4** should not delude us, if one will find method to reduce reflections down to zero; the losses remain still incredible big for amplifiers

The method of the calculation of total volume of TASE radiated out from the crystal during its pumping was developed by Chvykov et al. [25]. **Figure 5a** shows the evolution of normalized fluorescence of the crystals vs. pumping time when pumped by 100 ns-pulse for different crystal apertures. Here, Emax is the theoretical maximum of the extracted energy, and Eloss is the lost energy due to TASE. As seen from the plot, ASE grows dramatically after a certain time of pumping, even for 10 cm—crystal and soon becomes equal to the pumping energy. This means that further pumping is useless because all additional energy will be irradiated out of the crystal as ASE. The critical points of anomalous ASE (APs) are moving to the pumping process beginning with the growing crystal diameter. No more than 20–50% of the pump

Shortening of the pump pulse duration does not help to reduce the losses, at least until pulse duration becomes shorter than the time-length of the light distribution through the crystal in transverse direction. This is about several 100 ps, and thus such a pump would be useless due to the very low damage threshold. Fluorescence during pumping for different pump pulse

energy can be stored in the crystals with aperture of 15–20 cm as seen in **Figure 5**.

for solving this problem will be presented below in this chapter.

**2. Extraction during pumping (EDP) method**

amplifier volume [23].

with the large aperture.
