A scheme for shaping an incoherent radiation field may contain a set of laser cavities with subsequent periscopic addition of radiation into a piecewise continu­ous (mosaic) field. The field is then telescoped and focused onto the target.

The radiation of N laser channels may be added into a single (incoherent) mosaic beam using periscopes (Fig. 10.6). If divergence of laser beam from one channel is equal to ~5 x 10~4 rad, after telescoping a united light beam up to the dimensions of the transverse section of the RL, the radiation divergence may be <5 x 10~5 rad.

For systems with a large number of identical laser channels it is expedient to use serial, parallel, or mixed addition of radiation from individual channels to increase the intensity of the laser radiation. Figure 10.7 gives the principal schematics of this addition for three channels. As shown by the calculations, the total length of a “multipart” cavity with a high Q-factor may be 30-50 m (~10 channels). With parallel addition, the number of channels are limited by the losses in the semi-

transparent plates and also may be more than 10; with mixed (serial-parallel), respectively, more than 100 channels. In this case, the laser radiation is withdrawn from the RL by several (from 2-4 to 20-30) beams with a laser power of ~100- 200 kW each. These beams may either be added by external systems (including coherent), or each is directed to its target independently.

VNIIEF researchers are currently studying methods of “direct” addition (parallel and serial) in different laser setups (LUNA-2M, LM-4, and LM-8). The first results from these studies are given in studies [29—32]. Direct addition methods are preferable when system size increases. In this case, difficulties arise with the application of methods using a common mirror. On the other hand, direct addition methods introduce increased demands for optical quality (as to coefficients of refraction and transmittance) and for optical uniformity in the active laser medium.

With nuclear pumping, startup of the individual lasers of a multi-channel system occurs in a random way. Use of the direct addition method permits the first laser that starts lasing to operate in “master oscillator” mode. Thus, it is possible to achieve complete phasing of all lasers in a set. In this case, the radiation from all channels will have identical polarization, phase, and frequency.