Coherent addition
Coherent addition (or coherent combining) of lasers is a method of power scaling. It allows to increase the output power and brightness of single-transversal mode laser. Usually, the term coherent addition applies to fiber lasers. As the ability of pumping and/or cooling of a single laser is saturated, several similar lasers can be forced to oscillate in phase with common coupler. The coherent addition was demonstrated in power scaling of Raman lasers. Limits of coherent addition. The addition of lasers reduces the number of longitudinal modes in the output beam; the more lasers are combined, the smaller  is the number of longitudinal modes in the output. The simple estimates show that the number of output modes reduces exponentially with number of lasers combined. Of order of eight lasers can be combined in such a way. The future increase of number of combined lasers requires the exponential growth of the spectral bandwidth of gain and/or length of partial lasers. The same conclusion can be made also on the base of more detailed simulations. Nonlinear coherent addition of lasers. Nonlinear interactions of light waves are used widely to synchronize the laser beams in multichannel optical systems. Self-adjusting of  phases may be robustly achievable in binary-tree array of beam-splitters and degenerate four-wave mixing Kerr Phase conjugation in Chirped pulse amplification extreme light facilities. This phase-conjugating Michelson interferometer increases the brightness as formula_1, where formula_2 is the number of phase-locked channels. Talbot coherent addition. Constructive interference due to Talbot self-imaging forces the lasers in the array to transverse mode locking. The Fresnel number formula_3 of the one-dimensional formula_4 element laser array phase-locked by Talbot cavity is given by formula_5 For the two-dimensional formula_6 element laser array phase-locked by Talbot cavity  Fresnel number formula_3 scales as formula_8 as well. Talbot phase-locking techniques are applicable to thin disk diode-pumped solid-state laser arrays.