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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.  Practically, the combination of more than ten lasers with a passive combining arrangement appears to be difficult. However, active coherent combining of lasers has the potential to scale to very large numbers of channels.
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 , where 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 lock. The Fresnel number of the one-dimensional element laser array phase-locked by Talbot cavity is given by For the two-dimensional element laser array phase-locked by Talbot cavity Fresnel number scales as as well. Talbot phase-locking techniques are applicable to thin disk diode-pumped solid-state laser arrays.
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