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.
- A. Shirakawa, T. Saitou, T. Sekiguchi and K. Ueda: "Coherent addition of fiber lasers by use of a fiber coupler" Optics Express 10 (2002) 1167–1172
- D. Kouznetsov, J.F. Bisson. A. Shirakawa, K.Ueda "Limits of Coherent Addition of Lasers: Simple Estimate" Optical Review Vol. 12, No. 6, 445–447 (2005). (Also .)
- A.E.Siegman. Resonant modes of linearly coupled multiple fiber laser structures. Preprint of the Stanford University, 2005, 25 pages; http://www.stanford.edu/~siegman/coupled_fiber_modes.pdf
- Leo A. Siiman, Wei-zung Chang, Tong Zhou, and Almantas Galvanauskas, "Coherent femtosecond pulse combining of multiple parallel chirped pulse fiber amplifiers" Optics Express 20 (2012) 18097-18116