Coherent addition

Coherent addition (or coherent combining) of lasers is a way to increase the output power and brightness of single-transversal mode laser. Usually, it 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 considered as a way of power scaling of Raman lasers.^[1]

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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 number of longitudinal modes in the output. The number of output modes reduces exponentially with number of lasers combined. Of order of 8 lasers can be combined in such a way.^[2] 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 combinaiton of more than 10 lasers in such a way should be difficult if at all.

References

1. ^ 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.
2. ^ 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 [1].)

• A. E. Siegman: "Resonant modes of linearly coupled multiple fiber laser structures", Stanford University homepage (2004).