With the continuous increase of output power of double cladding fiber lasers, the researches of the techniques of fiber
laser beam combination have become a new focus. With the inherent characters of small volume, light weight, good
beam quality and system stability. Once the high power output laser was achieved, there would be broad applications in
industry, scientific researches and military field. Incoherent beam combination is much easier to be realized compared
with coherent beam combination, and with better system stability, thus it became an promising technique to achieve
small volume and high power laser source.
In an incoherent fiber laser beam combination system, the transform lens and diffractive grating play an important role in
determining the coupling efficiency. Through theoretic analysis and numerical simulations, it has been proved that the
optimum focal length of transform lens should be around 20cm in order to ensure a high system coupling efficiency
under some selected simulation parameters. Also the lower frequency of diffractive grating is preferred, and a
contradiction is present that is the higher frequency of diffractive grating is favorable in the consideration of maximum
fiber array width, thus a moderate value was chosen as 200mm<sup>-1</sup> with comprehensive consideration. The lower grating
thickness is helpful in improving the diffractive efficiency, while it faced the problem of heat resistance and
manufacturing difficulty, therefore the grating thickness between 2mm and 4mm is preferred. Under such optimized
parameters, the coupling efficiency would be 70% or even higher with small deviation angle and wavelength. An
example of effective jamming distance of combined fiber laser to visible light silicon CCD has been given, whose results
had proved the application foreground of combined fiber laser.
With the continuous increase of output power of double cladding fiber lasers, more effort is put into the researches of the technique of fiber laser beam combination, especially for incoherent laser beam combination because it is easier and with better system stability. Once high power output laser beam is achieved, there would be broad applications in industry, especially for manufacturing and material processing. The combination system's coupling efficiency plays an important role in determining the output power. Through theoretical analysis and numerical simulations, it has been proved that lower lateral off-set and higher grating period would be favorable, also an optimum spot radius exists which corresponds to a maximum value of coupling efficiency. Although lower focal length is helpful in improving the coupling efficiency, there is a contradiction that it makes a narrower fiber array width, which would limits the number of fiber lasers that could be utilized. Thus a moderate value of 20cm is chosen. Based on such optimized parameters, the beam quality M<sup>2</sup> is around 2, also a method of two parallel gratings is introduced, which ensures the M<sup>2</sup> factor to be around 1. Such combined fiber laser would be of great potential applications in manufacturing and material processing.