19 February 2009 Coherent fiber combining by digital holography
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Fiber lasers provide an attractive means of reaching high output laser power because of their advantages in terms of compactness, reliability, efficiency and beam quality. In order to obtain much higher output power than it is possible from a single fiber, beam-combining techniques have been investigated. In this communication, we present a new technique of coherent fiber combining, based on self adaptive digital holography that does not require any phase error measurement. A low power plane reference beam is first launched into the fiber amplifier array. The interference pattern between the beams with phase φ(x,y) issued from the fiber array and a plane reference beam is recorded on a digital camera and directly transferred to a Spatial Light Modulator (SLM) which acts as a programmable digital hologram. This hologram is read out simultaneously and a phase conjugate beam with phase -φ(x,y) is generated in order -1 of the diffraction pattern. This beam is then injected in the fiber amplifier array. At the output of the fiber amplifier array, the phase of each elementary beam are locked. Experimental demonstration of coherent beam combining by digital holography is demonstrated with polarization maintaining fibers operating at 1 μm. Digital holography is realized thanks to a CCD/CMOS camera and a liquid crystal SLM. Owing to the high resolution of existing SLMs and cameras, this technique could be applied to phase lock a large number of fiber amplifiers.
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C. Bellanger, C. Bellanger, A. Brignon, A. Brignon, J. Colineau, J. Colineau, J. P. Huignard, J. P. Huignard, } "Coherent fiber combining by digital holography", Proc. SPIE 7195, Fiber Lasers VI: Technology, Systems, and Applications, 71951N (19 February 2009); doi: 10.1117/12.807428; https://doi.org/10.1117/12.807428

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