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We develop a novel, femtosecond beam combination technique, which can coherently combine large numbers of ultrashort pulse beams using a diffractive optic pair. Existing methods of ultrashort pulse beam combining increase the number of combining optics with the number of beams. Diffractive combiners add many beams on one optic, but exhibit loss for femtosecond pulses due to pulse front tilt. We solve this problem by adding a second diffractive optic to cancel pulse front tilt. By selecting parameters, uncorrected temporal and spatial dispersions from the two DOEs can be made negligible for >30fs pulse widths.
We numerically model a proof-of-principle case of 1-D, 4-beam combination, showing that four 120fs beams can be combined with 92% efficiency. This has been demonstrated experimentally with the preservation of 120fs pulse duration and a relative combining efficiency of >85%. A 120fs Yb fiber oscillator output is amplified in a YDFA, split into four phase-controlled channels, and collimated to produce a beam array. This is sent to the DOE pair, forming a combined beam which is compressed and sent to FROG diagnostic. The measured output pulse duration is identical to the oscillator pulse duration.
Combining efficiency theory for a 2-D array of ultrashort pulse beams is developed, showing that hundreds of beams can be combined with >90% efficiency. We calculate that for a 224-beam case with practical optical parameters, and temporal dispersion causes 1% extra loss, while spatial dispersion causes 2.5% extra loss, in addition to possible DOE imperfections and beam aberrations.
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