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Highly coherent high-order harmonics in the soft X-ray region are generated by macroscopic phase matching. The high fringe visibility, which indicates the degree of coherence, of 0.73 is obtained at the 27th harmonic of a Ti:sapphire laser (the corresponding wavelength is 29.6 nm) by optimizing the 10-mm phase-matched propagation in 30 Torr argon gas, while the visibility of the pump laser is 0.84.
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Tamaki et al. (2001) studied this question.
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