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A directional coupler written in a glass sample by the focused 400-nm output from a 25-fs oscillator is reported. The coupler is single mode; the splitting ratio is 1.9 dB at 633 nm. A refractive-index profile of the waveguide with a magnitude of Dn = 4.5 x 10(-3) was retrieved from a near-field mode pattern.
Streltsov et al. (Mon,) studied this question.