Characterization of symmetric [3 x 3] directional couplers fabricated by direct writing with a femtosecond laser oscillator.

We demonstrate and characterize symmetric [3x3] three-dimensional directional couplers fabricated in glass using a high-pulse energy femtosecond laser oscillator. The characteristics of the [3x3] directional couplers closely agree with the theoretical prediction, except for small errors caused by the fabrication process. We show that deviations from symmetry are dominated by vertical position errors of the coupling waveguide.