Telecom-Band Directional Coupler Written With Femtosecond Fiber Laser

A femtosecond fiber laser with 1-MHz repetition rate was optimized to write low-loss waveguides and directional couplers inside bulk borosilicate glass. For the first time, this three-dimensional writing method has produced low insertion loss and near-unity maximum coupling ratio. Results are presented at telecommunication wavelengths of 1310 and 1550 nm