| A high repetition rate femtosecond fiber laser was applied to fabricate broadband directional couplers inside borosilicate glass for general power splitting application in the 1250 to 1650-nm wavelength telecom spectrum. The broadband response was optimized over the 4100-nrn bandwidth by examining the spectral response of the straight interaction region and the bent transition region separately. The spectral responses of the laser-written couplers were well-represented by coupled mode theory, permitting simple design and implementation of broadband couplers for bulk 3D optical circuit integration. Broadband directional couplers with various coupling ratio were fabricated. High spatial finesse of the femtosecond-laser writing technique enabled close placement (∼6 mum) of adjacent waveguides that underpinned the wavelength-flattened broadband responses. |
