Optical waveguide wiring using femtosecond laser pulses between multiple pieces of glass for optical sensor applications

We demonstrate optical waveguide wiring between multiple pieces of glass using high-repetition rate femtosecond laser pulses focused with a 1.4 NA oil-immersion microscope objective. In conventional waveguide wiring techniques, connecting optical waveguides in different pieces of glass requires special attention because of the very small core size of the waveguides. Connecting waveguides therefore necessitates highly accurate positioning stages and is a time consuming process. We demonstrate that it is possible to write optical waveguides across the gap between two pieces of glass with a separation up to 20 µm using high-repetition rate femtosecond laser micromachining. Lateral offset loss and angular misalignment loss, the two dominant loss mechanisms for waveguide wiring, are negligible because we eliminate differences in depth of focus across the gap by filling the gap with the index matching oil used between the objective lens and the glasses. Even when glasses of different indices of refraction are used, we can fabricate connected waveguides in the two pieces by correcting for the depth of focus, provided the power required for index modification is an order of magnitude smaller than the critical power for self-focusing. Our technique allows the simultaneous writing and wiring of waveguides and can thus be used to fabricate complex photonic devices. We will present the fabrication of optical sensors using this wiring technique.