Femtosecond Pulsed Laser for Advanced Photonic Packaging

The purpose of the advanced applications of glass laser micromachining presented in this paper is to solve the difficulty of both electrical and optical interconnects needed in electro-optical packaging. Glass interposers constitute an innovative approach to integrate different types of functionalities onto the same functionalized substrate. The distinctive interest of this packaging approach is to integrate both the electronic and photonic chips and to implement a passive alignment strategy of the external fiber by butt coupling and of the photonic chip using a conventional flip-chip technique with state-of-the-art registration accuracy in the micron range. Multiplexer/demultiplexer, Y junction, directional couplers, micro-channels and waveguide can also be fabricated in glass using laser photo-inscription. This technique is based on nonlinear absorption under exposition to a focused laser beam that results in permanent structural glass modifications. In this paper, we report on femtosecond laser direct inscription in glass caused by local densification. Two main properties of densified glass are exploited, namely, i) the increase of the index of refraction suitable to fabricate optical waveguides and ii) the dramatic increase of the etching rate in hydrofluoric acid (HF) suitable for structuring integrated mirrors. Here, we demonstrate the fabrication of a photonic mirror in a glass interposer for redirecting light from a buried waveguide. A slanted trench was fabricated by laser inscription and a HF etching step was performed to obtain the appropriate angle and depth of the mirror. SEM pictures show the mirror uniformity and the proof of 90° beam redirection from a buried waveguide. Optical waveguides were also structured by direct laser inscription using a 0.5 NA microscope objective to focus the laser beam in the bulk of a glass substrate (SCHOTT AF32). The resulting waveguides exhibit single mode operation and losses of 4. 35dB for a length of 2.5 cm including the coupling losses.