Low bend loss waveguides enable compact, efficient 3D photonic chips

Abstract

We present a novel method to fabricate low bend loss femtosecond-laser written waveguides that exploits the differential thermal stabilities of laser induced refractive index modifications. The technique consists of a two-step process; the first involves fabricating large multimode waveguides, while the second step consists of a thermal post-annealing process, which erases the outer ring of the refractive index profile, enabling single mode operation in the C-band. By using this procedure we report waveguides with sharp bends (down to 16.6 mm radius) and high (80%) normalized throughputs. This procedure was used to fabricate an efficient 3D, photonic device known as a "pupil-remapper" with negligible bend losses for the first time. The process will also allow for complex chips, based on 10's - 100's of waveguides to be realized in a compact foot print with short fabrication times.

Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications

“…The algorithm takes the limitation of the fabrication process such as accessible vertical real-estate, bends losses as well as minimum distance between waveguide to mitigate cross-coupling into account and creates an optimised waveguide layout with path length matching to within 100 nm [127]. Whilst the first generation of pupil remapping chips suffered from low throughput caused by bend losses [10], improved fabrication exploiting thermal annealing [67] vastly increased the transmission as well as closure phase stability [126].…”

“…Overwriting the waveguides several times can lead to an increase in index contrast [2,65,66] and thereby reduced bend losses. An alternative approach for reducing bend losses of waveguides inscribed in the cumulative heating regime is to thermally anneal the structures [67]. Unlike removing transient defects for reduction of propagation losses, this step exploits differential thermal stabilities of the different regions within the usually complex structures induced by high-repetition-rate femtosecond pulses in multicomponent silicates.…”

Control of waveguide properties by tuning femtosecond laser induced compositional changes

“…21 This provides a very large range of accessible index contrast for the production of highly confined structures and small radius of curvature waveguides. 23 The extent of this interval could potentially be broadened by modifying the initial La/K ratio of the glass composition. An estimate of the curvature radius that could be supported by the present structures for a 1.2 mm slit-shaped waveguide with a positive refractive index of 1.5 Â 10 À2 (Ref.…”

“…24) provides a value of $ 8 mm. 23 Although the relation between local ion concentration and refractive index is well known and modeled in the case of ion-exchanged waveguides, 25 the precise correlation between local concentration and refractive index in a fs-laser written waveguide is a very remarkable result. It must also be noted that local variations of the K content could potentially contribute to the local index of the structures.…”

Fabricating waveguide Bragg gratings (WBGs) in bulk materials using ultrashort laser pulses

“…The index profile is typically complex, consisting of regions of both positive and negative contrast [58,59]. However, WBGs created in the cumulative heating regime rely on the formation of nanosized voids, whose cross-sectional dimension cannot be controlled [28].…”