Abstract : Expanded-core structures based on layered increased index (type
I) waveguiding traces are fabricated by ultrafast laser photoinscription in
bulk optical glasses, with examples for fused silica and chalcogenide
glasses. The expanded-core waveguides can serve for large-mode-area
guiding concepts and their feasibility is experimentally investigated. A
parametric study of the geometry, number of traces and index contrast
indicates the possibility to design guided modes characteristics as
exemplified in fused silica. A specific arrangement consisting of 8 traces of
guiding layers with 6μm separation exhibit single-mode transport properties
with mode field area of ~805μm2. The condition of single mode operation is
also discussed in the frame of the dispersion relation of light guiding in
periodical dielectric structures. The supported supermode of expanded-core
structures can be controlled by careful design of the refractive index
change, the number of guiding layers and the thickness of the interlayers.
Inspection of the propagation characteristics shows equally low loss
features. A Y-branching splitter based on expanded-core concept
conserving single mode characteristics is fabricated. The optical design is
equally successfully tested in chalcogenide Gallium Lanthanum Sulfide
glass. Ultrafast laser inscribed expanded-core waveguiding provides
therefore an interesting path of fabricating large mode area waveguides
usable in near infrared and mid-infrared region beneficial for applications
requiring high power or large mode dimensions.
