Abstract
In this work, the fabrication of optical waveguides embedded in fused silica (Suprasil1) and boro-aluminosilicate glass (Eagle2000) is demonstrated with femtosecond laser direct writing, as well as their suitability to be brought to the surface, through wet etching, for enhanced evanescent coupling with the external dielectric medium. Fused silica demonstrated to be inappropriate in this particular application, as the guiding region is at the bottom of the induced modification, creating a barrier between the guided mode and the substrate’s boundary. Furthermore, the existence of nanogratings meant that, upon contact of the top of the induced modification with the substrate’s boundary, the waveguide is quickly etched. Eagle2000 demonstrated to be superior to fused silica due to its characteristic modification cross-section and absence of nanogratings, which allowed the placement of the guiding region as close to the substrate’s surface as required. However, surface roughness arising from the creation of insoluble products in the HF solution was found. The addition of HCl to dissolve these products was implemented.
Highlights
Femtosecond laser direct writing is a fabrication technique that relies on a very localized modification to manufacture three-dimensional structures
The modification translates into an increased refractive index, enabling the fabrication of optical circuits [1]
An array of optical waveguides was fabricated in fused silica (Suprasil 1) while increasing the inscription depth in steps of 0.5 μm
Summary
Femtosecond laser direct writing is a fabrication technique that relies on a very localized modification to manufacture three-dimensional structures. The modification translates into an increased refractive index, enabling the fabrication of optical circuits [1]. The creation of nanometric structures aligned along a preferential direction (nanogratings) is possible in fused silica, leading to an anisotropic etching reaction that enables the fabrication of microfluidic systems and, when in conjunction with optical circuits, optofluidic devices [2]. Being an inherently three-dimensional technique, it allows the fabrication of waveguides at any depth, including near the surface which is of great interest in sensing applications. We employ the femtosecond laser direct writing technique in the fabrication of optical waveguides at low depths in Suprasil 1 and Eagle2000 substrates, and discuss the outcome of etching them to the substrate’s surface
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