Low loss fiber-coupled volumetric interconnects fabricated via direct laser writing

• Published in: Optica 2024-07, Vol.11 (7), p.995
• Main Authors: Littlefield, Alexander J., Huang, Jack, Holley, Mason L., Duggar, Nikita B., Gao, Jingxing, Xie, Dajie, Richards, Corey A., Silberg, Truman, Purakayastha, Ujaan, Herr, Jesse, Ocier, Christian R., Deng, Xiangrui, Wang, Xiaoli, Braun, Paul V., Goddard, Lynford L.
• Format: Article
• Language: English
• ISSN: 2334-2536; 2334-2536
• DOI: 10.1364/OPTICA.525444

Photonic integrated circuits (PICs) are vital for high-speed data transmission. However, optical routing is limited in PICs composed of only one or a few stacked planes. Further, coupling losses must be low in deployed systems. Previously, we developed the subsurface controllable refractive index via beam exposure (SCRIBE) technique to write accurate 3D gradient refractive index (GRIN) profiles within a mesoporous silica scaffold. Here, we apply SCRIBE to fabricate low loss, broadband, polarization insensitive, fiber-coupled, single-mode volumetric interconnects that include waveguides traversing arbitrary 3D paths. By seamlessly integrating mode-matching subsurface lenses and GRIN waveguide tapers, calibrating for positional writing errors, implementing multipass exposure, automating alignment, and switching to antireflection coated fibers, we reduced the insertion loss for a fiber-PIC-fiber interconnect from 50 to 2.14 dB, or 1.47 dB, excluding the fiber array’s loss. Further, we establish an upper bound of 0.45 dB loss per coupler. We report quality factors of 27,000 and 77,000 and bending losses of 6 and 3 dB/cm for 15 and 30 µm radii microrings, respectively. We also demonstrate Bézier escalators, polarization-rotating and polarization-splitting interconnects, and a seven-channel 25 µm pitch volumetric interconnect. The SCRIBE platform presents a clear path toward realizing 3D PICs with unique functionality.