Broadband and polarization independent waveguide-fiber coupling

Waveguide-to-fiber coupling is one of the key challenges in silicon photonics. Many approaches have already been experimentally demonstrated, such as grating couplers, inverse tapers, sub-wavelength structures, lensed fibers, photonic wire bonds, small-core fibers and separate waveguide interposers. However, it is difficult to find a concept that simultaneously offers broadband operation, polarization independency, low optical loss, simple fabrication and easy assembly. In this paper, the pros and cons of different concepts are reviewed. Then two alternative concepts are introduced for coupling light between 3 µm thick SOI waveguides and standard single-mode fibers with ultra-broadband (>500 nm bandwidth) and polarization independent operation. Experimental results with 1-2 dB loss per waveguide-fiber interface are reported for 1) separate 12 µm SOI interposers and 2) polymer lenses directly written to the end facets of the 3 µm SOI waveguides. The insertion loss of the interposer concept includes both the waveguide-interposer and interposer-fiber interfaces, as well as the loss of the interposer itself. The loss of the polymer lens concept includes the losses of the waveguide-to-lens, lens-to-air and air-to-fiber interfaces, as well as the loss of the directly written lens. Polymer lenses were also integrated on top of up-reflecting mirrors to demonstrate vertical fiber coupling. The scalability of the two concepts for low-cost silicon photonics packaging is also analyzed, taking into account the ability to align fibers passively into V-grooves or others such structures on the SOI chips.