Efficient Suspended Coupler With Loss Less Than −1.4 dB Between Si-Photonic Waveguide and Cleaved Single Mode Fiber

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Benefiting from the 193-nm lithography, we have used one of the MPW platforms to realize narrow tip of width 100 nm with great repeatability and uniformity. Based on the new patterning technique, we optimized the suspended spot size converter (SSC) accordingly and the design result showed coupling loss less than −1 dB given with proper matching oil. We fabricated the device together with other photonic circuits and maintained the tip width around 105 nm across the wafers. First, we tested various matching oils, which are commercial product used for optical package but with unknown refractive index at C band, with one of them the SSC realized the state-of-the-art coupling loss less than −1.3 dB/facet with cleaved single mode fiber (SMF) for both TE and TM polarization state with a remarkable polarization-dependent loss less than 0.5 dB. Subsequently, we tried customized matching oils, which are not suitable for massive production but with known refractive index, to further confirm the effect of the refractive index of the matching oil. A similar trend with simulation result was verified. We also tested many chips randomly selected from different wafers to check the statistics and uniformity of the device, the measurement result showed variation within 0.4 dB across wafers, which is the best performance to the best of our knowledge realized in the available silicon photonics platforms worldwide. Finally, to test the feasibility of the design for package, we studied the alignment tolerance and conducted a simple packaging using ultraviolet (UV) curing to fix the fiber and the silicon chip. The 1-dB extra loss corresponded to 2-μm misalignment in cross section, similar to the tolerance between two cleaved SMF and the variation after UV curing was just 0.5 dB attributed to the loose alignment tolerance.