Abstract
For input coupling from single-mode optical fibers to photoreceivers made with silicon photonics integrated circuits (Si-PICs), we constructed a polarization-insensitive optical surface coupler with a vertically curved silicon waveguide. The polarization insensitivity was attained by the equalization of the radiative transmission losses of TE and TM polarizations in the vertically curved waveguide (VCW) structure while keeping the straight structure of the spot-size converter (SSC) connected above the curved structure. Numerical simulations revealed that the radius of curvature of 6 μm or larger nearly enabled the equalization, and thus we developed a bending technology using ion implantation to realize simultaneously a VCW radius of curvature of 6 μm and a straight SSC of length 7 μm by varying the implantation angles. The bent silicon core was subsequently coated with SiO2 by plasma-enhanced chemical vapor deposition to function as a collimation lens. The coupling properties of the fabricated coupler and the single-mode fiber with a mode-field diameter of 5 μm exhibited a polarization dependence loss of less than 0.25 dB, a 1-dB bandwidth of 180 nm, and coupling losses of about 1.6 dB at 1550 nm wavelength. A sharp inversely tapered Si waveguide that affects these characteristics significantly can be formed by applying ArF-immersion lithography on the 45-nm-node and hence fabrication is compatible with mass production processes.
Highlights
I NTENSE polarization sensitivity is an almost unavoidable attribute found in silicon photonics integrated circuits (Si-PICs)
The architecture of today’s Si-PIC device technology is based on it. This sensitivity is acceptable for the optical transmitter where a specific polarization is used for signal processing through outputs to the single-mode optical fibers (SMFs)
Compared with grating couplers (GCs), the elephant coupler has an inverse-tapered spot-size converter (SSC) at the top, with a lens-shaped SiO2 dome, and has advantages of low coupling loss, broadband coupling, and flexibility in beam direction. [8]–[10] Recently, we have reported on a coupler with a beam-spot size of 5 μm for coupling with high numerical aperture single-mode fibers (HNA-SMFs)
Summary
I NTENSE polarization sensitivity is an almost unavoidable attribute found in silicon photonics integrated circuits (Si-PICs). The architecture of today’s Si-PIC device technology is based on it This sensitivity is acceptable for the optical transmitter where a specific polarization is used for signal processing through outputs to the single-mode optical fibers (SMFs). The simulation results show that the radius of curvature of the VCW should be more than 7 μm, but the previous IIB method was unable to straighten the SSC structure at the tip at the same time To overcome this problem, the IIB method was improved by adding a new process in which the implantation angle was varied. We presented at a conference results reporting high-efficiency optical coupling with 2.3 dB minimum coupling loss and a 1-dB-bandwidth of more than 150 nm, with low polarization dependence produced by increasing the radius of curvature of the VCW section of the elephant coupler. We presented at a conference results reporting high-efficiency optical coupling with 2.3 dB minimum coupling loss and a 1-dB-bandwidth of more than 150 nm, with low polarization dependence produced by increasing the radius of curvature of the VCW section of the elephant coupler. [14] In this paper, we provide further details of the design and fabrication of the device, and report an improved result obtained after the conference
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