Abstract
Silicon photonics, with the unique advantages in cost-effective, mass-production and high dense integration resulting from the compatibility with the advanced CMOS technology, have developed rapidly in the recent years and been one of the key building blocks for optical interconnection. Various high performances of optical modulators and detectors have been implemented on silicon substrate for high speed E/O and O/E conversion. The plasma dispersion effect is the often seen approach applied to the optical modulation on silicon substrate over the past one decade. Compared with the injection type modulators, the depletion type modulators provide faster switching rate resulting from the virtue of short majority lifetime. But the modulation efficiency of carrier-depletion modulator is not superior to that of carrier-injection leading to a large device area. To address this problem, the location of heavily doped region of pn junction should be as close to the waveguide center as possible to obtain maximal overlap integral between modulated depletion region and optical mode so as to achieve best modulation efficiency. Nevertheless, the free carrier absorption inevitably increases because of the heavily doped region. Many attempts, to date, have been made for reducing phase shifter loss such as pipin diode and doping compensation method. But, each of them needs to stringently control the doping profile to keep the modulation efficiency while avoid strong free carrier absorption as well. In this study, we propose a new device structure to exploit fringe field pn junctions by deploying heavily doped regions just near the corners of the waveguide to mitigate free carrier absorption. In the meantime, the large fringe field at these doped regions effectively depletes the carriers resulting in extended depletion region across the waveguide center. These doped regions can be precisely controlled by self-aligned ion implantation as well as a post annealing process. Silicon integrated photonics shows great potential for applications in optical interconnect and optical signal processing. Many state-of-the-art active components such as Si modulators and Si/Ge photodetectors have been demonstrated for high-speed data transmission exceeding 40 Gbps. However, a large core-dimension discrepancy between the submicron silicon waveguides (or called silicon photonic wires) and single-mode fibers (SMF) introduces significant coupling loss. To solve this problem, many approaches have been proposed, for example, surface coupling by grating couplers, end-butt coupling by tapered lensed fibers or Si inverted nanotapers. Silicon inverted nanotapers have been studied widely with high coupling efficiency and small polarization- and wavelength-dependence. Nevertheless, through this method, another coupler with low index of refraction is usually employed to enclose the inverted taper as an inter-medium that confines light for better fiber-to-silicon-wire coupling. The core dimension of this extra coupler is usually limited to several m to achieve effective coupling. Though the lensed fiber has better mode matching with this extra coupler, the misalignment tolerance is very stringent, typically less than 1 m. A practical coupler design not only should be low-loss but ought to tolerate fiber misalignment as well. In this study, this 3D SU-8 taper is cascaded with an inverted nanotaper through the SiON waveguide for light coupling from SMF to silicon photonic wires. In the section 4-2, we have a brief discussion about the integration between fringe field junction modulator and wideband fiber coupler in the process point of view.