Significant effort in optical-fibre research has been put in recent years into realizing mode-division multiplexing (MDM) in conjunction with wavelength-division multiplexing (WDM) to enable further scaling of the communication bandwidth per fibre. In contrast, almost all integrated photonics operate exclusively in the single-mode regime. MDM is rarely considered for integrated photonics because of the difficulty in coupling selectively to high-order modes, which usually results in high inter-modal crosstalk. Here we show the first microring-based demonstration of on-chip WDM-compatible mode-division multiplexing with low modal crosstalk and loss. Our approach can potentially increase the aggregate data rate by many times for on-chip ultrahigh bandwidth communications.

We experimentally demonstrate quasi-phase-matched (QPM) four-wave-mixing (FWM) in silicon (Si) nanowire waveguides with sinusoidally modulated width. We perform discrete wavelength conversion over 250 nm, and observe 12 dB conversion efficiency (CE) enhancement for targeted wavelengths more than 100 nm away from the edge of the 3-dB conversion bandwidth. The QPM process in Si nanowires is rigorously modeled, with results explaining experimental observations. The model is further used to investigate the dependence of the CE on key device parameters, and to introduce devices that facilitate wavelength conversion between the C-band and mid-IR. Devices based on a superposition of sinusoidal gratings are investigated theoretically, and are shown to provide CE enhancement over the entire C-band. Width-modulation is further shown to be compatible with zero-dispersion-wavelength pumping for broadband wavelength conversion. The results indicate that QPM via width-modulation is an effective technique for extending the spectral domain of efficient FWM in Si waveguides.

We report a Ge-on-Si photodetector without doped Ge or Ge-metal contacts. Despite the simplified fabrication process, the device shows a responsivity of 1.14 A/W at -4 V reverse bias and 1.44 A/W at -12V, at 1550 nm wavelength. Dark current is less than 1µA under both bias conditions. We also demonstrate open eye diagrams at 40Gb/s.

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We present a compact and low loss 90° optical hybrid on a silicon-on-insulator (SOI) platform for coherent receiving systems. Our 90° optical hybrid uses a novel topology, comprising one Y-junction and three 2x2 multimode interference (MMI) couplers. The geometry of the 90° optical hybrid is fully optimized using particle swarm optimization (PSO). The fabricated 90° optical hybrid has a compact footprint of 21.6 µm x 27.9 µm, with an insertion loss less than 0.5 dB, a common mode rejection ratio (CMRR) larger than 30 dB, and phase error smaller than 3° in the C-band across 22 reticles on one wafer. The measured phase error (< 3°) in a packaged coherent receiver further confirms the excellent performance of the 90° optical hybrid.

We report error-free long-haul transmission of optical data modulated using a silicon microring resonator electro-optic modulator with modulation rates up to 12.5 Gb/s. Using bit-error-rate and power penalty characterizations, we evaluate the performance of this device with varying modulation rates, and perform a comparative analysis using a commercial electro-optic modulator. We then experimentally measure the signal integrity degradation of the high-speed optical data with increasing propagation distances, induced chromatic dispersions, and bandwidth-distance products, showing error-free transmission for propagation distances up to 80 km. These results confirm the functional ubiquity of this silicon modulator, establishing the potential role of silicon photonic interconnects for chip-scale high-performance computing systems and memory access networks, optically-interconnected data centers, as well as high-performance telecommunication networks spanning large distances.

While a number of Si waveguide (SiWG) integrated optoelectronic devices have been demonstrated in this wavelength range 8,9 , efficient photodetection remains an important and challenging task. Thus, spectral translation of mid-infrared signals to the telecom regime via four-wave mixing in SiWGs has been proposed for on-chip detection 10 , which makes use of the sensitive integrated photodiodes (PDs) in the telecommunications wavelength range 11 . However, this method requires a high-powered pump laser and long, on-chip-waveguide lengths to achieve efficient wavelength conversion. In addition, heterogeneous integration of both narrow-bandgap semiconductors 9,12-14 and graphene 15,16 with SiWGs has been demonstrated for on-chip mid-infrared detection. Though viable PDs have been demonstrated, 2 heterogeneous integration schemes present an inherent difficulty by imposing constraints on material quality and process integration. Extrinsic detectors, which utilize absorption transitions from dopant-induced trap states within the bandgap of a host material, present a simple solution for high-performance integrated mid-infrared PDs and alleviate the need for heterogeneous integration. These PDs can potentially be integrated into a standard CMOS process flow by adding an ion implantation and annealing step after activation of the source and drain implants, and prior to the deposition of back-end dielectrics and interconnect metallization.Alternatively, this additional fabrication step can be performed as a post-process, as is done here (see Methods). The Si dopants used in bulk photoconductors can potentially be integrated for detection from a range of wavelengths from 1.5 μm to greater than 25 µm (Fig. 1a) ). Subsequent to implantation, the PDs were annealed in atmosphere for a series of increasing temperatures, reaching a maximum of 350°C, and the responsivity was found to increase with annealing temperature.The detection mechanism of our p-Si:Zn-n PDs is due to substitutional Zn atoms in the Si lattice, which act as a double-acceptors and result in the two defect levels ( Fig. 2a) with energy levels of Ed1 ≈ Ev + 0.3 eV and Ed2 ≈ Ev + 0.58 eV 5,6 . While the position of the Fermi level in the Si:Zn region is not known, the excess carrier concentration is below that of the p and n regions, ensuring that the Si:Zn region will be fully depleted with the application of a reverse bias voltage. Photon-induced transitions occurs between Ed2 and the conduction band, which corresponds to a transition energy of Eg -Ed2 = 1.12eV -0.58eV = 0.54eV with a peak absorption wavelength of ≈ 2.3 µm. Photocurrent generation due to this transition is shown to 4 be a single-photon process by the linearity measurements in Fig. 2b. The presence of Ed1 does not contribute to photocurrent generation in the wavelength range of interest; however, its presence should impact the thermally assisted re-population rate of Ed2 and thus the internal quantum efficiency, ηi, of the PD. The responsivity, defined as R = Iph/Pin, where Iph is the photocu...