Xinru Wu

Fully suspended slot waveguides for high refractive index sensitivity

A fully suspended mid-infrared (FSMIR) slot waveguide is proposed and experimentally demonstrated on a silicon-on-insulator (SOI) platform for the first time. The slotted waveguide core is mechanically supported by lateral subwavelength grating claddings. The fabricated waveguides possess low propagation loss, which is measured to be 7.9  dB/cm at the wavelength of 2.25 μm. With the underlying buried oxide (BOX) removed, the FSMIR slot waveguide has a broad spectral range of transparency that is limited only by the absorption of silicon. Numerical simulation shows that its sensitivity, defined as the ratio between the change of the effective index and the ambient refractive index, can reach 1.123, which is 9.7% higher than the maximal sensitivity of conventional SOI slot waveguides on BOX.

128-Gb/s Line Rate OFDM Signal Modulation Using an Integrated Silicon Microring Modulator

Compared with Mach-Zehnder modulators, silicon microring modulators have the important advantages of smaller footprint and significantly lower energy per bit but suffer the disadvantages of inherent wavelength dependence and a modulation bandwidth that is limited by the resonator bandwidth. Here, we address the latter disadvantage of using an integrated silicon microring modulator with spectrally efficient modulation with 16-quadrature amplitude modulation on direct detection orthogonal frequency division multiplexing (OFDM). We demonstrate a line rate of 128-Gb/s and a net data rate of 107.6-Gb/s OFDM signal modulation using this tunable microring modulator. The 6.6- and 10-km single-mode fiber transmission of the modulated signal is presented. The measured bit error rates of back-to-back, 6.6-km, and 10-km fiber transmission are well below the forward error correction threshold.

Amplitude and Phase Modulation of UWB Monocycle Pulses on a Silicon Photonic Chip

We demonstrate both the amplitude and bi-phase modulation of ultrawideband (UWB) monocycle pulses using silicon microring modulator. The integrated silicon ring modulator generates the monocycle pulse train and modulates it simultaneously. By applying different driving voltages, the amplitude and phase-modulated UWB signal with a data rate of 2.5 Gb/s are achieved. The ring modulator can be thermally tuned for a free spectral range with a ~50-mW power consumption.

Mode-Division Multiplexing for Silicon Photonic Network-on-Chip

Optical interconnect is a potential solution to attain the large bandwidth on-chip communications needed in high-performance computers in a low-power and low-cost manner. Mode-division multiplexing (MDM) is an emerging technology that scales the capacity of a single wavelength carrier by the number of modes in a multimode waveguide, and is attractive as a cost-effective means for high bandwidth density on-chip communications. Advanced modulation formats with high spectral efficiency in MDM networks can further improve the data rates of the optical link. Here, we demonstrate an intra-chip MDM communications link employing advanced modulation formats with two waveguide modes. We demonstrate a compact single wavelength carrier link that is expected to support 2 × 100 Gb/s mode multiplexed capacity. The network comprised integrated microring modulators at the transmitter, mode multiplexers, multimode waveguide interconnect, mode demultiplexers, and integrated germanium on silicon photodetectors. Each of the mode channels achieves 100 Gb/s line rate with 84 Gb/s net payload data rate at 7% overhead for hard-decision forward error correction (HD-FEC) in the OFDM/16-QAM signal transmission.

Enhancement of self-phase modulation induced spectral broadening in silicon suspended membrane waveguides

We experimentally observed a possibly enhanced self-phase modulation (SPM) in silicon suspended membrane waveguides (SMWs) by measuring the spectral broadening of optical pulses. The nonlinear coefficient n 2 and the two-photon absorption coefficient β 2 of silicon SMWs were measured to be (4.6 ± 0.9) × 10−18 m2 W−1 and 0.46 cm GW−1 at 1555 nm wavelength. We also proposed a method of using SPM-induced spectral broadening to obtain the coupling loss of a single grating coupler and experimentally compared the spectra of two grating couplers in silicon SMWs and in silicon-on-insulator waveguides.

Fully suspended slot waveguide platform

A fully suspended slot waveguide (FSSWG) platform, including straight slot waveguides, 90° bends, high-Q racetrack resonators, and strip-to-slot mode converters, is demonstrated for broadband and low-loss operation in the mid-infrared spectral region. The proposed FSSWG platform has inherent merits of a broad spectral range of transparency which is limited only by the absorption of silicon, strong light–analyte interaction, good mechanical stability, and single lithography step fabrication process. By using asymmetric FSSWGs, the propagation loss, bending loss, and intrinsic optical Q factor are demonstrated to be 2.8 dB/cm, 0.15 dB/90°, and 12 600, respectively. The average conversion efficiency of a mode converter is 95.4% over a bandwidth of 170 nm and 97.0% at 2231 nm. The FSSWG platform would be promising for a long-range and cavity-enhanced light–analyte interaction.

Fully suspended mid-infrared racetrack resonator with subwavelength grating cladding

A fully suspended mid-infrared racetrack resonator is experimentally demonstrated. It has good mechanical stability and broad spectral range of transparency. The measured loaded optical Q factor is 16,440 at 2402.38 nm, with an extinction ratio of 11.83 dB.

Equalization of PAM-4 Signal Generated by Silicon Microring Modulator for 64-Gbit/s Transmission

We propose and demonstrate a 64-Gbit/s four-level pulse-amplitude-modulation transmission by using a limited modulation bandwidth silicon microring modulator (SiMRM) and the Volterra equalization scheme. Severe intersymbol interference caused by both the chromatic dispersion of the optical fiber and the bandwidth limitation of components can be compensated. Besides, several equalization schemes are compared and analyzed in terms of performance and complexity. In addition, the design and characteristics of the SiMRM used in the experiment are also discussed.

Scalable Ultra-Wideband Pulse Generation Based on Silicon Photonic Integrated Circuits

We demonstrate a scalable ultra-wideband (UWB) pulse generator using silicon photonic integrated circuits. A delay-and-superimpose principle for higher order UWB Guassian pulse generation is realized by monolithically integrating a pair of tunable microring resonators, a delayline waveguide, tap couplers, and an on-chip photodiode. The two microrings function as discrimination filters and generate the asymmetric monocycle pulses. For a proof-of-concept demonstration, the doublet pulses with a 10-dB fractional bandwidth of 100% and the triplet pulses with a 10-dB fractional bandwidth of 132% are generated by superposition of the two asymmetric pulses with a proper delay between them. The proposed scheme and the photonic chip can be scaled to generate more kinds of power-efficient pulses.

Multiplexing and switching for mode division multiplexed optical interconnects

We present some basic functional elements for mode division multiplexed optical interconnects including an inverse designed mode multiplexer, a thermo-optic based mode add-drop switch, and integrated modulators for mode division multiplexed interconnects. Optical interconnects for on-chip global interconnects have attracted research interest in recent years because they provide a potential solution to the problem of thermal limitations of copper interconnects, which already severely constrain the maximum clock frequencies that may be used in microprocessors. While state of the art CMOS transistors can switch at frequencies of many tens of gigahertz desktop microprocessors today are still limited to ~ 5 GHz clock frequencies because of the extreme heat density and associated practical problem of dissipating the heat generated if the dense electrical interconnects were operated at higher frequencies. The use of dense wavelength division multiplexing (DWDM) can provide an order of magnitude improvement in on-chip data throughput at the same power consumption as electrical interconnects. DWDM was necessary because of the limited area available for on-chip optical interconnects and the high density of interconnects that are needed. However DWDM is expensive because of its requirement of precise wavelength laser arrays (and temperature control of the wavelength selective elements). In this paper we review our work towards the development of functional elements for implementing on-chip optical interconnects that can adopt the lower cost approach of using a single wavelength source with mode division multiplexing (MDM) as an alternative to DWDM. We have developed some of the basic functional elements of MDM including a novel inverse designed mode multiplexer and demultiplexer, mode switches for MDM networks, and the integration of modulators for a MDM network on chip. Experimental performance on some of the fabricated devices will be presented at the conference.