Qiuhang Zhong

Focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces

We report on the design and characterization of focusing-curved subwavelength grating couplers for ultra-broadband silicon photonics optical interfaces. With implementation of waveguide dispersion engineered subwavelength structures, an ultra-wide 1-dB bandwidth of over 100 nm (largest reported to date) near 1550 nm is experimentally achieved for transverse-electric polarized light. By tapering the subwavelength structures, back reflection is effectively suppressed and grating coupling efficiency is increased to -4.7 dB. A compact device footprint of 40 µm × 20 µm is realized by curving the gratings in a focusing scheme.

A Low-Voltage 35-GHz Silicon Photonic Modulator-Enabled 112-Gb/s Transmission System

We present a silicon photonic traveling-wave Mach-Zehnder modulator operating near 1550 nm with a 3-dB bandwidth of 35 GHz. A detailed analysis of travelingwave electrode impedance, microwave loss, and phase velocity is presented. Small-and large-signal characterization of the device validates the design methodology. We further investigate the performance of the device in a short-reach transmission system. We report a successful 112-Gb/s transmission of four-level pulse amplitude modulation over 5 km of SMF using 2.2 Vp_p drive voltage. Digital signal processing is applied at the transmitter and receiver. 56-GBaud PAM-4 and 64-Gb/s PAM-2 transmission is demonstrated below a pre-FEC hard decision threshold of 4.4 x 10-3.

Analytical Modeling of Silicon Microring and Microdisk Modulators With Electrical and Optical Dynamics

We propose an analytical time-domain model for microring and microdisk modulators, which considers both their electrical and optical properties. Theory of the dynamics of microring/microdisk is discussed, and general solutions to the transfer matrix representation are presented. Both static and dynamic predictions from the model are compared to measurement results to demonstrate the accuracy of our model. Static predictions and measurements are presented for power and phase responses, whereas dynamic predictions and measurements are presented for small-signal and large-signal operations. The model verifies that the chirping and modulation bandwidth of the modulators depend on the detuning state. Finally, the accuracy and scalability of several techniques employed in the model are discussed.

Thermally controlled coupling of a rolled-up microtube integrated with a waveguide on a silicon electronic-photonic integrated circuit

We report on the first experimental demonstration of the thermal control of coupling strength between a rolled-up microtube and a waveguide on a silicon electronic-photonic integrated circuit. The microtubes are fabricated by selectively releasing a coherently strained GaAs/InGaAs heterostructure bilayer. The fabricated microtubes are then integrated with silicon waveguides using an abruptly tapered fiber probe. By tuning the gap between the microtube and the waveguide using localized heaters, the microtube-waveguide evanescent coupling is effectively controlled. With heating, the extinction ratio of a microtube whispering-gallery mode changes over an 18 dB range, while the resonant wavelength remains approximately unchanged. Utilizing this dynamic thermal tuning effect, we realize coupling modulation of the microtube integrated with the silicon waveguide at 2 kHz with a heater voltage swing of 0-6 V.

High-speed compact silicon photonic Michelson interferometric modulator

We present the detailed analysis and characterization of a silicon Michelson modulator with short 500 μm phase shifters and a low VπLπ of 0.72 V-cm under reverse bias. We investigate optical modulation of reverse biased p-n and forward biased p-i-n junctions. We demonstrate for the first time that error-free operation up to 40 Gbps is possible with lumped silicon interferometric modulators. For reverse bias operation, we show that even greater bandwidth can be obtained with lower impedance drivers. Forward bias operation with pre-emphasized signals is shown to have clean eye diagrams up to 40 Gbps, however, error counting reveals a strong dependence on test patterns and that error-free operation is achievable for short pattern lengths.

Characterization of azimuthal and longitudinal modes in rolled-up InGaAs/GaAs microtubes at telecom wavelengths

We report on theoretical and experimental investigation of azimuthal and longitudinal modes in rolled-up microtubes at telecom wavelengths. These microtubes are fabricated by selectively releasing a coherently strained InGaAs/GaAs bilayer. We apply planar waveguide method and a quasi-potential model to analyze the azimuthal and longitudinal modes in the microtubes near 1550 nm. Then we demonstrate these modes in transmission spectrum by evanescent light coupling. The experimental observations agree well with the calculated results. Surface-scattering-induced mode splitting is also observed in both transmission and reflection spectra at ~1600 nm. The mode splitting is in essence the non-degeneracy of clockwise and counter-clockwise whispering-gallery modes of the microtubes. This study is significant for understanding the physics of modes in microtubes and other microcavities with three-dimensional optical confinement, as well as for potential applications such as microtube-based photonic integrated devices and sensing purposes.

Analytical modeling for ultra-high-speed microring modulators with electrical and optical dynamics

We propose an analytical, time-domain model for microring modulators, which encompasses pn-junction and resonator dynamics. It shows excellent agreement with experiment for high frequency operation beyond 20 GHz. Pulse-amplitude modulation is predicted for up to 80 Gb/s.

Counterpropagating Whispering-Gallery-Modes of Rolled-up Semiconductor Microtubes

We report on the first experimental demonstration of counterpropagating whispering-gallery-modes (WGMs) of rolled-up semiconductor microtubes. We studied the thermo-optic effect of the microtube and achieved optical-optical modulation in both propagating and counterpropagating WGMs. We also realized a thermal wavelength-switchable fiber ring laser using the counterpropagating WGMs of the microtube.

A 4×4 fully non-blocking switch on SOI based on interferometric thermo-optic phase shifters

We developed a 4×4 fully non-blocking crossbar switch fabric based on interferometric thermal phase shifters, where heating is achieved using resistive elements around the silicon waveguide. We achieved switching times of 5 μs, with 41 mW power consumption in an individual switching element.

A lumped michelson interferometric modulator in silicon

We demonstrate the operation of a low V_πL_π, lumped, and compact Michelson modulator fabricated in SOI. The modulator operates up to 25 Gbps with measured error free operation up to 12.5 Gbps.