Rui Cheng

Polarization-insensitive, thermo-optic (TO), silicon photonic phase shifters using vertical slot waveguides are proposed. For such slot waveguides that support slot modes only for transverse-electric (TE) polarized modes, the TO coefficients of the fundamental TE and transverse-magnetic (TM) polarized modes (i.e., TE₀ and TM₀, respectively) can be easily engineered to be the same by designing the widths of the slot and the total waveguide. In the experiments, the TO coefficients of the TE₀ and TM₀ modes for various slot waveguides are characterized by measuring the electrical switching powers of dual-polarization MZI optical switches using the slot waveguides as the TO phase shifters. A small switching power difference of <0.3% between the two polarization modes is observed for the switch using the slot waveguide engineered to act as a polarization-insensitive phase shifter. The performance of the developed optical switch is also experimentally characterized to prove the potential of the proposed phase shifters to realize polarization-independent switches.

Grating-assisted, contra-directional couplers (GA-CDCs), owing to their four-port operations, can offer several important advantages over traditional, single waveguide-based Bragg gratings. However, how to flexibly design the spectral responses of GA-CDCs has been much less studied. We report the spectral tailoring methodology of GA-CDCs to achieve arbitrary, physically realizable, complex spectral responses. Silicon GA-CDCs with various customized responses are demonstrated using the methodology, including sidelobe-suppressed filters, single- and multi-channel flattop filters, sawtooth- and triangle-shaped filters, and three-channel photonic Hilbert transformers.

The degradation and failure of the urban water supply network may lead to serious safety hazards, including pipe breaks, water supply interruptions, water resource losses, and contaminant intrusions. The risk evaluation of water supply pipeline failure in a distribution network is a challenging task, because most of the available data cannot fully reflect pipeline failure events and many of the mechanisms still cannot be fully understood. Therefore, a predictive model is urgently needed to assess pipeline failure risk based on available data. In this paper, based on the traditional risk assessment theory, seven main factors affecting pipeline failure are selected and scored, and then a pipeline failure model is established by using the particle swarm optimization (PSO) neural network. The model uses the neural network training of historical data to evaluate the failure of the water supply pipeline, and the PSO is used to optimize the neural network to effectively improve the training time and accuracy. The model error and correlation coefficient are 0.003 and 0.987, respectively. The proposed model can be used as a powerful support tool to assist infrastructure managers and pipeline maintainers in their plans and decision-making.

We propose a novel narrowband, single-channel, wavelength-tunable, silicon photonic reflector based on a compact grating-coupled microring (MRR) incorporating an inline nanobeam. A bent grating-assisted contra-directional coupler (CDC) is used for the wavelength-selective coupling between the bus and ring waveguides of the MRR to allow only a single resonant mode to be excited. The nanobeam acts as an ultra-compact, broadband, inline mirror which enables coupling between the counter-propagating modes in the MRR, thereby generating backward-traveling resonant light. The combination of the grating-assisted CDC and the nanobeam ultimately results in the single-channel reflection response of the MRR. The proposed reflectors developed on silicon-on-insular (SOI) platforms were fabricated and experimentally characterized. Ultra-narrow reflection bandwidths of $\sim$15 pm and large side-lobe suppression ratios of up to 16.8 dB were demonstrated, and single-channel responses were observed over broadband wavelength ranges (60 nm). The thermal tuning of the reflector wavelength was also validated. Anticipated applications include sensing, laser engineering, and optical communications.

Grating-assisted contra-directional couplers (CDCs) as 4-port wavelength-selective devices hold great potential as add-drop filters, multiplexers, routers, etc, for various integrated-optics applications. However, these devices have inherent low coupling strengths due to the coupled modes confined in different separate waveguides, preventing from realizing broad operation bands. Here, we propose silicon photonic CDCs assisted by novel asymmetric photonic crystals (APhCs) on multimode waveguides with elliptical nanoholes. Such APhC-assisted CDCs, compared with the conventional grating-assisted ones, can achieve significantly larger coupling strengths, thus offering much wider operation bands. We investigate the impact of changes in various structural parameters on the spectral response of an APhC-assisted CDC, and then discuss the design and optimization strategy of the device. The possibility of using other nanohole shapes of the APhC is also explored. In the experiments, a series of broadband and flat-top filters based on APhC-assisted CDCs on silicon-on-insulator (SOI) were fabricated and measured, and the influence of variations in the filter parameters on the response was validated. Notably, a 3 dB bandwidth of $\sim$ 48.8 nm was demonstrated on an APhC-assisted CDC with a coupling length of 22.3 um, which is the largest bandwidth ever reported for unchirped, SOI-based CDC filters, to the best of our knowledge.

It is shown that the thermo-optic (TO) coefficients of various waveguide modes of a sub-wavelength grating (SWG)-assisted strip waveguide is closely dependent on the various waveguide parameters with different dependencies, including the SWG width, strip waveguide width, duty cycle, and pitch. This offers what we believe to be new degrees of freedom in the design of TO coefficients for integrated-optic waveguides, opening the door to engineering the TO coefficients of individual spatial modes or polarization states using sub-wavelength structures. Such a capability is expected to offer new design possibilities for a variety of integrated photonic, thermo-optic devices. To demonstrate the application of the concept, a mode-insensitive switch on silicon-on-insulator using a TO coefficient-engineered SWG as a mode-independent, thermo-optic phase shifter is designed and experimentally demonstrated. The experimental results show that the switching powers of the TE0-TE2 modes are only ∼29 mW, and the maximum extinction ratios for the cross (bar) states are 38.2 dB (31 dB), 37.9 dB (37 dB), and 31.9 dB (20.5 dB) for the TE0-TE2 modes, respectively, at the wavelength of 1550 nm.

We propose novel forward-dropping, grating-assisted contra-directional coupler (CDC)-based, silicon photonics add-drop filters. The CDC is designed to enable cross-coupling between the TE0 mode in the bus waveguide and the TE1 mode in the drop waveguide. An anti-symmetric photonic crystal (PhC) is placed at the input side of the drop waveguide to act as an ultra-compact, broadband, mode-conversion reflector, which reflects the backward TE1 mode into the forward TE0 mode. This mode conversion is essential to prevent the forward drop signal from coupling back to the bus waveguide. An asymmetric Y-branch serving as a broadband mode demultiplexer is used at the output of the drop waveguide to provide the signal add and drop functionalities of the filter. The proposed add-drop filter with a ~4.9 $\mu {\mathrm{ m}}$ -long PhC is experimentally demonstrated, which shows a center wavelength and a 3 dB bandwidth of ~1521 nm and ~4 nm, respectively. A 4-channel forward-propagating wavelength demultiplexer is also verified by cascading 4 stages of the filters.

Compact, multimode, silicon waveguide bends are essential building blocks for high-density, integrated-optics, mode-division-multiplexing (MDM) circuits. However, sharp, multimode, silicon waveguide bends can typically introduce considerable insertion losses and inter-mode crosstalk. Significant efforts have been made to realize compact and high performance multimode silicon waveguide bends, but most of those designs either require complicated fabrication processes or are not really compact. We show that the transmission performances of a regular, ultra-sharp, multimode, silicon waveguide bend can be drastically improved by directly optimizing its topology. The optimization can be efficiently accomplished by using an adjoint-based inverse design. Through this design strategy, we realize a three-mode, silicon waveguide bend that is ultra-compact, with a small radius of 5.5 μm, while having ultra-low average insertion losses of 0.023, 0.046, and 0.068 dB and small average mode crosstalk of −26, −27, and −27 dB for the input modes of TE0 − 2 (TE = transverse electric), respectively, in the wavelength range 1500 to 1600 nm. Also, the waveguide bend can be fabricated through a single electron-beam lithography step. The significant advantages in size and performance of the current waveguide bend show the great potential of the proposed inverse design strategy for designing multimode waveguide bends for MDM systems.

Integrated Bragg gratings (IBGs) developed on the silicon-on-insulator (SOI) platform, owning to their high spectral flexibility, are showing great potential for many optical applications such as optical telecommunications and optical signal processing. In this tutorial, we describe how to design SOI-based IBGs to achieve arbitrary, complex, spectral responses. The overall design flow of IBGs is first introduced, followed by detailed elaborations of each individual step of the design and implementation process. Then, design examples of two SOI IBG-based, advanced optical devices, which are a multichannel photonic Hilbert transformer and a flattop, dispersion-compensating filter, are described, to further illustrate the design methodology. Finally, a conclusion of this tutorial and an outlook of the future work are given.