Ring resonator based polarization diversity WDM receiver

Polarization-insensitive EDG demultiplexer combined with a polarization beam splitter

Polarization dependence is an inherent challenge for wavelength-division multiplexing transceivers on silicon photonic platforms, causing severe problems with polarization-dependent losses and hindering the implementation of monolithic integrated receivers. In this study, we developed a polarization-insensitive demultiplexer on a silicon nitride (Si3N4) platform, which provides a promising solution to the polarization challenge. Comprising an etched diffraction grating (EDG) and a polarization beam splitter (PBS), the demultiplexer can achieve polarization insensitivity by introducing an additional optical path difference for polarization compensation. The fabricated demultiplexers were experimentally measured to have minimum insertion losses of 1.5 dB, cross talks of better than -25 dB, and polarization-dependent losses of better than 0.7 dB. This is the first, to the best of our knowledge, proposed solution for a polarization-insensitive EDG demultiplexer combined with a PBS on a Si3N4 platform.

Design of wavelength division multiplexing devices based on tunable edge states of valley photonic crystals

Wavelength division multiplexing (WDM) devices are key photonic integrated circuit (PIC) elements. Conventional WDM devices based on silicon waveguides and photonic crystals have limited transmittance due to the high loss introduced by the strong backward scattering from defects. In addition, it is challenging to reduce the footprint of those devices. Here we theoretically demonstrate a WDM device in the telecommunication range based on all-dielectric silicon topological valley photonic crystal (VPC) structures. We tune its effective refractive index by tuning the physical parameters of the lattice in the silicon substrate, which can continuously tune the operating wavelength range of the topological edge states, which allows the designing of WDM devices with different channels. The WDM device has two channels (1475 nm-1530 nm and 1583 nm-1637 nm), with contrast ratios of 29.6 dB and 35.3 dB, respectively. We demonstrated highly efficient devices for multiplexing and demultiplexing in a WDM system. The principle of manipulating the working bandwidth of the topological edge states can be generally applied in designing different integratable photonic devices. Thus, it will find broad applications.

Co-packaged optics (CPO): status, challenges, and solutions

Due to the rise of 5G, IoT, AI, and high-performance computing applications, datacenter traffic has grown at a compound annual growth rate of nearly 30%. Furthermore, nearly three-fourths of the datacenter traffic resides within datacenters. The conventional pluggable optics increases at a much slower rate than that of datacenter traffic. The gap between application requirements and the capability of conventional pluggable optics keeps increasing, a trend that is unsustainable. Co-packaged optics (CPO) is a disruptive approach to increasing the interconnecting bandwidth density and energy efficiency by dramatically shortening the electrical link length through advanced packaging and co-optimization of electronics and photonics. CPO is widely regarded as a promising solution for future datacenter interconnections, and silicon platform is the most promising platform for large-scale integration. Leading international companies (e.g., Intel, Broadcom and IBM) have heavily investigated in CPO technology, an inter-disciplinary research field that involves photonic devices, integrated circuits design, packaging, photonic device modeling, electronic-photonic co-simulation, applications, and standardization. This review aims to provide the readers a comprehensive overview of the state-of-the-art progress of CPO in silicon platform, identify the key challenges, and point out the potential solutions, hoping to encourage collaboration between different research fields to accelerate the development of CPO technology.

Demonstration of polarization-insensitive optical filters on silicon photonics platform

We experimentally demonstrate a polarization-insensitive optical filter (PIOF) using polarization rotator-splitters (PRSs) and microring resonators (MRRs) on the silicon-on-insulator (SOI) platform with complementary metal-oxide-semiconductor (CMOS) compatible fabrication process. The PRS consists of a tapered-rib waveguide and an asymmetrical directional coupler (ADC), which realize the polarization rotation and splitting, to ensure the connected MRRs-based optical filter operating at one desired polarization when light with different polarizations are launched into the device. The measured results show that the optical transmission spectra of the device are identical for TE and TM polarization input. The box-like filtering spectra are also achieved with a 3-dB bandwidth of ∼0.15 nm and a high extinction ratio (ER) over 30 dB.

Optical performance monitoring using SOI-based spectral analysis

A novel optical performance monitoring (OPM) method based on Fourier transform spectrum analysis (FTSA) is designed for optical signal-to-noise ratio (OSNR) monitoring, modulation format and baud rate recognition in the presence of fiber nonlinearities. The interference intensities, which reflect spectral features of signals, are obtained by exploiting the FTSA consisting of two-stage Mach-Zehnder interferometer (MZI) arrays. Then, the mapping between the OPM parameters and modulated interference intensity (MII) is characterized using neural networks without prior knowledge of the configuration of the communication network. Results show that optical performance parameters are monitored simultaneously. Meanwhile, the accuracy of modulation format and baud rate recognition is 94.8% and most (over 86%) OSNR monitoring errors are less than ±1 dB under complex transmission conditions in presence of frequency offset and delay jitter. Besides, the FTSA can be fabricated on a silicon on insulator (SOI) platform with a large fabrication tolerance, and it has broad working bandwidth to support the full optical communication band. Therefore, the proposed OPM method is capable of integration and miniaturization, which can be ubiquitously applied in network intermediate nodes to support the construction of smart optical networks.

Measuring on-chip waveguide losses using a single, two-point coupled microring resonator

We demonstrate a method for measuring on-chip waveguide losses using a single microring resonator with a tunable coupler. By tuning the power coupling to the microring and measuring the microring's through-port transmission at each power coupling, one can separate the waveguide propagation loss and the effects of the coupling to the microring. This method is tolerant of fiber-chip coupling/alignment errors and does not require the use of expensive instruments for phase response measurements. In addition, this method offers a compact solution for measuring waveguide propagation losses, only using a single microring (230 µm×190 µm, including the metal pads). We demonstrate this method by measuring the propagation losses of silicon-on-insulator rib waveguides, yielding propagation losses of 3.1-1.3 dB/cm for core widths varying from 400-600 nm.

Automated control algorithms for silicon photonic polarization receiver

We demonstrate greedy linear descent-based, basic gradient descent-based, two-point step size gradient descent-based, and two-stage optimization method-based automated control algorithms and examine their performance for use with a silicon photonic polarization receiver. With an active feedback loop control process, time-varying arbitrary polarization states from an optical fiber can be automatically adapted and stabilized to the transverse-electric (TE) mode of a single-mode silicon waveguide. Using the proposed control algorithms, we successfully realize automated adaptations for a 10 Gb/s on-off keying signal in the polarization receiver. Based on the large-signal measurement results, the control algorithms are examined and compared with regard to the iteration number and the output response. In addition, we implemented a long-duration experiment to track, adapt, and stabilize arbitrary input polarization states using the two-point step size gradient descent-based and two-stage optimization method-based control algorithms. The experimental results show that these control algorithms enable the polarization receiver to achieve real-time and continuous polarization management.

High performance fully etched isotropic microring resonators in thin-film lithium niobate on insulator platform

We present our design, fabrication, and experimental results for very high-performance isotropic microring resonators with small radii (∼ 30 µm) based on single-mode strip waveguides and transverse magnetic (TM) polarization in a fully etched lithium niobate (Z-cut) thin-film on insulator. The loss of the devices is predicted to be < 10 dB/cm, and is measured to be ∼ 7 dB/cm. The measured optical responses of microring resonators exhibit an extinction of ∼ 25 dB (close to critical coupling), a 3 dB optical bandwidth of 49 pm (∼ 6 GHz) for all-pass structures, an extinction of ∼ 10 dB for add-drop structures, and a free spectral range of ∼ 5.26 nm, all of which are in excellent agreement with the design. This work is the first step towards ultra-compact and fully isotropic optical modulators in thin-film lithium niobate on insulator.