Silicon photonics beyond the singlemode regime is applied for enabling ultralow-loss waveguide propagation for the fundamental mode even without any special fabrication process. Here a micro-racetrack resonator is fabricated with a standard 220-nm-SOI (silicon-on-insulator) multiproject-wafer foundry and shows a record high intrinsic quality factor of 1.02×10 7 , corresponding to an ultralow propagation loss of only 0.065 dB cm −1 , which is about 20 times less than that of regular 450-nm-wide waveguides on the same chip. A state-of-the-art microwave photonic filter on silicon is then realized with an ultranarrow 3-dB bandwidth of 20.6 MHz and a tuning range of ≈20 GHz for the first time. A 100-cm-long delayline employed the broadened waveguides is also demonstrated with compact 90°Euler-curve bends, and the measured average propagation loss is about 0.14 dB cm −1 . The concept of silicon photonics beyond the singlemode regime helps solve the issue of high propagation loss significantly. In particular, it enables silicon photonic devices with enhanced performances, which paves the way for realizing large-scale silicon photonic integration. This concept can be extended further to any other material platforms, such as silicon nitride and lithium niobate. This also brings numerous new opportunities for various applications such as nonlinear photonics, large-scale photonic integration, quantum photonics, microwave photonics, etc.

A compact spectrometer on silicon is proposed and demonstrated with an ultrahigh resolution. It consists of a thermallytunable ultra-high-Q resonator aiming at ultrahigh resolution and an array of wideband resonators for achieving a broadened working window. The present on-chip spectrometer has a footprint as compact as 0.35 mm 2 , and is realized with standard multi-project-wafer foundry processes. The measurement results show that the on-chip spectrometer has an ultra-high resolution ∆λ of 5 pm and a wide working window of 10 nm. The dynamic range defined as the ratio of the working window and the wavelength resolution is as large as 1940, which is the largest for on-chip dispersive spectrometers to the best of our knowledge. The present high-performance on-chip spectrometer has great potential for highresolution spectrum measurement in the applications of gas sensing, food monitoring, health analysis, etc.

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First and foremost, waveguides allow for light propagating waves to be concentrated into a specific pathway to reduce the amount of attenuation as it travels along. Although spiral waveguides have become commonplace in realizing photonic integrated circuits, a new structure for these spiral waveguides which includes a Euler Curve S-Bend and optimized radius and width could decrease propagation loss to .28 dB/cm. These Euler S-Bend waveguides use the same machinery and production process as the standardized spiral waveguides, negating the cost of requiring new machinery and increasing the validity of using the Euler S-Bend waveguides in standard practice.

Chip-scale programmable optical signal processors are often used to flexibly manipulate the optical signals for satisfying the demands in various applications, such as lidar, radar, and artificial intelligence. Silicon photonics has unique advantages of ultra-high integration density as well as CMOS compatibility, and thus makes it possible to develop large-scale programmable optical signal processors. The challenge is the high silicon waveguides propagation losses and the high calibration complexity for all tuning elements due to the random phase errors. In this paper, we propose and demonstrate a programmable silicon photonic processor for the first time by introducing low-loss multimode photonic waveguide spirals and low-random-phase-error Mach-Zehnder switches. The present chip-scale programmable silicon photonic processor comprises a 1×4 variable power splitter based on cascaded Mach-Zehnder couplers (MZCs), four Ge/Si photodetectors, four channels of thermally-tunable optical delaylines. Each channel consists of a continuously-tuning phase shifter based on a waveguide spiral with a micro-heater and a digitally-tuning delayline realized with cascaded waveguide-spiral delaylines and MZSs for 5.68 ps time-delay step. Particularly, these waveguide spirals used here are designed to be as wide as 2 μm, enabling an ultralow propagation loss of 0.28 dB/cm. Meanwhile, these MZCs and MZ-Ss are designed with 2-μm-wide arm waveguides, and thus the random phase errors in the MZC/MZS arms are negligible, in which case the calibration for these MZSs/MZCs becomes easy and furthermore the power consumption for compensating the phase errors can be reduced greatly. Finally, this programmable silicon photonic processor is demonstrated successfully to verify a number of distinctively different functionalities, including tunable time-delay, microwave photonic beamforming, arbitrary optical signal filtering, and arbitrary waveform generation.