Broadband tunable microwave photonic phase shifter with low RF power variation in a high-Q AlN microring

Ultrahigh-Q lithium niobate microring resonator with multimode waveguide

Difficulty in etching lithium niobate (LN) results in a relatively high propagation loss, which necessitates sophisticated processes to fabricate high-quality factor (Q) microresonators. Here, we fabricate a multimode microring resonator with an intrinsic Q of 6 × 10⁶, which exhibits a propagation loss 50 times lower than that of a single-mode LN microring fabricated under the same process. Notably, the excitation of higher-order modes in the multimode microring is effectively suppressed by utilizing the Euler bend. The highly regular transmission spectrum of the resonator demonstrates a free spectral range (FSR) of 56 GHz. Based on this microresonator, we implement a bandpass microwave photonic filter with an ultra-narrow 3 dB bandwidth of 47.5 MHz and a large tuning range of 2-26.5 GHz. It can be anticipated that the combination of existing advanced etching techniques with this work will drive the propagation loss of a LN waveguide closer to the material absorption loss, significantly facilitating the optimization of performance in applications requiring ultrahigh-Q LN microresonators, such as frequency combs, frequency conversion, electro-optic modulation, and quantum photonics.

Supercontinuum in integrated photonics: generation, applications, challenges, and perspectives

Frequency conversion in nonlinear materials is an extremely useful solution to the generation of new optical frequencies. Often, it is the only viable solution to realize light sources highly relevant for applications in science and industry. In particular, supercontinuum generation in waveguides, defined as the extreme spectral broadening of an input pulsed laser light, is a powerful technique to bridge distant spectral regions based on single-pass geometry, without requiring additional seed lasers or temporal synchronization. Owing to the influence of dispersion on the nonlinear broadening physics, supercontinuum generation had its breakthrough with the advent of photonic crystal fibers, which permitted an advanced control of light confinement, thereby greatly improving our understanding of the underlying phenomena responsible for supercontinuum generation. More recently, maturing in fabrication of photonic integrated waveguides has resulted in access to supercontinuum generation platforms benefiting from precise lithographic control of dispersion, high yield, compact footprint, and improved power consumption. This Review aims to present a comprehensive overview of supercontinuum generation in chip-based platforms, from underlying physics mechanisms up to the most recent and significant demonstrations. The diversity of integrated material platforms, as well as specific features of waveguides, is opening new opportunities, as will be discussed here.

Ultra-narrow passband-tunable filter based on a high-Q silicon racetrack resonator

An ultra-narrow narrow passband-tunable optical filter employing a high-Q silicon racetrack resonator is proposed and experimentally demonstrated on a SOI platform. The high-Q silicon racetrack resonator is realized by utilizing the multimode waveguide racetrack, and the Q factor is measured as high as 8.1×10⁵. The structure of the device is based on a thermally tunable Mach-Zehnder interferometer coupled racetrack. The tunability of the bandwidth is realized by tuning the coupling coefficient between the racetrack resonator and the input or output ports. Finally, the bandwidth of the filter can be tuned from 1.92 to 11.00 pm (240 MHz to 1.375 GHz), and the free spectral range is about 0.28 nm (35 GHz), with the footprint of 0.21mm².

Ultraviolet to mid-infrared supercontinuum generation in single-crystalline aluminum nitride waveguides

We demonstrate ultrabroadband supercontinuum generation from ultraviolet to mid-infrared wavelengths in single-crystalline aluminum nitride waveguides. Tunable dispersive waves are observed at the mid-infrared regime by precisely controlling the waveguide widths. In addition, ultraviolet light is generated through cascaded second-harmonic generation in the modal phase-matched waveguides. Numerical simulation indicates a high degree of coherence of the generated spectrum at around the telecom pump and two dispersive waves. Our results establish a reliable path for multiple octave supercontinuum comb generation in single-crystalline aluminum nitride to enable applications including precision frequency metrology and spectroscopy.

Broadly tunable and low power penalty radio frequency phase shifter using a coupled silicon microcavity

We present a continuously tunable silicon photonics assisted radio frequency (RF) phase shifter using a coupled microring resonator. Using the coupled cavity, we demonstrate a sub-1 dB power penalty for a RF bandwidth of 34.5 GHz (9-43.5 GHz) and a phase shift of π over the reported frequency range. Rigorous optimization of the cavity design using the coupled-mode theory is carried out to realize ultranarrow resonance peaks with a low-extinction ratio and large phase shift. Thermal tuning of the cavity is used to tune the phase while all-optical tuning is exploited to achieve broadband operation. We present a detailed simulation and experimental study of the proposed configuration. The proposed device configuration exhibits a configurable resonance linewidth and extinction ratio that allows for a broad bandwidth and an extremely low power penalty microwave phase shifter. We believe the demonstration would allow better integration of the on-chip functional elements of integrated microwave photonics.

Ultrahigh Q microring resonators using a single-crystal aluminum-nitride-on-sapphire platform

Aluminum-nitride-on-sapphire has recently emerged as a novel low-loss photonics platform for a variety of on-chip electro-optics as well as linear and nonlinear optics applications. In this Letter, we demonstrate ultrahigh quality factor (Q_int) microring resonators using single-crystal aluminum nitride grown on a sapphire substrate with an optimized design and fabrication process. A record high intrinsic Q_int up to 2.8×10⁶ at the wavelength of 1550 nm is achieved with a fully etched structure, indicating a low propagation loss less than 0.13 dB/cm. Such high Q_int aluminum-nitride-on-sapphire resonators with their wide bandgap and electro-optical and nonlinear optical properties is promising for a wide range of low-power and high-power compact on-chip applications over a broad spectral range.

Integrated microwave photonic phase shifter with full tunable phase shifting range (> 360°) and RF power equalization

We report a novel microwave photonic phase and amplitude control structure based on a single microring resonator with a tunable Mach Zehnder interferometer reflective loop, which enables the realization of a continuously tunable microwave photonic phase shifter with enhanced phase tuning range while simultaneously compensating for the RF power variations. The complimentary tuning of the phase and amplitude presents a simplistic approach to resolve the inherent trade-off between maintaining a full RF phase shift while eliminating large RF power variations. Detailed simulations have been carried out to analyze the performance of the new structure as a microwave photonic phase shifter, where the reflective nature of the proposed configuration shows an effective doubling of the phase range while the amplitude compensation module provides a parallel control to potentially reduce the RF amplitude variations to virtually zero. The phase range enhancement, which is first verified experimentally with a passive only chip, demonstrates the capability to achieve a continuously tunable RF phase shift of 0-510° with an RF amplitude variation of 9 dB. Meanwhile, the amplitude compensation scheme is incorporated onto an active chip with a continuously tunable RF phase shift of 0-150°, where the RF power variations is shown to be reduced by 5 dB while maintaining a constant RF phase shift.

Aluminum nitride-on-sapphire platform for integrated high-Q microresonators

We demonstrate aluminum nitride (AlN) on sapphire as a novel platform for integrated optics. High-confinement AlN microring resonators are realized by adopting a partially etched (pedestal) waveguide to relax the required etching selectivity for exact pattern transfer. A wide taper is employed at the chip end facets to ensure a low fiber-to-chip coupling loss of ~2.8 dB/facet for both transverse-electric (TE) and transverse-magnetic (TM) modes. Furthermore, the intrinsic quality factors (Q_int) recorded with a high-resolution linewidth measurement are up to ~2.5 and 1.9 million at telecom band for fundamental TE₀₀ and TM₀₀ modes, corresponding to a low intracavity propagation loss of ~0.14 and 0.2 dB/cm as well as high resonant buildup of 473 and 327, respectively. Such high-Q AlN-on-sapphire microresonators are believed to be very promising for on-chip nonlinear optics.