High spectro-temporal compression on a nonlinear CMOS-chip

Single-drive electro-optic frequency comb source on a photonic-wire-bonded thin-film lithium niobate platform

Stable pulse and flat-top frequency comb generation are an indispensable component of many photonic applications, from ranging to communications. Lithium niobate on insulator is an excellent electro-optic (EO) platform, exhibiting high modulation efficiency and low optical loss, making it a fitting candidate for pulse generation through electro-optic modulation of continuous-wave (CW) light, a commonly utilized method for generating ultrashort pulses. Here, we demonstrate an on-chip electro-optic comb generation module on thin-film lithium niobate (TFLN) consisting of a Mach-Zehnder interferometer (MZI) amplitude modulator (AM) and a cascaded phase modulator (PM) system driven by a single-electrode drive. We show that when operated in the correct regime, the lithium niobate chips can generate frequency combs with excellent spectral power flatness. In addition, we optically package one of the pulse generator chips via photonic wire bonding. The pulses generated by the photonic-wire-bonded device are compressed to 840 fs pulse duration using an optical fiber and show extremely stable operation.

Frequency comb generation via synchronous pumped χ(3) resonator on thin-film lithium niobate

Resonator-based optical frequency comb generation is an enabling technology for a myriad of applications ranging from communications to precision spectroscopy. These frequency combs can be generated in nonlinear resonators driven using either continuous-wave (CW) light, which requires alignment of the pump frequency with the cavity resonance, or pulsed light, which also mandates that the pulse repetition rate and cavity free spectral range (FSR) are carefully matched. Advancements in nanophotonics have ignited interest in chip-scale optical frequency combs. However, realizing pulse-driven on-chip Kerr combs remains challenging, as microresonator cavities have limited tuning range in their FSR and resonance frequency. Here, we take steps to overcome this limitation and demonstrate broadband frequency comb generation using a χ(3) resonator synchronously pumped by a tunable femtosecond pulse generator with on-chip amplitude and phase modulators. Notably, employing pulsed pumping overcomes limitations in Kerr comb generation typically seen in crystalline resonators from stimulated Raman scattering.

Grayscale-patterned integrated multilayer-metal-dielectric microcavities for on-chip multi/hyperspectral imaging in the extended visible bandwidth

Pixelated filter arrays of Fabry-Perot (FP) cavities are widely integrated with photodetectors to achieve a WYSIWYG ("what you see is what you get") on-chip spectral measurements. However, FP-filter-based spectral sensors typically have a trade-off between their spectral resolution and working bandwidth due to design limitations of conventional metal or dielectric multilayer microcavities. Here, we propose a new idea of integrated color filter arrays (CFAs) consisting of multilayer metal-dielectric-mirror FP microcavities that, enable a hyperspectral resolution over an extended visible bandwidth (∼300 nm). By introducing another two dielectric layers on the metallic film, the broadband reflectance of the FP-cavity mirror was greatly enhanced, accompanied by as-flat-as-possible reflection-phase dispersion. This resulted in balanced spectral resolution (∼10 nm) and spectral bandwidth from 450 nm to 750 nm. In the experiment, we used a one-step rapid manufacturing process by using grayscale e-beam lithography. A 16-channel (4 × 4) CFA was fabricated and demonstrated on-chip spectral imaging with a CMOS sensor and an impressive identification capability. Our results provide an attractive method for developing high-performance spectral sensors and have potential commercial applications by extending the utility of low-cost manufacturing process.

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.

Integrated femtosecond pulse generator on thin-film lithium niobate

Integrated femtosecond pulse and frequency comb sources are critical components for a wide range of applications, including optical atomic clocks¹, microwave photonics², spectroscopy³, optical wave synthesis⁴, frequency conversion⁵, communications⁶, lidar⁷, optical computing⁸ and astronomy⁹. The leading approaches for on-chip pulse generation rely on mode-locking inside microresonators with either third-order nonlinearity¹⁰ or with semiconductor gain^11,12. These approaches, however, are limited in noise performance, wavelength and repetition rate tunability ^10,13. Alternatively, subpicosecond pulses can be synthesized without mode-locking, by modulating a continuous-wave single-frequency laser using electro-optic modulators^1,14-17. Here we demonstrate a chip-scale femtosecond pulse source implemented on an integrated lithium niobate photonic platform¹⁸, using cascaded low-loss electro-optic amplitude and phase modulators and chirped Bragg grating, forming a time-lens system¹⁹. The device is driven by a continuous-wave distributed feedback laser chip and controlled by a single continuous-wave microwave source without the need for any stabilization or locking. We measure femtosecond pulse trains (520-femtosecond duration) with a 30-gigahertz repetition rate, flat-top optical spectra with a 10-decibel optical bandwidth of 12.6 nanometres, individual comb-line powers above 0.1 milliwatts, and pulse energies of 0.54 picojoules. Our results represent a tunable, robust and low-cost integrated pulsed light source with continuous-wave-to-pulse conversion efficiencies an order of magnitude higher than those achieved with previous integrated sources. Our pulse generator may find applications in fields such as ultrafast optical measurement^19,20 or networks of distributed quantum computers^21,22.

A topological nonlinear parametric amplifier

Topological boundary states are well localized eigenstates at the boundary between two different bulk topologies. As long as bulk topology is preserved, the topological boundary mode will endure. Here, we report topological nonlinear parametric amplification of light in a dimerized coupled waveguide system based on the Su-Schrieffer-Heeger model with a domain wall. The good linear transmission properties of the topological waveguide arising from the strong localization of light to the topological boundary is demonstrated through successful high-speed transmission of 30 Gb/s non-return-to-zero and 56 Gb/s pulse amplitude 4-level data. The strong localization of a co-propagating pump and probe to the boundary waveguide is harnessed for efficient, low power optical parametric amplification and wavelength conversion. A nonlinear tuning mechanism is shown to induce chiral symmetry breaking in the topological waveguide, demonstrating a pathway in which Kerr nonlinearities may be applied to tune the topological boundary mode and control the transition to bulk states.

Optical characterization of deuterated silicon-rich nitride waveguides

Chemical vapor deposition-based growth techniques allow flexible design of complementary metal-oxide semiconductor (CMOS) compatible materials. Here, we report the deuterated silicon-rich nitride films grown using plasma-enhanced chemical vapor deposition. The linear and nonlinear properties of the films are characterized, and we experimentally confirm that the silicon-rich nitride films grown with SiD₄ eliminates Si–H and N–H related absorption. The performance of identical waveguides for films grown with SiH₄ and SiD₄ are compared demonstrating a 2 dB/cm improvement in line with that observed in literature. Waveguides fabricated on the SRN:D film are further shown to possess a nonlinear parameter of 95 W⁻¹ m⁻¹, with the film exhibiting a linear and nonlinear refractive index of 2.46 and 9.8 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document}× 10^–18 m²W⁻¹ respectively.

Dispersion compensation of high-speed data using an integrated silicon nitride ring resonator

Dispersion impairments are a well-known limitation in data center communications, limiting both the usable data rates and reaches. Several companies today adopt silicon photonics as a core technology in their transceiver products. This presents an opportunity for silicon photonics-based dispersion management technologies to be integrated with the transceiver transmitter or receiver. In this manuscript, we present a ring-resonator based, tunable dispersion compensation device, providing dispersion ranging as wide from  + 12.9 × 10³ps/nm to -12.3 × 10³ps/nm. Thermo-optic tuning from 20°C to 70°C is demonstrated to allow continuous wavelength tuning across 200 GHz. High-speed experiments using 25 Gb/s non-return-to-zero data propagating through 20 km of single mode fiber show that a significant improvement in the eye diagram is achieved after compensation with the ring-resonator device. We demonstrate a significant improvement in the BER from 10^-3 to 10^-11 for data rates of 25 and 25.78125 Gb/s.

Enhanced photonics devices based on low temperature plasma-deposited dichlorosilane-based ultra-silicon-rich nitride (Si8N)

Ultra-silicon-rich nitride with refractive indices ~ 3 possesses high nonlinear refractive index-100× higher than stoichiometric silicon nitride and presents absence of two-photon absorption, making it attractive to be used in nonlinear integrated optics at telecommunications wavelengths. Despite its excellent nonlinear properties, ultra-silicon-rich nitride photonics devices reported so far still have fairly low quality factors of [Formula: see text], which could be mainly attributed by the material absorption bonds. Here, we report low temperature plasma-deposited dichlorosilane-based ultra-silicon-rich nitride (Si₈N) with lower material absorption bonds, and ~ 2.5× higher quality factors compared to ultra-silicon-rich nitride conventionally prepared with silane-based chemistry. This material is found to be highly rich in silicon with refractive indices of ~ 3.12 at telecommunications wavelengths and atomic concentration ratio Si:N of ~ 8:1. The material morphology, surface roughness and binding energies are also investigated. Optically, the material absorption bonds are quantified and show an overall reduction. Ring resonators fabricated exhibit improved intrinsic quality factors [Formula: see text], ~ 2.5× higher compared to conventional silane-based ultra-silicon-rich nitride films. This enhanced quality factor from plasma-deposited dichlorosilane-based ultra-silicon-rich nitride signifies better photonics device performance using these films. A pathway has been opened up for further improved device performance of ultra-silicon-rich nitride photonics devices at material level tailored by choice of different chemistries.

Soliton-effect compression of picosecond pulses on a photonic chip

We report soliton-effect pulse compression of low energy (∼25pJ), picosecond pulses on a photonic chip. An ultra-low-loss, dispersion-engineered 40-cm-long waveguide is used to compress 1.2-ps pulses by a factor of 18, which represents, to our knowledge, the largest compression factor yet experimentally demonstrated on-chip. Our scheme allows for interfacing with an on-chip picosecond source and offers a path towards a fully integrated stabilized frequency comb source.