In wavelength division multiplexing (WDM) schemes, splitters must be used to combine and separate different wavelengths. Conventional splitters are fairly large with footprints in hundreds to thousands of square microns, and experimentally-demonstrated MMI-based and inverse-designed ultra-compact splitters operate with only two channels and large channel spacing ($>$100 nm). Here we inverse design and experimentally demonstrate a three-channel wavelength demultiplexer with 40 nm spacing (1500 nm, 1540 nm, and 1580 nm) with a footprint of 24.75 $\mu\mathrm{m}^2$. The splitter has a simulated peak insertion loss of -1.55 dB with under -15 dB crosstalk and a measured peak insertion loss of -2.29 dB with under -10.7 dB crosstalk.

Inverse design and demonstration of a compact on-chip narrowband three-channel wavelength demultiplexer

We propose a distorted optical waveguide around a microsphere to mimic curved spacetimes caused by the “gravitational fields”. Gravitational lensing effects analogues are experimentally demonstrated and this can be used to prospective light harvesting.

https://absimage.aps.org/image/MAR14/MWS_MAR14-2013-000010.pdf

Trapping light by mimicking gravitational lensing

Abstract Silicon photonic integration has gained great success in many application fields owing to the excellent optical device properties and complementary metal-oxide semiconductor (CMOS) compatibility. Realizing monolithic integration of III-V lasers and silicon photonic components on single silicon wafer is recognized as a long-standing obstacle for ultra-dense photonic integration, which can provide considerable economical, energy-efficient and foundry-scalable on-chip light sources, that has not been reported yet. Here, we demonstrate embedded InAs/GaAs quantum dot (QD) lasers directly grown on trenched silicon-on-insulator (SOI) substrate, enabling monolithic integration with butt-coupled silicon waveguides. By utilizing the patterned grating structures inside pre-defined SOI trenches and unique epitaxial method via hybrid molecular beam epitaxy (MBE), high-performance embedded InAs QD lasers with monolithically out-coupled silicon waveguide are achieved on such template. By resolving the epitaxy and fabrication challenges in such monolithic integrated architecture, embedded III-V lasers on SOI with continuous-wave lasing up to 85 °C are obtained. The maximum output power of 6.8 mW can be measured from the end tip of the butt-coupled silicon waveguides, with estimated coupling efficiency of approximately -6.7 dB. The results presented here provide a scalable and low-cost epitaxial method for the realization of on-chip light sources directly coupling to the silicon photonic components for future high-density photonic integration. 

/pdf/monolithic-integration-of-embedded-iii-v-lasers-on-soi-1ev4s97e.pdf

Monolithic integration of embedded III-V lasers on SOI

Multimode devices and components have attracted considerable attention in the last years, and different research topics and themes have emerged very recently. The multimodality can be seen as an additional degree of freedom in designing devices, thus allowing for the development of more complex and sophisticated components. The propagation of different modes can be used to increase the fiber optic capacity, but also to introduce novel intermodal interactions, as well as allowing for complex manipulation of optical modes for a variety of applications. In this roadmap we would like to give to the readers a comprehensive overview of the most recent developments in the field, presenting contributions coming from different research topics, including optical fiber technologies, integrated optics, basic physics and telecommunications.

Roadmap on multimode photonics

To overcome the capacity crunch of optical communications based on the traditional single-mode fiber (SMF), different modes in a few-mode fiber (FMF) can be employed for mode division multiplexing (MDM). MDM can also be extended to photonic integration for obtaining improved density and efficiency, as well as interconnection capacity. Therefore, MDM becomes the most promising method for maintaining the trend of “Moore’s law” in photonic integration and optical fiber transmission. In this tutorial, we provide a review of MDM works and cutting-edge progresses from photonic integration to optical fiber transmission, including our recent works of MDM low-noise amplification, FMF fiber design, MDM Si photonic devices, and so on. Research and application challenges of MDM for optical communications regarding long-haul transmission and short reach interconnection are discussed as well. The content is expected to be of important value for both academic researchers and industrial engineers during the development of next-generation optical communication systems, from photonic chips to fiber links.

/pdf/mode-division-multiplexing-from-photonic-integration-to-2omwvzdkrl.pdf

Mode division multiplexing: from photonic integration to optical fiber transmission [Invited]

On-chip optical interconnect has been widely accepted as a promising technology to realize future large-scale multiprocessors. Mode-division multiplexing (MDM) provides a new degree of freedom for ...

Key Multimode Silicon Photonic Devices Inspired by Geometrical Optics

The hybrid multiplexing technique reactivates optical interconnect as it offers multiple dimensions to dramatically enhance the data capacity of a single wavelength carrier. A straightforward method to realize hybrid multiplexing is to perform polarization multiplexing for mode-multiplexed signals, by utilizing a mode-transparent polarization beam splitter (MTPBS), which can process multiple modes simultaneously. However, present PBSs mainly work in the single-mode regime, and it is not easy to redesign the conventional PBS to accommodate multiple modes, due to the severe mode dispersion. Here, a novel MTPBS, which can tackle a group of modes simultaneously, is proposed and demonstrated. As a demonstration, the MTPBS supporting a total channel number of 13 is experimentally achieved, with low insertion loss and low modal/polarization cross talk. This work provides a new insight to realize hybrid multiplexing and represents a solution for high-density and large-capacity photonic integration.

/pdf/integrated-mode-transparent-polarization-beam-splitter-1agrv1e8g7.pdf

Integrated mode-transparent polarization beam splitter supporting thirteen data channels

/pdf/on-chip-optical-spatial-domain-integrator-based-on-fourier-3wn89phy34.pdf

On-chip optical spatial-domain integrator based on Fourier optics and metasurface

Quantum dot lasers on silicon have gained significant interest over the past decade due to their great potential as an on-chip silicon photonic light source. Here, we demonstrate multi-wavelength injection locking of InAs/GaAs quantum dot Fabry–Perot (FP) lasers both on GaAs and silicon substrates by optical self-injection via an external cavity. The number of locked laser modes can be adjusted from a single peak to multiple peaks by tuning wavelength dependent phase and mode spacing of back-injected light through a Lyot filter. The multi-wavelength injection locked laser modes exhibit average optical linewidth of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mo>∼</mml:mo> <mml:mn>20</mml:mn> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">kHz</mml:mi> </mml:mrow> </mml:math> , which are narrowed by approximately three orders of magnitude from their free-running condition. Furthermore, multi-wavelength self-injection locking via an external cavity exhibits flat-top optical spectral properties with approximately 30 stably locked channels under stable operation over time, where the frequency detuning is less than 700 MHz within 40 min. Particularly, FP lasers by direct epitaxial growth on silicon substrates are self-injection locked as a flat-top comb source with tunable free spectral range from approximately 25 to 700 GHz. The reported results emphasize the great potential of multi-wavelength injection locked lasers as tunable on-chip multi-wavelength light sources. 

Multi-wavelength Injection Locked Comb Laser Epitaxially Grown on Silicon

To exploit spatial dimension, on-chip optical modes with various spatial profiles have been utilized in optical interconnects and spatial analog computing. An integrated Fourier optical system is able to perform spatial operations. However, the reported schemes based on a subwavelength structure pose difficulty in fabrication, and the fabrication-friendly structure has been investigated only with a fundamental mode. With the complementary metal-oxide-semiconductor process, we propose an integrated 4-f system with simple geometry and a moderate minimum feature size to manipulate the mode’s spatial size and position in a mode-transparent way. A size magnification of 2.5 and center-to-center position offset of 7 µm are experimentally demonstrated. Reasonable insertion loss and low inter-mode crosstalk are measured over a 30 nm bandwidth. The work in this Letter paves the way for an on-chip Fourier optical system with convenient fabrication and broadband operation.

CMOS-compatible integrated 4-f system for mode-transparent spatial manipulation.

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