Optical Waveguide Theory

Quantum technologies comprise an emerging class of devices capable of controlling superposition and entanglement of quantum states of light or matter, to realize fundamental performance advantages over ordinary classical machines. The technology of integrated quantum photonics has enabled the generation, processing and detection of quantum states of light at a steadily increasing scale and level of complexity, progressing from few-component circuitry occupying centimetre-scale footprints and operating on two photons, to programmable devices approaching 1,000 components occupying millimetre-scale footprints with integrated generation of multiphoton states. This Review summarizes the advances in integrated photonic quantum technologies and its demonstrated applications, including quantum communications, simulations of quantum chemical and physical systems, sampling algorithms, and linear-optic quantum information processing. This Review covers recent progress in integrated quantum photonics (IQP) technologies and their applications. The challenges and opportunities of realizing large-scale, monolithic IQP circuits for future quantum applications are discussed.

Integrated photonic quantum technologies

Generations of technologies with fundamentally new information processing capabilities will emerge if microscopic physical systems can be controlled to encode, transmit, and process quantum information, at scale and with high fidelity. In the decade after its 2008 inception, the technology of integrated quantum photonics enabled the generation, processing, and detection of quantum states of light, at a steadily increasing scale and level of complexity. Using both established and advanced fabrication techniques, the field progressed from the demonstrations of fixed circuits comprising few components and operating on two photons, to programmable circuitry approaching 1000 components with integrated generation of multi-photon states. A continuation in this trend over the next decade would usher in a versatile platform for future quantum technologies. This Review summarises the advances in integrated photonic quantum technologies (materials, devices, and functionality), and its demonstrated on-chip applications including secure quantum communications, simulations of quantum physical and chemical systems, Boson sampling, and linear-optic quantum information processing.

Integrated Photonic Quantum Technologies

Phase‐change materials (PCMs) are seeing tremendous interest for their use in reconfigurable photonic devices; however, the most common PCMs exhibit a large absorption loss in one or both states. Here, Sb2S3 and Sb2Se3 are demonstrated as a class of low loss, reversible alternatives to the standard commercially available chalcogenide PCMs. A contrast of refractive index of Δn = 0.60 for Sb2S3 and Δn = 0.77 for Sb2Se3 is reported, while maintaining very low losses (k < 10−5) in the telecommunications C‐band at 1550 nm. With a stronger absorption in the visible spectrum, Sb2Se3 allows for reversible optical switching using conventional visible wavelength lasers. Here, a stable switching endurance of better than 4000 cycles is demonstrated. To deal with the essentially zero intrinsic absorption losses, a new figure of merit (FOM) is introduced taking into account the measured waveguide losses when integrating these materials onto a standard silicon photonics platform. The FOM of 29 rad phase shift per dB of loss for Sb2Se3 outperforms Ge2Sb2Te5 by two orders of magnitude and paves the way for on‐chip programmable phase control. These truly low‐loss switchable materials open up new directions in programmable integrated photonic circuits, switchable metasurfaces, and nanophotonic devices.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adfm.202002447

A new family of ultralow loss reversible phase‐change materials for photonic integrated circuits: Sb 2 S 3 and Sb 2 Se 3

Spatial light modulators are essential optical elements in applications that require the ability to regulate the amplitude, phase and polarization of light, such as digital holography, optical communications and biomedical imaging. With the push towards miniaturization of optical components, static metasurfaces are used as competent alternatives. These evolved to active metasurfaces in which light-wavefront manipulation can be done in a time-dependent fashion. The active metasurfaces reported so far, however, still show incomplete phase modulation (below 360°). Here we present an all-solid-state, electrically tunable and reflective metasurface array that can generate a specific phase or a continuous sweep between 0 and 360° at an estimated rate of 5.4 MHz while independently adjusting the amplitude. The metasurface features 550 individually addressable nanoresonators in a 250 × 250 μm2 area with no micromechanical elements or liquid crystals. A key feature of our design is the presence of two independent control parameters (top and bottom gate voltages) in each nanoresonator, which are used to adjust the real and imaginary parts of the reflection coefficient independently. To demonstrate this array’s use in light detection and ranging, we performed a three-dimensional depth scan of an emulated street scene that consisted of a model car and a human figure up to a distance of 4.7 m. By controlling two voltage gates separately from one another, a spatial light modulator has been made that can continuously vary the phase of 360 degrees while independently adjusting the amplitude.

All-solid-state spatial light modulator with independent phase and amplitude control for three-dimensional LiDAR applications

Optical-phased arrays (OPAs) enable complex beamforming, random-access beam pointing, and simultaneous scan and tracking of multiple targets by controlling the phases of two-dimensional (2D) coherent emitters. So far, no OPA can achieve all desirable features including large 2D arrays, high optical efficiency, wideband operation in wavelengths, fast response time, and large steering angles at the same time. Here, we report on a large-scale 2D OPA with novel microelectro-mechanical-system (MEMS)-actuated phase shifters. Wavelength-independent phase shifts are realized by physically moving a grating element in the lateral direction. The OPA has 160×160 independent phase shifters across an aperture of 3.1  mm×3.2  mm. It has a measured beam divergence of 0.042°×0.031°, a field of view (FOV) of 6.6°×4.4°, and a response time of 5.7 μs. It is capable of providing about 25,600 rapidly steerable spots within its FOV. The grating phase shifters are optimized for the near-infrared telecom wavelength bands from 1200 to 1700 nm with 85% optical efficiency.

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2D broadband beamsteering with large-scale MEMS optical phased array

Fast optical switches have been proposed as a promising alternative to enable continual scaling of data centers with increasing size and data rates. Silicon photonics is a compelling platform for large-scale integrated photonic switches, leveraging advanced manufacturing foundries for electronic integrated circuits. In the past decade, the port counts of silicon photonic switches have increased steadily to 128×128. Further scaling of the switch is constrained by the maximum reticle size (2–3 cm) of lithography tools. Here, we propose to use wafer-scale integration to overcome the die size limit. As a proof of concept demonstration, we fabricated a 240×240 switch by lithographically stitching a 3×3 array of identical 80×80 switch blocks across reticle boundaries. Stitching loss is substantially reduced (0.004 dB) by tapering the waveguide width to 10 μm. The fabricated switch on a 4 cm×4 cm chip exhibits a maximum on-chip loss of 9.8 dB, an ON/OFF ratio of 70 dB, and switching times of less than 400 ns. To our knowledge, this is the largest integrated photonic switch ever reported. The loss-to-port count ratio (0.04 dB/port) is also the lowest.

Wafer-scale silicon photonic switches beyond die size limit

Three-dimensional (3D) imaging sensors allow machines to perceive, map and interact with the surrounding world1. The size of light detection and ranging (LiDAR) devices is often limited by mechanical scanners. Focal plane array-based 3D sensors are promising candidates for solid-state LiDARs because they allow electronic scanning without mechanical moving parts. However, their resolutions have been limited to 512 pixels or smaller2. In this paper, we report on a 16,384-pixel LiDAR with a wide field of view (FoV, 70° × 70°), a fine addressing resolution (0.6° × 0.6°), a narrow beam divergence (0.050° × 0.049°) and a random-access beam addressing with sub-MHz operation speed. The 128 × 128-element focal plane switch array (FPSA) of grating antennas and microelectromechanical systems (MEMS)-actuated optical switches are monolithically integrated on a 10 × 11-mm2 silicon photonic chip, where a 128 × 96 subarray is wire bonded and tested in experiments. 3D imaging with a distance resolution of 1.7 cm is achieved with frequency-modulated continuous-wave (FMCW) ranging in monostatic configuration. The FPSA can be mass-produced in complementary metal-oxide-semiconductor (CMOS) foundries, which will allow ubiquitous 3D sensors for use in autonomous cars, drones, robots and smartphones. 

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A large-scale microelectromechanical-systems-based silicon photonics LiDAR

Abstract Efficient light energy transfer between optical waveguides has been a critical issue in various areas of photonics and optoelectronics. Especially, the light coupling between optical fibers and integrated waveguide structures provides essential input-output interfaces for photonic integrated circuits (PICs) and plays a crucial role in reliable optical signal transport for a number of applications, such as optical interconnects, optical switching, and integrated quantum optics. Significant efforts have been made to improve light coupling properties, including coupling efficiency, bandwidth, polarization dependence, alignment tolerance, as well as packing density. In this review article, we survey three major light coupling methods between optical fibers and integrated waveguides: end-fire coupling, diffraction grating-based coupling, and adiabatic coupling. Although these waveguide coupling methods are different in terms of their operating principles and physical implementations, they have gradually adopted various nanophotonic structures and techniques to improve the light coupling properties as our understanding to the behavior of light and nano-fabrication technology advances. We compare the pros and cons of each light coupling method and provide an overview of the recent developments in waveguide coupling between optical fibers and integrated photonic circuits.

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High-efficiency broadband light coupling between optical fibers and photonic integrated circuits

128×128 silicon photonic switch with 16, 384 MEMS-actuated switching elements and row/column addressing scheme have been successfully integrated on a 16x17mm2 chip. The maximum (minimum) on-chip loss is 22.7 dB (4.8 dB).

Kyungmok Kwon

Papers

2D broadband beamsteering with large-scale MEMS optical phased array

Wafer-scale silicon photonic switches beyond die size limit

A large-scale microelectromechanical-systems-based silicon photonics LiDAR

High-efficiency broadband light coupling between optical fibers and photonic integrated circuits

128×128 Silicon Photonic MEMS Switch with Scalable Row/Column Addressing