Large-scale Photonic Circuits Research Articles

Inverse-designed taper configuration for the enhancement of integrated 1 × 4 silicon photonic power splitters

Abstract Once light is coupled to a photonic chip, its efficient distribution in terms of power splitting throughout silicon photonic circuits is very crucial. We present two types of 1 × 4 power splitters with different splitting ratios of 1:1:1:1 and 2:1:1:2. Various taper configurations were compared and analyzed to find the suitable configuration for the power splitter, and among them, parabolic tapers were chosen. The design parameters of the power splitter were determined by means of solving inverse design problems via incorporating particle swarm optimization that allows for overcoming the limitation of the intuition-based brute-force approach. The front and rear portions of the power splitters were optimized sequentially to alleviate local minima issues. The proposed power splitters have a compact footprint of 12.32 × 5 μm2 and can be fabricated through a CMOS-compatible fabrication process. Two-stage power splitter trees were measured to enhance reliability in an experiment. As a result, the power splitter with a splitting ratio of 1:1:1:1 exhibited an experimentally measured insertion loss below 0.61 dB and an imbalance below 1.01 dB within the bandwidth of 1,518–1,565 nm. Also, the power splitter with a splitting ratio of 2:1:1:2 showed an insertion loss below 0.52 dB and a targeted imbalance below 1.15 dB within the bandwidth of 1,526–1,570 nm. Such inverse-designed power splitters can be an essential part of many large-scale photonic circuits including optical phased arrays, programmable photonics, and photonic computing chips.

Time-domain image processing using photonic reservoir computing

Photonic computing has attracted much attention due to its great potential to accelerate artificial neural network operations. However, the processing of a large amount of data, such as image data, basically requires large-scale photonic circuits and is still challenging due to its low scalability of the photonic integration. Here, we propose a scalable image processing approach, which uses a temporal degree of freedom of photons. In the proposed approach, the spatial information of a target object is compressively transformed to a time-domain signal using a gigahertz-rate random pattern projection technique. The time-domain signal is optically acquired at a single-input channel and processed with a microcavity-based photonic reservoir computer. We experimentally demonstrate that this photonic approach is capable of image recognition at gigahertz rates.

A brief review of design and simulation methodology in silicon photonics

Powerful electronic design automation tools have enabled the rapid development of electronic Integrated Circuits (ICs). Similar to electronic ICs, silicon photonics technology has sufficiently matured, and large-scale photonic circuits can now be implanted into a single chip. Design tools have also evolved from primary devices to complex photonic circuits. In this paper, we review the current state of photonic design automation in terms of device modeling methods and circuit simulation methodologies, and compare the photonics design flow with mature electronic design automation design flows. Key challenges and opportunities are also discussed.

All-optical tuning of a diamond micro-disk resonator on silicon

High-quality integrated diamond photonic devices have previously been demonstrated in applications from non-linear photonics to on-chip quantum optics. However, the small sample sizes of single crystal material available, and the difficulty in tuning its optical properties, are barriers to the scaling of these technologies. Both of these issues can be addressed by integrating micrometer-scale diamond devices onto host photonic integrated circuits using a highly accurate micro-assembly method. In this work a diamond micro-disk resonator is integrated with a standard single-mode silicon-on-insulator waveguide, exhibiting an average loaded Q -factor of 3.1 × 10 4 across a range of spatial modes, with a maximum loaded Q -factor of 1.05 × 10 5 . The micrometer-scale device size and high thermal impedance of the silica interface layer allow for significant thermal loading and continuous resonant wavelength tuning across a 450 pm range using a milliwatt-level optical pump. This diamond-on-demand integration technique paves the way for tunable devices coupled across large-scale photonic circuits.

Estimating the Indistinguishability of Heralded Single Photons Using Second-Order Correlation

High-visibility quantum interference (indistinguishability) among single photons is the key to scalable, high-fidelity linear optical quantum gates. Measuring indistinguishability by interference is laborious and time-consuming, though, and thus not scalable. The authors find that the faster, simpler second-order correlation functions provide results equivalent to the indistinguishability, and could be useful in rapid-prototyping source design of large-scale photonic circuits. Also, for mature guided-wave integrated optics such as silicon photonics, the high-visibility bottleneck is due to the physics of a process that tends to produce single photons in multiple spectral modes.

Polycrystalline diamond photonic waveguides realized by femtosecond laser lithography

In recent years, the perception of diamond has changed from it being a pure gemstone to a universal high-tech material. In the field of photonics, an increased interest is emerging due to its outstanding optical properties, such as its high refractive index, a spectrally wide transmission window, and high Raman coefficient. Furthermore, the capability to host color defects for room temperature single photon generation makes diamond an attractive platform for quantum photonics. Known as nature’s hardest material, the fabrication and handling of crystalline diamond for integrated optics remains challenging. Here, we report on the fabrication of three-dimensional Type III depressed cladding waveguides in polycrystalline diamond substrates by direct laser writing. Single mode waveguiding is demonstrated in the near-infrared telecommunication C-band. We believe that this enables the fabrication of three-dimensional large-scale photonic circuits, which are essential for advanced classical and quantum diamond photonics.

Tri-layer silicon nitride-on-silicon photonic platform for ultra-low-loss crossings and interlayer transitions.

We present a three-layer silicon nitride on silicon platform for constructing very large photonic integrated circuits. Efficient interlayer transitions are enabled by the close spacing between adjacent layers, while ultra-low-loss crossings are enabled by the large spacing between the topmost and bottommost layers. We demonstrate interlayer taper transitions with losses < 0.15 dB for wavelengths spanning from 1480 nm to 1620 nm. Our overpass waveguide crossings exhibit insertion loss < 2.1 mdB and crosstalk below -56 dB in the wavelength range between 1480 nm and 1620 nm with losses as low as 0.28 mdB. Our platform architecture is suited to meet the demands of large-scale photonic circuits which contain hundreds of crossings.

Silicon nanophotonic devices for chip-scale optical communication applications [Invited

This paper reviews recent work in the area of silicon photonic devices and circuits for monolithic and heterogeneous integration of circuits and systems on a chip. In this context, it presents fabrication results for producing low-loss silicon waveguides without etching. Resonators and add-drop distributed filters utilizing sidewall modulation fabricated in a single lithography and etching step are demonstrated. It also presents an optical pulse compressor that monolithically integrates self-phase modulation and anomalous dispersion compensation devices on a silicon chip. As an example of heterogeneous integration, we demonstrate vertical emitting metallo-dielectric nanolasers integrated onto a silicon platform. Future research directions toward large-scale photonic circuits and systems on a chip also are discussed.

Wannier-function based scattering-matrix formalism for photonic crystal circuitry

A guided-mode scattering matrix approach to photonic crystal integrated devices, based on the expansion of the electromagnetic field in Wannier functions is presented and its applicability to large-scale photonic circuits is demonstrated. In particular, we design two components typically used in wavelength division multi/demultiplexing applications, namely, a directional coupler and a Mach-Zehnder interferometer, and we analyze the transmission spectra as a function of the coupler length and/or delay line length, respectively. These examples demonstrate that by cascading basic functional elements, large-scale circuits can be accurately described and efficiently designed with minimal numerical effort.

Diamonds for scalable quantum information systems

Enormous potential lies in the weird quantum world for a new generation of information systems that address important and unsolved problems in secure communications, highperformance computing, data storage, and simulation. Surprisingly, single crystal diamond (SCD), long known for its allure as a gemstone, has just the right properties for fabricating critical components that will serve as building blocks of this new quantum technology. A key requirement for useful quantum information devices is the ability to create shared quantum entanglement among large numbers of quantum bits (qubits) independent of their physical locations. This is best done using optical photons, especially for long-distance secure quantum communication applications. However, because photons do not make good quantum memories, a system with long-lived qubits, such as spins and a highfidelity quantum interface to photons would be highly desirable. Although trapped ions are potentially capable of fulfilling this need, there is no substitute for solid-state implementation for ease of scaling and integration. Our approach is to use nitrogen-vacancy (NV) color centers in diamond. These consist of a nitrogen atom paired with a vacancy in the diamond lattice. They provide strong optical transitions that scatter photons efficiently enough to allow individual NVs to be easily visualized with a confocal microscope.1 Unlike most optical emitters, the NV has a ground state with three electron spin sublevels. Proximal nuclear spins provide additional degrees of freedom,2 resulting in numerous choices for qubit encoding. Spin coherence times for such qubits are unusually long, even at room temperature, rivaled only by ion qubits trapped in high vacuum. It is estimated that up to a million single qubit operations can be performed within the NV spin coherence lifetime. Through a fortunate interaction with the singlet excited states, the NV can be spin-polarized to a high degree by illumination with broadband light, such as an LED flashlight, even at room temperature. Equally fortuitous is the fact that one of the electron spin sublevels undergoes an optical cycling transition that is pure enough, at liquid helium temperatures, to allow single-shot spin readout.1 Even at room temperature, this cycling transition is adequate for such a readout if half of the emitted photons can be detected. Encouragingly, our recent theoretical estimates3 and pilot experiments4 with optical plasmon nanowires suggest this might soon be possible. At room temperature, we have demonstrated each of the key elements needed to build few-qubit quantum processors with enough functionality for quantum repeater nodes.5 Specifically, we showed that arbitrary electron spin coherences can be transferred to a nuclear spin where they can be stored for up to a fraction of second and recovered with high fidelity. Furthermore, the stored nuclear spin is sufficiently robust for optical re-initialization cycles of the electron spin, as required for a quantum repeater memory. We also observed multi-qubit interactions between the NV electron spin and nearby nuclear6 and electron7spins up to nanometers away. We are currently in the process of creating long-distance entanglement between pairs of NVs using optical measurements. So far, we have demonstrated narrowband, electrically tunable,8 optical transitions stable enough to allow spontaneous photons, emitted minutes apart, to produce optical interference fringes.9 We have also demonstrated optical Raman spin-flip transitions on one branch of the excited state while simultaneously maintaining an optical cycling transition on the other.10 In summary, we can report surprising progress in realizing scalable quantum information systems with NV color centers in diamond (see Table 1). In the future, we plan to develop large-scale photonic circuits that optically couple numerous NV qubits to construct advanced quantum processors, and highperformance spintronic devices.11