Silicon Photonics Platform Research Articles

Poincaré Rotator for Vortexed Photons

A Poincaré sphere is a powerful prescription to describe a polarized state of coherent photons, oscillating along certain directions. The polarized state is described by a vector in the sphere, and various passive optical components, such as polarization plates and quartz rotators are able to rotate the vectorial state by changing the phase and the amplitude among two orthogonal basis states. The polarization is originated from spin of photons, and recently, significant attentions have been made for optical Orbital Angular Momentum (OAM) as another fundamental degree of freedom for photons. The beam shape of photons with OAM is a vortex with a topological charge at the core, and the state of vortexed photons can be described by a hyper-Poincaré sphere. Here, we propose a compact Poincaré rotator, which controls a vortexed state of photons in a silicon photonic platform, based on Finite-Difference Time-Domain (FDTD) simulations. A ring-shaped gear is evanescently coupled to two silicon photonic waveguides, which convert optical momentum to OAM with both left and right vortexed states. By controlling the relative phase and the amplitude of two traveling waves in input ports, we can control the vortexed states in the hyper-Poincaré sphere for photons out of the gear. The impact of the geometrical Pancharatnam-Berry-Guoy's phase and the conservation law of spin and OAM for vortexed photons out of the gear are discussed.

Integrated avalanche photodetectors for visible light

Integrated photodetectors are essential components of scalable photonics platforms for quantum and classical applications. However, most efforts in the development of such devices to date have been focused on infrared telecommunications wavelengths. Here, we report the first monolithically integrated avalanche photodetector (APD) for visible light. Our devices are based on a doped silicon rib waveguide with a novel end-fire input coupling to a silicon nitride waveguide. We demonstrate a high gain-bandwidth product of 234 ± 25 GHz at 20 V reverse bias measured for 685 nm input light, with a low dark current of 0.12 μA. We also observe open eye diagrams at up to 56 Gbps. This performance is very competitive when benchmarked against other integrated APDs operating in the infrared range. With CMOS-compatible fabrication and integrability with silicon photonic platforms, our devices are attractive for sensing, imaging, communications, and quantum applications at visible wavelengths.

Selective area heteroepitaxy of low dislocation density antiphase boundary free GaAs microridges on flat-bottom (001) Si for integrated silicon photonics

Integrating III–V gain elements in the silicon photonics platform via selective area heteroepitaxy (SAH) would enable large-scale and low-cost photonic integrated circuits. Here, we demonstrate antiphase boundary (APB)-free gallium arsenide (GaAs) microridges selectively grown on flat-bottom (001) silicon (Si) inside a recess. This approach eliminates the need for etching the patterned Si to form trapezoid or v-groove shapes, often leveraged for eliminating APBs. A low surface dislocation density of 8.5 × 106 cm−2 was achieved for 15-μm-wide GaAs microridges, quantified by electron channeling contrast imaging. The avoidance of APBs is primarily due to their self-annihilation, influenced by the sufficiently low temperature GaAs nucleation and subsequent higher temperature buffer overgrowth. Dislocation filtering approaches, namely, thermal cycle annealing and strained-layer superlattices, have been applied to effectively reduce the dislocation density. SAH of GaAs on trapezoidal-shaped Si pockets is also reported to illustrate the differing growth conditions for GaAs on (001) and (111) Si microplanes.

D-Shaped Silicon-Cored Fibers as Platform to Build In-Line Schottky Photodetectors

We demonstrated a novel silicon photonic platform constituting a D-shaped silicon cored fiber (SCF) for Schottky photodetector (PD). By using a handheld polishing machine, D-shaped SCFs can be fabricated in a fast and simple way. D-shaped SCFs offer stable platforms for traditional semiconductor processing and we show that Schottky PDs could be easily made on the fibers. Experiment results confirm an improved responsivity of 0.577 mA/W and the modulation bandwidth could reach 170 MHz. The D-shaped SCFs may open a new approach of making all-in-fiber silicon photonic devices.

Tellurite glass microcavity resonators integrated on a silicon photonics platform

We report on the design and measurement of tellurium oxide microcavity resonators coupled to silicon bus waveguides on silicon photonic chips. The resonators are fabricated using a standard silicon photonics foundry processing flow in which the SiO2 top-cladding is etched in a ring shape and aligned next to a silicon bus waveguide. The resulting microtrench is coated in a tellurium oxide film by reactive sputtering in a post-processing step to form the waveguiding layer of the resonator. A 100-μm radius trench with a 1115-nm-thick TeO2 film is measured to have an internal Q factor of 0.9 × 105. Smoothing the etch wall surface with a fluoropolymer coating is shown to enhance the Q factor of several devices, with a trench coated in a 630-nm-thick TeO2 film demonstrating a Q factor of 2.1 × 105 corresponding to 1.7-dB/cm waveguide loss. These results demonstrate a potential pathway toward monolithic integration of tellurite glass-based nonlinear and rare-earth-doped devices compatible with silicon photonics platforms.

Engineering the light coupling between metalens and photonic crystal resonators for robust on-chip microsystems

We designed an on-chip transformative optic system with a broadband metalens coupler on a foundry compatible silicon photonic platform. By adjusting the on-chip metalens’ focusing length and mode dimension, the insertion loss between the metalens and the photonic crystal waveguide (PhC WG) structures is reduced to 2 dB by matching the mode on the metalens focal plane to the PhC WG mode. Alternatively, the integrated metalens allow for direct coupling from a multi-mode WG to the PhC cavity. The on-resonance transmission in a lens–cavity–lens microsystem achieves 60%. These micro-systems do not involve any single-mode silicon nanowire WG, and even a suspended PhC structure can be mechanically robust against vibrations. The proposed microsystem can be a new platform for miniaturized chemical and biosensor applications operating in air or solution environments.

Integrated Silicon Fourier Transform Spectrometer with Broad Bandwidth and Ultra‐High Resolution

AbstractAn ultra‐high resolution Fourier transform spectrometer (FTS) realized in silicon photonic platform that can operate with broad band, narrow band as well as a combination of broad band and narrow band signals is reported. The ultra‐high resolution of the spectrometer is achieved by exploiting multiple techniques: a Michelson interferometer (MI) structure to increase the optical path delay (OPD), a hybrid waveguide design to reduce insertion loss, an optimized heater and air trenches to achieve higher thermal efficiency. Moreover, to further increase the OPD of the spectrometer to increase its resolution, a novel multiple interferometers approach is employed which combines balanced MI with N statically imbalanced MIs, thereby increasing the OPD of a single MI by factor of N + 1. An FTS spectrometer consisting of N = 2 such MIs is fabricated and experimentally characterized using unknown broad bandwidth input signal spectra of about 180 nm centered around 1550 nm, a narrow line laser input signal, and a combination of broad and narrow band signals demonstrating spectral resolution of about 0.16 nm.

Towards Maximum Energy Efficiency of Carrier-Injection-Based Silicon Photonics

We present carrier-injection-based photonic switches, engineered for optical pulse distribution with maximum energy efficiency. We apply small-signal analysis and for the first time large-signal modelling to methodically optimize the switches for minimum energy consumption and to classify the electronic contributions from resistance, capacitance, and diode. We present optimized electronic switch activation, which yields a sixfold reduction in energy consumption and we show how static power consumption becomes a negligible factor for optical pulse switching. We demonstrate that with adjusted phase shifter dimensions, MZI-based switches can operate with additional 50% enhanced energy efficiency with down to 4 pJ per switching operation. We show even further efficiency improvement using ring-based designs, allowing an additional improvement of 50% in energy efficiency and we discuss the trade-off between efficiency and optical bandwidth associated to the Q-factor. We benchmark carrier-injection-based switches together with comparable technologies of the silicon photonics platform and identify carrier-injection to be the most suitable technology for pulse switching applications.

A universal 3D imaging sensor on a silicon photonics platform.

Accurate three-dimensional (3D) imaging is essential for machines to map and interact with the physical world1,2. Although numerous 3D imaging technologies exist, each addressing niche applications with varying degrees of success, none has achieved the breadth of applicability and impact that digital image sensors have in the two-dimensional imaging world3-10. A large-scale two-dimensional array of coherent detector pixels operating as a light detection and ranging system could serve as a universal 3D imaging platform. Such a system would offer high depth accuracy and immunity to interference from sunlight, as well as the ability to measure the velocity of moving objects directly11. Owing to difficulties in providing electrical and photonic connections to every pixel, previous systems have been restricted to fewer than 20 pixels12-15. Here we demonstrate the operation of a large-scale coherent detector array, consisting of 512 pixels, in a 3D imaging system. Leveraging recent advances in the monolithic integration of photonic and electronic circuits, a dense array of optical heterodyne detectors is combined with an integrated electronic readout architecture, enabling straightforward scaling to arbitrarily large arrays. Two-axis solid-state beam steering eliminates any trade-off between field of view and range. Operating at the quantum noise limit16,17, our system achieves an accuracy of 3.1millimetres at a distance of 75metres when using only 4milliwatts of light, an order of magnitude more accurate than existing solid-state systems at such ranges. Future reductions of pixel size using state-of-the-art components could yield resolutions in excess of 20megapixels for arrays the size of a consumer camera sensor. This result paves the way for the development and proliferation of low-cost, compact and high-performance 3D imaging cameras that could be used in applications from robotics and autonomous navigation to augmented reality and healthcare.

All-Optical Hybrid VO$_{2}$/Si Waveguide Absorption Switch at Telecommunication Wavelengths

Vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is one of the most promising materials for developing hybrid photonic integrated circuits (PICs). At telecommunication wavelengths, VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> exhibits a large change on the refractive index (Δn ~ 1 and Δκ ~ 2.5) between its insulating and metallic state. Such insulating-to-metal transition (IMT) can be triggered by light, which could enable all-optical hybrid VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -waveguide devices. Here, we experimentally demonstrate an all-optical absorption switch using a hybrid VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /Si waveguide fully compatible with the silicon photonics platform. All-optical characterization was carried out for TE polarization and at telecommunication wavelengths using an in-plane approach. The temporal dynamics were retrieved by means of pump-probe measurements. Our results show an extinction ratio of 0.7 dB/μm with a maximum switchable length of 15 μm, a switching speed as low as 318 ns, and an energy per switch of 15.8 nJ. The inherit large optical bandwith of a non-resonant waveguide poses this device as a promising candidate for developing all-optical and broadband silicon PICs.

Heterogeneously integrated ITO plasmonic Mach\u2013Zehnder interferometric modulator on SOI

Densely integrated active photonics is key for next generation on-chip networks for addressing both footprint and energy budget concerns. However, the weak light-matter interaction in traditional active Silicon optoelectronics mandates rather sizable device lengths. The ideal active material choice should avail high index modulation while being easily integrated into Silicon photonics platforms. Indium tin oxide (ITO) offers such functionalities and has shown promising modulation capacity recently. Interestingly, the nanometer-thin unity-strong index modulation of ITO synergistically combines the high group-index in hybrid plasmonic with nanoscale optical modes. Following this design paradigm, here, we demonstrate a spectrally broadband, GHz-fast Mach–Zehnder interferometric modulator, exhibiting a high efficiency signified by a miniscule VπL of 95 V μm, deploying a one-micrometer compact electrostatically tunable plasmonic phase-shifter, based on heterogeneously integrated ITO thin films into silicon photonics. Furthermore we show, that this device paradigm enables spectrally broadband operation across the entire telecommunication near infrared C-band. Such sub-wavelength short efficient and fast modulators monolithically integrated into Silicon platform open up new possibilities for high-density photonic circuitry, which is critical for high interconnect density of photonic neural networks or applications in GHz-fast optical phased-arrays, for example.

A Gradient-Oriented Binary Search Method for Photonic Device Design

Inverse design method is widely used in photonic device design. To accelerate the speed to find the possible solutions in design space, optimization method with high efficiency to exhaust the parameter space is pivotal. In this paper, we propose a gradient-oriented binary search method for photonic device design. It utilizes the gradient of refractive index change to the figure of merit to guide the flipping process between binary materials. This method can accelerate the optimization convergence speed compared with direct binary search method, while maintain the advantages of no additional binarization procedure and well critical dimension constraints. Based on this design methodology, we design a bimodal interferometer on silicon photonics platform, which can generate interference pattern of TE1 and TE2 mode using fundamental mode as input. The device layout is 4 μm × 8 μm. To verify the universality of the method, we design a TE1-TE0 mode converter and 3-dB optical power splitter. Results also verifies that the optimize speed of this method is much faster than direct binary search method. And the convergence iteration steps would not be significantly increased with the increase of unit cells. Our approach provides a promising supplement to the direct binary method, and can be implemented in a variety of device design.

Passive devices at 2 µm wavelength on 200 mm CMOS-compatible silicon photonics platform [Invited

As a promising spectral window for optical communication and sensing, it is of great significance to realize on-chip devices at the 2 µm waveband. The development of the 2 µm silicon photonic platform mainly depends on the performance of passive devices. In this work, the passive devices were fabricated in the silicon photonic multi-project wafer process. The designed micro-ring resonator with a 0.6 µm wide silicon ridge waveguide based on a 220 nm silicon-on-insulator platform achieves a high intrinsic quality factor of 3.0×105. The propagation loss is calculated as 1.62 dB/cm. In addition, the waveguide crossing, multimode interferometer, and Mach–Zehnder interferometer were demonstrated at 2 µm with good performances.

Electro-Optic Frequency Response Shaping using Embedded FIR Filters in Slow-Wave Modulators

A novel method is presented to embed finite-impulse-response filters in slow-wave Mach–Zehnder modulators. This allows to adjust the electro-optic frequency response to the designer's needs. The filter is embedded by adding optical delay lines and optical crossings between phase shifter segments. The position of the delay lines and crossing in the modulator and the delay line length determine the final response. In this work, we provide a full analysis and modeling approach of the proposed technique and apply it to a silicon photonic modulator. However, the technique is generally applicable to slow-wave modulators and thus not limited to a silicon photonics platform. The modeling is verified using measurements on the manufactured devices. A shaped modulator is used in C-band transmission experiments with 56 Gb/s NRZ data over 3 km fiber to counteract chromatic dispersion and show the advantage over a standard modulator.

Long cavity photonic crystal laser in FDML operation using an akinetic reflective filter.

A novel configuration of a Fourier domain mode locked (FDML) laser based on silicon photonics platform is presented in this work that exploits the narrowband reflection spectrum of a photonic crystal (PhC) cavity resonator. Configured as a linear Fabry-Perot laser, forward biasing of a p-n junction on the PhC cavity allowed for thermal tuning of the spectrum. The modulation frequency applied to the reflector equalled the inverse roundtrip time of the long cavity resulting in stable FDML operation over the swept wavelength range. An interferometric phase measurement measured the sweeping instantaneous frequency of the laser. The silicon photonics platform has potential for very compact implementation, and the electro-optic modulation method opens the possibility of modulation speeds far beyond those of mechanical filters.

Nonlinear silicon photonics on CMOS-compatible tellurium oxide

Silicon photonics is coming of age; however, it is still lacking a monolithic platform for optical sources and nonlinear functionalities prompting heterogeneous integration of different materials tailored to different applications. Here we demonstrate tellurium oxide as a complementary metal oxide semiconductor silicon photonics platform for nonlinear functionalities, which is already becoming an established platform for sources and amplifiers. We show broadband supercontinuum generation covering the entire telecom window and show for the first time to our knowledge third-harmonic generation in its integrated embodiment. Together with the now-available lasers and amplifiers on integrated TeO 2 this work paves the way for a monolithic TeO 2 -based nonlinear silicon photonics platform.

Photonic Arbitrary Waveform Generation With Three Times the Sampling Rate of the Modulator Bandwidth

A new method for high-speed photonic arbitrary waveform generation using optical sinc-pulse sequences and Mach-Zehnder modulators is presented, enabling a three- to four-fold increase in sampling rate compared to the bandwidth of the used modulator for generating the pulses and an N -fold increase compared to the bandwidth of the incorporated electronics, with N as the number of parallel branches. Proof of concept experimental results, using conventional off-the-shelf fiber components have been presented in this study. Due to its simplicity and the possibility to achieve high sampling rates with low bandwidth photonic and electronic equipment, this method is beneficial for integration on a silicon photonics platform.

Silicon photonics platforms for optical communication systems, outlook on future developments

Silicon photonics platforms for optical communication systems, outlook on future developments

Beam shaping for ultra-compact waveguide crossings on monolithic silicon photonics platform.

A beam shaping approach has been implemented to realize high-performance waveguide crossings based on cosine tapers. Devices with a compact footprint of 4.7µm×4.7µm were fabricated on the GLOBALFOUNDRIES 45 nm monolithic silicon photonics platform (45 CLO technology). Fabricated devices are found to be nearly wavelength independent (±0.035dB for 1260nm≤λ≤1360nm) with low insertion loss (∼0.2dB) and crosstalk (-35dB). The measured response of the devices is consistent with the three-dimensional finite-difference time-domain simulation results. The design stability is validated by measuring the device insertion loss on eight chips, which is found to be 0.197±0.017dB at the designed center wavelength of 1310 nm.

Wideband Rapidly Tunable Delay-Controlled Optoelectronic Oscillator

With the recent advancements in photonic integrated circuits, integrated RF photonics has increasingly been considered as a viable avenue to meet the demands of many emerging applications in telecommunication, sensing and signal processing. In this paper, an integrated, rapidly tunable, wideband optical delay line is used to implement an optoelectronic oscillator that can be continuously tuned from 594 MHz to 634 MHz. The oscillator exhibits a relatively flat modulation response at high frequencies (larger than 400 MHz), accommodating wideband fast frequency chirp generation with applications in radar and imaging. The on-chip variable delay line is characterized by continuously delaying an ultra-wide band 0.5-5 GHz monocycle pulse between 0-100 ps. The photonic integrated chip consists of silicon nanowaveguides, phase modulators, multimode interference coupler, a Mach-Zehnder interferometer, and a photodiode. The chip is fabricated on IME 180 nm Silicon Photonics platform and has an area of 0.55 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .