Silicon Photonic Systems Research Articles

Subwavelength adiabatic multimode Y-junctions.

Adiabatic multimode Y-junctions are extensively used for the design of multimode silicon photonics devices due to their straightforward principle in achieving mode splitting and manipulation. However, experimentally, the limited feature size achievable by lithography can greatly deteriorate the adiabatic branching property and result in large excess loss and crosstalk. Here, we propose a branch structure consisting of a circular-hole-based chirped subwavelength slot. By optimizing the holes' radii and positions under our fabrication constraint, we designed and experimentally demonstrated both symmetric and asymmetric approximately adiabatic four-mode Y-junctions with excellent performance close to ideal Y-junctions over a broad wavelength span. Such junctions are compact, CMOS-compatible, exhibit a relatively large fabrication tolerance, and could be extended for more modes; hence they can be potentially deployed for mode-division multiplexing silicon-photonic systems.

Monolithic integration of broadband optical isolators for polarization-diverse silicon photonics

Integrated optical isolators have been a longstanding challenge for photonic integrated circuits (PIC). An ideal integrated optical isolator for PIC should be made by a monolithic process, have a small footprint, exhibit broadband and polarization-diverse operation, and be compatible with multiple materials platforms. Despite significant progress, the optical isolators reported so far do not meet all these requirements. In this article we present monolithically integrated broadband magneto-optical isolators on silicon and silicon nitride (SiN) platforms operating for both TE and TM modes with record high performances, fulfilling all the essential characteristics for PIC applications. In particular, we demonstrate fully-TE broadband isolators by depositing high quality magneto-optical garnet thin films on the sidewalls of Si and SiN waveguides, a critical result for applications in TE-polarized on-chip lasers and amplifiers. This work demonstrates monolithic integration of high performance optical isolators on chip for polarization-diverse silicon photonic systems, enabling new pathways to impart nonreciprocal photonic functionality to a variety of integrated photonic devices.

Photoconductive heaters enable control of large-scale silicon photonic ring resonator circuits

A multitude of large-scale silicon photonic systems based on ring resonators have been envisioned for applications ranging from biomedical sensing to quantum computing and machine learning. Yet, due to the lack of a scalable solution for controlling ring resonators, practical demonstrations have been limited to systems with only a few rings. Here, we demonstrate that large systems can be controlled by using only doped waveguide elements inside their ringswhile preserving their area and cost. We measure the large photoconductive changes of the waveguides for monitoring the rings' resonance conditions across high-dynamic ranges and leverage their thermo-optic effects for tuning. This allows us to control ring resonators without requiring additional components, complex tuning algorithms, or additional electrical I/Os. We demonstrate automatic resonance alignment of 31 rings of a 16 x 16 switch and of a 14-ring coupled resonator optical waveguide, making them the largest, yet most compact, automatically controlled silicon ring resonator circuits to date, to the best of our knowledge.

Interband cascade laser on silicon

Mid-infrared (MIR) silicon photonic systems show great promise for miniaturizing a variety of sensing and detection technologies. Rapid progress has been made in recent years, and numerous passive and active MIR devices have now been constructed on various silicon-based platforms. We previously reported the heterogeneous integration on silicon of Fabry–Perot and distributed feedback quantum cascade lasers (QCLs) operating at 4.8 μm. Interband cascade lasers (ICLs) will be preferred for many on-chip sensing technologies because they operate in the 3–6 μm range with threshold drive powers 1–2 orders of magnitude lower than QCLs. In this work, we demonstrate the integration of ICLs on a silicon substrate. These lasers emit 3.6 μm light into silicon-on-insulator waveguides in pulsed mode at temperatures up to 50°C. This represents an important step toward MIR photonic integrated circuits on silicon that operate with much lower drive power and therefore an even smaller footprint.

Optomechanical antennas for on-chip beam-steering.

Rapid and low-power control over the direction of a radiating light field is a major challenge in photonics and a key enabling technology for emerging sensors and free-space communication links. Current approaches based on bulky motorized components are limited by their high cost and power consumption, while on-chip optical phased arrays face challenges in scaling and programmability. Here, we propose a solid-state approach to beam-steering using optomechanical antennas. We combine recent progress in simultaneous control of optical and mechanical waves with remarkable advances in on-chip optical phased arrays to enable low-power and full two-dimensional beam-steering of monochromatic light. We present a design of a silicon photonic system made of photonic-phononic waveguides that achieves 44° field of view with 880 resolvable spots by sweeping the mechanical wavelength with about a milliwatt of mechanical power. Using mechanical waves as nonreciprocal, active gratings allows us to quickly reconfigure the beam direction, beam shape, and the number of beams. It also enables us to distinguish between light that we send and receive.

The Effect of Temperature to the Formation of Optically Active Point-defect Complex, the Carbon G-centre in Pre-amorphised and Non-amorphised Silicon

The effect of the temperature ranging from cryogenics to room temperature were investigated on the formation of the optically-active point defect called the G-centre. The G- centre as an emissive point defect gained a lot of attention recently due to its sharp zero phonon luminescence peak at a wavelength of 1.28 μm (0.97 eV) with the evidence of lasing occurred in the structure. The emission of the G-centre is attributed to the carbon substitutional-carbon interstitial (CsCi) complex which interacts with silicon interstitials during the damage event. This complex is generated by implantation of carbon and followed by proton irradiation. Prior to the carbon implantation, two of the samples were pre-amorphised by germanium. Photoluminescence (PL) measurements were carried out at temperature ranging from 80 K up to room temperature to observe the intensity of the main peaks. The results confirm that the main peaks of point-defect centre in all of the samples including the G-centre suffer from the temperature quenching. However, the peak intensity for some of the wavelength especially the ones with high FWHM, do perform better at high temperature. The temperature quenching phenomena observed in the point-defect technique is the main problem that needs to be addressed and solved before realizing the method in the all silicon photonic system.

Bit-error ratio calculation for single-channel silicon optical interconnects utilizing return-to-zero pulsed signals

We introduce an accurate and efficient approach to compute the transmission bit-error ratio (BER) of single-channel silicon photonic interconnects when the optical signal consists of arbitrarily shaped optical pulses. The silicon photonic systems investigated in this study contain an optical transmitter, a silicon optical interconnect, which can be either a strip single-mode silicon photonic waveguide (Si-PhW) or a silicon photonic crystal (PhC) waveguide (Si-PhCW), and a direct-detection receiver. The input signal consists of a superposition of a train of Gaussian pulses and white noise, its propagation in the silicon waveguide being described by a rigorous theoretical model based on a modified nonlinear Schrodinger equation, which captures all relevant linear and nonlinear optical effects. The statistical properties of the output signal and the corresponding BER were determined using the Fourier series Karhunen–Loeve expansion method. Our analysis reveals that in the case of Si-PhWs the pulse width is the parameter that most strongly influences the BER, whereas for Si-PhCWs the main factor affecting BER is the pulse group velocity. Finally, we show that similar values of BER are achieved in Si-PhCWs with length of about 2 orders of magnitude smaller than that of Si-PhWs, a further reduction of the silicon interconnect footprint being possible if the Si-PhCW is operated in the slow-light regime.

Heterogeneous Integration for Mid-infrared Silicon Photonics

Heterogeneous integration enables the construction of silicon (Si) photonic systems, which are fully integrated with a range of passive and active elements including lasers and detectors. Numerous advancements in recent years have shown that heterogeneous Si platforms can be extended beyond near-infrared telecommunication wavelengths to the mid-infrared (MIR) (2-20 μm) regime. These wavelengths hold potential for an extensive range of sensing applications and the necessary components for fully integrated heterogeneous MIR Si photonic technologies have now been demonstrated. However, due to the broad wavelength range and the diverse assortment of MIR technologies, the optimal platform for each specific application is unclear. Here, we overview Si photonic waveguide platforms and lasers at the MIR, including quantum cascade lasers on Si. We also discuss progress toward building an integrated multispectral source, which can be constructed by wavelength beam combining the outputs from multiple lasers with arrayed waveguide gratings and duplexing adiabatic couplers.

Silicon waveguide optical switch with embedded phase change material.

Phase-change materials (PCMs) have emerged as promising active elements in silicon (Si) photonic systems. In this work, we design, fabricate, and characterize a hybrid Si-PCM optical switch. By integrating vanadium dioxide (a PCM) within a Si photonic waveguide, in a non-resonant geometry, we achieve ~10 dB broadband optical contrast with a PCM length of 500 nm using thermal actuation.

Exploiting high-order phase-shift keying modulation and direct-detection in silicon photonic systems.

A computational approach to evaluate the bit-error ratio (BER) in silicon photonic systems employing high-order phase-shift keying (PSK) modulation formats is presented. Specifically, the investigated systems contain a silicon based optical interconnect, namely a strip silicon photonic waveguide or a silicon photonic crystal waveguide, and direct-detection receivers suitable to detect PSK and amplitude-shaped PSK signals. The superposition of a PSK signal and complex additive white Gaussian noise passes through the optical interconnect and subsequently through two detection-branch receivers. To model the signal propagation in the silicon optical interconnects we used a modified nonlinear Schrödinger equation, which incorporates all relevant linear and nonlinear optical effects and the mutual interaction between free-carriers and the optical field. Finally, the BER is calculated by applying a frequency-domain approach based on the Karhunen-Loève series expansion method. Our computational studies of the BER reveal that the optical power, type of PSK modulation, waveguide length, and group-velocity are key factors characterizing the system BER, their influence on BER being more significant in a photonic system with larger nonlinearity. In particular, our analysis shows that the system performance is affected to a much larger extent when the signal propagates in the slow-light regime, despite the fact that this regime allows for a significantly reduced length of optical interconnects.

Mode-evolution-based coupler for high saturation power Ge-on-Si photodetectors.

We propose a mode-evolution-based coupler for high saturation power germanium-on-silicon photodetectors. This coupler uniformly illuminates the intrinsic germanium region of the detector, decreasing saturation effects, such as carrier screening, observed at high input powers. We demonstrate 70% more photocurrent generation (9.1-15.5mA) and more than 40 times higher opto-electrical bandwidth (0.7-31GHz) than conventional butt-coupled detectors under high-power illumination. The high-power and high-speed performance of the device, combined with the compactness of the coupling method, will enable new applications for integrated silicon photonics systems.

Tunable and Ultra-Small Graphene Integrated Silicon Racetrack Micro Resonator

We have demonstrated ultra-small and tunable graphene integrated racetrack silicon photonic resonators by harnessing the unique optoelectronic properties of graphene layer and the racetrack silicon resonators. Previous studies have been focused on the straight waveguide interacting with a graphene layer. To further explore how to effectively increase the interaction length between graphene layer and silicon photonic devices, this paper employs integrated racetrack silicon resonator interacting with graphene layer within the coupling region . These new configurations will reduce the footprints of the graphene integrated silicon photonics to micron size , instead of hundreds of microns or even millimeter size. We have demonstrated relatively large voltage tuning results and large modulation depth when putting the graphene layer within the coupling region of the racetrack microresonator close to the critical coupling condition. Our work offers flexibility and tunability for hybrid graphene silicon photonic systems, as well as future functional hybrid photonic devices and systems. Potential applications include emerging areas such as active silicon photonic devices for high-performance data processing, tele communications, integrated biophotonics, and low-power energy saving optoelectronic devices.

Tapered polysilicon core fibers for nonlinear photonics.

We propose and demonstrate a novel approach to obtaining small-core polysilicon waveguides from the silicon fiber platform. The fibers were fabricated via a conventional drawing tower method and, subsequently, tapered down to achieve silicon core diameters of ∼1 μm, the smallest optical cores for this class of fiber to date. Characterization of the material properties have shown that the taper process helps to improve the local crystallinity of the silicon core, resulting in a significant reduction in the material loss. By exploiting the combination of small cores and low losses, these tapered fibers have enabled the first observation of nonlinear transmission within a polycrystalline silicon waveguide of any type. As the fiber drawing method is highly scalable, it opens a route for the development of low-cost and flexible nonlinear silicon photonic systems.

A review of wireless-photonic systems: Design methodologies and topologies, constraints, challenges, and innovations in electronics and photonics

Photonic networks form the backbone for data communications. In particular, in current and future wireless communication systems, photonic networks are becoming increasingly popular for data distribution between the central office and the remote antenna units at base stations. As wireless-photonic systems become in increasing demand, low-cost implementation of such systems will be desirable. This paper describes how integrated photonics and electronics, on silicon, can be used to design such systems. Various building blocks of such silicon-photonics systems are reviewed. The emphasis is on a 60GHz wireless system which could be suitable for the emerging 5th-generation (5G) cellular networks. The implementation discussed here uses digital baseband optical transmission as opposed to the radio-over-fibre approach.

Metatronic analogues of the Wheatstone bridge

The Wheatstone bridge is a classical electrical circuit developed by Sir Charles Wheatstone in the middle of the 19th century. The operating principle of the Wheatstone bridge is based on the concept of a difference measurement, and it is one of the most popular circuits in the characterization of resistors. Here, we utilize the optical metatronic paradigm—metamaterial-inspired optical nanocircuitry—to extend the use of the Wheatstone bridge to different platforms and frequency regimes. Specifically, using numerical simulations we present three different designs in which the analogue of the Wheatstone bridge concept is implemented on all-optical metamaterial boards, microwave waveguides, and planar silicon photonic systems, operating in frequency bands ranging from microwaves to the visible. The proposed devices enrich the collection of available nanocircuitry, and it is shown that high accuracy and simplicity of the Wheatstone bridge can be transplanted and exploited in multiple nanophotonic applications including, e.g., the characterization of nanoparticles.

A silicon photonic wavelength division multiplex system for high-speed data transmission in detector instrumentation

We propose a new silicon photonics-based optical transmission system utilizing wavelength division multiplexing (WDM) . This technology has the possibility of reading out all raw data from a detector even without massive local data reduction. The transmitter in the detector volume consists of multiple integrated Mach-Zehnder modulators monolithically integrated with wavelength (de-)multiplexers. The first demonstrator currently under development aims for a data rate of 160 Gbit/s per fiber, scalable to 5 Tbit/s and beyond. We report on our recently developed Echelle grating WDM multiplexers with up to 45 channels on an area of 0.5 mm2 and electro-optic modulators providing a bandwidth of 18 GHz.

Lowering the energy consumption in silicon photonic devices and systems [Invited

We review current silicon photonic devices and their performance in connection with energy consumption. Four critical issues are identified to lower energy consumption in devices and systems: reducing the influence of the thermo-optic effect, increasing the wall-plug efficiency of lasers on silicon, optimizing energy performance of modulators, and enhancing the sensitivity of photodetectors. Major conclusions are (1) Mach–Zehnder interferometer-based devices can achieve athermal performance without any extra energy consumption while microrings do not have an efficient passive athermal solution; (2) while direct bonded III–V-based Si lasers can meet system power requirement for now, hetero-epitaxial grown III–V quantum dot lasers are competitive and may be a better option for the future; (3) resonant modulators, especially coupling modulators, are promising for low-energy consumption operation even when the power to stabilize their operation is included; (4) benefiting from high sensitivity and low cost, Ge/Si avalanche photodiode is the most promising photodetector and can be used to effectively reduce the optical link power budget. These analyses and solutions will contribute to further lowering energy consumption to meet aggressive energy demands in future systems.

Heterogeneously integrated 2.0μm CW hybrid silicon lasers at room temperature.

Here we experimentally demonstrate room temperature, continuous-wave (CW), 2.0μm wavelength lasers heterogeneously integrated on silicon. Molecular wafer bonding of InP to Si is employed. These hybrid silicon lasers operate CW up to 35°C and emit up to 4.2mW of single-facet CW power at room temperature. III-V tapers transfer light from a hybrid III-V/silicon optical mode into a Si waveguide mode. These lasers enable the realization of a number of sensing and detection applications in compact silicon photonic systems.

High-Efficiency Biwavelength Polarization Splitter-Rotator on the SOI Platform

We present the design of a highly efficient polarization splitter-rotator that operates at both 1310- and 1550-nm wavelength bands in a 220-nm silicon-on-insulator platform. The proposed polarization splitter-rotator has a simulated insertion loss of −0.9 dB at 1310 nm and −1 dB at 1550 nm. This is the first theoretical demonstration of a biwavelength polarization splitter-rotator. This letter is valuable for the implementation of a novel edge-coupling-based light duplexing silicon photonic system.

Optical and Electronic Packaging Processes for Silicon Photonic Systems

Fiber optic interconnection processes and hybrid integration of electronic devices for high speed Si photonic systems are presented. Thermal effects arising from these hybrid integration processes are also investigated. An overview of ePIXfab which offers affordable access to an advanced Si photonic foundry service is also presented. This includes the presentation of fundamental photonic packaging design rules which can greatly reduce the time and cost associated with the development of complex Si photonic devices.