Single-mode Silicon Waveguide Research Articles

Highly efficient fiber to Si waveguide free-form coupler for foundry-scale silicon photonics

As silicon photonics transitions from research to commercial deployment, packaging solutions that efficiently couple light into highly compact and functional sub-micrometer silicon waveguides are imperative but remain challenging. The 220 nm silicon-on-insulator (SOI) platform, poised to enable large-scale integration, is the most widely adopted by foundries, resulting in established fabrication processes and extensive photonic component libraries. The development of a highly efficient, scalable, and broadband coupling scheme for this platform is therefore of paramount importance. Leveraging two-photon polymerization (TPP) and a deterministic free-form micro-optics design methodology based on the Fermat’s principle, this work demonstrates an ultra-efficient and broadband 3-D coupler interface between standard SMF-28 single-mode fibers and silicon waveguides on the 220 nm SOI platform. The coupler achieves a low coupling loss of 0.8 dB for the fundamental TE mode, along with 1 dB bandwidth exceeding 180 nm. The broadband operation enables diverse bandwidth-driven applications ranging from communications to spectroscopy. Furthermore, the 3-D free-form coupler also enables large tolerance to fiber misalignments and manufacturing variability, thereby relaxing packaging requirements toward cost reduction capitalizing on standard electronic packaging process flows.

Ultra-low loss SOI waveguide crossings designed by a hybrid global optimization based on deep learning

We demonstrate an ultra-low loss single-mode silicon waveguide crossing and an ultra-compact and ultra-low loss polarization-insensitive silicon waveguide crossing using a hybrid global optimizer based on a physics-informed generative adversarial network with a direct binary search refinement. By adopting this method, a parallel waveguide crossing structure with a footprint of 3 × 4 μm2 for transverse electric(TE) mode which has an insertion loss better than 0.03 dB and has crosstalk lower than −27 dB in 1500∼1600 nm bands is designed. A polarization-insensitive waveguide crossing with the same footprint is also designed in 1530∼1570 nm bands, which can provide insertion loss lower than 0.2 dB and crosstalk lower than −26.3 dB for TE mode and insertion loss lower than 0.1 dB and crosstalk lower than −27.8 dB transverse magnetic(TM) mode. The results show that this global optimizer produced high-efficiency devices while avoiding being trapped in the local optimal solutions compared to the traditional stochastic inverse design algorithm.

Highly Efficient Broadband Adiabatic Mode Transformation Between Single-Mode Fibers and Silicon Waveguides

Highly Efficient Broadband Adiabatic Mode Transformation Between Single-Mode Fibers and Silicon Waveguides

Temperature-insensitive and low-loss single-mode silicon waveguide crossing covering all optical communication bands enabled by curved anisotropic metamaterial

Abstract We propose two designs of low-loss and temperature-insensitive single-mode waveguide crossing on silicon-on-insulator (SOI) platform with 415-nm operation bandwidth covering all optical communication bands. Both designs are enabled by subwavelength grating (SWG) modeled as an anisotropic metamaterial. The initial design applies straight SWG as the lateral cladding of the waveguide crossing to minimize the refractive index contrast and reduce the insertion loss (IL), but needs a relatively long taper. An improved design is then proposed where the curved SWG is introduced to replace the straight SWG to decrease the taper length and improve the performance. The waveguide crossing with the improved design achieves a calculated maximum IL of 0.229 dB and maximum crosstalk of −35.6 dB over a 415-nm wavelength range from 1260 nm to 1675 nm. The proposed devices are fabricated and characterized. Measured results of the improved design show a maximum IL of 0.264 dB and maximum crosstalk of −30.9 dB over a 230-nm wavelength range including O-, C-, and L-bands, which accord well with the simulation. Low temperature sensitivity has also been demonstrated in both simulations and experiments.

Monolithic integration of 1.3 μm asymmetric lasers grown on silicon and silicon waveguides with tapered coupling

We proposed a design scheme to enable the monolithic integration between a silicon waveguide and a 1.3 μm wavelength band III–V quantum dot laser, which is epitaxially grown on silicon with an asymmetric structure. The III–V laser is grown in a deep trench of a silicon-on-insulator wafer by the selective area epitaxy technique, and a GaAs coupling layer is inserted into the lower cladding layer of the laser, which can make the optical field distribution of the laser shift down. Besides, a mode-size converter with a three-segment tapered structure is designed to couple the output laser into the standard single-mode silicon waveguide. For the laser, the composition and the thickness of AlGaAs cladding layers and AlGaAs transition layer are optimized based on the optical waveguide theory. When the upper cladding layer is 0.6 μm Al0.7Ga0.3As, the lower cladding layer is 1.2 μm Al0.25Ga0.75As, and the transition layer is 20 nm Al0.45Ga0.55As, the optical confinement factors of the active region and the coupling layer are 45.34% and 40.69%, respectively. Then the length of the mode-size converter with a three-segment tapered structure is further optimized by the mode-matching method. When the lengths of the three tapered structures of the mode-size converter are 50 μm, 53 μm and 10 μm respectively, a coupling efficiency of 65% can be obtained between the laser and the Si waveguide. This scheme is expected to realize the efficient optical coupling between the silicon integrated light source and the silicon waveguide, which will promote the development of silicon monolithic photonic integration.

Ge2Sb2Se4Te1-assisted non-volatile silicon mode selective switch

A silicon waveguide mode selective switch based on optical phase-change material Ge2Sb2Se4Te1(GSST) is theoretically demonstrated. The mode switch formed by three cascaded asymmetric directional couplers (ADCs) allows the input TE11 mode to be selectively converted to TE21, TE31 or TE41 mode based on the state of GSST. Each ADC consists of the single-mode silicon waveguide and the GSST film covered multimode silicon waveguide. The phase change of GSST could adjust the effective index of GSST/Si hybrid waveguide, thereby implementing the mode conversion. The three-dimensional finite-difference time-domain (3D-FDTD) method is adopted for performance optimization. The simulation results show that the mode conversion efficiencies of TE11 to TE21, TE31 and TE41 mode are 89.96%, 92.87% and 90.94%, respectively. The proposed mode selective switch has good potentials in on-chip signal multiplexing.

Integrated Janus dipole source for selective coupling to silicon waveguide networks

The efficient selective and directional coupling of light to waveguiding circuitry at the nanoscale is one of the key challenges in nanophotonics, as it constitutes a prerequisite for many applications, including information processing, routing, and quantum technologies. Various exotic nanostructures and nanoparticle arrangements have been designed to achieve directional coupling with compact on-chip integration remaining one of the foremost hurdles to realizing many real-world devices. At the same time, selective coupling to one of several neighboring waveguides is much more difficult to achieve and control. To address this challenge, we demonstrate a subwavelength selective coupler integrated in a waveguide network, with selectivity controlled by wavelength, polarization, and angle of incidence. We utilize a Janus source, which is composed of a superposition of electric and magnetic dipoles, supported by a silicon nanocylinder. By placing the nanocylinder between identical single mode silicon waveguides, we successfully achieve selective coupling with a high contrast ratio between the waveguides. The operating wavelength of the Janus dipolar source can be easily tailored, and the coupling efficiency is also shown to be conveniently boosted by the addition of multiple nanocylinders. Our compact approach provides a direct path toward on-chip highly directional nanoscale sources for a plethora of applications, including information routing, metrology, and quantum technologies.

Broadband and high extinction ratio hybrid plasmonic waveguide-based TE-pass polarizer using multimode interference

A TE-pass polarizer based on multiple hybrid plasmonic waveguides (HPWs) is proposed, where the polarizer section consists of four symmetrically placed HPWs and a single-mode silicon waveguide in between. Owing to the strong polarization-dependent absorption loss of the multiple HPWs, the unwanted TM polarized light is strongly attenuated while TE polarized light passes through with little loss. In addition, the multimode interference (MMI) effect is further utilized to strip out TM mode, resulting in more compact device size. To improve the extinction ratio (ER) and operating bandwidth (OBW) of the device, a three-stage cascaded scheme is utilized. Results show that a compact device with a length of ∼ 26 µ m , an ultra-high ER of over 60 dB, and a low insertion loss (IL) of 0.15 dB is obtained at the wavelength of 1.55 µm, with a broad OBW of over 240 nm for an E R > --> 40 d B . Compared with the recently reported silicon TE-pass polarizers, the present device shows better performance in terms of ER and bandwidth with a comparable device footprint.

Automated control algorithms for silicon photonic polarization receiver.

We demonstrate greedy linear descent-based, basic gradient descent-based, two-point step size gradient descent-based, and two-stage optimization method-based automated control algorithms and examine their performance for use with a silicon photonic polarization receiver. With an active feedback loop control process, time-varying arbitrary polarization states from an optical fiber can be automatically adapted and stabilized to the transverse-electric (TE) mode of a single-mode silicon waveguide. Using the proposed control algorithms, we successfully realize automated adaptations for a 10 Gb/s on-off keying signal in the polarization receiver. Based on the large-signal measurement results, the control algorithms are examined and compared with regard to the iteration number and the output response. In addition, we implemented a long-duration experiment to track, adapt, and stabilize arbitrary input polarization states using the two-point step size gradient descent-based and two-stage optimization method-based control algorithms. The experimental results show that these control algorithms enable the polarization receiver to achieve real-time and continuous polarization management.

On-Chip Circular Polarization Splitter Using Silicon Photonic Nanoantenna Array

Control and sorting of quantum states of photons through the manipulation of polarization and spatial modes of light in integrated photonic circuits contributes important applications in optical communications and quantum-optical systems. We design and demonstrate a novel structure for a silicon nanoantenna array that can split the circular polarization states and couple them to separate single-mode silicon waveguides. Implemented using a CMOS-compatible photonic fabrication process, the array can be monolithically integrated with other photonic components for chip-scale optical signal processing. We also show that the polarization sorting property of the nanoantenna array can be flexibly controlled (by adjusting design parameters at subwavelength scale) to split any two arbitrary orthogonal polarization states.

Optical Versus Electronic Implementation of Probabilistic Graphical Inference and Experimental Device Demonstration Using Nonlinear Photonics

The probabilistic inference model has been widely used in various areas, such as error-control coding, machine learning, speech recognition, artificial intelligence, and statistics. In this paper, we study both computation and communications power consumption of optical-based and electronic-based implementations of the probabilistic inference algorithm used in solving large scale problems. Our analysis indicates that the optical implementation provides substantial reduction for power and area compare to the electronic-based solutions as problems become large. For a network with 1 million nodes and 100 alphabet size, our proposed wavelength multiplexed all-optical implementation requires approximately 200 kilowatts (kW) of power as compared with 1.47 gigawatts (GW) and 1.7 megawatts (MW) using CPU-based and subthreshold VLSI-based systems, respectively. The optical-based solution is tolerant to shot noise and imperfections of optical modules used in the architecture as well. We also performed an all-optical experimental verification of a graphical inference as the proof of concept and have demonstrated the essential mathematical operations, multiplication, and normalization (division), in photonics operations using nonlinear bulk materials. The normalization and multiplication are shown optically through a pump-probe saturation process and a logarithm-summation-exponential (log-sum-exp) operation, respectively. We used single mode silicon waveguide and single-wall carbon nanotube (SWCNT) as nonlinear optical materials to implement logarithm and exponential operations, respectively. The SWCNT is also used as the nonlinear component in the pump-probe saturation experiment to implement the normalization function.

High accuracy transfer printing of single-mode membrane silicon photonic devices.

A transfer printing (TP) method is presented for the micro-assembly of integrated photonic devices from suspended membrane components. Ultra thin membranes with thickness of 150nm are directly printed without the use of mechanical support and adhesion layers. By using a correlation alignment scheme vertical integration of single-mode silicon waveguides is achieved with an average placement accuracy of 100±70nm. Silicon (Si) μ-ring resonators are also fabricated and show controllable optical coupling by varying the lateral absolute position to an underlying Si bus waveguide.

Heterogeneously integrated III–V-on-silicon 2.3x μm distributed feedback lasers based on a type-II active region

We report on 2.3x μm wavelength InP-based type-II distributed feedback (DFB) lasers heterogeneously integrated on a silicon photonics integrated circuit. In the devices, a III–V epitaxial layer stack with a “W”-shaped InGaAs/GaAsSb multi-quantum-well active region is adhesively bonded to the first-order silicon DFB gratings. Single mode laser emission coupled to a single mode silicon waveguide with a side mode suppression ratio of 40 dB is obtained. In continuous-wave regime, the 2.32 μm laser operates close to room temperature (above 15 °C) and emits more than 1 mW output power with a threshold current density of 1.8 kA/cm2 at 5 °C. A tunable diode laser absorption measurement of CO is demonstrated using this source.

Hybrid integration of III-V semiconductor lasers on silicon waveguides using optofluidic microbubble manipulation.

Optofluidic manipulation mechanisms have been successfully applied to micro/nano-scale assembly and handling applications in biophysics, electronics, and photonics. Here, we extend the laser-based optofluidic microbubble manipulation technique to achieve hybrid integration of compound semiconductor microdisk lasers on the silicon photonic circuit platform. The microscale compound semiconductor block trapped on the microbubble surface can be precisely assembled on a desired position using photothermocapillary convective flows induced by focused laser beam illumination. Strong light absorption within the micro-scale compound semiconductor object allows real-time and on-demand microbubble generation. After the assembly process, we verify that electromagnetic radiation from the optically-pumped InGaAsP microdisk laser can be efficiently coupled to the single-mode silicon waveguide through vertical evanescent coupling. Our simple and accurate microbubble-based manipulation technique may provide a new pathway for realizing high precision fluidic assembly schemes for heterogeneously integrated photonic/electronic platforms as well as microelectromechanical systems.

Scattering of a plasmonic nanoantenna embedded in a silicon waveguide.

Plasmonic antennas integrated on silicon devices have large and yet unexplored potential for controlling and routing light signals. Here, we present theoretical calculations of a hybrid silicon-metallic system in which a single gold nanoantenna embedded in a single-mode silicon waveguide acts as a resonance-driven filter. As a consequence of scattering and interference, when the resonance condition of the antenna is met, the transmission drops by 85% in the resonant frequency band. Firstly, we study analytically the interaction between the propagating mode and the antenna by including radiative corrections to the scattering process and the polarization of the waveguide walls. Secondly, we find the configuration of maximum interaction and numerically simulate a realistic nanoantenna in a silicon waveguide. The numerical calculations show a large suppression of transmission and three times more scattering than absorption, consequent with the analytical model. The system we propose can be easily fabricated by standard silicon and plasmonic lithographic methods, making it promising as real component in future optoelectronic circuits.

Optically Monitored Electrical Switching in VO2

We demonstrate a hybrid silicon–vanadium dioxide (Si-VO2) electro-optic modulator that enables direct probing of both the electrically triggered semiconductor-to-metal phase transition in VO2 and the reverse transition from metal to semiconductor. By using a two-terminal in-plane VO2 electrical switch atop a single-mode silicon waveguide, the phase change can be initiated electrically and probed optically, separating the excitation and measurement processes and simplifying the analysis of the metal-to-semiconductor dynamics. We demonstrate a record switch-on time for high-speed electrical semiconductor-to-metal transition, with switching times less than 2 ns, and quantify the slower inverse transition, which is dominated by thermal dissipation and relaxation of the metallic rutile lattice to the monoclinic semiconducting phase. By limiting the current through the VO2 to reduce Joule heating, we enable inverse relaxation times as fast as 3 ns.

Fabrication of an electro-optical Bragg modulator based on plasma dispersion effect in silicon

In this paper we present a design as well as the fabrication procedure of a new type of electro-optical modulator based on plasma dispersion effect. The device consists from a regular single mode silicon waveguide and periodically patterned electrodes made out an indium thin oxide (ITO) located along both sides of the optical core through which the photonic signal is propagating. By applying voltage on the electrodes a periodic change in the free electrons is occurred along the optical core. This change affects both the imaginary and real part of the refractive index resulting in the generation of periodic change in the refractive index that creates an effective Bragg reflector. Different fabrication strategies are reported.

Direction-dependent optical modes in nanoscale Silicon waveguides

We show that in high-index-contrast nanoscale waveguides counter propagating waves can posses distinct spatial near-field profiles. Using transmission-based near-field scanning optical microscopy (TraNSOM), we identify and map the unique near-field intensity distributions of these counter-propagating modes in a single-mode silicon waveguide. Based on this phenomenon, we design and simulate an integrated device 45 µm in length that selectively attenuates reflected light with an insertion loss of -3.6 dB and an extinction of greater than -20 dB.

Waveguide dispersion effects in silicon-on-insulator coupled-resonator optical waveguides

The dispersion of the waveguides that constitute microring resonators can considerably affect the dispersion characteristics of coupled-resonator optical waveguides (CROWs). We derive expressions for CROW dispersion and group delay for silicon-on-insulator microring CROWs, showing both theoretically and experimentally the band-to-band dependence of the bandwidth and group delay on the dispersion properties of the constituent single-mode silicon waveguide.

Phase-Coupled Plasmon-Induced Transparency

We demonstrate the existence of electromagnetically-induced-transparency (EIT-)like spectral response in a system of nanoscale plasmonic resonator antennas coupled by means of a single-mode silicon waveguide. Our proposed scheme exploits the phase of the coupling between the antennas in contrast with the existing plasmonic approaches that rely on the strength of direct, near-field coupling of nanometallic elements. Quality factors of over 100 and group indices of over 10 are readily achieved at near-infrared frequencies by a single unit in ≈1 μm2 of total device footprint, representing a more than two orders size reduction over corresponding dielectric EIT structures. By obviating the need for a near-field interaction, the phase-coupling scheme also facilitates an improved access to the coupling medium between the resonators thereby paving the way toward dynamic control of their sharp EIT-like spectral response.