Silicon Waveguide Research Articles

Transparent conductive oxides and low-loss nitride-rich silicon waveguides as building blocks for neuromorphic photonics

Fully CMOS-compatible photonic memory holding devices hold a potential in the development of ultrafast artificial neural networks. Leveraging the benefits of photonics such as high-bandwidth, low latencies, low-energy interconnect, and high speed, they can overcome the existing limits of electronic processing. To satisfy all these requirements, a photonic platform is proposed that combines low-loss nitride-rich silicon as a guide and low-loss transparent conductive oxides as an active material that can provide high nonlinearity and bistability under both electrical and optical signals.

1-to-N terahertz integrated switches enabling multi-beam antennas

Implementing terahertz circuits and system designs relies on integrating reconfigurable devices such as switches, to fulfill a critical role in controlling and manipulating the flow of terahertz signals on the chip. Although there have been several demonstrations of on–off switching in the terahertz range, there have been no demonstrations of 1-to-N switching, to our knowledge. This pronounced lack of dynamically reconfigurable routing has severely limited the achievable complexity of terahertz systems. To address this, we propose 1-to-N switches made of cascaded disk resonators integrated into a substrateless silicon waveguide platform. A single switch can be controlled via photoexcitation using a low-power 658-nm laser focused onto the disk resonator, turning off the resonance and inhibiting coupling into a crossing port. The measurement results demonstrate that the proposed switch has low insertion loss, which can be attributed to the inherently low dissipation of the platform. The proposed on–off switch achieves a maximum insertion loss of 1.2 dB, and the maximum extinction ratio of the switch is 16.1 dB with 1.5 GHz of bandwidth. Furthermore, a 1-to-3 switch is monolithically integrated together with a Luneburg lens in order to project each of its output ports to a different far-field direction and, thereby, translate the switching operation into a form of reconfigurable beam control for future applications.

Cladding modulated silicon waveguide Bragg grating with TM-polarized light for optical true time delay line

An optical true time delay line (OTTDL) is an essential component in optical signal processing. A Bragg grating structure is usually employed in OTTDL design, which makes it possible to achieve a slow-light effect. Here, we present the design and experimental demonstration of a cladding modulated waveguide Bragg grating (CMWBG) that supports transverse magnetic polarization for OTTDL application. In contrast to traditional waveguide Bragg gratings that support transverse electric polarization, the proposed structure has low propagation loss and high fabrication tolerance characteristics. The slow-light effect of the CMWBG was experimentally demonstrated with a high group index value. An averaged maximum group delay value of up to 84.4 ps was achieved at a length of 1 mm CMWBG, and a tuning range of about 81.4 ps was obtained.

Negative capacitors and inductors enabling wideband waveguide metatronics

Waveguide metatronics, known as an advanced platform of metamaterial-inspired circuits, provides a promising paradigm for millimeter-wave and terahertz integrated circuits in future fifth/sixth generation (5/6G) communication systems. By exploiting the structural dispersion properties of waveguides, a lumped type of waveguide integrated elements and circuits could be developed in deep subwavelength scales with intrinsic low loss and low crosstalk. In this study, we focus on constructing negative capacitors and inductors for waveguide metatronics, effectively expanding the operating frequency range of waveguide integrated circuits. The incorporation of negative elements enables wideband impedance matching in waveguide, which have been both theoretically explored and experimentally validated within the waveguide metatronics paradigm. Furthermore, we have demonstrated that the negative elements can also be realized in the optical domain through the utilization of a silicon waveguide with photonic crystal cladding, indicating the feasibility and universality of wideband waveguide metatronics. The negative lumped elements could boost the progress of the waveguide metatronic technique, achieving superior performance on the conventional lumped circuits within waveguides that solely rely on positive elements.

Design and Characterization of Dispersion-Tailored Silicon Strip Waveguides toward Wideband Wavelength Conversion

One of cost-effective ways to increase the transmission capacity of current standard wavelength division multiplexing (WDM) transmission systems is to use a wavelength band other than the C-band to transmit in multi-band. We proposed the concept of multi-band system using wavelength conversion, which can simultaneously process signals over a wide wavelength range. All-optical wavelength conversion could be used to convert C-band WDM signals into other bands in a highly nonlinear fiber (HNLF) by four-wave mixing and allow to simultaneously transmit multiple WDM signals including other than the C-band, with only C-band transceivers. Wavelength conversion has been reported for various nonlinear waveguide materials other than HNLF. In such nonlinear materials, we noticed the possibility of wideband transmission by dispersion-tailored silicon-on-insulator (SOI) waveguides. Based on the CMOS process has high accuracy, it is expected that the chromatic dispersion fluctuation could be reduced in mass production. As a first step in the investigation of the broadness of wavelength conversion using SOI-based waveguides, we designed and fabricated dispersion-tailored 12 strip waveguides provided with an edge coupler at both ends. Each of the 12 waveguides having different widths and lengths and is connected to fibers via lensed fibers or by lenses. In order to characterize each waveguide, the pump-probe experimental setup was constructed using a tunable light source as pump and an unmodulated 96-ch C-band WDM test signal. Using this setup, we evaluate insertion loss, input power dependence, conversion bandwidth and conversion efficiency. We confirmed C-band test signal was converted to the S-band and the L-band using the same silicon waveguide with 3 dB conversion bandwidth over 100-nm. Furthermore, an increased design tolerance of at least 90 nm was confirmed for C-to-S conversion by shortening the waveguide length. It is confirmed that the wavelength converters using the nonlinear waveguide has sufficiently wide conversion bandwidth to enhance the multi-band WDM transmission system.

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

Stimulated intermodal Brillouin scattering in a hybrid photonic-phononic silicon waveguide

On-chip stimulated Brillouin scattering (SBS) has attracted extensive attention by introducing acousto-optic coupling interactions in all-optical signal processing systems. In this article, we demonstrate stimulated intermodal Brillouin scattering (SIMBS) through a hybrid photonic-phononic silicon waveguide on the silicon-on-insulator (SOI) platform. The designed photonic-phononic waveguide can independently control the optical and acoustic fields by introducing the ridge waveguide as a line defect into the honeycomb lattice phononic crystal slab. Small-signal Stokes gain of 0.7 dB is achieved in a 1-cm long straight waveguide device with an on-chip pump power of 42.5 mW. Positive net Brillouin amplification is also expected by effectively reducing the linear loss. Our proposed device offers a potential approach to implementing Brillouin amplifiers, nonreciprocal devices, and signal processing in planar photonic integrated circuits.

Silicon-Organic Hybrid Electro-Optic Modulator and Microwave Photonics Signal Processing Applications.

Electro-optic modulator (EOM) is one of the key devices of high-speed optical fiber communication systems and ultra-wideband microwave photonic systems. Silicon-organic hybrid (SOH) integration platform combines the advantages of silicon photonics and organic materials, providing a high electro-optic effect and compact structure for photonic integrated devices. In this paper, we present an SOH-integrated EOM with comprehensive investigation of EOM structure design, silicon waveguide fabrication with Slot structure, on-chip poling of organic electro-optic material, and characterization of EO modulation response. The SOH-integrated EOM is measured with 3 dB bandwidth of over 50 GHz and half-wave voltage length product of 0.26 V·cm. Furthermore, we demonstrate a microwave photonics phase shifter by using the fabricated SOH-integrated dual parallel Mach-Zehnder modulator. The phase shift range of 410° is completed from 8 GHz to 26 GHz with a power consumption of less than 38 mW.

Hybrid Si-GaAs photonic crystal cavity for lasing and bistability.

The heterogeneous integration of silicon with III-V materials provides a way to overcome silicon's limited optical properties toward a broad range of photonic applications. Hybrid modes are a promising way to integrate such heterogeneous Si/III-V devices, but it remains unclear how to utilize these modes to achieve photonic crystal cavities. Herein, using 3D finite-difference time-domain simulations, we propose a hybrid Si-GaAs photonic crystal cavity design that operates at telecom wavelengths and can be fabricated without requiring careful alignment. The hybrid cavity consists of a patterned silicon waveguide that is coupled to a wider GaAs slab featuring InAs quantum dots. We show that by changing the width of the silicon cavity waveguide, we can engineer the hybrid modes and control the degree of coupling to the active material in the GaAs slab. This provides the ability to tune the cavity quality factor while balancing the device's optical gain and nonlinearity. With this design, we demonstrate cavity mode confinement in the GaAs slab without directly patterning it, enabling strong interaction with the embedded quantum dots for applications such as low-power-threshold lasing and optical bistability (156 nW and 18.1 µW, respectively). This heterogeneous integration of an active III-V material with silicon via a hybrid cavity design suggests a promising approach for achieving on-chip light generation and low-power nonlinear platforms.

High optical contrast and multi-level storage of the ultracompact plasmonic device based on phase change materials

The combination of photonic devices and chalcogenide phase change materials (PCMs) provides a potential solution for photonic storage and computing. However, there are still some issues that need to be addressed for currently integrated phase-change photonic memory such as low speed, high energy consumption, and large footprint. To enable stronger interaction between light and PCMs, we utilize the plasmonic enhancement effect to amplify the local electric field, resulting in high energy efficiency in photonic storage and computation. This work proposes a nanoscale elliptical plasmonic nanoantenna that dramatically improves switching speed and energy by placing an Ag elliptical ring wrapped with Ge2Sb2Te5 (GST) on a silicon waveguide. The device exhibits crystalline and amorphous states with insertion losses of 0.3 dB and 1.7 dB, respectively, at a wavelength of 1550 nm. Write and erase operations can be completed by optical pulses of 1.5 ns–1 mW and 3 ns–3 mW, respectively. An ultra-small device volume of only 4.465 × 10−4 (µm3) is capable of achieving a switching contrast ratio of 28.5 %. Moreover, the non-volatile storage of multi-level intermediate states facilitates a wide weight-variation range and allows for precise weight updates, enhancing the overall performance and accuracy of the system. In the handwritten digit recognition task of the three-layer perceptron, the device demonstrates a recognition accuracy rate of 95 %, indicating that our approach can enable a more efficient neuromorphic photonic computing network.

On-chip multifunctional polarizer based on phase change material.

Polarizers are used to eliminate the undesired polarization state and maintain the other one. The phase change material Ge2Sb2Se4Te1 (GSST) has been widely studied for providing reconfigurable function in optical systems. In this paper, based on a silicon waveguide embedded with a GSST, which is able to absorb light by taking advantage of the relatively large imaginary part of its refractive index in the crystalline state, a multifunctional polarizer with transverse electric (TE) and transverse magnetic (TM) passages has been designed. The interconversion between the two types of polarizers relies only on the state switching of GSST. The size of the device is 7.5µm∗4.3µm, and the simulation results showed that the extinction ratio of the TE-pass polarizer is 45.37dB and the insertion loss is 1.10dB at the wavelength of 1550nm, while the extinction ratio (ER) of the TM-pass polarizer is 20.09dB and the insertion loss (IL) is 1.35dB. For the TE-pass polarizer, a bandwidth broader than 200nm is achieved with ER>20dB and IL<2.0dB over the wavelength region from 1450 to 1650nm and for the TM-pass polarizer, ER>15dB and IL<1.5dB in the wavelength region from 1525 to 1600nm, with a bandwidth of approximately 75nm.

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.

A new-generation approach to PAM-4 Based on the Fano resonance effect using a multimode interference structure

The goal of this research is to come up with a new optical structure for the generation of multilevel pulse amplitude modulation (PAM-4) signalling, which is used for optical interconnects and data center networks. The Fano effect is made with a structure made up of a 4x4 MMI coupler, feedback waveguides, and phase shifters. For the production of PAM-4, we employ the Fano effect. The new proposed approach can reduce power consumption and extend the linear region of the device transfer function compared with conventional approaches based on Mach Zehnder Interferometers (MZI) and ring resonators. We show that an extreme reduction of power consumption of 3 to 30 times, compared with the conventional structure based on MZI and ring resonators, is achieved. In addition, the proposed structure based on silicon wave guides has the advantages of extreme high bandwidth, large fabrication tolerance, and a compact footprint. The proposed structure can be applied to generate the PAM-4 based on the Fano effect with low power consumption, and it is suitable for complex systems with a lot of transceivers. The device is numerically simulated and designed using the Finite Difference Method (FDM) and Finite Difference Time Difference (FDTD). The PAM-4 structure based on silicon waveguides can be compatible with CMOS’s existing technologies.

Heuristic inverse design of integrated mode converter by directly reshaping silicon waveguide

Compact photonic devices for mode manipulation in silicon waveguide are of importance as fundamental building blocks in on-chip photonic integrated circuits. In this paper, we present a heuristic inverse design scheme based on directly reshaping the silicon waveguide for TE-polarized mode conversions. No additional complicated internal nanoscale structures are introduced to the waveguide. The boundary deformations of the 6 μm-long functional region are described by Bernstein polynomials and the optimal design is searched and determined by the gradient-based method of moving asymptotes. The TE0-to-TE1 (TE1-to-TE2) mode converter can be efficiently obtained after 19 (52) iterations from a straight waveguide. Quasi-3D simulations show the high mode purity (>0.99) at center wavelength of 1550 nm, while the lowest mode purity keeps higher than 0.97 within the operational bandwidth of 60 nm for all designs. Relatively high transmission efficiency is maintained as well. 3D simulations with SOI configuration validate the functionality and the satisfactory performances. For both 340 nm and 220 nm waveguide thicknesses, the conversion efficiencies respectively keep higher than 0.98 and 0.90 for TE0-to-TE1 and TE1-to-TE2 mode converters at center wavelength. By simply cascading the two proposed mode converters, reciprocal TE0-to-TE2 mode conversion can be achieved. Moreover, we demonstrate the robustness of proposed designs by adding ±10 nm geometric deviations.

Silicon Photonics Multi-Function Integrated Optical Circuit for Miniaturized Fiber Optic Gyroscope

Interferometric fiber optic gyroscope (IFOG) is advantageous for inertial navigation due to its high reliability as a solid-state technology with no moving parts. The size, weight, power, and cost of IFOG can be improved by employing photonic integration technology. Here, by leveraging the foundry-enabled silicon photonics platform, we demonstrate a silicon multi-function integrated optical circuit (Si-MIOC) with a fingertip size that can easily interfaces with light source and fiber coil to perform a tactical-grade gyroscope (0.1 °/hr bias instability). This achievement comes from the pros of high-index-contrast silicon waveguide that enables high polarization extinction ratio of > 60 dB and high electro-optic efficiency in silicon phase modulator, while the cons of the signature lead to non-negligible optical back-reflection at silicon/fiber interfaces and generate spurious bias voltage offset at the gyroscope outputs. Nevertheless, the results of earth rotation measurement and Allan deviation analysis confirmed that Si-MIOC based IFOG is a promising angular sensor and can be utilized to construct a compact and low-cost inertial measurement unit.

Plasmon-Assisted Si-ITO Integrated Electro-Optical Rib-Shape Modulator

An integrated plasmonic indium-tin-oxide (ITO)-based silicon electro-optical modulator has been designed, fabricated, and characterized. A sandwich structure shows a 40 GHz bandwidth at the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-2$</tex-math></inline-formula> dB level and an extinction ratio of about 1.8 dB. The device is fabricated on a silicon-on-insulator wafer. A silicon waveguide is excluded from the electric circuit. Instead, an ITO layer is used both as a bottom electrical contact and as an electric field-sensitive medium. As a gate oxide, we used a silicon dioxide layer capped with a gold top electrode to form a capacitor. The dielectric rib inside the ITO-Au capacitor forms a coupling interface between the TE photonic and plasmonic modes.

Complex-valued trainable activation function hardware using a TCO/silicon modulator

Artificial neural network-based electro-optic chipsets constitute a very promising platform because of its remarkable energy efficiency, dense wavelength parallelization possibilities and ultrafast modulation speeds, which can accelerate computation by many orders of magnitude. Furthermore, since the optical field carries information in both amplitude and phase, photonic hardware can be leveraged to naturally implement complex-valued neural networks (CVNNs). Operating with complex numbers may double the internal degrees of freedom as compared with real-valued neural networks, resulting in twice the size of the hardware network and, thus, increased performance in the convergence and stability properties. To this end, the present work revolves on the concept of CVNNs by offering a design, and simulation demonstration, for an electro-optical dual phase and amplitude modulator implemented by integrating a transparent conducting oxide (TCO) in a silicon waveguide structure. The design is powered by the enhancement of the optical-field confinement effect occurring at the epsilon-near-zero (ENZ) condition, which can be tuned electro-optically in TCOs. Operating near the ENZ resonance enables large changes on the real and imaginary parts of the TCO’s permittivity. In this way, phase and amplitude (dual) modulation can be achieved in single device. Optimal design rules are discussed in-depth by exploring device’s geometry and voltage-dependent effects of carrier accumulation inside the TCO film. The device is proposed as a complex-valued activation function for photonic neural systems and its performance tested by simulating the training of a photonic hardware neural network loaded with our custom activation function.

Inverse design of deformed Sb2Se3 stripes in silicon waveguide for reconfigurable mode converters

Reconfigurable photonic devices integrated with silicon waveguides are important building blocks for future on-chip photonic circuits. In this paper, we focus on the mode order conversion in silicon waveguides with non-volatile reconfigurable capability. Deformed phase change material Sb2Se3 (antimony triselenide) stripes are introduced at the edges of the functional region to provide the refractive index difference required by mode conversions. The shapes of stripes are inversely designed by a gradient-based iterative optimization strategy with 57 (19) iterations for TE0-to-TE1 (TE0-to-TE2) mode converter. The footprint of the functional region is as compact as square center wavelength. In the crystalline phase, TE0-to-TE1 and TE0-to-TE2 mode conversions are realized with conversion efficiencies of 98.5% and 96.3% at a center wavelength of 1550 nm, respectively. While in the amorphous phase, the input TE0 mode directly passes through the functional region with efficiencies of 93.0% and 92.4%, respectively. The output mode can be reconfigured by changing the phase of Sb2Se3 stripes. Moreover, after introducing ±10 nm geometrical deviations to the perfect Sb2Se3 stripe design, corresponding red and blue shifts of conversion efficiency spectra can be observed, and the simulation results reflect the reasonable robustness of the proposed mode converters.

High-Performance All-Optical Logic Operations Using Ψ-Shaped Silicon Waveguides at 1.55 μm.

We simulate with FDTD solutions a complete family of basic Boolean logic operations, which includes XOR, AND, OR, NOT, NOR, NAND, and XNOR, by using compact Ψ-shaped silicon-on-silica optical waveguides that are operated at a 1.55 μm telecommunications wavelength. Four identical slots and one microring resonator, all made of silicon deposited on silica, compose the adopted waveguide. The operating principle of these logic gates is based on the constructive and destructive interferences that result from the phase differences incurred by the launched input optical beams. The performance of these logic operations is evaluated against the contrast ratio (CR) metric. The obtained results suggest that the considered functions designed with the employed waveguide can be realized all-optically with higher CRs and faster speeds than other reported designs.

Nonvolatile phase change material based multifunctional silicon waveguide mode converters

Multimode photonics as a new research field of integrated photonics is closely relying on the waveguide mode, particularly for the higher-order waveguide mode. To efficiently generate higher-order waveguide modes in a single device, here we propose multifunctional silicon waveguide mode converters based on the nonvolatile and low-loss phase change material antimony triselenide (Sb2Se3). Such device is formed by two cascaded conversion regions along the silicon waveguide, where the first conversion region is formed by embedding a tapered Sb2Se3 layer into the silicon waveguide asymmetrically and the second one is formed by embedding two Sb2Se3 layers into the silicon waveguide symmetrically. When the embedded Sb2Se3 layers in two conversion regions undergo the phase transition between amorphous state and crystalline state, such device will achieve the mode conversions from input TE0 mode to output TE0, TE1, TE2, and TE1 + TE3 mode with mode conversion efficiency (CE) > 95%, mode crosstalk (CT) < −17 dB, and insertion loss (IL) < 0.4 dB at λ = 1.55 μm in a device length of only 10.3 μm where this multifunctional mode conversion in a single device has not been reported previously. To eliminate the output hybrid mode, we further revise the design and finally achieve the mode conversion from input TE0 mode to output TE0, TE1, TE2, and TE3 mode directly with a relatively good performance (CE > 85%, CT < −11 dB, and IL < 1 dB at λ = 1.55 μm). We believe the proposed multifunctional mode converters could be used as higher-order mode sources for the multimode photonics applications.