Low-loss Silicon Waveguides Research Articles

Low-loss and low-cross talk polarization-insensitive multimode silicon waveguide crossing.

A polarization-insensitive multimode silicon waveguide crossing is investigated and experimentally characterized in this Letter. By employing the particle swarm optimization (PSO) algorithm and finite difference time domain (FDTD) method, the lengths and widths of the waveguides in the proposed device are optimized for attaining wide bandwidth, small insertion loss (IL), low cross talk (CT), and compact size. Measurement results reveal that the footprint of the presented device is 11.92 μm × 11.92 μm. From 1520 to 1600 nm, the measured insertion loss and cross talk are smaller than 0.67 dB and -28.6 dB in the case of the TE0 mode, lower than 0.65 dB and -28.7 dB in the case of the TE1 mode, less than 0.48 dB and -36.3 dB in the case of the TM0 mode, and lower than 0.62 dB and -28 dB in the case of the TM1 mode.

Low-loss silicon waveguide and an ultrahigh-Q silicon microring resonator in the 2 µm wave band.

Silicon photonic-integrated circuits (PICs) operating in the 2 µm wave band are of great interest for spectroscopic sensing, nonlinear optics, and optical communication applications. However, the performance of silicon PICs in this wave band lags far behind the conventional optical communication band (1310/1550 nm). Here we report the realization of a low-loss waveguide and an ultrahigh-Q microring resonator in the 2 µm wave band on a standard 200 mm silicon photonic platform. The single-mode strip waveguide fabricated on a 220 nm-thick silicon device layer has a record-low propagation loss ∼0.2 dB/cm. Based on the low-loss waveguide, we demonstrate an ultrahigh-Q microring resonator with a measured loaded Q-factor as high as 1.1 × 106 and intrinsic Q-factor of 2 × 106, one order of magnitude higher than prior silicon resonators operating in the same wave band. The extinction ratio of the resonator is higher than 22 dB. These high-performance silicon photonic components pave the way for on-chip sensing applications and nonlinear optics in the 2 µm wave band.

A low loss silicon waveguide bend based on width and curvature variations

A simple, low loss and compact 90° SOI waveguide bend constructed by using an inner circular curve and an outer Euler-circular curve was proposed, optimal designed and fabricated. By adopting this simple structure, the width and curvature of the waveguide bend can be varied simultaneously, which can effectively suppress both the mode mismatch loss and the radiation loss. Three-dimensional finite difference time domain simulation results indicate a very low excess loss of about 0.0056 dB can be achieved for equivalent bending radius of 2 μm. By using the cut-back method, a low loss of about 0.0175 dB at 1550 nm was measured, which is less than one quarter of the traditional circular bend. In addition, the wavelength dependence for the proposed SOI waveguide bend was investigated and 0.0088 dB–0.0247 dB was measured in the wavelength range of 1480∼1580 nm.

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.

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.

Terahertz disk resonator on a substrateless dielectric waveguide platform.

Resonant cavities are fundamental to and versatile for terahertz integrated systems. So far, integrated resonant cavities have been implemented in relatively lossy terahertz platforms. In this Letter, we propose a series of integrated disk resonators built into a low-loss substrateless silicon waveguide platform, where the resonances and associated quality factor (Q-factor) can be controlled via an effective medium. The measurement results demonstrate that the Q-factor can reach up to 9146 at 274.4 GHz due to the low dissipation of the platform. Additionally, these resonators show strong tunability of the resonance under moderate optical power. These terahertz integrated disk resonators can be employed in sensing and communications.

Design of Low Loss Silicon Waveguide Bend with Width and Curvature Variations

Design of Low Loss Silicon Waveguide Bend with Width and Curvature Variations

Design of Compact, Broadband, and Low-Loss Silicon Waveguide Bends with Radius under 500 nm

Waveguide bend is an indispensable component in the on-chip compact photonic integrated circuits (PICs) and the minimum bend size greatly limits the increase of integration density of PICs. Here, we propose broadband and low-loss silicon waveguide bend schemes using air trenches on both sides and embedded germanium arc in the inner side of waveguide bend. Using these ways, the silicon waveguide bending radius can be greatly reduced to less than 500 nm and the obtained insertion loss (IL) can be as low as 0.12 dB compared with IL = 1.73 dB obtained by direct silicon waveguide bend under the same bending radius. Meanwhile, the working bandwidth can be extended over 500 nm covering the whole optical communication band by keeping IL < 0.5 dB. Therefore, the proposed device schemes could push the development of on-chip PICs toward higher integration density.

3 × 40 Gbit/s All-Optical Logic Operation Based on Low-Loss Triple-Mode Silicon Waveguide.

Information capacity of single-mode fiber communication systems face fundamental limitations imposed by optical nonlinearities. Spatial division multiplexing (SDM) offers a new dimension for upgrading fiber communication systems. Many enabling integrated devices, such as mode multiplexers and multimode bending with low crosstalk, have been developed. On the other hand, all-optical signal processing (AOSP) can avoid optical to electrical to optical (O–E–O) conversion, which may potentially allow for a low cost and green operation for large-scale signal processing applications. In this paper, we show that the system performance of AOSP can be pushed further by benefiting from the existing technologies developed in spatial mode multiplexing (SDM). By identifying key technologies to balance the impacts from mode-dependent loss, crosstalk and nonlinearities, three-channel 40 Gbit/s optical logic operations are demonstrated using the first three spatial modes in a single multimode waveguide. The fabricated device has a broadband four-wave mixing operation bandwidth (>20 nm) as well as high conversion efficiency (>−20 dB) for all spatial modes, showing the potential for a large-scale signal processing capacity with the combination of wavelength division multiplexing (WDM) and SDM in the future.

High-Q THz Photonic Crystal Cavity on a Low-Loss Suspended Silicon Platform

In this article, we present an ultrahigh-Q cavity at terahertz (THz) frequencies. The designed cavity is built on a low-loss suspended silicon (Si) waveguide. The substrate removal under the waveguide and the use of optimized deep reactive ion etching processing are the main reasons for observing very low losses of this design α <; 0.09 dB/mm. This very low-loss behavior of this designed platform is also demonstrated by the measurement of a one-dimensional photonic wire crystal cavity with Q > 18000. Different cavity layouts are adjusted in order to maximize the transmittance while maintaining high Q. A design with reduced number of etched crystal holes achieve Q > 1500 and high transmittance T > 70%. These structures are presented at sub-mm waves (around 600 GHz) for the design of a gas sensor in this frequency region, but the principles can be scaled and redesigned for other frequencies in the THz band.

Fabrication and analysis of low-loss silicon high-mesa waveguides.

Breath sensing is an effective tool for health monitoring. Previously, high-mesa waveguide structures have been proposed by our group for realizing a compact breath-sensing photonic circuit. By using the doped SiO2 as the waveguide core, 50% concentration CO2 has been detected. One issue of preventing parts per million (ppm)-order detection is the low portion of evanescent light (Γair=2.2%) in the doped SiO2 waveguides. In order to realize low propagation loss α and high Γair simultaneously, thin silicon (Si) waveguides with a Γair as high as 37.6% have been proposed and fabricated in this work. A thermal oxidation technique was applied to further reduce α, so that α was decreased from 1.45 to 0.84 and 0.29 to 0.2 dB/cm for the 0.5 and 3-µm-wide waveguide, respectively. According to our analysis, the significantly decreased α is attributed to recovering the damaged Si core and smoothing the waveguide sidewalls. The high Γair and effective loss reduction show a promising potential of applying Si high-mesa waveguides to realize ppm-order sensing.

Inverse-designed low-loss and wideband polarization-insensitive silicon waveguide crossing

Waveguide crossings are an essential component for constructing complex and functional on-chip photonic networks. Polarization-insensitive waveguide crossings are desired because photonic networks usually involve light with different polarizations. Here, we propose a polarization-insensitive waveguide crossing on a 250-nm silicon-on-insulator platform by using an inverse design method. In simulation, the designed waveguide crossing can maintain insertion loss below 0.18 (0.25)dB in the wavelength range of 1440-1640nm for the TE0 (TM0) mode and achieve minimal insertion loss as small as 0.08 (0.07)dB at the wavelength of 1550nm. The cross talk maintains below -32 dB and -35 dB for the TE0 and TM0 modes, respectively. Experimentally, the fabricated waveguide crossing achieves measured insertion loss less than 0.20 (0.25)dB for the TE0 (TM0) mode with minimal insertion loss as small as 0.1dB. The measured cross talk is below -28 dB and -31 dB for the TE0 and TM0 modes, respectively. Therefore, our proposed waveguide crossing can be widely applied in photonic integrated circuits to construct photonic systems with the capabilities of polarization control and mode (de)multiplexing.

Large scalable and compact hybrid plasmonic nanoantenna array

We propose a scheme for a large scalable and compact antenna array with subwavelength antenna spacing. In this scheme, the array consists of a series of hybrid plasmonic nanoantennas, which operate at 1550 nm and have a subwavelength footprint. In a wide bandwidth, the nanoantenna is highly compatible with a low-loss silicon waveguide, which feeds light from the bottom of the nanoantenna. Based on the proposed nanoantenna, two silicon photonic antenna arrays (1 × 8 and 8 × 8) are designed and investigated in detail. Both the one- and two-dimensional arrays can realize wide steering without grating lobes. For the 8 × 8 array, a high gain of 24.2 dB and wide steering range of 88.0 deg × 90.0 deg are achieved.

Silicon waveguide cantilever displacement sensor for potential application for on-chip high speed AFM

This paper reviews an initial achievement of our group toward the development of on-chip parallel high-speed atomic force microscopy (HS-AFM). A novel AFM approach based on silicon waveguide cantilever displacement sensor is proposed. The displacement sensing approach uniquely allows the use of nano-scale wide cantilever that has a high resonance frequency and low spring constant desired for on-chip parallel HS-AFM. The approach consists of low loss silicon waveguide with nano-gap, highly efficient misalignment tolerant coupler, novel high aspect ratio (HAR) sharp nano-tips that can be integrated with nano-scale wide cantilevers and electrostatically driven nano-cantilever actuators. The simulation results show that the displacement sensor with optical power responsivity of 0.31%/nm and AFM cantilever with resonance frequency of 5.4 MHz and spring constant of 0.21 N/m are achievable with the proposed approach. The developed silicon waveguide fabrication method enables silicon waveguide with 6 and 7.5 dB/cm transmission loss for TE and TM modes, respectively, and formation of 13 nm wide nano-gaps between silicon waveguides. The coupler demonstrates misalignment tolerance of ±1.8 μm for 5 μm spot size lensed fiber and coupling loss of 2.12 dB/facet for standard cleaved single mode fiber without compromising other performance. The nano-tips with apex radius as small as 2.5 nm and aspect ratio of more than 50 has been enabled by the development of novel HAR nanotip fabrication technique. Integration of the HAR tips onto an array of 460 nm wide cantilever beam has also been demonstrated.

Suspended silicon waveguides for long-wave infrared wavelengths.

In this Letter, we report suspended silicon waveguides operating at a wavelength of 7.67μm with a propagation loss of 3.1±0.3 dB/cm. To our knowledge, this is the first demonstration of low-loss silicon waveguides at such a long wavelength, with loss comparable to other platforms that use more exotic materials. The suspended Si waveguide core is supported by a sub-wavelength grating that provides lateral optical confinement while also allowing access to the buried oxide layer so that it can be wet etched using hydrofluoric acid. We also demonstrate low-loss waveguide bends and s-bends.

A III-V-on-Si ultra-dense comb laser

Optical frequency combs emerge as a promising technology that enables highly sensitive, near-real-time spectroscopy with a high resolution. The currently available comb generators are mostly based on bulky and high-cost femtosecond lasers for dense comb generation (line spacing in the range of 100 MHz to 1 GHz). However, their integrated and low-cost counterparts, which are integrated semiconductor mode-locked lasers, are limited by their large comb spacing, small number of lines and broad optical linewidth. In this study, we report a demonstration of a III-V-on-Si comb laser that can function as a compact, low-cost frequency comb generator after frequency stabilization. The use of low-loss passive silicon waveguides enables the integration of a long laser cavity, which enables the laser to be locked in the passive mode at a record-low 1 GHz repetition rate. The 12-nm 10-dB output optical spectrum and the notably small optical mode spacing results in a dense optical comb that consists of over 1400 equally spaced optical lines. The sub-kHz 10-dB radio frequency linewidth and the narrow longitudinal mode linewidth (<400 kHz) indicate notably stable mode-locking. Such integrated dense comb lasers are very promising, for example, for high-resolution and real-time spectroscopy applications.

Extreme field enhancement in nano-gap plasmonic cavity via 90% efficient coupling with silicon waveguide.

We propose a novel design for a sub-5-nm-gap plasmonic cavity to couple it efficiently with an integrated low loss silicon waveguide. We numerically obtain over 90% efficient coupling between a nano-gap plasmonic cavity with a modal volume of less than 10-7λ3 and a conventional silicon-on-insulator (SOI) waveguide by utilizing the anti-symmetric second-order resonance mode of the cavity and engineering its geometry to reduce the modal size to less than 5 nm. The electromagnetic field efficiently coupled to the small cavity, leading to extreme enhancement of the field intensity. For a 2-nm-gap cavity, the intensity enhancement was calculated to be more than 100,000,000 compared to that of light in an SOI waveguide.

Low-Loss Silicon Waveguides and Grating Couplers Fabricated Using Anisotropic Wet Etching Technique

We report low-loss silicon waveguides and efficient grating coupler to couple light into them. By using anisotropic wet etching technique, we reduced the side wall roughness down to 1.2nm. The waveguides were patterned along the [112] direction on a [110] SOI substrate. The waveguide boundaries are decided by the [111] planes which are normal to the [110] surface. Fabricated waveguides show minimum propagation loss of 0.85 dB/cm for TE polarization and 1.08dB/cm for TM polarization. The fabricated grating couplers show coupling efficiency of -4.16dB at 1570nm with 3dB bandwidth of 46nm.

Widely Tunable Narrow-Linewidth Monolithically Integrated External-Cavity Semiconductor Lasers

We theoretically analyze, design, and measure the performance of a semiconductor laser with a monolithically integrated external cavity. A ∼4 cm long on-chip cavity is made possible by a low-loss silicon waveguide platform. We show tuning in excess of 54 nm in the O-band as well as significant reduction in laser linewidth due to controlled feedback from the external cavity. The measured linewidth in full tuning range is below 100 kHz and the best results are around 50 kHz. Approaches to further improve the performance of such laser architectures are described.

Low-loss silicon waveguides with sidewall roughness reduction using a SiO2 hard mask and fluorine-based dry etching

High optical field confinement-possible silicon waveguides are used in various fields. The performance of a silicon waveguide depends on its sidewall roughness, which is responsible for scattering loss. The fabrication process for a silicon waveguide with a smooth sidewall for low loss was studied. The propagation loss of an optical silicon waveguide was reduced by using a SiO2 hard mask and reactive ion etching (RIE) with fluorine gases. The sidewall angle was controlled by Si etching with a SF6 and CF4 gas mixture. The thickness of the SiO2 hard mask affects the sidewall smoothing treatment. The roughness of the sidewall was reduced in the SiO2 etching process with a CF4 and O2 gas mixture as the thickness of the SiO2 hard mask was increased. The minimum propagation loss (0.89 dB cm−1) of the fabricated silicon waveguide was achieved by using the fabrication method introduced in this paper.