Air Trenches Research Articles

Low-loss silica waveguide 1×8 thermo-optic switch based on large-scale multimode interference couplers

Optical switches play key role in signal crossing and interconnection. Low-loss and compact optical switches are highly demanded in on-chip optical routing. A silica waveguide 1 × 8 thermo-optic switch based on cascaded 1 × 8 multimode interference (MMI) and 8 × 8 MMI couplers is experimentally demonstrated. Beam propagation method is adopted in the theoretical design and optimization. Standard CMOS technology fabrication is used in switch preparation. Air trenches are introduced to enhance the thermal modulation. At wavelength 1550 nm, the measured insertion loss and crosstalk of this 1 × 8 switch is less than 3.69 dB and −16.29 dB, respectively. And the extinction ratio is larger than 16.68 dB for all routing states. The rise time and fall time are 1.0 ms and 1.24 ms respectively. Compared with the 8-channel switch based on cascaded stages structure, the size reduces by 30 %. With future bandwidth improvement, this demonstrated switch has good potentials in the application of all-optical network routing.

High-efficiency thermo-optic control of a longitudinal radiation angle in a silicon-based optical phased array by leveraging the backside air trench structure.

We proposed a 2D 1 × 64 silicon optical phased array with a backside silicon-etched structure to achieve high tuning efficiency and a wide longitudinal steering range. At the radiator array, the n-i-n heater was implemented to steer the light in a longitudinal direction through the thermo-optic effect. The deep reactive ion etching process was utilized to generate the 600 µm depth air trench with a 1.8 cm2 area from the backside of the radiator array. We achieved almost 100% increment in terms of tuning efficiency, which is 1.56°/W for the proposed structure and 0.78°/W for the conventional structure.

Ultrahigh extinction ratio and a low power silicon thermo-optic switch.

The silicon thermo-optic switch (TOS) is one of the most fundamental and crucial blocks in large-scale silicon photonic integrated circuits (PICs). An energy-efficient silicon TOS with ultrahigh extinction ratio can effectively mitigate cross talk and reduce power consumption in optical systems. In this Letter, we demonstrate a silicon TOS based on cascading Mach-Zehnder interferometers (MZIs) with spiral thermo-optic phase shifters. The experimental results show that an ultrahigh extinction ratio of 58.8 dB is obtained, and the switching power consumption is as low as 2.32 mW/π without silicon air trench. The rise time and fall time of the silicon TOS are about 10.8 and 11.2 µs, respectively. Particularly, the figure of merit (FOM) has been improved compared with previously reported silicon TOS. The proposed silicon TOS may find potential applications in optical switch arrays, on-chip optical delay lines, etc.

Thermal flux manipulation on the silicon photonic chip to suppress the thermal crosstalk

The integration density of silicon photonic integrated circuit (PIC) is ultimately constrained by various crosstalk mechanisms on the chip. Among them, the most prominent limiting factor is the thermal crosstalk due to the wide use of the thermo-optic effect. High-density silicon PICs strongly demand an advanced structure with better thermal crosstalk suppression ability than the traditional air isolation trench. Inspired by the thermal-metamaterial based on the scattering-cancellation method, we demonstrate a closed heat shield (CHS) structure on a silicon PIC chip, which can manipulate the thermal flux to bypass the temperature-sensitive silicon photonics components. The on-chip CHS structure is a bilayer cylindrical shell fabricated by the standard silicon photonics processing flow. Its outer and inner shell layers are formed by a 6-μm-wide interconnection metal and 4-μm-wide air trench, respectively. Plenty of temperature-sensitive micro-ring resonators inside the CHS are used to probe the temperature profile. The measurement results show that the CHS can reduce the local temperatures by 50%/44%/36% at the locations 29/41/83 μm away from the external heater. In contrast, the conventional air trench of the same dimension reduces the local temperatures by 32%/28%/21% at the same positions. In addition, the response time of the thermal field inside the CHS is around one-half of that in the conventional air trench. Furthermore, the simulation result indicates that if the outer shell of the CHS can contact with the silicon substrate by utilizing the through-silicon-via structure, the thermal crosstalk suppression ability can be improved significantly.

Low crosstalk six-core five-mode fiber with high refractive index rings and air trench

A novel six-core five-mode fiber composed of an air trench and high refractive index ring is proposed in this paper. Numerical analysis shows that the fiber can stably transmit five LP modes in the C + L band. The effective refractive index difference of the five LP modes at 1.55 μm is more significant than 1.4 × 10-3, which is conducive to the independent transmission of the modes. The inter-core crosstalk of the fiber is less than -95 (dB / 100km). Therefore, the fiber can be well used to build a large-capacity optical fiber transmission system.

Optimization and comprehensive comparison of thermo-optic phase shifter with folded waveguide on SiN and SOI platforms

The thermo-optic phase shifter (TOPS) using metal heaters is widely used in various integrated photonics devices for its advantages of simple fabrication, low loss, and low cost. While folded waveguides are commonly employed to enhance the performance of these phase shifters, a comprehensive understanding of how varying the number of folds and the dimensions of heaters impact the device's performance is still lacking. In this paper, we conducted a comprehensive comparison and optimization into the impact of the number of folded waveguides and the width of heater on the performance of folded waveguide thermo-optic phase shifter (FW-TOPS) on silicon nitride (SiN) and silicon-on-insulator (SOI) platforms using the finite element method (FEM). The research has revealed that both the number of waveguides and the width of the heater exert a substantial influence on the performance of FW-TOPS, particularly on platforms with low refractive index contrast. In cases where the number of waveguides is the same, and the width of the heater electrode is identical, the comparison indicates that on the SiN platform, the FW-TOPS in waveguide Combination 2 (waveguide 1 is a single-mode waveguide, while waveguide 2 is a multi-mode waveguide) exhibits superior overall performance, compared to waveguide Combination 1 (both waveguide 1 and waveguide 2 are single-mode waveguides). However, on the SOI platform, FW-TOPS with waveguide Combination 1 demonstrates better overall performance compared to waveguide Combination 2. Furthermore, the simulation results indicate that although air trenches can significantly reduce the power consumption of the FW-TOPS, they also lead to a substantial decrease in the response speed. Therefore, on the 300 nm-thick SiN and 220 nm-thick SOI platforms, air trenches do not effectively enhance the overall performance of the FW-TOPS.

Broadband 32 × 32 Strictly‐Nonblocking Optical Switch on a Multi‐Layer Si3N4‐on‐SOI Platform

AbstractLarge‐scale silicon optical switches are essential to support the ever‐increasing data traffic. Among the various switching architectures, the well‐known switch‐and‐select (S&S) architecture has the advantages of low crosstalk and strict non‐blocking. However, it suffers from high path‐dependent losses as the waveguide crossings increase dramatically with the number of ports. In this paper, a large‐scale 32 × 32 S&S optical switch on a multi‐layer Si3N4‐on‐SOI platform is reported. The optical switch chip incorporates 1984 broadband thermo‐optic Mach‐Zehnder interferometer (MZI)‐based switch elements, 246 016 three‐dimensional (3D) waveguide crossings, and 2048 interlayer couplers. Both ≈1‐mdB‐loss 3D waveguide crossings and ≈0.3‐dB‐loss interlayer couplers are realized, significantly reducing the overall insertion loss and footprint of the switch chip. The measured average fiber‐to‐fiber insertion loss is 12.88 dB at the 1580 nm wavelength. In addition, the crosstalk is less than −20.7 dB over the 110‐nm wavelength range. The power consumption of the entire switch chip is only ≈0.98 W due to the air trenches and substrate undercut. High‐fidelity optical transmission of a 50 Gb s−1 quadrature phase‐shift keying signal verifies the high‐performance routing capability of this chip. These results indicate that the large‐scale optical switch with broadband, low crosstalk, and high‐power efficiency is promising for datacenter optical network applications.

Optimal design of a 4 × 4 MMI thermal optical switch with trapezoidal air trenches.

Optical switches are important in signal routing and switching. In this paper, a thermal optical switch with trapezoidal air trenches is proposed. The proposed structure consists of two cascaded 4×4 multimode interference (MMI) couplers. The beam propagation method is used to optimize the dimension and analyze the characteristics. Simulation results show excess loss (EL) and insertion loss (IL) are less than 0.14dB and 0.22dB at the wavelength of 1550nm, respectively. Besides, the extinction ratio (ER) is higher than 21dB. This design has the advantages of small size, low loss, and high flexibility, which is promising for application to all-optical network routing.

Low Power Consumption Hybrid-Integrated Thermo-Optic Switch with Polymer Cladding and Silica Waveguide Core

Taking advantage of the large thermo-optical coefficient of polymer materials, a hybrid-integrated thermo-optic switch was designed and simulated. It is also compatible with the existing silica-based planar light-wave circuit (PLC) platform. To further reduce the power consumption, we introduced the air trench structure and optimized the structural parameters of the heating region. This scheme is beneficial to solving the problem of the large driving power of silica-based thermo-optic switches at this stage. Compared with the switching power of all-silica devices, the power consumption can be reduced from 116.11 mW (TE) and 114.86 mW (TM) to 5.49 mW (TE) and 5.96 mW (TM), which is close to the driving power of the reported switches adopting polymer material as the core. For the TE mode, the switch's rise and fall times were 121 µs and 329 µs. For the TM mode, the switch times were simulated to be 118 µs (rise) and 329 µs (fall). This device can be applied to hybrid integration fields such as array switches and reconfigurable add/drop multiplexing (ROADM) technology.

Ultra-low loss SiN edge coupler interfacing with a single-mode fiber

In this work, an ultra-low loss silicon nitride (SiN) edge coupler was designed and fabricated to interface with a single-mode fiber (SMF). Unlike other works that focus on the core structure, this work focuses on the cladding structure. First, it is demonstrated that the cladding structure ultimately determines the size and shape of the mode when the taper tip width is small enough. Then, the thickness of the up-cladding is optimized to provide enough space for mode expansion in the vertical direction. Air trenches are added to confine the mode laterally. In addition, the refractive index (RI) of the up-cladding layer is slightly increased to prevent light from leaking into the Si substrate. This edge coupler is then fabricated on the SiN platform at Chongqing United Microelectronics Center. For the TE mode at 1630 nm, a coupling loss of 0.67 dB/facet was obtained. At 1550 nm, 0.85 dB/facet and 1.09 dB/facet were measured for the TE and TM modes, respectively, which means that the polarization-dependent loss is 0.24 dB. Although the design method and the structure are based on a pure SiN platform, they are applicable to a silicon-on-insulator platform as well.

Extremely Low-Profile Periodic 2-D Leaky-Wave Antenna: An Optimal Solution for Antenna-Frontend Integration

We investigate and present a very low-profile, high-efficiency, and high-gain 2-D leaky-wave antenna (2DLWA) implemented on a high permittivity substrate operating in the millimeter-wave range, paving the way for the seamless integration of a high-gain and high-efficiency antenna with a frontend. In contrast to the typical air-filled 2DLWA, where a perturbation of the first higher-order mode (TE1/TM1) results in highly directed broadside radiation, in the proposed antenna, the propagation and leakage of a quasi-TEM (Q-TEM) mode results in an extremely low-profile ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.065\lambda _{0}$ </tex-math></inline-formula> ), high-gain (~15 dBi) antenna. In this scenario, the transverse resonance condition for a Q-TEM mode is established by employing a capacitive partially reflective surface (PRS) in a thin ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.065\lambda _{0}$ </tex-math></inline-formula> ) substrate with a relative permittivity of 10.2. The proposed periodic 2DLWA, unlike the conventional uniform/quasi-uniform air-filled counterparts, radiates based on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathbf {n}=-\mathbf {1}$ </tex-math></inline-formula> space harmonic operation. Due to a strong mutual interaction between the PRS and ground plane, conventional design and analysis approaches are no longer relevant in this low-profile architecture. Instead, we employ the Floquet modal expansion theory to create an equivalent circuit model (ECM) for the PRS that takes into account the contributions and mutual interactions of the Floquet harmonics as well as the ground plane effect. By employing reflecting boundaries realized with edge truncation (air trenches) in a high permittivity substrate, we show that the aperture efficiency is enhanced by 15% compared to conventional counterpart.

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 &lt; 0.5 dB. Therefore, the proposed device schemes could push the development of on-chip PICs toward higher integration density.

Mid-infrared Ge-based thermo-optic phase shifters with an improved figure of merit

We demonstrate a power-efficient and fast thermo-optic phase shifter on a Ge platform for mid-infrared applications. Several approaches are implemented to improve the performance. Air trenches and Si-Ge multilayers with ultra-low thermal conductivity are introduced to provide thermal insulation. Few-layer graphene is used to enhance the thermal conductivity between the heater and the waveguide for efficient heat injection and subtraction. The optimized design has a power consumption of 3.9 mW and a time constant of 1.8 µs, resulting in a very small figure-of-merit of merely 7.0 mW·µs, 4 times smaller than the previously reported value.

Design of Air-Trenches/Holes Assisted Depressed-Core 12-Mode Fiber for Less MIMO Space Division Multiplexing

An air-trenches/holes assisted depressed-core few-mode fiber (AT-AH-DC-FMF) that features some asymmetrical air trenches, air holes and a depressed step-index core is proposed in this paper. The AT and DC can produce great contribution in separating the non-degenerated LP modes and spatial modes, thus suppressing the crosstalk efficiently. Based on AT and AH, the bending loss values of AT-AH-DC-FMF can be satisfied with the ITU-T recommendations of G. 654. The simulation results indicate that our proposed fiber can support 12 spatial modes with the effective index difference between adjacent LP modes (&#x0394;neff) larger than 1.03&#x00D7;10-3 and the effective index difference between adjacent spatial modes (&#x03B4;neff) larger than 1.24&#x00D7;10-4. The broadband characteristics such as &#x0394;neff, &#x03B4;neff, chromatic dispersion and effective mode field area (Aeff) over the whole C and L band are comprehensively investigated. Moreover, fabrication methods and birefringence are detailed discussed. Our proposed AT-AH-DC-FMF possesses great potential to be applied to less multiple-input multiple-output (MIMO-less) space-division multiplexing (SDM) enlarging optical communication capacity and simplifying the system complexity.

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.

Low Power and Compact Silicon Thermo-Optic Switch Based on Suspended Phase Arm Without Support Beams

A low power and compact thermo-optic switch based on $1\times 1$ Mach-Zehnder interferometer structure is demonstrated with the thermal isolation structure fabricated with a new backside etching process. The adjacent SiO2 and underlying silicon around phase arm are removed to suppress the thermal diffusion. Benefiting from the backside anisotropic etching process and front protecting photoresist used in underlying silicon removal, there are no support SiO2 beams in air trenches avoiding about 10% of extra thermal diffusion. At 1550 nm for TE mode, a low switching power (P $_{\pi } $ ) 0.48 mW is obtained with $450~\mu \text{m}$ thermo-optic interaction length ( $\text{L}_{\mathrm {TO}}$ ) representing a $\text{P}_{\pi }\cdot \text {L} _{\mathrm {TO}}$ product of only $0.22~\text {mW} \cdot \text {mm}$ , indicating a high thermal efficiency with a compact footprint. The 10% - 90% response time of the switch is $530~\mu \text{s}$ , including a rise time of 150 $\mu \text{s}$ , and a fall time of $380~\mu \text{s}$ . Compared with the switch without isolation structure, the switching power of the proposed switch is reduced more than 17 times, needing only 5.6% of the original value.

Energy-efficient thermo-optic silicon phase shifter with well-balanced overall performance

Silicon photonic integrated circuits (PICs) show great potential for many applications. The phase tuning technique is indispensable and of great importance in silicon PICs. An optical phase shifter with balanced overall performance on power consumption, insertion loss, footprint, and modulation bandwidth is essential for harnessing large-scale integrated photonics. However, few proposed phase shifter schemes on various platforms have achieved a well-balanced performance. In this Letter, we experimentally demonstrate a thermo-optic phase shifter based on a densely distributed silicon spiral waveguide on a silicon-on-insulator platform. The phase shifter shows a well-balanced performance in all aspects. The electrical power consumption is as low as 3 mW to achieve a π phase shift, the optical insertion loss is 0.9 dB per phase shifter, the footprint is 67×28µm2 under a standard silicon photonics fabrication process without silicon air trench or undercut process, and the modulation bandwidth is measured to be 39 kHz.

Low-DMD and low-crosstalk few-mode multi-core fiber with air-trench/holes assisted graded-index profile

We propose a novel scheme of air-trench (AT) assisted graded-index profile to design a heterogeneous few-mode multi-core fiber (hetero-FM-MCF) with high density of cores and low inter-core crosstalk (XT). By the using of finite element method (FEM), it is revealed that the AT structure can remarkably reduce XT and bending loss, and leaving no significant impact on other fiber properties. The numerical analyses show that the AT hetero-FM-MCF achieves a low XT of about -66 dB/km (15.7% lower than that of trench-assisted (TA) hetero-FM-MCF), the max differential model delay (DMD) over C+L band of 70 ps/km and relative core multiplicity factor (RCMF) of 41.7. To further improve the light confinement ability of individual cores, air-holes (AH) are introduced to the AT structure. The air-trench/hole assisted (AT-AH) hetero-FM-MCF generates a lower XT of -68 dB/km and 6% bending loss values optimization than AT hetero-FM-MCF.

Compact and Fabrication-Tolerant Waveguide Bends Based on Quadratic Reflectors

We propose and experimentally demonstrate a broadband, polarization-diverse compact bending design for low-index-contrast waveguides, where light is re-directed via total internal reflection (TIR) on an air-trench quadratic (elliptical or parabolic) reflector surface. Compared to prior work based on flat TIR mirrors, the quadratic reflector design contributes to minimized mode leakage and reduced optical losses, enabling high-density, scalable photonic architectures at the chip and board levels. Moreover, we develop a self-aligned fabrication process where the reflector and the waveguide segments are defined in a single lithography step, thereby circumventing the alignment sensitivity issue common to traditional air trench structures. Our simulations predict bending losses down to <0.14 dB per 90° and 180° bend at 850 nm wavelength, and we experimentally measure broadband losses of ∼0.3 dB per 90° and 180° bend for both TE and TM polarizations in structures fabricated using standard UV lithography.

Planar Highly Efficient High-Gain 165 GHz On-Chip Antennas for Integrated Radar Sensors

This letter demonstrates different planar highly efficient on-chip antennas at 165 GHz with high gain utilizing a standard silicon-Germanium bipolar-complementary metal-oxide-semiconductor (Bi-CMOS) process with a localized backside etching (LBE) feature that allows cutting air trenches in the silicon. A dipole antenna, a folded dipole antenna with air trenches around the radiator, and a single-ended patch antenna with air trenches at the radiating edges are designed, fabricated, and characterized in the D-band (110–170 GHz). The geometry of each individual antenna and the LBE air trenches are optimized to meet both process reliability specifications and radiation performance simultaneously. Metal fillings effects on the radiation pattern and matching are also studied.