Bent Waveguides Research Articles

CMOS-compatible ultra-compact silicon multimode waveguide bend based on inverse design method

Waveguide bend is one of the indispensable fundamental devices in building various integrated optical circuits. It is commonly used so that its footprints would affect the compactness of the whole circuit. However, in mode-division multiplexing systems, the radii of multimode waveguide bends are typically large (∼the order of 10 micrometers), which would limit the high-density integration of on-chip multimode devices. We designed a CMOS-compatible ultra-compact multimode waveguide bend based on the inverse design approach. The device ensures 90-degree high-performance transmission of TE0, TE1, and TE2 modes simultaneously with the footprint of 5.1μm×5.1μm in an equivalent bend radius of 4.4μm. Simulation results show that the insertion loss of the device is 0.045 dB, 0.062 dB, and 0.056 dB for TE0, TE1, and TE2 modes at 1550 nm, respectively, and the inter-modes crosstalk is lower than – 21.8 dB.

Shortcut to adiabaticity in bent waveguide couplers with a sign flip of the phase mismatch.

Bending the axis of a waveguide coupler can result in phase mismatch between the evanescently coupled waveguides. Using a bent waveguide coupler, we realize a shortcut to adiabatic light transfer between waveguides with a sign flip of the phase mismatch. Counterdiabatic driving with unitary transformation cancels non-adiabatic coupling in the waveguide coupler so that light evolution follows the adiabatic modes at short lengths. The counterdiabatic driving protocol is implemented in the waveguide coupler by engineering the waveguide spacing and axis bending profile. The bent waveguide coupler is compact and robust against parameter variations.

High-extinction-ratio Si optical modulator loaded with integrated polarizer

We propose a carrier-depletion type Si optical modulator for transverse electric (TE) polarization loaded with an integrated polarizer. The integrated TE-pass polarizer with a bent waveguide has a compact and simple structure and demonstrate a polarization extinction ratio of 41.3 dB and insertion loss of 0.67 dB. This Si optical modulator loaded with the integrated polarizer achieved a maximum extinction ratio of 54.5 dB. It is expected to have a wide range of applications in digital coherent optical communication and optical sensing fields.

A parallel design scheme for ultra-compact silicon waveguide crossing based on the tapered multimode interference coupler

We demonstrate two ultra-compact high efficiency silicon waveguide crossing devices based on the tapered multimode interference coupler in the entire wavelength range of 1520 to 1580 nm. A parallel waveguide crossing structure for transverse-electric (TE) mode with a footprint of only 1.05 × 2.6 μm2 is designed, which can provide single mode crossing with the insertion loss better than 0.12 dB and the crosstalk lower than −23 dB in the broadband. A polarization-insensitive waveguide crossing with a footprint of only 1.1 × 2.3 μm2 is also designed, which can provide insertion loss lower than 0.54 dB and crosstalk lower than −23 dB for both TE and transverse-magnetic modes. The proposed parallel crossing scheme can efficiently save rooms for building up sophisticated waveguide crossing networks by avoiding the waveguide bending before or after crossing. These devices can have great application potential in the dense photonic integrated systems.

3‐D printed circularly polarized multibeam antenna array with full azimuth scanning capability and wideband characteristic for 5G millimeter‐wave applications

This paper presents a wideband circularly polarized (CP) multibeam antenna array, which is with full azimuth scanning capability and operates in Ka band for 5G applications. It adopts a circular array configuration and consists of 18 elements with a simple structure. Regarding to each element, it is fed by a 90° bent rectangular waveguide and employs a simplified single dielectric slab polarizer to produce CP waves. Within the axial ratio bandwidth (ARBW), the proposed element can achieve a stable half power beamwidth (HPBW) of 29 ± 7°, while, the 3-dB AR beamwidth is larger than the HPBW. By switching the active elements, the proposed array can realize 360° beam steering in azimuth plane, while, the HPBW and the 3-dB AR beamwidth cover the entire plane. To simplify the fabrication process, dielectric and metal 3-D printing technologies are employed for processing. Experimental results indicate that the proposed array can achieve a 40.5% 3-dB ARBW which almost covers the entire Ka band from 26.2 to 39.5 GHz and a gain of 9.9 ± 1.5 dBic. The front-to-back ratio (FBR) of each beam is higher than 19 dB. In contrast to conventional designs, the proposed antenna features wide band, simple structure, and ease of fabrication.

Adaptable Optical Microwaveguides From Mechanically Flexible Crystalline Materials.

Flexible organic crystals (elastic and plastic) are important materials for optical waveguides, tunable optoelectronic devices, and photonic integrated circuits. Here, we present highly elastic organic crystals of a Schiff base, 1-((E)-(2,5-dichlorophenylimino)methyl)naphthalen-2-ol (1), and an azine molecule, 2,4-dibromo-6-((E)-((E)-(2,6-dichlorobenzylidene)hydrazono)methyl)phenol (2). These microcrystals are highly flexible under external mechanical force, both in the macroscopic and the microscopic regimes. The mechanical flexibility of these crystals arises as a result of weak and dispersive C-H⋅⋅⋅Cl, Cl⋅⋅⋅Cl, Br⋅⋅⋅Br, and π⋅⋅⋅π stacking interactions. Singly and doubly-bent geometries were achieved from their straight shape by a micromechanical approach using the AFM cantilever tip. Crystals of molecules 1 and 2 display a bright-green and red fluorescence (FL), respectively, and selective reabsorption of a part of their FL band. Crystals 1 and 2 exhibit optical-path-dependent low loss emissions at the termini of crystal in their straight and even in extremely bent geometries. Interestingly, the excitation position-dependent optical modes appear in both linear and bent waveguides of crystals 1 and 2, confirming their light-trapping ability.

Ultra-sharp silicon multimode waveguide bends based on double free-form curves

Mode-division multiplexing (MDM) can greatly improve the capacity of information transmission. The multimode waveguide bend (MWB) with small size and high performance is of great significance for the on-chip MDM integrated system. In this paper, an MWB with high performance based on double free-form curves (DFFCs) is proposed and realized. The DFFC is a combination of a series of arcs optimized by the inverse design method. The fabrication of this MWB only needs one-step lithography and plasma etching and has a large fabrication tolerance. MWBs with effective radii of 6 μm and 10 μm are designed to support three modes and four modes, respectively. The proposed method gives the best overall performance considering both the effective bending radius and the transmission efficiency. The fabricated MWB with four mode channels has low excess losses and crosstalks below − 21 dB in the wavelength range from 1520 to 1580 nm. It is expected that this design can play an important role in promoting the dense integration of multimode transmission systems.

Multi-faceted bending and multi-directional flexible optical waveguide of the elastic organic single crystal

Multi-faceted bending elastic organic single crystals are extremely rare and have potential applications in optical communication, mechanically tunable flexible fluorochromic materials, etc., which have attracted great attention of researchers. Herein, we reported two compounds with simple structures, namely 1,3-dimethylbarbituric acid (DMBA) and its derivative (TPABA). Under stress, both (200) and (002) faces of TPABA crystal could undergo reversible elastic deformation, possessing rare multi-faceted bending properties. The abundant weak isotropic intermolecular interactions (such as H-bonding, etc.) in crystal could buffer mechanical stress and prevent crystal fracture. Fortunately, TPABA crystals showed multi-directional flexible optical waveguide characteristics. The corresponding optical loss coefficients were 0.57 dBmm−1 (582 nm, straight line), 0.75 dBmm−1 (583 nm, (002) face bending), and 0.58 dBmm−1 (587 nm, (200) face bending). Unlike TPABA crystals, under stress, the (040) and (200) faces of the DMBA crystal underwent plastic deformation and fracture, respectively, showing the characteristics of single-faceted plastic deformation.

Inverse design of multimode silicon waveguide bends by topology optimization

Compact, multimode, silicon waveguide bends are essential building blocks for high-density, integrated-optics, mode-division-multiplexing (MDM) circuits. However, sharp, multimode, silicon waveguide bends can typically introduce considerable insertion losses and inter-mode crosstalk. Significant efforts have been made to realize compact and high performance multimode silicon waveguide bends, but most of those designs either require complicated fabrication processes or are not really compact. We show that the transmission performances of a regular, ultra-sharp, multimode, silicon waveguide bend can be drastically improved by directly optimizing its topology. The optimization can be efficiently accomplished by using an adjoint-based inverse design. Through this design strategy, we realize a three-mode, silicon waveguide bend that is ultra-compact, with a small radius of 5.5 μm, while having ultra-low average insertion losses of 0.023, 0.046, and 0.068 dB and small average mode crosstalk of −26, −27, and −27 dB for the input modes of TE0 − 2 (TE = transverse electric), respectively, in the wavelength range 1500 to 1600 nm. Also, the waveguide bend can be fabricated through a single electron-beam lithography step. The significant advantages in size and performance of the current waveguide bend show the great potential of the proposed inverse design strategy for designing multimode waveguide bends for MDM systems.

Compact and high Q-factor multimode racetrack ring resonator based on transformation optics.

The ring resonator is a versatile and functional component in the silicon-based integrated optical circuit. Most of the previously reported ring resonators work in the single-mode case. With the rapid development of mode division multiplexing technology, a multimode ring resonator (MMRR) has been proposed and the usage beyond the limit of a conventional single mode ring resonator has been explored. However, the reported MMRRs are either large in size or low in quality factor. In this paper, we designed a compact silicon MMRR with a small bending radius of 15µm, in which the three lowest TE modes all have high Q-factors. For suppressing the mode loss and inter-mode crosstalk in MMRR, a multimode waveguide bend (MWB) with mode adiabatic evolution was designed based on transformation optics and waveguide shape optimization. The independent excitation of each order mode of the MMRR is realized by using bending directional coupler and asymmetric directional coupler. We successfully fabricated the device on a silicon-on-insulator (SOI) platform using simple one-step lithography. The measured loaded Q-factors of the three lowest TE modes are 5.9 × 104, 4.5 × 104, and 4.7 × 104, respectively.

Modulation Response of Monolithically Integrated External Optical Injection-Locking Semiconductor Ring Laser Based on PPR Effect

The modulation response of monolithically integrated external optical injection-locking (EOIL) semiconductor ring lasers based on photon-photon resonance effect was systematically analyzed. The simulation results show that the modulation response of the EOIL laser is strongly dependent on the internal loss of the ring laser. Assuming that the waveguide was loss-free, the modulation bandwidth of as high as 67.5 GHz was predicted. However, due to the excessive loss associated with the bending waveguide of the circular-shaped ring laser, the modulation performance of such EOIL laser has no any advantage over its free-running counterpart. Instead, use of rectangular-shaped ring laser as the slave laser is proposed to circumvent the issue associated with the excessive waveguide loss, since the internal loss, in this case, is only dependent on the reflectivity of the total reflection mirrors. Simulation shows the modulation bandwidth of at least 38 GHz is achievable in such EOIL laser, provided the reflectivity of the total reflection mirrors is greater than 96%. It is believed that this theoretical study will facilitate the future experimental demonstration of the monolithically integrated EOIL lasers.

Broadband Silicon Nitride Power Splitter Based on Bent Directional Couplers with Low Thermal Sensitivity.

Directional couplers, as power splitters, have provided a significant contribution for light splitting and combining in silicon photonics. However, the splitting ratio of conventional directional couplers is very sensitive to wavelength, which limits the bandwidth and the transmission performance of the devices. In this work, a silicon nitride bent directional coupler with large bandwidth, large fabrication tolerance, and low thermal sensitivity is proposed and demonstrated through simulation analysis and experiments. Moreover, the fabrication process of 400 nm thick silicon nitride photonic devices is described, which are compatible with complementary metal–oxide–semiconductor technology. The 1 dB bandwidth of the bent waveguide coupler can reach 80 nm, and the thermal sensitivity is reduced by 85% compared to the silicon-based devices.

Compact waveguide bend with digital meta-structures on the silicon-on-insulator platform

Silicon devices have attracted much attention due to their application potential in high-density photonic integrated circuits (PICs). However, it is still challenging to realize light routing within a micro-sized region due to high optical losses introduced by sharp waveguide bends (WBs). Here, we propose and design an ultra-broadband WB with a bending radius of only 2 µm. The design is based on digital meta-structures which are topologically optimized through a genetic algorithm. The designed WB shows an insertion loss of 0.45 dB at the wavelength of 1.55 µm, while the 1 dB-bandwidth reaches 0.3 µm for TE0 mode. With TE0 mode input, modal crosstalk of less than − 20 dB is achieved over a spectral range from 1.5 µm to 1.8 µm. Our study is expected to enable flexible light routing with an ultra-wide bandwidth and compact device footprint, boosting the development of high-density PICs.

Subwavelength Grating Metamaterial Multimode Bend for Silicon Waveguides

AbstractIn this paper, a novel multimode waveguide implemented on a silicon‐on‐insulator (SOI) platform that supports several transverse (TE)/transverse magnetic (TM) modes across a broad range of wavelengths is experimentally demonstrated. The fully etched metamaterial design combines a gradient curvature bend with trapezoidal subwavelength grating segments and tapered concentric bridging strips. The simulations showed successful propagation of up to nine modes (5 TE and 4 TM) on the 340 nm SOI platform, with excess losses below 2.4 dB, and intermodal cross‐talk of less than −15.7 dB. Experimentally, a compact multimode bend with a radius of 10 µm on a 220 nm SOI platform is demonstrated that successfully supports four TE modes, with average signal‐to‐cross‐talk extinction ratio levels of 14.4 dB or better, across a wavelength band of 1500–1600 nm. This versatile multimode waveguide bend can be employed as a fundamental building block for densely integrated photonic circuits and mode multiplexing systems.

Low loss modified Bezier bend waveguide.

We proposed and experimentally demonstrated a low loss modified Bezier bend for silicon and silicon nitride photonic integrated circuits. Both simulation and experimental results confirm that the modified Bezier bend can effectively reduce the bend loss for silicon and silicon nitride platform. At a bend radius of 1 µm, the reduction of bend loss from 0.367 dB/90° of circular bend and 0.35 dB/90° of traditional Bezier bend to 0.117 dB/90° of modified Bezier bend for silicon platform was experimentally demonstrated. For a 12-µm radius silicon nitride bend, the bend loss reduction from 0.65 dB/90° of circular bend and 0.575 dB/90° of traditional Bezier bend to 0.32 dB/90° was achieved. The proposed modified Bezier bend design can also be applied to other material systems, such as InP, LN, GaAs, etc., to effectively reduce the bend waveguide loss.

Compact broadband ( O, E, S, C, L & U bands) silicon TE-pass polarizer based on ridge waveguide adiabatic S-bends.

A compact, ultra-broadband and high-performance silicon TE-pass polarizer is proposed and demonstrated experimentally. It is based on partially-etched (ridge) waveguide adiabatic S-bends, input/output tapers and side gratings on a silicon-on-insulator (SOI) platform. A compact footprint and weak back reflections are obtained due to the bent waveguide and the tapers, respectively. An extremely high extinction ratio is achieved by scattering the undesired light in the slab section using the side gratings. The 3D FDTD simulations show a TE loss less than 0.3 dB and an extinction ratio greater than 30 dB over a 500 nm wavelength range (1200 nm to 1700 nm). Measured results show a high TM loss (> 35 dB) and a low TE insertion loss (< 1.5 dB), over a 200 nm wavelength range (1450 nm to 1650 nm). The measured TE loss is < 0.6 dB at a communication wavelength of 1550 nm. The footprint of the optimized design is 65 µm × 20 µm.

Broadband Transformation Acoustic Waveguide With Anisotropic Density Based on Pentamode Metamaterials

Multiple layer anisotropic fluid medium is critical to the realization of transformation acoustic devices, such as cloak or bend waveguide. Pentamode metamaterials have attracted extensive attention as a solid artificial version with anisotropic modulus to approximate liquids. In this paper, we present an approach to realize fluid-like anisotropic density by using pentamode materials, and an underwater bend acoustic waveguide with anisotropic density is designed and fabricated to demonstrate the effectiveness of it. Simulation results indicate that, compared with anisotropic-modulus design by using pentamode materials, wider bandwidth acoustic modulation effect can be obtained. An in-depth and comprehensive analysis of the mechanisms of the broadband characteristics is provided by calculating the band structure of the pentamode metamaterials constituting the acoustic waveguides and analyzing their vibration modes. Finally, remarkable wavefront manipulation for underwater acoustics based on the acoustic waveguide with anisotropic density is experimentally verified.

Investigation on Ultra-Compact 2DPC Coarse Wavelength Division Demultiplexer

A two dimensional Photonic Crystal (2DPC) based eight-channel wavelength division de-multiplexer is proposed and designed for Coarse Wavelength Division Multiplexing (CWDM) applications. The circular ring resonator, channel selector, circulator rod, L bend waveguide and linear bus waveguide are essential parts of the proposed system. The system’s functional parameters such as Transmission efficiency, resonant wavelength, spectral width, channel spacing, Quality factor and crosstalk investigated this paper. The eight different wavelengths of channels are filtered out by altering the size of channel selector rod, setting the radius of the circle shaped cavity and relative refractive index of circulator rod. Initially the Photonic Band gap (PBG) is manipulated by applying Plane Wave Expansion (PWE) method of the 2DPC structure. The functional parameters are analysed by Finite Difference Time Domain (FDTD) method in periodic and non-periodic structure of the proposed system to arrive normalized transmission spectrum. The resonant wavelengths of designed eight paths of the device are varying from 1420nm to 1460nm with average spectral width and channel spacing are 5.8nm, 5.6nm respectively. The footprint of the device is 286.84μm2. Hence, this small device can be implemented for CWDM systems in Photonic Integrated Circuits (PIC)

Investigation of the curvature distribution for TE01-to-TM11 mode converter

An oversized TE-to-TM mode converter based on waveguide bend structure is designed for high power transmission line. The curvature distribution of the waveguide bend is analyzed and optimized using mode coupling theory. Analysis results show that the curvature near the output port plays a decisive role in the content of the parasitic modes, which indicates that the restriction on curvature far from output port could be relaxed. Therefore, the asymmetric waveguide axis with enhanced curvature near the front end and lessened curvature near the output port will have a high conversion efficiency and a broad bandwidth. Combining the curvature optimization and the phase rematching technology proposed in our preliminary work, the TE-to-TM mode converter realizes a 90 bend with a conversion efficiency over 95% in the range of 26–32.2 GHz (the relative bandwidth over 21%). To verify the designed mode converter, a prototype is manufactured and measured.

Ultra-Compact Mode (De)Multiplexer and Polarization Beam Splitter Based on Tapered Bent Asymmetric Directional Couplers

Wepropose and experimentally demonstrate the ultra-compact mode (de)multiplexers and polarization beam splitter (PBS) based on tapered bent asymmetric directional couplers. The strong coupling strength introduced by tapered bent waveguides significantly shortens the coupling length. As a result, high coupling and conversion efficiencies over a broad bandwidth can be obtained with small footprint. The coupling length of the fabricated PBS and mode (de)multiplexers are less than 4.9 μm and 8.7 μm, respectively. From 1534-1600 nm, the measured insertion loss (IL) is less than 1.4 and 1.55 dB for the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> mode (de)multiplexers, respectively, and the crosstalk (CT) is less than −10 dB. The fabricated PBS shows low insertion loss of 0.55 dB and decent crosstalk of < −14 dB for both polarizations over an 85 nm wavelength range from 1505-1590 nm. Meanwhile, the larger fabrication tolerance ensures that the proposed devices can be readily fabricated with the standard complementary metal-oxide-semiconductor (CMOS) process.