Fundamental TE Mode Research Articles

TE-polarized leaky-wave beam launchers: Generation of Bessel and Bessel–Gauss beams

The generation of focused beams in the millimeter- and submillimeter-wave ranges, with transverse-electric (TE) polarization, is investigated in the radiative near-field region. The desired field distribution is achieved through a leaky-wave beam launcher consisting of a grounded dielectric slab with an annular strip grating on top excited by a circular slot on the bottom ground plane. The latter is fed by a Marié transducer, which converts the input, fundamental TE10 mode of a standard rectangular waveguide into the higher-order TE01 mode propagating in the circular waveguide connected to the device. The generation of TE-polarized diffraction-limited Bessel and Bessel–Gauss distributions is achieved by suitably synthesizing the annular strip grating. Simulated results are in excellent agreement with those predicted by leaky-wave analysis providing a proof-of-concept for the generation of TE-polarized Bessel and Bessel–Gauss beams at 300 GHz with a beam size of 1.7 and 1.9 mm up to the nondiffractive range of about 25 and 15 mm from an aperture plane with radius of 12.75 mm, respectively.

Bidirectional multi-beam with multi-polarizations tensor holographic metasurface using bilayer anisotropic elements.

In this paper, we present and design a tensor holographic metasurface (THMS) capable of radiating multiple beams with different polarizations in both forward and backward directions. The proposed THMS manipulates the fundamental TE mode surface waves (SWs) to produce bidirectional leaky-wave radiation using bilayer anisotropic metal patches. To achieve the independent control of the direction and polarization of bidirectional beams, the nature of the capacitive impedance element supporting TE mode SWs is exploited. A dual-feed scheme exciting orthogonal TE SWs is proposed for THMS generating circularly polarized beams with good performance. As a proof of concept, a bidirectional THMS integrated with two WR-28 waveguide sources based on the orthogonal-feeds scheme, which simultaneously radiate linear- and circular-polarized four beams, is designed, fabricated, and measured at 35 GHz. The experiment results are in good agreement with the simulated ones, achieving a peak gain of 19.2 (19.6) dBi, a -3 dB gain bandwidth of 9.14% (4.3%), for circularly-polarized (linearly-polarized) beams and a 3 dB axial ratio bandwidth of 13% for circularly-polarized beams. The sidelobe levels and cross polarization levels of all beams are below -15 dB and -22 dB, respectively.

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

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

A 53–125 GHz chip to double‐ridge rectangular waveguide transition

AbstractA novel chip to double‐ridge rectangular waveguide transition, with an ultra‐broadband operation bandwidth exceeding three standard rectangular waveguide (WR) bands of WR15, WR12, and WR10 is demonstrated for the first time in this letter, to the best of authors' knowledge. This bandwidth is achieved by coupling the microstrip mode directly to the fundamental TE mode of the waveguide. A unique feature of this transition is that the transition design can be implemented on substrates with a high dielectric constant, which makes it suitable for integration with monolithic microwave integrated circuits (MMICs). A back‐to‐back transition is realized using a split‐block assembly and 254 thickness alumina substrate ( at 80 GHz). The latter acts as an MMIC substrate substitute, including a tapered microstrip and a 50 coplanar waveguide pad for probe‐based measurement. Two novel mechanical assembly concepts, involving die‐attach‐foil and indium solder, are used to build two split‐block prototypes. The simulated and measured S‐parameters of the transition are demonstrated from 35 to 125 GHz.

Low loss and ultra-broadband design of an integrated 3 dB power splitter centered at 2 µm.

Because chemical gas is sensitive to absorption in the 2µm band, and 2µm matches the absorption band of the remote sensing material, many remote sensors and optical sensors are designed to operate in the 2µm wavelength region. In this paper, we designed an integrated 3dB power splitter centered at 2µm. The study of this device is built on a silicon-on-insulator (SOI) platform. We introduced a subwavelength grating (SWG) to improve the performance of the device. We used the three-dimensional finite-difference time-domain (3D FDTD) method to analyze the effect of the structure on the power splitter. The insertion loss (IL) of the fundamental TE mode is only 0.04dB at 2µm and its bandwidth of IL <0.45d B is 940nm (1570-2510nm). It is suitable for multidomain and all-band photonic integrated circuits at 2µm.

Experimental demonstration of high-Q MRR based on a germanium-on-insulator platform with an yttria insulator in the mid-IR range

We experimentally demonstrate an all-pass microring resonator (MRR) based on a Y2O3 BOX germanium-on-insulator (GeOI) platform operating in the mid-IR region. The ring resonator was numerically designed to have a high quality (Q) factor in the 4.18 μm to 4.22 μm wavelength range in the fundamental TE mode. According to our design, the GeOI ring resonator was fabricated by the direct wafer-bonding technology with an yttria (Y2O3) buried oxide layer, which is transparent at the mid-IR region, for the bonding interface and the electron beam lithography. The experimental resonant characteristic was obtained using our fiber-based mid-IR measurement setup. The GeOI single MRR exhibited an extinction ratio (ER) of 15.28 dB and an insertion loss (IL) of 1.204 dB, and the racetrack showed an ER of 22.77 dB and an IL of 0.627 dB. Furthermore, the free spectral range of the device was 5.29 nm, and the loaded Q factor of 94,528 (176,158 of intrinsic Q factor) was extracted by the nonlinear least squares method. We believe this demonstration of our GeOI MRR offers a valuable opportunity to implement multipurpose devices such as optical sensors, switches, and filters in the mid-IR range.

Inverse-designed counter-tapered coupler based broadband and compact silicon mode multiplexer/demultiplexer.

A mode multiplexer/demultiplexer (MUX/DeMUX) is a crucial component for constructing mode-division multiplexing (MDM) systems. In this paper, we propose and experimentally demonstrate a wide-bandwidth and highly-integrated mode MUX/DeMUX based on an inverse-designed counter-tapered coupler. By introducing a functional region composed of subunits, efficient mode conversion and evolution can be achieved, greatly improving the mode conversion efficiency. The optimized mode MUX/DeMUX has a size of only 4 µm × 2.2 µm. An MDM-link consisting of a mode MUX and a mode DeMUX was fabricated on the silicon-on-insulator (SOI) platform. The experimental results show that the 3-dB bandwidth of the TE fundamental mode and first-order mode can reach 116 nm and 138 nm, respectively. The proposed mode MUX/DeMUX is scalable and could provide a feasible solution for constructing high-performance MDM systems.

Silicon-based compact eight-channel wavelength and mode division (de)multiplexer for on-chip optical interconnects.

A compact wavelength and mode division (de)multiplexer is proposed for multiplexing a total of eight guided TE modes of a 220nm thick silicon-on-insulator waveguide with input channels at two wavelengths of 1.55 and 2µm for wavelength division multiplexing. The (de)multiplexer is composed of four sequentially arranged sections with bus waveguides of increasing widths. The first section uses an asymmetric directional coupler to couple one TE mode at 1.55µm, while each of the next three sections consists of two collocated directional couplers to simultaneously couple two TE modes of the bus waveguide, one at each wavelength of 1.55 and 2µm. Three linear adiabatic tapers are designed to connect the consecutive bus waveguides. The fundamental TE mode of the bus waveguide at 1.55 or 2µm is coupled by using another adiabatic taper from a single-mode input waveguide. The simulation results show that over a broad bandwidth of >100n m the insertion loss and crosstalk for both wavelength bands is <1.15d B and <-27d B, respectively. In addition, a compact device footprint with a total coupling length of ∼61µm is achieved due to the use of collocated directional couplers in three sections.

Elimination of the fundamental mode hybridization on an x-cut lithium-niobate-on-insulator by using a densely packed bent waveguide array.

Lithium niobate (L i N b O 3, LN) is a promising material for integrated photonics due to its natural advantages. The commercialization of thin-film LN technology has revitalized this platform, enabling low-loss waveguides, micro-rings, and compact electro-optical modulators. However, the anisotropic birefringent nature of X-cut LN leads to mode hybridization of TE and TM modes, which is detrimental to most polarization-sensitive integrated optical waveguide devices. A novel structure, to the best of our knowldege, utilizing a densely packed bent waveguide array is presented in this paper to eliminate mode hybridization. The refractive index is modulated in a manner that eliminates the avoided crossing of the refractive index curves of the TE and TM fundamental modes; thus, mode hybridization is prevented. The structures are readily accessible in the full range of commercially available LN film thicknesses from 400 to 720nm and in any etching depth. The proposed structures give a polarization extinction ratio of -30d B across all bend radii, while simultaneously maintaining low excess loss of less than -1d B after reaching a 100µm bend radius.

A Silicon-Based On-Chip 64-Channel Hybrid Wavelength- and Mode-Division (de)Multiplexer

An on-chip 64-channel hybrid (de)multiplexer for wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) is designed and demonstrated on a 220 nm SOI platform for the demands of large capacity optical interconnections. The designed hybrid (de)multiplexer includes a 4-channel mode (de)multiplexer and 16-channel wavelength-division (de)multiplexers. The mode (de)multiplexer is comprised of cascaded asymmetric directional couplers supporting coupling between fundamental TE mode and higher-order modes with low crosstalks in a wide wavelength range. The wavelength-division (de)multiplexers consist of two bi-directional micro-ring resonator arrays for four 16-channel WDM signals. Micro-heaters are placed on the micro-resonators for thermal tuning. According to the experimental results, the excess loss is &lt;3.9 dB in one free spectral range from 1522 nm to 1552 nm and &lt;5.6 dB in three free spectral ranges from 1493 nm to 1583 nm. The intermode crosstalks are −23.2 dB to −33.2 dB, and the isolations between adjacent and nonadjacent wavelength channels are about −17.1 dB and −22.3 dB, respectively. The thermal tuning efficiency is ∼2.22 mW/nm over one free spectral range.

A TM11 High-Order Mode Leaky Wave Antenna

A novel leaky-wave antenna (LWA), which operates 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">$TM_{11}$ </tex-math></inline-formula> high-order mode, is proposed. This LWA achieves a 4.6 times higher gain than the traditional TE10-based LWAs. An analytical formulation is derived, which shows that a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$TM_{11}$ </tex-math></inline-formula> -based LWA couples more energy to free space through an appropriately designed slot than traditional TE10-based LWAs. Simulations are used to compare the performance of our proposed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$TM_{11}$ </tex-math></inline-formula> -based LWA with a similar LWA that operates at its fundamental TE10 mode. Specifically, it is shown that our <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$TM_{11}$ </tex-math></inline-formula> high-order mode LWA achieves 6.67 dB higher gain than a TE10-mode LWA that operates at the same frequency. Also, a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$TM_{11}$ </tex-math></inline-formula> -mode LWA is designed to mitigate the open-stopband (OSB) problem. This design achieves a fan beam with a maximum gain of 15.4 dBi and a scanning elevation range of 18° (through the broadside direction) in a bandwidth of 2 GHz. Prototypes of our LWA designs are manufactured using a cost-effective additive manufacturing method (i.e., selective laser melting), and their performance is measured. The measurements exhibit good agreement with simulations, thereby validating the proposed designs.

Reconfigurable THz Fixed-Frequency Beamscanning Through a Liquid-Crystal-Loaded Leaky Waveguide Operating in Its Fundamental TE Mode

Reconfigurable THz Fixed-Frequency Beamscanning Through a Liquid-Crystal-Loaded Leaky Waveguide Operating in Its Fundamental TE Mode

Simulation of Silicon Oxycarbide Waveguides for Shorter Band Photonics

In this research paper, we design and simulate Silicon Oxycarbide (SiOC) channel waveguides and photonic passive device such as directional coupler. SiOC channel waveguides are designed for different values of width and height at shorter wavelength band that is 1310 nm wavelength window with refractive index of SiOC (ncore) =2.2 μm and SiO2 (nclad) = 1.444 μm. TE and TM fundamental mode is achieved at width = 1.5 microns and height = 0.5 microns to understand the single mode operation of SiOC channel waveguides. The minimum bending radius of SiOC waveguide is found to be 30 microns that is large enough to integrate the large scale devices. Directional coupler is designed to measure the coupling power between waveguides with gap 0.6, 0.7 and 0.8 microns that gives the 3dB coupling at 20, 40 and 100 microns. In this research, silicon oxycarbide is presented as potential material platform for the highly efficient photonic devices.

Lithium Niobate Thin Film Polarization Beam Splitter Based on Asymmetric Directional Coupling

Polarization devices on lithium niobate thin film (LNTF) are vital components for photonic integrated circuits (PICs) on the novel platform. Because both linear and nonlinear devices on the anisotropic platform are intrinsically polarization-dependent. Here, we demonstrate compact polarization beam splitters (PBS) on LNTF based on two asymmetric directional couplers with an effective length of only 180 μm. The PBS is fabricated via a single-step electron beam lithography (EBL) and inductively coupled plasma (ICP) etching process. Our device shows efficient PBS operation at the telecom bands with an extinction ratio over 26 dB and 13 dB for fundamental TE and TM modes, respectively. The device holds promise for efficient on-chip polarization division multiplexing and polarization manipulation, which would become an indispensable component in future LNTF PICs.

Gain and Bandwidth Improvement Studies of Millimeter Wave Periodically Dielectric Loaded Gyro-Twystron Amplifier

In this article, the spurious oscillations in the output section of gyro-twystron have been analyzed and the design and modeling of its various subassemblies also been presented. The periodic dielectric loading (PDL) technique has been adapted in the output waveguide section to improve the device gain by suppressing the backward wave oscillations (BWOs). Further, to improve the performance of the amplifier all the cavities were staggered tuned. The particle-in-cell (PIC) simulation of the present PDL gyro-twystron predicted an RF output power of ~120 kW in the fundamental TE01 mode at 94 GHz. The conversion efficiency, 3-dB bandwidth, and saturated gain were calculated as ~34%, ~2.2 GHz, and ~51 dB, respectively. A triode magnetron injection gun (MIG) was designed for 60 kV and 6 A gyrating electron beam and optimized for the velocity spread of ~2.2% with a velocity ratio of 1.6. A single-stage depressed (SSD) collector has also been designed to enhance the efficiency of the amplifier to ~57%. A double-disk window was designed and optimized for the broader bandwidth of ~8 GHz.

Broadband and low-loss TM-pass polarizer using tilted subwavelength structures

Photonic systems built on the Silicon-on-Insulator platform exhibit a strong birefringence, and must thus be operated with a single polarization for most applications. Hence, on-chip polarizers that can effectively suppress an undesired polarization state are key components for these systems. Polarizers that extinguish TE polarized light while letting TM polarized light pass with low losses are particularly challenging to design for the standard 220 nm Silicon-on-Insulator platform, because the modal confinement is stronger for TE polarization than for TM polarzation. Here, we propose and design a broadband, low loss and high extinction ratio TM-pass polarizer by engineering a Bragg grating that reflects the fundamental TE mode into the first order TE mode using a subwavelength metamaterial which at the same time allows the TM mode to pass. Our device achieves an extinction ratio in excess of 20 dB, insertion losses below 0.5 dB and back-reflections of the fundamental TE mode of the order of -20 dB in a bandwidth of 150 nm as demonstrated with full 3D-FDTD simulations.

Generalized Bimode Equivalent Circuit of Arbitrary Planar Periodic Structures for Oblique Incidence

This work presents, for the first time, a generalized bimode Foster’s equivalent circuit for characterization of 2-D planar periodic structures (PPSs) with arbitrary geometry at oblique incidence. It considers the interactions between the fundamental TE and TM modes without any restriction within the bimode bandwidth of the geometry. The proposed circuit is only composed of frequency-independent LC elements, which can be extracted systematically from electromagnetic (EM) simulations. The reactive immittances obtained in the process fulfill Foster’s theorem, enabling the design process of PPS-based devices using standardized synthesis techniques from the circuit theory. To demonstrate its viability and general nature, equivalent circuits are extracted for different single-layer and multilayer PPS composed of rotated dipoles under oblique incidence <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula> = 20° and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> = 30° and including dielectrics. Excellent agreement is found between the response of the circuit model and the EM simulation in all cases. Finally, to validate experimentally the proposed equivalent circuit and highlight its applicability, a 90° reflective linear-polarization (LP) rotator centered at 25 GHz and under oblique incidence, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\theta $ </tex-math></inline-formula> = 30° and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varphi $ </tex-math></inline-formula> = 0° (TE), is designed, manufactured, and tested. The agreement between the circuit response, the EM simulation, and the measurement underlines the potential of the new equivalent circuit for PPS design under oblique incidence.

Design of a compact and highly sensitive modal interferometer using hybrid modes of a dielectric loaded plasmonic waveguide.

We show the presence of hybridization between fundamental TE and first higher-order TM modes in a dielectric loaded plasmonic waveguide of appropriately chosen core dimensions. Furthermore, a critical hybridization point is achieved at which both modes have nearly equal fraction of the TE and TM polarizations. Exploiting the interference among such modes, we propose the design of a compact and highly sensitive modal interferometer. The bulk and surface sensitivities of the proposed sensor are found to be ∼3-10µm/RIU for refractive index (RI) ∼1.33-1.36 and ∼0.7nm/nm for an adsorbed layer of RI 1.45, respectively. The proposed sensor gives robust performance against fabrication imperfections and is stable against temperature fluctuations due to extremely low temperature cross-sensitivity (∼10-15pm/∘C for a temperature change up to ∼100∘C).

Design and Optimization of a Compact Ultra-Broadband Polarization Beam Splitter for the SCL-Band Based on a Thick Silicon Nitride Platform

The polarization beam splitter is an essential photonic integrated circuit in applications where a high-performing on-chip polarization diversity scheme is required. The lower refractive index contrast of the silicon nitride material platform compared to silicon-on-insulator constitutes the separation of polarized light states a challenging task since for this purpose a large difference between the effective refractive indices of the fundamental TE and TM modes is highly desirable. In this paper, we present the design and optimization analysis of an ultra-broadband polarization beam splitter based on a thick silicon nitride platform through extensive 3D-FDTD simulations. The proposed device exploits two different Si3N4 thicknesses that enable the discrimination of the two polarizations at the proximity of an 800 nm thick slot and a 470 nm thick strip waveguide via directional coupling. The proposed two-stage PBS achieves higher than 30.6 dB polarization extinction ratio (PER) for both TE and TM polarizations across a 130 nm span at the SCL-band. The dimensions of the PBS are 94 × 14 μm2 and the insertion losses are calculated to be lower than 0.8 dB for both polarizations. The fabrication tolerance of the device is also discussed.

Design and Measurement of a Novel Overmoded TE01 Mode Converter for a Rectangular Gyro-TWT.

The rectangular gyrotron traveling wave tube (gyro-TWT) with a large aspect ratio (α) has the potential to achieve megawatt-class output power. As an essential component of gyro-TWT, a novel overmoded Ka-band mode converter with an α of 6.19 is designed, analyzed, and cold tested in this paper. Based on the magnetic dipole moment theory, the rectangular overmoded TE01 mode is excited via the rectangular fundamental TE10 mode. The cutoff waveguide is applied to prevent electromagnetic wave transport to the magnetron injection gun (MIG) region and also guarantee higher power electron beam transportation. Simulations predict an operation bandwidth higher than 4 GHz and greater than 99.8% mode purity between 33–37 GHz. To verify this design, the mode converter is manufactured and cold tested. The back-to-back measurement results exhibit a good agreement with the simulation. With similar topologies, the rectangular overmoded TE01 mode can be excited in a different α.