Fundamental Transverse Electric Mode Research Articles

Reverse design of multifunctional demultiplexing devices

A novel direct binary search algorithm based on rotation (RDBS) is proposed in this paper. A 1*2 ultra-compact 2.4*3.6µm2 multimode wavelength demultiplexer (DEMUX) is designed in reverse, which is roughly two orders of magnitude smaller than the size of a conventional waveguide device. It can simultaneously perform wavelength demultiplexing and mode conversion. This DEMUX separates the 1310 and 1550 nm wavelengths while converting the input light from the fundamental transverse electric mode (TE0) to the first-order transverse electric mode (TE1). The simulation results using RDBS show that the insertion loss(IL) of the upper channel (wavelength 1310 nm) is −0.9644 dB, the IL of the lower channel (wavelength 1550 nm) is −0.9752 dB, and the crosstalk values(CT) are −10.079 dB and −9.261 dB, respectively.

Compact and wideband multiport plasmonic coupler–splitter based on coding metamaterials

We propose plasmonic coupler–splitters based on embedded coding metamaterials. The fundamental transverse electric mode in a silicon waveguide is coupled to the surface plasmon polariton mode in plasmonic waveguides with high efficiency of nearly 95% and wide bandwidth of 1.45–1.65 µm, and then light is split into different output ports. To improve the performance of the devices, a multi-objective optimization scheme based on the non-dominated sorting genetic algorithm-II assisted by the direct-binary search method is applied in the design of coding metamaterials. The simulated results exhibit that utilizing our inverse design scheme, the plasmonic coupler–splitter can perform effective plasmonic coupling (total coupling efficiencies of more than 92%) and high-quality power splitting with various split ratios (split ratio errors lower than 1%) and output directions at a broad bandwidth (1.45–1.65 µm) simultaneously. The function of the frequency split can also be implemented in the same universal basic scheme. In comparison to previous works, we not only propose flexible coupler–splitter designs for various functions based on a universal design scheme, but also introduce effective algorithms for the inverse design of optical devices based on coding metamaterials.

1550‐nm Band Soliton Microcombs in Ytterbium‐Doped Lithium‐Niobate Microrings

AbstractMicro‐optical frequency combs are miniaturized coherent light sources exhibiting a tremendous influence on precision spectroscopy, optical clocks, and high‐speed optical communications. The rare‐earth‐doped lithium niobate (LN) is a promising platform to integrate the lasers and comb sources on the same chip of single material. However, microcombs generated in rare‐earth‐doped LN thin film (LNTF) have not yet been fully explored. To explore the protocols of generating optical combs in ytterbium‐doped LNTF, the comb‐like laser is studied in the 1060 nm band, and the dissipative Kerr soliton is experimentally demonstrated in the telecom band in a Z‐cut wafer. The over‐coupled fundamental transverse electric mode with anomalous dispersion is excited at an on‐chip pump power of 75.7 mW to generate the robust bright soliton. The broadband spectrum of the Kerr soliton ranges from 1480 to 1660 nm with 110 comb lines, whose repetition rate is 197.8 GHz. The mode‐locking of the soliton can be achieved as a result of the photorefractive effect of ytterbium‐doped LNTF instead of complex feedback schemes. Numerical simulations are highly accordant with the experimental results. This work expands the microcomb‐generating platform and increases the potential for integrated on‐chip applications including precision ranging and frequency division.

Compact and efficient polarization rotator using laterally asymmetric rib waveguides on a lithium-niobate-on-insulator platform.

Photonic devices based on a lithium-niobate-on-insulator (LNOI) are current research hotspots; however, owing to the high refractive index contrast of the LNOI platform and inherent birefringence of lithium niobate itself, such photonic devices are generally polarization sensitive, affecting their further wide application. This paper proposes a simple, compact, and efficient polarization rotator (PR) based on a laterally asymmetric rib waveguide by depositing a layer of semi-infinite silicon nitride dielectric material on one side of the rib waveguide. The results show that a PR with a polarization rotation region length of 15.77µm is achieved, and the polarization extinction ratio (PER), insertion loss (IL), and polarization conversion efficiency (PCE) are 38.57/68.95 and 0.2/0.22dB, and 99.99%/almost 100%, respectively, for the fundamental transverse electric mode (T E 0) and transverse magnetic mode (T M 0) at a 1.55µm wavelength. The operation bandwidth is around 120nm for the T E 0 mode and T M 0 mode when the PER, IL, and PCE are greater than 20dB, less than 0.32dB, and more than 99%, respectively. Fabrication tolerances to the key structural parameters are investigated in detail. In addition, the evolution fields of the T E 0 mode and T M 0 mode along the propagation direction through the proposed device are presented.

Compact, ultrabroadband and temperature-insensitive arbitrary ratio power splitter based on adiabatic rib waveguides.

We propose a compact, ultrabroadband and temperature-insensitive adiabatic directional coupler based on rib silicon waveguide-enabling arbitrary splitting ratios. Simulation results show that the device can achieve arbitrary splitting ratios from 1400 to 1600nm, equal to 50%:50%, 60%:40%, 70%:30%, 80%:20%, and 90%:10% for the fundamental transverse electric mode. The designed device has an excess loss of less than 0.19dB on the operational waveband. Furthermore, the proposed device shows a great robustness to fabrication imperfection, with a waveguide width deviation of 50nm and ambient temperature change from 0°C to 200°C. These properties make the design a potential candidate for ultrahigh-density photonic integration chips.

All-silicon and ultra-compact transverse electric-pass polarizer with large bandwidth and high polarization extinction ratio based on subwavelength metamaterials

In this paper, we have proposed an all-silicon and compact transverse electric (TE)-pass polarizer with large bandwidth and high polarization extinction ratio (PER), where the polarizer uses a semi-arc waveguide to cause polarization-dependent bending loss (PDBL). The combination of the subwavelength grating (SWG) and double adiabatic tapers serve as an equivalent cladding to get strengthened PDBL. The parameters of SWG and double adiabatic tapers are analyzed in detail to improve the performance of the device. In the proposed TE-pass polarizer, the injected fundamental TE mode (i.e., TE0) gets protection and the injected fundamental transverse magnetic mode (i.e., TM0) lets out. The footprint is only about 6 μm × 10 μm, which is substantially reduced compared with the previous counterparts. From the results, its bandwidth is as high as 228 nm (from 1.482 to 1.710 μm) with PER > 39 dB and low excess loss (EL < 1 dB), and it has an ultra-high PER of 42.5 dB and a low EL of 0.1 dB at 1.55 μm. In addition, large fabrication tolerances of 20 nm ( ± 10 nm) to the key structural parameters are analyzed.

Experimental characterization of an ultra-broadband dual-mode symmetric Y–junction based on metamaterial waveguides

Silicon photonic integrated circuits routinely require 3-dB optical power dividers with minimal losses, small footprints, ultra-wide bandwidths, and relaxed manufacturing tolerances to distribute light across the chip and as a key building block to form more complex devices. Symmetric Y–junctions stand out among other power splitting devices owing to their wavelength-independent response and a straightforward design. Yet, the limited resolution of current fabrication methods results in a minimum feature size (MFS) at the tip between the two Y–junction arms that leads to significant losses for the fundamental mode. Here we propose to circumvent this limitation by leveraging subwavelength metamaterials in a new type of ultra-broadband and fabrication-tolerant Y–junction. An exhaustive experimental study over a 260 nm bandwidth (1420 nm – 1680 nm) shows excess loss below 0.3 dB for the fundamental transverse-electric mode (TE0) for a high-resolution lithographic process (MFS ∼ 50 nm) and < 0.5 dB for a fabrication resolution of 100 nm. Subwavelength Y–junctions with deterministically induced errors of ± 10 nm further demonstrated robust fabrication tolerances. Moreover, the splitter exhibits excess loss lower than 1 dB for the first-order transverse-electric mode (TE1) within a 100 nm bandwidth (1475 nm – 1575 nm), using high-resolution lithography.

Ultracompact Polarization Splitter–Rotator Based on Shallowly Etched Subwavelength Gratings and Anisotropic Metasurfaces

Polarization splitter–rotators (PSRs) are an essential component in on-chip polarization-sensitive and polarization–division multiplexing systems. In this work, we propose an ultracompact and high-performance silicon-based polarization splitter–rotator utilizing anisotropic metasurfaces, which is the first to combine the two, to our knowledge. The tilted periodic metasurface structure has different modulation effects on different polarized light fields, such as the transverse–electric (TE) mode and the transverse–magnetic (TM) mode, which are beneficial for designing polarization management devices. According to the results, the entire length of the silicon PSR was ~13.5 μm. The TE-to-TM conversion loss and polarization conversion ratio ere −0.154 dB and 96.5% at 1.55 μm, respectively. In the meanwhile, the cross talk and reflection loss were −27.0 dB and −37.3 dB, when the fundamental TE mode was input. The insertion loss and cross talk were −0.19 dB and −25.01 dB at the central wavelength when the fundamental TM mode was input. In addition, the bandwidth reached up to ~112 nm with polarization conversion loss and insertion loss higher than −0.46 dB and −0.36 dB. The simulations also show that the designed devices had good fabrication tolerance.

Efficient mode converters and filters using asymmetrical directional couplers with subwavelength gratings.

Mode-division multiplexing (MDM) is a promising solution to improve data transmission capacity for future optical interconnect networks. Mode converters and mode filters play a key role in on-chip MDM systems. Here, we propose and experimentally demonstrate a device, enabling mode conversion and filtering simultaneously, which is composed of asymmetrical directional couplers with subwavelength gratings, in a small footprint of 14.7 µm × 1.42 µm. The device can realize optical mode conversion between the first-order transverse electric (TE) mode and the fundamental TE mode, and can also filter the fundamental TE mode efficiently. The conversion efficiency is over 95%, with a broad 1 dB bandwidth over 80 nm and a high mode extinction ratio of >29 dB. As a mode filter, strong mode elimination of >30 dB is achieved.

Inverse design of an air-cladding and fully-etched silicon polarization rotator based on a taper-based mode hybridization

In the regular photonic integrated circuits, many optical devices are designed on a silicon-on-insulator platform with a top silicon layer and an air cladding by the fully-etched procedure because of the simple fabrication. Based on a taper-based mode hybridization, a polarization rotator consisting of the polarization-conversion and mode-conversion parts is realized by the forward-design-assisted inverse design. The polarization rotator transforms the fundamental transverse magnetic mode to the first order transverse electric mode and then converted to the fundamental transverse electric mode. The polarization rotator with a total length of about 10μm exhibits the insertion loss of less than 1.1 dB and the extinction ratio of larger than 20.5 dB from 1540 nm to 1560 nm. In addition, we design another same-functionality polarization rotator with a total length of about 13μm. The insertion loss and the extinction ratio are less than 0.8 dB and larger than 24.3 dB, which investigate that appropriately improving the initial structure quality is conducive to the direct-binary-search algorithm convergence.

Terahertz Airy beam generated by Pancharatnam-Berry phases in guided wave-driven metasurfaces.

Metasurface antennas scatter traveling guided waves into spatial waves, which act as extendable subsources to overcome the size limitation on emission sources. With the use of a Pancharatnam-Berry phase metasurface stimulated by a circularly polarized wave in a waveguide, the local phase distributions of scattered spatial waves can be made consistent with those of an Airy beam, thereby allowing the generation of high-quality Airy beams. In a slab waveguide, circularly polarized waves are synthesized through superposition of in-plane transverse electric modes. Simulations demonstrate that a 20 mm × 20 mm footprint all-dielectric guided wave-driven metasurface generates a 2D Airy beam at a frequency of 0.6 THz. Furthermore, we employ a metasurface deposited on a strip waveguide to generate a 1D Airy beam under direct stimulation by the fundamental transverse electric mode. Our work not only provides a large-scale emitter, but it also suggests promising potential applications in on-chip imaging and holography.

Octave-spanning microcomb generation in 4H-silicon-carbide-on-insulator photonics platform

Silicon carbide has recently emerged as a promising photonics material due to its unique properties, including possessing strong second- and third-order nonlinear coefficients and hosting various color centers that can be utilized for a wealth of quantum applications. Here, we report the design and demonstration of octave-spanning microcombs in a 4H-silicon-carbide-on-insulator platform for the first time, to our knowledge. Such broadband operation is enabled by optimized nanofabrication achieving &gt; 1 million intrinsic quality factors in a 36-μm-radius microring resonator, and careful dispersion engineering by investigating the dispersion properties of different mode families. For example, for the fundamental transverse-electric mode whose dispersion can be tailored by simply varying the microring waveguide width, we realized a microcomb spectrum covering the wavelength range from 1100 nm to 2400 nm with an on-chip power near 120 mW. While the observed comb state is verified to be chaotic and not soliton, attaining such a large bandwidth is a crucial step towards realizing f − 2 f self-referencing. In addition, we also observed a coherent soliton-crystal state for the fundamental transverse-magnetic mode, which exhibits stronger dispersion than the fundamental transverse-electric mode and hence a narrower bandwidth.

Photonic crystal nanobeam cavities based on 4H-silicon carbide on insulator

The 4H-silicon carbide on insulator (4H-SiCOI) has recently emerged as an attractive material platform for integrated photonics due to its excellent quantum and nonlinear optical properties. Here, we experimentally realize one-dimensional photonic crystal nanobeam cavities on the ion-cutting 4H-SiCOI platform. The cavities exhibit quality factors up to 6.1×103 and mode volumes down to 0.63 × (λ/n)3 in the visible and near-infrared wavelength range. Moreover, by changing the excitation laser power, the fundamental transverse-electric mode can be dynamically tuned by 0.6 nm with a tuning rate of 33.5 pm/mW. The demonstrated devices offer the promise of an appealing microcavity system for interfacing the optically addressable spin defects in 4H-SiC.

Design and simulation of a compact three-mode (de)multiplexer based on a subwavelength grating multimode interference coupler

This paper reports, for the first time to the best of our knowledge, the design of a compact three-mode converter/(de)multiplexer (DE/MUX) based on a 3 × 3 subwavelength grating (SWG) multimode interference (MMI) coupler. The device uses a π/2 phase shifter and an asymmetric tri-junction coupler built in a silicon/silicon-on-insulator (SOI) platform. The inherent dispersion and anisotropy properties of the SWG structure are employed to increase the traditional MMI coupler operation bandwidth and reduce the overall dimensions of the DE/MUX. The proposed device can realize mode order converting and (de)multiplexing of the fundamental transverse electric mode TE0, the first-order transverse electric mode TE1, and the second-order transverse electric mode TE2 to three uniform TE0 with good performance and compact size compared with the conventional ones. The designed device's overall area is 4.5 × 120 µm2, and its performance is simulated using Rsoft photonics CAD suit 3D-BPM. The simulation results show that the device exhibits a maximum insertion loss of 1.3 dB, maximum crosstalk of − 15 dB, and 120 nm (3 dB-bandwidth) over the entire C-band.

Ultra-compact and broadband silicon polarizer employing a nanohole array structure.

An ultra-compact and broadband transverse magnetic (TM)-pass polarizer is experimentally demonstrated using a photonic crystal nanohole structure. By engineering the period of the circular holes, the fundamental transverse electric mode is suppressed due to the bandgap of the nanohole array, while the T M 0 mode propagates with a negligible insertion loss. Simulation results predict that the bandwidth of the device can reach 245 nm with a 33 dB polarization extinction ratio (PER). In the experiment, an insertion loss <1.2d B for the T M 0 mode and a PER over 20.8 dB are demonstrated in a 70 nm wavelength range from 1520 to 1590 nm, mainly limited by the grating couplers used for fiber-to-chip coupling. The fabricated device is compact with a total length of 7.21 µm.

Silicon Mode (de)Multiplexer Based on Cascaded Particle-Swarm-Optimized Counter-Tapered Couplers

In this paper, a design of a silicon mode (de)multiplexer based on cascaded counter-tapered couplers is proposed and investigated. By using the particle swarm optimization algorithm and finite difference time domain method, structural parameters of each counter-tapered coupler in our proposed mode (de)multiplexer are optimized so that high conversion efficiencies can be obtained and coupling lengths can be significantly shortened. The coupling lengths of counter-tapered couplers in the proposed silicon four-mode (de)multiplexer operating in transverse-electric polarization are 8 μm, 9 μm, and 10 μm. The corresponding conversion efficiencies are respectively 98.98%, 99.98%, and 98.68%. Experimental results show that, the fabricated device exhibits a demultiplexed crosstalk of -32.76 dB, -27.10 dB, -32.45 dB, or -33.29 dB at 1550 nm wavelength, when the fundamental transverse-electric mode is respectively launched into the ports Input0, Input1, Input2, or Input3. The measured demultiplexed crosstalk is less than -15.15 dB from 1525 to 1596 nm and lower than -18.12 dB from 1525 to 1561 nm.

Inverse-Designed Photonic Jumpers With Ultracompact Size and Ultralow Loss

Photonic integrated circuits are an ideal platform for chip-scale computation and communication. A complex photonic network usually requires components to direct light from one waveguide to another that is parallel to the former with an arbitrary separation d . Traditionally, this is achieved with S-shaped waveguide bends, which either occupy a large footprint or suffer from large insertion loss. To reduce the footprint and maintain ultralow insertion loss, here we propose and experimentally demonstrate inverse-designed photonic jumpers on a silicon-on-insulator platform. These photonic jumpers have ultracompact sizes by achieving the expected function within a footprint of d × 2.5 μm, while the simulated and measured insertion loss for the fundamental transverse-electric mode maintains below 0.14 and 0.30 dB, respectively, in the wavelength range of 1.50–1.60 μm. Therefore, these photonic jumpers can be used in large-scale photonic integrated circuits as a standard module to significantly enhance the integration density.

Ultra-compact polarization rotator based on mode coupling in a groove-like waveguide, assisted by subwavelength grating

A novel (to the best of our knowledge) scheme of an ultra-compact polarization rotator (PR) is based on an optical-axis-rotated section of a groove-like waveguide with a subwavelength grating (SWG) structure, which is used to replace one of the silicon strip waveguides in the polarization conversion section. In the proposed scheme, the mode coupling occurs, and the mode conversions between fundamental transverse magnetic mode ( T M 0 ) and fundamental transverse electric mode ( T E 0 ) are realized. At the operating wavelength of 1.55 µm, the simulated extinction ratio (ER) of ∼ 41 d B , insertion loss (IL) &lt; 1 d B , and reflection loss (RL) &lt; − 22 d B for the T M 0 - t o - T E 0 conversion with a total conversion length of 5.4 µm have been obtained. The simulated transmission spectra also show a relatively broad bandwidth of 53 nm (41 nm) for the T M 0 - t o - T E 0 ( T E 0 - t o - T M 0 ) conversion to keep the E R &gt; --> 25 d B ; as well as a high polarization conversion ratio of 99.95% (99.31%) for the T M 0 - t o - T E 0 ( T E 0 - t o - T M 0 ) process at 1.55 µm. The presence of SWG greatly enhances the device performance, and the tolerance to fabrication uncertainties has also been investigated.

Terahertz Beam Steering Based on Trajectory Deflection in Dielectric-Free Luneburg Lens

Free-space transmission of terahertz waves opens great opportunities for wireless applications including communications and radar, enabling higher information capacity, higher spatial resolution, and yet smaller apertures than using microwaves. Nevertheless, due to the shorter wavelengths, it involves a severe path loss. To compensate for the path loss, point-to-point transmission by beam steering is indispensable. However, the implementation of broadband and low-loss beam steering is still challenging in the terahertz range due mainly to the lack of practical phase shifters. To circumvent this issue, here we demonstrate a novel approach of terahertz beam steering based on trajectory deflection in a dielectric-free Luneburg lens. It converts a point excitation into a deflected beam in the fundamental transverse electric mode inside parallel conducting plates based on graded effective refractive index, which is then launched into free-space. The absence of dielectric medium contributes to reduce the insertion loss and also enables external control of the effective refractive index. Importantly, this approach is assisted by angular leverage; a very little change of the plate tilt results in very large deflection of the beam trajectory. We demonstrate beam steering from -25° to +25° by changing the plate tilt from -25' to +25' (-0.42° to +0.42°), which is transferred to the 60 times larger angles. Such a small tilt can easily be generated, for example, by using piezo or MEMS actuators. Since the excitation point can be fixed, the proposed device can easily be coupled to a wide range of chip sources and detectors, offering waveguide integration. As an application example, we implement high-resolution radar that identifies both the direction and range toward an object.

Sub-wavelength grating assisted mode order converter on the SOI substrate.

We have designed three mode order converters using the sub-wavelength grating on the silicon-on-insulator substrate. The proposed mode order converters can separately realize the mode order conversion from the fundamental transverse electric mode to the first-order transverse electric mode (TE0-to-TE1), the second-order transverse electric mode (TE0-to-TE2) and the third-order transverse electric mode (TE0-to-TE3) with compact device sizes and good performances. The simulation results show that the mode order conversion efficiencies of TE0-to-TE1, TE0-to-TE2 and TE0-to-TE3 are larger than 94.4%, 95.7% and 83.7% in the wavelength ranging from 1.5 µm to 1.6 µm, the corresponding device length are 8.72 µm, 4.98 µm and 14.54 µm. In addition, the mode order converter can be fabricated with only once etching which will be advantage to the device fabrication.