High Fabrication Tolerance Research Articles

High-tolerance reconfigurable MZI racetrack resonator on a 3-µm-thick SOI photonics platform

Integration of resonators impacts the utilization of the 3-µm-thick silicon-on-insulator (SOI) platform in photonics integrated circuits (PICs). We propose an integrated resonator leveraging a deep-etch silicon waveguide. Through the utilization of a tunable coupler based on multimode interferometers (MMIs), the resonator achieves high fabrication tolerance and reconfigurability. In a critical-coupling state, it serves as a filter with an extinction ratio (ER) of 23.5 dB and quality (Q) factor of 3.1×105, operating within the range of 1530–1570 nm. In an extreme over-coupling state, it functions as a large-bandwidth delay line, offering continuous change in delay time of 22 ps, nearly wavelength-independent. This work provides devices to the 3-µm-thick silicon photonics device library, enriching the potential applications of this technology platform.

A High-Efficiency Wideband Grating Coupler Based on Si3N4 and a Silicon-on-Insulator Heterogeneous Integration Platform.

To fully utilize the advantages of Si3N4 and Silicon-On-Insulator to achieve a high-efficiency wideband grating coupler, we propose and numerically demonstrate a grating coupler based on Si3N4 and a Silicon-On-Insulator heterogeneous integration platform. A two-dimensional model of the coupler was established and a comprehensive finite difference time domain analysis was conducted. Focusing on coupling efficiency as a primary metric, we examined the impact of factors such as grating period, filling factor, etching depth, and the thicknesses of the SiO2 upper cladding, Si3N4, silicon waveguide, and SiO2 buried oxide layers. The calculations yielded an optimized grating coupler with a coupling efficiency of 81.8% (-0.87 dB) at 1550 nm and a 1-dB bandwidth of 540 nm. The grating can be obtained through a single etching step with a low fabrication complexity. Furthermore, the fabrication tolerances of the grating period and etching depth were studied systematically, and the results indicated a high fabrication tolerance. These findings can offer theoretical and parameter guidance for the design and optimization of high-efficiency and broad-bandwidth grating couplers.

Inverse-designed broadband low-loss grating coupler on thick lithium-niobate-on-insulator platform

A grating coupler on 700-nm-thick Z-cut lithium-niobate-on-insulator platform with high coupling efficiency, large bandwidth, and high fabrication tolerance is designed and optimized by inverse design method. The optimized grating coupler is fabricated with a single set of e-beam lithography and etching process, and it is experimentally characterized to possess peak coupling efficiency of −3.8 dB at 1574.93 nm, 1 dB bandwidth of 71.7 nm, and 3 dB bandwidth of over 120 nm, respectively.

Low-loss hollow-core fiber with stadium-shaped nested tubes for near-infrared communications

We propose a hollow-core fiber structure consisting of multilayer nested stadium-shaped tubes (HC-SSF), designed to reduce attenuation in the near-infrared bands for communications. The HC-SSF exhibits low loss characteristics, resulting in a four-order-of-magnitude attenuation reduction in confinement loss. The proposed HC-SSF achieves a confinement loss of 3.42×10−9 dB/m at 1.30 μm, while maintaining favorable single-mode characteristics and excellent bend resistance. At a bend radius of 8 cm, the HC-SSF demonstrates a bend-introduced loss of 1.72×10−7 dB/m at 1.30 μm. The total loss of HC-SSF remains below 0.1 dB/km within the 1.15 μm to 2.0 μm range. Due to its high fabrication tolerance and simple structure, the HC-SSF offers valuable reference for the extensive application of hollow-core fiber in the field of communication. It has a wide range of promising applications in gas and liquid sensing, and optical communications.

Cladding modulated silicon waveguide Bragg grating with TM-polarized light for optical true time delay line

An optical true time delay line (OTTDL) is an essential component in optical signal processing. A Bragg grating structure is usually employed in OTTDL design, which makes it possible to achieve a slow-light effect. Here, we present the design and experimental demonstration of a cladding modulated waveguide Bragg grating (CMWBG) that supports transverse magnetic polarization for OTTDL application. In contrast to traditional waveguide Bragg gratings that support transverse electric polarization, the proposed structure has low propagation loss and high fabrication tolerance characteristics. The slow-light effect of the CMWBG was experimentally demonstrated with a high group index value. An averaged maximum group delay value of up to 84.4 ps was achieved at a length of 1 mm CMWBG, and a tuning range of about 81.4 ps was obtained.

Ultra-Compact Ultra-Broadband Two-Mode Transverse-Electric Based SWG Multiplexer Demonstrated at 64 Gbps

We designed and experimentally demonstrated an ultra-compact and ultra-broadband two-mode de-multiplexer based on a subwavelength grating (SWG) asymmetrical coupler (ADC) on a silicon-on-insulator (SOI) platform. The device consists of a cascaded strip waveguide tapered to a multimode segment and a SWG-based single-mode waveguide. Tapering the strip waveguide in multi-segments guarantees a high fabrication tolerance with a wider wavelength range of operation. The parameters of the periodic structure are chosen to meet the Bragg condition and phase-match the TE1 mode of the strip waveguide. These features produce a faster mode evolution with a shorter coupling length of only 8.5 µm and an entire mode division multiplexing (MDM) link (Mux and Demux) of only 55 µm. The MDM link operates over a wide wavelength range, covering the entire S-, C-, and L-bands. The insertion loss (IL) and crosstalk (CT) achieved at 1550 nm are better than 1 dB and -18 dB, respectively. The device also has low fabrication sensitivity to deviations in the waveguide's parameters. Additionally, the transmission performance of the link is demonstrated at 1550 nm wavelength using non-return-to-zero (NRZ) on-off keying (OOK) modulation, where clear eye diagrams are obtained at 64 Gbps.

3D Graphene Composite Foams for Efficient and Stable Solar Desalination of High‐Salinity Brine

Solar‐driven interfacial evaporation (SIE) is a promising desalination technology that utilizes solar energy and seawater. However, most SIE systems are limited by high cost, complex fabrication, and low salt tolerance. Herein, a simple method is presented for preparing a 3D graphene/carbon nanotubes/polypyrrole foam (GCPF) evaporator that exhibits high evaporation efficiency, excellent mechanical performance, and stable solar desalination of high‐salinity brine. The excellent processability of melamine foam enables the easy preparation of the 3D GCPF, which exhibits an enhanced evaporation rate of 4.012 kg m−2 h−1 and a photothermal conversion efficiency of 97.8% under one sun irradiation. Compared with the 2D sample, the GCPF‐3 evaporator utilizes solar energy more efficiently and maintains higher evaporation performance even when the light angle changes. Moreover, the 3D GCPF shows remarkable salt tolerance and durability, achieving a stable evaporation rate of 4.005 kg m−2 h−1 for 30 h with a salinity of 25 wt%, which is the best‐reported result among solar desalination systems. This work provides new insights into the design of 3D graphene composite foams and demonstrates their potential applications in continuous solar desalination.

On-Chip E00-E20 Mode Converter Based on Multi-Mode Interferometer.

Mode converters is a key component in mode-division multiplexing (MDM) systems, which plays a key role in signal processing and multi-mode conversion. In this paper, we propose an MMI-based mode converter on 2%-Δ silica PLC platform. The converter transfers E00 mode to E20 mode with high fabrication tolerance and large bandwidth. The experimental results show that the conversion efficiency can exceed -1.741 dB with the wavelength range of 1500 nm to 1600 nm. The measured conversion efficiency of the mode converter can reach -0.614 dB at 1550 nm. Moreover, the degradation of conversion efficiency is less than 0.713 dB under the deviation of multimode waveguide length and phase shifter width at 1550 nm. The proposed broadband mode converter with high fabrication tolerance is promising for on-chip optical network and commercial applications.

Fundamental optical logic gate operations using optically controlled two surfaces plasmonic polariton mode coupler based on graphene clad waveguide

Here, an optical pulse controlled 2 × 2 surface–plasmon–polariton (SPP) mode interference coupler based on graphene clad is designed for the operation of optical fundamental gates. Exploiting the light–matter interactions on graphene–silicon interface and nonlinear optical property of graphene, coupling characteristics are obtained as a function of extra phase change between the excited SPP modes (fundamental and first order) modes with incidence of optical pulse. The device with a very small coupling length of ∼4.3 μm shows optical NOT, AND, and OR gate operation. Our compact design with high fabrication tolerance opens an avenue for the development of a complex optical processor.

A 3-dB 90 degrees microstrip hybrid directional coupler at 2.27 GHz

A 90° hybrid-coupler in the 2.27 GHz range, offering high performance despite its size of 115.64 mm2 (18.36 λg), is introduced. The main factor that distinguishes this proposed design, positively, is that the device demonstrates high fabrication tolerances due to its simplicity, and uncomplicated length dimensions. At the center of the structure are 9 metal squares, all of equal size and equidistant. It has been observed that the S-parameters remain relatively the same after increasing or decreasing the side lengths, substrate parameters, and the vertical separation dimensions. Based on these results, it has been shown that the design will provide a successful fabrication tolerance. According to the simulation results, the S-parameters are as follows: S11 = −24.01, S12 = −3.063, S13 = −3.063, and S14 = –23.47 (all in dB). The phase deviation is only 0.8°. After the simulation studies, a prototype of the coupler was fabricated, and laboratory measurements were made. The simulated and measured results, have desirable agreement. Finally, a few similar studies presented in the literature are discussed with a comparative table, in terms of overall size, isolation, phase deviation, and structure complexity. Thus, the advantages of this presented structure, compared to these studies, have been revealed.

Low-loss fiber-to-chip edge coupler for silicon nitride integrated circuits.

Silicon nitride (SiN) integrated optical waveguides have found a wide range of applications due to their low loss, broad wavelength transmission band and high nonlinearity. However, the large mode mismatch between the single-mode fiber and the SiN waveguide creates a challenge of fiber coupling to these waveguides. Here, we propose a coupling approach between fiber and SiN waveguides by utilizing the high-index doped silica glass (HDSG) waveguide as the intermediary to smooth out the mode transition. We achieved fiber-to-SiN waveguide coupling efficiency of lower than 0.8 dB/facet across the full C and L bands with high fabrication and alignment tolerances.

Rapid Design Optimization and Calibration of Microwave Sensors Based on Equivalent Complementary Resonators for High Sensitivity and Low Fabrication Tolerance

This paper presents the design, optimization, and calibration of multivariable resonators for microwave dielectric sensors. An optimization technique for the circular complementary split ring resonator (CC-SRR) and square complementary split ring resonator (SC-SRR) is presented to achieve the required transmission response in a precise manner. The optimized resonators are manufactured using a standard photolithographic technique and measured for fabrication tolerance. The fabricated sensor is presented for the high-resolution characterization of dielectric substrates and oil samples. A three-dimensional dielectric container is attached to the sensor and acts as a pool for the sample under test (SUT). In the presented technique, the dielectric substrates and oil samples can interact directly with the electromagnetic (EM) field emitted from the resonator. For the sake of sensor calibration, a relation between the relative permittivity of the dielectric samples and the resonant frequency of the sensor is established in the form of an inverse regression model. Comparisons with state-of-the-art sensors indicate the superiority of the presented design in terms of oil characterization reliability. The significant technical contributions of this work include the employment of the rigorous optimization of geometry parameters of the sensor, leading to its superior performance, and the development and application of the inverse-model-based calibration procedure.

Proposal of mosaic-based 2 × 2 3-dB couplers for prospective on-chip communication in mid-infrared wavelengths around 2.1 µm

We propose two types of mosaic-based 2 × 2 3-dB couplers for wavelengths around 2.1 µm. For type I, the proposed device provides excess loss of < 0.22 dB and imbalance of < ± 0 . 07 d B over the wavelength range from 2.06 to 2.14 µm. Compared with the standard multimode-interference-based 2 × 2 3-dB coupler, the imbalance is improved by ∼ 0.4 d B . For type II, coupling length in the multimode-interference region is about three times shorter than the one based on multimode-interference theory. It provides excess loss of < 0.49 d B and imbalance of < ± 0 . 23 d B over the wavelength range from 2.06 to 2.14 µm. Both proposed devices show high fabrication tolerance to hole diameter variation. Especially for type II, the fabrication-tolerant direct binary search was effective to obtain such a structure. These mosaic-based 2 × 2 3-dB couplers will play an important role in highly integrated photonic circuits.

Thin-film lithium niobate polarization modulator without polarization diversity.

With the development of photonic integrated circuits and optical information processing on thin-film lithium niobate (TFLN), the realization of the TFLN-based polarization device is becoming more and more crucial. Here, we demonstrate a polarization modulator on the TFLN platform without polarization diversity. Without polarization manipulation elements, the device only composes a phase modulator and a two-dimensional grating coupler. The structure features small footprint and high fabrication tolerance. The device holds promise for polarization encoding telecommunication.

Silicon-based flat-topped wavelength filter for low crosstalk, high fabrication tolerance and wideband operating range for CWDM applications

A flat-topped wavelength filter with low crosstalk and high productivity is proposed and theoretically analyzed. The proposed filter scheme is composed of multiple cascade-connected delayed interferometers with several multimode interference couplers with 50:50 and 28:72 split ratios and a directional coupler with a 92:8 split ratio. Theoretical results based on analytic and numerical simulations exhibit flatband spectra over a wavelength range of > 100 nm and low crosstalk of < −20 [dB] with potentially better production yield for fabrication imperfections.

Mode division multiplexing on an InP membrane on silicon

A five-channel mode division multiplexed on-chip optical bus is designed and demonstrated on a submicron-thick InP membrane wafer bonded on a Si substrate. Dual-core adiabatic tapers are leveraged for realization of the mode (de)multiplexers. The optimized device shows low excess optical loss of maximum 0.14 dB compared to a reference waveguide and low cross talk of maximum −18.5 dB for all five channels over a broad optical bandwidth of 1510–1600 nm. High fabrication tolerance to width variations is also demonstrated, where a low excess loss of less than 1 dB and a low cross talk of less than −14 dB are maintained in the range of 1530–1585 nm, covering the C-band, when the width varies by up to 50 nm. The demonstrated results show an essential step toward a monolithic photonic layer on top of electronic chips for high-capacity on-chip optical interconnects.

Efficient Low Threshold Frequency Conversion in AlGaAs-On-Insulator Waveguides

A design study is presented for an efficient, compact and robust device to convert the frequency of single-photons from the near-infrared to the telecom C-band. The material platform aluminum gallium arsenide (AlGaAs)-on-insulator, with its relatively large second-order nonlinearity, is used to create highly confined optical modes. This platform can feasibly incorporate single-photon emitters such as indium arsenide (InAs) on gallium arsenide (GaAs), paving the way towards direct integration of single-photon sources and nonlinear waveguides on the same chip. In this design study, single-pass difference-frequency generation (DFG) producing C-band single-photons is enabled via form birefringent phase-matching between a 930 nm single-photon pump and continuous wave (CW) idler at 2,325 nm. In particular the idler and single-photons are combined with an on-chip directional coupler, and then tapered to a single waveguide where the three modes are phase-matched. The design is studied at a special case, showing high fabrication tolerances, and an internal conversion efficiency up to 41%.

Perfect vortex beam with polarization-rotated functionality based on single-layer geometric-phase metasurface.

Perfect vortex (PV) beam has seen significant advances in fields like particle manipulation, optical tweezers, and particle trapping, due to the fact that its ring radius is independent of the topological charge. Although geometric-phase metasurfaces have been proposed to generate PV beams, it always relies on circularly or elliptically polarized incident light, which hinders the miniaturization of compact optical devices. Here, using orthogonal decomposition of polarization vectors (ODPV), we proposed a geometric-phase metasurface, which breaks the dependence of circular polarization, to generate PV beam. In the design of the metasurface, we introduced PV phase profiles corresponding to the left-handed circularly polarized (LCP) component and the right-handed circularly polarized (RCP) component into the metasurface based on the principle of ODPV. We further determined the rotation angle of each nanostructure of the metasurface by calculating the argument of the composite vector of LCP and RCP in the transmission field. Simulation results show that the proposed geometric-phase metasurface can generate the PV beam upon the illumination of a linearly polarized incident. Moreover, the PV beam with polarization-rotated functionality is achieved by setting the polarization rotation angle. Furthermore, dual PV beams with orthogonal polarization states is realized at the same time by superimposing two sets of phase profiles on a single metasurface. It is also demonstrated that the PV beam parameters, such as ring radius and/or topological charge, can be set on demand in the metasurface design. The proposed metasurface has the exceptional advantage of high fabrication tolerance and is optical path miniaturization friendly, and will open a new avenue in advanced compact and integrated optical systems.

In-Fiber Butt-Coupled Spherical Microcavity With Whispering Gallery Mode and Fabry-Perot Resonances

An integrated in-fiber coupling configuration for simultaneously exciting whispering gallery mode (WGM) and Fabry-Perot (FP) resonances in one cavity is proposed and demonstrated experimentally. The WGMs traveling circularly in both clockwise and counter-clockwise directions are simultaneously excited by perpendicularly injecting the light from the single-mode-fiber (SMF) core into a polymeric microsphere embedded in the femtosecond laser micromachined coupling structure. While the standing wave between the two axial interfaces of the microsphere and its external media results in a spherical FP resonator, meaning that the WGM and the FP resonances coexist in the same spherical cavity. The superposition of WGM and FP resonances results in asymmetric Fano-like lineshape, which is of high importance for optical fiber sensors, filters, and switches due to its sharp slope around the resonant wavelength. Experimental results show that asymmetric spectral lineshape can be achieved even as the spatial coupling gap is larger than the wavelength, indicating that the proposed coupler has higher fabrication tolerance than the evanescent field-based coupling configuration. Such an integrated coupling configuration provides an effective method for the coupling between the optical fiber and WGM cavities made of any functional material by liberating them from the phase-matching condition and enriching the functionality by importing other resonance into the same cavity.

Ultrasensitive Broadband Refractometer Based on Single Stress-Applying Fiber at Dispersion Turning Point

Optical microfiber-based refractometer, which relies on modal interference operating near the dispersion turning point (DTP), has attracted a high level of research interest. The ultrahigh sensitivity can be achieved by mitigating the group effective refractive index difference between the modes in the coupling. Through modifying the microfiber diameter, the group effective refractive index difference can be modified to zero which is defined as DTP. However, all the DTP-based sensors report low fabrication tolerance and narrow operation bandwidth of probing wavelength for the high-sensitivity region where the refractive index (RI) sensitivities higher than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> nm/RIU can be achieved, which extremely limits the practical applications. To address the challenge, we manipulate and optimize the mode dispersion by using a tapered specially-designed single stress-applying fiber (SSAF). By introducing the stress-applying part (SAP) into the cladding, the wavelength sensitivity of the group effective RI difference can be significantly reduced through the flexibly engineerable control over the material index. In this way, the operation bandwidth is dramatically broadened for 500 nm in comparison with that of conventional microfiber with the same diameter based on DTP, indicating a broad range of optional probing wavelength and high fabrication tolerance. We experimentally verify the ultrahigh sensitivity and broadband operation around the DTP using an SSAF-based microfiber with a waist diameter of about 2.3 μm. The maximum RI sensitivity of 30563 nm/RIU is obtained.