Fabrication Tolerances Research Articles

Integrate and scale: a source of spectrally separable photon pairs

Integrated photonics is a powerful contender in the race for a fault-tolerant quantum computer, claiming to be a platform capable of scaling to the necessary number of qubits. This necessitates the use of high-quality quantum states, which we create here using an all-around high-performing photon source on an integrated photonics platform. We use a photonic molecule architecture and broadband directional couplers to protect against fabrication tolerances and ensure reliable operation. As a result, we simultaneously measure a spectral purity of 99.1±0.1%, a pair generation rate of 4.4±0.1MHzmW−2, and an intrinsic source heralding efficiency of 94.0±2.9%. We also see a maximum coincidence-to-accidental ratio of 1644±263. We claim over an order of magnitude improvement in the trivariate trade-off among source heralding efficiency, purity, and brightness. Future implementations of the source could achieve in excess of 99% purity and heralding efficiency using the lowest reported propagation losses.

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.

Integrated gallium phosphide-waveguide polarization rotator based on rotating a horizontal slot by 45 degree for 700 nm wavelength

Polarization manipulation elements operating at visible wavelengths represent a critical component of quantum communication sub-systems, equivalent to their telecom wavelength counterparts. The method proposed involves rotating the optic axis of the polarized input light by an angle of 45 degree, thereby converting the fundamental transverse electric (TE0) mode to the fundamental transverse magnetic (TM0) mode. This paper outlines an integrated gallium phosphide-waveguide polarization rotator, which relies on the rotation of a horizontal slot by 45 degree at a wavelength of 700 nm. This will ultimately lead to the conception of a mode hybridization phenomenon in the waveguide. The simulation results demonstrate a polarization conversion efficiency of 99.99% (99.98%) for TE0-to-TM0 (TM0-to-TE0) mode conversion, with an extinction ratio of 46.14 (39.62) dB and insertion loss below 1.6 dB at the specified wavelength. Additionally, the fabrication tolerance with regard to the width, height, and half-beat length of the proposed structure is investigated.

Enhancing single-mode guidance using avoided crossings in anti-resonant hollow-core fibers with five nested cladding tubes

We numerically analyze the nested anti-resonant nodeless fibers (NANFs) with five tubes to determine the best single-mode guidance and low confinement loss for the fundamental mode simultaneously. We derive the empirical relation between structural parameters that lead to avoided crossings between the core higher order modes (HOMs) and the cladding tube modes in a 5-tube nested design to achieve robust single-mode guidance. We report a low confinement loss of 1.17×10−3 dB/km and 0.23 dB/km, and huge HOM extinction ratio (HOMER) of the order of 105 and 104 at wavelengths of 650 nm and at 1550 nm respectively. We also investigate the fabrication tolerances of the confinement loss and HOMER to the changes in design parameters. These 5-tube NANFs exhibit a broad loss spectrum of 200 nm with loss of fundamental mode ¡ 0.003 dB/km and HOMER of 16000 centered around 650 nm. NANFs find applications in quantum communications, sensing and telecommunications and this work can be helpful in design optimization of such fibers.

Grating assisted temperature insensitive micro-ring resonator biosensor

A grating-assisted temperature-insensitive micro-ring resonator bio-sensor is reported using the finite-difference time-domain simulation method. In addition to choosing a negative thermo-optic coefficient (TOC) material, water, to nullify temperature-induced wavelength shift, optimization of the opto-geometric parameters provided a precisely zero-temperature sensitivity (TS) state of the structure. The negative TOC of water counterbalances the positive shift in resonance wavelength originating from Si waveguides. The grating structure delocalizes the core field into the analyte region, which enhances field–matter interaction to offer high bulk refractive index (RI) and affinity sensing. The advantage of incorporating grating structure in the ring has been demonstrated by comparing the bulk RI sensitivity and affinity sensitivity with and without the grating structure. Without the circular grating array, the bulk RI sensitivity is 214 nm RIU−1, and the affinity sensitivity is 0.139 nm nm−1. Using grating in the ring structure offers an improved bulk RI sensitivity of 525 nm RIU−1 and an adlayer sensitivity of 0.515 nm nm−1. We calculated the system limit of detection of the structure as ∼2 RIU. The fabrication tolerance of geometrical parameters, in the form of 10% variation in hole radius and 5% variation in other widths of our sensor, reveals a nominal TS of ∼6 pm ∘C−1. Changing the cladding analyte from pure water to other aqueous solutions, e.g. urine samples, also has a small impact on TS. The present work is useful in RI-dependent monitoring of the constituents present in aqueous medium-based bio-analytes samples in a variable ambient temperature environment.

A High-Sensitivity Fiber Biosensor Based on PVDF-Excited Surface Plasmon Resonance in the Terahertz Band

In this paper, a D-type photonic crystal fiber (PCF) with Zeonex material as the substrate and polyvinylidene fluoride (PVDF) material as the surface plasmon resonance (SPR) excitation layer is proposed for biosensing in the terahertz (THz) band. Analyzed with a finite element method, the proposed biosensor has shown excellent sensing properties for analyte refractive indices ranging from 1.32 to 1.45. With a maximum sensor resolution of 8.40 × 10−7 refractive index unit (RIU) and a figure of merit of 39.42 RIU−1, the maximum wavelength sensitivity and amplitude sensitivity can reach 335.00 μm/RIU and −66.01 RIU−1, respectively. A ±2% fabrication tolerance analysis is also performed on the biosensor to prove its practical feasibility. We conclude that our proposed PCF biosensor utilizing PVDF-excited SPR can provide high sensitivity, and thus a compact, label-free, and convenient solution for biomedical liquid sensing in the THz band.

Performance Study of Polarization Beam Splitters Based on Horizontal Slot Waveguide Operating at 700nm Wavelength

Abstract: Polarization beam splitter (PBS) integrated waveguides are the key components in the receiver of quantum key distribution (QKD) systems. Their function is to analyze the polarization of polarized light and separate the transverse-electric (TE) and transverse-magnetic (TM) polarizations into different waveguides. In this paper, a performance study of polarization beam splitters based on horizontal slot waveguide has been investigated for a wavelength of . PBS based on horizontal slot waveguide structure shows a polarization extinction ratio for quasi-TE and quasi-TM modes larger than with insertion loss below and a bandwidth of . Also, the fabrication tolerance of the structure is analyzed.

Optimization of hollow-core fibers with elliptical tubes for improved single-mode guidance

Hollow core fibers with elliptical tubes offer more design flexibility and can achieve better performance compared to their circular counterparts. We optimize the elliptical tubes of a hollow-core fiber to achieve a higher order mode extinction ratio (HOMER) exceeding 5500 along with a confinement loss of 0.012 dB/m in the telecommunication band; 4.7×10−4 dB/m with HOMER > 4300 in the visible band. We also derive an empirical relation between the structural parameters to obtain avoided crossings between the core higher order modes and the cladding modes to achieve the best single-mode guidance, besides achieving minimum confinement loss for the fundamental mode. We investigate the effect of bends on not just the confinement loss, but also on the HOMER. We have also included the dispersion and fabrication tolerances. These fibers find applications in quantum communication, sensing, and high-power laser delivery.

A new-generation approach to PAM-4 Based on the Fano resonance effect using a multimode interference structure

The goal of this research is to come up with a new optical structure for the generation of multilevel pulse amplitude modulation (PAM-4) signalling, which is used for optical interconnects and data center networks. The Fano effect is made with a structure made up of a 4x4 MMI coupler, feedback waveguides, and phase shifters. For the production of PAM-4, we employ the Fano effect. The new proposed approach can reduce power consumption and extend the linear region of the device transfer function compared with conventional approaches based on Mach Zehnder Interferometers (MZI) and ring resonators. We show that an extreme reduction of power consumption of 3 to 30 times, compared with the conventional structure based on MZI and ring resonators, is achieved. In addition, the proposed structure based on silicon wave guides has the advantages of extreme high bandwidth, large fabrication tolerance, and a compact footprint. The proposed structure can be applied to generate the PAM-4 based on the Fano effect with low power consumption, and it is suitable for complex systems with a lot of transceivers. The device is numerically simulated and designed using the Finite Difference Method (FDM) and Finite Difference Time Difference (FDTD). The PAM-4 structure based on silicon waveguides can be compatible with CMOS’s existing technologies.

Reconfigurable Generation of PAM-4 Signal Based on Fano Effect for Optical Interconnect Systems

We present a novel low-power architecture for the generation of multilevel pulse amplitude modulation (PAM-4) signal generation. A new structure of a micro-ring resonator based on a 4´4 MMI (Multimode interference) coupler is proposed to control the coupling coefficient and Fano shapes. Based on this structure, a high linearity of the transmission is created, compared with the Mach Zehnder Interferometer (MZI) conventional structure. Here, instead of using directional couplers and MZI as shown in the previous reports, in our method only two 4´4 multimode interference structures on silicon on insulator (SOI) waveguides has been used. The new design is compatible and suitable for the current CMOS fabrication technology. In this work, the special design is as follows: one of two MMI couplers is used to be a micro-ring resonator and two segmented phase shifters are used in the micro-ring resonator to generate 4 levels of the PAM-4 signal. The micro-ring resonator is controlled to work at the over-coupled region, so the Fano effect can be generated. This proposed PAM-4 architecture uses the generated Fano effect, therefore an extreme reduction in power consumption can be achieved. A large fabrication tolerance of ±500 nm and a compact footprint of 10´500 µm2 can be carried-out. The device is simulated and optimally designed using the FDTD (finite difference time domain) and EME (Eigenmode Expansion). This architecture can be useful for optical interconnects and data center network applications.

Inverse Design of Ultra-Compact and Low-Loss Optical Phase Shifters

In previous works, inverse design methods have primarily focused on manipulating the optical power to achieve specific design targets. In this paper, we use the inverse design method to enable the precise engineering of the optical phase. As a proof of concept, we present a series of phase shifters (PSs) with varying phase shifts, which are inversely designed and theoretically validated on the silicon-on-insulator (SOI) platform. The designed PSs exhibit remarkable accuracy, with deviations of below 1°. These proposed PSs feature an ultra-compact footprint of 3 µm in length and offer superior fabrication tolerances compared to conventional structures. Over the 1535–1565 nm bandwidth, the discrepancy between the actual and target phase shifts remains below ±1° for all phase shifters, while the insertion loss is consistently below 0.035 dB. Moreover, the feasibility of the designed five PSs is verified using 2 × 2 multimode interference couplers (MMI).

A Planar Shared-Aperture Co-Polarized Array Antenna With a High Isolation for Millimeter-Wave Full Duplex Communication

We provide the design of a co-polarized simultaneous transmit and receive (STAR) array antenna, aiming to address the issue of undesired isolation in the antenna domain of the millimeter-wave (mm-Wave) full-duplex co-polarized communication system. As the cornerstone for the design and development of the antenna, the guided-wave structures of the virtual-electric-wall (VEW)-based ridge substrate-integrated waveguide (RSIW) and ridge parallel plate waveguide (RPPW) are proposed and applied to the full-duplex antenna design. Compared with conventional RSIW, VEW-based RSIW has the merits of lower ohmic loss and releasing the fabrication tolerance in the array antenna. Additionally, on the basis of parallel plate waveguide (PPW), the proposed RPPW exhibits a Chebyshev filtering response. The radiating part of the proposed array employs a slot-pair array excited along orthogonal directions by using the preceding guided-wave structures. Among them, the RSIW array serves as the Tx antenna, and the RPPW array is regarded as the Rx antenna. To achieve co-polarized radiation patterns in the boresight direction, the substrate-integrated waveguide (SIW) feed network is integrated into the reverse side of the antenna, providing differential and common-mode excitations for the RSIW-excited and RPPW-fed arrays. Furthermore, the inter-port isolation is effectively ameliorated by inserting the mushroom-type metamaterial between linear arrays. As a proof-of-concept, a shared-aperture full-duplex array operating in the Q-band is developed and fabricated to demonstrate the above concept by using standard printed circuited board technology. The measured overlapped impedance bandwidth covers from 38.18 GHz to 39.13 GHz. The measured Tx-to-Rx isolation is greater than 25.5dB in the Q-band. The measured peak gains in Tx- and Rx-mode are 18.4 dBi and 17.8 dBi. The simulation and measurement results show good consistency, verifying the effectiveness of the proposed array in inter-port isolation, array size, planarization, and lightweight, making it suitable for mm-Wave shared-aperture full-duplex communication systems.

Effects of Porous Transport Layer Coating Inhomogeneities on PEM Water Electrolyzer

The porous transport layer (PTL) is a critical component of polymer electrolyte membrane (PEM) water electrolyzers, transporting water to the anode catalyst layer, conducting electrons, and removing the produced oxygen. Ti materials such as felts, mesh, and sintered powders are commonly used as PTLs at the anode due to the good corrosion resistance and conductivity of Ti. However, Ti forms an electrically resistive oxide surface layer that has detrimental effects on the performance and durability of PEM electrolyzer cells. In order to permit the use of Ti PTLs in PEM electrolyzers, platinum group metals (PGMs) such as Pt and Ir can be used as a conductive, protective layer [1-3]. However, use of PGM coatings adds to the cost of PEM electrolyzers, and it is likely that future systems will use thinner coatings to minimize cost. To implement the PEM water electrolysis on a larger scale, it is essential to minimize the cost of mass-manufactured cell components while ensuring that inhomogeneities do not adversely affect the performance and durability of the cell and ultimately the stack. Improved understanding of impact of inhomogeneities on cell performance and durability is key to enabling this goal.This study investigates the impacts of PTL coating inhomogeneities on the performance of PEM water electrolyzers. Coating inhomogeneities of various geometries were artificially made on Ti felt PTLs to investigate the effects on the cell performance and understand fabrication tolerances. In addition, a model is developed to analyze the impacts of coating geometry on the surface current density distribution and local electric potential of the cell. Reference Kang, Z., et al., Effects of interfacial contact under different operating conditions in proton exchange membrane water electrolysis. 2022. 429: p. 140942.Liu, C., et al., Constructing a Multifunctional Interface between Membrane and Porous Transport Layer for Water Electrolyzers. 2021. 13(14): p. 16182-16196.Liu, C., et al., Exploring the Interface of Skin ‐Layered Titanium Fibers for Electrochemical Water Splitting. Advanced Energy Materials, 2021. 11(8): p. 2002926.

Hollow-core pear-shaped conjoined-tube fiber with low loss in the ultraviolet band

PCTF (pear-shaped conjoined-tube fiber) is presented as a new ultraviolet (UV) guiding fiber with low loss. Results indicate that two PCTFs have better properties than that of previous studies in the UV band. The total loss of two PCTFs is less than 1 dB/km, and its bandwidth exceeds 150 nm between 0.2 and 0.4 μm. Furthermore, PCTF's single-mode performance is very promising, as evidenced by the higher-order mode extinction ratio (HOMER) over 103. The fabrication tolerance is discussed in this paper and results show that the tolerance is good enough to fabricate by normal fiber drawing process. This fiber is promising in applications for nonlinear optics, ultrafast optics, high power laser, and quantum optics.

Cost-efficiency enhancement of X- and Ku-band split waveguides for industrial accelerators

Compact microwave electron linear accelerators must operate at high frequencies in order to maintain tight SWaP (size, weight, and power) requirements, set by modern applications. Currently, the frequencies are practically limited to X- and Ku-bands because of the availability of MW-class magnetrons, required to achieve beam energies of several MeV, and tight fabrication tolerances. The latter is associated with micron-level accuracy and further waveguide frequency tuning process, both increasing its costs dramatically. By implying a novel split structure fabrication approach, it is possible to relax these tolerances to the values of 10–20 μm and eliminate the tuning step, greatly reducing their total costs. This paper presents the results of microwave measurements of split X- and Ku-band accelerating structures of different types and discusses the benefits of our fabrication approach.

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.

Arbitrary-ratio 1 × 2 optical power splitter based on thin-film lithium niobate.

Optical power splitters (OPSs) have been widely used in photonic integrated circuits, but an OPS with a large fabrication tolerance and free choice of power splitting ratio (PSR) is still highly desired for thin-film lithium niobate (TFLN) platform. Here, we propose and experimentally demonstrate several 1 × 2 OPSs with PSRs from 50:50 to 5:95 using TFLN platform. The proposed devices are built by multimode interference structure to achieve a broad bandwidth and large fabrication tolerance. Various PSRs can be obtained by adjusting the geometry structure of the multimode interference region. All of our fabricated devices feature an insertion loss lower than 0.3 dB at the wavelength of 1550 nm, and a PSR variation less than 3% in the range of 1520 nm to 1590 nm.

A universal and stable metasurface for photonic quasi bound state in continuum coupled with two dimensional semiconductors

The strong coupling of excitons to optical cavity modes is of immense importance when understanding the fundamental physics of quantum electrodynamics at the nanoscale as well as for practical applications in quantum information technologies. There have been several attempts at achieving strong coupling between excitons in two-dimensional semiconductors, such as transition metal dichalcogenides (TMDCs) and photonic quasi-bound states in the continuum (BICs). We identify two gaps in the platforms for achieving strong coupling between TMDC excitons and photonic quasi-BICs: firstly, in the studies so far, different cavity architectures have been employed for coupling to different TMDCs. This would mean that typically, the fabrication process flow for the cavities will need to be modified as one moves from one TMDC to the other, which can limit the technological progress in the field. Secondly, there has been no discussion of the impact of fabrication imperfections in the studies on the strong coupling of these subsystems so far. In this work, we address these two questions by optimizing a cavity with the same architecture, which can couple to the four typical TMDCs (MoS2, WS2, MoSe2, WSe2) and perform a detailed investigation on the fabrication tolerance of the associated photonic quasi-BICs and their impact on strong coupling.

High-Directionality Silicon Nitride Antenna Based on Distributed Bragg Reflector for Optical Phased Array

Optical phased arrays (OPAs) have great potential in the fields of integrated solid-state light detection and ranging. The ranging distance of an OPA can be further enlarged by improving the directionality of the grating antenna. A high-directionality silicon nitride grating antenna with a distributed Bragg reflector (DBR) is proposed. The DBR consists of a stack of silicon nitride and silicon dioxide layers, which are utilized as the bottom reflectors to further reduce downward radiation. In a simulation, the directionality of the antenna exceeded 71.6% within the wavelength range of 1420–1740 nm. Additionally, the directionality of the antenna can achieve 97.6% at 1550 nm. Compared to a grating antenna without a DBR, the directionality is improved by 1.52 dB. Moreover, the proposed silicon nitride grating antenna has a large fabrication tolerance and is compatible with CMOS fabrication techniques, showing great potential for enhancing the performance of the integrated optical phased array.

Design and numerical analysis of a novel photonic crystal fiber based chemicals sensor in the THz regime

We presented a decagonal cladding and hexahedron core-based photonic crystal fiber (PCF) to sense chemicals in the terahertz frequency (THz). Circular air holes (CAHs) in the cladding region make up the proposed sensor. A wide variety of frequencies were evaluated to analyze the sensor's performance in terms of sensitivity, confinement loss, and effective material loss respectively. We designed and quantitatively analyzed the optical properties of our proposed hexahedron-based PCF sensor using the finite element method (FEM). Square-shaped air hole length, strut, and core size have also been researched to improve the performance of the proposed sensor's sensing components and fabrication tolerance. At ideal conditions, the suggested PCF sensor has a maximum relative sensitivity of 94.65%, confinement loss of 6.01 × 10−8 cm−1, effective material loss (EML) of 9.16 × 10−4 cm−1, and effective mode area (EMA) of 1.35 × 10−7 m2. We are confident that the suggested sensor's optimized geometrical structure will be manufacturing-friendly, as well as the sensor's contribution to practical uses. Furthermore, our proposed PCF fiber will be ideal in the terahertz (THz) regions for various optical communication applications and medicinal signals.