Waveguide Core Research Articles

Metal-printing tunable interlayer waveguide coupler using low-loss fluorinated polycarbonate.

Tunable three-dimensional (3D) integrated optical waveguide chips with optical interconnection function are beneficial to expand the application of optical devices in a 3D integrated photonic module. Here, we propose a thermo-optic (TO) tunable interlayer waveguide coupler based on the metal-printing technique. Low-loss fluorinated polycarbonate (AF-Ali-PC MA) and poly (methyl methacrylate-glycidyl methacrylate) [P(MMA-co-GMA)] are synthesized as waveguide core and cladding layer, respectively. The thermal stability and optical adsorption characteristics of AF-Ali-PC MA are analyzed. Optical signal transmission features of the interlayer coupling waveguides are simulated. The optical response properties and fabrication process flows of a dynamic multilayer waveguide chip can be greatly improved by the metal-printing technique. The on-off time of the TO interlayer coupling chip is obtained as 250 µs, and the electrical power consumption is measured as 7.6 mW. To the best of our knowledge, this is the first time that a TO tunable interlayer waveguide coupler is achieved by an efficient metal-printing method, which is suitable for large-scale photonic integrated circuit (PIC) systems and multilayer optical interconnection (OXC) networks.

Liquid Core Waveguide Cell with In Situ Absorbance Spectroscopy and Coupled to Liquid Chromatography for Studying Light-Induced Degradation

In many areas, studying photostability or the mechanism of photodegradation is of high importance. Conventional methods to do so can be rather time-consuming, laborious, and prone to experimental errors. In this paper we evaluate an integrated and fully automated system for the study of light-induced degradation, comprising a liquid handler, an irradiation source and exposure cell with dedicated optics and spectrograph, and a liquid chromatography (LC) system. A liquid core waveguide (LCW) was used as an exposure cell, allowing efficient illumination of the sample over a 12 cm path length. This cell was coupled to a spectrograph, allowing in situ absorbance monitoring of the exposed sample during irradiation. The LCW is gas-permeable, permitting diffusion of air into the cell during light exposure. This unit was coupled online to LC with diode array detection for immediate and automated analysis of the composition of the light-exposed samples. The analytical performance of the new system was established by assessing linearity, limit of detection, and repeatability of the in-cell detection, sample recovery and carryover, and overall repeatability of light-induced degradation monitoring, using riboflavin as the test compound. The applicability of the system was demonstrated by recording a photodegradation time profile of riboflavin.

Low power optical phase shifter using liquid crystal actuation on a silicon photonics platform

Low-power and compact phase shifters are crucial for large photonic circuits, both to cope with variability and to create programmable waveguide circuits scaling to thousands of tuning elements. This work demonstrates a liquid crystal phase shifter where there is a lateral silicon electrode "rail" on one side of the waveguide core. Using this architecture, a strong quasi-static electric field E actuation can be applied over the gap, which is filled with liquid crystal cladding material, with modest voltages. Because the mode is largely confined in the waveguide, optical losses are limited, compared to earlier experiments with slot waveguides. The liquid crystal is deposited locally on three different device variations using inkjet printing. The local deposition avoids unwanted interference of the liquid crystal with other optical components such as grating couplers. Measurements show similar trends as simulations of the liquid crystal orientations. For one device with a length of 50 µm, a phase shift of almost 0.9π is shown at 10 VRMS. We also discuss the challenges with this first demonstration of this phase shifter geometry using a silicon side-rail as an electrode.

Design and optimization of optical power splitters for optical access networks

This paper aims to study the design, simulation, and optimization of low-loss Y-branch passive optical splitters up to 64 output ports for telecommunication applications. For a waveguide channel profile, the standard material silica-on-silicon is used. The Y-splitters are designed and simulated at telecommunication operating wavelength, lambda = 1550 nm. Except for the lengths of the used Y-branches, and a core size of the waveguides, design parameters such as port pitch between the waveguides and simulation parameters for all splitters are considered fixed. The simulation results are analyzed to determine the optimum length of the splitters and the optimum core size. Based on this optimization the total length of the highest designed 1times 64 Y-branch splitter was reduced by 41.14 % for a waveguide core (5times 5) upmu mathrm{m}^2 compared to the length of splitter with a standard (6times 6) upmu mathrm{m}^2 core size.

Demonstration of a flexible electro-optic polymer modulator with a low half-wave voltage for ECG signals acquisition

Polymer-based electro-optic (EO) modulators have the advantages of low half-wave voltages and a low-cost preparation process and thus have extensive application prospects. A flexible, low half-wave voltage SU-8 Mach–Zehnder (M-Z) EO polymer modulator was proposed for human electrocardiogram (ECG) signal detection; polydimethylsiloxane (PDMS) was used as the substrate, and crosslinked polymer disperse red 1 (DR1)/SU-8 was used as the waveguide core layer. PDMS surface modification and SU-8 polarization were achieved, and the device was prepared by spin coating, ultraviolet (UV) lithography, wet etching and low-temperature sputtering. The experimental results show that the modulator initially operates in a linear operating regime under unbiased voltage conditions, and direct linear modulation of the voltage can be realized. After the preparation process, the low half-wave voltage was measured to be 2 V with a core size of 4000 μm×22000 μm, and the loss of the device was −2.1 dB.

Interconnection of Few-Mode Fibers and Photonic Integrated Circuits Using Mode-Field Adapters

We propose a detailed method for the interconnection between optical fibers and waveguides of photonic integrated circuits. Appropriate modal transmission is accomplished by matching the mode field diameters from both waveguide structures. Links from one structure to another are created by an interconnecting waveguide, maintaining a fixed coupling efficiency as its size is modified to adjust to the target waveguide core. This tailored transition acts as a mode field adapter, equalizing the transmission among multiple modes and reducing the mode-dependent losses while coupling. We present an algorithm to design the mode field adapter based on matching the effective mode areas using the power overlap integral. A study case considering a polymer photonic integrated device immediately connected to a few-mode fiber is analyzed. Coupling efficiencies over 90% for every transmitted mode are achieved, showing an evident improvement compared to typical approaches only matching core sizes. Detailed comparison of the results for each transmission mode is presented. This same procedure can be used to interconnect optical waveguides with different refractive index profiles and core geometry.

Interlayer directional coupling thermo-optic waveguide switches based on functionalized epoxy-crosslinking polymers

In this study, interlayer directional coupling (DC) thermo-optic (TO) waveguide switches were designed and fabricated using functionalized epoxy-crosslinking polymers. Fluorinated SU-8 (FSU-8) with a photo-initiating epoxy-crosslinking network was self-synthesized as a waveguide core material. A copolymer of methyl methacrylate and glycidyl methacrylate P(MMA-co-GMA) with a thermo-initiating epoxy crosslinking structure was self-synthesized as a waveguide cladding material. Compared with commercial pure SU-8 and PMMA, FSU-8 exhibited a lower absorption loss and P(MMA-co-GMA) exhibited a higher thermal stability. Using epoxy-crosslinking functionalized polymers, the structure of the waveguides and electrode heaters were optimized, and the performance parameters of the interlayer DC TO switches were simulated. At a signal wavelength of 1550 nm, the insertion loss, extinction ratio, and power consumption of the actual interlayer devices were measured as 6.7 dB, 15.6 dB, and 9 mW, respectively. The rising and falling response times of the TO switches were obtained as 631.6 µs and 362 µs, respectively. The self-leveling ability and solvent resistance characteristic of the epoxy-crosslinking network for FSU-8 and P(MMA-co-GMA) may guarantee the realization of interlayer DC TO waveguide switches. The proposed technique will be suitable for photonic integrated waveguide chips with multilayer stacking dynamic optical information interactions.

Sapphire Photonic Crystal Waveguides with Integrated Bragg Grating Structure

This contribution demonstrates photonic crystal waveguides generated within bulk planar sapphire substrates. A femtosecond laser is used to modify the refractive index in a hexagonal pattern around the pristine waveguide core. Near-field measurements reveal single-mode behavior at a wavelength of 1550 nm and the possibility to adapt the mode-field diameter. Based on far-field examinations, the effective refractive index contrast between the pristine waveguide core and depressed cladding is estimated to 3·10−4. Additionally, Bragg gratings are generated within the waveguide core. Due to the inherent birefringence of Al2O3, the gratings exhibit two distinct wavelengths of main reflection. Each reflection peak exhibits a narrow spectral full width at a half maximum of 130 pm and can be selectively addressed by exciting the birefringent waveguide with appropriately polarized light. Furthermore, a waveguide attenuation of 1 dB cm−1 is determined.

Ultralow 0.034 dB/m loss wafer-scale integrated photonics realizing 720 million Q and 380 μW threshold Brillouin lasing.

We demonstrate 0.034 dB/m loss waveguides in a 200-mm wafer-scale, silicon nitride (Si3N4) CMOS-foundry-compatible integration platform. We fabricate resonators that measure up to a 720 million intrinsic Q resonator at 1615 nm wavelength with a 258 kHz intrinsic linewidth. This resonator is used to realize a Brillouin laser with an energy-efficient 380 µW threshold power. The performance is achieved by reducing scattering losses through a combination of single-mode TM waveguide design and an etched blanket-layer low-pressure chemical vapor deposition (LPCVD) 80 nm Si3N4 waveguide core combined with thermal oxide lower and tetraethoxysilane plasma-enhanced chemical vapor deposition (TEOS-PECVD) upper oxide cladding. This level of performance will enable photon preservation and energy-efficient generation of the spectrally pure light needed for photonic integration of a wide range of future precision scientific applications, including quantum, precision metrology, and optical atomic clocks.

Combining one and two photon polymerization for accelerated high performance (3 + 1)D photonic integration

Abstract Dense and efficient circuits with component sizes approaching the physical limit is the hallmark of high performance integration. Ultimately, these features and their pursuit enabled the multi-decade lasting exponential increase of components on integrated electronic chips according to Moore’s law, which culminated with the high performance electronics we know today. However, current fabrication technology is mostly constrained to 2D lithography, and thermal energy dissipation induced by switching electronic signal lines presents a fundamental challenge for truly 3D electronic integration. Photonics reduces this problem, and 3D photonic integration is therefore a highly sought after technology that strongly gains in relevance due to the need for scalable application-specific integrated circuits for neural networks. Direct laser writing of a photoresin is a promising high-resolution and complementary metal-oxide-semiconductor (CMOS) compatible tool for 3D photonic integration. Here, we combine one and two-photon polymerization (TPP) for waveguide integration for the first time, dramatically accelerating the fabrication process and increasing optical confinement. 3D additive printing is based on femtosecond TPP, while blanket irradiation with a UV lamp induces one-photon polymerization (OPP) throughout the entire 3D chip. We locally and dynamically adjust writing conditions to implement (3 + 1)D flash-TPP: waveguide cores are printed with a small distance between neighboring writing voxels to ensure smooth interfaces, mechanical support structures are printed at maximal distance between the voxels to speed up the process. Finally, the entire chip’s passive volume not part of waveguide cores or mechanical support is polymerized in a single instance by UV blanket irradiation. This decouples fabrication time from the passive volume’s size. We succeed in printing vertical single-mode waveguides of 6 mm length that reach numerical apertures up to NA = 0.16. Noteworthy, we achieve exceptionally low −0.26 dB injection losses and very low propagation losses of −1.36 dB/mm at λ 0 = 660 nm, which is within one order of magnitude of standard integrated silicon photonics. Finally, the optical performance of our waveguides does not deteriorate for at least ∼3000 h after printing, and remains stable during ∼600 h of continuous operation with 0.25 mW injected light.

A Microfluid Fiber Device for Trace Detection of Aggregation Induced Emission Molecules

An ultra-sensitive microfluid device for detecting aggregation of induced emission molecules is proposed and demonstrated. The minimum detectable concentration is greatly reduced by use of a micromachined fiber tip and a liquid core waveguide. The fiber tip is a tapered-microlens structure with optimized shape parameters. The liquid waveguide is formed with low refractive index Teflon capillary. The dependence of photoluminescence intensity on the concentration change is tested with the device. The limit of detection is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.316 \boldsymbol {\mu }\text{M}$ </tex-math></inline-formula> , the volume of the sample solution used is only <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.649 \boldsymbol {\mu }\text{L}$ </tex-math></inline-formula> and the minimum detectable concentration is down to 100 nM. It is a valuable result obtained with fiber spectrometer (a CCD-type machine), better than that obtained with traditional fluorospectro photometer (a photomultiplier tubes-type one). The reversibility and stability are also testified through experimental method. Together with the compact size, small sample volume and high sensitivity, the durable microfluidic sensing device has great potential for biosensing applications.

Ultrahigh-Q on-chip silicon–germanium microresonators

To date, integrated waveguides with the lowest losses have all relied at least in part on amorphous materials. In this work, we demonstrate fully crystalline, single-mode integrated microresonators comprising epitaxially grown S i 0.86 G e 0.14 waveguide cores with silicon claddings. This platform supports ultrahigh-quality-factor resonances, with Q reaching 1.71 ± 0.06 × 10 8 , corresponding to a loss rate of 0.39 ± 0.02 d B / m . This Q is nearly an order-of-magnitude improvement over that seen in prior integrated Si waveguides. Together with silicon’s strong Kerr nonlinearity, our results could unlock important new avenues for microwave photonics, optomechanics, and quantum transduction.

Optical modulator using ultra-thin silicon waveguide in SOI hybrid technology

We propose a detailed study of an on-chip optical modulator using a non-conventional silicon-based platform. This platform is based on the optimum design of ultra-thin silicon on insulator waveguide. This platform is characterized by low field confinement inside the core waveguide and high sensitivity to the cladding index. Accordingly, it lends itself to a wide range of applications, such as sensing and optical modulation. By employing this waveguide into the Mach–Zehnder interferometer configuration, an efficient optical modulator is reported using an organic polymer as an active material for the electro-optic effect. An extinction ratio of more than 20 dB is achieved with energy per bit of 13.21 fJ/bit for 0.5 V applied voltage. This studied platform shows promising and adequate performance for modulation applications. It is cheap and easy to fabricate.

Dispersion engineered mid-IR supercontinuum generation using multi-core silicon-rich nitride photonic integrated waveguide

The multi-core silicon-rich nitride photonic integrated waveguide is presented here to numerically model the spanning of supercontinuum generation in the mid-IR region beyond the 4 μm wavelength range by engineering the dispersion profile such that maximum phase-matching can be achieved. Tunable dispersive wave generation has been observed in a long mid-IR region up to 5.2 μm wavelength by the variation of gap spacing between the cores. This unique geometry enables a higher phase matching from 1.2 μm to 2.6 μm wavelength range resulting in the flat dispersion profile in the near-IR region by coupling the fundamental mode in narrow waveguide core to the higher-order mode in wide waveguide core that gives rise to anomalous GVD which plays an essential role in compensating chromatic dispersion in the mid-IR region.

Constructing thermal phonon insulators by using photonic crystals

This paper designs a thermal waveguide using thermal phonon insulators, to manipulate the loss of thermal radiation. Two-dimensional (2D) photonic crystals with dielectric background and air holes have been used in the waveguide clad to suppress thermal radiation so that thermal radiation cannot escape out from the waveguide clad. The photonic crystals may reduce the loss of thermal radiation through waveguide clad which surges the thermal energy confinement in the waveguide core and propagation along the longitudinal axis. The photonic crystals have been designed with the bandgap covering the thermal phonon frequency range and thus can be used to construct thermal phonon insulators to suppress or trap thermal emission generated through phonon wave propagation. The finite-difference-time-domain simulation results demonstrate that electromagnetic bandgap in photonic crystals has enough width in the thermal phonon frequency range to confine the thermal energy within the core of the waveguide and effective transmission of thermal radiation can be conquered within the waveguide core.

Extraordinary broadband impedance matching in highly dispersive media - the white light cavity approach.

Suppressing reflections from material boundaries has always been an objective, common to many disciplines, where wave phenomena play a role. While impedance difference between materials necessarily leads to a wave reflection, introducing matching elements can almost completely suppress this phenomenon. However, many impedance matching approaches are based on resonant conditions, which come at a price of narrow bandwidth operation. Although various impedance matching architectures have been developed in the past, many of them fail to produce a broadband and flat (ripple-free) transmission, particularly in the presence of strong chromatic dispersion. Here we propose and demonstrate an approach for designing an optimal matching stack capable of providing a flat broadband transmission even in the presence of significant group velocity dispersion. As an experimental example for the method verification, we used a strong modal dispersion in a rectangular waveguide, operating close to a mode cut-off. The waveguide core consists of alternating polymer sections with a variable filling factor, realized using additive manufacturing. As a result, a broadband matching in the range of 7-8GHz was demonstrated and proved to significantly outperform the standard binomial transformer solution. The proposed method can find use across different disciplines, including optics, acoustics and wireless communications, where undesired reflections can significantly degrade system's performances.

Understanding the Light Detection in an Integrated Novel Electrophotonic Wavesensor Photodetector

This work presents a novel integrable silicon photodetector which can only be conceived as part of a monolithic electrophotonic basic structure formed of a silicon light emitter, waveguide and light detector. That is, it cannot operate as a single electronic or photonic device. The detector presents current gain, and photons reach the depletion region straightforward, allowing the detection of low power light produced by silicon light sources currently in use, which is difficult for existing photodetectors. The waveguide core is made of silicon nitride, and it is simultaneously the insulator in a MOS-like device. The light detection unit is intended for novel seamless electrophotonic platforms, and it is called wavesensor. In spite that the device is a MOS-like structure, it is not a MOSFET neither a lateral bipolar transistor, and one of the main differences with the former is that this is a bulk device working in Punch-Through regime. Being a MOS-like structure, it is fully compatible with standard microelectronics technology. A development of the mathematics involved in its operation is carried out in order to understand the physics of the detector, showing a gain factor in the photocurrent. Computer simulations of the fabrication process and photoelectric response of the device confirmed photocurrent values higher than the expected for a photodiode with efficiency = 1, thus demonstrating a new integrable photodetector with gain, capable of detecting light in the range of nW for electrophotonic applications.

Broadband single-mode planar waveguides in monolithic 4H-SiC

Color-center defects in silicon carbide promise opto-electronic quantum applications in several fields, such as computing, sensing, and communication. In order to scale down and combine these functionalities with the existing silicon device platforms, it is crucial to consider SiC integrated optics. In recent years, many examples of SiC photonic platforms have been shown, like photonic crystal cavities, film-on-insulator waveguides, and micro-ring resonators. However, all these examples rely on separating thin films of SiC from substrate wafers. This introduces significant surface roughness, strain, and defects in the material, which greatly affects the homogeneity of the optical properties of color centers. Here, we present and test a method for fabricating monolithic single-crystal integrated-photonic devices in SiC: tuning optical properties via charge carrier concentration. We fabricated monolithic SiC n-i-n and p-i-n junctions where the intrinsic layer acts as waveguide core, and demonstrate the waveguide functionality for these samples. The propagation losses are below 14 dB/cm. These waveguide types allow for addressing color centers over a broad wavelength range with low strain-induced inhomogeneity of the optical-transition frequencies. Furthermore, we expect that our findings open the road to fabricating waveguides and devices based on p-i-n junctions, which will allow for integrated electrostatic and radio frequency control together with high-intensity optical control of defects in silicon carbide.

Interpreting light guidance in antiresonant and photonic bandgap waveguides and fibers by light scattering: analytical model and ultra-low guidance.

Here, we introduce a quasi-analytic model that allows studying mode formation in low refractive index core waveguides through solely focusing on the cladding properties. The model isolates the reflection properties of the cladding from the modes via correlating the complex amplitude reflection coefficient of the cladding to the complex effective index of the fundamental core mode. The relevance and validity of the model are demonstrated by considering a single-ring anti-resonant fiber, revealing unexpected situations of exceptionally low loss. Our model explains mode formation by light scattering, which conceptually provides deep insights into the relevant physics.

Polarization-independent electro-absorption optical modulator based on trapezoid polymer-graphene waveguide

A polarization-independent electro-absorption modulator based on the trapezoid polymer-graphene waveguide (PGW) was proposed. The modulator was constructed on a trapezoid polymer waveguide, and the insulting dielectric spacer sandwiched in the two graphene layers was placed on the surface of the trapezoid polymer waveguide core. The simulation results show that by applying different gate voltages on the graphene layers, effective mode index of the TE and TM modes in the PGW can realize the similar changes in the C-band, which provides the possibility for realizing the polarization-independent modulation. Here, through simulation and optimization, the presented modulator has a compact size and the extinction ratio (ER) of 37 dB can be achieved by reasonably setting working points of “OFF” and “ON” states with 800 μm long active graphene length. The ER variation between the two operating modes is about 0.46 dB. The corresponding power consumption of the modulator is about 23.6 pJ/bit.