Rib Waveguide Research Articles

A modular fabrication process for thin-film lithium niobate modulators with silicon photonics

We report advancements in the fabrication of electro-optic Mach-Zehnder modulators made by bonding an unetched thin film of lithium niobate (LN) to a second chip with rib waveguides in another material, such as silicon. Devices were fabricated after storing bonded silicon-LN chips in a common laboratory environment for more than three years. The chips survived the full processing flow and yielded modulators with greater than 50 GHz 3 dB electro-optic bandwidth, and less than 3 V cm at 1550 nm and equivalent performance to freshly-bonded and processed chips. Furthermore, we demonstrate the co-integration of hybrid bonded thin-film LN modulators and silicon photonics based high quality-factor ring resonators and higher-order coupled microring optical filters. The silicon microring resonators are used for photon-pair generation at 1550 nm using spontaneous four-wave mixing. These results show the feasibility of a modular modulator fabrication procedure, where the planarization and bonding steps are performed for a batch of chips at one time and smaller sub-batches are customized by end users at a much later time according to their needs and convenience.

Silicon nitride waveguide polarization rotator and polarization beam splitter for chip-scale atomic systems

The design, fabrication, and characterization of a silicon nitride waveguide polarization rotator and polarization beam splitter that operate with a polarization extinction ratio (PER) of ∼30 dB at the rubidium atomic transition of 780 nm wavelength are demonstrated. These polarization devices are fabricated on the same chip using a self-aligned process for integration of the rib and ridge waveguide structures. The polarization rotator is based on the mode evolution approach using adiabatic tapers and demonstrates a PER of ≥20 dB over a 100 nm bandwidth (730–830 nm wavelengths) with an insertion loss (IL) ≤1 dB. The polarization beam splitter is based on a cascaded tapered asymmetric directional coupler with phase matching between the fundamental and higher order TM modes, whereas the TE mode is separated by the through port. This provides a PER ≥ 20 dB with IL ≤ 1 dB over a 50 nm bandwidth for the cross port and a PER ≥ 15 dB with an IL ≤ 1 dB over an 18 nm bandwidth for the through port. These polarization control waveguide devices will enable photonic integrated circuits for saturated absorption spectroscopy of atomic vapors for laser stabilization on-chip.

Flexible source of correlated photons based on LNOI rib waveguides

Lithium niobate on insulator (LNOI) material has great potential for photonic integrated circuits, providing substantial versatility in design of various integrated components. To properly use these components in the implementation of different quantum protocols, photons with different properties are required. In this paper, we theoretically demonstrate a flexible source of correlated photons built on the LNOI waveguide with a special geometry. This source is based on the parametric down-conversion (PDC) process, in which the signal and idler photons are generated at the telecom wavelength and have different spatial profiles and polarizations, but the same group velocities. Distinguishability in polarizations and spatial profiles facilitates the routing and manipulating individual photons, while the equality of their group velocities leads to the absence of temporal walk-off between photons. We show how the spectral properties of the generated photons and the number of their frequency modes can be controlled depending on the pump characteristics and the waveguide length. Finally, we discuss special regimes, in which narrowband light with strong frequency correlations and polarization-entangled Bell states are generated at the telecom wavelength.

Integrated opto-mechanical cantilever sensor with a rib waveguide.

An integrated opto-mechanical cantilever sensor with a rib waveguide is reported in this paper. The device consists of a rib waveguide cantilever with buried waveguides on silicon. The rib cantilever is introduced to match better with the buried waveguide, further for increasing the interface coupling efficiency. With this configuration, single-mode operating can be achieved in transverse direction without decreasing the width of optical waveguide cantilever. The system sensitivity is 1.1 μm-1 which is increased by about 21%, compared with the conventional structure.

Design and demonstration of an efficient pump rejection filter for silicon photonic applications.

Photon pair generation via spontaneous four-wave mixing in silicon waveguides/microring resonators integrated with a high extinction pump rejection filter is very much in demand for futuristic large-scale integrated quantum photonics circuits. Ideally, a distributed Bragg reflector (DBR) can be designed to offer desired pump rejection. However, fabricated DBRs suffer degradation in pump extinction due to roughness-induced unwanted scattering waves in the forward direction around the Bragg wavelength. It is therefore inferred that the roughness-induced forward scattering can be reduced significantly by integrating a DBR structure in one of the sidewalls (instead of two sidewalls) of a multimode rib waveguide (instead of a single mode strip waveguide). Therefore, we studied a single-stage DBR filter with this design which exhibits a significantly higher stop band extinction (∼63 dB), in comparison with that of earlier reported results (<50 dB). To validate the pump rejection efficiency of such fabricated devices in quantum photonic applications, we have carried out on-chip stimulated four-wave mixing experiments and shown that the pump laser within the rejection band could be attenuated to the level of idler power.

Study on the single-mode condition for x-cut LNOI rib waveguides based on leakage losses.

Lithium niobate-on-insulator (LNOI) has recently emerged as a promising material platform for high-density and advanced photonics integrated circuits (PICs). And single-mode waveguides (SMW) are the most basic building blocks for structuring various PICs. In this paper, single-mode conditions (SMCs) for shallowly etched LNOI rib waveguides in x-cut LNOI wafer are investigated with the finite element method (FEM) in consideration of the lateral leakage and the magic width for the first time, to our best knowledge. Our results indicate that due to the lateral leakage and the magic width these shallowly etched x-cut LNOI rib waveguides have unique and complex SMCs. Our method and results provide a guidance in designing low-loss LNOI SMW and high-performance PICs.

Optical parametric generation in OP-GaAs waveguides pumped by a femtosecond fluoride fiber laser.

We report on mid-infrared optical parametric generation in the 4-5 μm and 9-12 μm bands by pumping custom-designed orientation-patterned gallium arsenide (OP-GaAs) rib waveguides with an ultrafast femtosecond fiber laser system. This pump source is seeded by a mode-locked fluoride fiber laser with 59 MHz repetition rate and can be tuned between 2.8 and 3.2 μm using a soliton self-frequency shifting stage. The single TE and TM modes OP-GaAs crystals feature quasi-phase-matched grating periods of 85 and 90 μm and different transverse sizes thus allowing a wide spectral tunability.

High-sensitivity integrated SiN rib-waveguide long period grating refractometer

In this research, we demonstrate a high-sensitivity integrated silicon nitride long period grating (LPG) refractometer based on a rib waveguide with sinusoidally modulated width. While integrated LPG architectures typically achieve ultrahigh sensitivity only over a narrow optical bandwidth using a phase-matching turning-point optimization technique, our sensor exhibits a very high refractometric sensitivity that was designed to remain constant over a broad operational optical spectral bandwidth. The proposed design method relies on multi-modal dispersion tailoring that consists of homogenizing the spectral behaviors of both group and effective indices of the coupling modes. Experimental results are in agreement with numerical simulations, demonstrating not only a sensitivity reaching 11,500 nm/RIU but, more significantly, also that this sensitivity remains almost constant over a broad spectral range of at least 100 nm around 1550 nm. Additional advantages of the proposed sensor architecture encompass a low temperature sensitivity, down to −0.15 nm/K, and simplicity of the fabrication process. These results demonstrate the feasibility of chip-scale photonic integration to achieve both high sensitivity and large dynamic range of the proposed refractometer.

Thermal dynamics of phase switching process of an SOI rib waveguide covered with a Ge2Sb2Te5 phase change material film

The combination of phase-change materials (PCMs) and integrated photonics promotes the development of new forms of all-optical devices and potentially provides large-scale all-optical arithmetic-logic units and neuromorphic processing chips using the large contrast in optical properties when switched between amorphous and crystalline phase states. Therefore, a proper understanding of the refractive index fluctuations during phase-switching is essential for controlled and optimized device operation. We present analytical results to simulate the temperature-field distribution while an optical pulse is injected into a waveguide with a phase change material cell. These results demonstrate good agreement with the calculations by the finite element method. Furthermore, the crystallization fraction and refractive index change of the Ge2Sb2Te5 PCM cell are investigated in calculation, along with the attenuation effect of the SOI rib waveguide during the phase switching and the dependence on the optical injection. Therefore, it is necessary to set the optical pulses appropriately for the phase-switching process for particular devices, such as optical memory and photonics neurons.

Compact MZI modulators on thin film Z-cut lithium niobate.

In this paper, we designed, implemented, and characterized compact Mach-Zehnder interferometer-based electro-optic modulators. The modulator utilizes spiral-shaped optical waveguides on Z-cut lithium niobate and the preeminent electro-optic effect which is applied using top and bottom electrodes. Optical waveguides are made of rib etched lithium niobate waveguides with bottom silicon oxide cladding, while SU8 polymer covers the top and sides of the rib waveguides. The proposed implementation resulted in low optical losses < 1.3 dB/cm. Moreover, we achieved compact modulators that fit 0.286 cm and 2 cm long optical waveguides in 110 µm × 110 µm and 300 µm × 300 µm areas, respectively. For single arm modulation, the modulators achieved a VπL of 7.4 V.cm and 6.4 V.cm and 3-dB bandwidths of 9.3 GHz and 2.05 GHz, respectively. Push-pull modulation is expected to cut these VπL in half. The proposed configuration avoids traveling wave modulation complexities and represents a key development towards miniature and highly integrated photonic circuits.

A perspective on integrated atomo-photonic waveguide circuits

Integrated photonic circuits based on suspended photonic rib waveguides, which can be used for coherent trapping, guiding, and splitting of ultra-cold neutral atoms in two-color evanescent light fields near their surfaces, are described. Configurations of quantum inertial sensors based on such integrated atomo-photonic waveguides, which are simultaneously guiding photons and atoms along the same paths, are presented. The difference between free-space and guided atom interferometers in the presence of external forces is explained. The theoretical and technological challenges, to be overcome on the way to the realization of such a platform for quantum technologies, are discussed.

Optimizations of Thermo-Optic Phase Shifter Heaters Using Doped Silicon Heaters in Rib Waveguide Structure

Optimizations of Thermo-Optic Phase Shifter Heaters Using Doped Silicon Heaters in Rib Waveguide Structure

Parallel directional coupler based dual-polarization electro-absorption modulator using epsilon near-zero material

Electro-optical modulation, where a radio frequency signal can be encoded in an optical field, is crucial to decide the overall performance of an integrated photonics system. Due to the growing internet penetration rate worldwide, polarization-division-multiplexing (PDM) technique has emerged to increase the link capacity, where polarization-independent modulators are desirable to reduce system complexity. In this study, we propose a novel parallel directional coupler based dual-polarization electro-absorption modulator based on epsilon-near-zero (ENZ) material. The proposed design is capable of independent and synchronized modulation of two fundamental modes viz. transverse magnetic (TM) and transverse electric (TE) mode of a standard silicon (Si) rib waveguide. Indium-tin-oxide (ITO)–Si based two parallel hybrid plasmonic waveguides (HPW1 and HPW2) are placed such that fundamental TM (TE) mode of the input bus waveguide can be coupled to HPW1 (HPW2). The ENZ-state of ITO, acquired upon two independent electrical gating, enables large modulation depth by utilizing enhancement of electric field at the absorptive carrier accumulation layer. With a 27 μm active length, the extinction ratio (ER) of the proposed design is 10.11 dB (9.66 dB) for TM (TE) modulation at 1550 nm wavelength. This results in a 0.45 dB ER-discrepancy and indicates the polarization-insensitive nature of the modulator. The insertion losses and modulation bandwidths of our design are less than 1 dB and more than 100 GHz, respectively, for both polarizations over the entire C-band of wavelength. The proposed design can find potential applications in the PDM-enabled integrated photonics systems and high speed optical interconnections at data center networks.

Leaky-mode long-period grating on a lithium-niobate-on-insulator waveguide

Lithium niobate on insulator (LNOI) is a promising platform for the construction of high-density integrated photonics circuits. In this paper, we propose compact long-period gratings (LPGs) formed on LNOI rib waveguides for achieving light coupling from the guide mode to the leaky mode. Our LPGs are polymer gratings coated on the surfaces of LNOI waveguides and can be designed to achieve strong mode coupling at specific resonance wavelengths for the TE-like and the TM-like polarization. By means of a numerical analysis, we find that the LPGs exhibit turning points in the phase-matching curves, which promises the realization of broadband wavelength filters and polarizers with such grating structures on the LNOI platform. To experimentally verify the theoretical results and demonstrate the LPG characteristics, we fabricate a number of LPG filters on LNOI rib waveguides, which include two broadband LPGs operating near the turning points: a TE-pass polarizer with a 20 dB extinction ratio over a bandwidth of 270 nm from 1430 to 1700 nm and a TM-pass polarizer with a 20 dB extinction ratio over a bandwidth of 90 nm from 1500 to 1590 nm. The insertion losses for the passing polarization are smaller than 2 dB in the C + L band. Our LPGs can serve as building blocks for the development of a wide range of integrated LNOI devices for application in on-chip signal processing.

High-aspect-ratio ZnSe microstructure generated by spatially shaped femtosecond laser writing assisted with wet chemical etching

A high-aspect-ratio microstructure in polycrystalline Zinc selenide (ZnSe) is important for waveguides and photonic crystals application. In this work, we demonstrated a successful approach for fabricating microstructure with Bessel beam inscription and etching using a mixed solution of hydrogen peroxide (H2O2) and ammonia hydroxide (NH4OH). The microstructure inscribed using the femtosecond laser back-side fabrication and selective etching, has a longitudinal length of 140 μm and a width of 12 μm. The aspect ratio of microstructures is ∼ 11.2. To showcase the potential of this technique, a rib waveguide with a longitudinal length and width of 60 μm was prepared and the rib waveguides supported a well confined mode at 1.55 μm. This process makes it possible to produce three-dimensional microstructures inside polycrystalline ZnSe, and thus offers a pathway for expanding the photonic crystal structure to the mid-to-far infrared field.

4-Bit All-Optical Serial-to-Parallel Converter With Sub-dB/cm Delay Lines Based on Rib Waveguides

This paper extends our previous work on the microring resonator-type serial-to-parallel conversion scheme aimed for all-optical label processing on the silicon photonic platform. In the previously proposed scheme, the architecture of the delay lines was silicon wires. This resulted in considerable propagation loss, confining multibit operation of serial-to-parallel conversion to a label packet of 2 bits. However, during a practical application, propagation loss at the delay lines has to be improved to support label packets comprising of a greater number of bits. The purpose of this article is twofold. First, we confirm that propagation loss can be lowered to sub-dB/cm values by introducing rib waveguides via simulations and experiment. Second, we demonstrate multibit serial-to-parallel conversion on a 4-bit label “1101” by utilizing a cascaded microring resonator device of which the straight sections of the delay lines are of rib waveguide architecture. In order to focus on free-carrier dispersion effect of silicon during operation, we left out a temporal delay between the pump pulses and probe bits to be extracted. As a result, serial-to-parallel conversion was viable even at peak pump power levels as low as ∼100 mW. We hereby verify that the proposed serial-to-parallel conversion scheme incorporated with rib waveguide delay lines is a realistic solution in realizing a photonics-based solution to all-optical label processing.

On-chip dual-band waveguide Bragg filter with identical subnanometer-bandwidth stopbands near 1310 and 1950 nm wavelengths

We propose the realization of an on-chip dual identical narrowband Bragg filter at ∼ 1310 and ∼ 1950 n m wavelengths simultaneously based on the silicon-on-insulator (SOI) platform. By taking advantage of subwavelength corrugation behavior at large wavelengths and the difference in the mode areas of the involved modes at the two widely separated wavelengths, undesired diffraction losses are circumvented while achieving Bragg resonances at the two wavelengths simultaneously. A double-corrugation Bragg grating rib waveguide filter is proposed, with two sets of gratings, the inner one close to the rib operating near 1310 nm wavelength, with the outer grating being designed to achieve a transmission dip around 1950 nm. Introducing a proper lateral misalignment to the set of inner grating indents, a dual identical narrowband Bragg filter with ∼ 0.47 n m 3 dB bandwidth at ∼ 1310 n m and ∼ 1950 n m is achieved. The proposed design strategy based on the SOI platform relies on a single-etching fabrication process and presents potential applications in situations where equal-bandwidth filtering is needed in on-chip communications and sensing.

A compact 1×3 two-mode selective silicon photonic router/switch using two tunable phase shifters

In this work, we propose a design in the proof-of-concept of a 1×3 two-mode selective silicon-photonics router/switch. The proposed device composes of a Y-junction coupler, two multimode interference (MMI) couplers, and two phase-shifters on the silicon-on-insulator (SOI) rib waveguides. The input m

Re-analysis of single-mode conditions for thin-film lithium niobate rib waveguides

Single-mode waveguide devices are necessary in photonic integrated circuits. Previous studies on the single-mode conditions (SMCs) of the thin film lithium niobate (TFLN) either only focus on wire waveguide, or lack comprehensive discussions on the influence of the rib waveguide with different cross-section area at a certain height. In this paper, a systematical study on the SMCs of rib waveguide based on TFLN is carried out by using finite difference method (FDM). By comparing the effective refractive index between the first-order mode in the central rib region and the fundamental mode in the slab waveguide, and the fraction of the mode power flux between the TFLN rib region and the whole waveguide region, the SMCs in the central rib region are analyzed in terms of geometric parameters of the TFLN rib waveguide. The simulation results suggest that no single equation could meet the SMCs of different waveguide structures simultaneously in a small TFLN with the height no more than 900 nm. Ultimately, new approximate expressions as functions of rib-waveguide width and etching depth are proposed to precisely describe the single-mode condition at these dimensions.

Progressive Poling of Large Area, High r33 Electro-Optic Polymer SEO100c

SEO100c, an EO-polymer, has been reported of having an r33 in excess of 100 pm/V. Experimental poling research was performed on rib waveguide modulator for device design and development. Reported is the determination of the impact that temperature and voltage have on the poling of a SEO100c waveguide device in order to maximize the r33 while avoiding damage to the device structure ensuring high yield in manufacture. The poling process is shown to have a nonlinear relationship between r33 and poling field aiding in the selection of achievable poling voltages for required r33 values. Device thermal stability is quantified and reported for the complete poling process and the impacts upon r33. Investigation into the possible relaxation of device r33 is measured over an extended period demonstrating desirable use within deployable devices.