High Index Contrast Waveguides Research Articles

X-Cut Lithium Niobate Optical Waveguide with High-Index Contrast and Low Loss Fabricated by Vapor Proton Exchange

Highly integrated and stable devices are appealing in optical communication and sensing. This appeal arises from the presence of high refractive index contrast and high-quality waveguides. In this study, we improved the vapor proton exchange (VPE) process, enabling large-scale waveguide fabrication and addressing the issue of liquid exchange during cooling. Additionally, we have prepared and characterized planar waveguides on X-cut lithium niobate (LN) crystals. The exchanged samples exhibit α and k1 phases, refractive index contrasts as high as 0.082, and exceptional refractive index uniformity. Furthermore, we utilized the same process to fabricate channel waveguides and Y-branch waveguides. We achieved low propagation losses in channel waveguides, accompanied by small mode sizes, and low-loss Y-branch waveguides with a highly uniform beam splitting ratio. All waveguides exhibited consistent performance across multiple preparations and tests, remaining free from aging effects for three months. Our results underscore the promising potential of VPE for creating Y-branch splitters and modulators in LN crystals.

A Theoretical Sub-0.1 dB Loss Single Mode Fiber-To-Chip Edge Coupler for Silicon Nitride Waveguides

A low loss optical interconnection between optical fibers and photonic integrated circuits is critical for high performance photonic systems. In the past decade, spot size converters, subwavelength waveguide grating (SWG) structures, and different refractive index materials have been applied to allow efficient coupling between the fiber and the photonic chips. However, it is still challenging to achieve low-loss coupling when interfacing high index contrast waveguides such as SiN with SMF-28 fibers. In this work, we report a multilayer edge-coupler using SiOxN materials with different indices to allow for efficient edge coupling between SMF-28 fiber and SiN single mode waveguides. A coupling loss of 0.068 dB for the TM mode was achieved theoretically at a 1550 nm wavelength, with a 1 dB alignment tolerance offset of 2.4 μm.

Refractive index profile renormalization for a silicon waveguide bend

An index profile renormalization method is proposed to study the effective refraction index increase in a high-index contrast waveguide bend such as a silicon waveguide bend. This method transforms a waveguide bend to an equivalent straight waveguide (ESW). The simulation results show that the ESW method can calculate the effective refraction index increase in a three-dimensional (3D) bend, costing much less time than the traditional finite difference time domain method and the index profile renormalization (IPR) method developed for small radius cases. In addition, the field center shift is studied. The relationships are established between the effective refraction index increase and field center shift from the bend axis along the radial direction for estimations based on the approximated IPR method. These results can lead to great convenience in estimating the effective refraction index increase in 3D bend, which could be used in conditions requiring critical phase control.

Edge Raman enhancement at layered PbI2 platelets induced by laser waveguide effect

As a two-dimensional (2D) layered semiconductor, lead iodide (PbI2) has been widely used in optoelectronics owing to its unique crystal structure and distinctive optical and electrical properties. A comprehensive understanding of its optical performance is essential for further application and progress. Here, we synthesized regularly shaped PbI2 platelets using the chemical vapor deposition method. Raman scattering spectroscopy of PbI2 platelets was predominantly enhanced when the laser radiated at the edge according to Raman mapping spectroscopy. Combining the outcome of polarized Raman scattering spectroscopy and finite-difference time domain simulation analysis, the Raman enhancement was proven to be the consequence of the enhancement effects inherent to the high refractive index contrast waveguide, which is naturally formed in well-defined PbI2 platelets. Because of the enlarged excited area determined by the increased propagation length of the laser in the PbI2 platelet formed waveguide, the total Raman enhancements are acquired rather than a localized point enhancement. Finally, the Raman enhancement factor is directly related to the thickness of the PbI2 platelet, which further confirms the waveguide-enhanced edge Raman. Our investigation of the optical properties of PbI2 platelets offers reference for potential 2D layered-related optoelectronic applications.

Hybrid Chalcogenide‐Germanosilicate Waveguides for High Performance Stimulated Brillouin Scattering Applications

AbstractOn‐chip stimulated Brillouin scattering (SBS) in arsenic trisulfide (As2S3) planar waveguides lead to a range of outstanding demonstrations in microwave photonics signal generation and processing. However, the lack of other integrated functionalities, high back reflections, and large in‐ and out‐fiber coupling losses in high index contrast waveguides cause a number of serious impairments and lessen the applicability of microwave photonic devices. In this report, a hybrid integration scheme is demonstrated where As2S3 waveguides optimized for SBS gain are coupled with very low losses via a vertical taper to a high index contrast and versatile germanosilicate (Ge:SiO2) platform. The Ge:SiO2 waveguide is optimally mode‐matched to commercially available high numerical aperture optical fiber to achieve very low coupling losses. The structure has very low back reflection due to the adiabatic nature of the taper and negligible refractive index difference across the fiber‐chip interface. The hybrid architecture exhibits a similar Brillouin gain coefficient to its monolithic counterpart but with an improvement of >3 dB/facet fiber‐to‐chip loss and >20 dB reduction in facet reflectivity. The hybrid structures demonstrated will bring chalcogenide‐based chip scale SBS devices closer to practical application.

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability

Unlike other crystals, the counter intuitive response of bismuth germanate crystals ({text {Bi}}_4{text {Ge}}_3{text {O}}_{12}, BGO) to form localized high refractive index contrast waveguides upon ultrafast laser irradiation is explained for the first time. While the waveguide formation is a result of a stoichiometric reorganization of germanium and oxygen, the origin of positive index stems from the formation of highly polarisable non-bridging oxygen complexes. Micro-reflectivity measurements revealed a record-high positive refractive index contrast of 4.25times 10^{-2}. The currently accepted view that index changes >1times 10^{-2} could be brought about only by engaging heavy metal elements is strongly challenged by this report. The combination of a nearly perfect step-index profile, record-high refractive index contrast, easily tunable waveguide dimensions, and the intrinsic high optical non-linearity, electro-optic activity and optical transparency up to 5.5,upmu {text {m}} of BGO make these waveguides a highly attractive platform for compact 3D integrated optics.

Lapping and Polishing of Crystalline KY(WO4)2: Toward High Refractive Index Contrast Slab Waveguides.

Rare-earth ion-doped potassium yttrium double tungstate, RE:KY(WO4)2, is a promising candidate for small, power-efficient, on-chip lasers and amplifiers. Thin KY(WO4)2-on-glass layers with thicknesses ranging between 0.9 and 1.6 μm are required to realize on-chip lasers based on high refractive index contrast waveguides operating between 1.55 and 3.00 µm. The crystalline nature of KY(WO4)2 makes the growth of thin, defect-free layers on amorphous glass substrates impossible. Heterogeneous integration is one of the promising approaches to achieve thin KY(WO4)2-on-glass layers. In this process, crystal samples, with a thickness of 1 mm, are bonded onto a glass substrate and thinned down with an extensive lapping and polishing procedure to the desired final thickness. In this study, a lapping and polishing process for KY(WO4)2 was developed toward the realization of integrated active optical devices in this material.

Heterogeneous integration of KY(WO4)2-on-glass: a bonding study

Rare-earth ion doped potassium yttrium double tungstate, RE:KY(WO4)2, is a promising candidate for small, power-efficient, on-chip lasers and amplifiers. There are two major bottlenecks that complicate the realization of such devices. Firstly, the anisotropic thermal expansion coefficient of KY(WO4)2 makes it challenging to integrate the crystal on glass substrates. Secondly, the crystal layer has to be, for example, <1 µm to obtain single mode, high refractive index contrast waveguides operating at 1550 nm. In this work, different adhesives and bonding techniques in combination with several types of glass substrates are investigated. An optimal bonding process will enable further processing towards the manufacturing of integrated active optical KY(WO4)2 devices.

Elimination of spurious modes in full-vectorial finite element method based acoustic modal solution.

Finite element method is a powerful technique for solving a wide range of engineering problems. However, the existence of the spurious solutions in full-vectorial finite element method has been a major problem for both acoustic and optic modal analyses. For emerging photonic devices exploiting light-sound interactions in high index contrast waveguides, this problem is a major limitation. A penalty function is introduced to remove these unwanted spurious modes in acoustic waveguides, which also identifies the acoustic modes more easily. Numerically simulated results also show considerably improved vector mode profiles. The proposed penalty method has been applied for the characterization of low index contrast single mode fiber and also for high index contrast silicon nanowire to demonstrate its effectiveness.

Femtosecond laser writing of photonic devices in borate glasses compositionally designed to be laser writable.

The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of La2O3 and Na2O can be straightforwardly used to produce high-index contrast (Δn) waveguides by femtosecond-laser-assisted ion migration. The positive Δn of the waveguides is caused by the local enrichment of La2O3 in the guiding region with a slope of 8·10-3(mol.%)-1. The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δn values can be controlled through the initial La2O3 content of the glass. Maximum Δn values >10-2 for samples with just 5.5mol. % of La2O3 have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.

Analysis of High-Index Contrast Lithium Niobate Waveguides Fabricated by High Vacuum Proton Exchange

High-index contrast waveguides fabricated with precise control and reproducibility are of high interest for nonlinear and/or electro-optical highly efficient and compact devices for quantum and classical optical data processing. Here, we present a new process to fabricate planar and channel optical waveguides on lithium niobate substrates that we called high vacuum proton exchange (HiVacPE). The main purpose was to improve the reproducibility and the quality of the produced waveguides by limiting and controlling the water traces in the melt, which is used for the ionic exchange. Moreover, we discovered that, when the acidity of the bath is increased, depending on the substrate orientation (Z-cut or X-cut) the waveguides are completely different in terms of crystallographic properties, index profiles, and nonlinearity. The best-obtained channel waveguides exhibit a refractive index contrast as high as 0.04 without any degradation of the crystal nonlinearity and state-of-the-art propagation losses (0.16 dB/cm). We have also demonstrated that the HiVacPE process allows fabricating waveguides on Z-cut substrate with high-index contrast up to 0.11 without degrading the crystal nonlinearity but high strain induced propagation losses. On top of that, we proposed an original and very useful method of analyzing waveguides with complex index profiles. This method can be used for the analysis of any waveguides, whose core contains several layers.

Vertically integrated spot-size converter in AlGaAs-GaAs.

We report on the demonstration of a spot size converter (SSC) for monolithic photonic integration at a wavelength of 850nm on a GaAs substrate. We designed and fabricated a dual-waveguide AlGaAs chip. The design consists of a lower waveguide layer for efficient end-fire coupling to a single-mode fiber, an upper waveguide layer for high refractive index contrast waveguides, and a vertical SSC to connect the two waveguide layers. We measured a SSC conversion efficiency of 91% (or -0.4 dB) between the upper and lower waveguide layers for the TE mode at a wavelength of 850nm.

Augmented low index waveguide for confining light in low index media

AbstractGuiding light in the low index region of a high refractive index contrast waveguide can be beneficial for many applications including sensing, nonlinear optics and electro‐optics. Existing methods to achieve this goal suffer from fabrication complexity, large loss, or poor optical confinement. We propose a simple structure to achieve a significant enhancement of light confinement in the low index medium. We explain the guiding principle of this structure using geometrical optics, and suggest a number of applications where this guiding structure can provide significant performance benefits. The combination of simplicity, ease of fabrication, and good confinement makes this waveguide an attractive choice for a wide range of applications. To illustrate this, we consider the integration of a nonlinear polymer with a silicon photonic waveguide, and show that significantly better performance with an easier fabrication process can be achieved using this new scheme. image

A vertical coupler for InP active components

A discussion about a coupler of a InP active component on a silicon-on-insulator (SOI) wafer is presented for applications in optical interconnects. A model for the design and evaluation of the coupler is developed using a coupling length based a model solver, the model solver is specifically suited for high index contrast waveguides with small area, which show more accurate for compact integrated optical devices. The photonic coupler is fabricated using microelectronics equipment for compatibility towards future generation electronic integrated circuit processing. Measured coupler efficiency is 65% to valid our model. This will be helpful for the design of InP active components.

Active phase correction of high resolution silicon photonic arrayed waveguide gratings.

Arrayed waveguide gratings provide flexible spectral filtering functionality for integrated photonic applications. Achieving narrow channel spacing requires long optical path lengths which can greatly increase the footprint of devices. High index contrast waveguides, such as those fabricated in silicon-on-insulator wafers, allow tight waveguide bends which can be used to create much more compact designs. Both the long optical path lengths and the high index contrast contribute to significant optical phase error as light propagates through the device. Therefore, silicon photonic arrayed waveguide gratings require active or passive phase correction following fabrication. Here we present the design and fabrication of compact silicon photonic arrayed waveguide gratings with channel spacings of 50, 10 and 1 GHz. The largest device, with 11 channels of 1 GHz spacing, has a footprint of only 1.1 cm2. Using integrated thermo-optic phase shifters, the phase error is actively corrected. We present two methods of phase error correction and demonstrate state-of-the-art cross-talk performance for high index contrast arrayed waveguide gratings. As a demonstration of possible applications, we perform RF channelization with 1 GHz resolution. Additionally, we generate unique spectral filters by applying non-zero phase offsets calculated by the Gerchberg Saxton algorithm.

Efficiency of evanescent excitation and collection of spontaneous Raman scattering near high index contrast channel waveguides.

We develop and experimentally verify a theoretical model for the total efficiency η0 of evanescent excitation and subsequent collection of spontaneous Raman signals by the fundamental quasi-TE and quasi-TM modes of a generic photonic channel waveguide. Single-mode silicon nitride (Si3N4) slot and strip waveguides of different dimensions are used in the experimental study. Our theoretical model is validated by the correspondence between the experimental and theoretical absolute values within the experimental errors. We extend our theoretical model to silicon-on-insulator (SOI) and titanium dioxide (TiO2) channel waveguides and study η0 as a function of index contrast, polarization of the mode and the geometry of the waveguides. We report nearly 2.5 (4 and 5) times larger η0 for the fundamental quasi-TM mode when compared to η0 for the fundamental quasi-TE mode of a typical Si3N4 (TiO2 and SOI) strip waveguide. η0 for the fundamental quasi-TE mode of a typical Si3N4, (TiO2 and SOI) slot waveguide is about 7 (22 and 90) times larger when compared to η0 for the fundamental quasi-TE mode of a strip waveguide of the similar dimensions. We attribute the observed enhancement to the higher electric field discontinuity present in high index contrast waveguides.

Tubular depressed cladding waveguide laser realized in Yb: YAG by direct inscription of femtosecond laser

We report on the fabrication of tubular depressed cladding waveguides in single crystalline Yb:YAG by the direct femtosecond laser writing technique. Full control over the confined light spatial distribution is demonstrated by the photoinscription of high index contrast waveguides with tubular configuration. Under optical pumping, highly efficient laser oscillation in depressed cladding waveguide at 1030 nm is demonstrated. The maximum output power obtained is 68 mW with a slope efficiency of 35% for an outcoupling transmission of 50%. A slope efficiency as high as 44% is realized when the coupling output ratio is 91% and a low lasing threshold of 70 mW is achieved with the output coupling mirror of 10%.

Broadband polarization independent nanophotonic coupler for silicon waveguides with ultra-high efficiency.

Coupling of light to and from integrated optical circuits has been recognized as a major practical challenge since the early years of photonics. The coupling is particularly difficult for high index contrast waveguides such as silicon-on-insulator, since the cross-sectional area of silicon wire waveguides is more than two orders of magnitude smaller than that of a standard single-mode fiber. Here, we experimentally demonstrate unprecedented control over the light coupling between the optical fiber and silicon chip by constructing the nanophotonic coupler with ultra-high coupling efficiency simultaneously for both transverse electric and transverse magnetic polarizations. We specifically demonstrate a subwavelength refractive index engineered nanostructure to mitigate loss and wavelength resonances by suppressing diffraction effects, enabling a coupling efficiency over 92% (0.32 dB) and polarization independent operation for a broad spectral range exceeding 100 nm.

Mode size converter between high-index-contrast waveguide and cleaved single mode fiber using SiON as intermediate material.

High-index-contrast (HIC) waveguide such as Si and Si3N4 has small mode size enabling compact integration. However, the coupling loss with single mode fiber is also remarkable owning to the mode mismatching. Therefore, mode size converter, as the interface between HIC waveguide and optical fiber, takes an important role in the field of integrated optics. The material with refractive index (RI) between HIC waveguide and optical fiber can be used as a bridge to reduce the mode mismatching loss. In this letter, we employ silicon oxynitride (SiON) with RI about 1.50 as the intermediate material and optimize the structure of the SiON waveguide to match with cleaved single mode fiber and HIC waveguide separately. Combined with inverse taper and suspended structure, the mismatching loss is reduced and the dependence to the dimension of the structure is also released. The coupling loss is 1.2 and 1.4 dB/facet for TE and TM mode, respectively, with 3 dB alignment tolerance of ± 3.5 μm for Si(3)N(4) waveguide with just 200 nm-wide tip. While for Si waveguide, a critical dimension of 150 nm is applied due to the higher index contrast than Si(3)N(4) waveguide. Similar alignment tolerance is realized with coupling loss about 1.8 and 2.1 dB/facet for TE and TM mode. The polarization dependence loss (PDL) for both platforms is within 0.5 dB.

Model of Vernier devices in silicon-on-insulator technology

In order to increase the number of channels that could be multiplexed or demultiplexed in the dense wavelength division multiplexed (DWDM) system based on the resonators on silicon-on-insulator (SOI) technology, the Vernier effect in the series-coupled racetrack resonators is presented to extend the free spectral range (FSR) of the DWDM systems. A method is developed based on a matrix approach to simulate Vernier devices. A three-dimensional full vectorial finite difference (FVFD) model, specifically suited for high index contrast and smaller size waveguides, for example, a waveguide in SOI technology, is developed to obtain the properties of a waveguide. Finally, the Vernier effect in the two series-coupled racetrack resonators is experimentally verified with an improved FSR and interstitial resonance suppression.