Single-mode Rib Waveguides Research Articles

Roughness Suppression in Electrochemical Nanoimprinting of Si for Applications in Silicon Photonics

Metal-assisted electrochemical nanoimprinting (Mac-Imprint) scales the fabrication of micro- and nanoscale 3D freeform geometries in silicon and holds the promise to enable novel chip-scale optics operating at the near-infrared spectrum. However, Mac-Imprint of silicon concomitantly generates mesoscale roughness (e.g., protrusion size≈45nm) creating prohibitive levels of light scattering. This arises from the requirement to coat stamps with nanoporous gold catalyst that, while sustaining etchant diffusion, imprints its pores (e.g., average diameter≈42nm) onto silicon. In this work, roughness is reduced to sub-10nm levels, which is in par with plasma etching, by decreasing pore size of the catalyst via dealloying in far-from equilibrium conditions. At this level, single-digit nanometric details such as grain-boundary grooves of the catalyst are imprinted and attributed to the resolution limit of Mac-Imprint, which is argued to be twice the Debye length (i.e., 1.7nm)-a finding that broadly applies to metal-assisted chemical etching. Last, Mac-Imprint is employed to produce single-mode rib-waveguides on pre-patterned silicon-on-insulator wafers with root-mean-square line-edge roughness less than 10nm while providing depth uniformity (i.e., 42.9±5.5nm), and limited levels of silicon defect formation (e.g., Raman peak shift<0.1cm-1 ) and sidewall scattering.

Optical loss characterization of SEO100C electro-optic polymer within single mode rib waveguides

Designing high-performance electro-optic polymer modulators requires insight into device-specific waveguide losses. Losses for rib waveguiding structures fabricated using SE0100C core and NOA73 cladding layers are reported. The waveguide loss was measured at 4.2 dB/cm, which is high compared to losses for different electro-optic rib waveguides reported at 1.2-2.9 dB/cm. The refractive index of unpoled SEO100C was measured at 1.655, expanding on the manufacturer’s reported optical constants of 1.65 (TE) and 1.7 (TM) for poled SEO100C. Utilizing the reported specific structure loss in device design for minimal loss will facilitate high performance electro-optic polymer modulator applications.

Design and fabrication of surface trimmed silicon-on-insulator waveguide with adiabatic spot-size converters.

Theoretical and experimental studies reveal that a predefined single-mode rib waveguide fabricated in silicon-on-insulator (SOI) substrate with a device layer thickness of 2μm can be adiabatically trimmed down to submicron waveguide dimensions (<1 μm), resulting in regional modification of waveguide properties. The fabrication process involves physical trimming/removal of a waveguide surface by plasma etchants that is spatially filtered by a shadow mask with a rectangular aperture inside a reactive ion etching system. The exact position of a shadow mask above a sample surface has been optimized (∼500 μm) to obtain the desired adiabatic spot-size converters of length up to 1mm at both ends of the trimmed waveguides. For experimental demonstration, three different sets of 15-mm-long single-mode waveguides fabricated in 2-μm SOI were adiabatically trimmed in the middle for three different lengths of 3, 5, and 7mm, respectively. Excess propagation loss and group index of a trimmed submicron waveguide section were extracted by analyzing the wavelength-dependent Fabry-Perot transmission characteristics of the device with polished input/output end facets. The insertion loss of a typical spot-size converter designed for the guidance of TE-like polarization has been recorded to be ∼0.25 dB for a wide range of wavelengths (1500 nm≤λ≤1600 nm). As predicted by numerical simulation, no polarization rotation has been observed in all the trimmed submicron waveguides. The proposed surface trimming technique can be potentially used to tune the waveguide cross-section/geometry for phase error correction and/or to avail stronger light-matter interactions at a desired location of an integrated optical circuit.

Large cross-section rib silicon-on-insulator (SOI) S-bend waveguide

S-bend SOI waveguide is known as the most critical part for SOI device design. Normalized output power for the different parameter of the S-bend waveguide has been analyzed using OptiBPM simulator in 1.55μm communication wavelength. Dimension of 5×5μm2 single-mode rib waveguide is chosen. The variable parameters are transition offset and lateral offset, given the waveguide length of 100μm–5000μm. The maximum normalized output power achieved at the waveguide length of 550μm for the 10μm S-bend offset is 95.81%. Moreover, the ideal lateral offset is 2.7μm with 2.52% normalized output power improvement.

Design and fabrication of a high-performance evanescently coupled waveguide photodetector

In this paper, we present the design, fabrication, and measurement of an evanescently coupled waveguide photodetector operating at 1.55 μm, which mainly comprises a diluted waveguide, a single-mode rib waveguide and a p—i—n photodiode with an extended optical matching layer. The optical characteristics of this structure are studied by using a three-dimensional finite-difference time-domain (3D FDTD) method. The photodetector exhibits a high 3-dB bandwidth of more than 35 GHz and a responsivity of 0.291 A/W at 1550 nm directly coupled with a cleaved fiber. Moreover, a linear response of more than 72-mW optical power is achieved, where a photocurrent of more than 21 mA is obtained at a reverse bias voltage of 3 V.

A proposal for digital electro-optic switches with free-carrier dispersion effect and Goos-Hanchen shift in silicon-on-insulator waveguide corner mirror

In a silicon-on-insulator (SOI) waveguide corner mirror (WCM) structure, with the quantum process of a frustrated total internal reflection (FTIR) phenomenon and the time delay principle in the two-dimensional potential barrier tunneling process of a mass of particles, we derive an accurate physical model for the Goos-Hanchen (GH) shift of optical guided-mode in the FTIR process, and in principle match the GH shift jumping states with the independent guided-modes. Then, we propose and demonstrate a new regime of 1 × N digital optical switches with a matching state between the free-carrier dispersion (FCD) based refractive index modulation (RIM) of silicon to create a GH shift jumping function of a photonic signal at the reflecting interface and the independent guided-modes in the FTIR process, where a MOS-capacitor type electro-optic modulation regime is proposed and discussed to realize an effective FCD-based RIM. At the critical matching state, i.e., the incident of an optical beam is at the vicinity of Brewster angle in the WCM, a mini-change of refractive index of waveguide material can cause a great jump of GH shift along the FTIR reflecting interface, and further a 1 × N digital optical switching process could be realized. For a 350–500 nm single-mode rib waveguide made on the 220 nm CMOS-compatible SOI substrate and with the FCD effect based RIM of silicon crystal, a concentration variation of 1018–1019 cm−3 has caused a 0.5–2.5 μm GH shift of reflected beam, which is at 2–5 times of a mode-size and hence radically convinces an optical switching function with a 1 × 3-1 × 10 scale.

Dramatic size reduction of waveguide bends on a micron-scale silicon photonic platform

We demonstrate theoretically and experimentally how highly multimodal high index contrast waveguides with micron-scale cores can be bent, on an ultra-broad band of operation, with bending radii below 10 µm and losses for the fundamental mode below 0.02 dB/90°. The bends have been designed based on the Euler spiral and fabricated on 4 µm thick SOI. The proposed approach enabled also the realization of 180° bends with 1.27 µm effective radii and 0.09 dB loss, which are the smallest low-loss bends ever reported for an optical waveguide. These results pave the way for unprecedented integration density in most semiconductor platforms.

Optimizing Design of a Single-Mode Optical Rib Waveguide

The modal characteristics of the optical rib waveguides are analyzed systematically based on the effective index method (EIM) and the improved normalized eigen equations of the waveguides are obtained by defined normalized parameters, which the thickness of the outer slab and the normalized outer ridge height of the waveguides are used as parameters in the single-mode condition. The achieved expressions are simpler and easier to optimize the structure parameters in designing a single-mode rib waveguide.

Single-mode rib waveguides in (Yb,Nb) : RbTiOPO4 by reactive ion etching

We present the fabrication of single-mode rib waveguides in (Yb,Nb) : RbTiOPO4 ((Yb,Nb) : RTP) by reactive ion etching (RIE) with a combination of SF6 and Ar gases. The influence of the gas pressure, RF power and gas ratio on the etch rate and surface quality was studied to optimize the etching of RTP. Optimized parameters were used to fabricate rib waveguides in a (Yb,Nb) : RTP film, grown by liquid phase epitaxy, with an etch rate of 10 nm min−1. Channel waveguide propagation was demonstrated for the first time in an RIE etched (Yb,Nb) : RTP rib structure.

Benzocyclobutene (BCB 4022-35) Single Mode Rib Waveguides

Design, fabrication and characterization of a photodefinable benzocyclobutene (BCB 4022-35 is a product of DowTM) single mode rib optical waveguide is presented in this work. At first, the refractive index of polymer film is measured by the method of prism coupling. We design the single mode rib waveguide based on the geometrical adjustment of rib width, total waveguide height using conventional R-soft BPM and effective index methods. Based on the design results, BCB waveguides were fabricated by using photolithographic process with wet etching, which is quite complicated and time consuming. In this work, the insertion and propagation losses of waveguide are measured by using the conventional cut back method. The insertion losses of waveguide are 3.95 dB and 4.08 dB for TE and TM modes respectively. The propagation losses of waveguide are 1.09dB/cm and 1.20dB/cm for TE and TM modes respectively. 

大截面单模脊波导的几何结构优化

大截面单模脊波导的几何结构优化

1.55 μm silicon-based reflection-type waveguide-integrated thermo-optic 2 × 2 switch

In this work a waveguide-integrated 2×2 switch operating at the infrared communication wavelength of 1550nm is proposed and theoretically discussed. The device is based on the total internal reflection (TIR) phenomenon and the thermo-optic effect (TOE) in hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si). It takes advantage of a bandgap-engineered a-Si:H layer to explore the properties of an optical interface between materials showing similar refractive indexes but different thermo-optic coefficients. In particular, thanks to modern plasma-enhanced chemical vapour deposition (PECVD) techniques, the refractive index of the amorphous film can be properly tailored to match that of c-Si at a given temperature. TIR may be therefore achieved at the interface by acting on the temperature. The device is integrated in a 4μm-wide and 3μm-thick single-mode rib waveguide. The substrate is a silicon-on-insulator (SOI) wafer with an oxide thickness of 500nm. We calculated an output crosstalk always better than 24dB and insertion losses as low as 3.5dB.

Solution-processed chalcogenide glass for integrated single-mode mid-infrared waveguides

Chalcogenide glass materials exhibit a variety of optical properties that make them desirable for near- and mid-infrared communications and sensing applications. However, processing limitations for these photorefractive materials have made the direct integration of waveguides with sources or detectors challenging. Here we demonstrate the viability of two complementary soft lithography methods for patterning and integrating chalcogenide glass waveguides from solution. One method, micro-molding in capillaries (MIMIC), is shown to fabricate multi-mode As(2)S(3) waveguides which are directly integrated with quantum cascade lasers (QCLs). In a second method, we demonstrate the ability of micro-transfer molding (µTM), to produce arrays of single mode rib waveguides (2.5 µm wide and 4.5 µm high) over areas larger than 6 cm(2) while maintaining edge roughness below 5.1 nm. These methods form a suite of processes that can be applied to chalcogenide solutions to create a diverse array of mid-IR optical and photonic structures ranging from <5 to 10's of µm in dimension.

Single-Mode Rib Waveguides Fabricated by Means of Sol-Gel Method

Rib waveguides were fabricated with the use of selective, wet chemical etching of two-component waveguide films SiO2:TiO2 which were obtained using sol–gel method. Photoresist was applied as a mask in the process. The etching of the films SiO2:TiO2 was carried out in water solution of ammonia fluoride. The paper presents the results of theoretical analysis as well as the power distributions in the fabricated strip waveguides.

Design and fabrication of Poly(dimethylsiloxane) arrayed waveguide grating

We have designed, fabricated and characterized poly(dimethylsiloxane) (PDMS) arrayed waveguide grating (AWG) with four-channel output for operation in the visible light wavelength range. The PDMS AWG was realized based on the single-mode PDMS rib waveguide. The device was designed for 1 nm channel spacing with the wavelength ranging from 639 to 644 nm. The measured insertion loss is 11.4 dB at the peak transmission spectrum and the adjacent crosstalk is less than -16 dB. The AWG device occupies an area of 7.5 × 15 mm(2). PDMS AWG has the potential for integration with microfluidics in a monolithic PDMS lab-on-a-chip device for visible light spectroscopy applications.

Modeling and analysis of birefringence in magneto-optical thin film made by SiO2/ZrO2 doped with ferrite of cobalt

The zero-birefringence condition is an important requirement for magneto-optical waveguide devices. It has been analyzed by means of Film Mode Matching method (FMM). The modelling is based on geometrical adjustments of the rib waveguide and especially the etch depth, which is a critical parameter in the design of rib waveguide. We determine the waveguide dimensions that would allow the same propagation constants for both polarizations TE and TM, and we have found that there are more choices for waveguide dimensions to produce birefringence-free waveguides and single-mode rib waveguide simultaneously.

Phase matched magneto optical single mode rib waveguides based on geometrical adjustments

In this work, we demonstrate via numerical simulation the design rules that must be imposed on the geometry of magneto optical rib waveguides to make them behave as single-mode and phase matched rib waveguides. Using film mode matching method, the results show that operation in both single mode and phase matching is possible under certain circumstances with selected geometries. We simulate the performance of such devices and we determined the waveguide dimensions (height, width, and etching depth) that satisfy single mode and phase matching conditions simultaneously.

Design and fabrication of Poly(dimethylsiloxane) single-mode rib waveguide

We have designed, fabricated and characterized poly(dimethylsiloxane) (PDMS) single-mode rib waveguides. PDMS was chosen specifically for the core and cladding. Combined with the soft lithography fabrication techniques, it enables an easy integration of microoptical components for lab-on-a-chip systems. The refractive index contrast, of 0.07% between the core and cladding for single-mode propagation was achieved by modifying the properties of the same base material. Alternatively, a higher refractive index contrast, of 1.18% was shown by using PDMS materials from two different manufacturers. The fabricated rib waveguides were characterized for mode profile characteristics and confirmed the excitation of the fundamental mode of the waveguide. The propagation loss of the single-mode rib waveguide was characterized using the cutback measurement method at a wavelength of 635 nm and found to be 0.48 dB/cm for of 0.07% and 0.20 dB/cm for of 1.18%. Y-branch splitter of PDMS single-mode rib waveguide was further demonstrated.

The Design and Fabrication of Poly(dimethylsiloxane) Single Mode Rib Waveguides for Lab-on-a-Chip Applications

This paper reports on the design and fabrication of Poly(dimethylsiloxane) (PDMS) single mode rib waveguide. PDMS was chosen as the base material for both the core and cladding of the waveguide. Such a PDMS waveguide allows an easy integration with microoptical components for lab-on-a-chip systems fabricated by soft lithography fabrication methods. The modification of the PDMS refractive index was achieved by diluting the base material with hexane. The fabricated PDMS rib waveguides were characterized for mode profile characteristics and the results confirmed the excitation of the waveguide's fundamental mode. The propagation loss was characterized at a wavelength of 635 nm and found to be 0.48 dB/cm. Such a low loss waveguide is useful as basic building block for complex microphonotnics device.

Large-core single-mode rib SU8 waveguide using solvent-assisted microcontact molding

This paper describes a novel fabrication technique for constructing a polymer-based large-core single-mode rib waveguide. A negative tone SU8 photoresist with a high optical transmission over a large wavelength range and stable mechanical properties was used as a waveguide material. A waveguide was constructed by using a polydimethylsiloxane stamp combined with a solvent-assisted microcontact molding technique. The effects on the final pattern's geometry of four different process conditions were investigated. Optical simulations were performed using beam propagation method software. Single-mode beam propagation was observed at the output of the simulated waveguide as well as the actual waveguide through the microscope image.