Propagation Loss Of Waveguide Research Articles

Plasmonic properties of silicon nitride encapsulated copper

We investigated the optical properties (i.e. plasmonic loss) of thin-film copper that was deposited and processed in a 300mm CMOS pilot line. The optical properties at 1550 nm (QSPP,Cu = 3921 ± 350) were evaluated by measuring the propagation loss of dielectric loaded plasmonic waveguides on samples with high and low root mean square surface roughness (Rq) with and without a silicon nitride diffusion barrier and at room temperature and cryogenic temperatures. In our fabrication result, the average grain size is 2.08±0.05μm, which is much larger than the mean free path of free electrons in copper, so the surface roughness becomes the main cause of the waveguide propagation loss from the fabrication constrains. Further, we show that copper can be encapsulated by a 15 nm silicon nitride diffusion and oxidation barrier without degrading the optical properties.

Computational spectrometer with multi-channel cascaded silicon add-drop micro-ring resonators.

The increasing demand for portable spectral analysis has driven the development of miniaturized spectrometers. Computational spectrometers, based on algorithmic reconstruction, are a potential solution to meet this demand. We report on the design and implementation of an integrated computational spectrometer on a silicon-on-insulator (SOI) substrate. The device is based on a 5-stage binary tree of cascaded silicon add-drop micro-ring resonators (MRRs). One of the 32 branches serves as the reference channel. Each of the other 31 branches has 4 cascaded MRRs with arbitrary coupling coefficients, cavity perimeters, and center distances. By using add-drop MRRs, we have 62 filter channels with 31 branches. It has no intrinsic structural reflection and scattering losses other than the excess loss in the 1 × 2 splitters and the waveguide propagation loss. The chip has a footprint of 1.5 mm2 and a resolution of 0.11 nm in the C-band. Broadband spectrum reconstruction with bandwidth >10 nm is also demonstrated.

300-nm-thick, ultralow-loss silicon nitride photonic integrated circuits by 8-in. foundry production

Silicon nitride (Si3N4) photonic integrated circuits are rapidly developing in recent decades. The low loss of Si3N4 attracts significant attention and facilitates a wide range of applications in integrated photonics. In this work, we demonstrate the foundry fabrication of a 300-nm-thick 8-in. wafer-scale Si3N4 platform, with a microresonator intrinsic quality factor of up to 15×106, corresponding to an ultralow loss of 2.2 dB/m. Leveraging this platform, we develop a mature process design kit, achieving a single-mode waveguide propagation loss of less than 5 dB/m, an edge coupler loss of 1.3 dB, and an insertion loss of 0.07 dB for multimode interference couplers. Utilizing the processed Si3N4 chip, we realize a hybrid integrated tunable external cavity laser with a tuning range from 1534 to 1602 nm, a record-high side-mode suppression ratio of up to 76 dB, an optical power of 26 mW, and an intrinsic linewidth of down to 314 Hz. Our work lays a solid foundation for the further development of applications, including nonlinear optics, quantum optics, optical communications, and ranging.

Preparation and loss analysis of Ge on Si SWIR optical strip waveguides

On-chip integrated photonics with operating wavelength at short-wave infrared region is becoming popular for its potential advantages in extending the telecommunication bandwidth and the application in gas sensing. As an important component of integrated optoelectronic chips, high-performance waveguides have attracted widespread attention. In this work, high-performance Ge waveguides were designed and fabricated on the Ge-on-Si platform, in which germanium films were epitaxially grown by the chemical vapor deposition process. The propagation loss for the waveguide was measured to be 2.34 dB/cm at the wavelength of 2 μm through the cut-back method. The quantitative relationship between the waveguide propagation loss and sidewall roughness as well as material defect density was established by introducing an empirical coefficient m. The m value was calculated to be 3.8 × 10−4 dB for the waveguide with a fixed Ge thickness of 1.5 μm by fitting the experimental propagation loss data at an operating wavelength of 2 μm. This formula provides a significant reference for future design of waveguide dimension structures, where defect induced absorption loss is non-negligible.

Er:LiYF4 planar waveguide laser at 2.8 μm

We report on a mid-infrared erbium planar waveguide laser operating on the 4I11/2 → 4I13/2 transition. It employs a heavily doped 10.6 at. % Er3+:LiYF4 single-crystalline layer grown by liquid-phase epitaxy. The waveguide laser delivers a maximum output power of 191 mW at ∼2809 nm with a slope efficiency of 15%, a linear polarization, and a laser threshold of 134 mW. The waveguide propagation losses are 0.4 ± 0.2 dB/cm. The polarized spectroscopic properties of the Er3+:LiYF4 layers are also investigated. The stimulated-emission cross section of Er3+ ions amounts to 0.87 × 10−20 cm2 at 2809 nm for π-polarization. Er3+:LiYF4 epitaxial layers represent a promising platform for integrated low-loss mid-infrared light sources.

Design of Horizontally Stacked AlN and Dielectric Cores Transverse Quasi‐Phase‐Matched Channel Waveguide for Squeezed Light Generation

AlN is a promising candidate for photonic integrated circuits. In this study, horizontally stacked transverse quasi‐phase‐matched (QPM) waveguides with an AlN center core and Si3N4, Ta2O5, and TiO2 side cores were designed to generate squeezed light at 920 nm. The squeezing level was calculated by considering the propagation loss at the wavelengths of the squeezed light and pump light. Using TiO2 for the side cores enhanced the electric field amplitude of the pump light in the AlN center core, and a high nonlinear coupling coefficient was obtained. Owing to the small propagation loss of the AlN waveguide on sapphire and high nonlinear coupling coefficient of the transverse QPM with TiO2 side cores, a squeezing level of over 8.1 dB was expected with a 1 cm long waveguide and pump power of 70 mW.

Femtosecond laser writing of robust waveguides in optical fibers with enhanced photosensitivity

We report the femtosecond laser writing of meter-long optical waveguides inscribed through the coating of specifically designed optical fibers. In order to improve the material photosensitivity and to ensure non-guiding optical fibers for subsequent laser processing of the waveguiding core, a depressed refractive index core design is implemented by co-doping a large portion of the optical fiber with germanium oxide and fluorine. The enhanced photosensitivity provided by further deuterium loading these fibers allows laser-writing of large refractive index contrast waveguides over wide cross sections. To mitigate the formation of photoinduced color centers causing high propagation losses in the photo-written waveguides, thermal annealing up to 400°C is performed on polyimide-coated laser-written fibers. Although the refractive index contrast decreases, the propagation losses are drastically reduced down to 0.08 dB/cm at 900nm allowing a robust single-mode guiding from visible to near infrared. Our results pave the way towards the development of a new generation of optical fibers and photonic components with arbitrarily complex designs.

Tunable and stable micro-ring resonator based on thin-film lithium tantalate

As ferroelectric materials, lithium tantalate and lithium niobate share similar material characteristics, such as a high Pockels effect and nonlinear optical coefficients. When compared to lithium niobate, lithium tantalate offers a higher optical damage threshold, a broader transparent window, and lower birefringence, making it a promising candidate for high-performance electro-optical photonic integrated devices. In this study, we design and successfully fabricate micro-ring resonators on an acoustic-grade lithium-tantalate-on-insulator wafer, demonstrating their tunability and dynamic modulation capabilities. Experimental results indicate that the achieved thin-film lithium tantalate based micro-ring resonator exhibits an intrinsic Q-factor of 8.4 × 105, corresponding to a waveguide propagation loss of 0.47 dB/cm and a tuning efficiency of 1.94 pm/V. More importantly, as compared to those based on thin-film lithium niobate, a much weaker photorefractive effect and drift phenomenon around the 1550 nm wavelength under a direct-current drive are observed in the present fabricated thin-film lithium tantalate micro-rings with a silicon oxide over-cladding and a tuning electrode on top.

GeAsSeTe/GeAsSe Pedestal Waveguides for Long-Wave Infrared Tunable on-Chip Spectroscopy

A dry-etched pedestal chalcogenide waveguide platform, designed for use in long-wave IR spectrometer applications, is demonstrated, fabricated and optically characterized. The optical layers were deposited on pre-patterned dry-etched silicon pedestals. An exceptionally low waveguide propagation loss was measured, at around 0.1 dB/cm at λ = 10 μm. The modal thermo-optic coefficient of the waveguide was experimentally estimated to be approximately 1.1 × 10−4 C−1 at λ = 1.63 μm, which is comparable to that of Si and GaAs. Waveguide spiral interferometers were fabricated, proving the potential for realization of more complex, chalcogenide-based, integrated photonic circuits. The combination of low propagation losses and a strong thermo-optic coefficient makes this platform an ideal candidate for utilization in on-chip tunable spectrometers in the long-wave IR wavelength band.

Propagation loss in polyfluorene waveguides due to nanometer-roughness at their interfaces, studied by amplified spontaneous emission measurements

The propagation loss in single-mode asymmetric waveguides due to interface nanometer-roughness was studied using amplified spontaneous emission (ASE) measurements. Poly(9,9-dioctylfluorene) (F8) was used as the organic gain medium and the structure of the asymmetric waveguide was quartz glass substrate/F8/air. The propagation loss was measured at the ASE wavelength (447 nm) of amorphous F8, and the surface roughness of the substrate and F8 was measured using an atomic force microscope. The propagation losses of F8 waveguides with different F8 slab thicknesses were in good agreement with those calculated using an analytical expression for single-mode asymmetric waveguides with nanometer-roughness interfaces. The results presented herein will be useful for the design of high-performance organic lasers and organic optoelectronic integration systems.

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.

Orange surface waveguide laser in Pr:LiYF4 produced by a femtosecond laser writing.

Depressed-cladding surface channel waveguides were inscribed in a 0.5 at.% Pr:LiYF4 crystal by femtosecond Direct Laser Writing. The waveguides consisted of a half-ring cladding (inner diameter: 17 µm) and side structures ("ears") improving the mode confinement. The waveguide propagation loss was as low as 0.14 ± 0.05 dB/cm. The orange waveguide laser operating in the fundamental mode delivered 274 mW at 604.3 nm with 28.4% slope efficiency, a laser threshold of only 29 mW and linear polarization (π), representing record-high performance for orange Pr waveguide lasers.

Low-loss operation of silicon-on-insulator integrated components at 2.6-2.7 µm.

Development of mid-infrared photonics is gaining attention, driven by a multitude of sensing applications requiring increasingly compact and cost-effective photonics systems. To this end, low-loss operation of µm-scale silicon-on-insulator photonic integration elements is demonstrated for the 2.6-2.7 µm wavelength region. The platform utilizes the 3 µm thick silicon core layer technology enabling demonstration of low-loss and low birefringence waveguides. Measurements of record low single mode waveguide propagation losses of 0.56 ± 0.09 dB/cm and bend losses <0.08 dB for various miniaturized bend geometries are presented and validated by simulation. Furthermore, a wavelength filter based on echelle grating that allows to select several operating channels within the 2.64-2.7 µm band, with a linewidth of ∼1.56 nm for each channel is presented.

Etchless photonic integrated circuits enabled by bound states in the continuum: tutorial

We provide a detailed tutorial demonstrating how the principle of “bound states in the continuum” (BICs) enables ultralow-loss guiding and routing of photons in photonic integrated circuits fabricated with an etchless process. Here, BICs refer to the nondissipative transverse magnetic (TM) polarized bound modes that exist in the transverse electric (TE) polarized continuum. First, we provide a theoretical analysis of BICs based on the coupling between the TM bound modes and the TE continuum, which is next verified by numerically simulated waveguide propagation loss of the TM bound modes for different waveguide geometries. Then, we present the experimental details, which include fabrication processes and characterization methods for various types of BIC-based integrated photonic devices. Finally, we discuss the superiority and versatility of the BIC-based integrated photonic platform, which can be adopted for different thin-film substrates, for different wavelength ranges, and for heterogeneous integration with different functional materials.

Toward point-of-care diagnostics: Running enzymatic assays on a photonic waveguide-based sensor chip with a portable, benchtop measurement system.

We demonstrate a portable, compact system to perform absorption-based enzymatic assays at a visible wavelength of 639 nm on a photonic waveguide-based sensor chip, suitable for lab-on-a-chip applications. The photonic design and fabrication of the sensor are described, and a detailed overview of the portable measurement system is presented. In this publication, we use an integrated photonic waveguide-based absorbance sensor to run a full enzymatic assay. An assay to detect creatinine in plasma is simultaneously performed on both the photonic sensor on the portable setup and on a commercial microplate reader for a clinically relevant creatinine concentration range. We observed a high correlation between the measured waveguide propagation loss and the optical density measurement from the plate reader and measured a limit-of-detection of 4.5 μM creatinine in the sensor well, covering the relevant clinical range for creatinine detection.

Wideband dispersion-free THz waveguide platform

We present a versatile THz waveguide platform for frequencies between 0.1 THz and 1.5 THz, designed to exhibit vacuum-like dispersion and electric as well as magnetic field enhancement. While linear THz spectroscopy benefits from the extended interaction length in combination with moderate losses, nonlinear THz spectroscopy profits from the field enhancement and zero dispersion, with the associated reshaping-free propagation of broadband single- to few-cycle THz pulses. Moreover, the vacuum-like dispersion allows for velocity matching in mixed THz and visible to infrared pump-probe experiments. The platform is based on the motif of a metallic double ridged waveguide. We experimentally characterize essential waveguide properties, for instance, propagation and bending losses, but also demonstrate a junction and an interferometer, essentially because those elements are prerequisites for THz waveform synthesis, and hence, for coherently controlled linear and nonlinear THz interactions.

Mid-infrared integrated silicon–germanium ring resonator with high Q-factor

We report the realization of a silicon–germanium on silicon ring resonator with high Q-factor at mid-infrared wavelengths. The fabricated ring exhibits a loaded Q-factor of 236 000 at the operating wavelength of 4.18 µm. Considering the combined waveguide propagation losses and bending losses, which are measured to be below 0.2 dB/cm, even higher Q-factors could be achieved on this platform. Furthermore, our dispersion engineering of the waveguides should make these microrings suitable for nonlinear optical applications. These results pave the way for sensing applications and nonlinear optics in the mid-infrared range.

Nonlinear optical materials based on fluorinated polyurethane-imides and their application in waveguide devices

Two monomers, a second-order nonlinear optical azo chromophore C containing a tricyanofuran electron acceptor and a dihydroxyethyl nitrogen electron donor, and a bisphenol AF-type diether dianhydride (BPAFDA), were designed and synthesized. Fluorinated polyurethaneimide (PUI) electro-optic (EO) waveguide materials were prepared using the synthesized monomers polymerized with 4,4′-diphenylmethane diisocyanate (MDI). The structures of the synthesized chromophore C, BPAFDA, and polymers of PUI were characterized by 1HNMR and FTIR, and the thermal properties of the polymers were characterized by DSC and TGA. The prepared PUI exhibited good film-forming properties with glass transition temperatures (Tg) between 160–169°C and over 300°C at 5% thermal weight loss in a nitrogen atmosphere. The experimental results showed that the fluorinated PUIs possessed an EO coefficient of 56–60 p.m./V at 1550 nm and the optical propagation loss of the polymer waveguide was between 1.3–1.4 dB/cm at 1550 nm. Using PUI as the core material of the waveguides, EO modulators with Mach-Zehnder (MZ) structure were designed and prepared, showing good EO modulation performance at 1550 nm.

Ultrahigh-Q lithium niobate microring resonator with multimode waveguide.

Difficulty in etching lithium niobate (LN) results in a relatively high propagation loss, which necessitates sophisticated processes to fabricate high-quality factor (Q) microresonators. Here, we fabricate a multimode microring resonator with an intrinsic Q of 6 × 106, which exhibits a propagation loss 50 times lower than that of a single-mode LN microring fabricated under the same process. Notably, the excitation of higher-order modes in the multimode microring is effectively suppressed by utilizing the Euler bend. The highly regular transmission spectrum of the resonator demonstrates a free spectral range (FSR) of 56 GHz. Based on this microresonator, we implement a bandpass microwave photonic filter with an ultra-narrow 3 dB bandwidth of 47.5 MHz and a large tuning range of 2-26.5 GHz. It can be anticipated that the combination of existing advanced etching techniques with this work will drive the propagation loss of a LN waveguide closer to the material absorption loss, significantly facilitating the optimization of performance in applications requiring ultrahigh-Q LN microresonators, such as frequency combs, frequency conversion, electro-optic modulation, and quantum photonics.

High-Q chalcogenide racetrack resonators based on the multimode waveguide.

We propose and demonstrate a highquality (Q) factor racetrack resonator based on uniform multimode waveguides in high-index contrast chalcogenide glass film. Our design features two carefully designed multimode waveguide bends based on modified Euler curves, which enable a compact 180° bend and reduce the chip footprint. A multimode straight waveguide directional coupler is utilized to couple the fundamental mode without exciting higher-order modes in the racetrack. The fabricated micro-racetrack resonator shows a record-high intrinsic Q of 1.31×106 for selenide-based devices, with a relatively low waveguide propagation loss of only 0.38dB/cm. Our proposed design has potential applications in power-efficient nonlinear photonics.