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This paper presents a new design for a 1 × 4 optical power splitter using multimode interference (MMI) coupler in silicon nitride (Si3N4) strip waveguide structures. The main functionality of the proposed design is to use Si3N4 for dealing with the back reflection (BR) effect that usually happens in silicon (Si) MMI devices due to the self-imaging effect and the higher index contrast between Si and silicon dioxide (SiO2). The optimal device parameters were determined through numerical optimizations using the beam propagation method (BPM) and finite difference time domain (FDTD). Results demonstrate that the power splitter with a length of 34.6 μm can reach equal distribution power in each output port up to 24.3% of the total power across the O-band spectrum with 0.13 dB insertion loss and good tolerance MMI coupler parameters with a shift of ±250 nm. Additionally, the back reflection range over the O-band was found to be 40.25–42.44 dB. This demonstrates the effectiveness of the incorporation using Si3N4 MMI and adiabatic input and output tapers in mitigating unwanted BR to ensure that a good signal is received from the laser. This design showcases the significant potential for data-center networks, offering a promising solution for efficient signal distribution and facilitating high-performance and reliable optical signal routing within the O-band range. By leveraging the advantages of Si3N4 and the MMI coupler, this design opens possibilities for advanced optical network architectures and enables efficient transmission of optical signals in the O-band range. [ABSTRACT FROM AUTHOR]

This paper presents a new design for a 1 × 4 optical power splitter using multimode interference (MMI) coupler in silicon nitride (Si3N4) strip waveguide structures. The main functionality of the proposed design is to use Si3N4 for dealing with the back reflection (BR) effect that usually happens in silicon (Si) MMI devices due to the self-imaging effect and the higher index contrast between Si and silicon dioxide (SiO2). The optimal device parameters were determined through numerical optimizations using the beam propagation method (BPM) and finite difference time domain (FDTD). Results demonstrate that the power splitter with a length of 34.6 μm can reach equal distribution power in each output port up to 24.3% of the total power across the O-band spectrum with 0.13 dB insertion loss and good tolerance MMI coupler parameters with a shift of ±250 nm. Additionally, the back reflection range over the O-band was found to be 40.25–42.44 dB. This demonstrates the effectiveness of the incorporation using Si3N4 MMI and adiabatic input and output tapers in mitigating unwanted BR to ensure that a good signal is received from the laser. This design showcases the significant potential for data-center networks, offering a promising solution for efficient signal distribution and facilitating high-performance and reliable optical signal routing within the O-band range. By leveraging the advantages of Si3N4 and the MMI coupler, this design opens possibilities for advanced optical network architectures and enables efficient transmission of optical signals in the O-band range.

The nanophotonic waveguide is gaining high popularity, because its significance is not only limited to the optical interconnects, but also it has been widely accepted for the sensor, filter, and other applications. The standard height of SOI-based nanophotonic waveguide is around 220 nm, which has already been explored extensively. However, some researchers have recently focused the attention on the reduced height waveguides (of 100 nm). Moreover, the photonic waveguides with the reduced height are suffering from high dispersion phenomena. To overcome this problem, the over-cladding technique, using silicon nitride (Si3N4) material, has been opted in the current work that improves the dispersion characteristics and other waveguide parameters. From the analysis, it has been observed that without the over-cladding layer, the dispersion is around −8000 ps/nm-km. Moreover, utilizing the over-cladding layer of thicknesses 200 nm and 100 nm, the dispersion values have been reduced respectively to around −3300 ps/nm-km and − 2000 ps/nm-km, with their respective strip widths of 500 nm and 800 nm, at the operating wavelength of 1.55 μm. Further, the current work can be extended for the design of optical filters and other related applications. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a cost-effective liquid refractive index (RI) sensor in polymer materials by employing a Mach-Zehnder interferometer (MZI) formed with the conventional strip waveguide. The proposed sensor can attain high sensitivity by choosing a suitable reference arm length to reduce optical path difference (OPD) between the two arms of the employed MZI. Our fabricated devices, which have the same sensing arm length of 7900 μm but different reference arm lengths of 7900.0, 7942.5, 7950.9, 7962.2, and 7969.5 μm, respectively, exhibit the maximum RI sensitivities of 33662.8 nm/RIU, and almost the same detection range (DR) of 0.0041 ± 0.0002 RIU.

We propose and demonstrate a cost-effective liquid refractive index (RI) sensor in polymer materials by employing a Mach-Zehnder interferometer (MZI) formed with the conventional strip waveguide. The proposed sensor can attain high sensitivity by choosing a suitable reference arm length to reduce optical path difference (OPD) between the two arms of the employed MZI. Our fabricated devices, which have the same sensing arm length of 7900 µm but different reference arm lengths of 7900.0, 7942.5, 7950.9, 7962.2, and 7969.5 µm, respectively, exhibit the maximum RI sensitivities of 33662.8 nm/RIU, and almost the same detection range (DR) of 0.0041 ± 0.0002 RIU. [ABSTRACT FROM AUTHOR]

Various photonic integrated components have been implemented by ultra-thin silicon-on-insulator (SOI) waveguides; therefore, it is desirable to couple ultra-thin SOI waveguides to plasmonic waveguides. In this paper, we present an ultra-thin SOI waveguide to a metal-dielectric-metal plasmonic waveguide based on a lens-funnel structure consisting of truncated Luneburg lens and metallic parabolic funnel. The lens is implemented by varying the guiding layer thickness. The effect of different parameters of the coupler's geometry is studied using the finite-difference time-domain method. The 1.13-μm-long coupler improves the average coupling efficiency in the C-band from 66.4 to 82.1%. The numerical simulations indicate that the coupling efficiency is higher than 69% in the entire O, E, S, C, L, and U bands of optical communication. [ABSTRACT FROM AUTHOR]

Two‐dimensional (2D) metasurface integrated with strip waveguide in silicon‐on‐insulator (SOI) has been designed to achieve high coupling efficiency for 3.8 μm wavelength. The optimisation in period and radius has been achieved using 3D finite‐difference time‐domain (FDTD). The calculated coupling efficiency in the in‐plane waveguide for the out‐of‐plane surface illumination is over 90% with a bandwidth of 1 μm. The design is consistent with the available lithography using 400 nm thick SOI for mid‐IR applications. Finally, monolithic integration can be achieved using standard multi‐project wafer run. [ABSTRACT FROM AUTHOR]

A miniaturized design of a Fabry–Perot resonator based on silicon strip and slot waveguide is presented. The corrugated Bragg gratings and a cavity are structured on both sides of a waveguide. The spectral characteristics of both the resonator designs are studied. The variation of the medium refractive index is detected by monitoring the shift of the spectral maxima of the Fabry–Perot cavity. The transmission spectrum and electric field distribution of the sensor structures are simulated using finite-element method. The resonator based on slot-waveguide shows better sensitivity and figure of merits as compared to strip waveguide. Our proposed sensor design has the potential to find further applications in biomedical science and nano-photonic circuits. [ABSTRACT FROM AUTHOR]

Waveguide bends are essential structures for designing an integrated optical waveguide modulator (IOWM). However, reducing bending loss for strip and slot waveguide bends and obtaining a small increment of footprint size by bending the radius are challenging issues. We aim to evaluate whether using strip waveguides with an adiabatic bend reduces the bending loss and to obtain a specific length suitable for a V-shaped mode converter with a small increment of footprint size in the IOWM. Furthermore, a new 180-deg-bend structure for slot waveguides, to reduce bending loss, is investigated. The results of reducing bending radius (R) with a large coverage angle (ac) for strip waveguide bends revealed a low bending loss and simultaneously a reduction of the footprint size in the IOWM. For the 180-deg slot waveguide bend, both the bending and coupling losses were reduced by optimizing the taper and distance offset, which enhanced transmission power efficiency. We validated the feasibility and efficiency of the proposed bending strip and slot structures in IOWM, which is important for integrated optical applications. [ABSTRACT FROM AUTHOR]

Highlights • A new pitch-catch scheme to excite and receive a relatively pure quasi-SH0 wave in the U-shape strip waveguide was proposed. • The viscosity of surrounding fluid can be quantitatively monitored based on the attenuation measurement of guided wave. • The performance of the U-shape waveguide sensor was evaluated in a wide range of viscosity (1cp-1600 cP). • The potential application of the U-shape waveguide sensor for high temperature (20℃–80℃) fluids was also presented. Abstract The shear horizontal (SH) wave in plate waveguides is of great advantage in fluid viscosity measurement as it only has in-plane displacements and causes a pure shear leakage in fluids, while the current excitation of non-dispersive SH waves in the strip with a finite width is very limited. In this work, we proposed a new pitch-catch scheme to excite and receive a relatively pure quasi-SH 0 wave in the U-shape strip waveguide. Based on the attenuation measurement on the quasi-SH 0 wave immersed in a viscous fluid, the viscosity of the fluid can be quantitatively monitored. Firstly, the wave field of the excited quasi-SH 0 was analyzed by using finite element simulations and the results demonstrate the effectiveness of the excitation method we proposed. Then, the U-shape waveguide sensor for viscosity measurement was presented, and a series of experiments were conducted on different concentrations of glycerol-water solutions with different viscosity (1cP-1600cP) under a wide temperature range (20℃–80℃). By calibrating the measured attenuation curves, the viscosity can be derived and the measurement uncertainty is less than 6% referring to a standard rotational viscometer. The proposed shear horizontal waveguide sensor is mechanically robust and quite suitable for online monitoring of fluid viscosity in industrial applications. [ABSTRACT FROM AUTHOR]

Trace gases such as H2O, CO, CO2, NO, N2O, NO2and CH4strongly absorb in the mid-IR (>2.5 μm) spectral region due to their fundamental rotational and vibrational transitions. CH4gas is relatively non-toxic, however, it is extremely explosive when mixed with other chemicals in levels as low as 5% and it can cause death by asphyxiation. In this work, we propose a silicon strip waveguide at 3.39 μm for CH4gas sensing based on the evanescent field absorption. These waveguides can provide the highest evanescent field ratio (EFR)>55% with adequate dimensions. Moreover, EFR and sensitivity of the sensor are highly dependent on the length of the waveguide up to a certain limit. Therefore, it is always a compromise between the length of the waveguide and EFR in order to obtain greater sensitivity. [ABSTRACT FROM AUTHOR]

We present a detailed study of Si-based optical waveguides, which can be used as evanescent field sensors for the quantitative analysis of various gases and liquids. Direct quantitative comparison of simulation with experimental results of directional coupling structures allows fine-tuning the material parameters and provides important input for future sensor design.

Gallium Nitride (GaN) based fully cut through free-standing planar photonic device was presented. Waveguide, grating couplers and supporting structure were designed by analytical calculation and finite element method (FEM) simulation. Optimized crossing structure and grating coupler with 0.4 dB intersection loss and 1 dB coupling loss at telecommunication band was obtained respectively. Mechanical stability was optimized by static and modal structure simulation. This work illustrates the potential of free-standing GaN photonic structure which will enable new functions for planar photonic devices. It also provides the possibility of monolithic integration of planar photonic and light sources on III–V active platforms. [ABSTRACT FROM AUTHOR]

Single-mode submicrometer-thick strip waveguides on silicon-on-insulator substrates, fabricated by silicon-planar-technology methods are considered. To solve the problem of 1.5-µm wavelength radiation input-output and its frequency filtering, strip diffraction gratings and two-dimensional photonic crystals are integrated into waveguides. The reflection and transmission spectra of gratings and photonic crystals are calculated. The waveguide-mode-attenuation coefficient for a polycrystalline silicon waveguide is experimentally estimated. [ABSTRACT FROM AUTHOR]

The dependence of propagation losses on waveguide width and polarization is measured for strip and rib waveguides made of tantalum pentoxide. For strip waveguides, the propagation losses are found to increase rapidly for widths <;3 μ m. The losses were significantly smaller for rib than for strip waveguides, as expected. A method is developed for estimating the dependence of propagation losses on waveguide width. The method is based on approximating sidewall imperfections with an area with complex refractive index in a 2D model and showed a good agreement with the measured dependence on waveguide width. The method is also used to predict that propagation losses will decrease rapidly for rib heights less than 20 nm for TM-polarization. [ABSTRACT FROM PUBLISHER]

The optical switch is a key component in photonic integrations that plays an important role in routing the optical signal within a photonic circuit. In this work, compact switches were proposed and demonstrated based on the interference of surface plasmons polaritons (SPP) on free metal surfaces and in waveguides. Thanks to the constructive and destructive interferences implemented in strip waveguides, 2 × 2 ports switch devices with a small footprint were achieved with a maximum modulation depth of about 80%. Moreover, an interesting composite interference was observed and analyzed in the waveguide device, which is considered to arise from a transmission/reflection interference and field superposition interference. The samples of Bragg gratings and slits with different parameters were investigated in detail, which provided convincing evidence supporting the composite interference model. This revealed that the mechanism would possibly inspire new designs and instructions in nanophotonic integrations. [ABSTRACT FROM AUTHOR]

We report a Ti:Er:LiNbO3 strip waveguide with high diffusion-doped surface Er3+ concentration. The waveguide was fabricated with a technological process in sequence of preparation of noncongruent, Li-deficient LiNbO3 substrate by performing Li-poor vapor transport equilibration treatment on a congruent Z-cut LiNbO3 plate, diffusion of 40-nm-thick Er metal film, and fabrication of 8-μm-wide Ti-diffused strip waveguide. The waveguide retains the LiNbO3 phase and shows the waveguiding characteristics similar to the conventional Ti:LiNbO3 waveguide. Secondary ion mass spectrometry study shows that the Er3+ diffusion reservoir was exhausted and the profile is Gaussian with a surface concentration two times larger than that of the conventional Ti:Er:LiNbO3 waveguide. The waveguide shows stable 1547-nm small-signal enhancement under the 1480-nm pumping without serious optical damage observed, and a 5-dB signal enhancement is obtained for the available coupled pump power of only 90 mW. A saturated net gain as much as 5 dB/cm is predicted theoretically. [ABSTRACT FROM PUBLISHER]

A compact crossing for silicon-based slot and strip waveguides is proposed by utilizing a strip-multimode waveguide (SMW) crossing at the center and two logarithmically tapered slot-to-strip mode converters at the ports with slot waveguides. For input/output ports with slot waveguides, the guided modes are efficiently transformed through the mode converter and then enter into the SMW (without mode converters for those ports with strip waveguides), where the fields converge at the center of the intersection due to the self-imaging effect. Hence, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to significant reductions of the crosstalk and radiation loss. The numerical results show that a hybrid waveguide crossing operating at the wavelength of 1.55 µm with the insertion loss, crosstalk, and reflection of 0.14/0.164, -35.88/-38.79, and -35.35/-40.5 dB for input ports with slot/strip waveguides, respectively, is achieved. Moreover, the fabrication tolerances to the structural parameters are investigated by using a finite-difference time-domain method and evolution of the injected field along the propagation distance through the crossing structure is also demonstrated. [ABSTRACT FROM AUTHOR]

Surface plasmon propagation in a nano metal strip waveguide with a biaxial substrate material was studied. The dispersion of the four fundamental modes propagating in the strip waveguide structure was analysed using a formalism based on the method of lines. The propagation properties of these modes with respect to the thickness of the metal film was studied, with two different biaxial materials as substrate for the waveguide. The results were then compared with the asymmetric strip waveguide structures with isotropic substrate and a uniaxial anisotropic substrate. Propagation characteristics as a function of longitudinal and transverse anisotropic components of the substrate material were also studied. The characteristics of the fundamental modes are observed to vary depending on the anisotropy of the biaxial material used. [ABSTRACT FROM AUTHOR]

Compact 48\,\times\,48 and 23\,\times\,23 arrayed waveguide grating (AWG) devices are realized by using SU-8 strip waveguides fabricated with the process of direct ultraviolet (UV) photolithography. The demonstrated 48-channel and 23-channel AWG devices operating around 1550 nm have a channel spacing of 0.8 nm (100 GHz) and 3.2 nm (400 GHz), respectively. Due to the high index-contrast of the SU-8 strip waveguide, the fabricated AWG has a very compact size of only about 0.22\,\times\,0.47 cm^2. For the fabricated 100 GHz-spaced AWG, the crosstalk between adjacent channels is less than -15 dB, and the polarization-dependent wavelength is about 0.72 nm (@1543.5 nm). The fabricated 400 GHz-spaced AWG device exhibits a crosstalk of less than -20 dB, a polarization-dependent wavelength of 0.02 nm (@1559.6 nm). Finally the temperature tenability of the present AWG device is also characterized. A 12 nm tuning range is observed as the temperature changes from 25^\circC to 115^\circC. [ABSTRACT FROM PUBLISHER]

The design and experimental demonstration of a large, ultrahigh-quality (Q) factor, and single-mode operation silicon ring resonator based on a strip silicon photonic waveguide are presented. The ring resonator comprises multimode straight strip waveguides (MMWG), single-mode strip waveguides (SMWG), 90° Bezier bends, and linear tapers. The SMWG are used in the coupling region to achieve mode coupling for the fundamental mode without exciting the higher-order mode. The MMWG, 1.6 μ m in width, are used to decrease the propagation loss. The taper smoothens the straight-to-bend transition and reduces the intermode crosstalk at the interface between the straight and bent waveguides. The design of the 90° Bezier bend ensures that only the maintained fundamental mode propagates at the bend with ultralow loss and low intermode interference. The resonator fabricated in this study had an ultrahigh-Q factor of 1.54 × 106, with a free spectrum range of 0.036 nm, and achieved an actual loss of unit transmission of approximately 0.25 dB/cm. [ABSTRACT FROM AUTHOR]