Asymmetric Waveguide Structures Research Articles

High performance on-chip polarization beam splitter at visible wavelengths based on a silicon nitride small-sized ridge waveguide.

Due to sensitive scaling of the wavelength and the visible-light absorption properties with the device dimension, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at the visible wavelengths. In this work, a simple and small polarization beam splitter (PBS) for a broad visible-light band, using a tailored silicon nitride (Si3N4) ridge waveguide, is presented, which is based on the distinct optical distribution of two fundamental orthogonal polarized modes in the ridge waveguide. The bending loss for different bending radii and the optical coupling properties of the fundamental modes for different Si3N4 ridge waveguide configurations are analyzed. A PBS composed of a bending ridge waveguide structure and a triple-waveguide directional coupler was fabricated on the Si3N4 thin film. The TM excitation of the device based on a bending ridge waveguide structure shows a polarization extinction ratio (PER) of ≥ 20 dB with 33 nm bandwidth (624-657 nm) and insertion loss (IL) ≤ 1 dB at the through port. The TE excitation of the device, based on a triple-waveguide directional coupler with coupling efficiency distinction between the TE0 and TM0 modes, shows a PER of ≥ 18 dB with 50 nm bandwidth (580-630 nm) and insertion loss (IL) ≤ 1 dB at the cross port. The on-chip Si3N4 PBS device is found to possess the highest known PER at a visible broadband range and small (43 µm) footprint. It should be useful for novel photonic circuit designs and further exploration of Si3N4 PBSs.

Polarization Splitting at Visible Wavelengths with the Rutile TiO2 Ridge Waveguide.

On-chip polarization control is in high demand for novel integrated photonic applications such as polarization division multiplexing and quantum communications. However, due to the sensitive scaling of the device dimension with wavelength and the visible-light absorption properties, traditional passive silicon photonic devices with asymmetric waveguide structures cannot achieve polarization control at visible wavelengths. In this paper, a new polarization-splitting mechanism based on energy distributions of the fundamental polarized modes in the r-TiO2 ridge waveguide is investigated. The bending loss for different bending radii and the optical coupling properties of the fundamental modes in different r-TiO2 ridge waveguide configurations are analyzed. In particular, a polarization splitter with a high extinction ratio operating at visible wavelengths based on directional couplers (DCs) in the r-TiO2 ridge waveguide is proposed. Polarization-selective filters based on micro-ring resonators (MRRs) with resonances of only TE or TM polarizations are designed and operated. Our results show that polarization-splitters for visible wavelengths with a high extinction ratio in DC or MRR configurations can be achieved with a simple r-TiO2 ridge waveguide structure.

High slope-efficiency quantum-dot lasers grown on planar exact silicon (001) with asymmetric waveguide structures

We report electrically pumped continuous-wave (CW) InAs/GaAs quantum dot lasers directly grown on planar exact silicon (001) with asymmetric waveguide structures. Surface hydrogen-annealing for the GaAs/ Si (001) templates and low-temperature growth for GaInP upper cladding layers were combined in the growth of the laser structure to achieve a high slope efficiency. For the broad-stripe edge-emitting lasers with 2-mm cavity length and 20-µm stripe width made from the above laser structure, a threshold current density of 203.5 A/cm2 and a single-facet slope efficiency of 0.158 W/A are achieved at ∼1.31 µm band under CW conditions. The extrapolated mean-time-to-failure reaches up to 21000 hours at room temperature, which is deduced from the data measured from C-mount packaged devices. Importantly, these results can provide a practical strategy to realize 1.3 µm wavelength band distributed feedback lasers directly on planar exact Si (001) templates with thin buffer layers.

Advanced Passive Silicon Photonic Devices With Asymmetric Waveguide Structures

Various passive photonic integrated devices have been developed successfully with silicon-on-insulator (SOI) nanowires in the past decade. It is well known that SOI-nanowire waveguides have ultrahigh index contrast ( $\Delta $ ) and ultrahigh birefringence. As a result, the structures and the design rules of a silicon photonic device are probably very different from the conventional case of using low- $\Delta $ optical waveguides. For example, some asymmetric waveguide structures have been used often to realize many silicon photonic devices developed recently. Furthermore, higher order modes have been involved in some of these silicon photonic devices. This paper discusses the special mode properties of silicon nanophotonic waveguides, including birefringence, mode dispersion, and mode hybridness. A review is then given on recent progress of these advanced passive silicon photonic devices with structural asymmetry, including on-chip polarization-handling devices, mode converters/(de)multiplexers, microring-resonator optical filters/switches, and Mach..Zehnder interferometer optical switches. Silicon photonic integrated circuits with these building blocks are also reviewed, including hybrid (de)multiplexers and reconfigurable optical add..drop multiplexers.

Generalized Mathematical Model of Thin Asymmetric Inductive Diaphragm in Rectangular Waveguide

Introduction. A significant number of microwave devices are constructed with applying the investigation results of thin asymmetric inductive diaphragm in rectangular waveguide. Increasing the requirements for the characteristics of these devices stipulates necessity to review the possibilities of existing electromagnetic methods in relation to increasing the accuracy of generalized scattering matrices calculation of this diaphragm. To obtain the high accuracy results of generalized scattering matrices calculation, a further study of integral equations method is represent considerable practical interest. Mathematical model of diaphragm. An accurate novel solution for general scattering matrix of infinitely thin asymmetric one-sided diaphragm in rectangular waveguide has been obtained. The problem is formed as the system of integral equations along the number of waves which is incident on the diaphragm. By applying the Galerkin’s method, each integral equation is reduced to system of linear algebraic equations relatively to coefficients of tangential electric field decomposition in diaphragm window by series of coordinate functions. The joint solution of all equations gives the distribution of tangential electric field in diaphragm window which is further used for finding the generalized scattering matrix. Numerical results. Two approaches for finding the coupling coefficients of coordinate functions of diaphragm window and eigen functions of waveguide are investigated. An estimation of limiting possibilities of developed realization of the integral equations method at approximation of tangential electric field in diaphragm window by series of eigen scalar functions has been carried out. It is shown that the calculated results of module and phase of fundamental wave transmission coefficient through diaphragm slowly go to exact values when the order of linear algebraic equation system is increased. Conclusions. The results of diaphragm investigation by using this algorithm have not only independent value but can be used to verify the accuracy of calculating the electromagnetic parameters of thin waveguide structures by using the general methods for solving electromagnetic problems, for example, FDTD. It is supposed that the results obtained in this work can be used in developing fast and high-precision calculation algorithms for frequency responses of multi-stage asymmetric waveguide structures.

Dynamically Manipulating the Plasmon-Induced Transparency—Like in Asymmetric Graphene-Based Waveguide Structures

We perform a theoretical and numerical investigation of the process of plasmon-induced transparency (PIT)-like in an asymmetric graphene-based waveguide system with two identical graphene-ribbon resonators. PIT-like phenomenon can be produced by the resonance hybridization between two graphene ribbons with frequency detuning. The hybrid resonance and resonant frequency detuning come from the variation in the phase of propagation waves at resonances and the coupling interaction between ribbons. These variations are induced by shifting the lateral distance between the two ribbons. Moreover, the transparency window can be actively manipulated by adjusting the Fermi energy of graphene without reoptimizing and refabricating the structure. The proposed graphene-based PIT-like structure may result in the development of new compact elements such as tunable sensors, modulators, and switchers.

A scheme for optical circulator by using asymmetric Y-branch waveguide structures

A novel scheme for optical circulator is proposed, in which the asymmetric Y-branch waveguide structures are employed to achieve the optical unidirectional propagation. The optical performance of our optical circulator is numerically investigated by using the beam propagation method. The simulated results show that not only high optical extinction ratio and low optical loss for both TE and TM modes can be easily realized, but the dependence of the operation wavelength and polarization is very weak in our configuration. Particularly, the proposed optical circulator has several advantages such as simple structure, reconfigurable port number and large fabrication tolerance, which is very useful for design and fabrication. This novel device provides a new way to achieve optical circulation propagation function, and has wide potential applications in photonic integrated systems.

Ultraslow-light effects in symmetric and asymmetric waveguide structures with moon-like scatterers

Ultraslow-light effects in two-dimensional hexagonal-lattice coupled waveguide with moon-like scatterers were theoretically studied using the plane-wave expansion method. For symmetric structures, simulations showed that slow light with high group index can be achieved by shifting the scatterers and adjusting the radius of moon-like scatterers. The maximum group index was over 8:0 × 104. For asymmetric structures, simulations showed that slow light with flat band and high group index can be obtained by shifting the scatterers, adjusting the radius of moon-like scatterers, and rotating the scatterers. The maximum group index was over 5:7 × 105 with a “saddle-like” relationship between the frequency and group index.

Surface plasmon propagation in a metal strip waveguide with biaxial substrate

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.

A triangular shaped channel MIM waveguide filter

A compact plasmonic branch-shaped metal–insulator–metal (MIM) waveguide with a triangular-annular channel is proposed and investigated. The results show that all-band waves can transmit through the structure when it has transmission symmetry. As the symmetry is broken, a typical filtering function is obtained and the transmissivity of the resonance frequency depends on the destroyed degree of symmetry, which results from interference between the two branched surface plasmon polaritons waves (SPPs) propagating along the branch-shaped waveguide. Moreover, the filtering wavelength can be selected by changing the length of the triangular-annular channel. These asymmetric waveguide structures may open up new avenues toward the control of SPPs and the development of filters based on plasmonic designs.

Guided modes in slab waveguides with negative index cladding and substrate

In this work, three normalized parameters are used to study the dispersion characteristics of three different asymmetric slab waveguide structures. Each structure contains at least one layer made of left-handed material (LHM). The first structure comprises a LHM cladding layer, the second contains a LHM substrate layer, and the third has a LHM cladding and substrate. The results show that the dispersion characteristics of these structures differ from conventional guided waves in waveguide structures comprising right-handed materials. A number of unusual properties are observed including the dispersion characteristics of the fundamental mode for the first structure is different for different μc and μs. The fundamental mode exists and can only propagate within a small frequency range in the second structure. In the third structure, the fundamental mode does not exist whereas the first-order mode exists in a narrow frequency range. No propagation modes in some frequency ranges is observed which enables the structure to behave as a bandpass filter.

非对称波导结构提高980 nm激光二极管电光效率的机理研究

The optical losses of asymmetric and symmetric waveguide structures were calculated theoretically for high power 980 nm diode lasers, by means of the continuity equation under strong injection and the Helmholtz equation of complex refractive index model. The epitaxial materials of the both waveguide structures were prepared by the metallorganic chemical vapor deposition growth technique, and corresponding laser devices were fabricated. The electro-optical performances of both devices were measured and analyzed by comparison. The theoretical calculations and experimental results indicates that, the asymmetric waveguide structure material does not reduce the optical loss, but decreases the series resistance of the diode, therefore, it reduces the joule heating, and enhances the conversion ef

Athermal High-Index-Contrast Waveguide Design

We present generalized design rules for athermal performance and materials compatibility in high-index-contrast (HIC) waveguides. Thermal stability of integrated photonic devices is one critical limitation in the development of commercially viable integrated optoelectronic circuits. Thermooptically neutral designs are achieved by choosing a cladding material whose thermooptic coefficient is opposite to that of the waveguide core. We derive analytical expressions of athermal conditions for both symmetric and asymmetric channel waveguide structures. The equations apply to general HIC systems such as silicon-on-insulator and SiN-based structures.

High-Efficiency 808-nm InGaAlAs–AlGaAs Double-Quantum-Well Semiconductor Lasers With Asymmetric Waveguide Structures

The InGaAlAs-AlGaAs double-quantum-well semiconductor lasers grown by molecular beam epitaxy show high quantum efficiency and high power conversion efficiency at continuous-wave power output using asymmetric waveguide structures. The threshold current density and slope efficiency of the device are 180 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 1.4 W/A, respectively. The internal loss and the internal quantum efficiency are 1.1 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 97%, respectively. The 75% maximum power conversion efficiency is achieved in 100-mum stripe widths 808-nm-emitting laser diodes with 1000-mum cavity length.

Equivalent rectangle approximation-staircase concatenation method for wedge shaped fiber

Wedge shaped fiber (WSF) is the key component to acquire high coupling efficiency between input/output ports of planar lightwave circuit chips and fiber arrays. It is very important to analyze the mode spot and the mode field evolution of the lightwave propagating in WSF using numerical simulation tools. A three_dimensional equivalent rectangle approximation-staircase concatenation method (ERA-SCM) is proposed to fractionize the wedged tip of WSF along the propagating direction and introduce rectangular waveguide approximation so that effective index of the segmentations is obtained and SCM analysis model is established. Lightwave propagation and mode field evolution is analyzed by ERA-SCM and compared with finite difference-beam propagation method (FD-BPM), showing that the former is more precise to solve mode field evolution for asymmetric fiber and waveguide structures. The measurement of the output optical spot verifies the ERA-SCM simulation error is within 1.9%, while that for FD-BPM method is 4.5%. Thus, ERA-SCM is an effective method for analyzing asymmetric fiber and waveguide structures.

810 nm非对称波导InGaAlAs/AlGaAs双量子阱半导体激光器

The 810-nm InGaAlAs/AlGaAs double quantum well (QW) semiconductor lasers with asymmetric waveguide structures, grown by molecular beam epitaxy, show high quantum efficiency and high-power conversion efficiency at continuous-wave (CW) power output. The threshold current density and slope efficiency of the device are 180 A/cm^2 and 1.3 W/A, respectively. The internal loss and the internal quantum efficiency are 1.7 cm^(-1) and 93%, respectively. The 70% maximum power conversion efficiency is achieved with narrow far-field patterns.

Analyzing multilayer optical waveguide with all nonlinear layers

We propose a general method for analyzing a multilayer optical waveguide with all nonlinear layers. This general method can be degenerated into some special cases, such as symmetric or asymmetric nonlinear optical waveguide structures, the intensity-dependent refractive index with self-focusing nonlinear medium, hollow waveguides, and multilayer systems. Based on this general method, the analysis and calculation of complicated multilayer optical planar waveguides can be achieved easily. The analytical and numerical results show excellent agreement.

The dynamics of amplified spontaneous emission in CdSe∕ZnSe quantum dots

We have used the variable stripe technique and pump-probe spectroscopy to investigate both gain and the dynamics of amplified spontaneous emission from CdSe quantum dot structures. We have found modal gain coefficients of 75 and 32cm−1 for asymmetric and symmetric waveguide structures, respectively. Amplified spontaneous emission decay times of 150 and 300ps and carrier capture times of 15 and 40ps were measured for the structures with high and low material gains respectively. The difference in the capture times are related to the fact that for the symmetric waveguide, carriers diffuse into the active region from the uppermost ZnMgSSe cladding layer, yielding a longer rise time for the pump-probe signals for this sample.

Free-carrier absorption and active layer heating in large optical cavity high-power diode lasers

An analytical theory is developed for the thermal properties of a large optical cavity semiconductor laser high above threshold. The performance of symmetric and asymmetric waveguide structures is analyzed and compared. Narrow asymmetric waveguides are predicted to be advantageous from the thermal point of view.

10 Gbit/s asymmetric waveguide APD with high sensitivity of −30 dBm

An asymmetric waveguide avalanche photodiode (APD) has been developed. Asymmetric waveguide structures are effective for achieving robustness under high input power operations, and high quantum efficiencies of 94% for 1.55 mum and 90% for 1.31 mum were achieved. A gain-bandwidth product of 110 GHz and a maximum 3 dB-down bandwidth of 20 GHz were also achieved. A high receiver sensitivity of -30.2 dBm at 10 Gbit/s for a receiver module with integrated TIA has also been demonstrated