Parallel microgenetic algorithm design for photonic crystal and waveguide structures

We have developed a powerful parallel genetic algorithm design tool for photonic crystal and waveguide structures. The tool employs a small-population-size genetic algorithm (microgenetic algorithm) for global optimization and a two-dimensional finite-difference time-domain method to rigorously design and optimize the performance of photonic devices. We discuss the implementation and performance of this design tool. We demonstrate its application to two photonic devices, a defect taper coupler to connect conventional waveguides and photonic crystal waveguides, and a sharp 90 degrees waveguide bend for low index contrast waveguides.     

Loss reduction in silicon nanophotonic waveguide micro-bends through etch profile improvement

Single mode silicon photonic wire waveguides allow low-loss sharp micro-bends, which enables compact photonic devices and circuits. The circuit compactness is achieved at the cost of loss induced by micro-bends, which can seriously affect the device performance. The bend loss strongly depends on the bend radius, polarization, waveguide dimension and profile. In this paper, we present the effect of waveguide profile on the bend loss. We present waveguide profile improvement with optimized etch chemistry and the role of etch chemistry in adapting the etch profile of silicon is investigated. We experimentally demonstrate that by making the waveguide sidewalls vertical, the bend loss can be reduced up to 25% without affecting the propagation loss of the photonic wires. The bend loss of a 2 μm bend has been reduced from 0.039dB/90° bend to 0.028dB/90° bend by changing the sidewall angle from 81° to 90°, respectively. The propagation loss of 2.7 ± 0.1dB/cm and 3 ± 0.09dB/cm was observed for sloped and vertical photonic wires respectively was obtained.     

Observation of amplitude and phase in ridge and photonic crystal waveguides operating at 1.55 microm by use of heterodyne scanning near-field optical microscopy

We apply heterodyne scanning near-field optical microscopy (SNOM) to observe with subwavelength resolution the amplitude and phase of optical fields propagating in several microfabricated waveguide devices operating around the 1.55 microm wavelength. Good agreement between the SNOM measurements and predicted optical mode propagation characteristics in standard ridge waveguides demonstrates the validity of the method. In situ observation of the subwavelength-scale distribution and propagation of optical fields in straight and 90 degrees bend photonic crystal waveguides facilitates a more detailed understanding of the optical performance characteristics of these devices and illustrates the usefulness of the technique for investigating nanostructured photonic devices.     

Arbitrary angle waveguide bends in two-dimensional photonic crystals

Arbitrary angle waveguide bends in two-dimensional photonic crystals are studied with modeling and calculation. The lattice orientation restriction to bending angles can be avoided by incorporating an annular air groove into the bending corner. Theoretical analysis shows that the sharp bends transmit guided lightwaves with a very slight difference of propagation properties between straight waveguides and bend sections. A transmission of larger than 90% with a bandwidth of wider than 52 nm is obtained in the vicinity of 1.55 μm for the sharp bends with bending angles from 0° to 165°.     

Transmission Characteristics of 90° Bent Photonic Crystal Waveguides

There are several key factors that affect the transmission characteristics of the 90° (L-shaped) bent photonic crystal waveguides. The first factor is the direct coupling efficiency from the incident lights into the waveguides. The second one is the bandgap deviations of the photonic crystals. And the third factor is the optical reflections in the bent corners. In this article, we compare three types of L-shaped bent photonic crystal waveguides. One is the original type, which has an abrupt right-angle bend. Another is an improved 90° bend with a 45°-mirror. The other is an L-shaped bent photonic crystal waveguide with a 45°-transitional section. We investigate their respective frequency responses and observe the improvements in the total transmission efficiencies provided by the latter types.     

Low-reflection photonic-crystal taper for efficient coupling between guide sections of arbitrary widths

We design and fabricate a new taper structure for adiabatic mode transformation in two-dimensional photonic-crystal waveguides patterned into a GaInAsP confining layer. The taper efficiency is validated by measurement of a reduction of the reflection between an access ridge and a photonic-crystal guide with one missing row from 6% to less than 1%. This taper is then incorporated into a 60 degrees bend; simulations demonstrate a 90% transmission between multimode ports.     

3 dB power splitter design based on coupled cavity waveguides

Using numerical simulations, we investigated the transmission properties for coupled cavity waveguides form in two-dimensional photonic dielectric rods in air with square lattice. The dispersion for the coupled cavity waveguides with different radius of the defects is given. The optimum design waveguide should be perfect transmission for pulse though with a quasi-flat dispersion band. Based on the coupled cavity waveguides, a 3 dB power splitter is designed. Our results may have an important role in the design of efficient power splitters in a photonic circuit.     

Ultracompact waveguide bends with simple topology in two-dimensional photonic crystal slabs for optical communication wavelengths

Ultracompact waveguide bends with simple topology in two-dimensional photonic crystal slabs are proposed by using an annular air groove at the bend corner to improve transmissions of the bends. Analysis indicates that the guided light wave experiences a very slight difference of propagation properties between straight waveguides and bends (with 60 degrees and 120 degrees bending angles). Transmissions of more than 90% can be achieved in the 60 degrees and 120 degrees bends for light waves at 1.55 microm with bandwidths of 101 and 74 nm, respectively.     

Low-loss amorphous silicon-on-insulator technology for photonic integrated circuitry

We report the fabrication of low-loss amorphous silicon photonic wires deposited by plasma enhanced chemical vapor deposition. Single mode photonic wires were fabricated by 193 nm optical lithography and dry etching. Propagation loss measurements show a loss of 3.46 dB/cm for photonic wires and 1.34 dB/cm for ridge waveguides.     

Transmission characteristics of various bent periodic dielectric waveguides

The transmission characteristics of various bent periodical dielectric waveguides (PDWGs) are analyzed by the finite-difference time-domain (FDTD) method in this work. The bending structures include a conventional 90° abrupt bend, a 90° transitional bend, and 90° circular arcs with different quantized radii of curvatures. The simulation results can make us to design a suitable bent PDWG in the photonic circuits.     

Large-bandwidth ultra-low-loss guiding in bi-periodic photonic crystal waveguides

For the first time, we experimentally demonstrate ultra-low-loss guiding in bi-periodic photonic crystal waveguides over a large bandwidth of more than 60 nm. In such bi-periodic waveguides, we achieve a propagation loss very similar to that of a dielectric ridge waveguide, which indicates that the dominant loss mechanism in these bi-periodic waveguides is due to fabrication imperfections. PACS 42.70.Qs; 42.79.Gn     

Coupling into slow-mode photonic crystal waveguides

We study efficient injectors for coupling light from z-invariant ridge waveguides into slow Bloch modes of single-row defect photonic crystal waveguides. Two-dimensional vectorial computations performed with a Bloch mode theory approach predict that very high efficiencies (>90%) can be achieved for injector lengths of only a few wavelengths in length, even for small group velocities in the range of c/100-c/400. This result suggests that photonic crystal devices operating with slow waves can be interfaced with classical waveguides without sacrificing compactness.     

Optimal tunability of waveguides based on silicon photonic crystals infiltrated with liquid crystals

In this work we study the optimization of the tunability range in waveguides based on two-dimensional silicon photonic crystal infiltrated with liquid crystal. The analyzed structure consists of a two-dimensional silicon photonic crystal with a triangular lattice of circular holes where a line of scatterers in the direction Γ–K has been replaced by a line of circular holes with different radius infiltrated by E7 liquid crystal. To this end, we use the plane-wave expansion method considering anisotropy and modelling supercells to account for the lattice defects that define the waveguide. Finally we study the field distributions of the guided modes in order to analyze their symmetries and confinement.     

Design and modeling of microwave photonic devices

Different types of finite element methods (FEMs) for microwave and optical waveguides are reviewed and are utilized for modeling of a traveling-wave (TW) optical modulator, as one of the typical microwave photonic devices. Using the quasi-TEM and the full-wave vector FEM solvers for microwave waveguides and the scalar FEM solver for optical waveguides, the behaviour of a TW Z-cut Ti:LiNbO3 Mach–Zehnder optical modulator with a ridge structure is investigated.     

III–V based-semiconductor photonic crystals

Three-dimensional (3D) and two-dimensional (2D) photonic crystals based on III–V semiconductors are described. On the 3D photonic crystals, the development of complete photonic crystals at optical wavelengths and their applications to ultrasmall optical integrated circuits including 3D sharp bend waveguide are described. On the 2D photonic crystals, two-unique device and/or phenomena are demonstrated.     

T-shaped optical circulator based on coupled magneto-optical rods and a side-coupled cavity in a square-lattice photonic crystal

We propose and numerically investigate a kind of high-efficiency T-shaped optical circulator in a two-dimensional square-lattice photonic crystal. In the T-shaped structure, the single-direction light transmission for 90° light-bending is achieved by coupling two magneto-optical rods. And aided with a side-coupled cavity, two paths of single-direction 90° light-bending are effectively combined, which realizes the nonreciprocal light transmission for two waveguides arranged and linked along a straight line. The optical properties of the system are investigated by finite element method. The circulator considered here can be used for isolating light reflections and improving system stabilization in designing photonic crystal integrated circuits.     

二维光子晶体全光异或门的设计及研究

分析了二维光子晶体马赫-曾德尔干涉仪的传输特性,将二维光子晶体波导、环形腔和马赫-曾德尔干涉仪有效结合,提出了一种基于二维光子晶体马赫-曾德尔干涉仪的异或门设计。用平面波展开法分析二维光子晶体能带结构,并用时域有限差分法验证光信号在该器件中的电场稳态分布。结果表明,该结构能够实现异或逻辑,且具有高逻辑对比度7.88 dB,快速响应周期0.388 ps和高传输速率7.87 Tbit/s;并且该器件结构尺寸仅为13 μm×14 μm,易于集成。该异或逻辑结构中引入了二维光子晶体马赫-曾德尔干涉仪,使得光子晶体逻辑门结构的设计更加多样,并为二维光子晶体半加器与全加器的设计提供了基础,具有重要的研究意义。     

Transmission of slow light through photonic crystal waveguide bends

The spectral dependence of the bending loss of cascaded 60 degrees bends in photonic crystal (PhC) waveguides is explored in a slab-type silicon-on-insulator system. An ultralow bending loss of (0.05 +/- 0.03) dB/bend is measured at wavelengths corresponding to the nearly dispersionless transmission regime. In contrast, the PhC bend is found to become completely opaque for wavelengths corresponding to the slow-light regime. A general strategy is presented and experimentally verified to optimize the bend design for improved slow-light transmission.     

Improvement of characterization accuracy of the nonlinear photonic crystals using finite elements-iterative method

We investigate nonlinear one- and two-dimensional photonic crystals by applying a finite element-iterative method. Numerical results show the essential influence of nonlinear elements embedded into a quarter-wave stack and the sharp photonic crystal waveguide bend on the spectral characteristics of these structures. We compare our results with those obtained in [21] from the discrete equation method for the case of a sharp waveguide bend. The comparison shows that neglecting the nonuniform field distribution inside the embedded nonlinear elements leads to overestimation of the waveguide bend transmissivity. PACS 42.65.-k; 42.70.Qs     

Photonic crystal channel drop filter based on ring-shaped defects for DWDM systems

This paper presents a novel configuration of channel drop filters based on two-dimensional photonic crystal slabs in silicon-on-insulator platforms. The structure is composed of two photonic crystal line-defect waveguides as input and output ports, along with an L3 cavity with ring-shaped border holes. The effects of structural parameters and fabrication errors on resonance frequency and drop efficiency are investigated. Band structure and propagation of electromagnetic field through device are calculated by plane wave expansion and finite-difference time-domain methods. The results show that the quality factor and line-width of output signal are ~5690 and 0.27 nm, respectively, indicating that the proposed filter can be properly used in dense wavelength division multiplexing systems with 0.8 nm channel spacing.