Normalized Delay Bandwidth Product Research Articles

Unit Cell Topology Optimization of Line Defect Photonic Crystal Waveguide

In this paper we utilize a systematic method to optimize the topology of a line defect Photonic Crystal Wave guide (PCW), a very popular and effective optical device for realizing optical buffer in room temperate operation. In order to do this, we divide the unit cell PCW to 25 squares and utilize a Binary Particle Swarm Optimization (BPSO) algorithm to find the optimum topology. During the optimization process, we apply some constraints to avoid band mixing as well. Calculation results show that Normalized Delay-Bandwidth Product (NDBP) for the structure optimized is 0.27. This structure optimized possesses a high bandwidth (20.9 nm) with group index of 21.7 as well.

Slow light in nonlinear photonic crystal coupled-cavity waveguides

Abstract Nonlinear photonic crystals can be formed by inserting Kerr-type nonlinear dielectric rods into perfect photonic crystals. Based on nonlinear photonic crystal, nonlinear photonic crystal coupled-cavity waveguide is constructed and its slow light properties are studied by using the Plane Wave expansion Method (PWM). Both single-defect coupled cavity and two-defect coupled cavity are proposed to optimize slow light properties. The result shows that using single-defect coupled cavity in waveguide is beneficial to obtain larger Normalized Delay-Bandwidth Product (NDBP) but it contributes little to decrease the group velocity of light and enlarging Q factor and delay time; While using two-defect cavity in waveguide can efficiently reduce the group velocity of light and enlarge Q factor and delay time. Compared to normal structures, our new designed nonlinear photonic crystal coupled cavity waveguide owns group velocity that is three magnitudes smaller than the vacuum speed of light. Delay time is of magnitude order of 10 ns and Q factor is of magnitude order of 1000, it means less loss and higher ability of storing energy.

Optical buffer performance enhancement using Particle Swarm Optimization in Ring-Shape-Hole Photonic Crystal Waveguide

Ring-Shape-Hole Photonic Crystal Waveguides (RSHPCW) is a popular structure in designing optical buffers attracted many attentions recently. There are some parameters such as the radii of holes and pillars next to the defect that have significant effects on slow light properties. Consequently, one of the promising methods for effectively slowing the light speed down and controlling dispersion is to optimize these parameters, which is the motivation of this study. Particle Swarm Optimization (PSO) algorithm is one of the best proposed heuristic optimization algorithms in Artificial Intelligence applied to many engineering problem. In this work, this algorithm is employed to find the best values of the aforementioned radii for maximizing Normalized Delay-Bandwidth Product (NDBP) of RSHPCW structure as the first systematic optimizer. Calculation results show that there are 34% and 41% improvement in NDBP and bandwidth compared to the previous works, very substantial achievements in this area.

Slow light engineering in periodic-stub-assisted plasmonic waveguide

We investigate the slow light engineering in periodic-stub-assisted plasmonic waveguide based on transmission line theory. It is found that the dispersion relationship of the proposed waveguide can be easily modified by tuning the stub depth and the period. The theoretical results show that a large normalized delay bandwidth product of 0.65 can be achieved at 1550 nm, meanwhile maintaining the group index of 35. In addition, the proposed waveguide shows "S-shaped" dispersion curve, which implies that the group velocity dispersion parameter at the inflection point equals zero and a dispersion-free slow light waveguide can be realized. Due to the excellent buffering capacity, the proposed compact configuration can find important applications on optical buffers in highly integrated optical circuits.

Slow light in an alternative row of ellipse-hole photonic crystal waveguide

High normalized delay-bandwidth product (NDBP) and wideband slow light are achieved in an alternative row of ellipse-hole photonic crystal waveguide. Two different criteria of flat ratio are adopted. Under a constant group index criterion, a high NDBP of 0.446 with a group index of 42 and a bandwidth of 16.4 nm are obtained by plane wave expansion calculations, while under a low dispersion criterion, the NDBP, group index, and bandwidth come to 0.352, 41, 13.1 nm, respectively. Low dispersion slow light propagation is numerically demonstrated by studying the relative temporal pulse-width spreading with the two-dimensional finite-difference time-domain method. As a whole, the presented results give indications about the "ultimate" possible improvement of slow light waveguide metrics by using noncircular holes.

Dispersionless slow light in MIM waveguide based on a plasmonic analogue of electromagnetically induced transparency

We have proposed a metal-insulator-metal (MIM) waveguide system, which exhibits a significant slow-light effect, based on a plasmonic analogue of electromagnetically induced transparency (EIT). By appropriately adjusting the distance between the two stubs of a unit cell, a flat band corresponding to nearly constant group index over a broad bandwidth of 8.6 THz can be achieved. The analytical results show that the group velocity dispersion (GVD) parameter can reach zero and normalized delay-bandwidth product (NDBP) is more than 0.522. Finite-Difference Time-Domain (FDTD) simulations show that the incident pulse can be slowed down without distortion owing to the low dispersion. The proposed compact configuration can avoid the distortion of signal pulse, and thus may find potential applications in plasmonic slow-light systems, especially optical buffers.

Improved design of photonic crystal waveguides with elliptical holes for enhanced slow light performance

We propose a new design for slow-light photonic crystal W1 waveguide, which uses a combination of circular and elliptical air holes arranged in a hexagonal lattice with background material of refractive index n = 2.84. Large value of normalized delay bandwidth product (NDBP = 0.3708) is obtained. We have also analyzed dispersion property for the structure and achieved nearly zero-dispersion for a very large bandwidth. Our proposed photonic crystal waveguide has slow light applications such as reduction in length and power consumption of all-optical and electro-optic switches at optical frequency.

Slow surface plasmon polaritons with a large normalized delay bandwidth product in an ultracompact metal gap superlattice

We propose a metal gap superlattice that can decrease the group velocity of surface plasmon polaritons (SPPs) with a large normalized delay bandwidth product (NDBP) and small high-order dispersion. The length of the superlattice is only 8.60μm and the transmission is 0.4. The multiple reflections of photons in the nanocavity result in slow SPP group velocity. The numerical analysis exploiting the transfer matrix method (TMM) is confirmed by the finite-difference time-domain (FDTD) numerical simulations. In addition, we also analyze the reason for pulse deformation.

Slow light engineering in polyatomic photonic crystal waveguides based on square lattice

In this paper, the slow light properties of the polyatomic Photonic Crystal (PhC) which has multiple different air holes in each primitive cell are investigated. A slow light waveguide with “U-type” group index–frequency curve, which results in nearly constant group index over large bandwidth, is achieved using this new photonic crystal geometry based on the square lattice. Also, the radius and position of the innermost rows of small air holes have been modified to investigate the feasibility of controlling the dispersion relation by subtle structural modification. Numerical results demonstrate that decreasing the group velocity effectively and meanwhile maintaining a large Normalized Delay-Bandwidth Product ( NDBP) can be achieved by only modifying the radius of the innermost rows of small air holes. Shifting the innermost rows of small air holes toward the waveguide core is highly beneficial to enlarge the slow light bandwidth, but it contributes nothing to the promotion of NDBP. Our results provide important theoretical basis for the potential application offered by the polyatomic photonic crystal in future optical networks.

Slow light miniature devices with ultra-flattened dispersion in silicon-on-insulator photonic crystal

We propose a silicon-on-insulator (SOI) photonic crystal waveguide within a hexagonal lattice of elliptical air holes for slow light propagation with group velocity in the range 0.0028c to 0.044c and ultra-flattened group velocity dispersion (GVD). The proposed structure is also investigated for its application as an optical buffer with a large value of normalized delay bandwidth product (DBP), equal to 0.778. Furthermore it is shown that the proposed structure can also be used for time or wavelength-division demultiplexing to separate two telecom wavelengths, 1.31 microm and 1.55 microm, on a useful time-scale and with minimal distortion.