Dual-frequency division de-multiplexer based on cascaded photonic crystal waveguides

A dual-frequency division de-multiplexing mechanism is demonstrated using cascaded photonic crystal waveguides with unequal waveguide widths. The de-multiplexing mechanism is based on the frequency shift of the waveguide bands for the unequal widths of the photonic crystal waveguides. The modulation in the waveguide bands is used for providing frequency selectivity to the system. The slow light regime of the waveguide bands is utilized for extracting the desired frequency bands from a wider photonic crystal waveguide that has a relatively larger group velocity than the main waveguide for the de-multiplexed frequencies. In other words, the wider spatial distribution of the electric fields in the transverse direction of the waveguide for slow light modes is utilized in order to achieve the dropping of the modes to the output channels. The spectral and spatial de-multiplexing features are numerically verified. It can be stated that the presented mechanism can be used to de-multiplex more than two frequency intervals by cascading new photonic crystal waveguides with properly selected widths.     

Dense wavelength division demultiplexing using photonic crystal waveguides based on cavity resonance

We demonstrated a photonic crystal waveguide based dense wavelength division multiplexing device using the resonances in the cavities. The demultiplexing is achieved through filtering. This filtering is achieved by varying the radii of the surrounding holes of the cavity, which in turn changes the resonant wavelength of the cavity. The four wavelengths demultiplexed in the design are 0.8 nm apart in the optical region centered on 1.55 and 1.56 μm. The device designed and simulated has easy to realize structure as well as high quality factor. Two-dimensional Finite Difference Time Domain (FDTD) is chosen to do the simulation of this work.     

Issues when designing hypoellipse photonic crystal waveguides

This work designs a type of line-defect photonic crystal waveguide (PCW) called hypoellipse PCW (HPCW) that considers two conflicting issues: group index and bandwidth. To do so, the recent multi objective framework called MoMIR is employed. A wide range of designs obtained demonstrates the advantage of considering group index and bandwidth simultaneously when designing HPCWs. Comparison of the proposed HPCW with the current best PCWs shows a nearly 7% improvement over the latter in terms of normalized delay-bandwidth product (NDBP). Analysis of the results reveals some of the physical rules about the structure of the HPCW. Finally, optical pulse propagation in obtained HPCWs and the process of designing an optical buffer by using an obtained design are explained.     

Contra-directional coupling between two-dimensional photonic crystal waveguides

The contra-directional coupling between two photonic crystal (PC) waveguides is studied, using the finite-difference time-domain (FDTD) method. A design of contra-directional coupler is presented and its transmission properties are investigated. The device can be used as an add/drop filter. It is also shown that the coupled mode theory is suitable to study the photonic crystal waveguide coupler.     

Transmission properties of two-dimensional photonic crystal channel waveguides

We study the transmission properties of straight channel waveguides designed in a two- dimensional (2D) photonic crystal patterned into an AlGaAs heterostructure. 2D dispersion calculations show the existence of small gaps occurring along the dispersion branch of the fundamental mode. We show that their location agree very well with mini-stop bands observed on the transmission spectra.     

Single mode lasers based on monolithic integration of ridge waveguides with 2D photonic crystal waveguides

The monolithic combination of active light sources with photonic crystal (PC) waveguide components is a key building block for future highly integrated photonic circuits. We demonstrate the coupling of light from an InGaAs/AlGaAs ridge waveguide laser to a monolithically integrated 2D PC waveguide. The PC guide is formed by removing three or five rows in a triangular lattice of air rods etched into the semiconductor. A tapered ridge waveguide geometry is demonstrated to improve coupling efficiency, so that maximum output powers of up to 10 mW from the PC waveguide are achieved. The resulting coupled cavity laser shows single mode emission with side mode suppression ratios > 35 dB over a broad range of injection currents.     

Numerical characterization of nanopillar photonic crystal waveguides and directional couplers

We numerically characterize a novel type of a photonic crystal waveguide, which consists of several rows of periodically arranged dielectric cylinders. In such a nanopillar photonic crystal waveguide, light confinement is due to the total internal reflection. A nanopillar waveguide is a multimode waveguide, where the number of modes is equal to the number of rows building the waveguide. The strong coupling between individual waveguides leads to the proposal of an ultrashort directional coupler based on nanopillar waveguides. We present a systematic analysis of the dispersion and transmission efficiency of nanopillar photonic crystal waveguides and directional couplers. Plane wave expansion and finite difference time domain methods were used to characterize numerically nanopillar photonic crystal structures both in two- and three-dimensional spaces.     

A three-channels WDM based on MMI in photonic crystal

A photonic crystal waveguide coupling structure can be constructed by putting three adjacent photonic crystal waveguides in parallel. Guided mode propagation method is employed to analyze the interference behavior of the approximate solution in such a coupling and the self-image phenomenon in multi-mode waveguides. A three-channel multimode interference wavelength division multiplexing can be obtained. The presented device has a high transmission rate as well as the advantage of multi-wavelength selection, thus it may have a potentially practical application in future photonic integrated circuit.     

Multilayer dielectric cylindrical mirrors based on omnidirectional reflection from a one-dimensional photonic crystal

A multilayer dielectric cylindrical mirror (MDCM) based on the one-dimensional omnidirectional reflection of a photonic crystal is presented. In this case, the refractive indices of the two materials are 1.6 (polystyrene) and 4.6 (tellurium), and the corresponding optimized thicknesses are 0.75a and 0.25a. A very high reflectance over a wide frequency range is observed. In this case, a is the lattice constant of the photonic crystal. In this band, the MDCM has good reflection and focal properties. Therefore, it is feasible to use the MDCM for integrated waveguide devices. As an example, an etched diffraction grating demultiplexer based on the MDCM is also proposed. Both the operational principle and design of the device are introduced. This provides a method for designing compact integrated waveguide devices.     

Microcavity filters based on hexagonal lattice 2-D photonic crystal structures embedded in ridge waveguides

Two-dimensionally periodic photonic crystal microcavity filters in a ridge waveguide format have been designed and fabricated. Transition mode-matching features were added to increase the optical throughput by more than a factor of two. An increase of Q-factor (more than 100%) was achieved by the addition of two further rows of photonic crystal holes to the microcavity filters. Attempts have also been made to tailor the filter response by applying design concepts used in other Bragg-grating optical filter technologies.     

Multiple slow light bands in photonic crystal coupled resonator optical waveguides constructed with a portion of photonic quasicrystals

Coupled resonator optical waveguides (CROWs) in complex two-dimensional (2D) photonic crystals (PCs) constructed with a portion of 12-fold photonic quasicrystals (PQs) are proposed. We show that enhanced transmission and slow light can be simultaneously achieved in such waveguides as well as general CROWs. Moreover, due to higher degree of flexibility and tunability of PQs for defect mode properties compared to conventional periodic PCs, multiple slow light bands can be flexibly obtained in CROWs constructed with complex 2D PCs. Our results may lead to the development of a variety of novel ultracompact devices for photonic integrated circuits.     

Different kinds of band-pass filters based on one-dimensional photonic crystal heterostructures

Three kinds of band-pass filters (BPFs), termed a TE BPF, a TM BPF and a BPF at oblique incidence, are designed by crossing the band edges of two different one-dimensional (1D) photonic crystals (PCs). The TE BPF, fulfilling the function of high reflectivity for p-polarization and high transmittance in a narrow pass-band for s-polarization, is constructed by two subPCs, one of which is obtained by decreasing the lattice constant of the other one while the other parameters keep the same. The TM BPF also comprises two subPCs, but the materials of one subPC are quite different with those of the other one. Similarly, the BPF at oblique incidence, from which both polarization waves can transmit in a narrow pass-band at oblique incidence, is composed of two different subPCs. Note that, the distribution of the wave impedance of material in each BPF should be periodic to prevent unexpected defect modes. The new filters not only provide narrow pass-bands and wide non-transmission bands, but also have simple structures. This is a result of the fact that the wave impedance ratio between composites is enlarged by introducing magnetic materials. The new designs have potential applications in optical filters and optical integrated circuits.     

A new design of tunable four-port wavelength demultiplexer by photonic crystal ring resonators

A four-channel wavelength demultiplexer based on photonic crystal ring resonators (PCRR), which can be used for photonic integrated circuits, is designed. Dropping efficiency and Q factor of single improved ring are 100% and 842, respectively. In order to achieve the structure of demultiplexer, three improved rings have been used, that every ring has an individual inner rod radius; it means that each ring has a varying resonant wavelength. The results of simulation using finite-difference time-domain (FDTD) method in our proposed structure reveals an average transmitted power higher than 90% for each output port, Channel spacing is about 8 nm and bandwidth for each individual channel is about 2.8 nm. The mean value of the crosstalk between output channels and the area of the proposed structure are about −29 dB and 317 μm², respectively. By changing the radius of inner rods, various wavelengths can be chosen, therefore this device is tunable.     

Modeling and design of 2D photonic crystal based Y type dual band wavelength demultiplexer

In this paper, photonic bandgap (PBG) induced wave guiding application of photonic crystals is exploited to design Dual Band Wavelength Demultiplexer (DBWD) for separating two telecommunication wavelengths, 1.31 and 1.55 μm. Two designs that use silicon rods in air and embedded air holes in silicon are realized for this purpose. Plane wave expansion (PWE) method and two dimension Finite Difference Time Domain (FDTD) methods are used to design and analyze the DBWD in Y type photonic crystal structure. Numerical analysis indicates that these designs enable the separation of two wavelengths with very high optical power extinction ratios. Other filter parameters like transmittance and quality factor are also calculated to confirm superior performance of the proposed design of photonic crystal based DBWD.     

A compact in-plane photonic crystal channel drop filter

This paper presents a novel in-plane photonic crystal channel drop filter.The device is composed of a resonant cavity sandwiched by two parallel waveguides.The cavity has two resonant modes with opposite symmetries.Tuning these two modes into degeneracy causes destructive interference in bus waveguide,which results in high forward drop efficiency at the resonant wavelength.From the result of numerical analysis by using two-dimensional finite-difference time-domain method,the channel drop filter has a drop efficiency of 96% and a Q value of over 3000,which can be used in dense wavelength division multiplexing systems.     

Optimization of a 2D photonic crystal add/drop multiplexer based on contra-directional coupling

We present a highly integrated add/drop multiplexer, where the contra-directional coupling is realized by phase matching two photonic crystal waveguides. The device band structure, the corresponding transmission and drop spectra, and the coupling length are carefully analysed. Different device configurations are discussed and by tailoring the coupling factor, we optimize the frequency response of the filter, obtaining a -sized channel selector, characterized by a very high drop efficiency.     

Ultra-compact polarization beam splitter based on photonic crystal waveguides

The authors design an ultra-compact all-PC-integrated polarization beam splitter which is only composed of three waveguides: one input waveguide and two output waveguides. The input waveguide can support both TM and TE modes, but one of the two output waveguides can only support TM modes while the other can only support TE modes. So an incident beam will be separated into two different polarization beams which emerge from different output waveguides. By the simulation of finite-difference time-domain method, we know that the polarization beam splitter really works the way as we predict.     

A novel wavelength dispersive device with a dispersive element based on staircase-like straight and parallel arrayed waveguides

We propose a new type of arrayed waveguide grating (AWG) multiplexer/demultiplexer based on modified group refractive index. This device is composed by an array of straight and parallel waveguides of equal length and each waveguide consist of two sections with different width. The length of the two sections are changed from a waveguide to the adjacent one following a linear dependence resulting in a wavelength dispersive waveguide array. An example of the device design for silicon-on-insulator (SOI) platform is provided and numerical simulations have been carried out for various arrayed waveguide parameters. We demonstrate that the group index modification can be used for tailoring device dispersion properties, and that it can also result in new dispersion characteristics predicted numerically not observed in conventional AWGs. Additional advantages are that the demultiplexer does not necessarily require bending waveguide sections as in a conventional AWG (de)multiplexers, and thus yields highly compact devices with potentially very low insertion loss. Channel spacing of 1 nm have been predicted for sub-micron waveguides sizes. In this paper it is also proposed a novel wavefront converter based on waveguide array lens-like element with waveguides broadened sections. Numerical results for different input/output geometries are analized.     

Compact,wide bandwidth,multi-channel power dividers based on one-dimensional photonic crystal waveguides

We propose novel compact multiport power dividers based on one-dimensional photonic crystal omnidirectional reflective waveguides. The proposed power dividers have advantages of wide bandwidth, flexible extended output channel number and compact size. The power dividers are numerical simulations using finite-difference time-domain method. Near-complete transmission and uniform at every branch of output power of multi-port power dividers are observed within wide frequency range. For a 1D PhC 1 × 6 power divider, the six output port achieved nearly 16.5% transmission at each arm from 1545 nm to 1553 nm and the size of 1 × 6 power divider is 14.3 μm × 14.3 μm at 1.55 μm operation wavelength.     

Design of a compact polarization beam splitter based on a deformed photonic crystal directional coupler

In this paper a compact polarization beam splitter based on a deformed photonic crystal directional coupler is designed and simulated. The transverse-electric （TE） guided mode and transverse-magnetic （TM） guided mode are split due to different guiding mechanisms. The effect of the shape deformation of the air holes on the coupler is studied. It discovered that the coupling strength of the coupled waveguldes is strongly enhanced by introducing elliptical airholes, which reduce the device length to less than 18.Sttm. A finite-difference tlme-domain simulation is performed to evaluate the performance of the device, and the extinction ratios for both TE and TM polarized light are higher than 20 dB.