Coupled Resonator Optical Waveguide Structures Research Articles

Mutlilayer Wedge Disks CROW for an Optical Delay Line

Optical delay lines are vital for the development of all optical telecommunications, optical computers and for many nonlinear and quantum optics processes. Periodic structures such as coupled resonator optical waveguides (CROW) achieve high delays in a compact manner and with a large bandwidth. Wedge disk resonators have demonstrated high quality factors compared to rectangular disks and rings while remaining integrable on chip. Vertical coupling is needed since the optical mode is pushed toward the disk center by the wedge. We propose an optical delay line made of wedge disks arranged in two overlapping layers. Disks are made of silicon dioxide on silicon pillars. Numerical calculations of the device performance and an experimental proof of concept are presented. A maximum delay of 85 ps for eleven disks is achieved with a loss per cavity of 3.91 dB. This is a first proof of concept of a CROW structure made of wedge disk coupled vertically.

The performance of CROW structures on detection of virus nanoparticles

Coupled resonator optical waveguide (CROW) structures were investigated as a whispering gallery mode optical resonator for biosensing applications. A single mode optical fiber taper was used to evanescently couple light into the microspheres. We have simulated the theoretical model of the crow structure with two and three microspheres. The coupled-resonator-induced transparency (CRIT) phenomenon was observed in the crow structures. In this paper, two crow structures and a single microsphere resonator are compared with each other. We have reported the high sensitivity of the crow structures compared to a single microsphere. We have demonstrated apparent shifts of CRIT transparent peak when the virus nanoparticles are placed on the surface of the microsphere. As a result, these crow structures can be used as a biosensing sensor with high sensitivity. The theoretical simulation was carried out at the operating wavelength of 1550 nm.

Photonic Delay Lines Based on Silicon Coupled Resonator Optical Waveguide Structures

We propose a Coupled Resonator Optical Waveguide Structure based on silicon microring resonators. The proposed structure is designed on Silicon on Insulator platform so as to scale down the device dimensions as silicon on insulator platform provides the advantage of large index contrast, miniaturization of device dimensions. Tunability is incorporated on the structure in the simulation environment by placing thermal heaters on the top of the rings which change the refractive indices of the rings thereby changing the resonance wavelength. Finite difference time domain method is used to simulate the structure. Variable delays are achieved by tuning the necessary number of ring structures to their resonant wavelengths. This structure can be helpful in realizing optical delay lines for on chip optical interconnects.

Photonic crystal power combiner based on hexagonal waveguides

A new 2×1 power combiner scheme based on two-dimensional photonic crystal has been designed using hexangular waveguides. These waveguides are created by reducing the radius of inner rods in alternate manner which subsequently leads to coupled resonator optical waveguide structure. In this design, the outer hexagon has two input ports and the output port is embedded in the inner hexagonal. Due to geometrical and metallurgical engineering, the photonic bandgap locates in the THz region. Three different radii of coupled resonator optical waveguide rods have been studied. It has been shown that the structure with largest inner rods provides better power output.

Multi-channel slow light coupled-resonant waveguides based on photonic crystal with rectangular microcavities

The light transmission characteristics through the coupled-resonator optical waveguides (CROWs) based on the two-dimensional square-lattice photonic crystal with rectangular microcavities are systematically studied by the finite-difference time-domain method. Owing to the multiple spatial symmetries of the defective modes localized by the rectangular microcavities, the guiding bands can be adjusted by changing the coupling region between the CROW and the traditional W1-typed input and output channels, and their corresponding group velocities can be quite different from each other through the same CROW structure. Through optimizing the coupling regions between the CROW and three output channels, a kind of device with both the abilities of light beam splitting and frequency selection can be obtained, which has potential applications in the future complex all-optical circuits.

Loop coupled resonator optical waveguides.

We propose a novel coupled resonator optical waveguide (CROW) structure that is made up of a waveguide loop. We theoretically investigate the forbidden band and conduction band conditions in an infinite periodic lattice. We also discuss the reflection- and transmission- spectra, group delay in finite periodic structures. Light has a larger group delay at the band edge in a periodic structure. The flat band pass filter and flat-top group delay can be realized in a non-periodic structure. Scattering matrix method is used to calculate the effects of waveguide loss on the optical characteristics of these structures. We also introduce a tunable coupling loop waveguide to compensate for the fabrication variations since the coupling coefficient of the directional coupler in the loop waveguide is a critical factor in determining the characteristics of a loop CROW. The loop CROW structure is suitable for a wide range of applications such as band pass filters, high Q microcavity, and optical buffers and so on.

Slow light propagating characteristics through a two-dimensional silicon-based coupled-resonator optical waveguide

The light propagating characteristics of the coupled-resonator optical waveguides (CROWs) are studied by the finite-difference time-domain method. The circular CROWs are constructed by arranging the micro-cavities at a certain distance, which is constructed by removing the air holes along the edge of a circle from a two-dimensional (2D) triangular-lattice photonic crystal (PC). With the increasing of distance between the adjacent cavities, the group velocities of the guiding modes reduce significantly. The circular CROW studied in the paper have much minibands within the band gap, and their respective group velocities can be quite different from each other. This kind of CROW structure can provide different group velocities for the light signals with different frequencies, and avails to the separating and controlling the light signals in the all optic integrated circuits.

Achieving Optical Tri-Stability Through Periodic Metal-Dielectric Structure

Through the modified transfer matrix method, the transmission properties of a one-dimensional coupled-resonator optical waveguide structure composed of metal layers and non-linear material layers is studied. Given proper incident frequency and structure parameters, an optical tri-stability has been achieved. The effect of loss has been considered.

Disorder in coupled-resonator optical waveguides

Disorder in coupled-resonator optical waveguides (CROWs) is modeled by exploiting the concept of the characteristic impedance of a periodic slow-light waveguide. Every imperfection in the CROW structure is modeled as an impedance discontinuity, and the related backreflection is evaluated by using well-known reflection rules. We demonstrate that backreflections induced by disorder scale with the square of the slowing factor and the square of the disorder parameter, both independently of the specific structure. The method is simple and accurate, holds even when the slowing factor of the CROW is modified by disorder, and can be applied to any slow-light structure where the characteristic impedance can be defined. Theoretical and numerical results are supported by an experimental investigation showing the effects of increasing disorder on both frequency and time domain responses of a ring resonator CROW. Pulse envelope distortions due to distributed backreflections along the disordered CROW arise as one of the main limiting factors for applications based on CROWs.

Micro-racetrack coupled-resonator optical waveguides in silicon photonic wires

Coupled-resonator optical waveguide (CROW) structures including coupled micro-racetrack resonators realized using silicon photonic wires are theoretically investigated in this paper. An accurate modelling procedure has been developed. It takes into account the differences in optical coupling between two identical resonators and between a straight waveguide (bus waveguide) and a resonator. The modelling approach is based on a mixed, very general procedure, based on the transfer matrix approach for CROW investigation, coupled mode theory in a generalized form for characterization of coupling between adjacent resonators and between bus waveguides and resonators and the full-vectorial finite element method for performing coupling coefficient calculation and waveguides modal analysis. This technique can be adopted to model a wide spectrum of different CROW structures. As an explicative example, CROW structures including three coupled micro-racetrack resonators have been investigated. A large quality factor (~7 × 105) has been predicted for a proposed CROW structure when the quasi-TE mode is excited. The influence of fabrication tolerances on CROW structures has been also investigated.

Coupled resonator optical waveguide structures with highly dispersive media

Analysis of photonic crystal coupled resonator optical waveguide (CROW) structures with a highly dispersive background medium is presented. A finite-difference time-domain algorithm was employed which contains an exact representation of the permittivity of a three-level atomic system which exhibits electromagnetically induced transparency (EIT). We find that the coupling strength between nearestneighbor cavities in the CROW decreases with increasing steepness of the background dispersion, which is continuously tunable as it is directly related to the control field Rabi frequency. The weaker coupling decreases the speed of pulse propagation through the waveguide. In addition, due to the dispersive nature of the EIT background, the CROW band shape is tuned around a fixed k-point. Thus, the EIT background enables dynamic tunability of the CROW band shape and the group velocity in the structure at a fixed operating point in momentum space.

Coupled resonator optical waveguides based on silicon-on-insulator photonic wires

Coupled resonator optical waveguides (CROWs) comprised of up to 16 racetrack resonators based on silicon-on-insulator (SOI) photonic wires were fabricated and characterized. The optical properties of the CROWs were simulated using measured single resonator parameters based on a matrix approach. The group delay property of CROWs was also analyzed. The SOI based CROWs consisting of multiple resonators have extremely small footprints and can find applications in optical filtering, dispersion compensation, and optical buffering. Moreover, such CROW structure is a promising candidate for exploration of low light level nonlinear optics due to its resonant nature and compact mode size (∼0.1μm2) in photonic wire.

Coupled-resonator optical waveguides for biochemical sensing of nanoliter volumes of analyte in the terahertz region

We present a detailed study of coupled-resonator optical waveguide (CROW) based sensors for biochemical sensing. The sensitivity dependence on the CROW structure parameters, such as intercavity distance and cavity type, is investigated for the effects in the THz region of the EM spectrum of introducing small quantities of molecules, such as DNA, in the holes. Introducing the absorptive material into the low-index medium greatly affects the shape of the propagating modes of the CROW and the transmitted E-field. The shift of the resonant frequency also depends linearly on the refractive index changes for off-resonant case (dispersive effect).

Coupled-resonator optical waveguides doped with nanocrystals

Microsphere resonators doped with semiconductor nanocrystals are explored as building blocks for coupled-resonator optical waveguides (CROWs). The evolution of individual cavity modes into coherently coupled waveguide modes is studied using polarization-sensitive microphotoluminescence spectroscopy. To demonstrate the formation of multisphere photon states, we use a bent linear array of microresonators and probe the properties of the cavity photon field by the spatially and spectrally resolved measurement of the nanocrystal emission. Photon mode coupling is evidenced by the observed mode splitting and emission intensity distributions along the CROW structure.