Planar Photonic Crystal Waveguide Research Articles

Propagation of optical pulses in photonic crystal waveguides

The dispersion properties of planar photonic crystal (PhC) coupled cavity waveguides are investigated. Both pulse broadening and compression have been demonstrated by propagating various chirped sub-picosecond pulses through the waveguides. The PhC waveguide exhibits large positive group-velocity dispersion (GVD), which is opposed to the negative GVD in a single-mode fibre (SMF). A maximum pulse width reduction of 40%, from 1.91 ps to 1.17 ps, is achieved, after transmission through an 8 /spl mu/m long planar PhC waveguide. The equivalent dispersion value is >10/sup 6/ times larger than that available from standard single-mode fibre at the pulse centre wavelength of 1536 nm.

Broadband photonic crystal waveguide 60� bend obtained utilizing topology optimization

Topology optimization has been used to design a 60 degrees bend in a single-mode planar photonic crystal waveguide. The design has been realized in a silicon-on-insulator material and we demonstrate a record-breaking 200-nm transmission bandwidth with an average bend loss of 0.43+/-0.27 dB for the TE polarization. The experimental results agree well with 3D finite-difference-time-domain simulations.

Comprehensive FDTD modelling of photonic crystal waveguide components

Planar photonic crystal waveguide structures have been modelled using the finite-difference-time-domain method and perfectly matched layers have been employed as boundary conditions. Comprehensive numerical calculations have been performed and compared to experimentally obtained transmission spectra for various photonic crystal waveguides. It is found that within the experimental fabrication tolerances the calculations correctly predict the measured transmission levels and other major transmission features.

Optimizing bending efficiency of self-collimated beams in non-channel planar photonic crystal waveguides

In this paper, we propose a device to bend light in non-channel planar photonic crystal (PhC) waveguides using the self-collimation phenomenon. The mode distribution in a non-channel planar PhC waveguide is investigated in detail in order to help understand the proposed bending mechanism. Three-dimensional finite-difference time-domain simulations show an over 80% bending efficiency for a 90 degree bend. As the first proposal for bending light in a non-channel planar PhC waveguide, the presented device enables the application of routing in non-channel planar PhC waveguides.

Planar photonic crystal structure with inherently single-mode waveguides.

A planar photonic crystal that allows inherently gap-guided single-mode waveguides is proposed and discussed. This novel structure consists of a two-dimensional lattice of silicon rods embedded on a thin silica slab sandwiched between two silica claddings whose refractive indices are slightly lower than the index of the silica core. The physical parameters of the structure, i.e., rod radius and core thickness, are optimized to maximize the bandgap width for odd modes. Lossless guided modes inside the bandgap and below the claddings' light cone are obtained by reducing the radius of a row of rods. The waveguide bandwidth can be increased by inserting a thin silicon dielectric waveguide instead of the row of rods. The proposed approach may overcome many of the common drawbacks in conventional holes-on-dielectric planar photonic crystal waveguides.

Photonic crystal waveguide-mode orthogonality conditions and computation of intrinsic waveguide losses.

We simulate the propagation of light in a W1 planar photonic crystal waveguide with the three-dimensional finite-difference time-domain method and apply an inner product against previously calculated mode profiles to the simulated field cross sections. We show that this inner product satisfies mode orthogonality for both photonic crystal waveguides and segmented waveguides and use the obtained data to evaluate waveguide losses.

Efficient propagation of TM polarized light in photonic crystal components exhibiting band gaps for TE polarized light

We have investigated the properties of TM polarized light in planar photonic crystal waveguide structures, which exhibit photonic band gaps for TE polarized light. Straight and bent photonic crystal waveguides and couplers have been fabricated in silicon-on-insulator material and modelled using a 3D finite-difference-time-domain method. The simulated spectra are in excellent agreement with the experimental results, which show a propagation loss as low as 2.5+/-4 dB/mm around 1525 nm and bend losses at 2.9+/-0.2 dB for TM polarized light. We demonstrate a high coupling for TM polarized light in a simple photonic crystal coupler with a size of ~ 20 m x 20 m. These promising features may open for the realization of ultra-compact photonic crystal components, which are easily integrated in optical communication networks.

Efficient photonic crystal Y-junctions

A highly efficient Y-junction based on a planar photonic crystal (PhC) platform is presented. The PhC consists of a triangular array of holes etched into a GaAs/AlGaAs heterostructure, with a typical period of 322 nm and ~35% fill factor. The Y-junction has smaller holes positioned at the centre of the junction, giving rise to very uniform splitting and high transmission. The performance is very encouraging, with experimental transmission of approximately 40% for each arm of the Y-splitter relative to a comparable single-defect PhC waveguide.

General methods for designing single-mode planar photonic crystal waveguides in hexagonal lattice structures

We systematically investigate and compare general methods of designing single mode photonic crystal waveguides in a two-dimensional hexagonal lattice of air holes in a dielectric material. We apply the rather general methods to dielectric-core hexagonal lattice photonic crystals since they have not been widely explored before. We show that it is possible to obtain single mode guiding in a limited portion of the photonic bandgap of hexagonal lattice structures. We also compare the potentials of different photonic crystal lattices for designing single-mode waveguides and conclude that triangular lattice structures are the best choice.

Planar photonic crystal waveguide devices for integrated optics

The concept of photonic crystal waveguides and related device applications in integrated optics are discussed, as well as some of the fabrication technology required to realise these structures in practise. Photonic crystal waveguides interact with light on a wavelength scale, which leads to a novel class of miniaturised devices that display designer dispersion.

Mode matching technique for highly efficient coupling between dielectric waveguides and planar photonic crystal circuits

In this paper, a mode matching technique for highly efficient coupling between dielectric silica waveguides (SWG) and planar photonic crystal (PPC) waveguides based on setting localized defects in a PPC tapered waveguide is reported. The introduction of multiple defects is designed properly depending on mode mismatching arising from the different widths of the SWG and the PPC waveguide. The procedure to obtain the optimum defects configuration is described. Transmission efficiencies above 80% at a wavelength of 1.55m are reported improving significantly the transmission efficiencies achieved with conventional PPC tapered structures without defects. Furthermore, the feasibility of the coupling technique for both input/output coupling over a large frequency band is shown.

Two-mode fringes in planar photonic crystal waveguides with constrictions: a probe that is sensitive to propagation losses

We analyze the transmission of planar photonic crystal channel waveguides, each of which consists of three missing rows in a triangular lattice of air holes and modified at both ends by constrictions. The structures are fabricated into a GaAs/AlGaAs heterostructure in which an internal source consisting of three layers of quantum dots is embedded. The constrictions induce peculiar spectral features that are used to improve the sensitivity of transmission measurements to propagation losses. Two effects are pointed out: (i) The constrictions act as mirrors, inducing Fabry–Perot fringes on the transmitted spectra, (ii) and the constrictions also induce a mode-mixing process, mostly between the fundamental and the third transverse modes of the waveguides. Using the visibility of the resultant two-mode fringes observed on the transmitted spectra, we extract a quantitative value for propagation losses at λ=1 µm:α1=25 cm-1 (1 dB/100 µm) for the fundamental mode.

Theoretical analysis of finite-height semiconductor-on-insulator-based planar photonic crystal waveguides

A planar photonic crystal waveguide based on the semiconductor-on-insulator (SOI) materials system is analyzed theoretically. Two-dimensional (2-D) calculations and comparison with dispersion relations for the media above and below the finite-height waveguide are used to obtain design guidelines. Three-dimensional (3-D) calculations are given for the dispersion relations and field profiles. The field profiles obtained using 2-D and 3-D calculations are qualitatively similar. However, we find that compared with 2-D calculations, the frequencies of the guided modes are shifted and the number of guided modes changes. The theoretically predicted frequency intervals, where the waveguide supports leakage-free guidance of light, are compared with an experimental measurement for propagation losses. Two out of three frequency intervals coincide with low-measured propagation losses. The poor guidance of light for the third frequency interval is explained theoretically by investigating the vertical localization of the guided modes.

Experimental and theoretical confirmation of Bloch-mode light propagation in planar photonic crystal waveguides

The dispersion diagram of the leaky modes in the planar photonic crystal waveguide is experimentally obtained for the wavelengths from 1440 to 1590 nm. A small stop band, around wavelength 1500 nm, is detected. The experimentally obtained results are in very good agreement with our three-dimensional finite difference time domain calculations. Propagation losses of the leaky modes are estimated and we have found that they decrease as we approach the ministop band.

Mode matching for second-harmonic generation in photonic crystal waveguides

The influence of localized photonic eigenstates on the second-harmonic generation efficiency in planar photonic crystal waveguides is studied with a simple, yet realistic model. We show that $g~{10}^{6}$ enhancements in the external conversion efficiency in specular reflection can be achieved due to a combination of ``generalized phase-matching'' and strong local fields in these periodically textured, nonlinear dielectric structures. The second-harmonic response involving the resonant modes of these membranelike photonic crystals is essentially a resonant cavity effect, and hence is strongly influenced by the modal quality factor.

High efficiency coupling technique for planar photonic crystal waveguides

A high efficiency coupling technique for photonic crystal waveguides is proposed. The technique is based on setting a single defect within a tapered photonic crystal waveguide. Simulation results show that the optimised scheme maximises the power transmission up to 87.5% at a wavelength of /spl lambda/ = 1.55 /spl mu/m for a 0.5 /spl mu/m-long photonic crystal taper.

Quantitative measurement of low propagation losses at 155 μm on planar photonic crystal waveguides

The Fabry-Perot resonance technique has been used to determine the propagation losses of planar photonic crystal (PC) waveguides. The structures are patterned into a GaInAsP confining layer on an InP substrate. Losses as low as 11 dB/mm have been measured on a guiding structure with three missing rows. The influence of the PC guide width and air-filling factor is demonstrated.