Photonic Crystal Coupled Waveguides Research Articles

Digital signal processing in coupled photonic crystal waveguides and its application to an all-optical AND logic gate

The realization of all-optical AND logic gates for pulsed signal operation based on the photonic bandgap transmission phenomenon is proposed. We are using realistic planar air-hole type coupled photonic crystal waveguides (C-PCWs) with Kerr-type nonlinear background medium. The novelty of our analysis is that the proposed AND logic gate operates with the temporal solitons, which maintain a stable envelope propagating in the nonlinear C-PCWs, enabling true ultrafast full-optical digital signal processing in the time-domain. The bandgap transmission takes place when the operating frequency is chosen at the very edge of the dispersion curve of one of the supermodes in the C-PCWs. In this regard, our original fast and accurate method is used to efficiently calculate the supermodes of the C-PCW system. The underlying semi-analytical full-wave modal analysis is based on the evaluation of the lattice sums for complex wavenumbers using the transition-matrix method in combination with the generalized reflection-matrix approach. As a proof of concept successful pulse operation of the all-optical AND logic gate is demonstrated in the framework of extensive full-wave finite-difference time-domain electromagnetics analysis.

Realization of true all-optical AND logic gate based on nonlinear coupled air-hole type photonic crystal waveguides

In this manuscript we propose an easily scalable true all-optical AND logic gate for pulsed signal operation based on band-gap transmission within nonlinear realistic air-hole type coupled photonic crystal waveguides (C-PCW). We call it "true" all-optical AND logic gate, because all AND gate topologies operate with temporal solitons that maintain a stable pulse envelope during the optical signal processing along the different C-PCW modules yielding ultrafast full-optical digital signal processing. We directly use the registered (output) signal pulse as new input signal between multiple concatenated nonlinear C-PCW modules (i.e. AND gates) to setup a multiple-input true all-optical AND logic gate. Extensive full-wave computational electromagnetic analysis proves the correctness of our theoretical studies and the proposed operation principle of the multiple-input AND logic gate is vividly demonstrated for realistic C-PCWs.

All-optical logic gates based on coupled heterostructure waveguides in two dimensional photonic crystals

In this study, all-optical logic gate with multifunctional performance have been designed in two-dimensional coupled photonic crystal waveguides (CPCWs) employing modulation of the refractive index in the coupling region. A two dimensional finite-difference time domain (2D-FDTD) was employed in our numerical simulations. It is shown that by switching the optical signal to different input waveguide ports, the proposed device can function as AND, OR, NOR and NOT gates operating at many or single wavelength. Our simulation results show that the optimized devices have very good transmission efficiencies, a broad frequency range and the contrast ratios between the output ports reveal a satisfactory level. Our results not only provide a simple on chip platform for next generation logic optical circuits, but also open up the possibility for the realization of ultrahigh speed signal processing, and future photonic crystal based all-optical integrated circuits.

Linear Amplification of Optical Signal in Coupled Photonic Crystal Waveguides

We introduce a weakly coupled photonic crystal waveguide as a promising and realistic model for all-optical amplification. A symmetric pillar type coupled photonic crystal waveguide consisting of dielectric rods periodically distributed in a free space is proposed as all-optical amplifier. Using, the unique features of the photonic crystals to control and guide the light, we have properly chosen the frequency at which only one mode (odd mode) becomes the propagating mode in the coupled photonic crystal waveguide, whereas another mode (even mode) is completely reflected from the guiding structure. Under this condition, the all-optical amplification is fully realized. The amplification coefficient for the continuous signal and the Gaussian pulse is calculated.

Ultra compact slow light photonic crystal directional coupler design with elliptical rods

We propose a novel design for a photonic crystal waveguide coupler, which uses elliptical shaped rods with refractive index equal to 3.4 arranged in square lattice. We investigate the performance of slow light in this photonic crystal coupler. By providing a slow-light region using elliptical cells, a coupling length of just 2.58μm is achieved at optical wavelength. This is extremely small coupling length then previously reported. The proposed structure would offer significant potential for novel compact high-speed optical signal processing devices.

基于光子晶体耦合波导的宽带慢光研究

基于光子晶体耦合波导的宽带慢光研究

Directional free-space coupling from photonic crystal waveguides

We present a general approach for coupling a specific mode in a planar photonic crystal (PC) waveguide to a desired free-space mode. We apply this approach to a W1 PC waveguide by introducing small index perturbations to selectively couple a particular transverse mode to an approximately Gaussian, slowly diverging free space mode. This "perturbative photonic crystal waveguide coupler" (PPCWC) enables efficient interconversion between selectable propagating photonic crystal and free space modes with minimal design perturbations.

Stopping of Light by the Dynamic Tuning of Photonic Crystal Slow Light Device

We propose a simple technique of stopping light pulses using a slow-light device based on photonic crystal coupled waveguide (PCCW). Dynamically tuning the material index chirp in the PCCW adiabatically transforms slow-light pulses into stopped ones. We demonstrate this in finite-difference time-domain simulation assuming ideal and actual tuning of the index chirp. In the ideal case, the group velocity of the almost stopped pulse is reduced to 190 times smaller than that of simple slow light pulse. The smallest limit is affected by the timing error of the tuning between wavelengths. Re-ordering and stopping of a pulse train are possible by optimizing the device length and timing. As a practical tuning method, we discuss carrier effects induced by photo-excitation. Taking into account carrier distribution and free carrier absorption, the actual behaviors of stopped light are estimated. We define and evaluate an effective delay-bandwidth product, which is affected by free carrier absorption.

Wide Range Tuning of Slow Light Pulse in SOI Photonic Crystal Coupled Waveguide via Folded Chirping

In this paper, we demonstrate state-of-the-art slow light in silicon-on-insulator photonic crystal coupled waveguide, which allows slow light pulse transmission and its tunable delay by means of structural chirping. The key idea of this study is the application of a folded chirping profile to the structure, instead of the conventional monotonous chirping. It suppresses unwanted spectral oscillation caused by structural disordering and expands the tuning range. By postprocessing an airhole-diameter-chirped device, we show that 0.9-ps-wide slow light pulses are delayed for 72 ps, corresponding to a buffering capacity of 80 bits. In a separate, unchirped device, we demonstrate a tunable delay by applying thermally induced index chirping. Here, a maximum tuning range of 103 ps and a tunable capacity of 22 bits are obtained.

Optomechanical coupling in photonic crystal supported nanomechanical waveguides

We report improved optomechanical properties of a nano-mechanical beam resonator embedded in an optical race-track resonator. Precise control of the mechanical resonator is achieved by clamping the beam between two low-loss photonic crystal waveguide couplers. The low insertion loss and the rigid mechanical support provided by the couplers yield both good mechanical and optical Q-factors for improved signal quality.

Spectroscopy of a tapered-fiber photonic crystal waveguide coupler

A spectroscopic technique based on a full four port modeling and measurement is developed and utilized to study a tapered- fiber photonic crystal waveguide coupler. The coupler is made by directly situating the tapered fiber on the defect region of a silicon membrane photonic crystal waveguide. The waveguide is lithographically terminated resulting a Fabry- Perot cavity. It turns out that the line-shape of the resonances is not merely a Lorentzian but can be constructed from that of the unloaded waveguide resonator. By knowing how the resonances broaden the experimental data is then fit and the coupling efficiency is extracted for the entire spectrum.

Zero-Dispersion Slow Light with Wide Bandwidth in Photonic Crystal Coupled Waveguides

By introducing an adjustment waveguide besides the incident waveguide, zero-dispersion slow light with wide bandwidth can be realized due to anticrossing of the incident waveguide mode and the adjustment waveguide mode. The width of the adjustment waveguide (W2) and the hole radii of the coupling region (r′) will change the dispersion of incident waveguide mode. Theoretical investigation reveals that zero dispersion at various low group velocity νg in incident waveguide can be achieved. In particular, proper W2 and r′ can lead to the lowest νg of 0.0085c at 1550 nm with wide bandwidth of 202 GHz for zero dispersion.

A New Formulation of Coupled Propagation Equations in Periodic Nanophotonic Waveguides for the Treatment of Kerr-Induced Nonlinearities

Nonlinear phenomena could be used to implement important signal processing functionalities in future nanophotonic integrated optical devices. In this paper, a semi-analytical model incorporating the influence of Kerr-induced nonlinearity in the propagation of an optical signal inside a periodic nanophotonic waveguide is derived. The approach consists of a system of nonlinear coupled mode propagation equations and is applicable to both single and multimode waveguides. The influence of the mode group velocity on the value of the self-phase modulation coefficient gamma is analyzed and the impact of higher order nonlinear terms is also investigated both at the middle and edge of the guided band. The model is also applied to estimate the nonlinear coupling coefficients of a photonic crystal waveguide coupler and provides an efficient method to analyze the influence of nonlinear phenomena in periodic nanophotonic waveguide devices.

Experimental observation of slow light in photonic crystal coupled waveguides

We experimentally demonstrate wideband dispersion-free slow light in chirped photonic crystal coupled waveguides (PCCW). In unchirped PCCWs, the zero group velocity can occur at an inflection point of a photonic band of even symmetric mode. The even symmetric mode is selectively excited by connecting the device with input and output waveguides through optimized branch and confluence structures. In the device fabricated on SOI substrate, a large increase in group delay was observed with a maximum group index of 140 and the zero group velocity dispersion at the inflection point. Photonic bands estimated from the group delay characteristics corresponded to calculated ones. In the chirped PCCWs, the group velocity dispersion was internally compensated and the nearly constant group index of 50-60 was obtained in a wavelength bandwidth of 10 nm. The dispersion compensation was also confirmed through the transmission measurement of sub-ps optical pulses.

New design of 2-D photonic crystal waveguide couplers

Based on couple wave equation and finite-difference time-domain (FDTD) algorithm, the strong couple characteristic of 2-D photonic crystal couplers is calculated. Theoretical analysis and numerical simulated results indicate that the energy in a 2-D photonic crystal coupler can not be totally transferred between two wave-guides. Compared with the result of weak coupling theory, our result is more accurate.

The analysis of interaction region of elliptical pillars of a directional photonic crystal waveguide coupler

Photonic crystals (PCs) have many potential applications because of their ability to control light-wave propagation. In this paper, the influence of interaction region of elliptical pillars on a directional photonic crystal waveguide coupler is studied by solving Maxwell's equations with use of finite-difference time-domain method. The PC waveguide coupler introduced two rows of elliptical pillars in the interaction region. Introducing elliptical pillars in the interaction region can reduce the coupling length of PC waveguide coupler. The coupling length of PC waveguide coupler was changed by rotating the rotation angle of elliptical pillars. These results may provide novel application in photonic integrated circuits.

Photonic crystal optical filter based on contra-directional waveguide coupling

Wavelength-selective operation of an optical filter (add/drop) based on a contra-directional photonic crystal waveguide coupler is demonstrated. The waveguides are defined as line defects in a two-dimensional triangular photonic crystal fabricated in an InP/GaInAsP heterostructure. The device is characterized using the end-fire method for the drop functionality. The experimental data are in good agreement with the theoretical results predicted by finite-difference time-domain simulations.

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.

Wavelength division multiplexing and demultiplexing with photonic crystal waveguide couplers

A multiplexer-demultiplexer (MUX-DEMUX) based on PC waveguide couplers is proposed, and its wavelength demultiplexing properties are theoretically investigated. First, a two-channel MUX-DEMUX is designed and characterized, and then, by cascading, two stages of photonic crystal (PC) waveguide couplers with different coupling coefficients are constructed. The device sizes are expected to be drastically reduced from a scale of a few tens of micrometers to a scale of a few hundreds of micrometers in a MUX-DEMUX.