Dominic F. G. Gallagher

A General Framework for the Finite-Difference Time-Domain Simulation of Real Metals

We present an efficient framework for the finite-difference time-domain simulation of real metals. The complex permittivity function of a metal is fitted to experimental data in the frequency domain using a non-linear least squares algorithm. A memory-efficient finite-difference time-domain (FDTD) scheme is presented for the simulation of the dispersive behavior of a metal in the frequency domain. The stability limit for the proposed scheme is determined and compared to the Courant limit. Excellent agreement between our FDTD formulation and the analytical solution for reflections from a thin metal sheet is found

A Finite-Difference Time-Domain Method for the Simulation of Gain Materials With Carrier Diffusion in Photonic Crystals

In this paper, we present a finite-difference time-domain formulation for active gain materials. Our scheme is based on a frequency-dependent conductivity. Experimental material gain is fitted with high accuracy to a multipole Lorentzian model using a semideterministic fitting algorithm. Because our model is an approximation to the full vectorial Maxwells system of equations, we include carrier diffusion into the rate equations for a two-level system. The material gain is included into the standard set of Maxwells equations by linking the frequency-dependent conductivity to the rate equations. Lasing is demonstrated for a vertical-cavity-surface-emitting-laser structure and photonic crystal lasers.

Gigabit pulse position bistability in semiconductor lasers

The paper briefly reviews the major forms of optical bistability in active optical devices compatible for use in gigabit optical communication systems, and reports an entirely new optical bistability for the first time. Unlike previous devices, the two bistable states of the optical device are each a series of picosecond optical pulses at 1 GHz or greater repetition rates, and are distinguished by a half period temporal shift between their temporal positions in relation to a clock pulse. The bistable device is based on a gain switched semiconductor laser. Theoretical studies suggest 100-ps switching speeds might be achieved, and experimental results are reported indicating optically triggered switching times of 500 ps.

Gain-coupled optical logic in semiconductor lasers

A new form of optical logic device has been devised giving NOT, NOR and NAND functions. It is based on gain competition between two optical modes in a semiconductor waveguide cavity. These modes are spatially separated at the input-output facet providing high input/output isolation. The input also sees very low reflection. Although the device relies on changes in carrier level, the switching speed is much greater than the carrier lifetime, with values of 70 to 100 ps being expected with multiquantum well devices. The device should have fan-outs of 5 to 15 and be fully cascadable. Because the input is not injected into a Fabry-Perot cavity, it is not wavelength sensitive.

Low-crosstalk fabrication-insensitive echelle grating multiplexers and passives for the silicon photonics toolbox

In this communication, we report about the design, fabrication, and testing of echelle grating (de-)multiplexers for the 100GBASE-LR4 norm and other passive architectures such as vertical fiber-couplers and slow-wave waveguides in the O-band (1.31-μm) for Silicon-based photonic integrated circuits (Si-PICs). In detail, two-point stigmatic 20th-order echelle gratings (TPSGs) on the 300-nm-thick SOI platform designed for 4x800-GHz-spaced wavelength division multiplexing featuring extremely low crosstalk (< -30 dB), precise channel spacing and optimized average insertion losses (~ 3 dB) are presented. Distributed Bragg reflectors (DBRs) are used to improve the grating facets reflectivity, while multi-mode interferometers (MMIs) are used in optimized perfectly-chirped echelle gratings (PCGs) for pass-band flattening. Moreover, 200-mm CMOS pilot lines processing tools including VISTEC variable-shape e-beam lithography are employed for the fabrication. In addition, wafer-level statistics of the multiplexers clearly shows the echelle grating to be inherently fabrication-insensitive to processing drifts, resulting in a minimized dispersion of the multiplexer performances over the wafer. In particular, the echelle grating spectral response remains stable over the wafer in terms of crosstalk, channel spacing and bandwidth, with the wavelength dispersion of the filter comb being limited to just 0.8 nm, thus highlighting the intrinsic robustness of design, fab pathways as well as the reliability of modeling tools. As well as that, apodized one-dimensional vertical fiber couplers, optimized multi-mode interferometers (MMIs) and extremely low-losses slow-light waveguides are demonstrated and discussed. The adiabatic apodization of such 1-D gratings is capable to provide band-edge group indices ng as high as 30 with propagation losses equivalent to the indexlike propagation regime.

Pulse-position modulation in gain-switched semiconductor injection lasers

This paper details some direct-delay time measurements on both long- and short-wavelength gain-switched injection lasers using a streak camera. The dependence of time delay on drive current is discussed and new applications of variable-delay optical-pulse generation are proposed. A comprehensive experimental and theoretical investigation of ‘alternate pulsing’ is reported. Finally optical delay measurements have for the first time been carried out on twin-stripe lasers and have determined a new type of optical switch whereby the temporal positions of picosecond pulses are strongly dependent on the relative phase of the electrical drive currents.

Numerical investigation of Littrow lasing in open resonator photonic crystal waveguides

We present a numerical analysis of in-plane lasing in open resonator photonic crystal (PhC) waveguides in the Littrow configuration. The intensity spectra corresponding to the resonant modes of PhC waveguide lasers are determined and agree closely with resonant modes in the band diagram of the waveguide. We investigate the dependence of the lasing behavior on the properties of the waveguide. In particular, our method allows for the determination of the lasing threshold, for which an optimal waveguide width is found. Because of the good reflectivity of the PhC mirrors the concept of open resonator waveguides holds promise for laterally small lasers.

Littrow Lasing in Photonic Crystal Waveguides

We propose Littrow based lasing of band edge modes in an open resonator formed by broad photonic crystal waveguide. The concept is developed by plane wave and FDTD simulations of bulk crystal and waveguide.

A comparative study of second-harmonic generation in plasmonic and dielectric gratings made of centrosymmetric materials (Presentation Recording)

We present a new numerical method for the analysis of second-harmonic generation (SHG) in one- and two-dimensional (1D, 2D) diffraction gratings containing centrosymmetric quadratically nonlinear materials. Thus, the nonlinear optical properties of a material are determined by its symmetry properties: non-centrosymmetric materials lack inversion symmetry and therefore allow local even-order SHG in the bulk of the material, whereas this process is forbidden in centrosymmetric materials. The inversion symmetry of centrosymmetric materials is broken at their surface whence they allow local surface SHG. Additionally, centrosymmetric materials give rise to nonlocal (bulk) SHG. Our numerical method extends the linear generalized source method (GSM), which is an efficient numerical method for solving the problem of linear diffraction in periodic structures of arbitrary geometry. The nonlinear GSM is a three-step algorithm: for a given excitation at the fundamental frequency the linear field is computed using the linear GSM. This field gives rise to a nonlinear source polarization at the second harmonic (SH) frequency. This nonlinear polarization comprises surface and bulk polarizations as additional source terms and is subsequently used to compute the nonlinear near- and far-field optical response at the SH. We study the convergence characteristics of the nonlinear GSM for 1D and 2D periodic structures and emphasize the numerical intricacies caused by the surface SH polarization term specific to centrosymmetric materials. In order to illustrate the practical significance of our numerical method, we apply it to metallic gratings made of Au and Ag as well as dielectric grating structures made of silicon and investigate the relative contribution of the bulk and surface nonlinearity to the nonlinear optical response at the SH. Particular attention is paid to optical effects that have a competing influence to the nonlinear optical response of the grating structures, namely the resonant local field enhancement and optical losses.

O-band wavelength (de-)multiplexers on SOI featuring quasi-athermal behavior and fabrication insensitiveness

We report about design, fabrication, and testing of wavelength (de-)multiplexers in the O-band. Echelle gratings (EGs) on SOI featuring high wafer- and lot-level uniformity are presented, while quasi-athermal Mach-Zehnder-based architectures are discussed as well.