Modelling of Photonic Wire Bragg Gratings

Abstract

Abstract. Some important properties of photonic wire Bragg grating structures have been investigated. The design, obtained as a generalisation of the full-width gap grating, has been modelled using 3D finite-difference time-domain simulations. Different types of stop-band have been observed. The impact of the grating geometry on the lowest order (longest wavelength) stop-band has been investigated -and has identified deeply indented configurations where reduction of the stop-bandwidth and of the reflectivity occurred. Our computational results have been substantially validated by an experimental demonstration of the fundamental stop-band of photonic wire Bragg gratings fabricated on silicon-on-insulator material. The accuracy of two distinct 2D computational models based on the effective index method has also been studied -because of their inherently much greater rapidity and consequent utility for approximate initial designs. A 2D plan-view model has been found to reproduce a large part of the essential features of the spectral response of full 3D models.

Closure of the stop-band in photonic wire Bragg gratings

“…The magnitude of the overall deviation between experimental and numerical data is consistent with the previously demonstrated large sensitivity to the geometrical dimensions [11]. Figure 4 also shows that the numerically evaluated reflectance values (crossed diamonds), obtained as the maximum value within the stop-band [6], behave consistently with the reduction of the gap widths, having a minimum at d = 160 nm.…”

“…The simulated gratings were designed as short as 16 periods, as a trade-off between computational load and visibility of the [6]. Figure 3 shows the calculated transmission (T ) and loss (L) curves (obtained as L = 1 − (T + R), with R standing for reflectance) relevant to the geometrical configurations that show evidence of the disappearance of the stop-band for TE polarization.…”

“…Bragg gratings) by engineering a periodic variation of the refractive index into their core or cladding. Thanks to the high refractive-index contrast of SOI, high reflectivity gratings as short as a few tens of micrometres can be fabricated for example by carving circular holes along the wire [4,5] or by the insertion of periodic indentation on the sidewalls [6,7]. Although not studied as extensively, the latter grating design (the photonic wire Bragg grating -PhWBG) has the advantage of facilitating a better flow and interaction with fluids, due to the exposed teeth (rather than laterally enclosed holes) resulting in easier fabrication (reduced RIE-lag effect), and better sensing performance [7].…”

“…1. In a previous publication [6], using a 3D finite-difference time-domain (FDTD) approach, we have studied numerically the properties of the stop-band of PhWBG in the wavelength region around 1.55 µm and found that, over a range of geometrical dimensions corresponding to large refractive index modulation, the gratings show an unexpected reduction in the stop-bandwidth and reflectance intensity. This phenomenon is counter-intuitive, since the reflectivity is usually expected to increase with increasing magnitude in the refractive index modulation as shown, for example, in [8] -when using Bloch and coupled-mode theories.…”

Integrated optical sensor utilizing slow-light propagation in grated-waveguide cavities

“…That happens at both interfaces, M1 and M2. For a perfect structure, i.e., a structure without scattering losses due to possible fabrication imperfections or without material induced absorption losses, the only inevitable losses occur by scattering due to modal mismatch at the transition between the input/output WG and the grated region [76,79]. …”