Discrepancies in the free spectral range (FSR) of one-dimensional (1D) photonic crystal/photonic wire coupled-cavities

Abstract

We present the simulation and experimental demonstration of a coupled-cavity 1D photonic-crystal/photonic-wire (PhC/PhW) structure that produces multiple resonance wavelengths. The combination of several cavities results in the assembly of a spectral response that exhibits multiple resonance wavelengths and potentially leads to the wavelength control required for wavelength division multiplexing (WDM) applications. By using a structure with three distinct in-line cavities, we have obtained three distinct resonance wavelengths—in conformity with the rule that the number of distinct resonance wavelengths is proportional to the number of cavities. The experimental photonic wire waveguide structure had cross-sectional dimensions of 600 nm (width) × 260 nm (height)—with an embedded photonic crystal (PhC) micro-cavity—all based on a silicon-on-insulator (SOI) platform. The embedded PhC structure was tailored to give resonance wavelengths in the C-band and L-band fiber telecommunication range. With the introduction of tapering in the multiple micro-cavity structure, it was possible to obtain three resonance wavelengths that correspond to WDM wavelengths of 1534.87, 1554.63 and 1594.86 nm—whereas, without tapering, the resonance wavelengths were 1645.60, 1670.76 and 1698.68 nm, respectively. We have observed an asymmetric free spectral range (FSR) situation with un-equal resonance wavelength spacing. The taper regions are also responsible for high optical transmission and lower Q-factor values at resonance. Transmission values of 0.17, 0.47 and 0.43 were obtained, together with Q-factor values of 1179.32, 930.05 and 970.35, respectively, without using tapered sections—while transmission values of 0.45, 0.74 and 0.43 were obtained, together with Q-factor values of 1083.24, 850.10 and 885.22, respectively, using tapered sections. (The normalisation values for the experiments were obtained with respect to an unstructured photonic wire). We have demonstrated that the taper structures used must be designed accurately, in order to maximize the transmission values at the desired resonance wavelengths. The demonstration of fabricated device structures that have measured properties that are in close agreement with predictions obtained using finite-difference time-domain (FDTD) computational software is an indication of the precision of the fabrication process. With the introduction of multiple cavities into the structures realised, the number of resonance wavelengths can be tailored for application as WDM components or other wavelength selective filters, such as arrayed-waveguide grating structures (AWGs) and Bragg gratings.