The silicon-based photonic crystal (PhC) is an artificially manufactured periodic dielectric material, whose unique band gap effect enables optical devices based on this structure to have the advantages of low loss and small size. In recent years, silicon-based PhC devices such as beam splitters, electro-optic modulators, optical switches, mode multiplexers, and optical add-drop multiplexers (OADMs) have received widespread attention from scholars in various countries due to their small size and easy cascading performance in the highly integrated optical communication system. Of various PhC-based devices, OADM, a key device in wavelength division multiplexing (WDM) systems, has attracted more and more attention from researchers. To meet the requirement of the highly integrated optical communication system, OADM design faces three possible challenges that cannot be ignored, namely, low insertion loss, compact size, and easy cascading performance. Moreover, with the advent of 5G, dense wavelength division multiplexing (DWDM) has become a key technology for increasing transmission capacity in optical fiber communication systems. As DWDM devices occupy an important position in optical communication systems, more requirements are posed for OADM design in channel spacing and crosstalk. In this study, we propose an OADM for DWDM systems based on PhCs. The device has low insertion loss, channel crosstalk, small size, and compact structure and can expand channels through cascading to achieve DWDM with channel spacing of 0.8 nm, which has great application potential in highly integrated large-capacity communication systems.

This paper designs an OADM on the basis of a two-dimensional (2D) PhC triangular lattice plate of air holes in silicon. In the designed PhC plate in silicon, the circular air holes are arranged in a triangular lattice and periodically distributed along the 2D X-Y planes. The designed structure contains two different Aubry-André-Harper (AAH) bichromatic potential cavities, i.e., the resonant cavity and the reflection cavity. The resonant cavity couples the light intensity at the working wavelength, and the reflection cavity reflects the light intensity at the working wavelength. First, we design a PhC AAH cavity, which is the key component of the proposed OADM device. It is composed of one-dimensional PhCs arranged according to different lattice constants based on the design principles of the AAH cavity model. Then, we model the basic structure of the designed OADM according to the coupled mode theory. Theoretical transmission spectra are derived to determine the optimal parameters of the OADM structure. After that, we calculate the parameters of the proposed device by the three-dimensional finite-difference time-domain (3D-FDTD) method for verification. In addition, we design a tapered structure for further optimization of the PhC OADM device on the basis of the modified step theory.

First, the theoretical spectra of the adding-dropping process based on Eqs. (12)-(17) present a clear trend that the transmission can reach resonant cavity 1 as is close to . The following four rules must be satisfied to achieve this ideal condition of the theoretical model: 1) two resonant cavities have the same resonant frequency ; 2) the amplitude coupling attenuation coefficients of the two resonant cavities to the bus waveguide are equal, which is ; 3) the phase delay of the light wave from one cavity to another is (n is a non-negative integer); 4) the amplitude coupling attenuation coefficient of resonant cavity 1 to the input waveguide and the bus waveguide is . Second, when the above four rules are met, the parameters of the design device are calculated by the 3D-FDTD method. The numerical results show that the proposed device can add/drop light intensity at the operation wavelength of 1556.2 nm and 1555.4 nm. The PhC AAH reflection cavity and tapered structure are designed to reduce the leakage of the light wave at the working wavelength on the bus waveguide and the mode mismatch loss at each port, which make the insertion loss and crosstalk lower than 0.51 dB and -29.54 dB, respectively. The line width is 0.2 nm due to the high Q value of the AAH cavity. However, the comparison of the theoretical and numerical spectra [Fig. 5 (b) and Fig. 6 (c)] demonstrates that the two transmission spectra overlap, but the highest transmittance obtained by the simulation is lower than the theoretical transmittance. This is because the simulation algorithm based on the 3D-FDTD method is more comprehensive than the coupled mode equation in the calculation of such loss as the coupling loss between waveguide and resonant cavity and that between silicon waveguide and PhC waveguide, the vertical direction loss of the resonant cavity, and the transmission loss of the PhC waveguide. In addition, the spectra of ports 1, 2, and 3 obtained by simulation are consistent with the spectral trend derived from the theoretical equations in Section 2.1.

An OADM based on PhCs for DWDM is proposed. The theoretical model of the three-port filter is built, and the transmission spectrum is derived on the basis of the coupled mode theory. The 3D-FDTD method is used to calculate transmission performance to verify theoretical results. The device has low insertion loss, channel crosstalk, and small size (19.35 μm×13.33 μm) and can expand channels through cascading to achieve DWDM with channel spacing of 0.8 nm, which has great application potential in highly integrated large-capacity communication systems.