Power splitter based on cascaded multimode photonic crystal waveguides with triangular lattice of air holes

Abstract

We propose a power splitter based on two-dimensional photonic crystals with triangular lattice of air holes. The structure can be divided into three sections consisting of the input waveguide, area-defect regions, and output waveguides. The operational principle of the splitter is that an incident field in the frequency range of interest is split into a twofold mode pattern with small distance in the first area-defect and then one of which is shifted by the secondary area-defect regions to output port. We optimize the size of area-defects for the mode-splitting and the shifting from the numerical analysis of distribution of time-averaged Poynting vectors by a finite-difference time domain method. And the transmittance over 45% per each output with a bandwidth about 0.06a/λ is achieved.

Introduction

Interest in photonic crystals (PCs) has been growing in recent years owing to their unique characteristics and potential ability to control lightwaves’ behaviors by introducing defects into them [1], [2], [3]. Currently, compact and efficient devices based on PCs such as co/contra-directional couplers, Mach–Zehnder modulators, power splitters, and wavelength multi/demultiplexers have been exploited as well as in conventional counterparts [4].

Power splitters as one of the most indispensable elements for photonic integrated circuits have been implemented with PC-based Y-junction structures, multimode-interference (MMI) effects, and directional-coupling effects [4], [5], [6], [7], [8], [9], [10], [11]. For Y-junction [4], [5], as problems of the mode-mismatch and bending loss, the bending regions have to be carefully optimized to obtain acceptable transmittance, which add difficulties to design routine and fabrication technique.

The directional-coupling structure in PCs proposed by Park et al. [6] is produced by placing two parallel W1 (single line defect in Γ–K direction) photonic crystal waveguides (PCWs) in close proximity to the input one. Note that although 47.6% transmittance per each output port is achieved, the coupling is wavelength-selective and the flat bandwidth is narrow. On the basis of similar operation mechanism, different structure has been proposed by Ren et al. [7], which consists of two parallel W1, one of which served as an input port as well as an output port. And transmission over 45% through each channel with a wide frequency range (about 0.04a/λ) is achieved. However, the strength of the mode-coupling effect decreases as the refractive index of dielectric is increased since the lightwaves are well confined within the dielectric waveguide. So it is much challenge to achieve strong coupling efficiency between two well-separated W1 PCWs, resulting in a longer length for efficient coupling.

Applications of MMI effects in PCWs, offering the advantages of substantial size-reduction, strong confinement compared with their convectional counterparts and subsequently leading to higher density of device integration, have been extensively studied theoretically in dielectric-rod system for either power-splitting or wavelength demultiplexing [8], [9], [10]. To our knowledge, the hole-type structures are seldom studied. One of the reasons is that compact and broadband mode-splitting in air hole structure based on MMI effects is not easily achieved using simple structure proposed in Ref. [8], as will be illustrated in Section 3.

In this paper, we prefer to choose a triangular lattice PCs of air hole structure. Since for a square lattice of air holes, many of the waveguide modes lie above the light line and hence are likely to be quite lossy. It is difficult to achieve simultaneously an in-plane bandgap and guided waveguide modes with significant bandwidths. Meanwhile, for optical communication windows, the ratio of R (radius of air holes) to a (the lattice constant) should be larger enough in square lattice PCs to obtain a significant bandgap, which turns out that the minimum feature size of “dielectric bridges” between two neighbouring air holes is much small, adding to the difficulty of fabrication.

It is worth mentioning here that the work in this paper is different from that of Ref. [8] not only on the choice of material system and lattice pattern, but the detailed structure. In hole-type structure, an incident field in wide bandwidth of interest is split into twofold mode pattern with small distance in the first area-defect and guided into two different directional waveguides, and then the secondary area-defects shift the split field into long output waveguides with designed structure. The power-splitter structure proposed in this paper overcomes the poor transmittance of Y-junction structure (if without delicate optimization) and the limited bandwidth/longer size of directional-coupling structure, which is one of the essential elements for compact PC-based optoelectronic circuit system such as power-splitting/combing and MZI-modulator. The structure can be applied to optical communication systems and also be integrated easily with other PC-based devices.