Elsevier

Optik

Volume 125, Issue 12, June 2014, Pages 2930-2933
Optik

Simplified design of low-loss and flat dispersion photonic crystal waveguide on SOI

https://doi.org/10.1016/j.ijleo.2013.12.081Get rights and content

Abstract

A low-loss and flat dispersion line-defect photonic crystal waveguide is proposed with a simplified waveguide-design on silicon-on-insulator (SOI) based on hexagonal lattice of 2D photonic crystals. A propagation loss of 3.6 dB/mm and flat dispersion over a large wavelength band is reported with an easy-to-implement design without disturbing the periodicity (keeping lattice constant and hole diameter fixed) in the line-defect photonic crystals. The combined effect of photonic crystals (in-plane periodicity) and the vertical layers on SOI results in efficient waveguiding and dispersion characteristics.

Introduction

In the past several years efforts have been made to control various optical properties of materials specially to confine the light in a specific area so to allow it to propagate only in a certain direction with certain frequency. In 1987, alternative arrangement of dielectric materials example Bragg mirrors giving rise to one dimensional light reflection infer to two dimensional and three dimensional photonic crystals [1], [2]. Photonic crystals are periodic optical nanostructure designed by patterning of dielectric material having different permittivity. They are artificial structures that allow controlling the motion of photons in similar way as semiconductor crystal control the motion of electrons. Basically photonic crystal occurs in nature and its different forms have been studied in last few decades. One of the major types of examined structures consists of two dimensional (2-D) slab structures [3]. Photonic crystal waveguides can be formed by introducing line defects which breaks the symmetry of slab and when operated at the wavelengths within the photonic band gap they are expected to provide low transmission losses and well confinement of light. Photonic crystal slabs (PCSs) are normally made up by periodically patterning columns of air holes in high index material slabs have attracted extensive attention responsible to their high potential in high-density photonic integrated circuits (PICs). PCSs are also gaining attention because of their ease of fabrication and their experimental agreeableness with microelectronic circuits. Large propagation loss and unusual dispersion characteristics remain important issues in realizing practical photonic crystal based integrated devices [4]. A unique feature of photonic crystal waveguide which make it different from conventional ones is that the dispersion can be significantly controlled by tailoring the waveguide parameters of the PCS [5]. This distinguished feature of dispersion may cause to the wide variety of optical nonlinearity phenomena and the same may also be adopted for various dispersion management devices that must be required by photonic information technologies. A tight control on the dispersion requires complicated disturbances in the waveguide structure which are not cost-effective and often result in large propagation loss [5], [6], [7].

It is convenient from the application point of view to realise photonic crystal effects in silicon [8]. There has been a trend of using silicon on insulator (SOI) wafers for making integrated silicon photonic devices [9]. A photonic crystal slab with high refractive index dielectric material (i.e. Si) having two dimensional periodic air-holes arranged in hexagonal array is one of the most attractive photonic crystal structure. Insertion of photonic crystals into SOI opened up new possibilities in the area of silicon photonics [10], [11], [12]. Photonic crystal based SOI is different from conventional photonic crystal slab in the sense that the former has three vertically oriented layers in addition to the in-plane periodic arrangement of the holes in silicon. Thus, it is possible to further tailor the properties of the waveguide by the optimisation of the thickness of the two layers – top Si and SiO2 under it. In this paper, a simplified design of a photonic crystal waveguide on SOI is proposed with low-loss and flat dispersion. A significantly low propagation loss of 3.6 dB/mm and flat dispersion over a large wavelength band is reported without disturbing the periodicity (keeping lattice constant and hole diameter fixed) in the line-defect photonic crystals. The propagation characteristics of the waveguide become more sensitive to any change in top silicon layer because of the vertical refractive-index contrast resulting from the presence of SOI. It is the combined effect of in-plane periodicity and the vertical layers of SOI which make it possible to realise efficient waveguiding and dispersion characteristics.

Section snippets

Proposed design of photonic crystal waveguide

The proposed design of photonic crystal waveguide on SOI is shown in Fig. 1. Optical guidance is affected by the photon crystals, line defect and the vertically oriented layers in SOI. The structure consists of three vertically oriented layers top-Si (refractive index n = 3.5) and SiO2 (refractive index n = 1.44) on Si substrate. The in-plane periodicity is introduced by creating the hexagonal array of air holes in the high refractive index Si layer. Thus line defect waveguide is created by

Conclusion

A low-loss and flat dispersion line-defect photonic crystal waveguide on silicon-on-insulator (SOI) is proposed. Propagation loss and dispersion characteristics have been investigated in a simple line-defect waveguide by keeping lattice constant and hole diameter fixed in photonic crystal. A low propagation loss of 3.6 dB/mm and flat dispersion over a large band without disturbing the periodicity is achieved. The flat dispersive nature which is one of the most important features in line defect

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