Silicon photonic Bragg-grating couplers for optical communications

Abstract

We discuss recent progress and challenges in realizing Bragg-grating devices on the submicron silicon-on-insulator platform for next-generation optical communications applications, such as on-chip optical interconnects and signal processing. In particular, we focus on grating-assisted, wavelength-selective couplers, known as contra-directional couplers (contra-DCs). In contrast to conventional two-port Bragg gratings operating in the reflection mode, contra-DCs are four-port devices with very weak backreflections and, therefore, can be easily integrated with other photonic components on a chip. In order to provide a reliable on-chip wavelength-division multiplexing (WDM) solution for high-speed optical interconnects, we have developed high-performance add-drop filters and, furthermore, wavelength multiplexers/demultiplexers with combined advantages of flat-top responses, low insertion loss (< 1 dB), and low crosstalk (< -23 dB). These WDM devices are ultra-compact and highly tolerant to temperature fluctuations (up to ± 50 0 C), showing great potential for large-scale integration and low-power consumption. We further discuss a novel four-port Bragg photonic resonator for high-speed, low-power optical switching. Using a coupler-chirped design with uniform Bragg gratings, we have recently achieved an on-chip, continuously tunable photonic delay line with low insertion loss. These system-orientated devices indicate great potential for large-scale integration of Bragg-grating-defined functions using CMOS-compatible silicon photonics technology.

Bi-Directional Coupler as a Mode-Division Multiplexer/Demultiplexer

“…As the effective indices of TE 1 and TE single 0 modes are approximately the same, these insertion losses cannot be avoided. However, if there exist other suitable choices of 1 = 0 and 2 that satisfy (22), these losses could be reduced. Another solution is to use TM 1 instead of TE 1 as illustrated in Example 2 below.…”

“…In this example, we select 1 = 0 and 2 = 1 in (22). Accordingly, we can determine the effective index and width of the single-mode waveguide as n single eff = 2.93 and d = 250 nm, respectively.…”

“…A similar structure, that uses two single-mode waveguides, has been adopted in other applications [21], [22]. Shi et al have demonstrated 4-port, electrically tunable photonic filters using silicon contra-directional couplers with uniform and phase-shifted waveguide Bragg gratings [21].…”

Hybrid multimode resonators based on grating-assisted counter-directional couplers

“…In this example, the waveguide widths w 1 and w 2 are chosen to allow single mode operation in waveguide 1 and multimode operation in waveguide 2, while the grating is designed to allow phase-matching between the forward-propagating fundamental mode (TE 0 ) of waveguide 1 and the backwardpropagating second order mode (TE 1 ) of waveguide 2 around a targeted wavelength λ 0 . At phase-matching, the grating period Λ is related to λ 0 and to the effective indices n 1 and n 2 of the two modes by the well-known relation [23][24][25][26][27] ( )…”

Effects of waveguide surface micro-structure on the transmission of light

“…8,9 Solid-state (silicon) and soft-matter photonics are opening exciting new opportunities for manipulating and controlling the flow of light. Silicon photonics has the potential to vastly improve on the current computation, sensing, and communication technology, [1][2][3]10 and soft-matter photonics has high potential to create tunable photonic devices. 11,12 Both solid-state waveguides such as in silicon photonics chips, 4,9 and soft-matter waveguides such as liquid crystal waveguides 13,14 have a certain degree of surface perturbations and roughness, which gives the waveguide a surface micro-structure.…”