High-performance 90° hybrid based on a silicon-on-insulator multimode interference coupler

Abstract

We propose a novel multimode interference coupler (MMI) design for high index contrast technologies, based on a shallowly etched multimode region, which is, for the first time, directly coupled to deeply etched input and output waveguides. This reduces the phase errors associated with the high index contrast, while still allowing for a very compact layout. Using this structure, we demonstrate a 2 × 4 MMI operating as a 90 • hybrid, with a footprint of only 0.65 mm × 0.53 mm including all the structures necessary to couple light to a fibre array. The hybrid exhibits a common mode rejection ratio (CMRR) better −20 dBe and phase errors better than ±5 • in a ∼ 50 nm bandwidth. • hybrids for coherent optical receivers [3][4][5][6]. The latter enable optical fibre based long haul transmissions with drastic increases in data rates without sacrificing additional bandwidth, by using complex quadrature and phase modulations like QPSK (quaternary phase shift keying). High index contrast technologies, such as Silicon-on-Insulator (SOI) and deeply etched Indium-Phosphide (InP) are attractive platforms for the implementation of such components, since they allow for very compact designs and small waveguide curvature radii, that allow for complex interconnections.However, the high index contrast of deeply etched InP and SOI platforms hinders the design of high performance MMIs, as explained in the following. The basic operation of MMIs consists in launching light from one of the access waveguides (numbered 1 and 2 in Fig. 1), into the wide multimode section where it expands into multiple modes. These travel with different propagation constants, and, at certain imaging distances, interfere to form one or several replicas of the input field, which are coupled to the output waveguides (numbered 3 to 6 in Fig. 1). The formation of these images is governed by the self-imaging theory [7], which essentially requires that all the modes excited in the multimode section exhibit quadratically related propagation constants, i.e. β m = β 1 − (m 2 − 1)π/(3L π ), where m = 1, 2, 3, ... is the mode number and L π is the beat length of the two lowest order modes. In waveguides with high lateral (x direction) index contrast, such as deeply etched InP ridges and silicon wires, this relation between the propagation constants only holds for the lower order modes, resulting in strong phase errors for the higher order modes [8]. These phase errors result in low quality imaging, and become especially detrimental as the number of MMI inputs or outputs grows. A number of techniques have been proposed to overcome this problem. First, by increasing the access waveguide width, only the lowest order modes, which exhibit almost ideal propagation constants, are excited [9][10][11][12]. This requires careful design of the access waveguide width, since increasing it too much results in unnecessarily large devices [9]. Second, using shallowly etched waveguides [8] reduces the index contrast, thus alleviating the phase error. However, this yields larger devices, ...

Silicon photonic network-on-chip and enabling components

“…By utilizing nanoscale lithographic techniques, passive devices based on sub-micron Si waveguides, such as filters [19−22], directional couplers [23,24], multi-mode interferometers (MMIs) [25,26], Mach-Zehnder interferometers (MZIs) [27,28], arrayed waveguide gratings (AWGs) [29,30], and photonic crystal devices [31−34], have been demonstrated. For a long time, Si has not been considered as an optical material because it lacks the commonly used physical effects such as the electro-optic effect and stimulated emission.…”

Fourier Based Combined Techniques to Design Novel Sub-Wavelength Optical Integrated Devices

“…An MMI is a well known structure widely used in photonic circuits, such us Mach-Zehnder interferometers [28] or 90 • hybrid for coherent optical receivers [29]. As shown in [29], MMI performance can be highly improved optimizing the index contrast.…”

“…As shown in [29], MMI performance can be highly improved optimizing the index contrast. While this solution required two etch steps, the use of SWG can reduce the index contrast employing only one etch step, which reduces the fabrication complexity.…”

“…This adjustment is needed because the actual device is different from the problem considered in the previous step: the SWG is not infinitely wide and the fundamental mode in the SWG is not a plane wave. In Figure 9(b), we have represented the modal phase error, as defined elsewhere [29], for three different pitches. To obtain high performance MMI, the modal phase error should be as low as possible.…”

Dual Photonic Generation Ultrawideband Impulse Radio by Frequency Shifting in Remote-Connectivity Fiber

“…At this stage, both MMI's width and etch depth need to be carefully chosen since there is an inherent tradeoff between device footprint and operation bandwidth. If one seeks to achieve high operation bandwidth, the propagation constants of the different modes in the multimode section should ideally follow a quadratic relationship [6]. In practice, deeply etched structures considerably deviate from this ideal behavior, specially when higher order modes are considered.…”