Ultra-low-loss CMOS-compatible waveguide crossing arrays based on multimode Bloch waves and imaginary coupling

Abstract

We experimentally demonstrate broadband waveguide crossing arrays showing ultra low loss down to 0.04 dB/crossing (0.9%), matching theory, and crosstalk suppression over 35 dB, in a CMOS-compatible geometry. The principle of operation is the tailored excitation of a low-loss spatial Bloch wave formed by matching the periodicity of the crossing array to the difference in propagation constants of the 1 st -and 3 rd -order TE-like modes of a multimode silicon waveguide. Radiative scattering at the crossing points acts like a periodic imaginary-permittivity perturbation that couples two supermodes, which results in imaginary (radiative) propagation-constant splitting and gives rise to a low-loss, unidirectional breathing Bloch wave. This type of crossing array provides a robust implementation of a key component enabling dense photonic integration. Silicon photonics is beginning to enable complex on-chip optical networks comprising hundreds of devices. One emerging application is energy efficient, chip-scale photonic interconnects for CPU-to-memory communication [1]. With increasing device density and complexity in a planar photonic circuit, efficient waveguide crossings are indispensible in many network topologies [1]. Crossing designs based on adiabatic aperture widening are large and relatively lossy (0.3-1 dB) [2-4], while resonant designs permit low loss and crosstalk in a compact footprint, but have narrow bandwidth [5] (e.g. ∼ 4 nm [6]). Multilayer processes allow reduced scattering in crossing waveguides [7] or their complete isolation through vertical displacement [8], but they require multiple lithographic steps and/or material layers. Multimode-interference (MMI) based crossings [9][10][11][12][13][14], despite ostensibly multimode behavior, have a number of attractive features, with individual crossings down to 0.18 dB loss and 41 dB crosstalk [14].In this Letter, we describe ultra-low-loss waveguide crossing arrays based on a periodic multimode structure. Popović et al. [12] proposed an efficient approach to design a crossing array (Fig. 1) by constructing a low-loss Bloch wave in a matched periodic structure where the optical field synthesizes periodic focii that jump across gaps and avoid diffraction loss and scattering at the crossing points. This concept is reminiscent of periodic lens-array microwave beam guiding [15]. Microphotonic implementations use a minimum of modes to implement focusing physics, eliminate reflections, and introduce new degrees of freedom. In the first experimental demonstration of this concept [16], we showed record low waveguide-crossing loss of 0.04 dB/crossing (0.9%), equal to theoretical design efficiency [12]. Another recent paper [17] demonstrated similar crossing arrays based on our proposal in Ref. 12, achieving 0.14 dB loss, and introduced an improvement based on subwavelength patterning of the sidewalls, reducing the loss further to below 0.02 dB. To our knowledge, these two results represent respectively the lowest achieved crossing loss in CMOS-compatible photolith...

Multilayer Silicon Nitride-on-Silicon Integrated Photonic Platforms and Devices

“…In Si, compact, fully-etched waveguide crossings fabricated using photolithography (i.e., without subwavelength features) at best have an insertion loss in the range of 0.04-0.1 dB and a crosstalk of −35 dB in the C-band [31], and using genetic algorithms, an insertion loss better than 0.04 dB can be achieved over a bandwidth of about 45 nm and crosstalk can be improved to − 40 dB [32]. To reduce the insertion loss, the index contrast of the Si waveguides is lowered at the crossing by using the partially-etched level at the expense of a larger crossing size [33], [34]; losses as low as 0.015 dB per crossing have been predicted [34].…”

Finite-difference complex-wavevector band structure solver for analysis and design of periodic radiative microphotonic structures

“…Periodic structures, such as photonic crystal microcavities and waveguides [1], fiber-to-chip grating couplers [2,3], and waveguide crossing arrays [4,5], are playing an increasingly important role in the design of integrated photonic circuits. With the prospect that silicon photonics can enable significant advances in a number of applications, including energy efficient processor-to-memory interconnects [6] and optical phased arrays [7], there is a need for efficient techniques for the rigorous design of periodic micro-and nanophotonic structures.…”

“…Next, we analyze the modal properties and band structures of a silicon linear photonic crystal waveguide, a unidirectional antenna-array-inspired fiber-to-chip grating coupler [3], and a low-loss Bloch-mode waveguide crossing array, where we demonstrate the first direct solution of the recently proposed open-system low-loss Bloch modes [4,5]. These examples illustrate the potential utility of a complex-wavevector band structure solver in device design.…”

Dielectric-loaded surface plasmon polariton crossing waveguides using multimode interference

“…To realize high-density interconnections in OEICs, low-loss and low-crosstalk crossing waveguides are required for plasmonic device miniaturization and flexible patterning of the optical interconnections. With reference to photonic integrated circuits, optical crossing waveguides have been proposed that are based on silicon-on-insulator (SOI) photonic wires [11][12][13]. Subsequently, crossing structures using the self-imaging properties of multimode interference (MMI) have also been proposed because they offer low insertion loss and ease of fabrication [11,12].…”

“…With reference to photonic integrated circuits, optical crossing waveguides have been proposed that are based on silicon-on-insulator (SOI) photonic wires [11][12][13]. Subsequently, crossing structures using the self-imaging properties of multimode interference (MMI) have also been proposed because they offer low insertion loss and ease of fabrication [11,12]. In these MMI crossing structures based on SOI photonic wires, the position of the input and output waveguides were set at the center part on both sides of the MMI structure.…”