Compact Multimode Polymer Waveguide Bends for Board-Level Optical Interconnects

Abstract

Multimode polymer waveguides are promising for use in board-level optical interconnects. In recent years, various on-board optical interconnection architectures have been demonstrated making use of passive routing waveguide components. In particular, 90 bends have played important roles in complex waveguide layouts enabling interconnection between non co-linear points on a board. Due to the dimensions and index step of the waveguides typically used in on-board optical interconnects, low-loss bends are typically limited to a radius of 10 mm. This paper therefore presents the design and fabrication of compact low-loss waveguide bends with reduced radii of curvature, offering significant reductions in the required areas for on-board optical circuits. The proposed design relies on the exposure of the bend section to the air, achieving tighter light confinement along the bend and reduced bending losses. Simulation studies carried out with ray tracing tools and experimental results from polymer samples fabricated on FR4 are presented. Low bending losses are achieved from the air-exposed bends up to 4 mm of radius of curvature, while an improvement of 14 m in the 1 dB alignment tolerances at the input of these devices (fibre to waveguide coupling) is also obtained. Finally, the air-exposed bends are employed in an optical bus structure, offering reductions in insertion loss of up to 3.8 dB. Index Terms-Board-level optical interconnects, multimode waveguide bends, polymer waveguides.

Stretchable optical waveguides

“…0.25dB per 90 • bend) when the bending radius becomes smaller than 4 mm. This corresponds with the results in [25], in which the bending losses of 50 µm x 50 µm waveguides (with an NA of 0.25 and 1.14) were simulated using ray tracing simulations. It was found that the bending loss of a single 90 • bend becomes higher than 0.25dB for bending radii between 3 and 9 mm, depending on the incoupling conditions and the numerical aperture of the simulated waveguide.…”

“…These are typical dimensions for multimode waveguides providing 1dB alignment tolerances larger than ±10 µm for in-and outcoupling [25]. To maximize the light confinement, the refractive index contrast between cladding and core was chosen as large as possible within the limits of available materials.…”

Properties of Large Core Polymer Optical Bend Waveguides

“…The total bend losses α were measured by a setup shown in Fig. 5 and then they were calculated from (10). The bend losses A modified with regard to the lengths of the bend waveguides were then calculated from (11):…”

“…Such basic building blocks have been developed for a couple of years and are also well characterized which concerns their utilization in the single-mode regime [3][4][5]. In the literature several types of optical bend waveguides have been already presented:  bend waveguides described by D. Israel [6] and S. Musa [7]; this will be in more details mentioned later,  single-mode polymer SU-8 2000 waveguide [8], with thickness around 1.7 µm and having different core widths (1.2, 1.6, 2.0, 2.4 and 2.8 µm) and bending radius (300, 200, 150, 100, 75, 50 and 25 µm),  multimode bend optical waveguides based on silicon rib structures (height of 28 μm and width 25 μm); these S-bend structures have the radius ranging from 2 to 20 mm and the bend offset 0.5 mm [9],  fully embedded and air-exposed curved waveguides with a 50 µm × 50 μm cross section having 10 mm bending radius limit [10],  90° bend and S-bend siloxane OE-4140 (core) and OE-414 (cladding) multimode waveguides having typical dimensions 50 µm × 50 µm [11],  multimode bend waveguides with dimensions 50 µm × 50 µm, 75 µm × 50 µm and 100 µm × 50 µm (wide × high) made of acrylate polymer Truemode TM from Exxelis Limited deposited on FR4 wafer intended for wavelength of 850 nm and the refractive index of the core was n core = 1.5560 and that of the cladding was n clad = 1.5264 [12],  polymer multimode waveguide splitter having input waveguide core width 100 µm and ended by S-bend waveguide with different core widths (20, 40, 60 and 80 µm); detailed study was presented in [13].…”

Free-Space 120 Gb/s Reconfigurable Card-to-Card Optical Wireless Interconnects with 16-CAP Modulation

“…While polymer-waveguides based interconnect technologies and fixed fiber based interconnects have been proven to be effective alternatives to copper-based card-to-card interconnects [2,3], providing high-speed reconfigurable optical interconnects that meet the emerging requirements of flexible connectivity has become a major challenge. In previous studies, reconfigurable optical interconnects architecture based on free-space optical technology has been demonstrated [4][5][6].…”