Multimode polymer waveguides have attracted considerable interest for use in high-speed on-board communication links as they provide low loss (<0.04 dB/cm at 850 nm) and high bandwidth (>40 GHz×m) and can be cost-effectively integrated onto standard PCBs. The fabrication of such waveguides on flexible substrates can provide additional advantages: shape flexibility, lightweight and reduced thickness which are particularly important in the aviation and automotive industries. Such flexible and lightweight optical connections will play an important role in next-generation airplanes and driverless cars connecting the multitude of peripheral sensors with the central processing unit at high speed and low latency. However, in such applications, flexible polymer waveguides are required to be bent to meet their stringent space requirements and twisted or stretched when connecting movable parts. Under sharp flexure, the bending or twisting loss dominates the waveguide loss limiting their practical use. In this work therefore, we present a new waveguide design for flexible polymer waveguides with improved bending performance and derive useful layout rules for minimizing twisting losses in such samples. The proposed waveguide structure only requires one additional fabrication step and achieves bending losses below 0.5 dB for a 3 mm bend. In comparison, the conventional waveguide design yields a 2 dB loss under the same bending radius and launch condition. Additionally, useful equations relating the maximum allowed number of twisting turns for low excess loss with sample thickness and width are proposed. Bending and twisting measurements on flexible waveguide samples are presented validating these methods and demonstrating the potential of this technology.
Multimode polymer waveguides have attracted strong interest for use in high-speed board-level interconnections as they can be cost-effectively integrated onto standard printed circuit boards (PCBs) and flexible substrates using conventional methods of the electronics industry and provide low-loss (<0.04 dB/cm at 850 nm) and high-bandwidth (>30 GHz×m) interconnection. Various high-capacity passive optical backplanes have been demonstrated using this technology while data transmission up to 40 Gb/s using NRZ modulation has been reported. Despite however the intensive research in this technology, very few studies have been reported on mode mixing effects in such multimode waveguides. Mode mixing is a very important phenomenon in highly-multimoded systems as it greatly affects the mode power distribution and therefore, light propagation inside these waveguides. Important transmission characteristics such as their loss and bandwidth performance are affected as well as the behaviour of passive waveguide components such as bends, crossings and couplers due to the different mode distribution at their input. In this work therefore, we present theoretical and experimental studies on mode mixing in polymer multimode waveguides used in board-level optical interconnects. Measurements are carried out on 24-cm long flexible waveguide samples to assess the strength of mode mixing using two common methods used in multimode fibre: mandrel wrapping and micro-bending, while a simple ray tracing model is developed to correlate mode mixing strength with waveguide sidewall roughness. The combination of experimental and theoretical studies can indicate the strength of the effect over the practical range of lengths (~1 m) which are relevant to the application.
Multimode polymer waveguides have attracted great interest for use in high-speed short-reach communication links as they can be cost-effectively integrated onto standard PCBs using conventional methods of the electronics industry and provide low loss (<0.04 dB/cm at 850 nm) and high bandwidth (>30 GHz×m) interconnection. The formation of such waveguides on flexible substrates can further provide flexible low-weight low-thickness interconnects and offer additional freedom in the implementation of high-speed short-reach optical links. These attributes make these flexible waveguides particularly attractive for use in low-cost detachable chip-to-chip links and in environments where weight and shape conformity become important, such as in cars and aircraft. However, the highly-multimoded nature of these waveguides raises important questions about their performance under severe flex due to mode loss and mode coupling. In this work therefore, we investigate the loss, crosstalk and bandwidth performance of such waveguides under out-of plane bending and in-plane twisting under different launch conditions and carry out data transmission tests at 40 Gb/s on a 1 m long spiral flexible waveguide under flexure. Excellent optical transmission characteristics are obtained while robust loss, crosstalk and bandwidth performance are demonstrated under flexure. Error-free (BER<10-12) 40 Gb/s data transmission is achieved over the 1 m long spiral waveguide for a 180° bend with a 4 mm radius. The obtained results demonstrate the excellent optical and mechanical properties of this technology and highlight its potential for use in real-world systems.