Brandon W. Swatowski

Development of Versatile Polymer Waveguide Flex Technology for Use in Optical Interconnects

We report on the implementation of novel flexible polymer waveguide interconnects. They are based on newly developed mechanically flexible low-loss silicone waveguides. In addition to meeting the generic requirements of rigid waveguide interconnects, several flex-material challenges were mastered: a) mechanical flexibility permitting waveguide flexing down to radii of 1.0 mm without cracking; b) minimization of waveguide curling induced by the CTE mismatch between flex substrates and polymer layers to enable assembly and connectorization; c) greatly improved cladding adhesion on standard PCB flex substrates, such as polyimide; and d) high environmental stability despite the reduced polymer cross-linking required for better mechanical flexibility. The new waveguides exhibit excellent stability in damp heat (2000 h in 85°C/85% rH) and under thermal shock (500 cycles from -40° to +120°C), and lead-free solder reflow up to 260°C. Using the newly engineered “Dow Corning WG-1017 Optical Waveguide Clad Dev Sample” and the established “Dow Corning WG-1010 Optical Waveguide Core”, we were able to develop a manufacturing process suitable for large areas and offering high process control and stability to produce waveguides having optical loss values of less than 0.05 dB/cm at 850 nm VCSEL wavelength and fulfilling requirements (a) to (d) above. We describe this manufacturing process and how we have overcome the material challenges mentioned. Furthermore, we present characterization and manufacturing results, show demonstrators, and outline the potential of flexible waveguides as versatile electro-optic assembly platform.

Flexible, stable, and easily processable optical silicones for low loss polymer waveguides

Photopatternable optical silicone materials have been developed that can be fabricated into flexible polymer waveguides using conventional film processing and photolithography techniques. Waveguides designed for multimode applications have demonstrated loss of 0.05 dB/cm at 850 nm. These waveguides have proven stable in 85% relative humidity and 85°C storage conditions for greater than 2000 hours to date with no degradation in optical performance. Further, this polymer waveguide system has shown sufficient adhesion to flexible polyimide substrates without any curling, and passed bending tests to a 1 mm radius without damage.

Laser Direct Writing of Single-Mode Polysiloxane Optical Waveguides and Devices

Using a custom-built laser direct writing system, single-mode polymer optical waveguides, and devices for board-level optical interconnects were fabricated. A novel photopatternable polysiloxane material was developed that combines low-loss, simple, and large-area processability, and reliability during manufacturing and system operation. The polysiloxane waveguides were designed with quadratic cross sections of 5.5 × 5.5 μm2 and a refractive index contrast of 0.0086 between core and cladding polymer for single-mode operation at the wavelength of 1.3 μm. A straight waveguide propagation loss of 0.28 dB/cm was achieved. A wide range of passive optical devices, including Y-splitters, directional couplers, and Mach-Zehnder interferometers were successfully fabricated and characterized. The results prove that the presented combination of material and process technology is a viable implementation for short distance board-level optical links.

Graded index silicone waveguides for high performance computing

We report on the optical functionality and key performance metrics of graded and step index multimode silicone waveguides. Results demonstrate optical losses of <;0.01 dB/crossing for graded index waveguides and 0.19-0.024 dB/crossing for step index.

Thermally Stable, Low Loss Optical Silicones: A Key Enabler for Electro-Optical Printed Circuit Boards

We report the development and characterization of a low loss polymer waveguide material with a polysiloxane backbone for enabling the new generation electrooptical printed circuit boards (PCBs). The polymer was designed to have low optical loss of <;0.05 dB/cm at 850 nm and low residual stress to withstand the harsh reliability requirements during PCB integration. The thermomechanical properties of the polymer were tuned to exhibit a residual stress of ~1 MPa over a temperature range of 25 °C to 200 °C in ambient air. Multimode polymer waveguides were fabricated using the polysiloxane polymer and embedded in a six-layer PCB architecture that was subjected to lamination, through-hole via drilling, plating, and IPC shock test (immersion) in solder bath at 288 °C. Eight channels of multimode polymer waveguide spirals of length 1.2 m were fabricated and the insertion loss measured after waveguide fabrication, lamination, and solder reflow. The deviation in insertion loss as a function of the PCB fabrication process was less than 3% with final insertion loss after solder reflow being 0.052 +/- 0.002 dB/cm. This finding presents an optical waveguide material which when embedded in a PCB fabricated using an industry standard process meets reliability requirements while maintaining optical performance.

Optical coupling between polymer waveguides and a silicon photonics chip in the O-band

We present a silicon photonics optical I/O interfacing solution based on adiabatic optical coupling between silicon and polymer waveguides. A transition loss of -0.9 dB and back-reflection <; -45 dB were found at 1310 nm.

Assembly and performance of silicone polymer waveguides

We report on the functionality and key performance properties of 50 μm x 50 μm flexible graded index silicone polymer waveguides. The materials show low optical propagation losses of < 0.04 dB/cm @ 850 nm over 1 m lengths as well as stability to 2000 hours 85°C/85% relative humidity and 5 cycles of 260°C solder wave reflow testing. Methods to fabricate large area panels are demonstrated for scaled manufacturing of polymer based optical printed wiring boards. The polymer waveguides are terminated with a passive direct fiber attach method. Fully MPO connectorized waveguide panels are realized and their optical performance properties assessed.

Polymer waveguide based optical backplanes and electro-optical assembly technology for computing applications

We report on recent developments of our board-level optical interconnect technology towards polymer waveguide flexes and on the adaption of connectorization and electro-optical assembly methods to be used in optical backplanes and high-density optical subassemblies.

Optimization of optical losses in waveguide component manufacturing

We report on the development and optimization of key performance properties of multimode silicone polymer waveguides, manufactured for 850 nm optical propagation. These developments are based on photopatternable, mechanically flexible, low-loss, gradient index waveguides. Cross sectional waveguide core sizes ranging from 40 μm x 50 μm to greater than 60 μm x 60 μm are assessed with optical analysis of component losses such as crossings and coupling between OM4 fiber and waveguide. Assessments of these values, led to optimization of waveguide size and lower total optical system losses. Methods of manufacture, preparation, and analysis are discussed in detail along with performance results.

Single-mode polymer waveguide PCBs for on-board chip-to-chip interconnects (Conference Presentation)

Pluggable optics are being pushed to their limits in terms of face plate density and power consumption requirements within emerging mega data centers and HPCs applications. Future applications seek silicon photonics based optical engines with ability for high channel count and throughput beyond 1Tb/s. In this paper, we show our results in development of single mode polymer-based optical-electrical PCBs (OEPCBs) supporting the emerging Si-Pho host PCB platforms with multi-terabit on-board routing capability for chip-to-chip communications. Single mode polymer waveguides (SM-PWGs) are fabricated using new photopatternable optical silicone materials (WG-2211/WG-2511-WG2711) on conventional PCBs. Test platform PCB shows designs with varying core sizes (20/15/12/9/7µm) and channel lengths (5-15cm). The measurements results show single-mode waveguides loss as less 0.4 dB/cm at 1310nm. Furthermore, the result show new waveguide material to be compliance for both rigid and flexible PCBs. OEPCB compliance evaluation test results shown in the paper includes results of lamination, chemical compliance, drilling, and plating tests. The results shown in the paper show first time ever fabrication of single mode polymer waveguide OEPCBs in production environment.