Optical PCB technology based on polymer waveguides
Prior work: multi-mode polymer waveguides for board-level optical interconnects
In the past decade, optical printed circuit boards (PCBs) based on multimode (MM) optical polymer waveguides (PWGs) attracted increasing attention in the field of board-level optical interconnects, mainly because optical PCBs are a key enabler for electro-optical integration. In detail, they can simultaneously provide electrical and optical signal routing capability as well as enable the simultaneous interfacing of electrical and optical connections. Generally speaking, they allow a tight integration of electrical and optical functions. Moreover, one-step mating of numerous optical interfaces becomes feasible. Finally, integrated PWGs help to avoid cumbersome cable handling at board level.
Table 1. Stringent requirements for MM PWGs used as optical interconnects. This illustrates why silicones are excellent base materials to realize optical PCBs. Within a close collaboration with our polymer materials provider Dow Corning Corporation, we were able to identify suitable silicone-based optical polymers and tailor them to fulfill all requirements.
The IBM photonics team has more than ten years’ experience in the research and development of such optical interconnects for board-level applications using silicone-based low-loss MM PWGs at 850-nm-VCSEL wavelength [1–6].
Within the scope of this work, we have established cost-efficient PCB-compatible manufacturing processes for panel-sized rigid or flexible optical PCBs, low-cost alignment and assembly concepts, and a standardized connector technology.
Furthermore, various passive and active optical interconnect demonstrators with high-bandwidth could be realized that use MM PWGs in or on top of mechanically rigid (Fig. 1a–b) as well as flexible (Fig. 1c) optical PCBs.
Figure 1. (a) Optical transmitter card of a 1210-Gbit/s optical link demonstrator containing 12 embedded PWGs. (b) High-speed and low-power link demonstrator TERABUS (funded by DARPA) with two “optochips” linked by 32 on-board high-density PWGs (with 62.5 µm pitch). (c) Optical backplane of 192 channels with complex channel shuffling based on eight stacked PWG flexes (after connectorization).
Current research focus: single-mode PWGs for Si-photonics packaging
Recently, in order to enable the required bandwidth increase of future data centers and HPCs and to meet system-cost and power constraints, we established a novel low-cost electro-optical packaging approach for emerging CMOS-compatible Si-photonics transceiver chips.
However, one of the main challenges of Si photonics is to optically connect the Si photonics chip with the external world.
In our packaging approach where the Si photonics chip sits next to the CPU on a joint carrier, we direct the light by adiabatic optical coupling from the Si waveguides to dedicated SM PWGs and from there to SM fibers at the carrier or board edge, as schematically shown in Fig. 2.
Figure 2. Novel electro-optical Si photonics packaging approach for high-bandwidth (up to 10 Tbit/s) off-carrier optical communication. Optical SM PWGs are used to connect a Si photonics transceiver chip with the external world.
This method allows us to overcome the problem of dimensional (Table 2) and modal mismatch between the SM fibers and the much smaller Si waveguides.
|Properties||MM PWGs||SM PWGs||Si waveguides||SM fibers|
|Typical dimensions [width × height]||35–50 × 35–50 µm2||6–8 × 6–8 µm2||300–500 × 140–250 nm2||∅ ca. 10 µm|
Table 2. Comparison of dimensions between multi-mode and single-mode polymer waveguides
To realize the required SM PWGs, we extended our well-established 850-nm VCSEL-based MM PWG technology to SM PWGs operating around 1310 nm as well as 1550 nm wavelength (i.e. in the O and C bands).
Processing of the SM PWGs (Fig. 3) is fully compatible with PCB technology. PWGs are reliable in harsh 85°C/85%rH tests (telcordia standard) as well as in temperature cycles of lead-free soldering (up to 260°C). Our SM PWG technology supports processing on chip-level, wafer-level and even on full panels (e.g. 450 × 300 mm2), as depicted in Fig. 4.
Depending on the application and the substrate choice, rigid as well as flexible PWG boards/chips can be implemented.
Figure 3. Schematic of SM PWG manufacturing process. (a) Deposition of lower cladding polymer followed by UV-flood curing. (b) Deposition of waveguide core polymer. (c) Waveguide patterning by UV-laser direct writing or proximity-mask lithography, followed by solvent-based development. (d) Deposition of upper cladding polymer. (e) UV-flood curing, or optionally UV patterning and solvent-based development of upper cladding. Finally, thermal curing step.
Figure 4. Photographs of SM PWGs realized on chip, wafer, and panel levels.
 B.J. Offrein et al., “Parallel optical interconnects in printed circuit boards,” Proc. SPIE 5990, 59900E (2005).
 T. Lamprecht et al., “Passive alignment of optical elements in a printed circuit board,” in Proc. 56th Electronic Components and Technology Conference, pp. 761-767 (2006).
 R. Dangel et al., “Polymer-waveguide-based board-level optical interconnect technology for datacom applications,” IEEE Trans. Adv. Packag. 31(4), 759-767 (2008).
 F.E. Doany et al., “160 Gb/s bidirectional polymer-waveguide board-level optical interconnects using CMOS-based transceivers,” IEEE Trans. Adv. Packag. 32(2), 345–359 (2009).
 D. Jubin et al., “Polymer waveguide based multi-layer optical connector,” Proc. SPIE 7607, 76070K (2010).
 R. Dangel et al., “Development of versatile polymer waveguide flex technology for use in optical interconnects,” J. Lightwave Technol. 31, 3915-3926 (2013).
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