Are You Ready for Five Gbps optical?

June 18, 2015 //By Markus Dittmann et al., TE Connectivity
Are You Ready for Five Gbps optical?
Plastic optical fiber (POF) has been the successful solution for "high-speed" data networks in the car for almost two decades. In that time, the data rate has increased from 5.6 Mbps to 150 Mbps. However, the bandwidth capability of the employed step index POF and light emitting diode (LED) is limited. As a result, the development roadmap to increase the data rate based on the LED and POF technology ends at around one Gbps even if advanced coding and modulation schemes are used.

All glass fibers (AGF) and vertical cavity laser (VCSEL) light sources provide a means to overcome this bandwidth limit with ease. A glass-fiber based physical layer solution with a five Gbps data transfer rate would be a giant leap for in-car data links. It overcomes the gulf that separates the data rate increase roadmap of POF and upcoming communications bandwidth demand. It is paving the way for a roadmap beyond the one to five Gbps data rates that are anticipated for the upcoming next generation of in-car data transmission. Hence, by combining VCSEL and AGF technologies, the foundation for the next generations of optical high-speed data links is established. It is an attractive technology as a successor to LED and POF since its bandwidth capability is unmet.

Although multi-Gbps communications systems based on AGF and VCSELs are common in the data communications industry, they are still new terrain in automotive. Getting such a new technology on the road is a long journey. Like every journey, it starts with the first steps. The first step was to come up with a suitable coupling and design concept to address the automotive environment conditions and harness topologies. The second step was to actually design and build components realizing the concept. The third step is now to verify the manufactured components. As a system is only as good as its weakest element, every integral part in a physical layer data link is an important building block. Thus analyzing the maturity of the five Gbps glass fiber link solution starts with the verification of each building block.

So what are the building blocks to be verified?

Describing a complete physical layer (PHY) link starts with the transmit side (TX) of the chipset that creates the data to be transported, possibly with encoding and modulation formats. In MOST Technology this has been an integral part of the network interface controller (NIC). At the other end of the link, there is the receive section (RX) of the PHY chipset. Between the PHY TX and the PHY RX, one finds all the building blocks required for an optical glass fiber data link as shown in figure 1. The building blocks are:

  • The transmitter engine as the electrical to optical media converter (EOC). It consist of a five Gbps directly modulated VCSEL and a six Gbps current driver integrated circuit (IC).
  • The receiver engine as the optical to electrical media converter (OEC) consists of a 10 Gbps photo diode (PD) as light detector and a six Gbps transimpedance amplifier (TIA) IC next to it.
  • Both the EOC and OEC are integrated into a common package, building the fiber optic transmitter/receiver (FOT). This FOT package includes a lens on the TX side to shape the optical beam for coupling towards the terminated cable. On the RX side a lens is integrated into the FOT package to refocus the collimated beam coming from the cable harness lensed connector. The FOT package also provides alignment features for the ferrules of the harness connector. The transceiver package consists of an overmolded lead frame and is designed to be integrated into a direct coupling device connector.
  • The fiber-optic cable consists of an AGF and several protective layers to make it robust enough for automotive use. It has a 125 µm cladding diameter and an 85 µm diameter graded-index core.
  • Another major building block in the optical link are the ferrules terminating the glass fiber. The molded polymer ferrules have an integrated lens to allow for an expanded beam coupling interface functionality.

Image 1: Between the PHY TX and PHY RX, one finds all the building blocks required for an optical glass fiber data link. For full resolution click here.
© TE Connectivity

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