Are You Ready for Five Gbps optical?
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.
© TE Connectivity
At TE Connectivity, all these fundamental building blocks for a five Gbps optical transmission link are now available as functional prototypes in the lab and the first verification results are available.
To program, operate and test the transceivers, TE Connectivity has designed and manufactured a PCB test board. The board provides separate filtered power supplies to the transmitter and receiver sections of the FOT. The traces between the FOT and SMA-connector are designed as single-ended 50 Ohm micro-striplines with 30 mm of length on a standard FR4 base material. A special fixture allows temporary programming and operation of a FOT without soldering it to the test board.
For the overall link testing and verification of the several building blocks, the following measurement setup was used: A differential pseudo random bit stream generator is used as a signal source to feed the fiber-optic transmitter. A glass fiber of several meters length, consisting of lensed ferrules on both ends, is inserted between the TX and RX FOTs, which are placed on separate test boards. In figure 2, the electrical receiver eye-diagrams are shown for data rates ranging from one Gbps to seven Gbps. One can see a slight degradation when going to higher data rates, with the VCSEL the most limiting factor with a bandwidth of five Gbps. The bandwidth reduction can be clearly seen at seven Gbps, where the eye starts to close. These measurements have been done with an optical input power resulting in a 76 µApp photodiode current at the transimpedance input stage of the receiver’s chipset, which refers to approximately -9 dBm of optical input power at the photodiode.
The receiver sensitivity is of interest for the evaluation of the system. The total jitter at five Gbps for a bit error rate (BER) of 10-12 has been measured and set against the receiver sensitivity (TIA) in µApp. The results are plotted in figure 3. The TIA guarantees a five Gbps transmission if the incoming peak-to-peak current is above 18 µApp. It can be seen that at this limit, the total jitter is about 0.42 UI. For higher input currents, the total jitter reduces. Hence, it is essential that the modulation amplitude of the optical input power in combination with the responsivity of the photodiode is higher than 18 µApp.
© TE Connectivity
To verify the molded optic lenses of the FOT, a scanning confocal microscope has been used to determine the lens surface quality and compliance to the defined lens shape. Since the field of view of the microscope is limited for the magnification factor required for the resolution, the scan results have to be stitched together to a complete surface data map. This surface map data has been filtered using a Gaussian low-pass filter algorithm to smooth the high-frequency surface roughness for the shape deviation analysis, see figure 4. Over 99% of all slope values of the transmitter lens have been measured within the specification. For the receiver lens, 89% of the slope values are within the specified limits.
To bring multi-gigabit glass-fiber technology on the road, more building blocks have to be created and verified. These building blocks are not only of a physical nature. Physical layer link specifications, suitable test- and measurement definitions and procedures for compliance and verification tests also have to be developed.
TE Connectivity has the fundamental building blocks that are ready for a five Gbps glass-fiber data link and is eager to work with interested parties to bring the automotive glass-fiber ecosystem to life. Because of the modular build, a possible future data-rate increment to 10 Gbps or above is straightforward and the first prototypes are currently being assembled. TE Connectivity is ready for the multi-gigabit age.
Authors:
Markus Dittmann is Development Engineer Fiber Optics and Project Coordinator – Advanced Infotainment at TE Connectivity Bensheim, Germany. (mdittmann@te.com)
Claus-Dieter Gräff is Product Manager Infotainment at TE Connectivity Bensheim, Germany. (claus-dieter.graeff@te.com)
Carlos Almeida is Manager Global Advanced Infotainment at TE Connectivity Bensheim, Germany. (calmeida@te.com)
Andreas Engel is Manager Business Development Infotainment at TE Connectivity Bensheim, Germany. (aengel@te.com)
Special thanks to the fiber optic team at TE Connectivity‘s Hertogenbosch, The Netherlands: Jeroen Duis; Rutger Smink; Michiel van Rijnbach; Sander Dorrestein; Joek Tuin.
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