Next-generation MOST PHY with 1Gbit/s keeping today's optoelectronics and fiber

May 31, 2012 //By Norbert Weber, Conrad Zerna, Fraunhofer IIS
Next-generation MOST PHY with 1Gbit/s keeping today's optoelectronics and fiber
The optical version of the MOST physical layer has several well-known advantages. It has virtually no EMI problems. The SI-POF has low weight and the fiber and optoelectronics are low-cost. As the need for higher data rates grows, the feasability of the next generation of MOST on the currently attractive link components becomes an urgent topic. The article describes the Physical Layer for future Gigabit MOST.

Extending the speed of MOST into the Gigabit range brings several challenges with it. The performance demands for every component are rising. On the transmitter side, it is necessary to modulate the LED fast. In the fiber, the intrinsic bandwidth cannot accomodate the signal natively and the high-frequency attenuation has to be dealt with. On the receiver side, a trade-off between bandwidth and sensitivity has to be addressed.

We will summarize the work on the EU-funded project POF-Plus [1], which has addressed several of the concerns about the components through new circuit techniques and the application of advanced signal processing.

We will apply the experience and new results of POF-Plus to the power budget of MOST150 and will extend it to 1.25Gbit/s. The remaining gap is small and a perspective on closing it will be outlined.

EU-Project POF-Plus

The target of the EU-Project POF-Plus was an “engineering solution of Gigabit Ethernet over 50m of SI-POF”. The focus was thus particularly on developing/using practical components that could be mass-produced.

For the transmitter, new driving techniques for LEDs were investigated. A current peaking technique in a non-50Ohm environment was applied to the LED to quickly populate and deplete the junction region. The feasibility of the concept was proven in a discrete circuit on a PCB [2]. A long-term test over 3500 hours of continuous operation with the first version of the discrete driver with LED revealed a reduction in the optical modulation amplitude (OMA) of only 6.5% over time. However, the loss occured almost entirely in the first 500 hours; after that point the OMA remained almost constant.

An improved version of the driver is in the making. The first prototypes display an improved performance over the first discrete version. The problem of bandwidth limitation has been solved in the transmitter. The Gigabit driver is not capable of producing the same extinction ratio (ER), but has only a slightly smaller OMA even

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