New gigabit POF standard for home networking, new possibilities for automotive applications Breaking the Gigabit Barrier

Most of the commercially available POF systems for home networks operate at 100 Mbit/s. For automotive networks this used to be fast enough, but with new applications like uncompressed video transmission for surround view cameras the demand for gigabit links over large-core step-index (SI) fibers increases as well. The few available gigabit systems can only operate over a few meters of SI POF, due to the bandwidth limitation of the fiber itself.

It is well known that bandwidth-efficient modulation formats can transmit up to 2 Gbit/s over 100 m of SI POF, but the use of these methods requires a worldwide standard in order to guarantee compatibility between manufacturers.  In September 2009, the new working group 412.7.1 [1] inside the DKE was founded in order to discuss the different related questions and to generate a common application guideline for the transmission of 1 Gbit/s over at least 50 m of SI fiber, which can also be interesting for automotive applications.

For the foreseen specification, some parameters are already set and serve as boundary conditions:

  • Step-index and multi-core fibers according to class A4a.2.
  • Eye-safe operation (≤1 mW fiber coupled power).
  • 24 dB optical link budget (including two in-line connections and 5 dB margin).
  • Low power consumption.

Polymer optical fibers with big core diameters and large numerical apertures have a limited transmission bandwidth. Therefore bandwidth-efficient modulation formats are very interesting for POF transmission links.

The bandwidth of a 50 m SI POF link is usually lower than 100 MHz. This is sufficient for the transmis- sion up to 100 Mbit/s. For data rates of 1 Gbit/s this seems to be much too low. This is a general problem and also valid for other media types being used inside of buildings like for example   wireless, copper cable or power line. Transmission schemes for these media make heavy use of signal processing to overcome the limiting bandwidth and multi-level modulation formats. If one employs signal processing of more efficient modulation formats, the transmission capacity of a link is not determined only by the 3 dB bandwidth, but also by the integral signal-to-noise-ratio (SNR) over the useable frequency range. In addition, there is a trade-off between efficiency and complexity, especially in a cost-sensitive environment like home networking or automotive applications.

Therefore, several transmission formats are under discussion, including simple binary amplitude modulation (Non-Return to Zero; NRZ), multi- level amplitude modulation (m-ary Pulse Amplitude Modulation; m-PAM), and multi-carrier modulation such as Discrete Multi-Tone (DMT) modulation.

The other limiting factor is set by the electro-optical components. The loss of polymer optical fibers has its minimum at around 570 nm. Unfortunately, only few and slow LEDs are available in this wavelength range. The second-best wavelength is between 450 and 530 nm. These LEDs are working very efficiently and fast, but there are nearly no (affordable) diodes available today. The loss minimum in the red spectral range is very small, leading to a strong increase of attenuation (about 150 dB/km higher than for green and blue) for wavelength-offset or broad spectra (like the LEDs). There are efficient LEDs and resonant-cavity LEDs (RC-LED) as well as lasers and VCSEL at 650 nm wavelength.

It is almost obvious that LD are the only source to come into consideration for non-return-to-zero transmission at 1 Gbit/s. The use of VCSEL would be the ideal solution, but they are not usable for temperatures over 70 °C. LED and RC-LED are not fast enough, but may have the ability to offer sufficient bandwidth for pulse amplitude modulation or discrete multi-tone modulation under certain circumstances. They also prove a transfer function linear enough for multi-level modulation. Especially for automotive applications, many developers avoid edge emitters because of the difficulties in mounting and their high and temperature-dependent threshold current.