Receivers were also investigated inside POF-Plus. Measurements with different off-the-shelf components (PD and TIA) were done to find the most performant combination. In the last year of the project, this work was overtaken by the availability of a commerical prototype of an integrated PD/TIA solution with high bandwidth (by A3Pics). It delivered the best performance of all compared options. The higher bandwidth comes at the cost of more noise and a smaller photo diode, which in turn leads to a higher coupling loss at the receiver. (The loss was measured with the prototypes in molded fiber optic transceiver (FOT) packages.) These losses, however, can be compensated for in the electronics, as will be discussed later.
To compensate for the low pass characteristic of the fiber equalization of the frequency transfer curve is applied. It is done at the receiver. In the case of limited optical transmission amplitude, equalization at the receiver results in a smaller SNR penalty than at the transmitter. (This is in contrast to an electrical channel.) Several architectures of different complexities have been investigated during the course of the POF-Plus project. We covered simpler solutions like self-adapting analogue peaking filters  for a laser-based channel as well as sophisticated structures like a combination of feed-forward and distributed feedback equalizer (FFE/DFE) implementation  for the RCLED-based channel. The best result was achieved with this FFE/DFE equalizer: we transmitted 1.25Gbit/s over the discrete LED driver, 50m of SI-POF  and the commerical PD-TIA prototype. The equalizer testchip compensated the channel and a bit error rate of less then 10-10 was measured. A photo of one of the testchips can be seen in figure 1.
Figure 1: Chip photograph of one of the equalizer prototypes
We also extended the equalizer approach and investigated the application of forward error correction (FEC) to the received signal. The particular implementation of a Reed-Solomon block code is capable of turning a BER