**6.3 5 Gb/s transmission over 10 m SI-POF**

In the next experiment, 405 (DL-5146-101S), 450, 515, and 639 nm laser diodes were used as WDM optical sources. To directly modulate the diodes with a higher modulation bandwidth, each diode was soldered to a 50 ohm SMA formable coax cable (**Figure 14**), which was connected to the output of an external bias tee with 6 GHz bandwidth and 0.1 MHz low cutoff frequency.

Due to the low impedance of the laser diodes (typically 2–5 ohms), a severe impedance mismatch was present. However, by using higher power of the modulating signals, the mismatch could be compensated. For mounting the diodes into the mounts, copper retainer rings for both TO-56 and TO-38 packages were fabricated (**Figure 15**). For better thermal conductivity between the TEC element of the mount, the retainer ring, and the laser diode housing, a heat-conductive paste was applied on the contact surfaces.

The bit rates in the individual channels were 1.25 Gb/s (405 nm channel), 1.05 Gb/s (450 nm channel), 1.25 Gb/s (515 nm channel), and 1.45 Gb/s (639 nm channel). The transmission parameters for the individual channels are listed in **Table 6**. In contrast to the previous measurement, no signal nonlinearities were present, and the maximum achievable data rates were limited by the ISI.

Taking into account the amplitude levels of the recorded eye diagrams and low interchannel crosstalk, like in the previous experiment, no power penalty due to crosstalk could be assumed if the laser diodes were modulated simultaneously. Therefore, it can be stated that 5 Gb/s transmission could be realized over 10 m SI-POF link at the BER <10–4 with four simultaneously active channels and no interchannel errors. Using a standard Reed-Solomon (255,247) FEC with 3.2% redundancy, 4.84 Gb/s transmission could be achieved at the BER<10–9 [28].
