**6.2 Measurement results and discussion**

### *6.2.1 Gb/s transmission over 10 m SI-POF*

The laser diodes providing the signals at four different wavelengths operated at 405 (DL-5146-101S), 450, 515, and 660 nm. Each diode was inserted into a laser socket of the TCLDM mount, which was also equipped with an internal 500 MHz bandwidth bias tee and had separate inputs for the bias and modulating current. The respective data rates achieved in the individual WDM channels were 0.5, 0.5, 0.7, and 0.8 Gb/s. The corresponding eye diagrams are represented in **Figure 14**. The effect of pulse shaping due to the low-pass characteristic of the fiber can clearly be recognized, e.g., in 405 nm channel.


#### **Table 5.**

*Optical power loss at the transmitter side when using 4 × 1 fused POF coupler.*

#### *Optical Fiber Applications*

The irregular signal trajectories, e.g., in the eye diagrams of 515 and 660 nm channels, indicate the presence of nonlinearities in the electrical domain (presumably introduced by the bias tee circuit). No BER measurement data were saved. However, due to the eye diagrams still opened wide enough, it can be reasonably assumed that the corresponding BERs were below the FEC threshold of 10–3, allowing for the error correction.

High optical isolation of the demultiplexer provided very low optical crosstalk between the WDM channels. The crosstalk of ≤35 dB, coming from 450 nm channel, was detected in 515 nm channel. In all other channels, the crosstalk lower than −45 dB was detected with a Melles Griot 13 PDH 005 integrating sphere. Considering the amplitude levels of the recorded eye diagrams and low interchannel crosstalk, no reduction in the SNR of the received signals could be assumed if the laser diodes were modulated simultaneously. Therefore, it can be stated that an aggregate bit rate of 2.5 Gb/s could be transmitted over 10 m SI-POF with four

#### **Figure 14.**

*Eye diagrams for 10 m SI-POF link at an aggregate bit rate of 2.5 Gb/s (note: The full time scale was automatically set by the oscilloscope and is smaller than two unit intervals of the signal).*

**Figure 15.** *TO-56 diode mounted in a retainer ring; (c) TCLDM9 mount with a butt-coupling unit.*

simultaneously active channels and no interchannel errors. The corresponding information rate after deduction of 7% FEC overhead would be 2.33 Gb/s.
