Philips Datacom Technology
For highspeed data transfer, there is no way around optical datacom technologies. These allow data transmission with a bandwidth in the GHz regime over long distances, without cross-talk.
For this purpose, electrical signals have to be translated into optical information - and back again. On the transmitter side, a driver processes the electrical signals and controls the electrical power going into the VCSEL, resulting in a fast modulation of the emitted light. In order to maintain clean signals, the electric characteristics of the VCESL, e.g. capacitance, resistance, ohmic contacts, have to be controlled precisely. Otherwise, signals are blurred, resulting in noisy data transmission. A common way to evaluate the quality of the data link is to record so-called eye diagrams. These are a superposition of randomly generated bit streams (signals), as monitored on an oscilloscope (see Fig 1). Eye opening and width provide information about any eventual presence of noise or jitter effects.
The VCSEL light is then coupled into an optical fiber, usually collimated by a micro lens. The low divergence of the VCSEL, one of its key features, thereby facilitates the coupling considerably. A further feature of VCSELs, namely the possibility of fabricating 2D laser arrays on the chip, allows the efficient fabrication of multiple links with fiber bundles.
On the receiver side, the incoming light is focused on photo diodes, generating an alternating photo current. A transimpedance amplifier then converts the current in a proportional voltage, thus the original electrical signal is reconstructed.
Eye diagram of a Philips VCSEL at room temperature
and a data rate of 12.5 Gbit/s