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At the heart of the module that converts RF signals to light is a laser diode. The basic principle is direct modulation of the incoming RF signal onto the output of the laser diode. An example of the intensity output from a typical laser diode is shown in Figure 1. The RF input signal directly modulates the laser diode bias current about the optimal DC working point, sometimes referred to as the quiescent point, which is typically 40mA. Modulation gains range from 0.02 to 0.2mW/mA and a monitor photodiode maintains the stability of the fixed operating point of the laser. For high performance (low noise and high dynamic range) Distributed Feedback (DFB) semiconductor lasers are used, although for less demanding lower cost applications Fabry-Perot (FP) lasers can be utilised.
The optical fiber is the transport medium for the signal and therefore an important part of the systems required for maintaining high signal integrity. Light emitted from the transmitter is coupled into a single mode optical fiber for cross-site transmission to the receiver location. Corning SMF28 single mode fiber is typically installed to transmit RF traffic at either 1310nm or 1550nm laser wavelengths, where the optical attenuation is 0.4dB/km and 0.25dB/km respectively. Figure 2 shows the optical transmission spectrum for typical signal mode fiber. Optical cable is factory terminated with industry standard FC/APC connectors. An 8° angled facet fiber endface ensures minimal return loss of 60dB.
Light emitted from the transmitter emerges from the single mode optical fiber at a receiver location and is coupled into the receiver module using FC/APC connectors. Inside the receiver module a high speed PIN photodiode performs an O/E conversion operation, to deliver an RF electrical signal output. The behaviour of a typical photodiode is represented by the response curve, shown in Figure 3, yielding typically 0.9A/W.
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