ABSTRACT
This chapter describes the fundamental understanding of the detection of optical signals using direct techniques. In this detection process, the lightwave power is absorbed by the semiconductor active area in which the absorption would result in the generation of positive and negative carriers which are then collected by either the cathode or anode of the photodetector (PD) junction under reverse bias. On the other hand, the detection of the optical input —eld can be mixed with a local oscillator so that the total —eld can be detected in the PD under the square-law relation. This square law results in a few DC components and two AC components whose frequency can be either the sum or difference of the signal and the local laser. The difference term will bring the detected signals back to the baseband when the carrier frequency of the local laser and the signal carrier is the same, the homodyne coherence detection. A schematic of the structures of the direct detection and coherent detection subsystems is shown in Figure 7.1a and b, respectively, in which the —elds of the signals are local oscillators denoted as E Es LOand . It is obvious in direct detection that the generated current source is proportional to the power of the signal source while that current is proportional to the square of the total optical —eld of the signal and the local oscillator. This coherent mixing allows the boosting of the magnitude of the generated current by the magnitude of the local oscillator and preserve the phase ϕs of the signals as shown in the diagram.
