chapter 2, Basic Structure of Optical Transceivers

Excerpt

The role of an optical receiver is to convert an optical signal into an electrical signal. The goal of an optical transmitter is to convert an electrical signal into a modulated optical signal. These requirements define digital transceivers as well as analog receivers and transmitters. However, they differ from each other in respect of design requirements and design considerations, irrespective of digital or analog. The digital transceivers deal with large signals, while the analog receivers handle smaller signals per channel and overall large loading due to multi tone transport. On the other hand, both topologies have common requirements that can be analyzed similarly, such as sensitivity and jitter, with minor differences.

Digital transceivers, however, differ from analog with respect to interfaces, controls, status, reports, and indications due to the differences in mode of operation. As for transmission, an analog transmitter handles fewer signals per channel since the optical modulation index (OMI) is low, about 4% per channel at maximum. But it has to transmit many channels; hence, the loading is similar to that of a large signal. The modulation depth for a digital transmitter is that of a large signal, driving the laser between threshold to high conduction and high optical power. Further discussion on OMI is provided in Secs. 6.9, 8.2.4, and 21.2. Additional material about community access TV (CATV) signals, standards, and broadcast methods are provided in Chapter 3.

2.1 Analog CATV Receiver and Coax Cables

An analog receiver consists of a photodetector (PD), input matching network, and RF chain [front end low-noise amplifier (LNA), automatic gain control (AGC, if needed), and an output stage]. The statuses from the analog receiver are; PD monitor, AGC voltage, and optional received signal strength indication (RSSI). The PD monitor indicates responsivity. Generally voltage is read over a photocurrent sampling resistor of 1 KΩ at 1 mW input power. Then the responsivity in mA∕mW is calculated. The RSSI monitor provides the RF reading at a given optical level.