Chapter 12 provided an extensive explanation about automatic gain control (AGC) topologies for RF leveling in both feedforward (FF) and traditional feedback methods. It was explained that amplitude control using the feedback approach is a linear approximation around the operating point to which the leveling system is locked. Amplitude control is used in signal-leveling systems such as receiver systems, whereas AGC is used in transmitter systems where automatic level control (ALC) is used. Additional amplitude leveling systems that use feedback optically control power to stabilize the emitted power from a laser. This system is called automatic-power-control (APC) loop. An additional control system utilizing feedback is for leveling the amount of laser cooling, using a thermoelectric cooler (TEC) to achieve tight wavelength control. This is required for DWDM systems per the international telecommunications union (ITU) grid specifications as provided in Chapter 1. In the TEC loop, the temperature is controlled for wavelength stabilization, as well as for optimizing the laser's environmental conditions. Temperature control is done in order to deliver the required optical power in community access television (CATV) transmitters and WDM under extreme ambient conditions. The RF power leveling used in AGC is dual to a TEC loop, where RF power is equivalent to the heat capacity denoted as Q. In APC, RF power is dual to the optical level. In this short chapter, analogies to AGC models are provided as well as an introduction to APC and TEC loop concepts. Many of these TECs and APC commercial controllers are available. Both TEC and APC loops in current advanced small-form pluggable (SFP) and analog designs are realized by using microcontrollers. TEC controllers and APC controllers are part of laser drivers, and loop filter implementations are done in software. However, it is beneficial to understand the design considerations of these kinds of loops. This chapter is intentionally placed after Chapter 12 to provide a review about APC and TEC loops used for both analog and digital applications.
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