Automatic power control circuit for 2.5Gb/s burst-mode optical transmitter

Abstract

A novel automatic power control (APC) method for burst mode laser driver is proposed and the proposed method is simulated in this dissertation. Recently, $\emph{burst-mode optical transmitters have been required for subscriber systems}$, such as the ATM-PON and high speed computer data links. In such scheme, transmission and reception of data is a burst-mode format, as encountered in data links. Typically, this kind of data transmission is characterized by short packets or bursts of digital data, followed by dead zones where no data is present. Therefore for burst-mode laser drivers, the average photodiode current is not a suitable control means. Instead output power control has to be achieved by peak power, and the APC must hold infinitely peak value information to enable simultaneous response. Because analog peak detector has a limited holding time which may lead to incorrect power levels at the beginning of the burst, we use a digital counter to have unlimited holding time. This APC has two external trimpots. One controls reference voltage and the other controls tolerance limit. Counter clock is generated only when the degradation of output power exceeds the tolerance limit. If counter clock transits low to high, a counter ups its current status and also laser modulation current increases since the current driving laser diode is controlled by 6bits word from counter. The power degradation is compensated for by this process. This APC can control photodiode current ($I_pd$) from 67$\muA$ to 1.67mA and maximum tolerance limit of $I_PD$ is 67$\muA$.