I. Introduction
With the rapid growth of data throughput in various industries, quad-channel optical transmitters has been widely studied and used for 100G to 400G optical communication applications. In these systems, ADC plays an important role in detecting working conditions based on some voltage or current parameters [1]. Vertical-cavity surface-emitting laser (VCSEL) is a typical directly modulated laser (DML). It has a positively biased PN junction structure which generates stimulated emission by a bias current and outputs optical signal by a modulation current [2]. However, the static optical power varies with temperature and device aging, which requires an automatic power control (APC) method to maintain the desired working condition [3]. One typical method is to use a monitor photodiode (mPD) to generate a current signal proportional to the output power of VCSEL. An ADC is integrated in the VCSEL transmitter to measure the current signal for subsequent control of VCSEL bias current through digital logic circuits and DACs.