This current signal can be transformed into voltage and compared by an operational amplifier with a voltage reference that represents the photodiode current signal-to-voltage at the optical power required. The error signal, resulting from the comparison of voltages, is amplified by the operational amplifier gain and injected to modify the bias current [18] (similar to a proportional controller). Furthermore, there exists other possible configurations in analog APC circuits referred to as noninverting laser driver [19] and inverting laser driver [19; 20]. Both control circuits integrate the error signal (defined with slight differences in each circuit) and inject it as a control current. This signal is added or subtracted to the bias current to modify the supply current of the laser. The design of analog APC requires a bias current and a voltage reference for each operating temperature. Identifying these values can be a time-intensive process [20] and can restrict the laser's operation to a particular range of temperatures. In addition, analog APCs are implemented as analog circuits, which are difficult to modify. An alternative to the traditional analog APC design is to use a digital APC to stabilize the emitted optical power using the probing photodiode current as a feedback signal. This type of controller does not require the a priori estimation of the bias current and a voltage reference to operate (equivalent values for the controller are calculated internally by themselves). Furthermore, a controller implemented in a microprocessor can change the operating condition or set point condition by an operator easier than in an analog circuit. In addition, if the designed controller is simple, it can be implemented in miniaturizedlow-power microprocessors, which is a key feature for small satellite applications where simplicity is associated with the robustness of the system. For these reasons, we chose a proportional-integral (PI) controller as the base for a digital APC. The design of a PI controller is focused on _finding_ the control parameters that guarantee the emitted power stability under operating conditions. Control parameters can be obtained for some systems using a set of theoretical or experimental rules [21]. Most of these methods require the knowledge or modeling of the transfer function that defines its temporal evolution. To find the control parameters in a PI controller for a first-order system or the stability condition of the feedback for a higher-order system requires that the system exhibits explicit temporal dynamics [22]. On the other hand, due to the laser being modeled without a temporal dynamic, common methods can not be used to estimate the controller parameters. Thus, the parameters should be obtained by algebraically resolving the differential equation of the PI controller. The implementation of a proportional-integral (PI) controller in a laser power driver, which operates with a fixed optical power output at a stable temperature, is described in [11]. However, there are no general design rules for the controller (proportional gain \(K_{p}\) and integral gain \(K_{p}/T_{i}\)). In addition, there are no studies on the settling time and stability of the controller. This makes it difficult to design a PI controller for a broad operating temperature range and to estimate the settling time a priori. Thus, in this article, we focus on developing the theoretical control rules of PI controllers to stabilize the emitted optical power around a set point. In addition, we implemented the designed PI controller in a microcontroller to evaluate the validity of the theoretical findings. The proposed driver design is based on uses the lightt-current relationship, which represents the steady state response of the laser and can be obtained either from the datasheet of the laser or experimentally. Additionally, we determine the stability conditions of the laser driver simulating a digital controller for different operation conditions.