Citation: Pedreros, J.; Becerra, A.; Rojas, J.; Pavez, C.; Diaz, M. Design and Stability Analysis of a Digital Automatic Power Control Based on a PI Controller for Laser Drivers. Machines 2023, 11, 516. https:// doi.org/10.3390/machines11050516 Academic Editor: Ahmed Abu-Siada Received: 23 January 2023 Revised: 7 March 2023 Accepted: 7 March 2023 Published: 1 May 2023 Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). machines Article Design and Stability Analysis of a Digital Automatic Power Control Based on a PI Controller for Laser Drivers Jose Pedreros 1,2 , Alex Becerra 1,2 , Javier Rojas 1,2 , Cristian Pavez 3,4 and Marcos Diaz 1,2, * 1 Electrical Engineering Department, Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile; jose.pedreros@ug.uchile.cl (J.P.); abecerra@ing.uchile.cl (A.B.); jrojascatalan@ing.uchile.cl (J.R.) 2 Space and Planetary Exploration Laboratory (SPEL), Faculty of Physical and Mathematical Sciences, University of Chile, Santiago 8370448, Chile 3 Comisión Chilena de Energía Nuclear, Center for Research in the Intersection of Plasma Physics, Matter and Complexity, P2mc, Nueva Bilbao 12501, Las Condes, Santiago 7600713 , Chile; cristian.pavez@cchen.cl 4 Departamento de Ciencias Físicas, Facultad de Ciencias Exactas, Universidad Andres Bello, Republica 220, Santiago 8370035, Chile * Correspondence: mdiazq@ing.uchile.cl Abstract: Laser diodes are widely used in research and industrial applications in areas such as measurements, communications and health. In most of these applications, stability in the emitted light power is required. This can be realized by modifying the internal parameters, such as the current supply, by using an analog automatic power control (APC). This research presents the design and analysis of a feedback laser driver (digital APC system) based on a proportionall–integral (PI) controller. The controller’s theoretical design acting on the supply current in a laser was obtained by algebraically solving the general equations of a PI controller over a laser described as a steady-state system. The required steady-state model can be determined from the lightl–current curve obtained either from the laser data sheet or experimentally. A posterior numerical analysis shows that the proportional gain of the PI controller is only limited numerically by the reciprocal of the slope efficiency of the laser when the characteristic time of the system is greater than the sampling period. Finally, the APC model was tested in an experimental setting using a laser diode ADL-65052TL at several temperatures. The results show that the proposed relations for the proportional gain and the integral time are valid, achieving the desired power stability with a drift of less than 0.1%. Keywords: control design; laser applications; laser stability; lighting control; PI controller 1. Introduction Semiconductor lasers and laser diodes are widely used in different areas, such as systems of cutting, welding, and joining of materials [1]; fiber optic or free space com- munications, including space communications [2,3]; or in scientific fields in instruments, inertial confinement to nuclear fusion [1], atomic clocks [4,5], and space micro propulsion systems [1,6,7]. Laser operation itself and/or changes in the environmental conditions can produce fluctuations in the laser’s operation temperature, which generate several unwanted ef- fects [8]. Thermal variations generate a shift of the emission spectrum (wavelengths) and a reduction in the quantum efficiency. The emission spectrum undergoes displacements on the order of tenths of nm (wavelengths) by each Celsius degree of variation in the laser operating temperature [8–10]. In addition, decreases in the quantum efficiency modify the relation between the laser supply current with the emitted optical power [8,9,11]. These changes can impact the performance of the specific applications. For instance, in optics communications, the signal-to-noise ratio is reduced due to high-frequency instability in Machines 2023, 11, 516. https://doi.org/10.3390/machines11050516 https://www.mdpi.com/journal/machines