Closed-loop laser stabilization system

European XFEL is a free electron laser capable of generating light pulses with wavelength as small as 0.05 nm, which will enable observation of objects at the atomic scale. Laser diode driver (LDD) is a significant element in the beam monitoring system, which will be used in the European XFEL laser. Its role is to provide constant current source, linear, noiseless, and accurate, that delivers exactly the current to the laser diode that it needs to operate for a particular application. To ensure the stable power output to the laser diode, PI controller was implemented. The basic goal of locking the laser frequency is to reduce the frequency fluctuations between the laser and cavity. When the feedback bandwidth is wider than the dominant noise spectrum of the laser, reduction of the fluctuations the laser is tightly locked to the cavity. The code of the PI routine was optimized to enable higher frequency of its execution, which should improve the regulation parameters. This paper presents the microprocessor-based laser diode driver that can be used for a field electron laser system. It focuses on the firmware based on a real-time operating system (RTOS) developed to provide good stability of PI regulation combined with high flexibility in terms of system control.