Linien: A versatile, user-friendly, open-source FPGA-based tool for frequency stabilization and spectroscopy parameter optimization

We present a user-friendly and versatile tool for laser frequency stabilization. Its main focus is spectroscopy locking, but the software is suitable for lock-in techniques in general as well as bare PID operation. Besides allowing for sinusoidal modulation (up to 50 MHz), triangular ramp scanning, IQ demodulation (1 $f$ to 5 $f$), IIR and PID filtering, Linien features two different algorithms for automatic lock point selection; one of them performs time-critical tasks completely on FPGA. Linien is capable of autonomously optimizing spectroscopy parameters by means of machine learning and can measure the error signal's power spectral density. The software is built in a modular way, providing both a graphical user interface as well as a Python scripting interface. It is based on the RedPitaya STEMLab platform but may be ported to different systems.