PyRPL (Python Red Pitaya Lockbox) — An open-source software package for FPGA-controlled quantum optics experiments

The Red Pitaya [1] is an affordable (<; 250 Euros) field-programmable gate array (FPGA) board with fast analog inputs and outputs (sampled at 125 MHz). This makes it useful for quantum optics experiments, in particular as a digital feedback controller for analog systems. Based on the open source software provided by the board manufacturer, we have created the software package PyRPL [2] (Python RedPitaya Lockbox) which implements many devices that are needed for optics experiments with the Red Pitaya. The user interface and all high-level functionalities are written in Python, but an essential part of the software is contained in a custom FPGA design written in the hardware description language (HDL) Verilog. The HDL part of PyRPL implements various digital signal processing (DSP) modules: a two-channel oscilloscope for measurement and diagnostics; two arbitrary function generators for waveform synthesis and noise generation; four proportional-integral-derivative (PID) controllers with up to fourth-order filters for feedback control; three demodulation modules for the generation of Pound-Drever-Hall-like error signals and the realization of narrow, phase-tunable band-pass filters; an infinite impulse response (IIR) filter able to realize transfer functions with up to 24 poles and 24 zeros; a network analyzer for the characterization of analog and digital systems and in-loop tuning of feedback parameters; a spectrum analyzer for the measurement and minimization of noise in closed-loop systems.