A flexible and scalable, fully software-based lock-in amplifier for nonlinear spectroscopy.

We demonstrate a cost-effective, fully software-based lock-in amplifier (LIA) implemented on a commercial computer. The device is designed for application in nonlinear spectroscopy, such as transient absorption and coherent multidi-mensional spectroscopy, but may be also used in any other application. The performance of our device is compared to a state-of-the-art commercial LIA with nearly identical results for both devices. Advantages of our device over commercial hardwired electronic lock-in amplifiers is the improved flexibility in the data analysis and the possibility of arbitrary up-scaling of the number of LIA channels.