  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.
