Hi guys~ I have a question about lock-in detection method that commonly used in optical spectroscopy experiments, for example, the optical pump-probe reflectivity exeperiments on some semiconductor surfaces, or kinda like that. Usually, the sinario is: you use laser pulses, divided as pump and probe, for inducing and detection, not so hard to get that picture, and you need an optical chooper or some other modulation methods, also the frequency of your chopper is connected to a lock-in amplifier, of course, to get the signal buried in noises. I think some of you are definitely pretty familiar with this sort of experiments. here comes my question, which confused me alot, let me put it this way: it's said that lock-in amplifier will ideally and only select the signal that equals to your reference signal, other components are filtered and reduced. but how do you know that your reference signal from the optical chopper is comparable to the real signal you want to measure? specifically, let's say you want to measure some really fast phenemenon such as phonon oscillations in semiconductor, with ultrafast laser pump-probe setup, in this case, how to choose the frequency of your chopper (and also it's the reference frequency for lock-in)? is that the case that if your signal is oscillating at GHz level, you are going to choose the chooper frequency at the same level? that sounds rediculous so that I don't get it. I don't know if I made myself clear, hopefully someone would enlighten me on this. Thx in advance.