I am in need of suggestion regarding a frequency modulation experiment. I am splitting a 80 MHz repitition rate laser using a 50 : 50 beam splitter. Both arms are frequency shifted using two AOM's driven at 40 MHz and 40.66 MHz. After that they are recombined using another beam splitter. I need to confirm the resulting frequency that is 660 kHz. However, when I use a oscilloscope, the only signal it displayes is the laser rep rate that is 80 MHz. Is there a way to just see the modulated optical frequency ?