If you are interested in the spectrum of the sideband signal from mixing the lasers, your best bet is to simply digitize the photodiode output with a digital oscilloscope and fourier transform that data. If you are interested in only lower frequency components modulated on that signal, then the PLL is an excellent filter, and you can digitize the PLL oscillator output with an oscilloscope. In either case, a calibrated oscilloscope is your best data capture tool (if you have access to one, of course).