for doppler or FM you need an RF source to modulate the beam, 100Mhz for instance. a PLL connected to the receive side gives you an error signal output on the VCO control voltage, which can be slowly varying DC for a doppler speed radar, or audio for a laser mic. it's very easy to get a reference signal, because the RF for the transmitter is generated locally. this is a bit more complicated than just tapping the audio off of the photodetector (AM demodulation of the light beam). in the AM detection method, you take advantage of the movement of the target changing the deflection of the beam, and therefore the amount of light returning from the target. narrow optical filters between the target and the photodetector will reduce the amount of noise from unwanted light sources (sunlight, fluorescent lights, etc...)