As people have replied more than once, beyond 8km, RADAR is used for measuring distance, even if you are in OLS - only mode. They are integrated sensors. It does so in short pulses every now and then, as to try and avoid detection. IRL, you could avoid this by turning the radar of comletly, or by flying in a MiG-29UB, that doesn't have the radar at all ;)
By contrast, the MiG-23's OLS doesn't have a laser ranger at all, and it uses radar pulses for ranging all the way.