There are various definition for a "photonic radar" so it can be very confusing for most people
The first is Phased array radar where ferrite or bit phase shifters are replaced with Optic fiber, thus making use of "True time delay" principle. Called as Photonics true time delay. The advantage is that it removes bottleneck of operational bandwidth of active array radar. (Phase shifters do limit radar operational frequency due to phase mismatch in the phase shifter element)
This technique is so far limited by difficulties of implementation as hundreds of precisely matched fiber optic threads must be manufactured for each elements.
One example of a brute force approach:
The second definition of Photonic radar is the one that use photon entanglement principles ( or they also call it a quantum radar ), this one is still mostly in theoretical stage as far as i know
The third definition of photonic radar is a kind of radar where they used photonic method to generate RF signal. For current radar , transceivers can only generate digital signals at frequencies of 2 GHz or lower To get the higher frequencies needed for better resolution, those signals have to be upconverted by other electronics.When you do that you got a bit noise ( called internal noise of the system ) which will affect your detection range ..etc. For this kind of radar , they select two optical resonant frequencies (modes) in a mode-locked laser and fired it at a photodiode sensor. The electrical output from the sensor is an RF signal that can reach up to 40 GHz without upconversion. Because there is no up convention there is little noise ( about 10 dB less than current system if i recall correctly )