Drones have transformed the war in Ukraine. Commanders sit in bunkers scanning banks of screens, as surveillance drones hunt for targets on the ground. Once they are spotted, artillery or mortars might blast them. Or bomb-dropping or kamikaze drones zoom in for the kill. The problem is that jamming and accidents down well over half of the drones in the air. But a new addition to the arsenal of both sides is proving more effective. They are fibre-optic drones. With no radio signal to detect or jam they are proving hard to stop.