By matching the phases, the light from the coupler is input to the fiber traveling in a circular direction, i.e., the path of {HEAD ~ the light} down {TAIL ~ the fiber} forms a corkscrew 
By matching the phases , the light from the coupler is input to the fiber traveling in a circular direction , i.e. , the path of { HEAD ~ the light } down { TAIL ~ the fiber } forms a corkscrew
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH