good (normal operation, green) 5V > U > 240 mV

nality not guaranteed, red) U < 120 mV

This is a measure about the light signal strength. You cannot "improve" the signal.
You should look at the quality of the fiber optic (correct fiber type, length inside allowed ranges,coorect mounting of the fiber (bending radius) and correct type of connectores and mounting ofthe connectors, etc...).You could measure the fibers with appropriate tools to find out if there is an unacceptable loss at some place.