DFM will not be able to distinguish between a fiber cut, and a switch that is powered off. Both will cause the switch to appear unreachable to DFM, and both will cause DFM to generate an OperationallyDown event (which can be turned into an email, syslog, or trap notification).
Assuming there is a syslog message generated from an upstream device which can notify you of a fiber cut, you could use RME's syslog analyzer feature to notify you when that occurs. However, there is no way to know a device was just powered off (versus some other event which made it unreachable).