disable interface TX laser (JUNOS)
Just in case if you want to simulate a fiber cut, you can enter a command from the FPC shell that cuts the power to the laser of an interface
start shell pfe network fpcX
test xfp <index> laser off
You can find the XFP index by doing "show xfp list" from the shell prompt. I haven't seen a method that works for SFP interfaces.
For sonet interfaces, this hidden configuration is also an option:
set interfaces so-x/y/z sonet-options insert-los