Link issues from an optical tap

I am trying to set up an optical tap feed in a FreeBSD server for traffic monitoring. The NIC is using the igb() driver but will not come up when I connect a strand to the RX port. The the interface comes up without any problems when I've tested it with a spare Cisco switch. I've tried forcing the interface up ( ifconfig igb0 up monitor) and I've tried all the available media and mediaopts settings too but it remains "status: no carrier". The light levels have been tested and are pretty good (-11dbm to -17dbm). The link on the far end is a Juniper 1000BaseSX port that has gigabit auto-negotiation enabled.

Any suggestions on how I might coax the interface to come up?
 
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