Is the Cat4507 software at 12.2(20)EW or higher?
Can you verify the Mode Conditioning Patchcords (MCP) are for 62.5 micron fiber and that the singlemode fiber cord is attached to the transmit port on each end?
Verify that the transmit fiber on one end gets connected to the receive end at the other end.
Are the fiber connectors and ports clean?
When you said the fiber was tested, what was tested... any results for loss (dB) on each fiber?
What are the transceiver LEDs showing?
What does a Show Interface command show for each transceiver on each end?
Do you have an Optical Power Meter?
Thanx!