EXFO FTB-8500 Series for FTB-500 - Manual (Page 37)

Physical Interfaces and LEDs
Ethernet and Fibre Channel Application 23
FTB-8525 and FTB-8535 Models

LEDs
LASER LED: The red LASER LED is on when the FTB-8500 Series and
FTB-8120NGE/8130NGE is emitting an optical laser signal.
LINK/ACT LED: The LINK/ACT LED is on when the link is up, off when
the link is down, and flashing when frames are transmitted and/or
received.