I always thought a SFP was a physical link layer only device?
If so you need to ask the PHY or MAC rather then the SFP for link status, the SFP will only tell you that it sees modulated light, not that the light encodes ethernet frames.
The SFP modules I have played with output a PECL pair that is pretty much an amplified and limited version of the signal from the photodiode, you want the PHY or MAC levels to give you link status, the SFP does not decode things that far.
Regards, Dan.