You are receiving that error message because the light level being received on the SFP transceiver (-40.0 dBm) is below that transceiver's low optical power alarm threshold value of -28.8 dBm. Normally, when a transceiver says it is at -40 dBm on the receiving side, that usually means it is not getting any light at all on one strand. In your case, though, you only have one strand. So it may be getting light—just not in the wavelength range it is expected to receive light at.
If you have two GLC-BX40-U-I SFPs connecting to each other, that one possible source of the problem. The -U-I SFP at one end is transmitting at a wavelength that the -U-I SFP at the other end is not designed to receive at. You need a GLC-BX40-D-I at the other end for the connection to work.
The -U-I transmits around 1310nm (1260–1360nm) and receives around 1550nm (1540–1560nm), while the -D-I transmits around 1550nm (1540–1560nm) and receives around 1310nm (1260–1360nm). They are complementary.
Replace one of those -U-I SFPs with a -D-I SFP and your link should come up.
If you have a -U-I at one end and a -D-I at the other end and the SMF distance is less than 40 km, the cable strand may have a break in it somewhere along the path. You can check this with an Optical Time Domain Reflectometer (OTDR).
If the fiber span is longer than 40 km, you are probably losing too much optical power in the 1310nm transmission direction. Attenuation of the optical power signal at 1310nm is something like 0.4dB/km, while at 1550nm it is more like 0.25dB/km which is much less. You would need to upgrade to GLC-BX80-U-I and GLC-BX80-D-I SFPs to go the extra distance. These transceivers will reach out to 80 km.
Hope this helps.
-Larry
