When your SFP module keeps rebooting: the problem is often the power supply

You plug your SFP/ONU module into your USB-to-TTL adapter, open your serial terminal, and suddenly you see this repeating over and over again: U-Boot 2011.12-lantiq-gpon-1.2.24 (Nov 03 2014 - 22:46:28), Build: falcon_sfp_linux Board: SFP DRAM: 64 MiB Now running in RAM -