You can just connect a normal fiber between a 10G SFP and a 40G QSFP and let the 40G end auto negotiate to 10G. Fanout cables are better for density though.
Does 40G QSFP transponder be able to even communicate with 10G SFP? Do they even have the same encoding? OOK? QPSK?
Seems to work mostly fine: https://community.fs.com/blog/40g-qsfp-to-10g-sfp-configuration-guide.html
Considerations for QSFP+ to SFP+ Split
Usually, this split configuration is supported on most switches provided by different vendors regardless of standalone and stacked ones. Certainly, you still need to check this function in the instruction of your switch or consult your vendor, as it’s the most basic condition of splitting QSFP+ to SFP+ configuration. Sometimes, most ports on a switch support split, but some don’t. In other cases, when a switch is deployed as a leaf switch, some ports do not support the split. Or when the port is already used as a stack port, this may also limit split configuration. Thus the conditions can be quite different based on your applications.
