A "modem" is used for modulating and demodulating a broadband signal to/from a cable (DOCSIS) or telephone wire (xDSL). On fiber, there's no modulation.
What you're referring to is probably the router that's providing your network with Internet access. Fiber-capable routers ususally have modular fiber ports (SFP) where various transceivers can be plugged in.
If your router (likely) doesn't have an SFP port for the LAN side, you can either
- replace the router with a model featuring an SFP port where you can plug the fiber transceiver of your choice (assuming 1000BASE-SX)
- use a media converter from e.g. 1000BASE-T to 1000BASE-SX
1000BASE-SX is gigabit Ethernet for short reach, there's a number of different PHYs for other speeds and ranges. Also, make sure you know what kind of connectors you need (ST, SC, or LC).