Is optical fibers used for distributed systems?

Fiber has been used for over twenty years to connect storage systems to compute resources. Recently, Ethernet over fiber is becoming more common.

As @talidayo points out, the fiber connections are unlikely to be saturated. What hurts distributed systems is the overhead of message sending, the packing and unpacking of transmission data structures. As a rudimentary rule, individual nodes / services should spend only a small fraction of the resources and time for marshaling data and the rest on useful computation.

In my experience latency is more of a problem than throughput, but then again, totally depends on the kind of application you're building.

Network switching can become a bottleneck of it's own, especially once you reach fiber-scale. It's generally better to avoid fiber if you can these days, especially since there are fleetingly few applications that will saturate a modern fiber optic connection.