Three new fiber-optic components are described: an in-line filter, an optical switch, and an in-line attenuator. The in-line filter inserts an optical filter into any fiber line containing a connector while imposing very little cost in additional power loss. The optical switch allows a single output fiber to transmit input from one of two different fibers and permits switching back and forth between the input fibers as needed. The in-line attenuator attenuates signals in fiber lines over a large range of about -30 dB with infinite variability. All three components are rugged, reliable, accurate, and inexpensive. They were developed for the prompt diagnostics used on underground nuclear tests and have successfully performed in this harsh, demanding environment.