When working with fiber optics, you need some special tools to check fiber connections. Continuity checking makes certain the fibers are not broken and to trace a path of a fiber from one end to another through many connections. You can use a visible light “fiber optic tracer” or “pocket visual fault locator“. It looks like a flashlight or a pen-like instrument with a light bulb or LED source (sometimes laser) that mates to a fiber optic connector.
Attach the light source a fiber optic cable to test to the visual tracer and look at the other end to see the light transmitted through the core of the fiber. If there is no light at the end, go back to intermediate connections to find the bad section of the cable. Warning: Avoid looking at other fibers that could have signal in them, because active communications fibers can have so much invisible laser power in them that it can damage your eyes!
EDN design idea LED flasher checks fiber-optic strands allows you to verify fiber-optic strands. It uses two high-intensity flashing LEDs (red and yellow) that you can see at the far end of multi-mode fiber link (up to 1 km). Basically the circuit is LED flasher with some extra features (like auto power-off after 50 minutes). The fiber transmitter module is constructed simply: the T1-3/4 LEDs fit nicely into ST barrel connectors with some glue to hold them in place. If you need some other fiber connector type, you can use a patch cable to match the fiber connectors in your network.
Nice circuit idea but the circuit looks a little complicated for a LED flasher (three ICs). If you want to make something simpler, I think that you could also try the same idea with just one simple 555 timer based or other simple LED flasher circuit.
It is even possible to use a flash light for this application with suitable adapter or you can make your own light source if the commercial devices look too expensive. I own an a simple plastic adapter that connects a Mini Maglite to many common types of fiber optic connectors. It is an Universal adaptor for connectors with 2.5mm ferrules (works with FC/SC/ST connectors). It was given out for free at some fiber optics fair years ago and made from cheap plastic. It is not ideal, but it works and does the job many times.
There are also higher power versions of such tracers. A higher power version of the tracer uses red laser light that is powerful enough (typically few mW) to show breaks in fibers or high loss connectors. You can actually see the loss of the bright red light even through many yellow or orange simplex cable jackets. The red laser light can also be seen from the other end of the fiber cable that can be many kilometers long (ranges typically of up to 5km). This kind of gadget can be also used to optimize mechanical splices or pre-polished-splice type fiber optic connectors. Laser pointers are really cheap nowadays, but it is a bit hard to focus the laser beam from them well to the thin fiber.
Invisible infrared radiation normally carried in optical fiber can be made visible with a video camera. Many video cameras will detect IR radiation quite well, both black&white and color cameras (usually as RED but can be other color as well). The camera on your cell-phone can also usually work as a really cheap “laser” detector that nearly every tech carries. For example N73 cellular phone camera showed 1300 nm light as weak blue color on the fiber that has the signal in it. Because I can see IR remote control signals (850 – 950nm) on cellular phone camera, should 850 nm IR signals from fiber show as well.
The primary tools for serious fiber optics testing are a fiberscope for visual connector inspection and a power meter with laser source, a set of laser goggles (if you plan to work on single-mode or long-haul multimode fiber), an assortment of fiber patch cables, and a mini-maglite with fiber adapter (for simple continuity tests). There are many tutorials on the web showing how to use these tools, most from equipment vendors themselves, and some are even high quality video presentations.
More fiber optic testing ideas and information can be found at Fiber Optic Testing FAQ, LANshack.com Fiber Optic Testing, FOA Reference Guide To Fiber Optics Fiber Optic Testing, Measuring Power in Fiber Optics and Can You Build a Fiber Test Kit On a Budget?
Here is picture of most common fiber optic connector types (from Asara web site): LC, MU, MT-RJ, SC, ST and FC