Every strand in a fiber network is proven with light before it carries traffic. Two instruments do the proving: the OTDR, which maps the entire strand and every event on it, and the light source and power meter pair, which measures exactly how much light survives the trip. Understanding what each one says — and what it can't say — is the difference between documentation and decoration.
The OTDR: radar for glass
An optical time-domain reflectometer fires laser pulses down the strand and times the faint reflections that return. The result is a trace — distance on one axis, signal on the other — where every splice, connector, bend, and break appears as an event at a measurable footage. That's how a technician finds a backhoe cut within a few feet from miles away, and how a splicer proves each splice lost 0.05 dB and not 0.5.
One honest limitation: an OTDR measures reflections, not true end-to-end loss, and a single-direction trace can misstate a splice where fiber types change. That's why specifications call for bidirectional OTDR testing — shoot both ways, average the events.
Insertion loss and the loss budget
The light source and power meter (an OLTS) answers the only question the network electronics care about: how many dB disappeared between here and there? Designers compute a loss budget — fiber attenuation per kilometer plus an allowance per splice and connector — and the measured link either fits it or doesn't. A link can pass OTDR review and still fail its budget; acceptance testing uses both instruments for that reason.
What acceptance documentation should contain
Bidirectional OTDR traces per strand, insertion-loss readings against the stated budget, splice-loss tables, and as-built strand assignments — delivered in the tester's native files, not just PDFs. That package is what lets the next crew troubleshoot in minutes instead of days; it's the standard our splicing crews deliver, and what any network owner should demand. The full closeout picture is in our as-builts guide.