What Tier 1 proves
Tier 1 testing certifies that a link's insertion loss, length and polarity meet the design. It is contractual: the numbers go into the handover package and gate payment. The reference methods live in IEC 61280-4-1 (multimode, with encircled-flux launch conditions) and 61280-4-2 (single-mode), adopted by TIA-526.
Doing it right
Set the correct fiber type and wavelengths. Make a one-cord reference with inspected, cleaned, reference-grade cords; verify the reference stays within tolerance when you add the second cord. Test at both required wavelengths (850/1300 MM, 1310/1550 SM — add 1625 where the spec says), in both directions, and record the average. Wavelength-pair behavior is diagnostic in itself: loss much worse at the longer wavelength points at bending.
Judging results
Compare against the calculated budget per link, not a generic number. Investigate suspicious *goodness* too — a 4 km link reading 0.4 dB means a referencing error or wrong fiber under test. Negative loss always means the reference is wrong.
The paperwork that pays you
Per link: ID, length, per-wavelength/direction results, margin vs budget, instruments and calibration dates, reference method, operator, date. Exported instrument files (not retyped tables) are the professional standard and the only records a dispute will accept.