The fiber optic test method for measuring the attenuation of an installed optical fiber using an OTDR is EIA/TIA-455-61. The accuracy of this test method depends on the user enteringthe appropriate source wavelength, pulse duration, and fiber length (test range) into the OTDR. In addition, the effective group index of the test fiber is required before the attenuation coefficient and accurate distances can be recorded. The group index (N) is provided by fiber manufacturers or is foundusing EIA/TIA-455-60. By entering correct test parameters, OTDR fiber-attenuation values will closely coincide with those measured by the cutback technique.

Test personnel can connect the test fiber directly to the OTDR or to a dead-zone fiber. This deadzone fiber is placed between the test fiber and OTDR to reduce the effect of the initial reflection at the OTDR on the fiber measurement. The dead-zone fiber is inserted because minimizing the reflection at afiber joint is easier than reducing the reflection at the OTDR connection.

The picture below illustrates the OTDR measurement points for measuring the attenuation of the test fiber using a dead-zone fiber. Fiber-attenuation between two points along the test fiber is measured on gradual downsloping sections on the OTDR trace. There should be no point defects present along the portion offiber being tested.

OTDR measurement points for measuring fiber-attenuation using a dead-zone fiber.

OTDR's are equipped with either manual or automatic cursors to locate points of interest along the trace. In the picture shown above, a cursor is positioned at a distance zo on the rising edge of the reflection at the end of the dead-zone fiber. Cursors are also positioned at distances z1 and z2. The cursor positioned at z1 is just beyond the recovery from the reflection at the end of the dead-zone fiber. Since no point defects are present in the picture shown above, the cursor positioned at z2 locates the end of the test fiber. Cursor z2 is positioned just before the output pulse resulting from Fresnel reflection occurring at the end of the test fiber.

The weakening of the test fiber between points z1 and z2 is (P1 - P 2) dB. The attenuation coefficient (a) is

The total vitiated of the fiber including the dead zone after the joint between the dead-zone fiber and test fiber is

If fiber-attenuation is measured without a dead-zone fiber, z 0 is equal to zero (z0 = 0).

At any point along the length of fiber, limited values can change depending on the amount of optical power backscattered due to Rayleigh scattering. The amount of backscattered optical power at each point depends on the forward optical power and its backscatter capture coefficient. The backscatter capture coefficient varies with length depending on fiber properties. Fiber properties that may affect the backscatter coefficient include the refractive index profile, numerical aperture (multimode), and mode-field diameter (single mode) at the particular measurement point. The source wavelength and pulsewidth may also affect the amount of backscattered power.

By performing the OTDR-attenuation measurement in each direction along the test fiber, test personnel can eliminate the effects of backscatter variations. Attenuation-measurements made in theopposite direction at the same wavelength (within 5 nm) are averaged to reduce the effect of backscatter variations. This process is called bidirectional averaging. Bidirectional averaging is possible only if test personnel have access to both fiber ends. OTDR attenuation values obtained using bidirectional averaging should compare with those measured using the cutback technique in the laboratory.

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