Fiber length tests approved
WILLIAM B. GARDNER
During 1996, the Telecommunications Industry Association (TIA) Subcommittee FO-6.6 balloted two fiber-optic test procedures (fotps) for measuring fiber length. The first method is fotp-60, "Measurement of Fiber or Cable Length Using an otdr," authored by Felix Kapron of Bellcore in the United States. The second method is fotp-133, "Length Measurement of an Optical Fiber or Cable by the Phase-Shift Method," authored by Andrew Hallam of York Technology in the United Kingdom.
A variety of commercial test equipment is available for implementing these methods. The optical time-domain reflectometer (otdr) used in the fotp-60 test method measures the transit time for a pulse propagating through the fiber under test. The phase-shift test set used in the fotp-133 test method measures the change in the phase of sinusoidal modulation as light propagates through the fiber. The otdr and the phase-shift test set are also the same instruments used for measuring optical loss and chromatic dispersion, respectively.
The fiber`s group-index characteristic (an inverse measurement of the velocity with which signals travel through the fiber) must be known, however, before either of the test methods can be used for measuring fiber length. The group-index value is typically a percentage point or two greater than the refractive index of silica, and like the latter, it varies with wavelength.
Moreover, an increase in the accuracy of fiber attenuation values requires a corresponding increase in the accuracy of fiber-length values. For instance, measuring a loss of 0.350 dB/km to an uncertainty of ۪.001 dB/km requires knowing the fiber`s length to ۪.07%, which in turn, requires knowing the group index value to ۪.001.
If the group index is not known, the test technician can use either of two measurement options, the first of which calls for the use of a mechanical (as opposed to an optical) method for measuring fiber length. A common mechanical approach involves counting the turns while winding the fiber onto a drum of known diameter. Appropriate calculations are then made to determine fiber length.
A TIA fiber-optic test procedure for this test method is in preparation; however, this method is frequently impractical to perform in the later stages of fiber and cable processing and installation.
The second option incorporates an optical length measurement method, such as fotp-60 or fotp-133, but entails a separate measurement of the group index. For example, fotp-133 includes an annex that describes the following group index test method:
Use a calibrated meter ruler and measure the length of a 2- or 3-meter-long fiber.
Then measure the phase shift that this fiber produces in a sinusoidally modulated optical signal.
Calculate the group-index value from the phase-shift measurement.
The apparatus required is identical to that used in fotp-133 for fiber-length measurement. Other valid methods for measuring group index are available, such as interferometry, but they have not yet been documented by the TIA.
Fortunately, the group-index measurement does not have to be performed on every fiber. Different fiber designs lead to different group indices, but fiber manufacturing processes are generally reproducible enough to ensure that all fibers of a given design have the same group-index value.
The fotp-60 and fotp-133 test procedures work equally well on cabled or uncabled fibers. When measuring cables, however, note that cable constructions generally result in the fiber length exceeding the sheath length. The cabling process, how ever, does not have a substantial effect on the group-index value. q