Figure 2-6.

Figure 2-7.

Coupling loss as a function of beam splitter reflectivity.....
Coupling loss for a glass plate beam splitter

4

6

7

Figure 4-20. Group index measurements for five commercial

fibers. Measurements on three fibers repeated

to within 0.1 percent. From Franzen and Day [48]....
Example of a scatter-like fault signature.
Gaussian probe pulse.....

Figure 5-1.

Figure 5-2.

Example of an absorption-like fault signature.
Gaussian probe pulse....

Figure 5-3.

Figure 5-4.

Example of a composite fault signature consisting
of both scattering and absorption loss. Gaussian
probe pulse...

Backscatter signature for a graded-index fiber with

Figure 5-5.

Figure 5-6.

Figure 5-7.

a fusion splice at the midpoint. The three other
irregularities are unidentified. Fiber J......
Backscatter signatures resulting from a microbending
experiment (see text). Lower curve exhibits effect
of pressure-induced microbending. Upper curve
represents the response in the relaxed state. Fiber K...
Backscatter signature for a fiber wrapped on drums
of differing diameter. The input half of the fiber
is on a drum of diameter 30 cm, the output half of
the fiber is on a drum of 10 cm diameter. Fiber H.....
Backscatter signature under conditions similar to
figure 5-6, except the final 7 percent of the fiber
is on the smaller drum. Fiber H.....

35

37

37

38

Figure 5-8.

Computer simulation of radiation loss with an excess F value of 0.0006, the other parameters being similar to fiber H. Excess radiation loss begins at time 345.... Fault signature for a bubble in a graded-index fiber. This is an expanded scale of figure 2-4. Fiber H.... Figure 5-10. Signature for a commercial coupler. The scatter at 1.2 us is off scale. Fiber E.

Figure 5-9.

38

40

40

BACK SCATTER MEASUREMENTS ON OPTICAL FIBERS

B. L. Danielson

An optical time domain reflectometer (OTDR) and its components are described in detail. The system performance for this device is examined.

Experimental

methods are described for the measurement of several parameters of interest in
the characterization of optical fibers using the OTDR. These parameters include
scattering loss and capture fractions for unperturbed fibers. Experimental cap-
ture-fraction values are reported for several step and graded-index fibers and
these results are compared with theoretical predictions. Rayleigh backscatter
signatures are also presented for several fibers from different manufacturers.
Fault signatures are shown for some intrinsic and extrinsic fiber perturbations.

KEY WORDS: Backscattering; capture fractions; fiber scattering: optical time
domain reflectometry; Rayleigh scattering.

1. INTRODUCTION

The optical time domain reflectometer (OTDR) is an instrument that was developed in 1976 [1,2] which has proved to be very useful in testing optical fibers. It is similar in operation to the time domain reflectometer that has been used for many years to examine and locate irregularities and mismatches in cables. In the optical case, the inherent Rayleigh backscattering which occurs in the fiber material also provides a visualization of the attenuation and scattering properties of the waveguide as a function of length. Not only can the attenuation of the fiber be estimated, but anomalies may be located and characterized. An excellent review article by Rourke [3] contains much useful information on the OTDR system and its application to a number of optical fiber measurement problems.

If a rectangular optical pulse of width W and peak power is injected into a fiber, the time-dependent response of the backscatter power at the input end of the fiber (t) can be shown to be [4]

n

where Vq is the group velocity, as the Rayleigh scattering attenuation coefficient, and F is the Rayleigh capture fraction, that is, the fraction of the scattered radiation which is trapped in the fiber and returned in the backward direction. The quantities refer and F an to the corresponding variables for non-Rayleigh type scattering processes. The total attenuation coefficient, ar is the sum of the scattering coefficients, as and and the absorption coefficient representing the conversion of pulse energy into heat, a. Equation (1-1)

an,