
more accurate than direct visual measurement, its suitability is limited by the
wavelength range of available spectrum analyzers, together with that of suitable
polarizers and sources. For this reason, the technique is usually restricted to
measurement at 1550 nm (see Fig. 8.21).
8.6.7 Extinction Ratio (ER)
Whereas beat length provides an invaluable means by which the PM perfor-
mance of different PM fibers may be compared, the influence of environment is so
significant that absolute performance may only be determined experimentally—
by deploying the fiber in a way that is representative of the intended application
and measuring the purity of the output ...