Page 446 - Fiber Bragg Gratings

9.3 Phase and temporal response of Bragg gratings 423

data has been measured with 1 pm resolution, and the GDR is adequately
resolved. We can measure the 3-dB optical (6-dBe) reflectivity bandwidth
by comparing the transmitted signal level with R p, remembering that the
measurements are in dBe.
This grating is designed to compensate the dispersion for ~80 km of
standard optical fiber, as has been presented in Chapter 7. With unapod-
ized gratings the low resolution masks the detail and a comparison is
shown in Fig. 9.17. Here two measurements of the same grating made
with 10 pm and 1 pm are compared. The GDR is apparent with the higher
resolution. With longer gratings this factor becomes even more critical.
As we have seen in Chapter 7, for 1-meter-long unapodized gratings,
the GDR frequency has a period of —1.5 pm. To resolve the GDR, other
means have to be adopted, such as the use of a multisection DFB laser
[17], which can be electrically tuned. This is a very time-consuming task,
since other factors such as the ambient temperature need to be controlled
very accurately. The drift of the wavelength with temperature of a chirped
grating is an issue. The grating should be temperature controlled in order
to get an accurate measurement. With a change in the local Bragg wave-
length of several pm/°C, a stable environment is essential for the measure-
ment.

Figure 9.17: Resolution dependence of the group delay of an unapodized
chirped grating (after Ref. [17]).