Page 445 - Fiber Bragg Gratings

422 Chapter 9 Measurement and Characterization of Gratings

Figure 9.15: The spectra of a 30-mm-long grating measured using the vector
voltmeter.

dB (optical). The grating reflectivity can be measured in transmission, to
normalize the peak of the reflectivity spectra. The results for an identical
but apodized grating are equally good.
We now move on to the spectrum of a longer grating, of the type
used in dispersion compensation. There are a few points that need to be
remembered. In Chapter 7 the delay response of long gratings is consid-
ered in detail, and it is found that long gratings need to be characterized
with high wavelength resolution if the GDR (group delay ripple) spectrum
is to be resolved. In making measurements, a wavelength resolution of 1
pm is generally sufficient. The measured reflectivity and group delay of
a raised cosine apodized, 100-mm-long grating is shown in Fig. 9.16. The

Figure 9.16: The reflectivity and delay of a 100-mm-long chirped grating
with a chirped bandwidth of 0.75 nm [19], measured with a resolution of 1 pm.