Page 434 - Fiber Bragg Gratings

9.1 Measurement of reflection and transmission spectra of Bragg gratings 411

Figure 9.1: Apparatus to measure the transmission and reflection spectrum
of Bragg gratings.

grating (see Fig. 9.2). For a uniform grating with a reflectivity R, we can
calculate the coupling coefficient, K acL, as

With this information and the measured bandwidth, 2AA, between the
first zeroes (FWFZ), the grating length L is uniquely defined as per Eq.
(4.6.14),

From the length, we may calculate the refractive index modulation vkn
as in Eqs. (4.6.3) and (4.3.6), with a fringe visibility v,

Eqs. (9.1.1) and (9.1.2) are plotted in Fig. 9.3 for three different values
of grating length, 1, 2, and 8 mm, as a function of the coupling constant
K ac. The data has been plotted for a Bragg wavelength of 1550 nm. The
wavelength shift <5A as the grating grows can be calculated from Eq. (4.6.4),