Page 447 - Fiber Bragg Gratings

424 Chapter 9 Measurement and Characterization of Gratings

Finally, we consider the drift of the Bragg wavelength with the out-
diffusion of hydrogen for a fiber after the grating has been written. With
a typical period of measurement of tens of minutes to an hour, the out-
diffusion of hydrogen (or deuterium) must be taken into consideration,
as has been shown in Chapter 2 [20,21], In Fig. 9.18 we see the drift in
the Bragg wavelength of a chirped grating monitored over a period of 45
days from the inscription of the grating, immediately after removal from
the cold storage. The fiber is deuterium soaked at 200 bar at the start.
By the end of 45 days, the total drift in the wavelength is approximately
— 1.65 nm, and it continues to shift very slowly.
With technological improvements, it will be necessary to measure
even longer gratings, perhaps longer than 10 meters. The measurement
of one of these gratings (reflection spectrum of a 2-meter-long WDM chan-
nelized grating is shown in Fig. 9.19) can take several hours at picometer
resolution. Here it becomes important that the grating be collectively
maintained at the same temperature for the duration of the measurement.
A fast technique has been reported by Ouellette et al. [221, which relies
on the intrinsic birefringence B of the fiber. By alternately measuring the
orthogonal polarization reflected, S2, and launched, SI, signals from the
spectrum analyzer, the group delay is shown to be

This method requires the calibration of the fiber birefringence. This
technique may prove to be valuable, since it is simple, although there is
no information available on the resolution.

Figure 9.18: The drift in the Bragg wavelength of a chirped grating with
time due to deuterium out-diffusion [17].