Page 449 - Fiber Bragg Gratings

426 Chapter 9 Measurement and Characterization of Gratings

and the Bragg wavelengths are therefore easily separated in a spectrum
analyzer. By rotating the polarization of the UV writing beam so that it
is along the fiber axis, the UV-induced birefringence can be reduced from
~5% of the UV-induced refractive index change to around 0.5% [24]. This
area has received more attention recently, with the application of setup
similar to that shown in Fig. 9.14 to measure the group delay difference
of two polarization states. Also applied is the Jones matrix approach to
map out all the polarization states in the fiber. The comparison between
the two methods is good. Measured polarization-induced delay is reported
to be —28 and 7 psec for two chirped gratings written in different fiber
[25].
Clearly this is an area that will receive attention as the deployment
of fiber gratings for dispersion compensation becomes widespread.

9.3.1 Measurement of the grating profile
The use of simulation is an excellent method for characterizing the mea-
sured spectrum of a grating. By knowledge of the physical length, the
reflectivity and the shape of a grating, it is possible to identify a couple
of parameters to choose in order to allow the simulation. This has been
demonstrated for the spectra shown in Figs. 9.13 and 9.15. More complex
structure can be analyzed theoretically, as has been amply demonstrated
by Ouellette et al. [27] for stitching errors produced by phase masks. For
example, random errors in the stitching cause the bandwidth of a uniform-
period grating to broaden and acquire noise. The errors can be calculated
from the simulation. Alternative techniques exist for assessing the refrac-
tive index profile of a grating. These include optical low-coherence reflec-
tometry (OLCR), originally used to detect small flaws in optical
waveguides [28,29], applied to fiber gratings by Lamblet et al. [30], and
the method of side scatter from a grating, demonstrated by Krug et al.
[31]. An interesting and very simple technique is the application of "heat
scan" to probe the chirp in a grating [32,33]. The grating is probed with
a fine hot wire while the reflection characteristics are being monitored.
A detuning of the Bragg wavelength as a function of probe position mea-
sures the chirp. These are in addition to other methods already mentioned
as being appropriate for assessing phase masks [9,14]. It is important to
know the sources of "noise" on a grating, or correct for a flaw [34-36],
since the out-of-band spectra may deteriorate. It should be mentioned
that optical time-domain reflectometry (OTDR, or back-scatter) has also