chapter 1 - Bentham Science

Fiber Bragg Gratings: Advances in Fabrication Process and Tools Fiber Bragg Grating Sensors 11
There are three basic types of grating associated with UV inscription and these are known as Type I, Type II and
Regenerated. Type I gratings are associated with refractive index changes that occur below the damage threshold of
glass whereas Type II gratings are associated with changes in refractive index above the damage threshold of glass.
Each type is summarized below. There are several Type I sub-classes that are identified according to the writing
conditions and properties of the gratings.
(Fig. 2) shows three different types of gratings written with the same period phase mask. The three gratings reflect
light at different wavelengths. This difference in the reflected wavelength occurs because the refractive index
change produced by each process is associated with a different mechanism and has a different value.
Figure 2: The spectra of Type I (blue), Type IHp (red), and Type In (green) gratings written in B/Ge fiber [14].
Type I
Type I – These are currently the most common gratings used and often referred to as standard gratings. These
gratings can be fabricated in most types of germanosilicate fiber. In the reflected spectrum there is negligible light
lost to the cladding modes or by absorption. The refractive index change associated with this process is usually
(although not in the case of Type In) positive i.e. the average refractive index in the exposed area is higher than the
unexposed core.
The underlying mechanism, at least for small refractive index changes, is a one-photon absorption effect [5]. The
process works by exciting defect centers where the energy levels correspond to oxygen deficiency centre (ODC)
absorption bands at wavelengths around 244 nm and 320 nm. These same bands can also be excited by multi-photon
processes using femtosecond lasers, this will be discussed later in the chapter. For large refractive index changes it is
likely that some type of densification change is also involved [15].
Erdogan showed that Type I Bragg gratings exhibited a temperature dependent decay [16]. It was shown that the
decay in the reflectivity could be characterized using a power law. Immediately after the inscription the decay rate
was at its maximum and over time this decay rate decreased. The starting rate of decay was dependent on the
surrounding temperature. This decay is due to the thermal depopulation of the trapped excited states created during
the UV irradiation. Elevating the temperature gives the carriers in the shallowest traps enough energy to escape and
return to the ground state. The carriers remaining are associated with more stable traps, however if the temperature
is steadily increased progressively more of these will decay resulting in a further decrease in the grating strength.
Since it is the most unstable carriers that decay at low temperature Bragg gratings can be annealed at an elevated
temperature prior to use and this removes the least stable part of the induced refractive index change. Post-anneal the
grating exhibits a much greater level of stability over the longer term. This annealing process can also be used to
stabilize other Type I grating sub-types although for each type the related decay properties must be individually
characterized.