Page 245 - Fiber Bragg Gratings

222 Chapters Apodization of Fiber Gratings

at the same time, a smaller subgrating must be written. The maximum
chirp that can be produced from a subgrating length of SL g is

A grating with a bandwidth of 10 nm near the Bragg wavelength of
~1.55 /ctm requires a subgrating length of ~300 jam. The second point to
remember is that the refractive index modulation remains almost un-
changed with the chirp induced using the MPF scheme, which can only
be used with a pulsed laser. A reduction in the refractive index modulation
occurs when a chirp is induced with the MPM method. It can be compen-
sated for to some extent by adjusting the irradiation intensity or by chang-
ing the movement velocities of the fiber or phase mask and UV beam,
but it requires a CW writing beam. Both methods allow the inscription
of long gratings but do require a movement stage with a translation
capability at least as long as the grating to be written.
The complication of saturation effects [27,28] has not been addressed
in the case of strong gratings written using either of these methods,
although certain fibers show a linear response to the time of exposure to
UV radiation at a longer wavelength of 334 nm and a much increased
writing time [29]. Undoubtedly these will play an important role as the
requirements for the types of gratings become more demanding. The effect
of linearity of the photosensitive response of the fiber to, for example, the
change in the local intensity is as yet unknown. The further, more serious
issue of the out-diffusion of hydrogen/deuterium from long gratings has
also not been discussed. Out-diffusion causes a reduction in the refractive
index of both the UV-exposed and the unexposed regions [30,31]. A differ-
ential change in the refractive index between the two regions will lead
to a degradation in the transfer characteristics of long gratings, since the
phase change accumulates over its entire length.
Both the MPF and MPM methods are flexible and capable of apodizing
gratings with arbitrary refractive index modulation profiles and are capa-
ble of the production of identical grating characteristics.
The issues related to the other schemes, such as the apodized phase
mask, although convenient, are limited flexibility allowing only the repli-
cation of the type of apodization programmed in the phase mask. There
is a restriction on the maximum size of the phase mask as well as on the
reproducibility of the apodized phase mask. Although, as with the step-
chirped phase mask [6], it is possible to combine apodization and chirp