Page 104 - Fiber Bragg Gratings

3.1 Methods for fiber Bragg grating fabrication 83

tion modes in the counterpropagating direction. This is no longer true for
coupling to copropagating radiation modes, and care needs to be taken
to ensure that the periods of the gratings are not identical, by choosing
an appropriate fiber. Intermodal coupling has been demonstrated using
internally [62,69], as well as externally written gratings for different order
mode coupling [61], as well as similar order modes [64]. The functioning
of these devices is discussed in Chapter 6.

3.1.10 Single-shot writing of gratings

Single-shot writing of fiber gratings has been demonstrated using pulses
from an excimer laser [118-121]. Higher reflectivity gratings have also
demonstrated in boron-germanium codoped optical fiber [123]. Although
the quality of these gratings has not been comparable with those written
with other methods, the principle has led to a novel scheme of writing
grating in the fiber while it is being drawn from a preform.
The process of writing a grating in an optical fiber generally requires
the stripping of the protective polymer coating, which is opaque to short-
wavelength UV radiation. Stripped fiber is weakened owing to mechanical
processing (see Chapter 9) during grating inscription and should ideally
be recoated. However, grating inscription on a fiber-drawing tower enables
the fiber grating to be coated immediately after fabrication. Further, an
array of gratings may be fabricated sequentially in a length of fiber by
stepping the Bragg wavelength after each inscription. Figure 3.20 shows
a schematic of grating fabrication during fiber drawing that was originally
proposed by Askins et al. [122] and subsequently demonstrated [123].
Stepped-wavelength gratings were demonstrated using an interferometer
that was tuned to a different Bragg wavelength between pulses from the
excimer laser [122]. Although the gratings can be written during fiber
drawing, the quality and repeatability remains poor, owing to problems
of beam uniformity, mechanical alignment, and stability. For some applica-
tions in which a simple reflection is required, the quality of the grating
may not be a critical parameter.
Advances in polymer coating materials have resulted in perfluori-
nated polymers that are essentially transparent in the longer UV wave-
length regions of 266-350 nm [70,71]. Grating inscription at these
wavelengths has been demonstrated as well, indicating that high-quality
and high-reflectivity gratings may be possible without stripping the coat-
ing off the fiber. The fabrication of gratings in fiber with the primary