Page 121 - Fiber Bragg Gratings

100 Chapters Fabrication of Bragg Gratings

such that its bandwidth is —50% greater than the chirp AA^ of the grating,
irrespective of length of the grating. Making this choice results in a devia-
tion of the characteristics of the grating that differ <1% from those of a
continuously chirped grating. There must be an integer number of periods
in each subsection to ensure that phase mismatch does not occur. A result
of this requirement is that the lengths of the subsections are only approxi-
mately equal.
The step-chirped grating is ideally implemented in the phase mask
using e-beam lithography [23]. Splitting a grating into small fields is
exactly how the e-beam process works [20], so that it is naturally suited to
the fabrication of step-chirped phase masks. Wide and narrow bandwidth
(1-50 nm) phase masks have been fabricated with linear and quadratic
step chirps [23] and used for pulse recompression of femtosecond pulses
transmitted over optical fiber [106]. Step-chirped phase masks 100 mm
long, useful for dispersion compensation in telecommunications transmis-
sion links [107], have also been demonstrated [108,74].
There are other techniques that mimic the step-chirped gratings, for
example, with a limited chirp capability, using stretch and write [103]
discussed in Section 3.1.14. Riant and Sansonetti have also shown that
by the use of a focusing lens and a phase mask, small sections with
different wavelengths can be built up to create a step-chirped grating
[109]. Focusing or defocusing a beam at a phase mask changes the wave-
length of the inscribed period in the fiber immediately behind the phase
mask. Thus, stepping the spot along the mask while adjusting the focus
allows the inscribed wavelength to be altered at each step. Using this
method, 50-mm long gratings have been demonstrated with chirp values
of ~lnm[109].
There is great advantage in the use of the step-chirped phase-mask,
since it allows not only the definition of the grating wavelength, but also
any value of chirp rate. The minimum chirp bandwidth possible is, of
course, limited by the natural bandwidth of a grating of length L g.
An extension of the step-chirped principle is the concept of super-
step-chirped gratings [75]. This technique allows even longer gratings to
be assembled using a set of short step-chirped phase masks. This over-
comes a major limitation of e-beam fabrication: the writing of masks
longer than 100 mm is difficult and expensive. An alternative is to fabri-
cate several gratings on a single-phase mask plate, each with a fixed
chirp. Each phase mask is designed to begin at a wavelength SA longer
than the wavelength at which the last phase mask finished. The gratings
on the phase mask are aligned one above the other with exactly the