Page 335 - Fiber Bragg Gratings

312 Chapter 7 Chirped Fiber Bragg Gratings

pulse amplification [13], chirp compensation of gain-switched semiconduc-
tor lasers [14], sensing [15], higher-order fiber dispersion compensation
[16], ASE suppression [17], amplifier gain flattening [18], and band-
blocking and band-pass filters [19].

7.1 General characteristics of chirped
gratings

It has been recognized for a long time that gratings can be used for
correcting chromatic dispersion [20-22]. Winful [23] proposed the applica-
tion of a fiber grating filter for the correction of nonlinear chirp to compress
a pulse in transmission in a long grating. It was also suggested that since
these gratings display negative group-velocity dispersion, their applica-
tion in dispersion compensation could be tailored by chirping of the grating
[23]. These are known as dispersion compensating gratings (DCGs). Non-
linear transmission is also a subject of renewed interest in long gratings
[24-28]. Kuo et al. [29] measured the negative-group-velocity dispersion
of Bragg gratings in the visible wavelength region. While the dispersion
in these gratings in transmission is large, a limiting factor is the narrow
bandwidth over which they can be used to correct for linear dispersion
[30,31]. The drawback of such an arrangement is that the gratings have
to be used on the edge of the band and consequently introduce a loss
penalty, since some of the light is reflected, although it has been shown
that strong, long, highly reflective gratings can be used to compensate
for dispersion in communication links in transmission with negligible loss
[32], by proper design of the grating. For high-bit-rate systems, higher-
order dispersion effects become important, dissipating the advantage of
the grating used in transmission. The criteria used for the design of the
grating to compress pulses in a near ideal manner are a compromise
between the reduction of higher-order dispersion and pulse recompression.
Bandwidths are limited with this configuration by the strength of the
coupling constant and length of a realizable uniform period grating. Uni-
form period DCGs may find applications in closely spaced WDM systems.
If the coupling constant K^ is ramped linearly as a function of grating
length, the grating exhibits strong dispersion. Used in reflection, an 81-
mm-long grating had a coupling coefficient that was varied from zero to
1
12 cm" to achieve a dispersion of ~3.94 nsec/nm [6] but over a bandwidth
of only ~0.2 nm. These gratings typically have >99% in-band reflectivity.