Page 363 - Fiber Bragg Gratings

340 Chapter 7 Chirped Fiber Bragg Gratings

using a half-Gaussian refractive index modulation induced chirped grat-
ing [2], to the measurement of dispersion in a grating [5], to the first
report of DC in a transmission through a fiber [51,52]. In the last demon-
stration, 400-fsec pulses at a bit rate of 100 Gb/sec was transmitted
through 245 m of standard telecommunications fiber and were recom-
pressed with an 8-mm-long 12-nm bandwidth DCG to 450 fsec after dis-
persing to 30 psec, a compression ratio of—65. Since these demonstrations
many more measurements of DC have been reported using a variety of
gratings: notably, dispersion-tunable chirped gratings at 10- and 20-Gb/
sec transmission rates with a 5-cm-long grating and 80 km of fiber using
strain tuning [53], as well as 220 km at 10 Gb/sec and 100-mm-long
temperature-tuned chirped gratings [54]. A novel offset core fiber grating
has also been reported for strain tuned dispersion compensation of 270
km of standard fiber at 10 Gb/sec [55,56]. Fixed wavelength, 100-mm-
long, chirped gratings have also been used at 10 Gb/sec transmission with
up to 500 km of standard fiber [57,11]. Longer gratings, up to 400 mm
[58], have been used at 40 Gb/sec over 109 km of fiber, and in excess of
1-m-long gratings with a bandwidth of—10 nm at 10 Gb/sec over 100 km of
standard fiber in a WDM transmission system with up to 11 wavelengths
simultaneously [48]. Other WDM experiments at 10 Gb/sec have shown
DC at four wavelengths over 100 km using a single superstructure chirped
grating (see Chapters 3 and 6) [59].
The long transmission lengths at high bit rates are possible with
multiple chirped gratings, either lumped [60,61] or cascaded [12]. In the
latter scheme, 8 X 20 Gb/sec transmission over 315 km used four 1-meter-
long continuously chirped gratings at 3 X 80 km + 1 X 75 km hops, and
8 X 10 Gb/sec over 480 km was demonstrated using six 1-meter-long
continuously chirped, 6.5-nm bandwidth gratings [62,63] at 80-km hops.
The demonstrated results showed near ideal operation at 10 Gb/sec, de-
spite the 4 to 10 psec polarization mode dispersion of each grating, al-
though at 20 Gb/sec there was some polarization dependence. A pseudo
31
random bit sequence of 2 — 1 was used for the 10 Gb/sec bit stream (and
multiplexed for the 20 Gb/sec) for each wavelength spaced at nominally 0.8
nm.
The schemes used for lumped gratings are shown in Fig. 7.24. Either,
band-pass filters can be used with identical pairs of gratings as in Fig.
7.24a, or circulators with four or more ports may be used.
Cascading of chirped gratings reduces the available bandwidth and
hence system tolerance in multihop routes. This effect is due to filtering