314 CHAPTER 7. DISPERSION MANAGEMENT
(a) (b)
Figure 7.17: (a) Reflection spectrum and (b) total GVD as a function of voltage for a fiber
grating with temperature gradient. Inset showsτg(λ)at several voltages. (After Ref. [164];
©c2000 IEEE; reprinted with permission.)
Distributed heating of the Bragg grating requires a thin-film heater deposited on the
outer surface of the fiber with an intracore grating [164]. The film thickness changes
along the grating length and creates a temperature gradient through nonuniform heating
when a voltage is applied across the film. A segmented thin-film heater can also be used
for this purpose [167]. Figure 7.17 shows the reflection spectra of a 8-cm-long grating
at three voltage levels together with the total dispersionDgLgas a function of voltage.
The inset showsτg(λ)for several values of the applied voltage. The grating is initially
unchirped and has a narrow stop band that shifts and broadens as the grating is chirped
through nonuniform heating. Physically, the Bragg wavelengthλBchanges along the
grating because the optical period ̄n(z)Λbecomeszdependent when a temperature
gradient is established along the grating. The total dispersionDgLgcan be changed in
the range−500 to−2200 ps/nm by this approach. Such gratings can be used to provide
tunable dispersion for 10-Gb/s systems.
When the bit rate becomes 40 Gb/s or more, it is necessary to chirp the grating
so that the stop band is wide enough for passing the signal spectrum. The use of a
nonlinear chirp then provides an additional control over the device [160]. Such chirped
gratings have been made and used to provide tunable dispersion compensation at bit
rates as high as 160 Gb/s. Figure 7.18 shows the measured receiver sensitivities in the
160-Gb/s experiment as a function of the residual (preset) dispersion with and without
the chirped grating with tunable dispersion [165]. In the absence of the grating, the
minimum sensitivity occurs around 91 ps/nm because the DCF used provided this value
of constant dispersion. A power penalty of 4 dB occurred when the residual GVD
changed by as little as 8 ps/nm. It was reduced to below 0.5 dB with tunable dispersion
compensation. The eye diagrams for a residual dispersion of 110 ps/nm show that the
system becomes inoperable without the grating but the eye remains wide open when
tunable dispersion compensation is employed. This experiment used 2-ps optical pulses
as the bit slot is only 6.25 ps wide at the 160-Gb/s bit rate. The effects of third-order
dispersion becomes important for such short pulses. We turn to this issue next.