7.6. FIBER BRAGG GRATINGS 299
Figure 7.13: Schematic illustration of dispersion compensation by two fiber-based transmission
filters: (a) chirped dual-mode coupler; (b) tapered dual-core fiber. (After Ref. [77];©c 1994
IEEE; reprinted with permission.)
transmission peak within its stop band [76]. The bandwidth of such transmission filters
is relatively small.
7.6.3 Chirped Mode Couplers
This subsection focuses on two fiber devices that can act as a transmission filter suitable
for dispersion compensation. A chirped mode coupler is an all-fiber device designed
using the concept of chirped distributed resonant coupling [77]. Figure 7.13 shows the
operation of two such devices schematically. The basic idea behind a chirped mode
coupler is quite simple [78]. Rather than coupling the forward and backward propagat-
ing waves of the same mode (as is done in a fiber grating), the chirped grating couples
the two spatial modes of a dual-mode fiber. Such a device is similar to the mode con-
verter discussed in Section 7.4 in the context of a DCF except that the grating period
is varied linearly over the fiber length. The signal is transferred from the fundamental
mode to a higher-order mode by the grating, but different frequency components travel
different lengths before being transferred because of the chirped nature of the grating
that couples the two modes. If the grating period increases along the coupler length, the
coupler can compensate for the fiber GVD. The signal remains propagating in the for-
ward direction, but ends up in a higher-order mode of the coupler. A uniform-grating
mode converter can be used to reconvert the signal back into the fundamental mode.
A variant of the same idea uses the coupling between the fundamental modes
of a dual-core fiber with dissimilar cores [79]. If the two cores are close enough,
evanescent-wave coupling between the modes leads to a transfer of energy from one
core to another, similar to the case of a directional coupler. When the spacing between
the cores is linearly tapered, such a transfer takes place at different points along the
fiber, depending on the frequency of the propagating signal. Thus, a dual-core fiber
with the linearly tapered core spacing can compensate for fiber GVD. Such a device
keeps the signal propagating in the forward direction, although it is physically trans-
ferred to the neighboring core. This scheme can also be implemented in the form of