8.2. WDM COMPONENTS 349
Several kinds of add–drop filters have been developed since the advent of WDM
technology [84]–[94]. The simplest scheme uses a series of interconnected directional
couplers, forming a MZ chain similar to that of a MZ filter discussed earlier. However,
in contrast with the MZ filter of Section 8.2.1, the relative delayτmin Eq. (8.2.3) is
made the same for each MZ interferometer. Such a device is sometimes referred to as
aresonant couplerbecause it resonantly couples out a specific wavelength channel to
one output port while the remainder of the channels appear at the other output port. Its
performance can be optimized by controlling the coupling ratios of various directional
couplers [86]. Although resonant couplers can be implemented in an all-fiber con-
figuration using fiber couplers, the silica-on-silicon waveguide technology provides a
compact alternative for designing such add–drop filters [87].
The wavelength selectivity of Bragg gratings can also be used to make add–drop
filters. In one approach, referred to as thegrating-assisteddirectional coupler, a Bragg
grating is fabricated in the middle of a directional coupler [93]. Such devices can
be made in a compact form using InGaAsP/InP or silica waveguides. However, an all-
fiber device is often preferred for avoiding coupling losses. In a common approach, two
identical Bragg gratings are formed on the two arms of a MZ interferometer made using
two 3-dB fiber couplers. The operation of such an add–drop filter can be understood
from Fig. 8.12(b). Assume that the WDM signal is incident on port 1 of the filter. The
channel, whose wavelengthλgfalls within the stop band of the two identical Bragg
gratings, is totally reflected and appears at port 2. The remaining channels are not
affected by the gratings and appear at port 4. The same device can add a channel at
the wavelengthλgif the signal at that wavelength is injected from port 3. If the add
and drop operations are performed simultaneously, it is important to make the gratings
highly reflecting (close to 100%) to minimize the crosstalk. As early as 1995, such
an all-fiber, add–drop filter exhibited the drop-off efficiency of more than 99%, while
keeping the crosstalk level below 1% [88]. The crosstalk can be reduced below−50 dB
by cascading several such devices [89].
Several other schemes use gratings to make add–drop filters. In one scheme, a
waveguide with a built-in, phase-shifted grating is used to add or drop one channel from
a WDM signal propagating in a neighboring waveguide [84]. In another, two identical
AWGs are connected in series such that an optical amplifier connects each output port
of one with the corresponding input port of the another [85]. The gain of amplifiers
is adjusted such that only the channel to be dropped experiences amplification when
passing through the device. Such a device is close to the generic add–drop multiplexer
shown in Fig. 8.12(a) with the only difference that optical switches are replaced with
optical amplifiers.
In another category of add–drop filters, optical circulators are used in combination
with a fiber grating [92]. Such a device is simple in design and can be made by connect-
ing each end of a fiber grating to a 3-port optical circulator. The channel reflected by
the grating appears at the unused port of the input-end circulator. The same-wavelength
channel can be added by injecting it from the output-end circulator. The device can also
be made by using only one circulator provided it has more than three ports. Figure 8.13
shows two such schemes [94]. Scheme (a) uses a six-port circulator. The WDM signal
entering from port 1 exits from port 2 and passes through a Bragg grating. The dropped
channel appears at port 3 while the remaining channels re-enter the circulator at port 5