
impulse response. Ideally, this impulse response is the inverse of the impulse
response that created the ISI, i.e., h
EQ
(t) = F
1
{1/F{h
ISI
(t)}}. To create a tapped
delay line in optics, one can use MZIs. A simple one was demonstrated in [79], and
a more complex one, using lattice filters, was demonstrated in [80].
Figure 9.60 shows a simple OEQ—a two-tap OEQ, with one pre-cursor tap and
one post-cursor tap. Fo r many ISI impairments, it is found that the best time tap
spacing is 0.7 times the bit period of the digital data signal. Two-tap OEQs have
been demonstrated for both 40- and 100-Gb/s signals, correcting modulation
bandwidth limitations, extinction ...