
that it involves monitoring on the actual data signal as it has propagated along the
impairment path of the signal itself.
In fact, the pilot-tone-based monitoring technique can monitor many other
physical parameters cost-effectively in the optical domain. This technique typically
utilizes a small amplitude-modulated pilot tone dedicated to each WDM channel, as
shown in Figure 7.19. Because the tone is at a single, low frequency, it is easy to
generate and process using conventional electronics. In addition, it can monitor the
optical parameters without using the expensive demultiplexing filters such as tun-
able optical filter and diffraction grat ...