What,why and How of Pilot Tone in DWDM system?
There have been many efforts to use pilot tones (i.e., small sinusoidal components added to WDM signals) for the monitoring of WDM signals directly in the optical layer. For example, it has been reported that pilot tones could be used to monitor various optical parameters of WDM signals such as optical power, wavelength, and optical signal-to-noise ratio (OSNR), and so on .
The pilot-tone-based techniques could be used to monitor these parameters without the expensive demultiplexing filters (such as tunable optical filter and diffraction grating). Thus this technique could be extremely cost-effective. In addition, this technique is well suited for use in a dynamic WDM network, since the pilot tones are bound to follow their corresponding optical signals anywhere in the network. Thus the optical path of each WDM signal could be monitored simply by tracking its tone frequency
Although the pilot-tone-based monitoring technique has many advantages, it also has some limitations owing to the following problems.
First, the pilot tone could impose unwanted amplitude modulation on the data signal and degrade the receiver sensitivity .
Second, the performance of the pilot-tone-based monitoring technique could be deteriorated by ghost tones caused by cross-gain modulation (XGM) and stimulated Raman scattering (SRS) . These problems could be mitigated by using proper amplitudes and frequencies of pilot tones. However, for use in a long-haul network with a large number of channels, it may still be necessary to restrict the number ofWDM channels to be monitored at a time (by using an optical bandpass filter).
Operating Principle
Figure shows the operating principle of the pilot-tone-based monitoring technique .We assume that an optical signal is transmitted from node A to node C via node B. A small sinusoidal component (i.e., pilot tone) is added to the optical signal at node A. This pilot tone can be extracted at node B by use of a simple electronic circuit and can be used for monitoring various optical parameters such as optical power, wavelength, OSNR, and so on. Pilot tones can also be used to monitor the optical paths of WDM signals even in a dynamically reconfigurable network. This is because once the pilot tone is attached, it is bound to follow the optical signal throughout the network. Thus we can monitor the optical path of each WDM signal simply by tracking its corresponding tone frequency.
For practical applications, pilot tones should be added into and extracted from the optical
signal anywhere in the network.
