Optical Channel Monitoring Using Two Photon Absorption

Optical channel monitoring is used for managing and conditioning dense wavelength division multiplexed (DWDM) signals in optically transparent transmission systems. In addition to measuring channel power and wavelength, more detailed channel identification information is desirable such as modulation format and bit rate. This application is particularly challenging in optically transparent mesh transport systems because the transparent nature of the network lends itself to the use of diverse modulation formats and bit rates. Furthermore, the loss of signals propagating over long distances can result in spectral noise peaks that can be equal in power to DWDM signals. In order for nodes to independently recover from these conditions, the resident channel-monitoring device must be able to distinguish such noise peaks from signal bearing channels. Two photon absorption monitoring has received attention recently as a simple and robust measurement that is sensitive to group velocity dispersion, polarization mode dispersion, and the optical signal to noise ratio (OSNR). In this paper, the use of two-photon absorption in a channel-monitoring configuration is studied. Given that two-photon absorption, which is proportional to an average over the intensity squared, is a measure of the signal variance, the authors demonstrate that the two-photon absorption normalized to the square of the single photon absorption provides an indication of channel presence and also explore the conditions under which the channel modulation format can be distinguished.