Abstract
Channel monitors that are based on scanning the wavelength division
multiplexing (WDM) spectrum with a tunable filter must have sufficient resolution
to resolve the ASE floor between channels to enable the OSNRs to be determined.
If the bandwidth of the scanning filter is too broad then adjacent channels
in the measurement become smeared together thus obscuring this level of ASE.
However, in practice it is desirable to use as broad a filter as possible
since this will increase mechanical robustness and reduce cost. In this paper,
we describe and experimentally demonstrate a signal processing technique that
can successfully extract the OSNR out of such smeared measured spectra. Such
inverse problems are notoriously ill-posed. To deal with this our technique
employs two stages of Tikhonov regularization and uses the known spectral
response of the scanning filter together with the knowledge of the expected
spectral structure of a single channel. As with all ill-posed inverse problems
it can be expected that the extracted OSNR would be extremely sensitive to
measurement errors and inaccuracies in the known filter and expected channel
shapes. We investigate the sensitivities in OSNR to such errors and our results
show the technique to be fairly robust, yielding uncertainties in OSNR below
1 dB over a wide range of realistic system noise levels and spectral variations.
© 2009 IEEE
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