This paper presents and experimentally demonstrates
a novel method for the estimation of optical signal-to-noise ratio
(OSNR) based on the comparison of an asynchronous histogram of
the signal under analysis with a reference asynchronous histogram.
The latter is acquired from the signal under analysis at a calibration
stage. The proposed method allows the use of optical amplification
to increase the sensitivity of the optical monitoring system
(OMS) by a factor 20 dB, when using an erbium doped fiber preamplifier.
In addition, the use of a semiconductor optical preamplifier,
initially designed for nonlinear operation at 2.5 Gb/s is used in the
OMS to preamplify 40-Gb/s signals, achieving a sensitivity gain of
10 dB. It will be experimentally demonstrated that the proposed
method is applicable to 40-Gb/s nonreturn to zero (NRZ) signals
arbitrarily degraded by group velocity dispersion (GVD). Furthermore,
accurate monitoring of the OSNR of return-to-zero (RZ)
signals will also be possible using a simple RZ-to-NRZ converter
based on narrow-band optical filtering within the OMS. The proposed
method also allows estimating of the GVD-induced OSNR
penalty between the signal under analysis and the signal at the calibration
stage.