Monitoring the signal quality in service is important in optical amplifier transmission systems to accommodate the explosion of data traffic. To check the quality of these systems without any conversion to electrical signal, the performance monitoring of signal quality in the optical domain is essential. At present, the transmission quality estimation method is to measure the optical signal to noise ratio(OSNR) or to measure statistically Q by sampling the signal in transmission. In this thesis, we propose an estimation method of the optical transmission quality based on the clock amplitude and investigate the relationship between the eye penalty and the clock amplitude of the transmitted signal.
This method is simpler than the statistical Q measurement method. The clock amplitude extracted from the output of each optical amplifier is compared with the eye penalty. For the verification of this method, the linear and nonlinear transmission characteristics of both the return-to zero(RZ) signal and the non-return-to zero(NRZ) signal are studied by numerical simulations. The major results of simulations are as follows.
In the case of RZ transmission for 10Gb/s signals, the clock amplitude in linear transmission regime has good correspondence to the eye penalty regardless of transmission length. In the nonlinear transmission regime, the clock amplitude has good correspondence to the eye penalty up to the transmission length of 400 km.
Next, in the case of NRZ transmission for 10 Gb/s signals, the correspondence between the clock amplitude and the eye penalty depends on which of the two clock extraction methods is used. The time-delayed extraction method yields a better correspondence than the frequency-doubling extraction. The clock amplitude has a good correspondence to the eye penalty in the linear transmission regime. In the nonlinear transmission regime, the correspondence between the clock amplitude and the eye penalty may be maintained if the input optical power is less than 6 dBm.
For WDM systems, the eye penalty can be estimated from the extracted clock amplitude with small input optical power and large channel spacing. As criteria for eye penalty monitoring in WDM systems, the difference between the directly measured eye penalty and one estimated from the clock amplitude measurement must be smaller than 0.15 dB in all channels. On the basis of this criteria, the eye penalty can be estimated up to the transmission length of 720 km in the time-delayed extraction method when the input optical power is 3 dBm.