gaalas gaas micromachined thermally tunable vertical cavity filter with low tuning voltagewhere variable excitation of the principal states of polarisation is neglected). It is interesting to evaluate the mean outage rate R , , , , i.e. mean number of outage events per unit time, and the mean outage i.e. mean duration of an outage event. duration To,,,, The calculation of the mean outage rate is a simple application of the ‘level crossing’ problem [7] and can be performed by the following integral evaluated for AT = tmnr: M tion that, as far as we h o w , has never been calculated until now for a buried cable. Conclusions: Repeated PMD measurements on a buried optical cable have been performed and full statistical analysis of the measured DGD spectra against time lead to two important conclusions. First, DGD spectra measured at 24 h intervals present strong correlations. While……