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An optical digital cross-connect system (DCS) comprised of both photonic switching networks and electric DCSs is proposed for applications in network-failure restorations. Optical DCSs exhibit excellent characteristics in restorations of multiple failures and when compared against their electrical counterparts, size-reduced apparatus can be expected. The design of DCSs for applications in metropolitan-area networks are investigated and it is shown that such DCSs can be realized using LiNbO3 photonic switch matrices together with traveling wave optical amplifiers (TWA). With respect to the problem arising from the polarization dependency of TWA gain, it was solved using the output power controller (APC) which is shown theoretically to cause no difficulty in the noise characteristics. The optical DCS is designed and manufactured using LiNbO3 switch matrices and TWA, by which its basic operation properties are verified. Using this trial system, it is confirmed that the network failures can be restored by changing the optical routes. [ABSTRACT FROM AUTHOR]