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Fiber-to-the-home (FTTH) has now reached the stage where it is providing real-time broadband services. Since even a temporary suspension of broadcasting and telephone services cannot be permitted, it has become important to improve the reliability of FTTH network operation. We have developed an advanced optical line switching system that diverts transmission signals from an in-service line to a spare line without interrupting communication. This system is realized by equalizing the transmission times of the signals sent through dualized lines composed of an in-service line and a spare line. The transmission times are equalized by employing two techniques in the dualized lines. One is a technique that uses a free space optics system to tune the optical line length continuously. The other is a technique for measuring a wide range of time differences between signals transmitted through the dualized lines. Since the dualized lines have a Mach-Zehnder interference structure, the differences can be detected with high accuracy by employing dual-light interferometry. We applied this switching system to a gigabit Ethernet passive optical network (GE-PON) during transmission and succeeded in switching in-service lines without inducing any frame loss.