Frame-Loss-Free Optical Line Switching System for In-Service Optical Network

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.

I. Introduction

Fiber-To-The-Home (FTTH) systems, which are capable of transmitting vast amounts of data, are mainly designed by passive optical network (PON) topology [1], [2] and support today's “information society”. When FTTH systems have to cease operating during work related to upgrading services and network operation, many users suffer a big disadvantage. To deal with such situations, approaches designed to protect PONs are being actively studied [3]–[5]. Since the introduction of optical fiber cable, we have also developed an optical fiber cable transfer splicing system [6], [7] to minimize cable switching time and to prevent any deterioration in the service. However, neither the PON protection technique nor the transfer splicing system can avoid communication failures because transmission ceases during line switching.