Robotic Waveguide by Free Space Optics

Abstract

Road construction and work on the water supply often require the relocation of aerial/underground telecommunication cables. Each optical fiber leading from an optical line terminal (OLT) in a telephone office to a customer’s optical network unit (ONU) must be cut and reconnected. Customers expect real-time transmission for high-quality communications to continue uninterrupted, especially for video transmission services. Some electrical transmission apparatus can maintain communication without interruption, even when optical cables are temporarily cut. The system is complicated and any transmission delay during O/E conversion is fatal to real-time communication. Although it is desirable to directly switch the transmission medium itself, it had been thought that some data bits would inevitably be lost during the replacement of optical fibers. An optical fiber cable transfer splicing system [1] has been developed to minimize the disconnection time. It takes 30 ms to switch a transmission line, and more than 2 seconds * to restore communications with, for example, GE-PON [2]. We have developed an interruption-free replacement method for in-service telecommunication lines, which can be applied to the current PON system equipped with conventional OLTs and ONUs [3, 4]. Two essential techniques were presented; a measurement method and a system for adjusting the transmission line length. The latter continuously lengthens/shortens the line over very long distances without losing transmitted data based on free space optics (FSO). The former distinguishes the difference between the duplicated line lengths by analyzing signal interference. The mechanism that automatically coordinates both these two functions and referred to as a robotic waveguide in this paper compensates for the traveling time difference of a transmitted pulse. Interferometry is the technique of diagnosing the properties of two or more lasers or waves by studying the pattern of interference created by their superposition. It is an important investigative technique in the fields of astronomy, fiber optics, optical metrology and so on. Studies on optical interferometry are reported to improve tiny optical devices [5-7]. We have applied the technique to measure length of several kilometers of optical fibers with a 10 mm resolution. This paper describes the design of our robotic waveguide system. An optical line length measurement method is studied to distinguish the difference of two lines by evaluating interfered optical pulses. An optical line switching procedure is designed, and a line length adjustment system is prototyped. Finally, we applied the proposed system to a 15 km GE-