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3 - All-optical long-distance soliton-based transmission systems
- Edited by J. R. Taylor
-
- Book:
- Optical Solitons
- Published online:
- 21 October 2009
- Print publication:
- 23 April 1992, pp 61-72
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Summary
Introduction
The theoretical studies of long-distance soliton transmission in optical fibres with periodic Raman gain were outlined in Chapter 2 ‘Solitons in Optical Fibres: An Experimental Account’ by L. F. Mollenauer. From this work, it was anticipated that the overall information rates made possible by the all-optical nature of the system were potentially orders of magnitude greater than in conventional communication systems. Rate–length products were predicted to be as high as approximately 30000 GHz · km for a single wavelength channel. In the following sections we will describe the first set of experiments designed to explore the fundamentals of such long-distance repeaterless soliton transmissions.
Experimental investigation of long-distance soliton transmission
In the following experiments, a train of soliton pulses was injected into a closed loop of fibre, the length of which corresponded to one amplification period in a real communication system. The pulses propagated around the loop many times until the net required fibre pathlength had been reached (Mollenauer and Smith, 1988). The experimental configuration is shown schematically in Figure 3.1. A 41.7 km length (L) of low-loss (0.22 dB/km at 1.6 μm) standard telecommunications single-mode fibre was closed on itself with a wavelength selective directional coupler (an all-fibre Mach–Zehnder (FMZ) interferometer). The interferometer allowed efficient coupling of the bidirectional pump waves at approximately 1.5 μm (provided by the 3 dB coupler at the pump input) into the loop. At the same time, the signals at approximately 1.6 μm (50 ps sech2 intensity profile pulses at a 100 MHz repetition rate from a synchronously mode-locked NaCl:OH– colour centre laser) were efficiently recirculated (<5% coupled out).
2 - Solitons in optical fibers: an experimental account
- Edited by J. R. Taylor
-
- Book:
- Optical Solitons
- Published online:
- 21 October 2009
- Print publication:
- 23 April 1992, pp 30-60
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- Chapter
- Export citation
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Summary
Introduction
In optical fibers, solitons are non-dispersive light pulses based on non-linearity of the fiber's refractive index. Such fiber solitons have already found exciting use in the precisely controlled generation of ultrashort pulses, and they promise to revolutionise telecommunications. In this chapter, I shall describe those developments, and the experimental studies they have stimulated or have helped to make possible. Thus, besides the first experimental observation of fiber solitons, I shall describe the invention of the soliton laser, the discovery of a steady down-shift in the optical frequency of the soliton, or the ‘soliton self-frequency shift’, and the experimental study of interaction forces between solitons.
As early as 1973, Hasegawa and Tappert (1973) pointed out that ‘single-mode’ fibers – fibers admitting only one transverse variation in the light fields – should be able to support stable solitons. Such fibers eliminate the problems of transverse instability and multiple group velocities from the outset, and their non-linear and dispersive characteristics are stable and well-defined. The first experiments (Mollenauer et al., 1980), however, had to wait a while, for two key developments of the late 1970s. The first was fibers having low loss in the wavelength region where solitons are possible, and the second was a suitable source of picosecond pulses, the mode-locked color center laser.
But the first experiments led almost immediately to further developments.