Dynamic circulating-loop methods for transmission experiments in optically transparent networks

Abstract

Recent experiments incorporating multiple fast switching elements and automated system configuration in a circulating loop apparatus have enabled the study of aspects of long-haul WDM transmission unique to optically transparent networks. Techniques include per-span switching to measure the performance limits due to dispersion compensation granularity and mesh network walk-off, and applied constant-gain amplification to evaluate wavelength reconfiguration penalties. Keywords: transparent networks, circulating loop experiments, dispersion compensation granularity, wavelength reconfiguration. INTRODUCTIONOptically transparent mesh networks take advantage of photonic cross-connects and reconfigurable optical adddrop multiplexers (ROADMs) to enable flexible wavelength routing in the physical layer. Neighbouring channels in the optical spectrum can traverse diverse routes through the network. The transmission performance of these channels will depend on the route physical parameters and on the different channel configurations encountered along the route. Physical parameters that influence transmission include factors such as the fiber span length, fiber type, and the dispersion management characteristics. Much is known about the propagation of signals over regular spans of similar types and lengths with well controlled dispersion maps [1]. In transparent networks, the number of different combinations of route parameters can quickly become too large to address explicitly in the system design. Understanding how the transmission varies as the route characteristics deviate from the design targets is important for accurately predicting performance and thereby reducing regeneration. In general, this requires thorough testing and validation for a specific system design [2]. Extensive simulation studies have been used to account for mesh transmission variations [2-4]. Circulating loop experiments, which are commonly used for long haul transmission, traditionally have not had the flexibility to examine a wide range of transmission characteristics [1]. Multi-loop experiments have been used to vary the fiber type over different transmission segments [5]. We introduced a dynamically reconfigurable circulating loop that enables long haul transmission experiments that examine the impact of dispersion map variations [6]. This new apparatus uses fast switches synchronized with the loop circulation in order to vary the length of fiber in each span. Further accounting for variations in the channel loading is complicated due to the strong dependence of the channel power on the amplifier dynamics and control. Typically channel reconfiguration is enabled in transparent networks by using optical amplifiers that are controlled to maintain a constant gain condition as the channel configuration is varied. Due to gain ripple and tilt, complex power dynamics can arise even for well controlled amplifiers [7]. In circulating loop experiments constant gain amplifier control is not desirable because the amplifiers are connected in a ring a...