Due to the explosive growth of data traffic, there is an urgent need to develop the survivable high-capacity transport network. The transmission capacity of optical fiber has been increased to tens of terabits per second by using the wavelength-division-multiplexing (WDM) technologies. However, the complicated electronics required to process the enormous data traffic at each node still imposes a serious bottleneck for the expansion of network capacity. To solve this problem, there have been substantial efforts to route and switch the signals in the optical layer. For example, a large number of all-optical WDM networks have been demonstrated using various types of optical cross-connects (OXC's). These networks are transparent, reconfigurable, and capable of restoration from network failures without using the SONET layer.
The objective of this dissertation is to design and implement an all-optical WDM network, with a special emphasis on the fast restoration, and evaluate its performances. For this purpose, we have implemented KAIST All-Optical Network (KAON) which consisted of four OXC's, two in-line amplifiers, and 800 km of single-mode fiber. Each OXC was made of four pairs of 2.5 Gb/s transceivers, three pairs of multiplexers and demultiplexers, three pairs of pre-amplifiers and post-amplifiers, four 4 x 4 thereto-optic space switches, three 1.51-pm supervisory transceivers, a network monitoring system (which continuously monitored various network elements and qualities of optical signals at 130 points), and a network management system. In this network, the optical path monitoring and the channel identification were achieved by using dual tones. To minimize the restoration time, we implemented the OXC's with thereto-optic polymer switches (switching time: -1.5 ms) and used hardware interrupt for the detection of loss-of-signals (LOS). The restoration time composed of the failure detection time (0.2 ms), TCP/IP processing time (0.4 ms), management information base (MIB) processing time (0.1 ms), propagation delay time (0.4 ms over 80 km), and switching time (1.5 ms). As results, the network could be restored from any single link failure within 6 ms, even when the restoration path was 400 km.
In all-optical WDM network, the networks performance could be seriously affected by the closed cycles, which represent the closed optical paths formed by the wavelength-selective optical cross-connects and erbium-doped fiber amplifiers. Once the closed cycles is formed, lasing could occur at the corresponding wavelength, as the amplified spontaneous emission noise becomes re-circulated within the closed cycles. This ASE lasing would result in the signal power variations (induced by amplifier saturation), the signal power transients (induced by relaxation oscillation), and the signal power fluctuations (induced by mode-locking-like oscillation). There have been many efforts to eliminate the closed cycles in all-optical WDM network by using unused add/drop ports, dilation of switches, and installation of circuit breakers. However, these techniques require a large number of additional optical switches, tunable optical filters, and/or unused ports. In this dissertation, we proposed and demonstrated a simple technique to eliminate the closed cycles by using a null-port placed only at one OXC in the all-optical WDM network. The closed cycles were first combined into a large one by using the property of Eulerian network, and then eliminated simply by switching their paths and directing to the null-port of OXC. Unlike the previous techniques, the proposed technique does not require any additional component except an extra null port in one of the OXC's.
There have been substantial efforts to develop the efficient optical add/drop multiplexer (OADM) using arrayed waveguide gratings (AWG's) for the use in all-optical WDM networks. For example, it has been proposed to implement the OADM by using a pair of 1 x N AWGs. However, this technique requires two AWGs operating at the same wavelength. To avoid this problem and minimize cost, it has been proposed to implement the OADM by using an AWG in loop-back configuration. This OADM utilizes only one Nx N AWG and could support up to N-1 channels. However, this technique is inherently sensitive to the first-order intraband crosstalk. The OADM could also be implemented by using a 2 x N multiplexer in fold-back configuration. However, this technique could utilize only N/2 channels, although it is immune from the first-order crosstalk. Recently, this technique has been improved to support all the N channels at the cost of additional N WDM filters. In this dissertation, we proposed and demonstrated a simple OADM using one 2 X N AWG and couplers. Unlike the previous techniques, the proposed technique can support all the N channels without using the WDM filters. In addition, we found that the suppression of the first-order crosstalk would not necessarily improve the performance of OADM because of the crosstalk caused by reflection from inside of the AWG.
