In order to accommodate the explosive growth of data traffic in recent years, the transmission speed of the time-division-multiplexed (TDM) transmission system has been increased beyond 10 Gbps. However, it would be difficult to increase the speed of TDM system further than 40 Gbps due to the limited performances of electrical and optical devices and the chromatic dispersion of optical fiber. To overcome this problem, it has been proposed to use the wavelength-division-multiplexed (WDM) transmission system. The WDM system could increase the transmission capacity per optical fiber significantly by transmitting multiple optical signals operating at slightly different wavelengths from each other, even though each laser operates at relatively low speed.
The objectives of this dissertation are to design and implement high-capacity WDM long distance transmission system, and develop the essential techniques required for the realization of such systems. Accordingly, this dissertation report on the laser power and frequency stabilization technique, monitoring techniques for the optical powers, frequencies, and optical signal-to-noise (OSNR) of WDM signals, performance measurement technique, and the techniques to compensate the chromatic dispersion and loss of optical fiber. These techniques were used to implement a high-capacity WDM long-distance transmission system.
In a high-capacity WDM system, the optical signals can be degraded by the dispersion and nonlinearity of transmission fiber. However, it is difficult to analyze the effects of dispersion and fiber nonlinearities precisely. In general, the effect of dispersion can be compensated by using dispersion-compensating fiber or chirped fiber grating, and the effects of fiber nonlinearity can be minimized by reducing of the optical power per channel. Thus, the system was designed by considering the optical signal-to-noise ratio of each channel, while maintaining the optical output power per channel below the threshold power for fiber nonlinearities. The WDM system was then evaluated by using the fiber-optic transmission simulator.
WDM systems would require each transmitter laser to operate at a specific frequency to the end of the system′s lifetime. A simple technique is developed to make an etalon filter which provides a set of equally spaced references at the standardized frequencies. This filter is used to demonstrate a cold-start WDM system that can start data communication without any manual adjustment of laser frequencies or optical powers. This system is advantageous because there is no need to precisely adjust the frequencies of WDM lasers in the factory or in the field, thus simplifying the installation and maintenance of WDM systems.
For the proper management of WDM networks, it is essential to monitor the optical frequency and power of each channel. Previously, several techniques have been proposed for this purpose using gratings (such as arrayed-waveguide gratings, fiber gratings, and diffraction gratings) or tunable filters (such as acousto-optic tunable filters and Fabry-Perot tunable filters). However, these techniques require expensive optical components and/or precise tuning mechanisms. To solve this problem, two different techniques have been developed in this dissertation. The first technique utilizes a temperature-tuned etalon filter. The frequency and power of each channel can be easily monitored by measuring the filtered optical power at several temperatures. The measured power and frequency errors are less than 2 dB and 1 GHz, respectively. The second technique monitors the frequencies of multiple WDM channels by using pilot tones and a pair of spectrally interleaved solid etalon filters. The frequency of each WDM channel is monitored by comparing the intensities of pilot tones in WDM signals passed through two etalon filters. The results show that the frequencies of multiple WDM channels can be monitored simultaneously with an accuracy better than ±3 GHz in most spectral regions even after transmission over 640 km of SMF.
Future optical networks are envisioned to be dynamically reconfigurable, i.e., the optical channels are added/dropped or cross-connected frequently in the optical layer using WDM technologies. In these networks, however, the OSNR′s of WDM channels could be different from each other. This is because WDM channels arrive at their destinations via different optical paths. In addition, the effective optical bandwidth of WDM channel depends on the specific configuration of each node and the amplified spontaneous emission spectrum may not be flat. Thus, for the proper operation and management of these networks, it is desirable to monitor the OSNR of every WDM channel at every node. Recently, several OSNR monitoring techniques have been proposed. However, these techniques can not monitor the OSNR′s accurately in the dynamically reconfigurable optical network due to the limited resolution and unflat ASE spectrum. In this dissertation, a simple OSNR monitoring technique has been demonstrated by using the receiver noise characteristics. This technique can be implemented simply by using two photodetectors and a DSP board. The results show that the proposed technique could monitor the OSNR with accuracy better than 1 dB.
These techniques have been used to demonstrate a high capacity (20 Gbps × 16 channels) WDM transmission over 640 km of single mode fiber. This systems consists of sixteen externally modulated DFB lasers, eight 80-km long single mode fiber spools, and nine EDFA modules with dispersion compensating fibers. This system is also used to demonstrate a directly modulated 2.5 Gbps x 16-channel WDM transmission system over 640-km of conventional single mode fiber using the dispersion compensating fiber. Thus, the capacity of this system could be increased easily without adjusting the outside plant by replacing the directly modulated 2.5-Gbps channels with the externally modulated 10-Gbps channelsThe feasibility of this capacity upgrade was demonstrated by implementing a 130-Gbps (10 Gbps × 12 channels + 2.5 Gbps × 4 channels) WDM transmission system using the same fiber link.
