To accommodate the explosive growth of data traffic, it is essential to increase the capacity of optical fiber using the wavelength-division-multiplexing (WDM) technology. Accordingly, WDM system has been widely deployed for not only long haul (LH) but also metro applications during the past several years. For a LH system, it is important to maximize the transmission capacity by increasing the bit rate per channel and reducing the channel spacing. On the other hand, for a regional metro network, it is necessary to enhance the cost-effectiveness by enabling the use of inexpensive transmitters and avoiding dispersion compensators. These requirements necessitate the development of new types of optical fibers. In this dissertation, a short-period dispersion-managed fiber (SPDMF), capable of suppressing both chromatic dispersion and fiber nonlinearities, is proposed for the use in LH systems. In addition, a negative dispersion fiber (NDF) with small dispersion, which enables the use of inexpensive directly modulated lasers without dispersion compensation, is proposed to implement a cost-effective metro network. At present, there are several types of optical fibers in the market including conventional single-mode fiber (SMF) and non-zero dispersion shifted fiber (NZDSF). However, SMF requires significant dispersion compensation for the systems operating at high speed (> 10 Gb/s). On the other hand, NZDSF may not be able to support a system with narrow channel spacing (< 50 GHz) due to fiber nonlinearities. To overcome these problems of chromatic dispersion and fiber nonlinearities, a dispersion-management technique has been proposed by using two types of fibers with alternating dispersion (i.e. positive and negative dispersion of fibers). However, this technique requires relatively long length (typically > 20 km) of positive and negative fiber sections. Thus, it becomes impractical to deploy such a dispersion-managed fiber in the field since it should use two types of fiber cables (as most of the fiber cable is shorter than 10 km). In addition, it would increase the complexity of fiber maintenance significantly.
In this dissertation, a short-period dispersion-managed fiber (SPDMF) is proposed to suppress the effects of chromatic dispersion and fiber nonlinearities simultaneously. Unlike the previously reported dispersion-managed fiber, SPDMF could be accommodated within a single cable since it utilizes short (<5 km) positive and negative dispersion fibers. Thus, there will be no confusion of installing two different types of fiber cables.
The proposed SPDMF utilizes a conventional SMF (dispersion: 17 ps/km/nm, effective area: 80㎛²) for the positive section. A detailed theoretical analysis shows that the section length of both positive and negative fibers can be as short as 3 km, when the dispersion and effective area of the negative section were -15 ps/km/nm and 65㎛², respectively. Thus, the average dispersion of the proposed SPDMF was merely 1 ps/km/nm.
The performance of SPDMF was evaluated by using 320-Gb/s (32 x 10 Gb/s) and 640-Gb/s (32 x 20 Gb/s) WDM systems with 50-GHz channel spacing, and compared with NZDSF. When NZDSF were used, the Q-factor for a 10-Gb/s channel was measured to be less than 11 dB after 480-km transmission. However, when the proposed SPDMF were used, the average Q-factor was measured to be 18 dB even after 564-km transmission without using DCM modules. SPDMF also showed better performance than NZDSF when the bit rate was increased to 20 Gb/s while maintaining the channel spacing to be 50 GHz. The average Q-factor was measured to be 11.25 dB after transmission over 160 km of NZDSF. The transmission distance could not be increased further than this due to the effects of four-wave mixing and cross-phase modulation. However, when the proposed SPDMF were used, the average Q-factor for a 20-Gb/s signal was measured to be as high as 18.23 dB even after transmission over 280 km. In addition, it has been shown that the proposed SPDMF is also suitable for the transmission of 40-Gb/s based WDM signals through a successful demonstration of 320-Gb/s (40 Gb/s x 8 channels) WDM transmission experiment. After transmission over 320 km of SPDMF, the average Q-factor was measured to be 20.4 dB (which was 2 dB higher than the values obtained by using medium dispersion fiber and conventional SMF). These results confirmed that the SPDMF could substantially reduce the required DCM for 40-Gb/s based WDM systems, since its average dispersion was merely 1 ps/km/nm. In addition, SPDMF was robust to both the intra-channel and inter-channel nonlinearities due to its small average dispersion and large local dispersion.
Recently, there have been many efforts to develop a cost-effective metro network by using directly modulated lasers (DML's). These include the 10 Gb/s x 200 km transmission using mid-span spectral inversion, 10 Gb/s x 38.5 km transmission using spectral filtering, and 20 Gb/s x 53 km transmission using electrical equalizer. However, these techniques typically increase the system's complexity and/or reduce the usable bandwidth. To overcome these problems, a $MetroCor^TM$ fiber has been recently proposed for the transmission of directly modulated 10-Gb/s signals. However, even when this fiber were used, the maximum transmission distance for the directly modulated 10-Gb/s signals would still be limited to less than 100 km due to its large dispersion. In this dissertation, a negative dispersion fiber (NDF) with small dispersion (-0.5 ~ -3.3 ps/km/nm @ C-band) was proposed to overcome this problem. The proposed NDF was used for the transmission of directly modulated 10-Gb/s WDM signals over 320 km without any dispersion compensation or spectral filtering. The Q-factor was measured to be 20.2 dB after transmission over 320 km of NDF. This result represents the longest transmission distance ever reported for directly modulated 10-Gs signals. The proposed NDF was also used for the successful transmission of directly modulated 20-Gb/s signals over 160 km without dispersion compensation, and over 320 km by compensating the accumulated dispersion with 30-km long conventional SMF. These results indicate that the proposed NDF could be used to implement a regional metro WDM network cost-effectively by using directly modulated lasers.
