The capacity of dense wavelength-division-multiplexed (DWDM) optical fiber communications systems is increasing rapidly. To maximize the transmission capacity in DWDM system, the bit rate per channel should be increased, while the channel spacing should be reduced. However, when the bit rate per channel is increased, the chromatic dispersion of the optical fiber should be compensated to avoid the dispersion-induced power penalty. Reducing the channel spacing causes the system to be sensitive to various fiber nonlinearities.
To evaluate the effects of dispersion and various nonlinearities in a high-capacity WDM transmission system, we developed a WDM simulator in this thesis. The WDM simulator, based on a conventional split step Fourier method, was designed to improve the simulation speed by using adaptive step size and provide graphic user interface (GUI) environment.
Using the WDM simulator developed in this thesis, we evaluate the dispersion-managed fiber optic cable that can support the narrow channel spacing of 50 GHz. This cable consists of positive and negative dispersion fibers to minimize the average dispersion, while maintaining the local dispersion above some minimum value. The design objectives of this cable are as follows: (1) The cable length should be as short as possible. (2) The four-wave mixing (FWM) effects should be suppressed effectively. (3) The average dispersion should be minimized.
We use the conventional single mode fiber (SMF) as the positive subsection of the proposed cable. The SMF, due to the large effective area and dispersion, is robust to various fiber nonlinearities. We assume that the length of positive subsection is equal to that of negative subsection. We used the WDM simulator to optimize the subsection length and the chromatic dispersion of negative subsection of the proposed cable by estimating the signal-to-FWM tone power ratio. This is because FWM is the dominant nonlinear phenomenon for the WDM system with narrow channel spacing. The optimized subsection length and dispersion of negative subsection was found to be 4 km and -15 ps/nm/km, respectively. The results show that signal-to-FWM tone power ratio is more than 25 dB (FWM-induced power penalty less than 1 dB) even when the channel spacing was 50 GHz.
We estimated the Q-factor of the WDM transmission system using the proposed cable. We assumed that this system transmits sixteen 10 Gb/s channels spaced at 50 GHz. The optical power per channel was 5 dBm and the transmission distance was 320 km. The results show that, when the proposed cable is used, the Q-factor was estimated to be 23 dB, which is 8 dB larger than the system using the nonzero dispersion shifted fiber (NZDSF). Thus, we believe that the proposed dense periodic dispersion-managed cable could be used to maximize the capacity of WDM systems.
