To accommodate the explosive growth of data traffic, there have been many efforts to increase the capacity of wavelength-division-multiplexed (WDM) transmission system. For this purpose, it is necessary to increase the bit rate per channel and the number of channels. However, if the operating speed of each channel is increases to >40 Gb/s, the chromatic dispersion of fiber link could impose a serious limitation on the system’s performance. Thus, the chromatic dispersion of such high-speed fiber link must be managed with extreme care. However, even if the dispersion of transmission fiber is compensated precisely by using DCF modules, there could be a non-negligible residual dispersion due to the changes of ambient temperature. In addition, the value of chromatic dispersion is slightly different from transmission fiber to fiber due to the imperfect manufacturing process, while the DCF module typically has a specific value of negative dispersion depending on the required compensation length. Thus, a small amount of chromatic dispersion is unavoidable even after the compensation by DCF modules. This residual dispersion could also vary due to the ambient temperature changes or reconfiguration of optical paths. Thus, it is extremely difficult to satisfy the stringent requirement of chromatic dispersion for the high-speed signals operating at >40 Gb/s by using only the DCF modules.
Previously, the residual dispersion caused by ambient temperature was evaluated by assuming that the entire transmission link would experience the same temperature changes simultaneously or each fiber section would experience the statistically independent temperature changes. However, in practice, the temperature changes along a long-haul fiber-optic link would be quite different from these assumptions due to the seasonal and regional factors. In this dissertation, an accurate temperature model was developed by using the soil temperatures measured in 47 different regions of U.S.A. for one year. Using this temperature model together with a simple equation to describe the distribution of residual dispersion arising from temperature variations, the system’s outage probability was evaluated. The results showed that the maximum transmission distances for 40-Gb/s NRZ and RZ signals could be limited to 3120 km and 1500 km, respectively, assuming that the system’s outage probability (@ dispersion penalty: >1dB) should be less than $5.5\times10^{-5}$.
To evaluate the residual dispersion caused by the statistically distributed chromatic dispersion of optical fiber, the distribution of chromatic dispersions in 1000 spools of single-mode fiber and nonzero dispersion-shifted fiber obtained from five different fiber manufacturers was investigated. In addition, a simple equation was developed to describe the statistical distribution of residual dispersions by using the means and standard deviations of the zero-dispersion wavelengths and the dispersion slopes. The results show that when the chromatic dispersion of transmission link was compensated by using DCF modules (and the effect of ambient temperature change was neglected), the maximum transmission distance for 40-Gb/s RZ signal could be limited to 27 km ~ 153 km. However, this distance would be further reduced to 27 ~ 146 km if both the ambient temperature changes and statistically distributed fiber dispersions were considered. These results could be used to determine when the tunable dispersion compensators should be used in a high-speed WDM network.
The transmission capacity of fiber-optic transmission system could also be increased by utilizing a larger number of WDM channels at narrow channel spacing. In this case, however, various fiber nonlinearities could seriously deteriorate the system’s performance. To overcome this problem, there have been many efforts to increase the spectral efficiency by using the polarization multiplexed (PM) or polarization-shift-keying (PolSK) techniques. In PM system, the spectral efficiency can be doubled since two optical signals operating in the orthogonal polarization states, but at the same wavelength, carry independent traffics. In case of PolSK system, the number of WDM channels can be increased by reducing the channel spacing since it is relatively insensitive to the fiber nonlinearities such as cross-phase modulation (XPM) and self-phase modulation (SPM) due to the constant power envelope. However, the polarization-dependent loss (PDL) in a fiber-optic transmission link can impair the orthogonality between the signals in two polarization states and generate crosstalk components on the signal operating in the opposite polarization state. In this dissertation, the effect of the PDL-induced crosstalk and its corresponding power penalty were analyzed in both PM and PolSK systems. The results show that the crosstalk component caused by PDL could degrade the system’s performance by increasing the in-band crosstalk and decreasing the extinction ratio in the PM and PolSK systems, respectively. In PM systems, 1-dB PDL could generate the crosstalk of -25 dB, which, in turn, degrades the receiver sensitivity by ~1 dB. In PolSK systems, the PDL of 3 dB could cause 1-dB power penalty.
It has been reported that the PDL in a long-haul transmission link has Maxwellian distribution. Thus, to evaluate the effect of PDL, a variable PDL emulator capable of generating a wide range of PDL values is needed. In this dissertation, a novel variable PDL emulator based on the LiNbO3 modulator was proposed and demonstrated. The proposed PDL emulator could generate a wide range of PDL values, up to 35 dB, simply by adjusting the bias voltage of the modulator. The wavelength dependency was measured to be less than 0.03 dB in the range of 1520 nm ~ 1590 nm. Using a simple feedback circuit, the long-term stability of the proposed PDL emulator was ensured to be better than ±0.02 dB.
본 논문에서는 시스템의 전송 용량을 증가시키는 위하여 초고속 광신호를 이용하거나 채널 수를 증가 시키는 방법을 이용할 경우, 시스템의 확장성을 제한하는 온도변화 및 색분산 분포 특성에 의한 잔여색분산과 편광의 변화의 영향을 분석하고 실험을 통하여 검증하였다. 이를 위하여, 실제 광링크에서 발생하는 온도변화를 정확하게 나타낼 수 있는 새로운 온도변화 모델을 제시하였으며 실제 전송용 광섬유로 주로 사용되는 단일모드광섬유와 비 영분산천이 광섬유의 색분산의 온도 변화율을 직접 측정하였다. 또한, 상용화 된 단일모드 광섬유와 비 영분산천이 광섬유의 색분산 분포를 측정하였으며, 광링크를 위하여 사용된 광섬유들의 영분산 파장(zero dispersion wavelength)와 영분산 기울기(dispersion slope at zero dispersion wavelength)의 평균값과 표준편차 만으로 잔여색분산 분포를 쉽게 계산할 수 있는 방법을 제안하였다. 광링크 주위의 온도변화와 광섬유 색분산 분포 특성에 의하여 발생하는 잔여색분산 분포를 동시에 고려하기 위하여 제안한 온도변화 모델을 색분산 분포 특성에 의하여 발생하는 잔여색분산을 계산하는 식에 적용시킴으로써 실제 광링크에서 발생하는 잔여색분산 분포를 더욱 정확하게 분석할 수 있는 방안을 제시하였다. 이러한 방법을 이용하면, 광링크에서 광섬유의 색분산 분포나 온도 변화에 의한 색분산 변화의 영향을 정확하게 예측할 수 있으며 파장분할 다중방식 초고속 광통신망에서 가변 분산보상기를 사용해야 할 시점과 보상 범위를 쉽게 결정 할 수 있을 것으로 판단된다. 한편, 시스템의 전송 용량을 증가시키기 위하여 광신호의 편광을 이용하는 경우, 광링크에 존재하는 편광의존손실이 편광 다중화 방식 광전송 시스템과 편광 변조 방식 광전송 시스템에 미치는 영향에 대하여 분석하였다. 이를 위하여, 편광의존손실로 인하여 발생하는 광신호의 수직한 두 편광상태의 변화와 이로 인한 누화의 발생을 간단한 수식으로 모델링 하였다. 또한 편광 다중화 시스템과 편광 변조방식 시스템에서 이러한 영향으로 발생하는 페널티를 계산하였으며 실험으로 검증하였다. 또한, 광링크에 존재하는 편광의존손실을 정확하게 모사하기 위한 새로운 가변 편광의존손실 에뮬레이터를 제안하였다. 제안한 에뮬레이터는 광신호의 TE 모드 편광과 TM 모드 편광을 모두 전달할 수 있는 $LiNbO_3$ 변조기를 이용하였으며 변조기의 바이어스 전압을 바꾸어 줌으로써 편광의존손실을 매우 큰 값까지 발생시킬 수 있었다. 본 논문에서는 제안한 편광의존손실 에뮬레이터를 이용하여 광링크에 존재하는 편광의존손실의 영향을 쉽고 빠르게 분석 할 수 있었다.
