Fiber Raman amplifier (FRA) has long been considered as one of the most important elements for the development of the next-generation fiber-optic communication systems. This is because FRA can improve the optical signal-to-noise ratio, reduce the impacts of fiber nonlinearities, and provide broad bandwidth. However, the performance of FRA can be seriously affected by various problems such as polarization-dependent gain (PDG), pump-to-signal relative-intensity noise (RIN) transfer, and four-wave mixing (FWM) caused by pump-pump, pump-signal, and signal-signal interactions. Thus, there have been substantial efforts to understand these effects and overcome their limitations.
In this dissertation, we have analyzed the PDG RIN transfer, and FWM (caused by signal-signal interaction) in a Raman-amplified WDM system. The results of these analyses were used to identify the requirements of FRA for the suppression of these deleterious effects. In addition, using the FRA designed with above requirements, we have demonstrated a repeaterless transmission of directly modulated WDM signals over 200 km of negative-dispersion fiber (NDF) for the use in a cost-effective metro network.
Firstly, we have developed an analytical model for the PDG statistics in both co-pumped and counter-pumped FRA's. This model considered various design parameters of FRA including the fiber length, fiber loss, polarization-mode dispersion (PMD), Raman gain efficiency, wavelength difference between the pump and signal, pump power, and degree-of-polarization (DOP) of pump. The validity of this model was confirmed by experimental data and numerical simulations. The proposed analytical model was also used to evaluate the PDG statistics of FRA. The results showed that the PDG of FRA could be suppressed by using the pump laser having low DOP, a large wavelength difference between the signal and pump, and low-loss fiber with high PMD. The results also showed that the PDG in a counter-pumped FRA is about 3 times smaller than that in a co-pumped FRA, unlike the previous estimation published in other literatures. The proposed analytical model was used to identify the requirements of various parameters for FRA-based transmission systems. For example, to suppress the PDG within 1 dB in 8000-km long transmission system, it would be necessary to use the counter-pumped FRA's implemented by using pump lasers with low DOP (DOP < 14 %).
Since FRA has fast gain dynamics, the RIN of the pump laser could be transferred to the signal and degrade the system's performance. In general, FRA is implemented by using polarization-multiplexed pump lasers to obtain high pump power and minimize the polarization-dependent characteristics. In this dissertation, we have developed analytical expressions for the pump-to-signal RIN transfer in both co-pumped and counter-pumped FRA's using polarization-multiplexed pump lasers. We confirmed the validity of this model by using experimental data. The results showed that the shape of RIN transfer in FRA was similar to that of the first-order low-pass filter (with an extinction of 20 dB per decade) and the corner frequency of RIN transfer in a co-pumped FRA was about $10^4$ times higher than that in a counter-pumped FRA. This was because the RIN transfer in a co-pumped FRA was suppressed due to the slight difference between the group velocities of the pump and signal (caused by the chromatic dispersion of transmission fiber), while in a counter-pumped FRA, the pump and signal propagate in the opposite direction and the effect of the counter-propagation dominates over the effect of group velocity difference. By using these analyses, we have identified the requirements of RIN for pump lasers in both co-pumped and counter-pumped FRA's. For example, to suppress the RIN-induced penalty to be smaller than 0.1 dB, the RIN of pump laser should be smaller than -114 dB/Hz and -77 dB/Hz for the for the co-pumped and counter-pumped FRA's, respectively (assuming that the fiber dispersion is 17 ps/km/nm, fiber length is 80 km, gain of FRA is 16 dB, and Q factor without RIN transfer is 10).
Recently, there have been many efforts to utilize FRA's for the transmission of WDM signals over low-dispersion fiber such as dispersion-shifted fiber (DSF). This is because the use of FRA could substantially reduce the signal power within fiber and suppress the effects of fiber nonlinearities. However, for the optimization of Raman gain and signal power in such a system, it is necessary to estimate the FWM crosstalk accurately - which typically requires a time-consuming simulation. In this dissertation, we have developed a simple analytical model to evaluate the FWM crosstalk in a WDM system using FRA's. The validity of this model was confirmed by experimental and simulation data. Using this model, we have estimated the FWM crosstalk in a WDM system using DSF and Raman amplifiers. The results showed that, even when DSF was used, the counter-pumped Raman amplifier would not increase the FWM crosstalk by more than 3 dB. On the other hand, the co-pumped amplifier could increase the FWM crosstalk by more than 25 dB. Thus, in case DSF were used, it would be advantageous to use the counter-pumped Raman amplifier for the transmission of WDM signals. The developed model could also be used for the optimization of the signal powers and Raman gain for WDM transmission systems using either DSF or non-zero dispersion-shifted fiber (NZ-DSF).
Recently, there have been many efforts to develop a cost-effective metro network by using directly modulated lasers (DML's). For this purpose, we have already proposed to use the NDF with a small dispersion (-2.5 ps/km/nm at 1550 nm) in the regional metro network. Because of its small negative dispersion, there is no need for using dispersion compensating modules even when DML's were used. The cost-effectiveness of such a network could be further improved if we simplify the outside plant by eliminating the in-line amplifiers. To demonstrate this possibility, we have implemented a 200-km long repeaterless transmission system by using a counter-pumped FRA. To minimize the performance degradations caused by PDG and pump-to-signal RIN transfer in FRA, we used pump lasers having low DOP (<10 %) and low RIN (<-125 dB/Hz). As a result, we could successfully demonstrate an error-free repeaterless transmission of directly modulated 10-Gb/s WDM signals over 200 km of NDF by using a counter-pumped FRA of 22-dB gain. In addition, we showed that the maximum transmission distance of this system could be extended to 260 km simply by using forward-error correction codes and increasing the gain of FRA to 28 dB.
광섬유 라만 증폭기는 차세대 광통신 시스템의 개발을 위한 핵심 기술로 각광을 받고 있다. 이는 광섬유 라만 증폭기를 이용하는 경우 전송 신호의 광신호대 잡음비를 개선할 수 있고, 광섬유 내에서 발생하는 비선형 현상의 억제가 가능하며, 넓은 이득 대역폭을 제공하기 때문이다. 그러나 광섬유 라만 증폭기에 의한 성능 개선은 편광 의존 이득, 펌프 잡음의 신호로의 전달, 사광파 혼합 등에 의해 열화될 수 있다.
본 학위 논문에서는 WDM 방식 초고속 광통신 시스템을 위한 광섬유 라만 증폭기의 특성과 응용에 관하여 기술한다. 우선, 광섬유 라만 증폭기의 이득에 영향을 주는 파라미터인 펌프와 신호의 파장 간격, 광섬유의 손실과 편광 모드 분산, 그리고 펌프의 편광 정도(DOP: degree of polarization) 등을 고려하여 신호와 펌프의 편광 상태에 따른 편광 의존 이득을 순방향 및 역방향 라만 증폭기에 대해서 이론적으로 분석하였다. 그리고 이론적 분석 결과를 실험과 시뮬레이션을 통하여 검증한 내용에 대하여 기술하였다. 이러한 분석 결과를 바탕으로 광섬유 라만 증폭기의 편광 의존 이득을 효율적으로 억제하기 위한 방안을 제시하였다. 다음으로, 편광 결합된 펌프 광원을 사용하는 광섬유 라만 증폭기에서 펌프 변동의 신호로의 누화를 분석하고, 실험을 통해 검증한 내용을 기술하였다. 이러한 분석을 바탕으로 순방향 및 역방향 라만 증폭기를 위한 펌프 광원의 잡음 조건을 제시하였다. 또한, 광섬유 라만 증폭기가 사용된 WDM 전송 시스템에서의 사광파 혼합 효율을 이론적으로 분석하고, 실험을 통해 검증한 내용을 기술하였다. 그리고 이러한 분석 결과를 바탕으로 DSF나 NZ-DSF와 같이 낮은 색분산을 갖는 광섬유가 전송 선로로 사용된 광통신 시스템에서 WDM 전송을 위한 라만 증폭기의 이득과 광신호의 입력 전력의 최적화 방안에 관하여 논의하였다. 마지막으로 편광 의존 이득과 펌프 잡음의 신호로의 누화에 의한 성능 열화를 억제할 수 있는 광섬유 라만 증폭기를 사용하여 직접 변조된 10 Gb/s WDM 광신호의 무중계 전송 실험에 대한 결과를 기술하였다.
