다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

Wavelength-division multiplexing (WDM) technologies are expected to play a key role in realizing the next generation scalable and flexible passive optical networks (PONs). One candidate is WDM-PON, in which each optical network unit (ONU) uses a different wavelength, i.e., a unique wavelength, in each direction to communicate with the optical line terminal. Another candidate is WDM/time-division multiplexing (TDM)-PON; it combines WDM with TDM technology. This paper reviews recent state-of-the-art research on the enabling technologies needed to realize future WDM-PON and WDM/TDM-PON systems, and discusses future directions toward practical PON systems.

Enabling Technologies for Future Scalable and Flexible WDM-PON and WDM/TDM-PON Systems

We propose a simple self-polarization-stabilization technique for the wavelength-division-multiplexed passive optical network implemented with reflective semiconductor optical amplifiers (RSOAs) and self-homodyne coherent receivers. By placing a 45° Faraday rotator in front of the RSOA in the optical network unit, the state-of-polarization of the upstream signal becomes orthogonal to that of the linearly polarized seed light at the input of the coherent receiver regardless of the birefringence in the transmission link. Thus, we can achieve the polarization stability of the upstream signal at the input of the coherent receiver. We first implement a self-homodyne receiver by using the proposed self-polarization-stabilization technique and measure its sensitivity by using 2.5-Gb/s binary phase-shift keying signals in the laboratory. The result shows an excellent receiver sensitivity of -46.4 dBm. We also confirm the efficacy of the proposed technique in the transmission experiment over 68-km long link partially composed of installed (buried and aerial) fibers. No significant degradation in the receiver sensitivity is observed during the 10-h experiment despite the large polarization fluctuations occurred in these installed fibers.

Long-Reach Coherent WDM PON Employing Self-Polarization-Stabilization Technique

Colorless gigabit wavelength-division-multiplexing (WDM)/time-division-multiplexing (TDM) hybrid passive optical network (PON) using a remote protocol terminator is proposed and experimentally demonstrated. The system consists of reflective semiconductor optical amplifier-based WDM-PON and a legacy TDM-PON, combined with a simple and efficient remote node structure in order to provide high-speed fiber-to-the-home service to a large area with a high population density. The demonstrated system can support 512 subscribers with a single feeder fiber. The performance and feasibility of the system were evaluated by transmitting gigabit per second data including the effects of Rayleigh backscattering and 20 km attenuation. The system has been used by Korea Telecom to provide commercial services in Gwangju, Korea, since March 2009, after successfully passing through the benchmark test. This is the first commercial deployment of a WDM/TDM hybrid PON. It can accommodate field-deployed TDM-PON link without any modification and extend the maximum total reach of PON link by the use of remote protocol termination. Number of subscribers and the allocated bandwidth to the subscribers can be designed by simply changing the splitting ratio.

First Commercial Deployment of a Colorless Gigabit WDM/TDM Hybrid PON System Using Remote Protocol Terminator

We investigate the effects of the discrete reflection on the performances of upstream and downstream signals in the wavelength-division-multiplexed passive optical network (WDM PON) implemented in a single-fiber loopback configuration using the reflective semiconductor optical amplifiers (RSOAs). We first analyze the optical beat interference (OBI) noise caused by the discrete reflection, and clarify the relation between the reflection tolerance and the network's operating conditions such as the RSOA gain, the link loss, and the location of the reflection point, etc. The results show that the impact of the reflection can be expressed by using the effective crosstalk level. We then measured the reflection tolerance of the RSOA-based WDM PON, in which the downstream signal operating at 1.25 Gb/s is remodulated by the RSOA at the subscriber's site for the transmission of 155-Mb/s upstream signal. The reflection tolerances are measured to be in the range of -42 to - 35ndB for the downstream signals and -29 to -19ndB for the upstream signals, depending on the RSOA gain. These small reflection tolerances are caused by the fact that the reflected light is re-amplified by the RSOA. We also show that the dependence of the reflection tolerance on the RSOA gain can be explained by using the effective crosstalk level. These results are used to evaluate the impacts of the unwanted discrete reflections on the RSOA-based WDM PON.

Effects of Reflection in RSOA-Based WDM PON Utilizing Remodulation Technique

We demonstrate an RSOA-based WDM PON operating at the symmetric rate of 1.25 Gb/s by using a low-frequency subcarrier (2.5 GHz). The performance is robust against the remodulation noise as well as the reflection noise.

Demonstration of RSOA-based WDM PON Operating at Symmetric Rate of 1.25 Gb/s with High Reflection Tolerance

We investigate the effects of back-reflection in RSOA-based WDM PON utilizing the remodulation technique. The downstream signal is likely to have severe reflection tolerance (-42 ~ -35 dB), which highly depends on the RSOA gain.

Reflection tolerance of RSOA-based WDM PON

We report on the recent progresses in the wavelength-division-multiplexed passive optical network (WDM PON) based on the reflective semiconductor amplifiers (RSOAs), including the enhanced operable temperature range, effects of reflection and its mitigation methods, 10 Gb/s operation by using electronic equalizers and forward error-correction (FEC) codes, and increased maximum reach to ≫100 km by using a novel self-homodyne receiver.

Recent progresses in RSOA-based WDM PON

We investigate the effectiveness of optical frequency dithering to enhance the reflection tolerance of RSOA-based WDM PONs. We show that the reflection-induced penalty can be substantially reduced in both directions by the same dithering frequency.

Reflection tolerance enhancement of RSOA-based WDM PON by using optical frequency dithering

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