Due to the explosive growth of the data traffic caused by various types of data/multimedia services, it is critical to rapidly increase the transmission capacity of the fiber-optic communication system. For this purpose, the wavelength-division-multiplexing (WDM) technique has been widely utilized. However, since the capacity of the WDM system is limited by the gain bandwidth of the optical amplifier, it is now necessary to increase the amplifier’s bandwidth and improve the spectral efficiency (i.e., utilize high-speed WDM signals with a narrow channel spacing). To achieve these objectives, various state-of-the-art techniques have been proposed such as the distributed Raman amplifier (DRA), differential multilevel phase-shift-keying (DxPSK) format, polarization-division-multiplexing (PDM) technique, and digital coherent receiver. However, if these new techniques are adapted to the next-generation fiber-optic transmission system, there could be many unexpected problems such as the multi-path interference (MPI) noise and the optical signal-to-noise ratio (OSNR) imbalance between the polarization states, etc. Thus, to ensure the reliability of the next-generation fiber-optic transmission system, it is needed to develop the optical performance monitoring (OPM) capable of monitoring these new parameters. In this dissertation, various monitoring techniques have been proposed and demonstrated for this purpose. Firstly, the effects of the MPI noise and the amplified spontaneous emission (ASE) noise generated in DRA systems have been evaluated. For simultaneously monitoring both of these noises, a new technique based on the polarization-nulling method has been proposed. Secondly, the characteristics of the DxPSK signals have been investigated. By using this result, monitoring techniques for OSNR and chromatic dispersion of DxPSK signals have been proposed. Finally, a new OPM technique based on the asynchronous amplitude and phase histograms has been proposed. The performance of this technique has been evaluated for both single- and dual-polarization signals.
In DRA systems, the system’s performance could be limited by the MPI noise as well as the ASE noise. Thus, for the proper operation of DRA systems, it would be desirable to have the capability of monitoring both of these noises. However, the MPI noise, which is a replica of the optical signal, has been considered as a difficult parameter to monitor. In this dissertation, a new technique which can monitor simultaneously the ASE and MPI noises is proposed. This technique is based on the polarization-nulling technique and utilizes the fact that the degree-of-polarization (DOP) of the MPI noise is 1/9 and its state-of-polarization (SOP) is identical with that of the signal in DRA systems (assuming that the MPI noise is generated mostly by double Rayleigh backscattered signals). Thus, if the signal is nullified by using the polarization-nulling technique and the in-band noise power is measured, the measured value should have contributions from both the ASE and MPI noises. However, these contributions can be easily separated by using the deterministic DOP of the MPI noise (since the ASE noise is, in principle, unpolarized). For demonstration, the MPI noises included in the 10-Gb/s and 40-Gb/s signals (modulated in NRZ and RZ formats) are measured at various OSNR values in the ranges of $15 dB \sim 30 dB$. In addition, the MPI noise is generated in a real DRA system and measured as a function of the on-off gain of the Raman amplifier. The results show that the proposed technique can accurately measure both the ASE and MPI noises in DRA systems regardless of the data rates and modulation formats.
Recently, there have been growing interests in the phase-modulated formats such as differential phase-shift-keying (DPSK) and differential quadrature phase-shift-keying (DQPSK) for the use in the next-generation high-speed optical network. For the efficient operations of these networks, it would be highly desirable to have the capability of monitoring the ‘in-band’ OSNR. However, the conventional interpolation technique cannot be used in such networks due to the extremely high spectral density of these phase-modulated signals. For the phase-modulated signals, this problem can be solved by using the OSNR monitoring technique based on the radio frequency (RF) spectrum analysis. This technique utilizes the fact that the OSNR is proportional to the signal-ASE beat noise. In principle, DPSK/DQPSK signals have no intensity modulation component. Thus, when the OSNR is high, the signal-ASE beat noise can be estimated directly from the RF spectrum. However, when the OSNR is low (which is the most important case in practice), the RF noise level becomes no longer proportional to the OSNR due to the contribution of the ASE-ASE beat noise. Thus, to compensate for this nonlinear relation between the measured noise level and the true value of the OSNR, a delicate calibration should be performed by using the values obtained by experiments. In this dissertation, a new OSNR monitoring technique for the phase-modulated signal by using the self-heterodyne detection with an optical frequency shifter is demonstrated. This technique can measure not only the background noises but also the beat signal at the shifted frequency. Thus, by using these two measured quantities, the OSNR can be estimated accurately without the complicate calibration. The results show that the proposed technique can accurately monitor the OSNR’s of the phase-modulated signals (such as DPSK and DQPSK signals) even when the signals are polarization-scrambled. The performance of this technique is not sensitive to both chromatic dispersion and polarization-mode dispersion (PMD).
Chromatic dispersion is one of the most important limiting factors in the high-speed optical network. However, it has been reported that the chromatic dispersion can be substantially changed by seasonal temperature variations. In addition, in a dynamic WDM network, each channel can be exposed to different amount of chromatic dispersion whenever the network is reconfigured. Thus, there have been numerous efforts to overcome this limitation by developing an adaptive dispersion compensation technique. However, for this purpose, it is vital to have the capability of monitoring the chromatic dispersion accurately. In this dissertation, a simple technique to monitor the chromatic dispersion of DxPSK signals by using an AM pilot tone generated by using a broadband light source (BLS) such as a reflective semiconductor optical amplifier (RSOA) is proposed. The AM pilot tone, obtained by modulating the RSOA gain with a small sinusoidal current, is automatically added to every WDM channel simply by coupling the output of the RSOA to the transmission fiber at the transmitter’s site. Thus, unlike the conventional pilot-tone based monitoring techniques, the proposed technique does not require any modifications of the WDM transmitters. In addition, since the effect of chromatic dispersion is more sensitive for the optical signal with a broader spectral width, it is possible to utilize a low-frequency pilot tone compared to the conventional AM-tone based techniques. As a result, the performance of the proposed technique can be less sensitive to PMD. In addition, since the BLS has a broad spectral width ($\sim 30 nm$), the proposed technique can monitor the chromatic dispersion of multiple WDM signals by using only one BLS (and, consequently, single pilot tone).
It is now apparent that multilevel modulated signals, such as the QPSK and quadrature amplitude modulation (QAM) signals, will play an important role in the next-generation high-speed optical networks. Thus, for the efficient operation and management of these networks, we should be able to monitor the quality of multilevel signals directly in the optical layer. So far, the OSNR has been used as one of the essential monitoring parameters since it can be directly related to the bit-error rate (BER) of the conventional intensity-modulated binary signal. However, in the case of the multilevel signal, the OSNR may not be able to properly represent its quality by itself since it can be seriously affected by the nonlinear phase noise as well (caused by the use of the high signal power required for the transmission of multilevel signal). In this dissertation, a new optical performance monitoring technique for the multilevel modulated signals based on the amplitude and phase histograms is proposed. This technique utilizes a front-end of the intradyne receiver and the asynchronous delay-tap sampling method. Thus, there is no need to use the complicate optical phase-locked loop (since the intradyne receiver utilizes a free-running laser as LO). In addition, due to the use of the asynchronous delay-tap sampling method, this technique can acquire only the data located at the center of the bit of the monitoring signal without using the synchronized trigger signal. Using the proposed technique, the amplitude and phase histograms of the monitoring signal are obtained first by using the delayed and non-delayed parts of the in-phase and quadrature components. From these histograms, the amplitude and phase Q-factors are evaluated. In addition, these Q-factors can be used to estimate the BER of the multilevel modulated signal. For a demonstration, the amplitude and phase Q-factors of 20-Gb/s QPSK signal are measured by using the proposed technique and its BER is estimated by using these Q-factors. The results show that the estimated BERs agree well with the directly measured values.
Recently, there have been numerous efforts to increase the transmission capacity of fiber-optic communication systems by utilizing the polarization-division-multiplexing (PDM) technique. However, when this PDM technique is used, the system’s performance can be degraded by the polarization-dependent loss (PDL) since it can cause an imbalance between the OSNRs of two polarization tributaries. In fact, it has been already reported that this PDL-induced OSNR imbalance can lead to the overestimation of the signal’s quality if the PDM signal is monitored simply by measuring its total OSNR of two polarization tributaries. Thus, for the proper operation of such PDM signals in a modern dynamic network, it is necessary to monitor the individual OSNR of each polarization tributary of the PDM signal. In this dissertation, a new technique for monitoring the individual OSNR of each polarization tributary of the PDM-QPSK signal based on the asynchronous delay-tap sampling technique is proposed. The proposed technique utilizes a polarization-diversity front-end, an LO laser, and the electronics involved in the asynchronous delay-tap sampling process. This technique has the advantages of using a coherent receiver, while the speed requirements for the electronics (such as the analog-to-digital converter and real-time memory) are substantially reduced. However, it is not simple to utilize the asynchronously sampled data for the polarization demultiplexing of the PDM signal because the constant modulus algorithm (CMA) does not work well without using consecutive data symbols. To solve this problem, a new algorithm is proposed, designated as “unitary matrix transformation method,” to separate two polarization tributaries without the prior knowledge of the signal. By using this algorithm, the CMA can be utilized even for the asynchronously sampled data. For a demonstration, the OSNRs of two polarization tributaries of the 40-Gb/s PDM-QPSK signal are monitored by using the proposed technique. The results show that the performance of the proposed technique is not sensitive to the PMD and PDL.
본 논문은 차세대 광통신망에 적용될 가능성이 높은 최신의 기술들을 논의하고, 차세대 광통신망의 효율적인 운영/유지/관리를 위한 새로운 감시 기술들을 제안하고, 차세대 광통신망에의 적용 가능성을 확인하는 것이다. 본 논문의 세부적인 목적과 동기는 다음과 같다.
첫째, 라만 증폭기를 사용하는 경우 증폭기의 이득이 증가할수록 ASE 잡음에 의한 OSNR은 개선되지만 MPI 잡음에 의하여 OSNR이 더 이상 개선되지 않고 오히려 감소하게 된다. 이와 같이 라만 증폭기를 사용하는 시스템에서는 ASE 잡음과 MPI 잡음이 광신호의 성능을 제한하는 요인이 된다. 특히 MPI 잡음은 광신호와 동일한 스펙트럼을 가지는 복제 신호이기 때문에 직접적으로 이 잡음의 크기를 측정하는 것은 매우 어려운 일이다. 본 논문에서는 광신호와 ASE 잡음, MPI 잡음의 서로 다른 편광 특성을 이용함으로써 ASE 잡음과 MPI 잡음을 광신호와 분리하고, 간단한 계산 과정을 통해 두 잡음을 동시에 측정할 수 있는 기술을 제안한다.
둘째, 차세대 초고속 WDM 광통신망의 효율적인 구현을 위해 수신 감도가 뛰어나고, 스펙트럼 효율성이 좋은 광신호의 첨단변조방식의 발전에 따라, 이러한 첨단변조방식을 사용하는 광신호의 OSNR과 색분산의 감시가 필수적으로 요구된다. 기존의 OSNR 감시 기술은 광스펙트럼분석기를 사용하여 광신호 스펙트럼의 바깥 대역에서 ASE 잡음을 측정하고 이를 선형적으로 근사하여 광신호에 포함된 잡음의 크기를 유추하였다. 그러나 이러한 OSNR 감시 방법은 DWDM 시스템에서는 더 이상 유용하지 않으며, 광통신망이 동적으로 발전해감에 따라 이 기술을 대체할 새로운 OSNR 감시 기술들이 요구되고 있다. 따라서, 본 논문에서는 DPSK, DQPSK 변조방식을 사용하는 광신호의 OSNR을 감시하기 위해 self-heterodyne 수신기를 이용하여 측정한 RF 스펙트럼을 분석하는 방법을 제안한다. 그리고 이러한 위상변조방식 광신호의 색분산을 감시하기 위해 BLS에 파일럿 톤을 사용하면서, 시스템의 송신기는 변화시키지 않는 새로운 방법을 제안한다.
셋째, 멀티레벨 위상변조방식 및 PDM 기술의 연구가 활발해 짐에 따라 수신 성능이 뛰어난 디지털 코히어런트 수신기에 대한 연구가 많이 진행되고 있다. 또한 이러한 기술들이 사용되는 차세대 초고속 광통신망을 효율적으로 유지/운영/관리하기 위해서는 광신호의 성능을 제한하는 여러 열화 요인들의 감시가 필수적으로 요구된다. 본 논문에서는 간단한 구조의 코히어런트 수신기와 비동기식 지연 탭 샘플링 방법을 이용하여 광신호의 변조방식에 무관하게 광신호대 잡음비 및 색분산을 감시할 수 있는 새로운 감시 기술을 제안한다. 또한, 제안된 감시 기술을 이용하여 광신호의 성능에 미치는 여러 열화 요인들을 감시할 뿐 아니라, 광신호의 성능을 나타내는 비트오율을 직접 감시할 수 있다.
넷째, 광전송 시스템의 전송용량 증대를 위해 각광을 받고 있는 PDM 기술이 시스템에 사용될 때, 광선로에 존재하는 PDL에 의해서 PDM 신호의 두 편광 축에 있는 광신호의 OSNR은 서로 달라질 수 있다. 그러나 지금까지 제안된 OSNR 감시 기술은 각각의 편광 축을 분리하여 OSNR을 측정할 수 없으므로, 기존의 감시 기술을 이용하여 PDM 신호의 OSNR을 감시하게 되면 PDM 신호의 성능은 실제보다 더 좋게 예측할 수 밖에 없다. 따라서, 앞서 제안된 코히어런트 수신기를 이용한 성능 감시 기술을 이용하여, PDM 신호를 위한 새로운 OSNR 감시 기술을 제안한다.