The optical performance monitor (OPM) has become an indispensable component in the modern optical transport network implemented with reconfigurable optical add/drop multiplexers (ROADMs). The major applications of OPM in such a dynamic network include (1) adaptive impairments compensation, (2) dynamic resources allocation, (3) link setup, control and optimization, (4) automatic fault detection and localization, and (5) resilience mechanism activation. These applications have become increasingly important as the conventional capacity and distance limitations are mitigated substantially due to the recent introduction of the advanced modulation formats and digital coherent receivers.
The main objective of this dissertation is to develop practical OPM techniques for the next-generation ultrahigh-speed optical transport networks. For this purpose, various OPM techniques have been proposed and demonstrated including (1) a simple optical signal-to-noise ratio (OSNR) monitoring technique based on RF spectrum analysis for the polarization-division-multiplexed quadrature phase-shift keying (PDM-QPSK) signals, (2) a novel phasor monitor based on the software-based synchronization technique, which can be used to obtain the constellation diagram and to control the modulator’s bias voltages in the optical trans-mitter, (3) an asynchronous sampling-based monitoring technique, which is transparent to data rates and modulation formats and capable of monitoring multiple network impairments simultaneously, and (4) a nonlinearity-tolerant OSNR estimation technique based on amplitude correlation function between two symbols for the use in the coherent optical transmission systems.
The objective of the first chapter is to provide a comprehensive overview of the OPM technology, in-cluding its applications in various optical systems and networks. In the following four chapters, various OPM techniques are proposed and demonstrated. In particular, the technical merits and limitations of four specific OPM techniques are discussed in each chapter. The last chapter summarizes the dissertation and provides future perspectives and challenges of OPM techniques.
In the second chapter, the principle of the OSNR monitoring technique based on RF spectrum analysis for QPSK signal is discussed. The QPSK format has become widely used in the high-speed long-haul optical networks. Thus, for the proper management of these networks, it is necessary to monitor the quality of the QPSK signal directly in the optical layer. The OSNR is one of the most important performance indicators that can be used for this purpose. Previously, an optical spectrum analyzer (OSA) has been typically used for monitoring the OSNR of the transmitted signal. However, since this technique merely estimates the OSNR by measuring the noises in between the channels, it cannot be used for the accurate monitoring of the ‘in-band’ OSNR. To solve this problem, the possibility of using the OSNR monitoring technique based on the RF spectrum analysis for the QPSK signal is investigated. Since the performance of this technique is bound to be affected by amplitude modulation components (generated by Mach-Zehnder modulator (MZM)), the effect of this residual amplitude modulation components on the monitoring performance is evaluated. In addition, the approach to improve the monitoring accuracy is discussed.
In the third chapter, a novel phasor monitor is proposed and its applications are discussed. Due to the recent developments in the advanced modulation formats such as multilevel phase-shift keying (PSK), it is needed to evaluate the constellation diagram of the phase-modulated signals on the complex plane. Thus, several types of phasor monitors have been proposed by utilizing both coherent and non-coherent detection techniques. Among them, the phasor monitor based on the non-coherent detection technique appears to be attractive since it can be implemented by using only the passive optical components. However, this phasor monitor still requires the use of high-speed analog-to-digital convertors (ADCs) and memories operating at the speed faster than the symbol rate. As a result, it may be difficult to realize even the non-coherent phasor monitor cost-effectively. To solve this problem, the use of software-based synchronization technique is proposed. This enables us to utilize low-speed, free-running ADCs in the phasor monitor. In addition, the proposed technique will be compared with previous techniques in terms of technical merits and monitoring performance. Then, a phase-adjustment-free phasor monitor by using the proposed technique is implemented. It is also provided that the applications of the proposed phasor monitor, including bit-error rate (BER) estimation and automatic bias control. In particular, the principles of the automatic bias control for optical QPSK and 16QAM transmitters are discussed in detail. To ensure the long-term stability, it is essential to optimize the bias voltages of the modulators by using automatic bias controllers. In the proposed technique, the feedback signal for the bias control is obtained by calculating the difference between the ideal phasor and the phasor under improper bias conditions. Thus, this technique can be applied to any modulation formats.
Since the future optical networks should be able to support the optical signals modulated in different formats and data rates, the ability to accommodate the various modulation formats and multiple data rates is one of the most desired features as a future OPM module. Thus, in the fourth chapter, the current data rate and modulation format-transparent OPM techniques are discussed and two different novel techniques are proposed. Firstly, amplitude histogram-based monitoring technique is discussed. The OPM technique based on the asynchronous amplitude histogram (AAH) is attractive since it is not only transparent to the data rate and modulation format, but also capable of monitoring multiple impairments. However, since this technique samples the signal’s intensity asynchronously, the AAH is bound to have some unwanted samples collected at the symbol’s transitions. In this chapter the problem associated with the samples collected at the symbol’s transitions is discussed. To solve this problem, it is also proposed to utilize the software-based synchronization technique for the OSNR and CD monitoring of the non-return-to-zero (NRZ)-QPSK and NRZ-16QAM signals. This technique enables us to obtain the synchronous amplitude histogram (SAH) from the asynchronously sampled data without using the clock extraction circuitry. Thus, the proposed technique is transparent to the data rates and modulation formats. Secondly, the data-rate transparent CD monitoring technique for DQPSK signal is proposed using the software-based clock phase-shift detection technique. DQPSK format with the direct-detection technology is considered as a plausible solution for the metropolitan and regional networks due to its low power consumption and low cost. However, for the proper operation of these networks, it is essential to utilize the dynamic CD compensation technique as the CD can be continuously changed due to the variations in the environmental conditions such as the ambient temperature as well as the network reconfigurations. Thus, it is inevitable to monitor the residual CD for the use of the DQPSK format. For this purpose, various optical CD monitoring techniques will be reviewed in terms of their technical merits and limitations. In addition, a new CD monitoring techniques is proposed based on software-based clock phase-shift detection which enables to monitor sign information of the residual CD, as well as data rate transparent operation. The principle of this technique is discussed and the effectiveness of the proposed CD monitoring technique is demonstrated by using 40-Gb/s NRZ- and return-to-zero (RZ-) DQPSK signals. It is also shown that the proposed technique is robust to effects of OSNR and polarization mode dispersion (PMD).
In the fifth chapter, OPM techniques in long-haul coherent optical transmission systems are discussed. The PDM-QPSK modulation format and digital coherent detection have become a standard solution for the transport of 100-Gb/s signals in the long-haul optical networks. For the efficient operation and management of these networks, it is highly desirable to monitor the OSNR of the PDM-QPSK signal since it can be used not only for the estimation of the BER performance but also to identify the optimum coefficients of the digital signal processing (DSP) algorithms at the coherent receiver. However, these long-haul coherent optical net-works have some unique characteristics such as no dispersion compensation and fiber nonlinearity. Thus, in view of the system requirements, the OSNR monitoring techniques for coherent optical transmission systems are discussed. In particular, the monitoring issues relevant to fiber nonlinearities are discussed in detail and nonlinearity-tolerant OSNR estimation technique is proposed. The proposed technique utilizes amplitude correlations between the polarizations as well as the neighboring symbols to remove the contribution of the nonlinearity-induced signal distortions. By using this technique, the OSNR can be estimated accurately without the prior knowledge of the system configurations such as the transmission distances, number of WDM channels, and fiber launch powers.
조절형광분기결합장치를 사용하는 현대의 광전송망에서 광신호의 품질을 보증하기 위해 광신호의 성능을 감시하는 것은 매우 중요하다. 광신호의 성능 감시 기술은 다양한 목적으로 사용 될 수 있다. 광신호의 성능 감시 기술은 채널의 장애요인을 자동으로 보상하고, 능동적으로 채널 자원을 할당하며, 광전송망을 유지/보수 및 최적화하는데 사용할 수 있다. 또한 시스템에 장애가 발생하는 경우 장애의 위치와 원인을 감지하고 이를 자동으로 복구하는데 사용할 수 있다. 이러한 광신호의 성능 감시 기술의 중요성은 첨단 변조 방식과 디지털 코히어런트 수신기가 사용됨에 따라 복잡도가 증가하는 광전송망에서 더욱 증대되고 있다.
본 논문의 주된 목적은 차세대 초고속 광통신망을 위한 실용적인 광신호의 성능 감시 기술을 개발하는 것이다. 이를 위해, 다양한 광신호의 성능 감시 기술을 제안하고 그 성능을 검증한다. 먼저 RF 스펙트럼 분석법에 기반한 PDM-QPSK 신호의 광신호대 잡음비 감시 기술에 대해 논의한다. 또한 소프트웨어 기반의 동기화 기술을 이용한 페이저 감시 방법을 설명하고 이를 이용하여 광신호의 constellation diagram과 비트오율을 감시하고 송신기의 바이어스 전압을 제어하는 방법에 대해 논의한다. 다음으로 비동기 샘플링 기술을 이용하여 전송 속도와 변조 방식에 무관하게 적용 가능하고 복수의 네트워크 장애요인을 동시에 감시할 수 있는 기술에 대해 논의한다. 마지막으로 코히어런트 기반의 장거리 광전송망에 적합한 광신호대 잡음비 감시 기술에 대해 논의한다. 이 기술은 심볼 간의 진폭 상관관계를 이용하여 광섬유 비선형성이 존재하는 경우에도 광신호대 잡음비를 정확하게 추정할 수 있는 기술이다.
