648 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 4, FEBRUARY 15, 2013 Performance Monitoring and Measurement Techniques for Coherent Optical Systems Bogdan Szafraniec, Senior Member, IEEE, Senior Member, OSA, Todd S. Marshall, Member, IEEE, and Bernd Nebendahl (Invited Tutorial) Abstract—Modern spectrally efficient optical communication systems utilize polarization-multiplexed coherent transmission in complex modulation format. Coherent receivers used in these systems measure the amplitude and phase of the optical signals for both orthogonally polarized components carrying information. Knowledge of the amplitude and phase of the optical field, in combination with digital signal processing, gives the receiver an inherent metrology and performance monitoring capability. As the optical signal propagates from the transmitter over optical fiber to the receiver, a signal transformation and degradation is expected. The receiver observes the properties of the transmitted optical signal as degraded by the impairments of the transmission medium. The details of monitoring optical signal parameters and link impairments are the focus of this paper. The optical signal parameters include polarization state and residual carrier phase; optical link impairments include chromatic dispersion and polar- ization mode dispersion. Two distinct techniques are presented: one based on Stokes space analysis, and the other on Kalman filtering. The Stokes space techniques are modulation-format independent and do not require demodulation. The Kalman fil- tering provides optimal estimation of the physical quantities that describe the optical signal and the optical medium. Index Terms—Chromatic dispersion (CD), optical fiber disper- sion, optical fiber testing, optical modulation, optical polarization, Stokes parameters. I. INTRODUCTION O PTICAL performance monitoring estimates parameters of the optical signal or of the optical channel in order to maintain the operation and to predict the performance of the op- tical transmission system [1]. Among multiple parameters that are often monitored, the signal parameter that is of great interest is the optical signal-to-noise ratio (OSNR). The OSNR changes as the signal is attenuated and noise is accumulated. The OSNR can be used to predict the bit error rate (BER) under the assump- tion of additive white Gaussian noise dominated channels. The Manuscript received June 06, 2012; revised August 01, 2012; accepted Au- gust 02, 2012. Date of publication August 08, 2012; date of current version January 16, 2013. B. Szafraniec and T. S. Marshall are with the Measurement Research Laboratory, Agilent Technologies, Santa Clara, CA 95051 USA (e-mail: bogdan_szafraniec@agilent.com; marshall-public@agilent.com). B. Nebendahl is with the Photonic Test and Measurement Division, Agilent Technologies, 71034 Böblingen, Germany (e-mail: bernd_nebendahl@agilent. com). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JLT.2012.2212234 BER serves as a direct measure of the system performance. An alternative measurement to the OSNR is the Q-factor. However, the latter is becoming more challenging to estimate in the sys- tems that utilize complex modulation formats with complicated eye diagrams. Recently, in coherent systems, efforts were made to use error vector magnitude (EVM) as an alternative way of predicting the BER [2], [3]. The EVM is estimated from the demodulated signals often illustrated in constellation plots. Im- pairments of the optical link that lead to degradation of perfor- mance are equally important. Among them are chromatic dis- persion (CD), polarization mode dispersion (PMD), and polar- ization-dependent loss. The classical approaches for measuring optical signal and optical link (channel) include spectrum anal- ysis, polarimetry, AM and PM pilot tones injection, and mea- surement [1]. Often the optical performance monitor is associ- ated with an external device that is not a part of the transmis- sion system. The external monitor is capable of estimating one or more parameters of interest. The concept of performance monitoring has dramatically changed since the adoption of coherent optical transmission. In coherent systems, many impairments such as CD and PMD can be estimated and compensated within the receiver itself [1], [4], [5]. Thus, the optical receiver can be considered to serve as an optical performance monitor in addition to its primary function of receiving data. A conceptual diagram of the coherent receiver is shown in Fig. 1. This diagram includes many functions that are commonly performed in the processor of the receiver, including transformation of the received signal (e.g., polarization alignment) and compensation of impairments (e.g., CD and PMD compensation). The coherent receiver has also become part of some test instruments that are designed to analyze the quality of optical signals and to estimate the parameters of an optical link. These test instruments, unlike the receivers in communication systems, are designed to perform primarily metrological functions. Consequently, they employ tools that are optimized from the metrology point of view. In metrology, the estimation of the optical signal parameter or of the impairment is often as important as its compensation. This differs from the classical receiver where accomplishment of equalization is often sufficient. Therefore, the techniques that are described in this paper are selected to match the needs of metrology of coherent optical signals and optical links rather than of data reception alone. This is the key reason justifying the use of Stokes space and the Kalman filter, the two subjects of this study. 0733-8724/$31.00 © 2012 IEEE