New methods for measuring and monitoring chromatic dispersion in optical communication systems

This PFC main goal is to introduce new approaches on the chromatic dispersion measurement field, based on the big range of possibilities the setup of a general standard RF-tone modulation chromatic modulation method provides, and to show its good performance pointing towards a real-time on-line monitoring system for optical communication networks. The project’s objectives are defined considering two well-delimited stages. First, we will study some standard RF-tone-addition techniques for measuring chromatic dispersion, specifically the Modulation Phase Shift Method (MPSM) [2] and the Peucheret`s Method [3]. We will analyze their operating principles, recognize all the variables involved in their basic configurations and evaluate their performances under different measurement conditions. We will also study the implications of real-time on-line monitoring of chromatic dispersion in optical networks. We have to consider that the test signal has to travel together with the data; therefore, it is a priority to keep the optical carrier unaltered in the transmission and reception procedures. This background will help us to identify the main drawbacks of both methods which motivate the proposal of a new improved technique based on a similar mathematical basis but with better performance in terms of accuracy and cost trade-off. The general features of this new approach will be exposed on a basic setup designed for a laboratory environment, so that we can contrast it with the conventional techniques. This method dubbed Asymmetric Modulation Bias-Controlled Method (ABCM) will focus on RF modulated signal amplitude and will take advantage of its direct relation with chromatic dispersion. One of the basic building blocks of these standard methods is the device that imposes the RF pure-tone modulation to the optical signal, namely the Mach-Zehnder interferometric modulator usually in the conventional push-pull configuration and biased at the quadrature point. In the context of the new improved CD measurement methods, we will observe how the Mach-Zehnder modulator Bias Voltage concept gains relevance; becoming the main variable to be handled by the use of a dual drive Mach-Zehnder modulator in asymmetric configuration. Finally, we will analyze this ABCM method performance while some fixed parameters (RF Frequency, Nominal Dispersion, resolution) take different values in order to find out the optimum operating conditions. The problem when trying to apply the ABCM to the real-time on-line monitoring of optical networks is that it relies in the eventual cancellation of the optical carrier which in a network monitoring application is shared with the data and it is essential for a proper data recovery. We must find an alternative where this optical carrier cancellation is not essential for the monitoring function and that would be the ABCM-SC (SC for suppressed carrier) Therefore, on a second stage, we will focus on giving this new perspective about dispersion measurement a direct application in optical communications field. We will restructure the ABCM into a practical dispersion monitoring system for optical communication networks. This improved monitoring technique will be based on a proof-of-concept study (no real data transmission considered) to evaluate the method’s performance in terms of accuracy, robustness and adaptability, building the basis for data transmission experiments in future projects. An important aspect to take into account will be the way we carry out the RF tone addition procedure without altering the optical carrier (transmitted data). To accomplish this requirement we will use a Bessel function analysis to achieve a carrier-suppressed modulation of the RF tone, which introduces another important handling parameter: the RF Tone Amplitude. We will also be concerned about isolating the emitter part (where data is transmitted) from the monitoring point (where dispersion is measured), but at the same time complementing each other to operate in a real-time situation. Finally, we will study the requirement of including the second RF harmonic detection together with the first harmonic as it adjusts better to a real-time monitoring system and increases the accuracy level in chromatic dispersion measurement