Effects of Optical Filtering on Signal Characterization in Coherent Systems

Abstract

There is a big need for standardizing signal quality tests of transmitters for 100 Gb/s PM-QPSK fiber optic communication systems. Such systems are currently being deployed and still, there is no standard for transmitter testing. Several approaches for doing this in an efficient manner have been proposed. These include different methods of detecting the signal and different approaches for filtering the signal. The signal can be sampled electrically or optically, both over- and undersampled. The choices of filtering range from optical and electrical hardware filters to filtering in digital signal processing. All filtering and sampling techniques are not compatible. The focus in this project will be on optical filtering that can be used together with optical sampling. The reason for using optical sampling is to achieve high measurement bandwidth. The only filtering technique which is easily applicable when sampling optically is optical filtering. This is because the signal is undersampled. The project is an exploratory study to investigate if optical sampling together with optical filtering is a good approach for signal quality testing. Experiments were made to assess the accuracy of optical filtering by measuring on QPSK signals and comparing to DSP filtered measurements. Impulse response measurements were also made to compare the impulse responses of an optically and DSP filtered system. Also, theoretical work has been done to determine the equivalence to optical filtering in the DSP domain. The accuracy of the optical filtering is found to be sufficient under certain conditions on the filter characteristics. It is found that optical filtering together with optical sampling is a viable method for fast and efficient transmitter testing. The optical filtering method should be included in future standardization studies as an alternative to DSP filtering used in real-time sampling approaches.