An In-Band OSNR Monitoring Method for Polarization Multiplexed QPSK Signals Using Stokes Parameters

Abstract

The optical signal-to-noise ratio (OSNR) is an important parameter for measuring signal quality in optical communications systems. Due to the recent development of polarization multiplexed systems, Nyquist filtered systems and reconfigurable systems, the traditional optical spectrum analysis method for estimating the OSNR cannot be used. Several other methods have been proposed, with various shortcomings, so there is a need for other methods. In this work, an in-band OSNR monitoring method for polarization multiplexed signals based on a Stokes polarimeter has been investigated through theoretical studies, simulations and measurements. For the measurements, a 90 degree hybrid based polarimeter was constructed and used for measurements on noise loaded 28 GBd DP-QPSK signals. The impact of chromatic dispersion (CD) and polarization-mode dispersion (PMD) on the method was also investigated. The method was successfully used to estimate the OSNR within 1dB for OSNR values up to 25 dB. Through simulations it was shown that the tolerable amount of CD increased if the bandwidth used for the ADC was decreased. In the measurements of this work, a bandwidth of 10 MHz was used, which should tolerate over 1000 km of SMF with D = 17 ps/nm/km according to the simulations. However, the method was shown to be sensitive to PMD, tolerating a differential group delay of less than a tenth of the symbol time. Also developed in this work was a method for compensating for a non-ideal 90 degree hybrid. A provisional patent application has been filed for the method.