Research Of Optical-Signal-To-Noise-Ratio Monitoring Tecniques Based On Mach-Zehnder Interferometers

Monitoring optical signal performance in installed optical networks is of key importance to manage the transparency of the networks and locate the failure positions in the networks. Among kinds of characteristics, optical-signal-to-noise ratio (OSNR) is one of the most important diagnostic characteristics to be measured for impairment-aware dynamic routing and intelligent compensation networks, especially in wavelength-division-multiplexed (WDM) reconfigurable networks. Therefore, it is important to have a reliable method to monitor OSNR over a large range of signal to noise ratio. Traditionally, the OSNR of a channel is determined by measuring the noise level between channels and performing a linear interpolation in order to determine the noise level at the channel frequency, i.e., out-band OSNR measurement. However, in a dynamic WDM network with intermediate filtering in the optical add-drop modules (OADM), adjacent channels may have vastly different OSNR values, making these out-band OSNR measurement methods invalid. Therefore, there have a number of techniques been reported to monitor the in-band OSNR.This dissertation mainly focuses on the optical performance monitoring, especially the in-band OSNR monitoring techniques. The main research efforts are summarized as follow.1. A simulation platform of an OSNR monitor based on a single Mach-Zehnder Interferometer (MZI) is demonstrated. It was found that in the simulation the choices of parameters affect the performance of OSNR monitor significantly, and there is a relationship among OSNR range, Δτ, ΔDsig, FSR, OBPF and α. The investigated input signals were10Gb/s and40Gb/s NRZ/RZ33/RZ50/RZ66-OOK/DPSK signals. To optimize the performance of the MZI-based OSNR monitor, we studied the impact of delay time (Δτ) between the arms of interferometer, and found that the monitoring range of the OSNR monitor can be improved by up to10dB for different signals if choosing appropriate Δτ.2. A novel scheme by using two parallel Mach-Zehnder interferometers to monitor the in-band OSNR is proposed for the first time. This scheme, which comprises a pair of parallel MZIs with different optical delays, maintains the advantage of all MZI-based OSNR monitor which is immune to the influence of the CD, PMD and polarized noise. However, the superiority of this scheme is no requirement of prior knowledge of the signal amplitude autocorrelation function, and unlike the other interference based systems, the proposed scheme can be integrated. The two-MZIs-based monitor allows measurement of OSNR ranging from10to28dB for10Gb/s NRZ-OOK/RZ33-OOK/NRZ-DPSK/RZ33-DPSK signals and40Gb/s16QAM signal within an accuracy of±0.5dB. By verifying the fabrication feasibility of the two-parallel-MZIs in simulation, we found that our scheme can be manufactured in semiconductor devices, which is potential to be integrated, thus makes the scheme practical in the future high-speed optical networking applications.3. A novel and robust in-band OSNR monitoring scheme based on two-section Lyot-Sagnac interferometer is proposed and experimentally demonstrated. The schematic of the proposed OSNR monitor is composed of a length of PMF and a PMF-pigtailed phase modulator. With different polarization alignment between the PMF and the phase modulator, optical signals passing in two directions experience different time delays, due to the intrinsic birefringence inside the loop induced by the PMF and the phase modulator. This monitor allows measurement of OSNR ranging from5to25dB for40Gbaud NRZ-QPSK signals within an accuracy of±0.5dB. Experimental results also show that the effect of CD, PMD and the state of polarization of noise on the OSNR measurement can be neglected. By introducing two different time delays in the Sagnac loop, the scheme requires no prior knowledge of the noise-free coherence properties of the signal, which makes it practical. Further investigation proved that this OSNR monitor is not affected by low input optical power and is wavelength (C band) independent. Due to the fact that the OSNR monitoring scheme is wavelength insensitive, we believe that the scheme can be used to monitor OSNR of multiple channel signals in WDM network.