Chromatic dispersion monitoring for next generation optical networks
Date of Issue2008
School of Electrical and Electronic Engineering
Chromatic dispersion (CD) is one of the major factors limiting optical transmission distance. Therefore, it is essential to monitor the residual CD of individual wavelength channels so as to ensure that the residual CD does not exceed the designed tolerance. This thesis documents the investigations of cost-effective and high performance CD monitoring techniques. Different techniques for CD monitoring are thoroughly reviewed in the thesis. Among them, tracking the power of radio frequency (RF) tones is relatively simple and effective for CD monitoring. However, this technique may be influenced by the power loss or gain of optical transmission links. To address this issue, an improved CD monitoring method based on the RF power ratio is proposed. It is shown that, with an inserted dispersion offset, the CD monitoring range and accuracy (or sensitivity) can be greatly improved by selecting appropriate RF frequencies and dispersion offsets. Adequate modulation index of the RF tone should be chosen so as to acquire a large monitoring range with a small power penalty. By employing an RF tone removal scheme, the power penalty induced by the RF tones can be further reduced. Polarization mode dispersion (PMD), self-phase modulation (SPM), cross-phase modulation (XPM) and chirp parameter of external modulator may influence the accuracy of the CD monitoring. The effects of PMD, SPM, XPM and chirp on CD monitoring are investigated in the thesis. It is shown that the presence of these effects induces significant CD monitoring errors. To tackle this problem, a new CD monitoring technique is proposed, which can effectively suppress the impacts due to PMD, SPM, XPM and chirp. Both experiments and simulations show that the PMD, SPM, and XPM effects can be eliminated or suppressed by optically sideband filtering and DSB/SSB RF power ratio detection, and the monitoring error induced by the small chirp fluctuation can be suppressed using two RF tones and a CD offset. Other degradation effects on the CD monitoring are also discussed. Furthermore, the thesis presents a new CD monitoring technique for DPSK signals by detecting the power of the clock tone as well as an added RF tone. A new transmitter configuration is proposed for simultaneous generation of an RF tone and a DPSK data signal using only a single dual electrode Mach-Zehnder modulator (DE-MZM). The RF tone induced power penalty to the DPSK signal is investigated. By introducing one bit delay interferometer (DI) used for DPSK demodulation, the RF tone influence can be suppressed. Experimental and simulation results show that the technique can significantly improve the CD monitoring capability and the DI can greatly suppress the RF tone induced power penalty if the RF tone has a frequency close to the half data bit-rate. The proposed transmitter configuration is applied to optical label switching systems, and it is shown that the use of the proposed transmitter can simultaneously generate a DPSK payload and an SCM label. This SCM label can be exploited for CD monitoring. Investigations on the impacts of the subcarrier frequency, label bit-rate, and modulation index on both the DPSK payload and the label demonstrate that the technique provides good transmission performance for both the DPSK payload and the label signal without requiring an additional label eraser.
DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics