In order to increase network capacity and achieve longer transmission distance, advanced optical performance monitoring and mitigation techniques provide promising solutions. These techniques can improve the control of transmission and physical layer fault management, and they are essential in building a high capacity and reliable all-optical network, which are expected to be transparent and dynamic reconfigurable. However, technology challenges exist to achieve these goals. In this dissertation, we propose and demonstrate three novel techniques for optical performance monitoring and impairment mitigation. The major contribution of this dissertation are summarized as follows.
In Chapter 2, an adaptive post-detection method is proposed for chromatic dispersion monitoring. The method uses an optical delay-and-add filter, whose two arms have a differential optical delay equal to a half period of a pilot tone or a half-data-bit/one-data-bit period. This method does not require a pilot tone if the transmitted data has a symmetrical spectrum with respect to its optical carrier.
Adaptive feedback schemes, such as a scheme to accurately align the DAF quadrature point with the optical carrier of a monitored wavelength and a scheme to automatically adjust a VDC based on the monitored clock or pilot-tone power, are proposed to form a complete dispersion equalization apparatus. We have shown that the post-detection scheme works well even when a residual chirp exists due to a finite MZ modulator DC extinction ratio or SPM. The proposed scheme was also verified by VPI simulation to work well for various modulation formats such as NRZ, RZ50, CS-RZ, RZ-DPSK and CSRZ-DPSK. The dependence of the dispersion monitoring window and dispersion resolution on data formats has also been thoroughly studied.
In Chapter 3, we have experimentally demonstrated an automatic timing alignment method for an RZ-DPSK transmitter using an optical frequency discriminator. Compared with previously published monitoring schemes, our proposed method achieved a significantly improved monitoring power dynamic range of ~17.5 dB within a timing alignment range of a half-bit period. An additional advantage of this method is that the optical frequency discriminator can also serve as a wavelength locker if the discriminator is temperature stabilized.
In Chapter 4, we have also demonstrated, both analytically and experimentally, that by offsetting the bias point of a MZI modulator from its inflection point, one can
obtain sufficient CSOs from the MZI-based transmitter to compensate those CSOs generated from an in-line SOA. The SOA carrier-modulation-induced CSOs were suppressed by as much as 9 to 16 dB over the entire CATV band (from 50 to 550 MHz). We believe that the same technique can also be applied to electro-absorption modulators.
The research can be studied consequently in following aspects.
(a) We have demonstrated our proposed chromatic dispersion monitoring technique by 10.61 Gb/s RZ signal and a half-bit-delay DAF. In addition, one published result [20] shows its feasibility using 10 Gb/s NRZ signal and an one-bit delay DAF. It is worthy to carry out experiments for various DAF differential delay and modulation formats, which have attracted much attention recently.
(b) For our proposed alignment monitoring technique for pulse carver and data modulator by using an optical frequency discriminator, the demonstrated experiment shows flat-top response of an optical filter will degrade the monitored power dynamic range, and therefore make the monitoring results more sensitivity to the filter center frequency stability. In order to increase the detuning range between the optical carrier frequency and center wavelength of the optical frequency discriminator, other optical filters can be used to replace the thin-film filter in our previous experiment. In our simulation, both Fabry Perot and Gaussian filters are good candidates for achieving this goal, as shown in Fig. 5.1. An experiment can be performed to verify its feasibility.
-40 -30 -20 -10 0 10 20 30 40 0
5 10 15 20 25 30
Dynamic Range (dB)
Frequency Detuning (GHz)
Gaussian Filter FP Filter
Fig. 5.1 Simulated Monitored power dynamic range (MPDR) as a function of the frequency detuning between the CW laser and the center frequency of the optical filter using Gaussian filter and Fabry Perot filter, respectively.
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