The effect of PMD on the three axes of the degree of the polarization (DOP) ellipsoid is discussed in the different
modulation format systems. The results show that the minimum axis of DOP ellipsoids is suitable for a feedback signal
in the compensation systems. In addition, the mitigation performances of DPSK formats are superior to that of OOK
format.
Automatically controlled polarization controllers (PC) are the essential integral parts in an automatic compensator for
polarization mode dispersion (PMD). It is suggested by Prof. M. Karsson et al. at Charlmers University of Technology,
and afterward widely accepted that we need to adjust only two waveplate PC (two degrees of freedom, DOF) for each
stage PMD compensator to complete PMD compensation. It is equivalent to say that only two of degrees of freedom for
each PC are needed to complete the state of polarization (SOP) transformation from any input state into any other state
covering the entire Poincare Sphere. In this paper will take two types of commercial available PC as examples to prove
theoretically and experimentally that except for the reset-free problem it is necessary to adjust at least three instead of
two waveplates in order to transform any input state into any other state covering the entire Poincare Sphere. Therefore
we can achieve complete PMD compensation at least using 3 DOF instead 2 PC in each stage PMD compensator.
A kind of voltage-controlled polarization controller made in electro-optic materials is studied in the experiment. By
changing voltages added on the different plates, the corresponding out states of polarization are gotten which spread all
over the Poincare. Results show that the phase retardation angle in each plate is a quadratic function of the applied
voltages. And the degrees of polarization ellipsoids are obtained with the same the principal state of polarization. PC, a
key element in the communication system, has great advantage for Polarization mode dispersion compensation and
accurate controlling to the states of polarization.
We established the DPSK generating and receiving system, and realized the different kind of DPSK formats. Then we
demonstrated the experiment of automatic PMD compensation in 10Gb/s NRZ-DPSK, RZ33-DPSK, RZ50-DPSK, and
RZ67-DPSK systems. Using the particle swarm optimization algorithm in these DPSK systems, the optical signals were
successfully recovered instantly from PMD induced distortion after compensation.
We study the polarization mode dispersion (PMD) compensation performance of different modulation formats. Firstly,
we analyze the relationship between the signal's degree of polarization (DOP) and differential group delay (DGD) of
different modulation formats. The results show that the differential phase-shift keyed (DPSK) modulation formats are
more suitable for PMD compensation system than the ON-OFF keying (OOK) modulation formats because the DOP-DGD
curves of DPSK signals are monotonic and it is easy to search the global maximum. Secondly, we study the PMD
compensation performance of the mentioned modulation formats with DOP as feedback signal. It is shown that the
modulation formats with smaller bandwidth have better PMD compensation performance, and that PMD compensation
performance of DPSK formats are better than that of OOK formats. CSRZDPSK combines the carrier-suppressed
characteristic of CSRZ format and specific spectrum characteristic of DPSK format, thereby it has the best performance
of PMD compensation.
The performance of 40Gb/s three different formats (return-zero format (RZ) with 33% and 50% duty cycle respectively and carrier-suppressed RZ (CSRZ)) systems are compared when polarization mode dispersion (PMD) is considered. Analytic results indicate that CSRZ modulation format is the best in mitigating PMD and improving system performance among three formats.
Using the degree of polarization (DOP) ellipsoid method, differential group delay (DGD) is obtained from
limited sampling data in the experiment, which is important for the feed-forward compensation scheme. Results
indicate the values of DGD obtained from the experiment are accordant with the theoretical values when the
DGDs are less than 30ps.
The polarization mode dispersion (PMD) monitoring system is the key integral part of an adaptive PMD compensator. The degree of polarization (DOP) ellipsoid obtained by using a polarization scrambler can be used as either a feedback or feedforward monitoring signal for automatic polarization mode dispersion compensation. Generally, more than several thousands of sampling data of states of polarization (SOP) must be collected to insure getting a correct DOP ellipsoid. This would result in an unacceptable time consuming for adaptive PMD compensation. In this paper, for the first time, we introduce the particle swarm optimization (PSO) algorithm in determining the real-time DOP ellipsoid with high precision, requiring only 100 sampling data of SOPs. Experimental results confirm that the PSO algorithm is effective for ellipsoid data fitting with high precision within 250ms using our hardware environment.
In this paper, the degradation of signal degree of polarization (DOP) by first and second order polarization mode dispersion (PMD) in 40Gb/s RZ and NRZ optical communication is analyzed by numerical simulation. The simulation results show that the degradation of signal DOP by first order PMD is monotony, but which is fluctuated by second order PMD. The influence of two components of second order PMD on DOP for NRZ code and RZ are also investigated by numerical simulation method. The results also show that the influence of the depolarization component and the PCD component on NRZ and RZ data formats are different. Those results will provide the theory basis for how to select proper compensation arithmetic. A compensation system is founded to compensate the first and high order PMD effectively by DOP as feedback signal and particle swarm optimizer (PSO) as compensation arithmetic. Here, we introduce an intelligent method PSO as a searching algorithm to multi-DOF (degree of freedom) PMD compensation, The PSO algorithm used here is described as: (1) Local neighborhood structure is employed to avoid being trapped into sub-optima. (2) 20 particles are employed. So 20 time units (less than 20 ms) are required in one iteration. (3) The maximum iteration number is set to 50. The experiment result show that the first and second PMD can be compensated at the same times for 10 Gb/s RZ and NRZ by using DOP as feedback signal and PSO as searching algorithm.
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