Electronic Dispersion Compensation

The OptSim example depicted above demonstrates a 10 Gigabit multimode link. The multimode fiber is a 300-m graded-index fiber with a 50-micron core diameter. The fiber refractive index profile has a dip defect in the center of the core ñ one of the typical profile distortions due to manufacturing imperfections. The fiber profile used is:

The deviations of index profile from an ideal graded-index may cause increase in differential modal delay, and as a result the performance degradation.

The output signal from the transmitter has a single-mode Gaussian pulse profile with 7 micron FWHM. The connector between transmitter and fiber adds 10 micron offset from the fiber core center. As a result, the received eye diagram at the PIN/TIA receiver is distorted and the computed link BER for this eye diagram is as high as 3.1x10-3.

To improve the eye diagram and BER of the link, we apply EDC to electrical signal after the first receiver in the form of equalization filter implemented as an electrical finite-impulse response (FIR) filter. This equalization scheme is also known as linear or forward-feedback equalization (LE or FFE). The FIR filter model used applies the following transformation to the electrical signal:

where x(t) is incoming signal, y(t) the outgoing signal, (2N+1) the number of taps, Ci- weight coefficients or taps, and DT the taps delay.

For the eye diagram before EDC, we found that a FFE parameters setting with 3 taps, tap delay of ?TB, and taps coefficients {-0.3, 1.0, -0.3} is sufficient to improve the eye diagram quality, and hence, the BER. The eye diagram after applying EDC shows a significant improvement. The corresponding computed BER is equal to 3.7x10-13, i.e. satisfies the link BER requirement of 1x10-12 or better.

The electrical signal waveforms before and after the EDC:

The transfer function for the applied FIR filter:

In this example the parameters of equalization filter are optimized for the given offset of laser-fiber alignment. To demonstrate that we can perform the parameters scan simulation for different values of offsets. Figure 7 shows simulated link BER and Q-factor before and after EDC for different values of offset, ranging from 0 to 20 microns. One can see that the biggest gain from EDC is observed at the 10-micron offset ñ i.e. the one we use in the example. Thus, if one wants to use different offset values, then the FFE parameters have to be modified (optimized) for the best performance.