# Adaptive Pre/Post-Compensation of Cascade Filters in Coherent Optical Transponders
## 6 Experimental Setup and Results  
A set of experiments was conducted to get the quantitative benefits in real-world settings using the setup in Figure 7. We tested channel propagation with PDL emulator, PMD and Polarization Controller. PC1 and PC2 used to scramble the two polarizations, emulating state-of-polarization (SOP) variation, at a rotation rate of 800 Hz. Noise was added, with 50/50% combiner, by controlling the variable optical attenuator (VOA) setting at the egress of the amplifier spontaneous emission (ASE) source.  
The LCoS-based 1 GHz resolution waveshaper was used to mimic two different optical links, as shown in Figure 1b, with the frequency responses of both CMD Mux and Demux subtracted. Instead, we used real AWG-based CMD at both sides, which helped to filter the noise before the signal is received. PMD emulator was programmed to introduce 30 ps. Variation in PMD, along with state of polarization, can be corrected at the receiver with no penalty. Its introduction here for completeness to mimic both random birefringence in fiber links and not directly related to WSSs. As well, based  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **SSMF Value** \\ \hline Span Length & 60 km \\ \hline Kerr Coefficient & \(2.6\times 10^{-20}\) m\({}^{2}\)/W \\ \hline Dispersion & 16 ps/nm/km \\ \hline Attenuation & 0.2 dB/km \\ \hline EDFA Gain & 12 dB \\ \hline EDFA Noise Figure & 6 dB \\ \hline Laser Linewidth & 100 KHz \\ \hline \hline \end{tabular}
\end{table}
Table 1: Fiber parameters.  
on results shown in [40], using a high-resolution measurement technique, a MEMS-based WSS has an upper-bound PMD value of 30 femto-second per port. The commercial card was set in loopback through the experimental work, therefore once acquired, the laser was centered at ITU wavelength of 1546.92 nm (equivalent to 193,800 GHz), and only dithering within a frequency range of 200 MHz since it is locked to itself. An optical spectrum analyzer with a fine resolution of 300 MHz, using 90/10% splitter, was used in the measurement of the transmitted spectrum with and without pulse shaping, and filtering effects programmed at the waveshaper. The sweeping of OSNR was done with varying the setting of VOA. The setup was characterized according to IEC 61280-2-9 standard. To minimize error, all reported OSNR values are the average of 10 consecutive measurements, with accuracy within 0.1 dB.  
In Figure 8, using two commercial cards, the advantage of using the proposed method is presented for different jitter profile and various Tx EO responses. A maximum gain of 0.38 dB in Required-OSNR is measured when the total jitter root-mean-square (RMS) is 1 picosecond (ps), equivalent to 3.5% of unit-interval (UI). With DAC PLL is jittery and the EO response has steep roll-off at high-frequencies, equal 6 dB loss at 15 GHz, balancing the compensation of the channel filtering has a high-gain compared to performing only pre-compensation. Especially that the clipping effects, at the Tx filter, introduce a bias noise that impacts the SNR. The clipping is caused by the necessity to peak more than the maximum allowed magnitude per frequency bin. For lower jitter (i.e., better clocking quality), as we will discuss in detail in Section 7.1.3 below, and relaxed Tx EO response of 3 dB loss at 15 GHz, the gain is less obvious. With the same card, i.e., same EO responses to ease comparison, when jitter RMS is 1.8% (0.5 ps) and 2.7% (0.75 ps) of UI, the gain is up to \(\sim\)0.26 and \(\sim\)0.15 dB, respectively.  
Figure 8: Measured required-OSNR gain versus pre-compensation for different cards (with comparison to simulation).  
Figure 7: Block diagram of experimental setup.