## 4 Impact On System Performance: Experiment  
This section expands on the results published in [13], where carrier induced frequency noise was experimentally investigated using a variety of CPR algorithms. In this paper, we focus on BPS algorithm and compare system performance for different number of taps. Figure 6 shows the experimental setup. The transmitter consists of a pulse pattern generator (PPG), a 4 levels pulse amplitude modulation (PAM) generator, and an optical IQ modulator. A LiNbO3 phase modulator is used to manipulate the frequency PSD of an external cavity laser, as illustrated in the Figure 6a. The receiver consists of an integrated coherent receiver, an ECL and a digital storage oscilloscope (DSO). Data demodulation is performed offline using standard DSP algorithms as described in [22]. The CPR algorithm used is again BPS [21]. More details about the experimental setup can be found in [13]. Results for back to back (B2B) are shown in Figure 7. We use the same volumetric plot as in Section 3 to represent the penalty _vs._ carrier induced frequency noise. 27 semiconductor lasers were emulated corresponding to each point in the volumetric plots. The receiver sensitivity is set to 21.7 dB for a BER of 3.8\({}^{-3}\), which we use as forward error correction (FEC) threshold (As per guidelines given in _ITU-T Recommendation G.975.1, Appendix I.9 (2004)_). We evaluate the system performance for different tap-lengths of the BPS algorithm. We can observe the same tendency as in simulated results,  
Figure 4: Simulation setup for a semiconductor laser. Figure 5: Simulation results for carrier induced frequency noise from transmitter laser. where lower values of \(K\) and \(F_{r}\) yield higher penalties. We found that the optimum number of taps is different depending on which frequency noise profile is considered, resulting from a different trade-off between OSNR and phase noise. Another interesting observation is that, for the lowest level of white frequency noise \(\Delta v_{int}\) = 500 kHz, the lowest penalty is found at the center of the plane, (\(F_{r}\) = 3 GHz, \(K\) = 0.3 ns). For \(\Delta v_{int}\) = 2.5 MHz, the lowest penalty is shifted to (\(F_{r}\) = 5 GHz, \(K\) = 0.3 ns). And for \(\Delta v_{int}\) = 5 MHz, the lowest penalty is located at (\(F_{r}\) = 5 GHz, \(K\) = 0.6 ns). This trend holds true for all other configurations of the BPS algorithm, suggesting there may be an interplay between white frequency noise and carrier induced frequency noise. When transmission is considered, we distinguish between two cases. The first, when the emulated laser is placed at the transmitter. The second, where the emulated laser is placed in the receiver, as a local oscillator. In both cases the transmission length was 520 km. In the first case, we observe negligible penalties overall, as dispersion was digitally corrected at the receiver and launch power was kept under the non-linear regime. Results for the second case are shown in Figure 8. Penalties are  
Figure 6: Experimental Setup: inset (**a**) shows the power spectral densities for emulated and measured frequency noise. Figure 7: Experimental results for B2B. computed against the same OSNR value as in the B2B case: 21.74 dB. In this case, only results from a BPS configuration with 15 taps are shown, as the results are mostly independent of the number of taps used. This is due to nonlinear intermixing of the dispersed signal and the laser phase noise, through a process known as equalization enhanced phase noise (EEPN) [23], as illustrated by the constellations in Figure 8. In this case, we can observe that carrier induced frequency noise is most relevant at low frequencies. These results are in agreement with the findings published in [24], where it was shown that low frequency noise is critical for system performance when considering local oscillator (local oscillator (LO)) lasers. In this case, we observe a similar trend to the B2B case, with two significant differences: the first, penalties are augmented due to EEPN.