## 3 Results  
The OSNR improvement using distributed Raman amplification was first evaluated. Figure 2 illustrates the measured OSNR versus the power of the first-order Raman pump for two pumping schemes, first using backward pumping with 95% of the pump power and second using bidirectional pumping, using the remaining 5% pump power for co-pumping. The experiments were done at two launched powers, 5 and 2 dBm, into the 200 km SSMF link. From Figure 2 it is observed that with 95% backward pumping, the received signal OSNR increased with the pump power compared to the reference case when the Raman pump was off. In addition, with the extra 5% of the power in the forward pumping, the OSNR enhancement was higher, especially at high power. It is important to note that the maximum forward pump used was only 40 mW. In coherent optical communication systems using multi-level modulation formats, a high OSNR only is not enough to ensure a good transmission quality since other impairments such as the phase noise have a large impact on the system performance [18]. Therefore, we characterized the received signal amplitude and phase noise induced by the Raman pump RIN transfer. Figure 1: Experimental setup of distributed Raman amplified coherent transmission link. PPG: pulse pattern generator; DAC: digital-to-analog converter; WDM: wavelength division multiplexer; SSMF: standard single mode fiber; LO: local oscillator; DSO: digital storage oscilloscope; DDPLL: decision-directed phase-lock-loop; BPS: blind phase search; QPSKP: QPSK partitioning. Figure 3 shows the measured relative frequency noise and amplitude noise power spectrum density (PSD) of the received signal after bidirectional Raman amplification for different pump powers. All selected cases are with a 200 km SSMF length and a 5 dBm launched power. In this measurement, the transmitter laser was unmodulated and transmitted over the Raman amplified link with different pump configurations, as shown with the green dots in Figure 1. It is noted that this method characterizes the combined noise properties from both the received signal and the narrow linewidth LO. From the frequency noise PSD shown in Figure 3a, one can see that with a low Raman pump power, the noise shows a property that can be closely approximated as white phase noise. However, at a high pump power, the frequency noise level in the low frequency regime rises, while at the high frequency side the level decreases, which is due to the improvement of the signal OSNR (as shown in Figure 2). It is noted that there are spurs rising at around 11 GHz at the high pump power, which we attribute to the stimulated Brillouin scattering. Similarly, from Figure 3b it is observed that as the pump power increases, the amplitude noise level at the high frequency side decreases, which is also due to the improvement of the signal OSNR. However, at the lower frequency regime a rise of the noise level at a higher pump power is observed. This noise enhancement can be attributed to the RIN transfer from the Raman pump. It is noted that the frequency regime of the enhanced noise is important for a potential effective mitigation by optical or digital means, as it defines the mitigation bandwidth of the optical or digital filter. The Q factor (calculated from the measured bit error rate) is presented in Figure 4 as a function of the Raman pump power. The results for 200 km at a launched signal power of 5 and 2 dBm with  
Figure 3: (**a**) Measured frequency noise and (**b**) relative amplitude noise power spectrum density for different pump powers. For all selected cases, the launched power = 5 dBm, fiber length = 200 km. Figure 2: OSNR versus Raman pump power for two pumping schemes, backward pumping only with 95% and backward pumping with 95% together with co-pumping with 5% for two different launching powers. the two different pumping schemes were taken after demodulation by the DDPLL, QPSKP and BPS algorithms, respectively.