# Unrepeated 240-km 64-QAM Transmission Using Distributed Raman Amplification over SMF Fiber +
## 2 Experimental Set-Up of the Raman Amplifier  
Figure 1 represents the experimental set-up for a 28-GBaud Nyquist-shaped optical 64-QAM signal. The transmitter consisted of two synchronized 50-GSa/s arbitrary waveform generators (AWGs), an optical IQ modulator, and an external cavity laser (ECL) with less than 100-kHz linewidth (LW). A pseudo-random bit sequence with a word length of \(2^{15}-1\) (PRBS15) was generated and then Gray-mapped to generate a 28-GBaud 64-QAM signal followed by Nyquist pulse shaping with a 0.15 roll-off factor. The sequence was then resampled to match the sampling rate of the AWGs. At the output of the IQ modulator, the optical signal was amplified using an erbium-doped amplifier (EDFA). Then, the signal was fed into the preamplifier and band-pass filtered followed by the coherent receiver with an integrated local oscillator laser having less than 100-kHz LW. A digital storage oscilloscope (DSO, 80 GSa/s, 33 GHz) was used to convert the signal into the digital domain with offline demodulation where chromatic dispersion (CD) and nonlinear noise were compensated for digitally using digital back propagation with simulated Raman power profiles, specific to each configuration.  
The transmission fiber used in the experiment was a standard SMF-28 with approximately 0.2-dB/km loss. The measured loss in 160-km (4 \(\times\) 40 km), 200-km (5 \(\times\) 40 km), and 240-km (6 \(\times\) 40 km) links including splices was 33.6 dB, 41 dB, and 52.7 dB, respectively. The loss from the forward and backward wave division-multiplexer (WDM) was 0.6 dB and 0.8 dB, respectively. In unrepeetered 160-km and 200-km transmissions, a first-order distributed backward-pumped (BP) Raman amplification with the pump centered at 1455 nm was sufficient to achieve a bit error rate (BER) below the soft forward error correction (FEC). However, given the strong OSNR constraints of the system at longer distances links, in the 240-km experiment, we used a second-order hybrid  
Figure 1: Experimental set-up of a 28-GBaud Nyquist-shaped optical 64-quadrature amplitude modulation (QAM) signal (left), transmission line with Raman amplification schemes for each distance (middle), and receiver with DSP schematic (right).  
dual/URFL bidirectional Raman pumping scheme with the highly depolarized forward and backward pumps centered at 1366 nm. Random distributed feedback (rDFB) lasing in the forward direction was generated, seeded by the reflected Stokes-shifted light by a high-reflectivity (95%) fiber Bragg grating (FBG) centered at 1455 nm with a 0.5-nm bandwidth and fed back by distributed Rayleigh backscattering in the long link [11; 12]. Backward pumping (BW) was provided by two Raman pumps at 1366 nm and 1455 nm, respectively. All pump and signal launch powers used in experiments are listed in Figure 3 and Figure 4 in Section 3. To combine and demultiplex the Raman pumps and the signal, we used \(1\times 3\) WDM couplers at the beginning and at the end of the span. In all set-ups, there was an EDFA implemented before and after the transmission line.  
Digital post-processing included full simulation of distributed Raman power profiles where each spectral component was calculated numerically using an extended model that accounts for the residual Raman gain shift from the lower-order pumps to the transmitted signal and pump depletion from all components. Individual signal power profiles were then fed into the DBP code, realized by a fixed step algorithm followed by resampling to one sample per symbol [13]. A multi-modulus algorithm (MMA) equalizer was applied to compensate for linear polarization effects and, finally, a filtered blind phase search (F-BPS) for carrier frequency and phase recovery followed by error counting [14].