# A Pulse Shaping Based Optical Transmission System of 128 QAM for DWDM with N \(\times\) 904 Gbps
## 3 System Architecture and Transceiver Units
The two signals output from the M-ary pulse generator 1 are processed as bias via the EB 1 and EB 2. The two biased signals modulate a light-wave produced by a CW laser via LiNb-DPMZ modulator 1. The generating methods of the light-waves in LiNb-DPMZ modulator 1, LiNb-DPMZ modulator 2, LiNb-DPMZ modulator 3, and LiNb-DPMZ modulator 4 are same. We use the light-wave which is input into the LiNb-DPMZ modulator 1, for example, to illustrate the modulating process. The light-wave produced by the CW Laser 1 with the center frequency of (1550 \(\pm\) 0.7 \(\times\) N) nm and the line-width of 100 Hz is divided into two light-waves via PS 1, and the two divided light-waves are processed with the same way. These two light-waves are generated by the light-wave output from PS 1 via optical coupler 3, which are input into the LiNb-DPMZ modulator 1 and LiNb-DPMZ modulator 2 as modulated laser light-waves, respectively. The modulated photonic carrier-wave signal output from the LiNb-DPZM modulator 2 is processed via an optical phase shifter at 90\({}^{\circ}\), and the photonic carrier-wave is coupled with the output of the LiNb-DPMZ modulator 1 via optical coupler 5. The modulated photonic carrier-wave output from the LiNb-DPMZ modulator 4 is processed via an optical phase shifter at 90\({}^{\circ}\), and the photonic carrier-wave is coupled with the output of LiNb-DPMZ modulator 3 via optical coupler 6. The output of coupler 5 is combined with the output of coupler 6 via PC 1. The output of the coherent photonic transmitter with the modulation format of 128 QAM are then obtained. It is noted that the parameters of the LiNb-DPMZ modulator used above are configured as, extinction ratio: 20 dB, switching RF voltage: 3 V, switching bias voltage: 3 V, insertion loss: 5 dB, modulation voltage 1:2 V, bias voltage 1:0 V, bias voltage 2:0 V, and modulation voltage 2:2 V.  
The function block diagram of the photonic heterodyne coherent receiver is shown in Figure 4. The photonic signals transmitted from the coherent photonic transmitter are split into two paths by PS 3 in the photonic heterodyne coherent receiver. The two photonic signals are processed with the same way. One of the two photonic signals is split into two new photonic signals via coupler 7; the new two photonic signals are processed with the same method. One of the new two photonic signals is input into coupler 11; and the other one is input into coupler 12. The continuous laser-wave emitted from  
Figure 3: Setup for coherent photonic transmitter (LiNb-DPMZ Modulator: LiNb-Dual Port Mach-Zehnder Modulator; CW Laser: Continuous Wave Laser; EB: Electrical Bias; PS: Polarization Splitter; EG: Electrical Gain; ON: Optical Null; PC: Polarization Combiner). CW Laser 2 with the center frequency of (1550 \(\pm\) 0.7 \(\times\) N) nm and the line-width of 100 Hz is split into two paths via PS 2, and these two photonic signals are processed with the same method. One of the two photonic signals is split into two new photonic signals, again via coupler 8. One of the two new photonic signals is processed via the optical phase shifter at 90\({}^{\circ}\), and one is input into coupler 12, while the other is input into the coupler 11. The two photonic signals output from coupler 11 are detected by two positive intrinsic negative photodiodes (PIN-PD 1 and the PIN-PD 2); two detected signals are processed by ES 1, and then amplified via EA 1. It is noted that the processing procedures and processing methods of coupler 11, coupler 12, coupler 13, and coupler 14 are same. Figure 5 shows the flow diagram of the algorithm design for the UDSP which is used for the compensation of transmission impairments and signal reconstruction. The noise is removed by a Bessel filter with 3 rd order. The sampled signal is re-sampled at a rate of \(2\times samples/symbol\). A cubic interpolation is utilized to adapt the waveform of the sampled signal to the resampling rate. The first and \((N/2+1)\) sampled signals are used for re-sampling, where \(N\) is the number of the samples per symbol.