# A Pulse Shaping Based Optical Transmission System of 128 QAM for DWDM with N \(\times\) 904 Gbps
## 1 Introduction  
With the rapid development of the Internet of Things, mobile internet, and cloud computing, massive data of Petabyte (\(10^{15}\) bytes) is generated every day. The mass data makes the capacity of the backbone transmission networks increase rapidly. The optical networks with ultra-large capacity shoulder the fundamental service-transmission function of the backbone transmission networks. The sustained multiplication of the bandwidth requires higher capacity for the backbone transmission networks. The transmission systems with large capacity and high bandwidth become the development direction of future optical networks. Consequently, upgrading the transmission capacities of the backbone networks with the technique of dense wavelength division multiplexing (DWDM) on a large scale is imperative. One of the key techniques is to enhance the transmission capacity of a single wavelength. Major network operators around the world have initiated experimental tests and engineering verification of single-carrier optical fiber communication system with a rate of 400 Gbps. Currently, the transmission rate of a single wavelength is evolving from 100 Gbps to 400 Gbps. Analyzing the core mechanisms and solving the key problems of single-wavelength optical fiber communication system with 400+ Gbps has become a recent focus in academia and industry [1; 2; 3; 4; 5; 6; 7]. In 2013, Bell laboratories reported a dual-carrier transmission system with the transmission distance of 320 km; the code-type of the transmission system was 64 quadrature amplitude modulation (QAM) format; and the capacity was up to 1.5 Tbps [8]. In 2014, Bell laboratories reported a multi-carrier transmission system with the single-channel rate of 1 Tbps again; the code-type of the system was 16 QAM; the transmission distance was 480 km; and the spectral efficiency of thesystem was 5 bps/Hz [9]. Compared with optical communications with direct-detection, coherent optical communications [10; 11; 12; 13; 14; 15] have the advantages of high receiving sensitivity and anti-polarization dispersion. In addition, the coherent detection can support M-ary modulation format and effectively utilize the bandwidth of the spectrum. Consequently, the technique of coherent optical communication is widely used in DWDM systems.  
By analyzing the latest research in this field [1; 2; 3; 6; 8; 9], we find that: (1) when the channel spacing is less than 50 GHz in a DWDM system, the cost of the array waveguide grating (AWG) and the difficulty of the production engineering will be greatly increased, and the interference between adjacent channels will be very serious; (2) the ratio of the transmission speed of a single wavelength to the spacing between two adjacent channels is relatively small, that is to say, the utilization efficiency of the spectrum is low; and (3) the coherent modulation and coherent detection are the core techniques in DWDM systems. For these reasons, we combine high-order coherent modulation with Gaussian optical filtering in an optical transmission system, and a DWDM system with ultra-high-speed and large capacity is realized. In this DWDM system, the two adjacent wavelengths spacing is 0.7 nm, the transmission rate of the single-wavelength is 904 Gbps, and the spectral efficiency is 10.33 bps/Hz. We also analyze the transmission performances of the DWDM system from the angles of the optical signal-to-noise ratio (OSNR) and the bandwidth of Gaussian optical filter (GOF).