## 1 Introduction  
Recent, new technologies, such as fifth-generation wireless networks (5G), artificial intelligence (AI), virtual reality (VR), the Internet of things (IoT), and self-driving cars, require a large bandwidth and fast response time. To support the needs of such new technologies, the core network is evolving from 100 GE to 400 GE and the research and development of 400 Gbps optical transceivers continues to progress [1]. With increased data consumption, increased bandwidth is required. For this purpose, long-distance optical transceivers that use a variety of modulation methods such as polarization-multiplexing and quadrature phase-shift-keying (PM-QPSK or DP-QPSK), and quadrant amplitude modulation (QAM) [2] have been developed. However, this modulation method requires an expensive photomultiplier and the corresponding sophisticated technology. A commonly used non-return-to-zero (NRZ) modulation scheme can transmit one bit per symbol. A level-four pulse-amplitude modulation (PAM4) is advantageous because it can transmit two bits per symbol, twice that of the traditional NRZ [3]. However, the PAM4 scheme has a characteristic lower bit error rate (BER) because it is more noise-sensitive than the NRZ method. Therefore, increased caution is required when designing optical transceivers using PAM4. The Institute of Electrical and Electronics Engineers(IEEE) 802.3 cd standard document defines the physical layer and associated parameter management of 50 Gbps PAM4 optical transmitters and receivers for 50 GBASE(Gb/s PHYs)-FR (far reach, 2 km) and LR (long reach, 10 km) [4]. However, the specification for the longest-distance transmission, which is 40 km for PAM4 signals, has been proposed for some enterprises but have yet to be established as a standard [5]. Table 1 shows the existing optical receiver solutions for long-distance transmission. Experiments were conducted using an avalanche photodiode (APD) optical receiver for 40 and 60 km long-distance PAM4 signal transmissions [6; 7; 8]. However, the method using APD receivers requires additional circuitry to drive the APD and the cost of parts is higher than that of positive-intrinsic-negative photo diode (PIN-PD) receivers. The semiconductor optical amplifier (SOA) is another technology that can be used for long-distance transmissions greater than 40 km [9; 10]. SOA has been applied to the C form-factor pluggable (CFP), specifically the CFP2 optical transceiver, which is a large form-factor type owing to the high cost of parts, the additional thermoelectric cooler (TEC) control, and its slarge size; however, it can amplify the optical signal to be lower than the APD method to the level of the receiver sensitivity. These existing studies have successfully long-distance transfers but have not experimented with packaged transceiver types. The study in [11] proposed a method for transmitting long distances using a QSFP28-type transceiver. However, it used a method to amplify signals outside the transceiver by adding the erbium-doped fiber amplifier (EDFA) to the transmission system rather than inside the transceiver. We designed and implemented the QSFP28 optical transceiver using PAM4. This study makes the following contributions: (1) 50 Gbps high-capacity long-distance transmission, only PIN-PD was used to minimize circuit complexity and was implemented at low cost over traditional approaches. To overcome the limitations of the PIN-PD receiver, the PAM4 signal transmitter output BER was minimized to demonstrate that 40 km transmission is feasible. (2) A hardware and firmware design scheme in which the optical transceiver's function and performance meet standard specifications is proposed. The optical transceiver manufacturers do not publicly share their methodology or techniques. By implementing optical transceiver using the proposed design scheme, quality can be ensured, and cost and development time can be reduced.