## 1 Introduction  
Data traffic is increasing worldwide in the 5G era. As virtual reality (VR), augmented reality (AR), and Internet of Things (IoT) technologies are expected to become mainstream, the data capacity requirement is predicted to increase. The advent of artificial intelligence (AI) demands faster and more accurate signal processing. Transmission of a large volume of data requires the system to be simple. As the network becomes increasingly complex, the number of nodes increases, and more transmitters are required. As each transmitter should be simple in order to ensure the high stability and flexibility of the network, technologies for transmitting high data traffic with simpler structures are urgently required. Several methods have been proposed to allow increased data traffic. Among these, the simplest method is to use a wider bandwidth. However, because of the bandwidth limitation of the transmitter, it is impossible to extend the bandwidth to infinity. Therefore, modulation methods that increase spectral efficiency have been investigated. On-off keying (OOK) is a representative modulation method because of its simple structure. However, owing to its low spectral efficiency, it cannot satisfy the demand for increasing data traffic. Therefore, M-pulse amplitude modulation (MPAM), which divides the signal amplitude into M levels, has been proposed as an alternative to OOK. However, to increase by 1 bit, MPAM requires two times more levels. Because a higher level requires more power to maintain bit error rate (BER) performance, MPAM has limited scalability. Moreover, because of channel characteristics such as dispersion and nonlinearity, it is difficult to increase M indefinitely [1]. To overcome these problems, modulation methods using light properties such as polarization, wavelength, phase, and mode have been proposed [1; 2; 3; 4]. Modulation methods using phases have been proposed to increase data capacity [1; 5; 6; 7; 8; 9]. Coherent detection is a representative detection method [6; 7]. However, coherent detection inherently requires an optical phase-locked loop (OPLL) owing to the channel-induced phase noise (PN). This makes the link more complex. Instead, modulation called differential phase-shift keying (DPSK) has been researched. Because DPSK does not require an OPLL, it has a simpler structure. However, DPSK has limited performance when the channel is dynamically changed because it uses an optical delay line that meets the symbol length. Thus, the symbol rate should be changed in line with the channel. Modulation methods that use modes have been proposed as alternatives [2, 10, 11]. However, these require complex structures. Moreover, the mode can be distorted as light passes through the channel, which causes inter-channel interference [11, 12]. Wavelength has attracted attention for several years as an effective light resource for increasing data capacity. Multiplexing, also known as wavelength division multiplexing (WDM), is a representative method. However, because WDM can be used independently of other methods, it was not considered as a comparative method in this study [4]. Polarization is an attractive resource for data transmission. By using the orthogonality of each polarization, data transmission can be increased two-fold. Therefore, various modulation methods have been proposed [1, 5, 13, 14]. These traditional polarization modulation methods require at least two or more modulators to modulate the signal because each modulator modulates a single polarization intensity. However, using the birefringence of commercial LiNbO\({}_{3}\) MZM, dual polarization intensity could be modulated simultaneously with a single modulator. In this case, however, each polarization intensity is interdependent. Therefore, dual polarization intensity can be modulated simultaneously but not independently. In this study, a new intensity modulation scheme for dual polarization in a single LiNbO\({}_{3}\) MZM is proposed and analyzed.