# Two-Channel OTDM System for Data-Center Interconnects: A Review
## 1 Introduction  
Data centers have become essential infrastructure for society due to the rapid exploration of data traffic through high-definition video and audio streaming services, cloud services, and social networks [1; 2; 3; 4]. The 400 gigabit Ethernet will employ multiple 50 or 100 Gb/s/\(\lambda\) 4-level pulse-amplitude modulation (PAM-4) signals as specified in the Ethernet standard [5; 6]. Considering the steep growth in traffic [7; 8], it is necessary to develop a way to implement Ethernet beyond 400 gigabits (such as 800 gigabit or 1.6 terabit Ethernet) implemented by multiple \(\geq\)100 Gb/s/\(\lambda\) signals soon [3; 9; 10]. Since the use of a cost-effective modulation scheme is still preferred [1; 3; 11], various studies have applied PAM-8 to implement beyond 400 gigabit Ethernet, which can be made cost-effective by using a 3-bit digital-to-analog converter (DAC) and reducing the bandwidth requirements of components [2; 12; 13]. However, PAM-8 is sensitive to impairments caused by the limited bandwidth of transceivers and chromatic dispersion (CD) [14]. Thus, PAM-8 may require various techniques, such as Tomlinson-Harashima precoding [15], a machine-learning-based equalizer [16], or maximum-likelihood sequence estimation [13; 14], to improve the signal-to-noise and distortion ratio (SINAD), which can be considered extremely complex and cost-intensive for data-center interconnects (DCIs). Consequently, the use of the probabilistically shaped PAM-8 signal has also been considered for stable operation by reducing the spectral efficiency to less than three [16; 17; 18]. Therefore, it is unlikely that the modulation level of PAM exceeds eight, which makes the implementation of high-speed DCIs based on intensity-modulation and direct-detection (IM/DD) systems challenging. [PERSON] et al. proposed using optical time-division multiplexing (OTDM) to increase the operating speed of DCIs [19]. They demonstrated the transmission of a 208 Gb/s OTDMPAM-4 signal using a four-channel OTDM system [19], which was actively researched in the 1990s and 2000s [20]. Although such a system is more complex than the conventional IM/DD system that uses a single pair of transceivers, it makes high-speed operation possible by overcoming the insufficient bandwidth of the modulator [21]. An optical pulse source with a high repetition rate is required on the transmitter side to implement the four-channel OTDM system [20]. The optical pulse should have a high extinction ratio (ER) and a narrow pulse width, commonly generated using an actively mode-locked laser consisting of a Fabry-Perot resonator, a gain medium, and a modulator [20; 22]. Among the multiple longitudinal modes determined by the cavity, only the harmonics of a certain frequency (determined by the modulation frequency) can be locked. However, stabilizing the pulse with a high repetition rate is challenging because it requires high-precision timing distribution systems [23; 24]. An alternative method involves the use of a comb generator, which typically consists of an intensity modulator, a phase modulator, and a dispersion compensating fiber (DCF) [25; 26]. However, this method requires high power consumption for comb generation and a few hundred meters of DCF to increase the ER and reduce the pulse width [26]. Recently, we proposed the implementation of high-speed DCIs with a two-channel OTDM-PAM-\(N\) signal [21]. In this system, the pulse source can be simply generated using a modulator and a sinusoidal electrical clock. In the previously proposed two-channel OTDM system, the OTDM signal was generated using the sinusoidal optical pulse and detected using a single photodiode (PD), which is valid because a PD has a larger bandwidth than a modulator [27; 28]. The crosstalk between two modulated signals can be compensated using a \(2\times 2\) multiple-input multiple-output (MIMO) equalizer realized through matured digital signal processing (DSP), enabling the use of sinusoidal pulses with low ER. This paper reviews recent research on two-channel OTDM systems.