As cloud computing, the Internet of Things, and 5G technologies rapidly develop, global network traffic has experienced exponential growth. This surge in traffic, both within and between data centers, has fostered an ever-increasing demand for high-speed and high-performance optical fiber transmission systems for short- and medium-reach distances. Currently, the intensity-modulation and direct-detection (IMDD) system employing four-level pulse amplitude modulation (PAM4) is the primary solution for cost-sensitive short- and medium-reach transmission scenarios. The IMDD system features a simple structure, low power consumption, and low cost. However, it utilizes only the amplitude dimension of the optical carrier to transmit information, leaving other optical domain dimensions untapped. Additionally, the IMDD system's limited receiver sensitivity poses a challenge when higher-order modulation formats are tried to improve spectral efficiency. Coherent detection systems with higher receiver sensitivity are characterized by utilizing the polarization, phase, and amplitude of optical carriers to transmit information, which leads to higher spectral efficiency. However, their practical implementation in short- to medium-reach transmission scenarios brings about challenges including increased system complexity, higher power consumption of digital signal processing (DSP) chips employed in coherent detection systems, and the need for a high-performance narrow linewidth laser as a local oscillator (LO). These factors limit the widespread adoption of coherent detection in such scenarios.

To this end, researchers have explored simplified coherent schemes, including self-homodyne coherent detection (SHCD) and differential self-coherent detection (DSCD) schemes for new-generation short- and medium-reach transmission systems. These schemes strike a balance between system performance and complexity, with higher receiver sensitivity than IMDD systems, and less complexity and costs than standard coherent detection. Among these schemes, the SHCD scheme has caught considerable attention. The SHCD system eliminates the need for a narrow linewidth laser as an LO on the receiver side by splitting the laser power at the transmitter between the transmitted signal and a remote LO. This allows utilizing an uncooled large linewidth laser in SHCD systems while the receiver sensitivity remains high. Extensive research efforts have been devoted to advancing the development of this scheme. The DSCD scheme, based on a differential modulation format, provides an alternative approach. It utilizes the relative phase information between two adjacent signals for self-coherent signal demodulation. A notable advantage of this scheme is its high tolerance to laser linewidth, which eliminates the need for LO and carrier phase recovery at the receiver side. Consequently, it enables the utilization of large linewidth lasers for coherent detection to reduce system cost and improve receiver sensitivity. In contrast to the SHCD scheme, the DSCD scheme overcomes the performance degradation caused by mismatched transmission paths of the signal and the remote LO. Recent research findings presented in our paper highlight that, in systems where receiver electrical noise is the primary impairment, the theoretical performance of DSCD is equivalent to that of SHCD. Additionally, DSCD outperforms SHCD in systems dominated by optical noise introduced by optical amplifiers. As a result, the DSCD technology provides a promising solution for high-speed and high-performance optical fiber transmission systems. Its advantages include high receiver sensitivity, low-cost implementation, and low power consumption, thus making itself an appealing choice in the field.

In terms of receiver sensitivity, implementation complexity, and performance in optical power-limited and optical signal-to-noise (OSNR) limited regimes, we review and compare the optical transmission schemes, including IMDD employing PAM4, SHCD employing quadrature phase shift keying (QPSK) modulation, and DSCD employing differential quadrature phase shift keying (DQPSK) modulation. In recent years, the IMDD system faces challenges in improving system transmission rates, while the SHCD system has gained attention as a low-cost, and high-performance solution. Sowailem's group from McGill University demonstrates a bidirectional SHCD scheme employing optical circulators for short-reach systems. Deming Liu's research group from Huazhong University of Science and Technology presents an SHCD system leveraging a large linewidth distributed feedback (DFB) laser as a downstream transmission solution for optical access networks. Ming Tang's research group from Huazhong University of Science and Technology proposes a real-time 400 Gbit/s bidirectional SHCD transmission by employing low-cost uncooled large linewidth DFB lasers for data center interconnects.

However, the practical implementation of an SHCD system still encounters challenges. Bidirectional transmission of signals and remote LOs requires additional optical circulators in SHCD transceivers (Table 2). Furthermore, the sensitivity of the SHCD system to transmission path differences increases with the utilization of larger laser linewidth (Fig. 4). In contrast, the DSCD system exhibits high tolerance for laser linewidth and is unaffected by transmission path differences. In optical power-limited systems, the DSCD-DQPSK system yields comparable performance to the SHCD-QPSK system with optimal power separation ratio (Fig. 6), which is significantly better than the IMDD-PAM4 system (Fig. 7). In OSNR-limited systems, the remote LO quality is inevitably affected by optical noise, which influences the optimal laser power separation ratio (Fig. 8) and the receiver sensitivity of the SHCD system (Fig. 9). Implementing a narrow bandwidth optical filter for the remote LO can filter out a portion of the noise and enhance system performance but at the expense of additional costs. Conversely, in OSNR-limited systems, the receiver sensitivity of the DSCD-DQPSK system is superior to that of the SHCD-QPSK system, and it does not require an additional narrow bandwidth optical filter.

In conclusion, both the SHCD and DSCD schemes realize a significant improvement in receiver sensitivity compared to the IMDD scheme. However, the increased DSP complexity and power consumption for coherent detection is a price for this improvement. Additionally, the SHCD system faces challenges from transmission path differences and noise within the remote LO, and addressing the challenges will increase the system implementation costs. Thus, further reducing DSP power consumption, system complexity, and cost is an important direction for future research for simplified self-coherent schemes. However, compared with the IMDD-PAM4 system and the SHCD-QPSK system, the proposed DSCD-DQPSK system is inherently advantageous and promising for short- and medium-reach optical fiber transmissions.