Big data services have yielded explosive growth of capacity in short-reach optical networks. The intensity modulation direct-detection (IMDD) systems with simple schemes and power-efficient digital signal processing (DSP) are typically preferred in short-reach scenes. However, they are unable to satisfy the demand for continually increasing interface speed. The Ethernet interface data rate is approaching 800 Gbit/s and 1.6 Tbit/s. Hence, the conventional IMDD will suffer from serious technical challenges, including dispersion-induced power fading, rapidly increasing cost, and limited sensitivity.

As an alternative, coherent technology can provide high spectral efficiency, high sensitivity, and good tolerance to the chromatic and polarization-mode dispersion. However, for short-reach application, this technology is considered overly costly and power-consuming. These drawbacks originate from two main challenges. On one hand, power-consuming DSP is required for solving various impairments on the received signal in traditional coherent schemes. Moreover, with the fading of the Moore’s law, the node gain brought by new footprints of application specific integrated circuits (ASICs) tends to be marginal. Using only advanced DSP in the development of coherent technology for short-reach high-speed interconnections is quite difficult. On the other hand, traditional coherent technology is associated with complex hardware structures, especially the adoption of narrow-linewidth, frequency-stable, and tunable lasers, such as external cavity laser and integrable tunable laser assembly (ILTA). Consequently, coherent technology is still inapplicable to short-reach networks.

Apart from conventional IMDD and coherent technology, many self-coherent schemes have been proposed with certain tolerance to laser linewidths and less implementation complexity (than that of the conventional systems). The Kramers-Kronig receiver (KKR) and Stokes vector receiver are two typical schemes, each receiving considerable attention. In terms of the product of analog to digital converter ADC bandwidth and number of ADCs, these schemes are more expensive while achieving the same capacity as that of the conventional coherent technology. The self-homodyne coherent (SHC) scheme has been proposed as another "coherent-lite" scheme, including polarization division multiplexing and space division multiplexing as the categories. The key feature of this scheme is that the signal lights transmit simultaneously along with their pilots in links. At receiver, coherent detection will be conduced though the remotely delivered pilot to achieve optical domain phase recovery of signal. Thus, the cost and the power consumption are reduced, and the use of low-cost and uncooled distributed feedback (DFB) laser and baud-rate-sampling receivers is realized. The advantages of the coherent technology are therefore inherited and the scheme is simplified, and hence this technology is considered one of the most promising technologies for future short-reach optical networks. Despite the excellent characteristics of SHC schemes, many key implementational issues must be solved prior to deployment.

In space division systems, the relative time delay will induce a unique phase noise and degrade the system performance, which may prevent use of the low-cost DFB laser in the SHC scheme. Fortunately, the derivative of such phase noise is a colored frequency modulation noise. Utilizing this characteristic, we proposed and demonstrated an in-service high-precision and large-dynamic-range estimation method of relative time delay (RTD), contributing to the realization of the SHC technique. Besides, the random birefringence in the optical fiber will lead to changes in the state of polarization (SOP) during delivery of the pilot and signal. Another implementational issue is that automatic compensations of such randomly changed SOP is required for real fiber links. By leveraging our in-house-developed adaptive polarization controller (APC), we solved both problems of polarization demultiplexing in polarization division multiplexed (PDM) SHC systems and pilot SOP locking in space division multiplexed (SDM) SHC systems. The APC technique allows further simplification of the DSP algorithms. Utilizing only one APC device and its symmetry property, we also demonstrated the first multi-input and multi-output (MIMO)-free SDM-SHC transmission and PDM-SHC transmission in bidirectional (BiDi) scenes. The APC technique paves the way for low-cost, power-efficient, high-speed BiDi optical interconnections. We present an overview of the progress that our group has realized for SHC systems, including the APC techniques, the in-service RTD estimation techniques, and the simplest BiDi SHC transmission architectures based on APC techniques.

The SHC scheme capable of optical-domain equalization, high spectrum efficiency, and compatibility with current ASIC architecture has been demonstrated. This scheme provides a promising method for future low-cost short-to-medium-reach optical interconnections. Moreover, the SHC technology will generate new demands from other technical areas, including photonic integration, special optical fibers, and DSP. This technology will promote and accelerate innovations in multiple fields.