# Special Issue on Optical Communications and Networking: Prospects in Industrial Applications
## 2 Special Issue Papers  
In this Special Issue, we have carefully selected nine articles, including one invited and eight contributed papers. These papers cover advanced techniques for short-reach applications, such as data centers, long-distance applications such as trans-oceanic links from systems that use single mode fibers, and the systems use free-space transmission, such as visible light communication (VLC) systems and satellite-to-sea laser communications. The following are the summary of these papers:  
* Phase Noise Cancellation in Coherent Communication Systems Using a Radio Frequency Pilot Tone [3]  
In long-haul optical fiber communication, DSP-based (digital signal processing) dispersion compensation can be distorted by the equalization-enhanced phase noise (EEPN), due to the reciprocities between the dispersion compensation unit and the local oscillator (LO) laser phase noise (LPN). In this invited paper, the authors demonstrated a unique approach for phase noise cancellation using a radiofrequency pilot tone. They showed that the RF pilot tone can entirely eliminate the LPN and efficiently suppress the EEPN when it is applied prior to the carrier phase recovery. \(\bullet\) 400 GbE Technology Demonstration Using CFP8 Pluggable Modules [4]  
In this paper, the authors reviewed the current status of 400 GBASE client-side optics standards and multi-source agreements (MSAs). They then compared different form factors for 400 GE modules, including CFP8, OSFP, and QSFP-DD. The essential techniques used to implement 400 GE, such as pulse amplitude modulation (PAM4), forward error correction (FEC), and a continuous time-domain linear equalizer (CTLE), are also discussed. In addition, a 400 GE physical interface card (PIC) in Juniper's PTX5000 platform has been developed, conforming to the latest IEEE802.3 bs standard. To validate the PIC's performance, a commercial optical network tester (ONT) and the PIC are optically interconnected through two CFP8-LR8 modules. The CFP8-LR8 module utilizes eight optical wavelengths through coarse wavelength division multiplexing (CWDM). Each wavelength carries a 50 Gb/s PAM4 signal. The signal transmits through 10 km of single mode fiber (SMF). The ONT generates framed 400 GE signal and sends it to the PIC through the first CFP8 module. The PIC recovers the signal, performs an internal loopback, and sends 400 GE signal back to the ONT through the second CFP8 module. The optical spectrum, eye diagram, receiver sensitivity, long time soaking results, and internal digital diagnosis monitoring (DDM) result are fully characterized. The pre-FEC bit error rate (BER) is well below the KP4 FEC threshold of \(2.2\times 10^{-4}\). After KP4 FEC, an error-free performance over 30 km of SMF is achieved. At the end, the authors demonstrated both the interoperation between the PIC and the ONT, as well as the interoperation between the two CFP8 modules. This demonstration represents the successful implementation of the 400 GE interface in the core IP/MPLS router. \(\bullet\) Power and Signal-to-Noise Ratio Optimization in Mesh-Based Hybrid Optical Network-on-Chip Using Semiconductor Optical Amplifiers [5]  
To address the performance bottleneck in metal-based interconnects, hybrid optical network-on-chip (HONoC) has emerged as a new alternative. However, as the size of the HONoC grows, insertion loss and crosstalk noise increase, leading to excessive laser source output power and performance degradation. In this paper, the authors proposed a low-power scalable HONoC architecture by incorporating semiconductor optical amplifiers (SOAs). An SOA placement algorithm is developed considering insertion loss and crosstalk noise. Moreover, the authors establish a worst-case crosstalk noise model of SOA-enabled HONoC and induce optimized SOA gains with respect to power consumption and performance, respectively.