# Performance Monitoring of Hybrid All-Optical Fiber/FSO Communication Systems
###### Abstract  
The demand for network capacity has increased due to the introduction of new digital applications and services, which rely heavily on optical communication networks. While fiber networks serve as the optical networks' backbone, deploying fiber in certain scenarios is not feasible, making it necessary to use other technologies conjointly. A hybrid all-optical fiber/free space optic (FSO) link is proposed to avoid such a challenge. The all-optical system avoids using electronics that have limited bandwidth. Hence, it supports high-capacity communication. However, the all-optical system comes with challenges arising from fiber and FSO channel impairments. To monitor the amount and type of distortion in the optical channel, machine learning (ML) techniques are exploited. In this work, Gaussian process regression (GPR) is utilized as an ML technique to predict three main channel impairments that arise in the hybrid all-optical fiber/FSO channels, which are turbulence, optical signal-to-noise ratio (OSNR), and chromatic dispersion (CD). The model's performance is evaluated using boxplot graphs, root mean square error (RMSE) metric, and R-squared metric. The results indicate that the model can predict the various impairments with high accuracy, except under strong amplified spontaneous emission (ASE) noise, where the model demonstrated lower accuracy in predicting light turbulence parameters. The proposed approach provides a self-aware and self-adaptive communication system and can optimize network resources in the future. all-optical network; hybrid fiber/FSO; machine learning; performance monitoring Article  
Performance Monitoring of  
as a backup for fiber in availability-sensitive networks [4; 5]. FSO also offers several other advantages, including cost-effectiveness, rapid deployment, enhanced security compared to RF links, and the use of unlicensed spectrum. However, there are some challenges associated with FSO links. For example, link blockage by obstacles such as construction work may require re-planning of the FSO link. The possibility of unauthorized access to the communication link is another concern, although it is harder than the RF link to gain access because FSO uses a line-of-sight link. FSO link alignment is another challenge, as it requires precise alignment due to its narrow beam. The safety of living creatures such as birds is also a concern, which can be addressed by respecting the recommended maximum transmitted power. In a traditional intensity modulation/direct detection (IM/DD) FSO system, when the fiber is connected to the FSO system on the transmitter side, the optical signal carried by the fiber should be converted to an electrical signal first to drive the laser. This is achieved using a photodetector (PD), which is an optoelectronic device. The drive circuit modulates the laser light to produce an output-modulated optical signal. Such conversion operation in the transmitter of the traditional hybrid fiber/FSO links from an optical signal to an electrical signal and then to an optical signal increases the system cost and reduces the available bandwidth due to using electronics that have limited bandwidth [6]. To overcome such a limitation, a hybrid all-optical communication fiber/FSO system is proposed [6; 7]. In such a system, the optical signal delivered by the fiber in the transmitter side is coupled to the free space using a light collimating lens. Hence, there is no need for optical/electrical/optical conversion. In the FSO receiver side, the light is collimated to the fiber using another collimating lens. Hence, the signal over the entire fiber/FSO link remains optical. The advantage of a hybrid all-optical fiber/FSO link comes with challenges arising from the channel impairments. In the fiber channel, the optical signal is prone to different types of impairments such as chromatic dispersion (CD) and amplified spontaneous emission (ASE) noise due to signal amplification. In the FSO channel, the signal is prone to turbulence and atmospheric attenuation.