# Integrated Fiber-FSO WDM Access System with Fiber Fault Protection
## 1 Introduction  
Recently, with the increase in the access data traffic numerously owing to the speedy progress of cloud service, quantum computing, artificial intelligence (AI), data center, big data, 4K/8K video, online game, and 3D holographic application, the network service providers need to provide the wide network capacity, low latency, high-speed rate, and cost-effective deployment for end user [1; 2; 3]. Thus, the passive optical network (PON) would be the best candidate for the last mile data traffic access [4; 5]. Here, the time-division-multiplexing (TDM) and wavelength-division-multiplexing (WDM) access are currently the most used technologies. The TDM- and WDM-PONs are the point to multi-point (PTMP) and point to point (PTP) access system [6; 7], respectively. The relative development and standard TDM-PON systems have also been demonstrated fully, such as the Ethernet-PON (EPON, IEEE 802.3 ah), Gigabit-PON (GPON, ITU-T G.984), next-generation PON (NG-PONI, ITU-T G.987) and NG-PON2 (ITU-T G.989) [8; 9; 10; 11]. The traffic rates of these TDM-PONs were from 1 to 40 Gb/s for broadcasting and power-sharing downstream data due to the PTMP operation. To achieve higher data capacity and network security, the PTP-based WDM-PON systems with the traffic rate from 1 to 60 Gb/s and even to 100 Gb/s have been studied and demonstrated extensively [12; 13; 14; 15]. Due to the high amount of data transmitted in WDM system, the stability and reliability of the fiber access network is extremely important [16]. Therefore, under high-speed and wide capacity signal transmission in such PON networks, if the connected fiber path is broken, it will affect the network connection of the related end-users. To prevent the occurrence of fiber breakpoint, the self-healing operation in WDM-PON network is also an important research topic [17; 18; 19; 20]. However, the previous works needed to add extra more passive and active devices in the PON architecture to avoid the fiber fault occurrence. Furthermore, in some situations, the conventional PON might not be suitable for the installation of fiber owing to the geographical restrictions [21]. In PON network, the fiber can be replaced by using the free space optics (FSO) communication in some parts of the optical distribution network (ODN), where installing fiber is hard and expensive. Therefore, use of the hybrid free space optics (FSO) communication and PON access have also been proposed and proven [21; 22; 23]. In addition to being used in environments where optical fibers cannot be deployed, the FSO transmission also makes the access network more flexible and reliable [24]. In this paper, an integrated fiber-FSO WDM-PON system with self-protected mechanism against fiber fault is proposed. 10 Gb/s on-off keying (OOK) bidirectional WDM fiber and FSO signals in the same PON architecture are achieved simultaneously. Here, we apply C- and L-band wavelengths regarding as the fiber and FSO data traffics, respectively. To demonstrate the signal performances of fiber and FSO access, the eight C- and L-band wavelengths with 0.8 nm channel spacing of 1535.04 to 1540.56 nm (\(\lambda_{\text{1C}}\) to \(\lambda_{\text{8C}}\)) and 1560.61 to 1566.31 nm (\(\lambda_{\text{1L}}\) to \(\lambda_{\text{8L}}\)) are exploited for presentation, respectively. The access point (AP) can be deployed independently or integrated into optical network unit (ONU) to access the FSO signal according to the actual environment and demand. The proposed WDM-PON system not only can deliver the fiber and FSO signals simultaneously, but also can provide the fault protection for guaranteeing fiber signal access. All the obtained power budgets of testing signals can meet under the forward error correction (FEC) level in the presented PON architecture. Moreover, all the redundant power budget of FSO traffic also can support bidirectional free space transmission of 800 m without signal amplification based on the previous optical design system [25].