# Hybrid Self-Protected Fiber-FSO WDM-PON System with Fiber Breakage Prevention
###### Abstract  
In this research, we present and design a blended wavelength-division-multiplexing passive optical network (WDM-PON) architecture to deliver both baseband and free-space optical (FSO) signals by using two wavelength bands over an available bandwidth to avoid the signal interference. Only using few additional components in the optical line termination (OLT) and optical network unit (ONU)/optical wireless unit (OWL) can achieve self-protected operation in the presented hybrid fiber-FSO access network against fiber breakage. In the measurement, eight WDM wavelengths with 24.3 Gbit/s on-off keying (OOK) modulation format are applied to verify and discuss the bit error rate (BER) performances of baseband and FSO signals, respectively, through 25 km fiber and 2 m wireless FSO connections. Therefore, the presented PON not only can deliver both baseband and FSO traffics, but can also provide a self-restored mechanism against fiber fault.  
self-healing; wavelength division multiplexing (WDM); free-space optical (FSO); passive optical network (PON); Rayleigh backscattering (RB) +
Footnote †: journal: _Photonics_  
studies have been also proposed via the hybrid fiber-FSO access network, together with RB mitigation [22]. In addition, once a fiber breakage has occurred in the feeder or distribution fiber of a PON system, it will damage the bidirectional baseband and FSO signal transmissions between the optical line termination (OLT) and optical network unit (ONU)/optical wireless network (OWU) [23; 24; 25; 26]. The fiber breakage on PON access is a very important issue for data connection, especially in high-speed and high-capacity baseband or FSO signal transmissions. To evade this problem, employing the alternative fiber connections between OLT and a remote node (RN), or RN and ONU/OWU to a route transmission path for reconnection baseband or FSO data are the most common methods. The only consideration for these usage methods is how to apply the least optical components in OLT, RN and ONU/OWU, respectively.  
In this paper, to satisfy the demand of broad capacity and seamless signal access, a hybrid fiber-FSO WDM-PON architecture is presented and performed experimentally. We apply 24.3 Gbit/s OOK modulation format to provide the symmetric fiber and FSO WDM signal transmissions in PON access simultaneously. In this design, we can separate the fiber and FSO wavelengths into two bands (A and B bands) over the available bandwidth for bidirectional transmissions, respectively. Besides, based on the periodic wavelength arrangement of arrayed waveguide grating (AWG), we can assign different wavelengths of the A and B bands for downstream and upstream traffics to avoid the Rayleigh backscattering (RB) noise, simultaneously. Additionally, all the fiber and FSO signals can be transmitted through the same AWGs used for data link in the proposed WDM-PON architecture. Here, to reach the fiber fault protection, we only require the addition of two components in the optical line termination (OLT) and optical network unit (ONU)/optical wireless unit (OWU), respectively. In this demonstration, we select eight wavelengths in the A and B bands for fiber and FSO traffics to exhibit the downstream and upstream performances in the normal and fault protection status, respectively, after passing through 24 (or 25 km) fiber link and 2 m free space transmission. The obtained power budgets of the selected eight wavelengths can meet with total insertion loss induced by the designed PON under the forward error correction (FEC) level when the corresponding bit error rate (BER) is \(\leq\)3.8 \(\times\) 10\({}^{-3}\). According to the obtained redundant power budget of FSO signal, the free space transmission length can be between 860 and 1000 m according to the previous optical system design [27]. As a result, the presented hybrid PON system can not only transmit fiber and FSO signals simultaneously, but can also provide a self-protected operation to prevent the fiber breakage on the feeder and distribution fibers.