## 1 Introduction  
The wavelength-division-multiplexed passive optical network (WDM-PON) will play a promising role in future broadband access solutions, owing to its wide bandwidth, high security, easy upgradeability, and network flexibility [1, 2, 3]. The low-cost WDM-PON schemes using tunable lasers have become practical for deployment in the access and transport networks and are considered in the standardization process [4, 5]. These schemes can provide the needed bandwidth and performance for future mobile networks as well as reach extension and service provisioning for thousands of users. Due to the extended transmission distance and large power splitting, optical amplifiers are crucial for the network planning and deployment needs.  
There are three basic types of optical amplifiers that may be used at different positions of an optical network. A post-amplifier placed immediately after optical transmitters can be used to boost the signal level to extend reach. It features high input power, medium optical gain, and high output power. The noise figure (NF) of a booster amplifier is not usually a critical parameter [6, 7, 8]. At the receiving end of an optical link, a pre-amplifier may be required to amplify the weak optical signal before a photodetector in order to improve the signal-to-noise ratio (SNR). A large gain with low NF is needed [6, 7]. To compensate for the transmission loss of a link of long distance or large number of power splitting, in-line amplifiers are placed within an optical link. An in-line amplifier typically requires a moderate gain, a low NF, and a high output power [6, 7]. Because of its high gain, high power, low NF, and insensitivity to polarization of the signal, the erbium-dopedfiber amplifier (EDFA) has become the most popular optical amplifier and a necessary component in optical fiber communication systems [9].  
The protection scheme for optical networks is vital to assure network reliability and service quality. This is even a critical issue in the access networks connecting mobile networks and data centers due to the drastically increasing traffic in the links. Moreover, the present WDM-PON architecture is expected to play an important role in the infrastructure of optical millimeter-wave (MMW) data access and 5G/B5G mobile fronthaul and backhaul links [10; 11]. Table 1 shows the failure rates and repair times for some network components [12; 13]. Because of its high failure rate and poor reliability, the EDFA fails easily, possibly causing a great loss of the optical signal that passes through it. Therefore, a protection scheme for optical amplifiers is a vitally important topic. The protection for optical amplifiers in WDM-PONs is extremely important since an EDFA can serve tens or even hundreds of wavelength channels.  
To date, most of the protection schemes for WDM-PONs have stressed the protection of working feeder fibers or distribution fibers [14; 15; 16; 17]. Nevertheless, in [13], the proposed optimal protection architecture for WDM-PONs based on various design rules can protect the fixed transceivers, the EDFA in an optical line terminal (OLT), and the trunk between the OLT and remote node (RN). However, this protection architecture does not meet the protection requirement of the uplink EDFA.  
To provide a consummate protection for EDFA, we propose a protection scheme for WDM-PONs with reconfigurable optical amplifiers (ROA) whereby the working EDFAs on both downlink and uplink paths can be protected. In our scheme, the ROAs are implemented in the OLT to enable switching in different protection scenarios. The detailed operation principles and the implementation of ROAs are discussed in Sections 3 and 4, respectively. The feasibility of the scheme is verified by simulations with 10 Gb/s downstream and upstream transmissions in Section 5.