本發明提供之可逆光放大器包含一光增益元件及一光路徑控制單元。該光增益元件具有一輸入端及一輸出端，該輸入端接收一光訊號，並於該輸出端輸出一放大光訊號。該光路徑控制單元具有四個輸入/輸出埠，分別為一第一端點、一第二端點、一第三端點以及一第四端點，該第三端點及該第四端點分別連接於光增益元件該輸入端及該輸出端。該光路徑控制單元用於將該光訊號由該第一端點/該第二端點傳送至該第三端點，並且導入光增益元件，該放大光訊號由該第四端點傳送至該第二端點/該第一端點；此外並提供第一端點與第二端點間不經光增益元件之返回路徑。 A reversible optical amplifier provided by the present invention includes an optical gain element and an optical-path control unit. The optical gain element has an input terminal and an output terminal. The input terminal receives an optical signal and outputs an amplified optical signal. The optical-path control unit has four I/O ports, respectively being a first end, a second end, a third end, and a fourth end. The third end and the fourth end are respectively couple to the input terminal and the output terminal of the optical gain element. The optical-path control unit is utilized to transfer the optical signal from the first/second end to the third end and to lead it into the optical gain element, and then to transfer the amplified optical signal from the fourth end to the second the first/second end. Moreover, a return path that does not pass through the optical gain element is provided between the first end and the second end.