The present invention provides a method for producing a Mach-Zehnder filtering device. The method includes the following steps: (a) providing an optical fiber having a jacket and a core containing rare earth element dopants; (b) stripping the jacket of a segment of the optical fiber; and (c) performing a fused-tapering to the stripped segment of the optical fiber to form a first and a second necks simultaneously, wherein the core exists in both the first and the second necks to form a Mach-Zehnder interferometer.