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We propose the in-band optical signal-to-noise-ratio (OSNR) monitoring method using a Mach-Zehnder interferometer. This interferometer is implemented by inserting a piece of Yb3+-doped optical fiber (YDF) between two strands of thermally expanded core (TEC) fibers: the former to give optical phase delay to one of the two arms (core and cladding modes of the YDF) and the latter to give two arm paths. By using TEC fibers to split the input signal and combine again, the fringe pattern necessary for OSNR monitoring shows polarization independence and wideband characteristics. The phase of the optical signal propagating along the core mode of YDF is changed by flowing a laser light of 976 nm to the core of YDF. Compared with the OSNR measured by optical spectrum analyzer (OSA), the errors are less than 0.5 dB over the OSNR range of 20 dB. Also, the dynamic range showed over 20 dB at the wavelength range of 1541-1566 nm.