Critical Wavelength in the Transmission Spectrum of SMS Fiber Structure Employing GeO $_{2}$ -Doped Multimode Fiber

We demonstrate and explain the existence of a critical wavelength in the transmission spectrum of single-multi-singlemode fiber structures, employing GeO₂-doped multimode fibers (MMFs), such that the spectrum shows opposite spectral shift on either side of this wavelength with increasing temperature. Further, the shift is maximum for peaks/dips nearest to this wavelength. The variation of critical wavelength with temperature is also measured. A theoretical account of the observed behavior has been presented and the results are found to be in excellent agreement with the experiments. It is shown theoretically that for a given MMF there should be two critical wavelengths of the opposite nature separated by a large wavelength difference.