The present invention provides an optical fiber excellent in nonlinearity, especially an optical fiber having a low dispersion stable in a wide wavelength region near 1550 nm and a high nonlinearity characteristic are disclosed. An optical signal processing device using this optical fiber, especially an optical wavelength converter and a pulse compressor are disclosed. The optical fiber comprisesa core and a clad provided around the core. The optical fiber is characterized in that the dispersion slope at 1550 nm wavelength is in the range from -0.01 to 0.01 ps/nm<2>/km, the absolute value ofthe dispersion at 1550 nm wavelength is in the range below 10 ps/nm/km, and the nonlinearity coefficient is in the range above 30x10<-10>/W. Further, another mode of an optical fiber having a polarization holding characteristic, a low loss, a high optical signal process efficiency, and a high nonlinearity is disclosed.