Longitudinal zero-dispersion wavelength (ZDW) fluctuations in long waveguides impose a fundamental limit on the achievable parametric mixer bandwidth. We demonstrate for the first time that the precisely measured ZDW profile can be taken advantage of by applying spatially controlled tension along the fiber length for fluctuation reduction and synthesis of a wide mixer response. The technique was experimentally validated by measuring local dispersion and tension maps and synthesizing a 145-nm-wide fiber parametric amplifier.