A parametric process is provided for producing visible light at a second wavelength including pumping an optical fiber (4) with input light (1) at a first wavelength of less than one micron, which is longer than the second wavelength, and wherein the optical fiber (4) has zero group velocity dispersion at a third wavelength in the visible or near infrared region and longer than the first wavelength. An optical amplifier that uses the parametric process for producing light at second wavelength is also provided. The optical fiber (4) may be a photonic crystal fiber. The parametric process is tuneable by adjusting the frequency of the zero group velocity dispersion, the frequency of the first wavelength or the birefringence of the optical fiber (4).