The influence of incoherent background illumination produced by light-emitting diodes (LED´s) of different average
wavelengths and laser diode emitting in blue region of visible on diffraction characteristics of narrow coherent light
beams of He-Ne laser due to refractive index changes of Fe-doped lithium niobate sample are studied. It has been
experimentally demonstrated that nonlinear diffraction of red beams with wavelength 633 nm and diameters on full
width of half maximum (FWHM) near to 15 μm may be totally compensated using background light with average
wavelengths 450 – 465 nm. To provide the necessary intensity of incoherent background, the combinations of spherical
and cylindrical concave lenses with blue LED and laser diode module without focusing its beam have been used.