To modulate the laser pulses are used crystals of niobate lithium LiNbO3 with the properties of gyrotropy and birefringence. Cadmium thiogallate CdGa2S4 has a coefficient of nonlinear susceptibility is five times greater than the coefficient of nonlinear susceptibility of niobate lithium [1]. The dispersion of birefringence in CdGa2S4 and PbGa2S4 crystals studied at room temperature in the region of transparency (0,5 - 3 eV). In both crystals, the intersection of the spectral characteristics nII and n⊥ are observed. In crystals of cadmium thiogallate refractive indices intersect at a wavelength 4857Å. When passing through this point in the long wavelengths CdGa2S4 crystal, from an optically positive (nII > n⊥) becomes optically negative (nII < n⊥), and at λ = 485Å7 becomes optically isotropic (nII = n⊥).figureFig.1. The transmission spectra of CdGa2S4 crystals in the crossed polarizers and the refractive indices of PbGa2S4 crystals. In the transmission spectra of CdGa2S4 crystals with a thickness 5 mm, placed between crossed polarizers, series of lines а1-а11 and b1-b17 are observed, who come running to the isotropic wavelength λ0 = 4857Å, fig.1. This device is a comb filter in the visible wavelengths range. Similar effects are observed in the transmission spectra of lead thiogallate PbGa2S4 crystals. From the interference spectra, spectral dependence of the refractive index nII and n⊥ and their difference Δn = nII - n⊥ are calculated. With the passage of light through a crystal, light waves “fast-nII” and “slown⊥”, polarized parallel to the crystal axis have some delay between the components [2 The phase delay, proportional to the birefringence of the crystal, to create narrow-band filters are needed with a large birefringence crystals Δn = nII - n⊥. In the lead thiogallate crystals this value varies with the wavelength in the range 0-0,2, which is close to the record values. [1] L. Suslikov, Z. Goldmasi, I. Kopinet, V. Slivka, Optica and spectroscopia, 51 (2), (1981), [2] A. Yariv, P. Yen, Optical wavesw in crystals, Edition published by John Wiley&Sons, Inc.1984