Tunable narrowhyphen;band optical filters over a wide spectral range are constructed. These use the efficient coupling of orthogonally polarized optical waves near the isotropic wavelength in gradedhyphen;composition mixed crystals of CdS1minus;xSexand Zn1minus;xCdxS. In one crystal of CdS1minus;xSex,xvaried continuously between 0.04 and 0.29 and the central wavelength lgr;iof the corresponding tunable filter from 5130 to 5700 Aring;. In one sample of Zn1minus;xCdxS,xvaried from 0.41 at one end to 0.70 at the other and lgr;ishifted from 4272 to 4584 Aring;. In both cases the transmission spectra were narrow peaks with peak transmission of 10ndash;20percnt; and widths at halfhyphen;maximum of approximately 10 Aring;.
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