Narrowband grating filters were fabricated in GeO2 thin-film optical waveguides by optically induced changes in the refractive index. Intense counterpropagating laser beams inside the waveguide resulted in distributed feedback reflectors having a peak efficiency of 40%. The spectral response of the filters was measured by thermal tuning of the center frequency and agreed with the results of a coupled-mode analysis. The filter bandwidths were <0.01 nm.
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