Abstract: Assuming the incident light was a plane wave, the feedback characteristics of double volume Bragg gratings (VBGs) were studied based on Kogelniks theory. According to the analysis, with the structure of the double VBGs, the bandwidth could be broadened and the diffraction efficiency could be improved through choosing the wavelength differences between the two VBGs. For a system consisting of two VBGs whose bandwidth and diffraction efficiency were 100 pm and 20% respectively, the diffraction efficiency was 33.3% when their central wavelengths had the same value, and the bandwidth could be tuned to 1.8 times (180 pm) of a single VBGs bandwidth. Since there was a thermal shift in VBGs central wavelength, changes in diffraction efficiency and bandwidth could be realized by adjusting the VBGs temperature. External cavities with double VBGs offers an efficient method for optimizing pumping source.