US Patent 7295748 Variable optical attenuator with wavelength dependent loss compensation

The invention relates to compensating for the wavelength dependent loss (WDL) in a variable optical attenuator (VOA) system by using a chromatic dispersion wedge, which is inserted between the mirror and the lens. Different wavelength components, e.g. shorter blue and longer red wavelengths, are redirected at different angles by the chromatic dispersion wedge and now arrive at the output fiber at different positions. Thus the smaller mode field diameter (MFD) or mode spot size of the blue light, overlaps more of the core of the output fiber by being closer thereto and now has less insertion loss (IL) as compared to a VOA without WDL compensator.