Ideal wavelength-selective filters with a flat transmission passband, large out-of-band rejection, and reduced sensitivity to fabrication can be realized by use of adiabatic coupling. The physical construction of the filter limits adiabatic power exchange to a specific range of wavelengths, thus yielding a boxlike response. Such robust response, however, requires an increased device length compared with that of optimized filters that achieve the same filter characteristics. No sophisticated synthesis method is required here, and an accurate description of filter response follows from simple physics.
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