Gratings of different period spacings are fabricated on quantum well infrared photodetectors having two‐stack multiple quantum wells(MQW) to determine the grating coupling efficiency in multicolor detection. The peak wavelength of each MQW is 4.8 and 9.4 μm, respectively, covering the two atmospheric windows. 2D gratings of box‐shaped cavities with grating period either in 3 or 4.6 μm spacing are used to coupled normal incident light into the detector. Comparing detectors with and without gratings at either normal or 45° facet incidence, the detectors with an optimized grating show efficient light coupling for both wavelengths due to the fact that the longer wavelength in the present case is an integral multiple of the shorter wavelength. The optimized long wavelength grating couples the shorter wavelength through additional higher orders of diffraction.