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We study experimentally and numerically the polarization sensitivity of quantum well infrared photodetectors coupled to a diffraction grid. The polarization extinction ratio of such system is determined by two factors: polarization sensitivity of the diffraction grid and the intrinsic polarization sensitivity of the photodetector itself. The combined effect of these factors result in non-monotonic dependence of the polarization extinction ratio on the parameters of the diffraction grid. By varying the grid parameters, i.e., increasing the height and tuning the grid period, a maximum value for the polarization extinction ratio can be achieved. Both front side and back side illuminations of the photodetector are studied. The strongest polarization sensitivity is achieved under front side illumination.