A high-efficiency optical filter, based on a counter-propagating wavelength converter in ZnTe, is theoretically demonstrated. The filter has extremely narrow bandwidth of 0.5 A corresponding to pump 0.3 Wpower and 1 cm length of quasi-phase-matching (QPM) grating, tunable over several nanometers of bandwidth by combination of the grating length and pump power variation. The presented scheme is also a candidate for a wavelength-dependant switch.