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A new technique to generate all-optical negative coefficients in photonic signal processors is presented. It is based on using a dual-output Mach-Zehnder modulator, excited with two wavelengths, to obtain radio-frequency (RF) signal modulated optical carriers in opposite phase, together with a wavelength selection scheme to separate each wavelength and to perform the function of producing negative taps. This eliminates the need to convert to the electrical domain to obtain subtraction and also eliminates the speed, noise, and distortion limitations of previous optical cross-gain modulation approaches. Experimental results on a high-skirt-selectivity bandpass filter demonstrate the all-optical negative coefficient generation mechanism and show the synthesis of high-speed negative taps in the optical domain.