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A technique that enables two or more infinite impulse response optical delay lines to be connected in series without the coherent interference problem is presented. It is based on inserting an optical frequency shifter into the infinite impulse response optical delay line structure. Signal processors based on the cascaded optical delay line structure have a higher frequency response performance than the single optical delay line structure. Experimental results are presented, which demonstrate a dual series frequency shifting amplified recirculating delay line can realise a high-resolution, high skirt selectivity, high stopband rejection level bandpass filter response, and a narrow-passband, flat-top bandpass filter response using a non-commensurate delay line approach.