A new technique that can suppress the coherent interference effect in an optical delay line signal processor is presented. It is based on single tone phase modulation of the optical carrier to translate the phase-induced intensity noise at baseband into high frequencies. This technique enables a narrow-linewidth telecommunication-type laser to be used as an optical source for a delay line signal processor with robust operation. It has the advantage of not affecting the signal processor and the link configurations. Experimental results demonstrate a robust delay line notch filter frequency response using a narrow-linewidth optical source.