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We propose a novel differential phase technique for improving the performance of surface plasmon resonance (SPR) biosensor systems. Typical prism-coupled schemes use a Mach-Zehnder interferometer to analyze the phase change caused by the SPR effect. In the conventional case, since the probe beam and the reference beam are required to separate into two different optical paths, mechanical drift and vibrations introduces a high level of unwanted noise. In the new scheme, we use a differential phase sensing technique, which means that both the probe beam and reference beam traverse identical optical paths in the system. Experimental results obtained from water/glycerin mixtures show good measurement sensitivity (5.48 /spl times/ 10/sup -8/ refractive index unit) thus demonstrating the performance merit of our technique.