- Surface plasmon resonance biosensors based on grating coupling exhibiting two plasmons are less known because usually thick gratings and thick metal films are used. In this paper we show that when thin dielectric grating is used on top of thin metal film two surface plasmons are generated at the two boundaries of the metal film represented as two dips in the reflectivity or peaks in the absorption. One of the plasmons is sensitive to the analyte refractive index (sensitivity 580nm/RIU) while the other is sensitive to the refractive index of the substrate; hence it can be used as a reference. This self-reference makes the measurement more accurate and less sensitive to temperature fluctuations and optomechanical drifts. Field distribution calculations show that the plasmon excited at the metal-substrate interface is a long range plasmon with large penetration depth.