The Resonant Mirror (RM) Sensor is a leaky planar waveguide optical sensor that uses frustrated total internal re ̄ection (FTR) to couple light into and out of a leaky waveguiding layer. RM modes are dispersive as well as leaky, causing the coupling angle for a particular mode to shift as the illumination wavelength changes. This is a particular problem when using coherent illumination from a laser diode, as these are prone to mode hopping as the diode temperature changes. This leads to sudden changes in the wavelength of the laser output, causing a similar abrupt change in the measured resonance angle. Without some form of referencing, it is impossible to determine whether a change in resonance angle is a result of a wavelength change or a change in surface refractive index.To overcome this problem, an additional buried RM waveguide layer was incorporated into the sensor structure. By using a slightly different thickness for the buried waveguide layer, a second resonance could be obtained for one polarisation which was much less sensitive to surface refractive index changes, but had the same dispersion as the conventional surface RM resonance. As a result, the difference in resonance angle between the buried and surface modes was only sensitive to changes in the refractive index of the overlayer above the surface RM layer. The device is slightly sensitive to changes in temperature, although the response is dominated by the temperature coef®cient of refractive index of the aqueous overlayer. Since the RM layers are very thin( ~1.3 mm in total), thermal equilibrium between the surface and buried modes should be established very rapidly.To demonstrate the utility of this approach, modi®ed RM sensors were fabricated using CVD of silica and silicon nitride. The sensors were tested in two ways; ®rstly by changing the wavelength of illumination using a series of interference ®lters, and secondly by placing materials of different refractive index on the surface of the device. In the ®rst case, the two resonance peaks moved by the same angle, while in the second case, the surface mode moved approximately 25 times further than the buried mode.