Long-Period Grating (LPG) Sensors Liquid 489 Input optical fiber Thin metal grating on silica Output optical fiber Figure 10.41 Possible SPP refractive index sensor with thin metal film grating [93]. demonstration, the metal grating was sandwiched between two dielectric layers, making it impossible to access the SPP on the metal grating. By not depositing a polymer film on the surface of the grating, it would be possible to measure the refractive index by placing a drop of liquid on it, as shown in Fig. 10.41. Self-referencing in SPP sensors can be done using phase information. In this approach, one arm of a Mach­Zehnder interferometer is used as an SPR sensor, whereas the other arm in close proximity is used as reference. Any temperature change is therefore seen by both arms and to a large extent is canceled. The sensing is achieved by the interference of the unchanged signal in the reference and the phase change sensed by the SPP. Using a grating to excite the SPP enhances the sensitivity of the SPP sensors, as the coupling is strongly de-tuned by the presence of the measurand. The dispersion of the phase is linear close to resonance and is therefore given the refractive index directly. However, the sen- sitivity enhancement is restricted to a region close to the resonance condition, as