Grating-coupled multimode fiber optics for filtering and chemical-sensing applications

A multimode fiber optic coupled to a planar diffraction grating offer a very compact and sensitive sensing method to detect and monitor chemicals and their reactions. Starting with dispersion studies of the fiber/grating system, we proceed to show its applications in detecting refractive index of liquids. We show that two different types of sensors result, depending on the coupling arrangements between the fiber and the grating. In the first arrangement, the fiber cladding is removed and the fiber core is pressed against the planar grating. In the second type, the fiber is located directly above the grating at a certain angle. In both cases, the fiber optic acts as a slit permitting only a certain range of the wavelengths to be transmitted between the fiber and the grating. In both cases, the fiber-optic signal shows an asymmetric spectrum, which extends more gradually into shorter wavelengths and diminishes abruptly in the longer-wavelength portion of the spectrum. These phenomena are explained by using the well-known diffraction equations. Furthermore, applications of these devices as narrowband filters and chemical or gas sensors are discussed.