Design of the Bimodal Grating Sensor with a Built-In Mode Demultiplexer

This new sensor design provides good volume sensitivity (around 1600 nm/RIU) via collinear diffraction by the asymmetric grating placed in the waveguide vicinity. It provides the mode transformation between the fundamental TE0 and the first TE1 modes of the silicon wire (0.22 mu m by a 0.580 mu m cross-section) in the water environment. In order to provide the wavelength interrogation with a better extinction ratio for the measuring signal, the grating design is incorporated with the mode filter/demultiplexer. It selects, by the compact directional coupler (maximum 4 mu m wide and 14 mu m long), only the first guided mode (close to the cutoff) and transmits it with small excess loss (about -0.5 dB) to the fundamental TE0 mode of the neighboring single mode silicon wire, having variable curvature and width ranging from 0.26 mu m to 0.45 mu m. At the same time, the parasitic crosstalk of the input TE0 mode is below -42 dB, and that provides the option of simple and accurate wavelength sensor interrogation. The environment index is measured by the spectral peak position of the transmitted TE0 mode power in the output single mode silicon wire waveguide of the directional coupler. This type of optical sensor is of high sensitivity (iLOD similar to 2.1 x 10(-4) RIU for taking into account the water absorption at 1550 nm) and could be manufactured by modern technology and a single-step etching process.

Automated summary

This new sensor design utilizes collinear diffraction by an asymmetric grating to achieve good volume sensitivity. It transforms the modes of the silicon wire in a water environment and incorporates a mode filter/demultiplexer for wavelength interrogation. The sensor demonstrates high sensitivity and can be manufactured using modern technology.