Application of resonant subwavelength gratings to a rotary position encoder

Resonant subwavelength gratings have proven to be excellent devices for producing narrow resonances useful for filtering applications. In this paper we discuss the use of RSGs in a rotary position encoder intended for use in harsh environments. To avoid problems with routing electrical signals to the encoder, a single fiber optic connection is used to address the device with multiplexed wavelengths corresponding to position bits. Each wavelength has a corresponding RSG that is patterned in the appropriate position locations. A demonstration device utilizing RSGs with TiO2 and SiO2 films on a silicon substrate will be presented. The design and modeling effort provided several RSGs with resonances addressable by a single tunable laser source. Since multimode fiber is used to route the optical signals, the gratings were designed to be polarization insensitive. Additionally, the individual RSGs accommodate significant wavelength shifts to simplify the integration of the encoder system. The fabrication of the devices was based on electron beam lithography and details of this work will be presented. Measurements of the individual RSGs as well as a demonstration of the determination of rotary position using these gratings will be shown.