Resonant THz sensor for paper quality monitoring using THz fiber Bragg gratings

Abstract

We report fabrication of THz fiber Bragg gratings (TFBG) using CO 2 laser inscription on subwavelength step-index polymer fibers. A fiber Bragg grating with 48 periods features a ~4 GHz-wide stop band and ~15 dB transmission loss in the middle of a stop band. The potential of such gratings in design of resonant sensor for monitoring of paper quality is demonstrated. Experimental spectral sensitivity of the TFBG-based paper thickness sensor was found to be ~-0.67 GHz / 10 µm. A 3D electromagnetic model of a Bragg grating was used to explain experimental findings. Terahertz (THz) waves offer unique opportunities that are not available at other wavelengths. Firstly, THz radiation is non-ionizing, which can be employed for safe imaging. Secondly, many materials, such as ceramic [1], plastic [2], and paper-based materials [3] are relatively transparent for THz radiation. Hence, innovative security and quality-control applications are envisioned by utilizing THz waves. Most of the current THz sensors are realized in the non-resonant configurations where sample is interrogated directly by the THz light. In resonant sensors, changes in the sample properties are measured indirectly by studying variations in the optical properties of a resonant structure coupled to a sample. In this letter, we study the use of THz fiber Bragg grating for resonant THz sensing. Fabrication of the THz fiber Bragg gratings (TFBGs) requires availability of the low-loss waveguides. One of the simplest examples of a low-loss THz waveguide is a subwavelength step-index plastic fiber [4]. In such fibers, a large fraction of the modal power is found outside of the fiber core, which not only significantly reduces the transmission losses, but also makes such fibers a promising platform for sensing applications. Recently, fabrication of several fiber-based components such as TFBGs [5] and THz notch filters [6] was reported using laser inscription on THz fibers. Very precise, however, expensive excimer laser system was used for inscription. In this work, we demonstrate fabrication of TFBGs using a cost effective CO2 laser inscription system, which is widely used for long period grating fabrication on silica fibers in the near-IR [7, 8]. Optical response of such gratings and their application in sensing were studied theoretically using GratingMOD module from RSoft Design Group. Fabricated TFBGs were then used to build prototypes of resonant multi-measurand sensors for monitoring paper quality, which is an important manufacturing problem [9]. For the lack of space, in this letter we only present the use of such sensors for paper thickness monitoring, while only indicating the other sensing modalities. The subwavelength step-index fibers with a diameter of 400 µm were drawn in-house from the low density polystyrene (LDPE) rods. LDPE has a relatively low material loss (~0.2 cm-1) and almost constant refractive index (RI) of 1.54 in the 0.2-0.5 THz region studied in this work [4]. TFBGs were then inscribed point-by-point on the 5 cm-long pieces of step-inde...

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“…The approach that was developed by us (analysis of the spectral line dynamics) can be used under real conditions together with other THz sensors platforms (for example, [68][69][70][71]) used for the identification and detection of the substance. It might essentially increase a probability of the substance detection.…”

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