We propose and demonstrate a novel temperature-insensitive bio-sensor for accurate and quantitative detection of Escherichia coli (E. coli) bacteria in water. Surface sensitivity is maximized by operating the long-period fiber grating (LPFG) closest to its turnaround wavelength, and the temperature insensitivity is achieved by selectively exciting a pair of cladding modes with opposite dispersion characteristics. Our sensor shows a nominal temperature sensitivity of ∼1.25  pm/°C, which can be further reduced by properly adjusting the LPFG lengths, while maintaining a high refractive index sensitivity of 1929 nm/RIU. The overall length of the sensor is ∼3.6  cm, making it ideally suitable for bio-sensing applications. As an example, we also show the sensor's capability for reliable, quantitative detection of E. coli bacteria in water over a temperature fluctuation of room temperature to 40°C.

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...

No abstract

We propose a novel kind of the low-loss THz Waveguide Bragg Grating (TWBG) fabricated using plasmonic two-wire waveguide and a micromachined paper grating for potential applications in THz communications. Two TWBGs were fabricated with different periods and lengths. Transmission spectra of these TWBGs show 17 dB loss and 14 dB loss in the middle of their respective stop bands at 0.637 THz and 0.369 THz. Insertion loss of 1-4 dB in the whole 0.1-0.7 THz region was also measured. Finally, TWBG modal dispersion relation, modal loss and field distributions were studied numerically, and low-loss, high coupling efficiency operation of TWBGs was confirmed.The terahertz (THz) spectrum has seen significant technological advances over the past few decades, which were mainly driven by applications in sensing [1], industrial characterization [2], and fundamental photonics [3]. Recently, THz communications became an active research topic, driven by the availability of unregulated bandwidth and a promise of much higher transmission rates as compared to microwave communications. Communication applications rely heavily on the availability of high quality signal delivery and signal processing components such as waveguides and waveguide Bragg gratings. However, design of lowloss, high coupling efficiency optical components in THz spectral range proved to be challenging due to high losses and high material dispersion of the materials. Recently, fabrication of the THz fiber Bragg gratings was reported in [4, 5] using laser inscription on plastic subwavelength fibers. However, subwavelength fibers suffer from low operational bandwidth and high dispersion [6], which limits the THz communication bandwidth and the signal transmission length.In this work, we demonstrate fabrication and optical characterization of a novel kind of low-loss, high coupling efficiency THz Waveguide Bragg Grating using the combination of a low-loss, low-dispersion plasmonic twowire waveguide and a low-loss micromachined paper grating. We believe that such TWBGs can pave the way for novel high quality signal processing components for demanding THz communication applications.We start by discussing optical properties of the fundamental mode of a THz Waveguide Bragg Grating. In our calculations we use COMSOL Multiphysics FEM software to solve for the modal complex effective refractive indices and field profiles. The TWBG considered in this work is composed of a paper grating inserted between two copper wires (see Fig. 1(a)). For simplicity of presentation, the paper grating with pitch =225 μm is replaced by the uniform paper layer of the same thickness H=100 μm, while Bragg condition is imposed via simple reflection of the modal dispersion relation at the edge of the first Brillouin zone / BZ

We report fabrication of THz fiber Bragg gratings (TFBG) [3] are relatively transparent for THz radiation. Hence, innovative security and qualitycontrol 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-index LDPE fibers using a class IV Synrad CO2 laser operating at 10.6 µm with average output power of 1.5 W and repetition rate of 20 kHz. The TFBG presented in this report consists of 48 notches that are ~110 µm-deep and ~170 µm-wide (see Fig. 1 (a)). Grating period is designed to be 340 µm, in order to place the TFBG stop band in the low-loss transmission window of the subwavelength fiber which is centered at ~0.3 THz. TFBG total length is 17 mm. The TFBG transmission spectra were recorded using a THz-time domain spectroscopy (TDS) setup modified for fiber measurements [10]. In our studies we used ~600 pslong scans that resulted in spectral resolution of ~1.5 GHz. A ~4 GHz-wide stop band (Full Width at Half Maximum,