Label-Free DNA Detection Using Etched Tilted Bragg Fiber Grating-Based Biosensor.

A label-free-based fiber optic biosensor based on etched tilted Bragg fiber grating (TFBG) is proposed and practically demonstrated. Conventional phase mask technic has been utilized to inscribe tilted fiber Bragg grating with a tilt angle of 10°, while the etching has been accomplished with hydrofluoric acid. A composite of polyethylenimine (PEI)/poly(acrylic acid) (PAA) has been thermally deposited on the etched TFBG, followed by immobilization of probe DNA (pDNA) on this deposited layer. The hybridization of pDNA with the complementary DNA (cDNA) has been monitored using wavelength-dependent interrogation. The reproducibility of the probes has been demonstrated by fabricating three identical probes and their response has been investigated for cDNA concentration ranging from 0 µM to 3 µM. The maximum sensitivity has been found to be 320 pm/µM, with the detection limit being 0.65 µM. Furthermore, the response of the probes towards non-cDNA has also been investigated in order to establish its specificity.

Mach-Zehnder interferometer based fiber-optic nitrate sensor.

A biocompatible, reliable and quick responsive fiber-optic sensor based on Mach-Zehnder interferometer (MZI) is demonstrated for nitrate analytes tracing. The sensor was constructed by collapsing the air holes of a short length photonic crystal fiber (PCF) with the single-mode fibers (SMFs) on both ways. The proposed sensor has been coated with a graphene-PVA (polyvinyl alcohol) membrane using the thermal coating technique to make the sensor attractive to the nitrate ions in the aqueous solution. The maximum response is found to be 0.15 pm/ppm on the nitrate measurement scale of 0 ppm to 100 ppm with an average reaction time of ∼10 s. Also, a short length of FBG (fiber Bragg grating) is implanted with SMF to improve the sensing accuracy of the presented sensor.

PCF based modal interferometer for lead ion detection.

A compact, reliable, and fast responsive PCF (photonic crystal fiber) based modal interferometric sensor for lead ion detection is proposed and experimentally demonstrated. The sensor has been fabricated by splicing a small section of PCF with SMF (single mode fiber) followed by collapsing the air holes of PCF at its tip. The interferometer is dip coated with chitosan-PVA (polyvinyl alcohol) and glutathione functionalized gold nanoparticles. Three probes have been fabricated, and the maximum sensitivity has been found to be 0.031 nm/ppb for lead ions whereas the detection range has been considered from 0 ppb to 50 ppb. The probe has been found to have a faster response time of ∼ 10 s. Furthermore, the sensor has been found to be less responsive towards other heavy metal ions, thereby demonstrating its selectivity towards lead ions. Besides, a section of FBG (fiber Bragg grating) has been embedded into the interferometer and the temperature response of FBG peak along with interference spectra has been investigated for better accuracy.

Low gas pressure sensor based on a polymer optical fiber grating.

A fiber Bragg grating (FBG) inscribed Zeonex-based novel, to the best of our knowledge, side hole polymer optical fiber (SHPOF) is proposed and demonstrated for low gas pressure measurement above and below the atmospheric pressure. Two different grades of Zeonex have been used to fabricate the core and cladding of this fiber, thereby making it dopant free. The side hole introduced in the cladding is parallel to the fiber core. A few ultrashort pulses with nanosecond duration have been used to write the FBG in the core of this fiber. The incorporation of the side hole leads to enhancement of pressure sensitivity as well as low hysteresis and performance repeatability compared to Zeonex-based conventional polymer optical fiber (CPOF). Above the atmospheric pressure, the proposed probe shows a pressure sensitivity of 0.47 pm/kPa, which is 80% more compared to the Zeonex-based CPOF and 0.48 pm/kPa for regime below atmospheric pressure. The sensor has a resolution of 2.12 kPa and exhibited very low hysteresis.

Hydrogel based Fabry-Pérot cavity for a pH sensor.

A simple, reliable, and quick reactive Fabry-Pérot (FP) structure-based fiber optic pH sensor is presented. The pH-sensitive hydrogel and single-mode fiber (SMF) are placed inside a fused silica capillary to form the FP cavity. The gel thickness is characterized by the spin coating method with respect to different spin speeds. The proposed sensor shows a pH sensitivity of 0.30 nm/pH along with a fast response time of 15 s to 20 s for different pH solvents in the acidic range. Also, the temperature sensitivity of the FPI sensor is found to be -0.56 nm/°C.

Silicone Rubber Based Highly Sensitive Fiber-Optic Fabry-Perot Interferometric Gas Pressure Sensor.

A simple, compact, and highly sensitive gas pressure sensor based on a Fabry-Perot interferometer (FPI) with a silicone rubber (SR) diaphragm is demonstrated. The SR diaphragm is fabricated on the tip of a silica tube using capillary action followed by spin coating. This process ensures uniformity of its inner surface along with reproducibility. A segment of single mode fiber (SMF) inserted into this tube forms the FPI which produces an interference pattern with good contrast. The sensor exhibits a high gas pressure sensitivity of -0.68 nm/kPa along with a low temperature cross-sensitivity of ≈ 1.1 kPa/°C.

Fabry-Perot cavity-based contact force sensor with high precision and a broad operational range.

A novel, compact, and robust contact force sensor based on a micro-length single-mode fiber (SMF) incorporated in a cleaved micro-air cavity (MAC) is proposed. The fabrication process involves splicing of the SMF with a hollow-core fiber (HCF) followed by cleaving of the MAC and insertion of a SMF into the MAC. The force sensing mechanism is based on the movement of the micro-SMF inside the cleaved MAC. The total length of the probe varies between 300 and 500 µm, making it bend proof. Due to the all-silica-based structure, the sensing capability of the probe is demonstrated for a low (0-1000 mN), as well as a high range of force (1-10 N) measurements. The optimized structure shows a maximum force sensitivity of 14.2 pm/mN with a negligible temperature dependence of 0.4 pm/°C. The performance of the sensor is verified using an FEM-based software. The proposed probe has a linear response, negligible hysteresis, and repeatability error, making it suitable for biomedical sensing and robotic applications.

Vortex Shedding Optical Flowmeter based on Photonic Crystal Fiber.

In the present work we propose a PCF (photonic crystal fiber) based Modal interferometer detector for sensing low flow velocity by detecting the frequency of vortices shed from a bluff body. The proposed novel design encapsulates the interferometric arm inside a metal casing to protect the sensor from harsh process fluids. The characterization of the developed probe is conducted under no flow conditions using a piezo actuator to evaluate the sensor response over wide frequency range (0-500 Hz). The developed sensors shows a reasonably flat response in the tested frequency range. Experiments are conducted by employing the developed sensor behind a bluff body of a vortex flowmeter to measure the frequency of the shed vortices and hence, the fluid flow rate. The low flow rate sensitivity of the vortex flowmeter is improved many folds by using the present sensor and the minimum Reynolds number detected is Re = 5000. A linear trend is observed between the frequency of the vortices and the flow velocity which is desirable for fluid flow measurement. The PCF based interferometric sensor with metal encapsulation makes the vortex flowmeter, sensitive at low flow rates, robust and economical to be used in industrial application.

Nano-displacement sensor based on photonic crystal fiber modal interferometer.

A stable nano-displacement sensor based on large mode area photonic crystal fiber (PCF) modal interferometer is presented. The compact setup requires simple splicing of a small piece of PCF with a single mode fiber (SMF). The excitation and recombination of modes is carried out in a single splice. The use of a reflecting target creates an extra cavity that discretizes the interference pattern of the mode interferometer, boosting the displacement resolution to nanometer level. The proposed modal interferometric based displacement sensor is highly stable and shows sensitivity of 32 pm/nm.

Fabry-Perot based strain insensitive photonic crystal fiber modal interferometer for inline sensing of refractive index and temperature.

We report a highly stable, compact, strain insensitive inline microcavity-based solid-core photonic crystal fiber (SCPCF) modal interferometer for the determination of the refractive index (RI) of an analyte and its temperature. The interferometer is fabricated by splicing one end of SCPCF with single-mode fiber (SMF) and the other end with hollow-core PCF. This is followed by cleaving the part of the solid glass portion possibly present after the microcavity. The formation of the cavities at the end faces of the SCPCF results in reduction of the period of the interference pattern that helps in achieving distinctiveness in the measurement. Three sensor heads have been fabricated, and each has a different length of the collapsed region formed by splicing SMF with SCPCF. The response of the sensors is found to be sensitive to the length of this collapsed region between SMF and SCPCF with a sensitivity of 53 nm/RI unit (RIU) and resolution of 1.8×10(-4) RIU. The temperature response of the sensor is found to be linear, having a temperature sensitivity of 12 pm/°C. In addition to these findings, the effect of strain on the proposed structure is analyzed in both wavelength and intensity interrogation.