Photonic crystal fiber sensor based on hybrid mechanisms: Plasmonic and directional resonance coupling

We propose a special refractive index sensor design based on a photonic crystal fiber. Two analyte channels are introduced, with one analyte channel coated with gold layer and the other one without gold layer. A hybrid resonance method is used in the sensor to achieve a large dynamic index range, where surface plasmon resonance occurs when the analyte index is lower than that of the fiber material, while the core mode couples with the resonant mode of the adjacent analyte-filled cylinder when the analyte index is larger than the fiber material. When considering fluorinated polymer fibers, a broad index range of analyte refractive index from 1.25 to 1.45 with high sensitivity can be achieved. The maximal sensitivities reach 1.4 × 10⁴ nm/RIU and 2.7 × 10⁴ nm/RIU respectively when refractive index is in the range of 1.25 to 1.383 and 1.383 to 1.45. The sensor characteristics, make this simple sensor very interesting for detecting a wide range of fluid's refractive index or chemical agent concentration.     

Simultaneous measurement of refractive index and temperature using thinned fiber based Mach–Zehnder interferometer

A novel method for simultaneous measurement of refractive index and temperature based on a small core and cladding diameters thinned fiber Mach–Zehnder interferometer (MZI) using singlemode-multimode-thinned-multimode-singlemode (SMTMS) fiber structure is proposed. Experiments indicate that the selected two interference orders have sensitivities of ?16.1936 nm/RIU and 0.0534 nm/°C, and ?23.0473 nm/RIU and 0.0575 nm/°C, respectively, among RI range from 1.3325–1.3720 and temperature range from 22 °C–82 °C. We can thus use the coefficient matrix of these two peaks to simultaneously determine the surrounding refractive index and temperature. The fabrication is easy, safe and cost effective, includes only the fusion splicing, making the device properly attractive for practical sensor applications.     

Fabry–Pérot cavities based on chemical etching for high temperature and strain measurement

In this paper, two hybrid multimode/single mode fiber Fabry–Pérot (FP) cavities were compared. The cavities fabricated by chemical etching are presented as high temperature and strain sensors. In order to produce this FP cavity a single mode fiber was spliced to a graded index multimode fiber with 62.5 μm core diameter. The Fabry–Pérot cavities were tested as a high temperature sensor in the range between room temperature and 700 °C and as strain sensors. A reversible shift of the interferometric peaks with temperature allowed to estimate a sensitivity of 0.75 ± 0.03 pm/°C and 0.98 ± 0.04 pm/°C for the sensor A and B respectively. For strain measurement sensor A demonstrated a sensitivity of 1.85 ± 0.07 pm/μ? and sensor B showed a sensitivity of 3.14 ± 0.05 pm/μ?. The sensors demonstrated the feasibility of low cost fiber optic sensors for high temperature and strain.     

Study of the sensing characteristics of light emitting conjugated polymer MEH-PPV for nitro aromatic explosives

The sensing characteristics of light emitting conjugated polymer MEH-PPV for nitro aromatic explosives were studied in this paper. MEH-PPV was deposited on the surface of U-shaped plastic optical fiber (POF) using dip-coating techniques. The influences of MEH-PPV concentrations and bending radius of the U-shaped sensor heads on the sensitivity of the sensor were studied. The sensor was found to be most sensitive to TNT range from 0 to 4 mg/100 ml and the sensitivity was about 4 ng/ml. The limit of detections was around 1–10 ng/ml. It was also found that the conjugated polymer changes color from red to brownish black when the sensor head was put into TNT solution which provided high selectivity for sensing TNT at room temperature.     

Optical fiber strain and temperature sensor based on an in-line Mach–Zehnder interferometer using thin-core fiber

We propose and demonstrate a fiber in-line Mach–Zehnder interferometer using thin-core fibers. This in-line interferometer is composed of a short section of thin-core fiber inserted between two single mode fibers (SMF), and demonstrated as a strain and temperature sensor in this study. A strain sensitivity of ?1.83 pm/με with a measurement range of 0?2000 με, and the temperature sensitivity of ?72.89 pm/°C with a temperature variation of 50 °C are achieved. We also discussed that the influence of strain and temperature change on the relative power ratios among the excited cladding modes in thin-core fibers.     

Single-mode fiber design proposal using evolutionary technique with ultra low bending-loss appropriate in FTTH application

A proposal for the new single mode optical fiber containing four cladding layer with ultra low bending loss is presented. The suggested design method is based on the Genetic Algorithm optimization technique. Compared to the work reported in [1], our designed structure exhibits very small bending loss over the wide communication band (1.3–1.65 μm). Simulation results show bending loss of 6.78e?14 dB/turn at 1.55 μm for single turn of 5 mm radius. The best value reported in [1] was 2e?3 dB/turn for the same wavelength and radius of curvature.     

Properties of a gold-deposited surface plasmon resonance-based glass rod sensor with various light-emitting diodes and its application to a refractometer

The performance of a simple sensor system prepared using gold (Au)-deposited glass rods of 1 to 4 mm in diameter with a deposition length of 100 mm based on surface plasmon resonance (SPR) is presented. The sensor properties of the Au-deposited glass rods of 2 mm in diameter with deposition lengths of 10 to 100 mm are also presented. The sensor system consists of a light-emitting diode (LED) as the light source and a photodiode (PD) as the detector. The response curves and sensor properties of the Au-deposited glass rod with a Au film thickness of 45 nm obtained by using three LEDs with yellowish green (563 nm), red (660 nm), and infrared (940 nm) emissions were investigated. The results were compared with those of a corresponding Au-deposited optical fiber sensor with a core diameter of 0.2 mm. Though the sensitivity, response, and detection limit of the Au-deposited glass rod sensor are lower than those of the optical fiber sensor, the fabrication and handling of the Au-deposited glass rod sensor are easier because of the robustness. Since the dielectric constant of Au changes with the light wavelength, the sensor properties of both the Au-deposited glass rod sensor and the optical fiber sensor depend strongly on the wavelength of the incident light and can be controlled by changing the LED emission wavelength. This sensor system is a new SPR-based refractometer with easy construction and operation. Ethanol concentrations in various spirits were measured with this SPR-based refractometer and the results agreed well with those measured with an Abbe refractometer.     

Wavelength conversion based on high nonlinear microstructured fiber

A wavelength conversion based on high nonlinear microstructured fiber is demonstrated. Core diameter and pitch of the microstructured fiber used in this wavelength conversion method are 2.05 μm and 5.0 μm, respectively. Diameter of the air-holes in the fiber cladding is 4.50 μm, the nonlinear coefficient of the microstructured fiber is 112.2 W^?1 km^?1 and it is 60 times higher than that of a conventional dispersion-shift fiber, the length of the fiber is 100 m. Four-wave-mixing effect is improved in the high nonlinear microstructure fiber and then the efficiency of the wavelength conversion is improved also. 10 Gbps Not-Return-to-Zero (NRZ) modulation format and 10 Gbps Return-to-Zero (RZ) modulation format are converted effectively by our method. This study can provide a new alternative solution for high effective all-light wavelength conversion in high speed optical communication systems with multi-wavelengths and all-light optical networks.     

Fiber sensor based on Lophine sensitive layer for nitrate detection in drinking water

A new characterization of Lophine as a sensitive layer to measure Nitrate in drinking water is presented in this paper. The characterization was performed with a standard slide and a standard multimode fiber coated with a Lophine sensitive layer (2,4,5-Triphenylimidazol (C₂₁H₁₆N₂)). Spectral characterization has been conducted in the wavelength range from 300 to 1100 nm. We have demonstrated that Lophine can be used as a fiber sensor for the detection of Nitrate in drinking water. The sensing properties of the fiber sensor were analyzed at room temperature. Successful results were achieved when sensing Nitrate in the range between 1 mg/l and 70 mg/l. The response time was 20 ms and the recovery time was 40 ms.     

Hydrostatic pressure sensor based on a gold-coated thin-core fiber modal interferometer and ripple shift measurement

We propose a hydrostatic pressure sensor based on a gold-coated thin-core fiber modal interferometer (TCFMI). A thin-core fiber is spliced to a single mode fiber forming a single-end fiber modal interferometer (FMI) due to the core mismatch and the fiber end reflection. Relative reflection spectra of TCFMIs based on thin-core fibers with different lengths are investigated. The TCFMI is gold-coated to enhance the optical reflectivity, which also results in the ripple of the relative reflection spectra of TCFMI. A high hydrostatic pressure sensor test system is proposed and the performance of the pressure sensor has been experimentally investigated. A pure-ripple-shift measurement method is used to achieve the demodulation of the sensor. The proposed pressure sensor has a sensing range up to 40 MPa and a sensitivity of 44.8 pm/MPa.     

Reduction of fiber facet reflection by a curved core termination

A method to reduce end facet reflectivity of optical fibers by curving the fiber core is presented. The curved end core is obtained by first angle cleaving the fiber end and then balling up the fiber tip by a fusion process. The light escapes from the fiber core at the curve and follows his path through the cladding material up to the balled up fiber surface. Thus, the fiber termination acts as a beam expander and at the same time the facet reflectivity is reduced by >38 dB.     

Investigation on micro/nanofiber Bragg grating for refractive index sensing

We propose a refractometric sensor based on micro/nanofiber Bragg grating (MNFBG). The refractive index (RI) sensing performance dependence on the fiber radius and Bragg grating period of the sensor, as well as the temperature cross-sensitive effect, is investigated theoretically. The simulation results demonstrate that 400 nm-radius MNFBG has a linear response to RI ranging from 1.3 to 1.39 with a sensitivity as high as 992.7 nm/RIU and half temperature cross-sensitivity of normal FBG. A maximum sensitivity of up to 1200 nm/RIU and an outstanding RI resolution of 8.3 × 10^-6 can be achieved. MNFBG has high potential in various types of optical fiber sensor applications.     

Novel bending sensor based on a meniscus shaped beam with LPG

A novel bending sensor based on a long period fiber grating (LPG) is presented. A LPG was glued into a V-shaped groove, which lies on the lower surface of a meniscus shaped beam. It is found that the transmission optical power of the LPG changed linearly with the variation of the bending of the beam. The bending applied on the beam can be measured by detecting the intensity variation of the LPG's resonant dip wavelength. Under a relative large bending measured range from 0 to 7.5 m^?1, the sensitivity of 3.003 dB m^?1 and curvature resolution of 0.001 m^?1 have been achieved for the proposed bending sensor.     

A mid-infrared carbon monoxide sensor system using wideband absorption spectroscopy and a single-reflection spherical optical chamber

A mid-infrared carbon monoxide (CO) sensor system based on a dual-channel differential detection method was developed using a broadband light source in the 4.60 µm wavelength region and a single-reflection spherical optical chamber with ∼0.373 m absorption path length. CO detection was realized by targeting the wideband strong absorption lines within 4.55–4.65 µm. A dual-channel pyroelectric detector as well as a self-developed digital signal processor (DSP) based orthogonal lock-in amplifier was employed to process CO sensing signal. A minimum detection limit of ∼0.5 ppm in volume (ppmv) was achieved with a measurement time of 6 s, based on an Allan deviation analysis of the sensor system. The response time (1000 → 0 ppmv) was determined to be ∼7 s for the CO sensor operation. Due to the characteristics of low detection limit, fast response time and high cost performance, the proposed sensor has relatively good prospect in coal-mining operation.     

Experimental research on an arbitrary pulse generation system for imaging VISAR

Imaging VISAR is an important diagnostic tool for a variety of shock-related phenomena in laser-driven experiments. To adapt to various types of shaped driven pulse, the imaging VISAR needs an illuminating light with good shaping capability. Here, a flexible laser probe system was experimentally studied. Being generated from a 1064-nm DFB laser, the continuous wave was modulated by a waveguide amplitude modulator driven by 10 GS/s arbitrary waveform generator. After being amplified by fiber amplifiers and Nd:YAG rod amplifiers, the signal pulse was frequency-converted to 532-nm green light by a thermally controlled LBO crystal with a final output energy larger than 10 mJ. Finally, the green light was coupled into a 1-mm core diameter, multimode fused silica optical fiber and propagated to the imaging VISAR. The probe laser could realize accurate pulse shaping with time resolution below 100 ps. Uniformity in intensity and capability of arbitrary pulse shaping provides great convenience for the analysis of experimental data.     

Temperature self-compensation fiber-optic pressure sensor based on fiber Bragg grating and Fabry–Perot interference multiplexing

A high performance multiplexed fiber-optic sensor consisted of diaphragm-based extrinsic Fabry–Perot interferometer (DEFPI) and fiber Bragg grating (FBG) is proposed. The novel structure DEFPI fabricated with laser heating fusion technique possesses high sensitivity with 5.35 nm/kPa (36.89 nm/psi) and exhibits ultra-low temperature dependence with 0.015 nm/°C. But the ultra-low temperature dependence still results in small pressure measurement error of the DEFPI (0.0028 kPa/°C). The designed stainless epoxy-free packaging structure guarantees the FBG to be only sensitive to temperature. The temperature information is created to calibrate the DEFPI's pressure measurement error induced by the temperature dependence, realizing effectively temperature self-compensation of the multiplexed sensor. The sensitivity of the FBG is 10.5 pm/°C. In addition, the multiplexed sensor is also very easy to realize the pressure and the temperature high-precise high-sensitive simultaneous measurement at single point in many harsh environmental areas.     

Temperature and strain discrimination using two parallel connected Sagnac loops based on character-1 shaped polarization-maintaining fiber

A fiber sensor configuration suitable for discrimination of temperature and strain is presented. The sensor head is composed of two parallel concatenated Sagnac loops based on character-1 shaped polarization-maintaining fiber (PMF). The two Sagnac loops include different sections of character-1 shaped PMFs, and show different sensitivity to temperature and strain. By monitoring the wavelength shift of the two dips in the transmission spectrum, simultaneous measurement of temperature and strain is obtained. The sensitivity for strain and temperature are measured to be 14.46 pm and ? 0.54 nm.     

Analysis of multimode BDK doped POF gratings for temperature sensing

We report a temperature sensor based on a Bragg grating written in a benzil dimethyl ketal (BDK) doped multimode (MM) polymer optical fiber (POF) for the first time to our knowledge. The thermal response was further analyzed in view of theory and experiment. In theory, with the order of the reflected mode increasing from 1st to 60th order, for MM silica fiber Bragg grating (FBG) the temperature sensitivity will increase linearly from 16.2 pm/°C to 17.5 pm/°C, while for MM polymer FBG the temperature sensitivity (absolute value) will increase linearly from ?79.5 pm/°C to ?104.4 pm/°C. In addition, temperature sensitivity of MM polymer FBG exhibits almost 1 order larger mode order dependence than that of MM silica FBG. In experiment, the Bragg wavelength shift will decline linearly as the temperature rises, contrary to that of MM silica FBG. The temperature sensitivity of MM polymer FBG is ranged from ?0.097 nm/°C to ?0.111 nm/°C, more than 8 times that of MM silica FBG, showing great potential used as a temperature sensor.     

Fiber curvature sensor based on spherical-shape structures and long-period grating

A novel curvature sensor based on optical fiber Mach–Zehnder interferometer (MZI) is demonstrated. It consists of two spherical-shape structures and a long-period grating (LPG) in between. The experimental results show that the shift of the dip wavelength is almost linearly proportional to the change of curvature, and the curvature sensitivity are −22.144 nm/m⁻¹ in the measurement range of 5.33–6.93 m⁻¹, −28.225 nm/m⁻¹ in the range of 6.93–8.43 m⁻ and −15.68 nm/m⁻¹ in the range of 8.43–9.43 m⁻¹, respectively. And the maximum curvature error caused by temperature is only −0.003 m⁻¹/°C. The sensor exhibits the advantages of all-fiber structure, high mechanical strength, high curvature sensitivity and large measurement scales.