In this paper, the most recent progress as well as challenges of distributed optical fiber sensing (DOFS) in industrial applications is discussed. Compared to the vast market of sensors used to measure strain or temperature, the success of distributed optical fiber sensing (DOFS) at the industrial level is very limited, at best. One of the reasons for this lack of the wider acceptance is the mismatch between the commercially available systems and actual industrial requirements, especially for the spatial resolution and precision. These requirements are organized and clarified in the paper. It also describes the hybrid Brillouin-Rayleigh system, which exhibits capabilities surpassing those of strain gauges. The principles of the system are illustrated considering the fiber calibration methodology. Formulas required for determining strain, temperature, and hydro-pressure are derived and discussed. Finally, the examples of applications are presented.

This paper describes the development and applications of a fiber-optic electric current sensing technique with the stable properties and compact, simple, and flexible structure of the sensing device. The special characteristics of the sensors were achieved by use of the special low birefringence fiber as the Faraday-effect sensing element and were also achieved with creation of sensing schemes which matched with the features of the fiber. Making use of the excellent features of the sensing technique, various current monitoring devices and systems were developed and applied practically for the control and maintenance in the electric power facility. In this paper, the design and performance of the sensing devices are introduced first. After that, examples of the application systems practically applied are also introduced, including fault section/point location systems for power transmission cable lines.

A multimode interference refractive index (RI) sensor based on the coreless fiber was numerically and experimentally demonstrated. Two identical single mode fibers (SMF) are spliced at both ends of a section of the coreless fiber which can be considered as the equivalent weakly guiding multimode fiber (MMF) with a step-index profile when the surrounding refractive index (SRI) is lower than that of the coreless fiber. Thus, it becomes the conventional single-mode multimode single-mode (SMS) fiber structure but with a larger core size. The output spectra will shift along with the changes in the SRI owing to the direct exposure of the coreless fiber. The output spectra under different SRIs were numerically studied, as well as the sensitivities with different lengths and diameters of the coreless fiber. The predication and calculation showed the good agreement with the experimental results. The proposed RI sensor proved to be feasible by verification experiments, and the relative error was merely 0.1% which occupied preferable sensing performance and practicability.

The key factor of the sensitivity in the FBG-based pH sensor is analyzed in detail. A multi-thin-layer structure of the gel coated cover was proposed and implemented with a special process. The sensors with the coated thickness of 420 μm, 500 μm, and 580 μm were built up, respectively. The corresponding spectral shifts of 0.08 nm, 0.13 nm, and 0.22 nm were detected when the pH sensors were soaked in the pH value of 3-9. Meanwhile, the sensor with the gel layer thickness of 580 μm was measured in the optimum measurement time period with the pH value changing from 3-12, in which the detected sensitivity of 52 pm/pH was achieved in the pH range of 6-12.

In recent years, the silica-on-silicon based multimode interference (MMI) optical waveguide is an interesting research topic. It is being advanced various researches on the silica based MMI coupler. This paper represents the considerations of the optimal design of the silica-on-silicon based MMI optical coupler for better performance. For that, we have illustrated the simulation results on a particular case of the 4×4 silica-on-silicon based MMI coupler. From the simulation results, it is seen that the performance of the MMI coupler depends on multiple width and length combinations of the MMI waveguide. The results also show that the width of the multimode waveguide could not be too small or too large for optimal performance, and at the widths, 100 μm, 120 μm and 130 μm, the performance could be optimized and be almost 0.62 - 0.64 in a given length range. Finally, the results have been compared with the optical coupler presently available in the market and show that the silica-on-silicon based MMI coupler is much more efficient in terms of losses and the performance associated with it and the size of the coupler.

The fiber-optic displacement sensor based on the distributed Bragg reflector fiber laser is proposed, that is, the fiber laser cavity is attached to the measured object, when the measured object is stretched or contracted, and the length of the fiber laser cavity is also stretched or contracted accordingly. In view of the nonlinearity of the fiber-optic displacement sensor, the calibration based on piezoelectric ceramics is applied to improve the linearity of the displacement sensor. Experiment results show that the fiber-optic displacement sensor has a linear response with the nominal working distance of 90 μm.

In this paper, we propose a Fresnel reflection-based optical fiber sensor system for remote refractive index measurement using the optical time domain reflectometry technique as an interrogation method. The surrounding refractive index from a long distance away can be measured easily by using this sensor system, which operates based on testing the Fresnel reflection intensity from the fiber-sample interface. This system is a simple configuration, which is easy to handle. Experimental results showed that the range of this measurement could reach about 100.8 km, and the refractive index sensitivities were from 38.71 dB/RIU to 304.89 dB/RIU in the refractive index (RI) range from 1.3486 to 1.4525.

A novel dual-loop technique was proposed for single-mode selection in an optoelectronic oscillator (OEO). It consisted of a pump laser and a feedback circuit including an intensity modulator, a Fabry-Perot (FP) etalon, two optical fiber delay lines, two photodetectors, and an amplifier. By inserting the Fabry-Perot etalon, the proposed dual-loop OEO realized a single mode oscillation ranging from 0 Hz to 20 GHz. The strong oscillation mode was present at 15 GHz, and the side modes suppression ratio (SMSR) exceeded 140 dB. More over the length of the two fiber loops were just 5 meters and 36 meters.

The propagation characteristics of the surface-plasmon-polariton (SPP) mode in the single interface of silver (Ag) and gallium lanthanum sulfide (GLS) have been studied both analytically and numerically. The obtained numerical results show an excellent agreement with the analytical ones. The locations of the spatial resonance point along the direction of propagation were determined for the dielectric and the metal.

The purpose of this paper is to compare the performance of two spectrometers that are manufactured from the same company. In this work, heavy metals like lead Pb and copper Cu in the KBr matrix were analyzed using the laser induced breakdown spectroscopic technique. A Q-switched Nd:YAG laser with 90 mJ per pulse operating at the fundamental wavelength of 1064 nm and pulse duration of 10 ns was used to generate plasma at the focal region. The important experimental parameters such as the laser energy, integration time, distance between the lens and sample, distance and angle of the optical fiber from the target were optimized. Two spectrometers manufactured by Ocean Optics namely as Maya2000Pro and USB 4000 were employed for anlyzing the spectral lines. The experimental setup and conditions were remained the same for both experiments. The production of spectral lines from each of the interested elements was analyzed and compared with the NIST (National Institute of Standards and Technology) database. The sensitivity, repeatability and limit of detection for each of the systems are discussed in detail.

This paper presents a novel readout system for wireless passive pressure sensors based on the inductively coupled inductor and cavity (LC) resonant circuits. The proposed system consists of a reader antenna inductively coupled to the sensor circuit, a readout circuit, and a personal computer (PC) post processing unit. The readout circuit generates a voltage signal representing the sensor’s capacitance. The frequency of the reader antenna driving signal is a constant, which is equal to the sensor’s resonant frequency at zero pressure. Based on mechanical and electrical modeling, the pressure sensor design based on the high temperature co-fired ceramic (HTCC) technology is conducted and discussed. The functionality and accuracy of the readout system are tested with a voltage-capacitance measurement system and demonstrated in a realistic pressure measurement environment, so that the overall performance and the feasibility of the readout system are proved.

A highly sensitive in-situ turbidity sensor with the low power consumption was proposed and evaluated in this study. To meet the practical requirements of the in-situ detection, we have designed the light scattering path, watertight mechanical structure, and ultra-weak scattering light detecting method. Experiments showed that the sensor had a sensitivity of 0.0076 FTU with the concentration range of 0 - 25 FTU and the R-square of 0.9999. The sensor could withstand the water pressure in depth of 1000 m and had the low power consumption in the active mode 10.4 mA, sleep mode 65 μA with a supply voltage of 8.4 V. Southern China Sea buoy experiments indicated that the sensor could work well in the actual in-situ environment. In comparison with sensors of other companies, our sensor had relatively more comprehensive performance.

The impact of microwave sweeper power fluctuation in the BOTDA system has been theoretically analyzed and experimentally tested. And a novel method comparing real-time acquisition of probe wave power with a new algorithm to realize probe wave power normalization for eliminating this impact was proposed. The principle of the proposed method was described theoretically. And the contrast test between our new method and conventional one was carried out. The experiment results indicated that the temperature accuracy was effectively improved from ±5 ℃ to ±2 ℃.

A long period fiber grating (LPFG) fabricated upon the all-solid photonic bandgap fiber by CO2 laser irradiation was investigated, and its resonance wavelength was at 1335.76 nm with a modulation depth of 15 dB and a 3-dB bandwidth of 2.6 nm. We studied its strain, temperature, and index sensor characteristics, the strain sensitivity of 0.992 pm/με was obtained by using linear fit, and the relationship between the refractive index and wavelength obeyed the distribution of quadratic function. Also, we demonstrated its temperature response was relatively insensitive (21.51 pm/℃).