Fiber Loop Mirror Based on Optical Fiber Circulator for Sensing Applications.

In this paper, a different Fiber Loop Mirror (FLM) configuration with two circulators is presented. This configuration is demonstrated and characterized for sensing applications. This new design concept was used for strain and torsion discrimination. For strain measurement, the interference fringe displacement has a sensitivity of (0.576 ± 0.009) pm‧µÎµ-1. When the FFT (Fast Fourier Transformer) is calculated and the frequency shift and signal amplitude are monitored, the sensitivities are (-2.1 ± 0.3) × 10-4 nm-1 µÎµ-1 and (4.9 ± 0.3) × 10-7 µÎµ-1, respectively. For the characterization in torsion, an FFT peaks variation of (-2.177 ± 0.002) × 10-12 nm-1/° and an amplitude variation of (1.02 ± 0.06) × 10-3/° are achieved. This configuration allows the use of a wide range of fiber lengths and with different refractive indices for controlling the free spectral range (FSR) and achieving refractive index differences, i.e., birefringence, higher than 10-2, which is essential for the development of high sensitivity physical parameter sensors, such as operating on the Vernier effect. Furthermore, this FLM configuration allows the system to be balanced, which is not possible with traditional FLMs.

Static Ice Pressure Measuring System Based on Fiber Loop Ring-Down Spectroscopy and FPGA.

Hydraulic engineering built in the cold region, such as reservoirs and hydropower stations, is often threatened by static ice pressure from nature. Therefore, it is of vital significance to research the pressure variation in the growth and melting processes of the ice layer for the design and protection of hydraulic structures in cold regions. This paper introduces an optical fiber sensor system based on the fiber loop ring-down spectroscopy technology and field-programmable gate array (FPGA) pulse modulation technology. An electro-optic modulation scheme that relied on FPGA to generate optical pulses with adjustable pulse width and period is proposed, which is more suitable for the in-situ observation. In addition, the temperature stability and repeatability of the system are also discussed. This system was applied to the real-time detection of static ice pressure on the sidewall and bottom of the polyvinyl chloride (PVC) pipe during the ice growth and melting processes. The results indicate that the system has favorable stability and sensitivity, and the relationship obtained between the static ice pressure and temperature could provide some references for the field application in the future.

Multipoint Fiber Loop Ringdown Sensors for Large Strain Measurement Using Frequency-Shifted Interferometry.

A novel multipoint fiber loop ringdown (FLRD) strain sensing system using frequency-shifted interferometry (FSI) is proposed and experimentally validated. Compared to conventional multipoint FLRD techniques, this scheme measures the decay rate of the continuous wave (CW) light in the space domain and thus greatly reduces the cost without the requirement of expensive devices. A serial dual-point strain sensing system was experimentally constructed and a biconical tapered multimode fiber (MMF) as the sensor head was used for obtaining the large measuring range. By applying different strains on the sensor heads through translation stages, a linear response between strain and additional loss induced by strain sensor was obtained, and the static strain sensitivities of 0.13676 dB/mε and 0.19665 dB/mε were achieved, corresponding to the detection limit of 0.0123 dB and 0.0360 dB, respectively. Moreover, a large measuring range of approximately 6 mε was achieved for both strain sensors. The experimental results indicate that our proposed method offers a promising multipoint strain sensor which has the advantages of low cost, a simple sensing structure and a large measuring range.

Ambient Refractive-Index Measurement with Simultaneous Temperature Monitoring Based on a Dual-Resonance Long-Period Grating Inside a Fiber Loop Mirror Structure.

In this work, we report the experimental results on optimizing the optical structure for ambient refractive index measuring with temperature changes monitoring. The presented optical structure is based on a dual-resonance long-period grating embedded inside a fiber loop mirror, where the long-period grating acts as the head of the refractive-index sensor, whereas the section of polarization maintaining fiber in the loop mirror ensures suitable temperature sensing. The optimization process was comprised of tuning the resonance and interferometric peaks by changing the state of polarization of propagating beams. Experimental results establish that the response of the proposed sensor structure is linear and goes in opposite directions: an increase in the ambient refractive index reduces the signal response, whereas a temperature increase produces an increased response. This enables us to distinguish between the signals from changes in the refractive index and temperature. Due to the filtering properties of the interferometric structure, it is possible to monitor variation in these physical parameters by observing optical power changes instead of wavelength shifts. Hence, the refractive index sensitivity has been established up to 2375.8 dB/RIU in the narrow RI range (1.333⁻1.341 RIU) and temperature sensitivities up to 1.1 dBm/°C in the range of 23⁻41 °C. The proposed sensor is dedicated to advanced chemical and biological sensor applications.

A Loop All-Fiber Current Sensor Based on Single-Polarization Single-Mode Couplers.

Low current sensitivity and insufficient system stability are two key problems in all-fiber current sensor (AFCS) studies. In order to solve the two problems, a novel AFCS combining single-polarization single-mode (SPSM) couplers and a loop structure is presented in this paper with a design that incorporates the advantages of both SPSM couplers and a loop structure. SPSM couplers are shown to simplify the AFCS system and reduce the risk of interference, and the loop structure can enhance the current sensitivity. Both theory and experiment prove that the new AFCS can simultaneously overcome two prevalent obstacles of low current sensitivity and low stability.

D-Shaped Polarization Maintaining Fiber Sensor for Strain and Temperature Monitoring.

A D-shaped polarization-maintaining fiber (PMF) as fiber optic sensor for the simultaneous monitoring of strain and the surrounding temperature is presented. A mechanical end and edge polishing system with aluminum oxide polishing film is utilized to perform sequential polishing on one side (lengthwise) of the PMF in order to fabricate a D-shaped cross-section. Experimental results show that the proposed sensor has high sensitivity of 46 pm/µÎµ and 130 pm/°C for strain and temperature, respectively, which is significantly higher than other recently reported work (mainly from 2013) related to fiber optic sensors. The easy fabrication method, high sensitivity, and good linearity make this sensing device applicable in various applications such as health monitoring and spatial analysis of engineering structures.

An intrinsic fiber-optic single loop micro-displacement sensor.

A micro-displacement sensor consisting of a fiber-loop made with a tapered fiber is reported. The sensor operation is based on the interaction between the fundamental cladding mode propagating through the taper waist and higher order cladding modes excited when the taper is deformed to form a loop. As a result, a transmission spectrum with several notches is observed, where the notch wavelength resonances shift as a function of the loop diameter. The loop diameter is varied by the spatial displacement of one end of the fiber-loop attached to a linear translation stage. In a displacement range of 3.125 mm the maximum wavelength shift is 360.93 nm, with 0.116 nm/µm sensitivity. By using a 1,280 nm broadband low-power LED source and a single Ge-photodetector in a power transmission sensor setup, a sensitivity in the order of 2.7 nW/µm is obtained in ≈ 1 mm range. The proposed sensor is easy to implement and has a plenty of room to improve its performance.

Chemical sensing using fiber cavity ring-down spectroscopy.

Waveguide-based cavity ring-down spectroscopy (CRD) can be used for quantitative measurements of chemical concentrations in small amounts of liquid, in gases or in films. The change in ring-down time can be correlated to analyte concentration when using fiber optic sensing elements that change their attenuation in dependence of either sample absorption or refractive index. Two types of fiber cavities, i.e., fiber loops and fiber strands containing reflective elements, are distinguished. Both types of cavities were coupled to a variety of chemical sensor elements, which are discussed and compared.

Fiber loop ringdown - a time-domain sensing technique for multi-function fiber optic sensor platforms: current status and design perspectives.

Fiber loop ringdown (FLRD) utilizes an inexpensive telecommunications light source, a photodiode, and a section of single-mode fiber to form a uniform fiber optic sensor platform for sensing various quantities, such as pressure, temperature, strain, refractive index, chemical species, biological cells, and small volume of fluids. In FLRD, optical losses of a light pulse in a fiber loop induced by changes in a quantity are measured by the light decay time constants. FLRD measures time to detect a quantity; thus, FLRD is referred to as a time-domain sensing technique. FLRD sensors have near real-time response, multi-pass enhanced high-sensitivity, and relatively low cost (i.e., without using an optical spectral analyzer). During the last eight years since the introduction of the original form of fiber ringdown spectroscopy, there has been increasing interest in the FLRD technique in fiber optic sensor developments, and new application potential is being explored. This paper first discusses the challenging issues in development of multi-function, fiber optic sensors or sensor networks using current fiber optic sensor sensing schemes, and then gives a review on current fiber optic sensor development using FLRD technique. Finally, design perspectives on new generation, multi-function, fiber optic sensor platforms using FLRD technique are particularly presented.

Recent Advances in High-Birefringence Fiber Loop Mirror Sensors.

Recent advances in devices and applications of high-birefringence fiber loopmirror sensors are addressed. In optical sensing, these devices may be used as strain andtemperature sensors, in a separate or in a simultaneous measurement. Other describedapplications include: refractive index measurement, optical filters for interrogate gratingsstructures and chemical etching control. The paper analyses and compares different types ofhigh-birefringence fiber loop mirror sensors using conventional and microstructured opticalfibers. Some configurations are presented for simultaneous measurement of physicalparameters when combined with others optical devices, for example with a long periodgrating.