Juan Kang

Temperature-Independent Fiber Bending Sensor Based on a Superimposed Grating

A compact fiber bending sensor has been presented by superimposing a uniform fiber Bragg grating (FBG) into a tilted fiber Bragg grating (TFBG). The reflection of FBG is modulated by the same bandwidth of the cladding modes of TFBG, which is sensitive to the bending of fiber. By monitoring the differential Bragg reflection between FBG and TFBG, the sensor is insensitive to ambient temperature changes and power fluctuations from light source or fiber link during the measurement. The sensitivity of the sensor is also doubled because the signal from FBG passes through the TFBG twice.

Broadband near-infrared luminescence in bismuth borate glasses

We report the results of our investigation on the optical properties of the bismuth borate glass (75B2O3-25Bi2O3). Unusual near-infrared (NIR) and visible band luminescence was observed when the sample was excited by lasers working at 532 and 808 nm. The NIR fluorescent lifetime of the sample measured at room-temperature was longer than two hundred microseconds. This indicated that this glass system could be a new candidate for the broadband optical amplification and laser material covering the wavelength from 1100 to 1300 nm. The influences of preparation conditions and glass compositions on the luminescence properties of glasses were investigated. It was found that the heat-treatment under air and hydrogen atmosphere can both weaken the infrared luminescence of the materials. Furthermore, with the addition of oxidation agent CeO2, the sample did not show any NIR luminescence upon the excitation of the lasers working at 532 or 808 nm. All the results indicated that the infrared luminescence center should be Bi+ ions.

Reflective long-period fiber grating-based sensor with Sagnac fiber loop mirror for simultaneous measurement of refractive index and temperature

In this paper, we propose a reflective long-period grating-based sensor with a Sagnac fiber loop mirror (SFLM) for simultaneous measurement of refractive index (RI) and temperature. By cascading the SFLM to the end of a long-period fiber grating (LPFG), the LPFG works as a reflection operation, which is convenient in some applications. Further, the SFLM and the LPFG have different sensitivities to RI and temperature. As a result, RI and temperature measurement can be simultaneously achieved by monitoring the wavelength shifts of the LPFG and the SFLMs dips in the reflection spectrum. Experimental results show that the temperature sensitivity can reach 1.533 nm/°C, and the RI sensitivity is from 16.864 nm/RIU (refractive index unit) to 113.142 nm/RIU when the RI range is from 1.333 to 1.430. The application for 40 km long-distance RI and temperature measurement shows that the sensor has potential application in long-distance sensing.

Compact micro-displacement sensor with high sensitivity based on a long-period fiber grating with an air-cavity

We proposed and experimentally demonstrated a compact micro-displacement sensor with high sensitivity based on a long-period fiber grating (LPG) with an air-cavity. The sensor head is obtained by composing an air-cavity with the ends of a LPG and a single mode fiber (SMF). The wavelength shift of the LPG has a linear relationship with the length of the air gap which agrees well with the theoretical analysis. The experimental results show that the sensitivity is ~0.22 nm/µm within the micro-displacement range of 0 to 140 µm.

A fiber loop mirror temperature sensor demodulation technique using a long-period grating in a photonic crystal fiber and a band-pass filter

A fiber loop mirror (FLM) temperature sensor using a long-period grating (LPG) written in a photonic crystal fiber (PCF) and a band-pass filter as a demodulator is proposed. By utilizing the stable filtering function of the LPG in the PCF, the resonant wavelength variation of the FLM with temperature is transferred effectively to the intensity variation of the output light. By monitoring the light intensity of the band-pass of the filter, temperature applied on the FLM is deduced by an optical power meter. Experiment results show that the temperature sensitivity is high as ~1.742 dB/ °C when a filter with a full width at half maximum 3 nm and the center at 1545 nm is used.

Multiplex and simultaneous measurement of displacement and temperature using tapered fiber and fiber Bragg grating

A simple method to work out the multiplexing of tapered fiber based sensors is proposed and demonstrated. By cascading a tapered fiber with a fiber Bragg grating (FBG), the sensor head is provided with a wavelength identification, different FBGs provide the sensor heads with different reflective peaks and they can be distinguished in optical spectrum. By compositing several such sensor heads with a multi-channel beam splitter, a star-style topological structure sensor for multipoint sensing is achieved. At the same time, the output intensity at the peak wavelength is sensitive to one external physical parameter applied on the related FBG-cascaded tapered fiber and the central wavelength of the peak is only sensitive to temperature, so that that parameter and temperature can be measured simultaneously. A sensor for dual-point measurement of the displacement and temperature simultaneously is experimentally demonstrated by using a 2 × 2 coupler in this paper. Experiment results show that the sensor works well and the largest sensitivities reach to 0.11 dB/μm for displacement in the range of 0-400 μm, and ∼0.0097 nm/°C for temperature between 20 °C and 70 °C.

Photonic Crystal Fiber Loop Mirror-Based Chemical Vapor Sensor

A compact chemical vapor sensor based on a fiber loop mirror (FLM) inserted with a high-birefringent photonic crystal fiber (HiBi-PCF) is proposed and experimentally demonstrated. Because the resonant dips of the HiBi-PCF FLM are sensitive to the phase difference between two counter-propagating waves in the FLM, a highly sensitive chemical vapor can be achieved by measuring the resonant dip shifts when chemical vapor diffuses into the air holes of the HiBi-PCF. Experimental results show a high sensitivity of 15.5 pm/ppm for ethanol is obtained when a 5.1 cm long HiBi-PCF is used in the FLM. Three different mechanisms for the high sensitivity of the proposed sensor are discussed. A liquefaction model with a full-vector finite element method is proposed and is regarded as the main contribution.

High-sensitivity temperature sensor based on a droplet-like fiber circle

A low-cost yet high-sensitivity temperature fiber sensor is proposed and demonstrated in this paper. A single-mode fiber with coating is simply bent in a droplet-like circle with a radius of several millimeters. The strong bending induces mode interferences between the silica core mode and the excited modes propagating in the polymer coating. Many resonant dips were observed in the transmission spectra and are found to shift to a shorter wavelength with the increase of environmental temperature. Our linear fitting result of the experimental data shows that the proposed sensor presents high temperature sensitivity up to -3.102  nm/°C, which is even comparable with sensors based on selective liquid-filled photonic crystal fibers. Such high temperature sensitivity results from the large thermo-optical coefficient difference between the silica core and the polymer coating. The influence of a circle radius to the sensitivities is also discussed.

Simultaneous measurement of trace organic vapors and temperature by use of zeolite thin film-coated fiber spherical end face and fiber Bragg grating

Abstract. An optical fiber spherical end face coated with zeolite thin film is used together with a fiber Bragg grating (FBG) for simultaneous measurement of trace organic vapor and temperature. The fiber spherical end face is fabricated by using electrical arc discharge on single mode fiber and then coated with zeolite thin film. The FBG is placed in front of the zeolite film-coated fiber spherical end head. The coated fiber spherical end essentially forms an intrinsic Fabry–Perot (F–P) cavity. The trace chemical vapor concentration is measured by monitoring the interference wavelength shift of the coated optical fiber spherical F–P sensor head, due to the zeolite film adsorption to organic vapor molecules. Meanwhile, temperature is measured by monitoring the Bragg wavelength shift of the FBG. The experimental results show that the proposed trace organic vapor sensor exhibits a high sensitivity, which is 1.199  nm/ppm for isopropanol within the range from 0 to 20 ppm, and the temperature sensitivity is 10.2  pm/°C within the range from 30°C to 70°C. Such a sensor has high sensitivity, excellent repeatability, fast response, and real-time monitoring capability.

Simultaneous measurement of refractive index and temperature based on intensity demodulation using matching grating method

A solution refractive index (SRI) and temperature simultaneous measurement sensor with intensity-demodulation system based on matching grating method were demonstrated. Long period grating written in a photonic crystal fiber (LPG-PCF), provides temperature stable and wavelength dependent optical intensity transmission. The reflective peaks of two fiber Bragg gratings (FBGs), one of which is etched then sensitive to both SRI and temperature, another (FBG2) is only sensitive to temperature, were located in the same linear range of the LPG-PCFs transmission spectrum. An identical FBG with FBG2 was chosen as a matching FBG. When environments (SRI and temperature) change, the wavelength shifts of the FBGs are translated effectively to the reflection intensity changes. By monitoring output lights of unmatching and matching paths, the SRI and temperature were deduced by a signal processing unit. Experimental results show that the simultaneous refractive index and temperature measurement system work well. The proposed sensor system is compact and suitable for in situ applications at lower cost.