Xiaoyong Zhong

High-sensitivity strain sensor based on in-fiber improved Fabry–Perot interferometer

We demonstrated a high-sensitivity strain sensor based on an in-fiber Fabry-Perot interferometer (FPI) with an air cavity, which was created by splicing together two sections of standard single-mode fibers. The sensitivity of this strain sensor was enhanced to 6.0  pm/με by improving the cavity length of the FPI by means of repeating arc discharges for reshaping the air cavity. Moreover, such a strain sensor has a very low temperature sensitivity of 1.1  pm/°C, which reduces the cross sensitivity between tensile strain and temperature.

Simultaneous measurement of strain and temperature by employing fiber Mach-Zehnder interferometer

We demonstrated a novel fiber in-line Mach-Zehnder interferometer (MZI) with a large fringe visibility of up to 17 dB, which was fabricated by misaligned splicing a short section of thin core fiber between two sections of standard single-mode fiber. Such a MZI could be used to realize simultaneous measurement of tensile strain and temperature. Tensile strain was measured with an ultrahigh sensitivity of -0.023 dB/μɛ via the intensity modulation of interference fringes, and temperature was measured with a high sensitivity of 51 pm/°C via the wavelength modulation of interference fringe. That is, the MZI-based sensor overcomes the cross-sensitivity problem between tensile strain and temperature by means of different demodulation methods. Moreover, this proposed sensor exhibits the advantages of low-cost, extremely simple structure, compact size (only about 10 mm), and good repeatability.

Long Period Fiber Gratings Inscribed by Periodically Tapering a Fiber

A promising technique for inscribing long period fiber gratings (LPFGs) was demonstrated by only using a commercial splicer. The commercial splicer was developed secondarily to build up a new program for periodically tapering a single mode fiber. High-quality LPFGs with a low insertion loss of ~1 dB and a large resonant attenuation of more than -30 dB were achieved. The achieved periodic tapers exhibited an excellent reproducibility with a small error of less than ±0.3 μm. To the best of our knowledge, it is the minimum reproducibility error of tapers achieved by arc discharge technique so far. Near mode fields of three LPFG samples with different pitches were observed to investigate the mode coupling in the taper-inscribed LPFGs. In addition, the resonant wavelengths of our taper-inscribed LPFGs exhibited a blue shift first and then red shift with an increased number of grating periods, resulting from residual stress relaxation together with physical deformation.

High-sensitivity strain sensor based on inflated long period fiber grating.

We demonstrated a high-sensitivity strain sensor based on an inflated long period fiber grating (I-LPFG). The I-LPFG was inscribed, for the first time to the best of our knowledge, by use of the pressure-assisted CO(2) laser beam scanning technique to inflate periodically air holes of a photonic crystal fiber. Such periodic inflations enhanced the sensitivity of the LPFG-based strain sensor to -5.62 pm/με. After high temperature annealing, the I-LPFG achieved a good repeatability and stability of temperature response with a sensitivity of 11.92 pm/°C.

Intensity-Modulated Strain Sensor Based on Fiber In-Line Mach–Zehnder Interferometer

We demonstrated a novel intensity-modulated strain sensor based on a fiber in-line Mach-Zehnder interferometer with a large fringe visibility of up to 17 dB, which was fabricated by splicing a section of thin core fiber between two sections of single mode fibers with one misalignment-spliced joint. Such a strain sensor exhibited an ultrahigh sensitivity of -0.023 dBm/με within a measurement range of 500 με, which is about one order of magnitude higher than that reported in references. Displacement and stress distributions at the misalignment spliced joint were simulated by use of finite element method. In addition, the proposed strain sensor has an advantage of compact size of ~10 mm.

Tunable phase-shifted fiber Bragg grating based on femtosecond laser fabricated in-grating bubble

We present a type of phase-shifted fiber Bragg gratings based on an in-grating bubble fabricated by femtosecond (fs) laser ablation together with a fusion-splicing technique. A microchannel vertically crossing the bubble is drilled by fs laser to allow liquid to flow in or out. By filling different refractive index (RI) liquid into the bubble, the phase-shift peak is found to experience a linear red shift with the increase of RI, while little contribution to the change of phase shift comes from the temperature and axial strain. Therefore, such a PS-FBG could be used to develop a promising tunable optical filter and sensor.

Simultaneous Refractive Index and Temperature Measurement With LPFG and Liquid-Filled PCF

We proposed and demonstrated a novel scheme for simultaneous measurement of temperature and refractive index (RI) by cascading a liquid-filled photonics crystal fiber (PCF) to a long period fiber grating (LPFG) written in a single-mode fiber. The liquid-filled PCF showed a high-temperature sensitivity of 1.695 nm/°C, which is two orders of magnitude higher than that of the LPFG (0.0642 nm/°C). The LPFG exhibited a blue shift with the increased RI. In contrast, liquid-filled PCF showed a total immunity to the surrounding RI. So, the temperature was solely measured by the liquid-filled PCF, and the RI information was extracted from the total response of the LPFG with the temperature effect having been compensated by the liquid-filled PCF.

High-Sensitivity Gas Pressure Sensor Based on Fabry-Pérot Interferometer With a Side-Opened Channel in Hollow-Core Photonic Bandgap Fiber

We demonstrate a high-sensitivity gas pressure sensor by use of an in-fiber Fabry-Pérot interferometer (FPI) based on hollow-core photonic bandgap fiber (HC-PBF) with a side-opened channel. The FPI was constructed by splicing a thin piece of HC-PBF between two stander single-mode fibers. Then, a side-opened channel was drilled through the hollow core of the HC-PBF by use of a femtosecond laser. Such an FPI with a side-opened channel greatly enhanced the gas pressure sensitivity up to 4.24 nm/MPa, which is two orders of magnitude higher than that of FPI with an enclosed cavity. In addition, the effects of cavity length on the gas pressure sensing performance were also studied. A shorter cavity gives rise to broader measurement range while offering larger detection limit, and vice versa. The structure size is tens of micrometers, which makes it possible to develop an ultracompact high-sensitivity gas pressure sensor.

Thin-Core-Fiber-Based Long-Period Fiber Grating for High-Sensitivity Refractive Index Measurement

We experimentally demonstrated the fabrication of asymmetric long-period fiber gratings (LPFGs) in thin core fiber by use of focused CO2 laser beam. The proposed device exhibits a high extinction ratio of over 25 dB at the resonant wavelength and a narrowed 3-dB bandwidth of only 8.7 nm, which is nearly one order of magnitude smaller than that of LPFGs in conventional single-mode fibers. It also exhibits a high polarization-dependent loss of over 20 dB at resonant wavelength. The temperature and external refractive index (RI) sensitivity of the proposed structure are measured to be 46 pm/°C, within a temperature range from 25 °C to 100 °C, and 1047.3 nm/RIU, within the RI range from 1.400 to 1.440, respectively. The temperature induced error is ~8% for RI measurement. Such long LPFGs may find potential applications of highly sensitive RI sensors in the fields of chemical and biomedical sensing.

Long Period Fiber Gratings Inscribed With an Improved Two-Dimensional Scanning Technique

We demonstrated a promising CO2 laser irradiation system based on an improved 2-D scanning technique. Such a system could be used to inscribe high-quality long period fiber gratings (LPFGs) with good reproducibility of grating inscription, which attributes to the fact that our system includes a CO2 laser with an excellent power stability of less than ±2% and a 3-D ultraprecision motorized translation stages with an excellent bidirectional repeatability value of 80 nm. Moreover, a control program with an easy-to-use operation interface was developed in our system so that a high-quality LPFG could be achieved as soon as grating parameters, such as grating pitch and number of grating periods, are entered, which has a widespread commercial value and prospects for development. Additionally, near mode fields of the CO2-laser-induced LPFG were observed and simulated to investigate mode coupling in the gratings.