Zhiyong Bai

Liquid modified photonic crystal fiber for simultaneous temperature and strain measurement

A liquid modified photonic crystal fiber (PCF) integrated with an embedded directional coupler and multi-mode interferometer is fabricated by infiltrating three adjacent air holes of the innermost layer with standard 1.48 refractive index liquids. The refractive index of the filled liquid is higher than that of background silica, which can not only support the transmitting rod modes but also the “liquid modified core” modes propagating between the PCF core and the liquid rods. Hence, the light propagating in the liquid modified core can be efficiently coupled into the satellite waveguides under the phase-matching conditions, resulting in a dramatic decrease of the resonant wavelength intensity. Furthermore, there is a multi-mode interference produced by modified core modes and rod modes. Such a compact (∼0.91  cm) device integrated with an embedded coupler and interferometer is demonstrated for high-sensitivity simultaneous temperature (∼14.72  nm/°C) and strain (∼13.01  pm/μϵ) measurement.

Nano silica diaphragm in-fiber cavity for gas pressure measurement

We demonstrate an ultrahigh-sensitivity gas pressure sensor based on the Fabry-Perot interferometer employing a fiber-tip diaphragm-sealed cavity. The cavity is comprised of a silica capillary and ultrathin silica diaphragm with a thickness of 170 nm, with represents the thinnest silica diaphragm fabricated thus far by an electrical arc discharge technique. The resulting Fabry-Perot interferometer-based gas pressure sensor demonstrates a gas pressure sensitivity of about 12.22 nm/kPa, which is more than two orders of magnitude greater than that of a similarly configured fiber-tip air bubble sensor. Moreover, our gas pressure sensor has a low temperature cross-sensitivity of about 106 Pa/°C, and the sensor functions well up to a temperature of about 1000 °C. As such, the sensor can potentially be employed in high-temperature environments.

Long period fiber grating based on periodically screw-type distortions for torsion sensing

A novel long-period fiber grating (LPFG), fabricated by periodically cascading a series of screw-type distortions, is proposed and experimentally demonstrated. These screw-type distortions are induced by twisting the fiber during CO2 laser beam exposure. The resulting LPFG will either be left- or right-hand helical, depending on the twist rate and direction used during fabrication, with a certain frozen shear strain. Due to the independence between grating pitch and twist rate, this type of LPFG could be more flexible than the helical- or chiral-fiber gratings reported previously. During LPFG twisting, the device displays good directional dependence and an enhanced torsion sensitivity of 0.1604 nm/(rad/m), which implies the structure could be an excellent candidate for torsion sensors.

High-Sensitivity Gas-Pressure Sensor Based on Fiber-Tip PVC Diaphragm Fabry–Pérot Interferometer

We demonstrate a novel polyvinyl chloride (PVC) diaphragm-based fiber-tip Fabry–Perot interferometer for gas-pressure measurements with ultrahigh sensitivity. The PVC diaphragm has been coated to the end facet of a well-cut standard single-mode fiber by use of a plastic welder. An ultrahigh-pressure sensitivity of ∼65.5 nm/MPa at 1565 nm and a low-temperature cross sensitivity of ∼–5.5 kPa/°C have been experimentally demonstrated. The proposed sensor has advantages of high pressure sensitivity, miniature size, low cost, and easy fabrication.

Femtosecond laser microprinting of a polymer fiber Bragg grating for high-sensitivity temperature measurements

We demonstrate the microprinting of a novel suspended polymer fiber Bragg grating for high-sensitivity temperature measurements. The proposed sensor was developed using a femtosecond laser-induced multiphoton polymerization technique. The grating was cured in a single-groove silica tube spliced between two single-mode fibers. Its transmission spectrum, mode field, and temperature response were thoroughly investigated. A sensitivity of -220  pm/°C was achieved over a temperature range of 24°C to 40°C, which is meaningful in biosensing applications. This all-in-fiber polymer Bragg grating exhibits high temperature sensitivity, excellent mechanical strength, and ultrahigh integration. As such, a temperature sensing element of this type would be a beneficial tool for biological measurements.

Influence of Side-Polished Fiber Surface Topography on Surface Plasmon Resonance Wavelengths and the Full Width at Half-Maximum

The influence of surface topography of side-polished fiber on resonance wavelengths and the full-width-at-half-maximum (FWHM) of surface plasmon resonance was evaluated in this work, based on the power spectrum density, wavelet, and finite-difference time-domain methods. The abrasive size determined the surface topography with various spatial period components. Coarse abrasives induced obvious low-frequency waviness features and a blue shift in the resonance wavelength. Fine abrasives introduced middle frequency microwaviness (0.5–1.0 μ m), which led to a blue or red shift, depending on the special period extent between 0.5 and 0.75 μ m. All waviness components broadened the FWHM because of the superimposed effect and the introduction of a high-order coupling model. High-frequency roughness components were able to shift the resonance peaks toward shorter wavelengths. Larger coupling energy tended to decrease the FWH, while high-order coupled modes tended to broaden the FWHM. We established a roughness model with Maxwell–Garnett theory incorporated into fractal dimensions. Experimental results demonstrated the feasibility of such a model. Small abrasive particles were able to narrow the FWHM, which is beneficial for identification of resonance wavelengths and evaluation of the surface plasmon resonance effect.

Sensing Characteristics of Tilted Long Period Fiber Gratings Inscribed by Infrared Femtosecond Laser

We propose a novel tilted long period fiber grating (TLPFG) design, inscribed using a line-by-line inscription technique and an infrared femtosecond (Fs) laser. The responses of this TLPFG to external refractive index, temperature, torsion, and strain were systematically investigated to determine its sensing characteristics. The external refractive index (RI) was measured to be −602.86 nm/RIU at an RI of ~1.432. The TLPFG was used to accurately measure temperatures up to 450 °C with a sensitivity of 103.8 pm/°C. The torsion and strain sensitivity of the device were 48.94 nm/(rad/mm) and −0.63 pm/µε, respectively. These results demonstrate that the proposed TLPFG could be used as sensors in a series of application fields including high temperatures and external environments.

Temperature Insensitivity Polarization-Controlled Orbital Angular Momentum Mode Converter Based on an LPFG Induced in Four-Mode Fiber

We propose a novel method for generating ±1-order orbital angular momentum (OAM) in long-period fiber gratings (LPFGs) by adjusting a polarization controller (PC). An LPFG, inscribed in a four-mode fiber (4MF) using a CO2 laser, was used to generate OAM±1 across a broad range of wavelengths from 1530 nm to 1630 nm. Additionally, the OAM vortex phase remained stable while the temperature increased from 23 °C to 50 °C. The LPFG, as a temperature sensor, and its temperature sensitivity was measured to be 38.6 ± 0.37 pm/°C at the resonant wavelength of 1625 nm. This design offers simple fabrication and several properties which are highly beneficial for all-fiber optical communications based on the OAM mode-division multiplexing technique.