Youfu Geng

High-Sensitivity Mach–Zehnder Interferometric Temperature Fiber Sensor Based on a Waist-Enlarged Fusion Bitaper

An all-fiber high-sensitivity temperature fiber sensor based on a Mach-Zehnder interferometer in standard single-mode fibers (SMFs) is described. The interferometer consists of two concatenated waist-enlarged fusion bitapers which are fabricated simply by cleaving and fusion splicing. It is demonstrated that such an all-fiber Mach-Zehnder interferometer incorporates intermodal interference between the LP01 mode and a high-order cladding mode of LP07 mode. Its response to temperature is investigated and a high sensitivity of 0.070 nm/°C is obtained by a 7.5 mm interferometer. This simple, low-cost and easy-to-fabricate core-cladding modal interferometer with entire SMF-based structure also has great potential in diverse sensing applications.

Some features of the photonic crystal fiber temperature sensor with liquid ethanol filling.

We introduce a novel photonic crystal fiber (PCF) temperature sensor that is based on intensity modulation and liquid ethanol filling of air holes with index-guiding PCF. The mode field, the effective refractive index and the confinement loss of PCF were all found to become highly temperature-dependent when the thermo-optic coefficient of the liquid ethanol used is higher than that of silicon dioxide and this temperature dependence is an increasing function of the d/Lambda ratio and the input wavelength. All the experiments and simulations are discussed in this paper and the temperature sensitivity of transmission power was experimentally determined to be 0.315 dB/ degrees C for a 10-cm long PCF.

Compact and Ultrasensitive Temperature Sensor With a Fully Liquid-Filled Photonic Crystal Fiber Mach–Zehnder Interferometer

We propose a compact and ultrasensitive all-fiber temperature sensor based on an in-line fully liquid-filled photonic crystal fiber (PCF) Mach-Zehnder interferometer (MZI). It consists of a small piece of index-guiding PCF fully infiltrated by fluid and two standard single-mode fibers offset spliced with PCF. Two core modes LP01 and LP11 are conveniently used as optical arms to form the in-line MZI-type interferometer. Experimental and theoretical investigations of its response to temperature confirm that high temperature sensitivity up to -1.83 nm/°C could be realized with such a compact interferometeric PCF temperature sensor.

Sensitivity-enhanced high-temperature sensing using all-solid photonic bandgap fiber modal interference

A wavelength-encoded interferometric high-temperature sensor based on an all-solid photonic bandgap fiber (AS-PBF) is reported. It consists of a small piece of AS-PBF spliced core offset with standard single-mode fibers. Two core modes LP(01) and LP(11) are conveniently utilized as optical arms to form Mach-Zehnder-type interference at both the first and the second photonic bandgaps, and the maximum extinction ratio exceeds 25 dB. Experimental and theoretical investigation of its response to temperature confirms that high temperatures up to 700 °C can be effectively sensed using such an AS-PBF interferometer, and benefiting from a large effective thermo-optic coefficient of fiber structure, the sensitivity can be significantly enhanced (71.5 pm/°C at 600 °C).

Experimental realization of D-shaped photonic crystal fiber SPR sensor

A novel surface plasmon resonance sensor based on a D-shaped, all-glass, endless single-mode photonic crystal fiber is experimentally demonstrated in this paper, which provides a new approach to realizing a high-performance photonic crystal fiber surface plasmon resonance sensor. In order to achieve the best performance, the side-polished position of the D-shaped photonic crystal fiber is theoretically and experimentally obtained. The proposed sensor can be used in wavelength and intensity interrogations simultaneously, and the experimental results of wavelength interrogation agree well with theoretical results. By combining the two interrogation methods, we present a two-feature interrogation method to improve the resolution. As a new interrogation method, the two-feature resolution is determined to be 6.53 × 10−5 RIU, which is higher than those of the wavelength and intensity interrogations.

Mode-beating-enabled stopband narrowing in all-solid photonic bandgap fiber and sensing applications.

In this paper, core-cladding modal beating in a short piece of all-solid photonic bandgap fiber (AS-PBF) is observed in longitudinal propagation direction. It is demonstrated that at the stopband range of AS-PBF, the power could transfer back and forth between the fiber core and the first layer of high-index rods. Both experimental results and the theoretical analysis from transverse coupled mode theory confirm that the 3-dB width of the sharp stopband could be significantly narrowed by multicycles of such core-cladding modal couplings, which is of great benefit to the high-resolution sensing applications. Based on such a guiding regime, a high-temperature sensor head is also made and its response to temperature is tested to be of 59.9 pm/°C.

Polymer Microbubble-Based Fabry–Perot Fiber Interferometer and Sensing Applications

A polymer microbubble-based Fabry-Perot fiber interferometer (FPI) for pressure and temperature measurement is proposed and demonstrated. By splicing a small segment of photonic bandgap fiber to a single-mode fiber and immersing such a fiber tip into an optical adhesive, a micro air bubble could be buried into the formed polymer diaphragm. Size of air bubble and polymer diaphragm thickness can be controlled by adjusting arc discharge intensity at the fiber splice point. In order to achieve the wide dynamic range and high-resolution measurement, a demodulation algorithm based on absolute phase analysis is adopted and present high sensitivity for simultaneous pressure and temperature sensing. This simple and reproducible fabrication method of such a sensor gives an alternative way to construct FPI-based biomedical and microfluidic sensors.

Temperature Sensor Based on Quantum Dots Solution Encapsulated in Photonic Crystal Fiber

A luminescence temperature sensor was proposed based on quantum dots aqueous solution encapsulated in a photonic crystal fiber and sandwich structure by fusion splicing two multimode fibers end-to-end. The relationships of the luminescent peak, intensity, and full-width at half-maximum (FWHM) of the luminescent emission band with the temperature are investigated. A self-referenced method is used to optimize the relationship between intensity and temperature. The experimental results indicate that the sensitivities are 130.9pm/°C, 67pm/°C and -0.008/°C, respectively, with favorable repeatability for the luminescent peak, the FWHM, and the self-referenced intensity in the temperature range from -10 °C to 120 °C.

V-groove all-fiber core-cladding intermodal interferometer for high-temperature sensing

Novel V-groove all-fiber core-cladding intermodal interferometers fabricated by CO2 laser irradiation on a standard single-mode fiber are described. The high-order cladding modes are excited due to the special V-groove structure. The interferometers are classified as Mach-Zehnder and Michelson type based on the way they are structured. Benefiting from the large difference of thermal coefficients of the core and high-order cladding modes, both types receive high temperature sensitivity by monitoring the wavelength shift of the interference spectrum, and their responses to temperature are similar. Compared with the Mach-Zehnder interferometer, the Michelson interferometer is more compact and more flexible in application.

UV-Curable Polymer Microhemisphere-Based Fiber-Optic Fabry-Perot Interferometer for Simultaneous Measurement of Refractive Index and Temperature

A fiber-optic Fabry-Perot interferometer based on UV-curable polymer microhemisphere is proposed and demonstrated. The polymer microhemisphere is formed by adhering and solidifying a liquid microdroplet of UV-curable adhesive to the end face of a cleaved single-mode fiber. The height of polymer microhemisphere could be flexibly controlled by adjusting the diameter of a single-mode fiber. The theoretical and experimental results demonstrate that the refractive index (RI) and the temperature of external environment can be simultaneously measured by the fringe contrast variation and the wavelength shift of reflection spectra separately, alleviating the cross sensitivity effectively. The obtained temperature and RI sensitivities are about 0.19 nm/°C and 260 dB/RIU in the RI range of 1.38-1.42.