Jian Ju

Long period gratings in air-core photonic bandgap fibers

Long period fiber gratings in hollow-core air-silica photonic bandgap fibers were produced by use of high frequency, short duration, CO2 laser pulses to periodically modify the size, shape and distribution of air holes in the microstructured cladding. The resonant wavelength of these gratings is highly sensitivity to strain but insensitive to temperature, bend and external refractive index. These gratings can be used as stable spectral filters and novel sensors.

Properties of a highly birefringent photonic crystal fiber

We report the results of our recent investigation on the properties of a highly birefringent photonic crystal fiber including modal birefringence, mode field diameter, divergence angle, and polarization mode dispersion, which are important for sensing and communication applications.

Strain and Temperature Characteristics of a Long-Period Grating Written in a Photonic Crystal Fiber and Its Application as a Temperature-Insensitive Strain Sensor

Strain and temperature characteristics of a long-period grating (LPG) written in an endless-single-mode photonic crystal fiber (ESM-PCF) are investigated theoretically and experimentally. By use of a dispersion factor , a deeper understanding of the behavior of LPG in the ESM-PCF is achieved. The negative strain sensitivity of the LPG is explained by the negative value of the dispersion factor . Our analysis clearly reveals the significant effect of the waveguide dispersive characteristics of the cladding modes on the strain and temperature characteristics of the LPG in the ESM-PCF. By selecting an appropriate grating period, a simple, low-cost LPG sensor with approximately zero temperature sensitivity but large strain sensitivity is realized.

A Compact Fiber-Tip Micro-Cavity Sensor for High-Pressure Measurement

A novel fiber-tip micro-cavity pressure sensor was fabricated by use of a fusion splicer and a pressurizing gas chamber. The fabrication process is simple and efficient without the need for careful cleaving, chemical etching, and bonding. Micro-cavities with wall thickness of a few micrometers demonstrated a pressure sensitivity of ~ 315 pm/MPa . The sensors have compact size, good mechanic strength, and high temperature stability up to 600°C, and may be potentially used for pressure sensing in a high-temperature environment.

Fiber-tip micro-cavity for temperature and transverse load sensing

A low cost fiber-optic micro-cavity interferometric sensor is presented. The micro-cavity is fabricated at the fiber tip by splicing a silica capillary to a single mode fiber and then heating/melting the capillary to form a microsphere with an internal air cavity. Applications of the micro-cavity sensor for temperature and traverse load measurements are demonstrated. The sensor has small size and good mechanical strength, and may be used in high temperature environment.

Photonic bandgap fiber tapers and in-fiber interferometric sensors

Nonadiabatic tapers in hollow-core air-silica photonic bandgap fibers (PBFs) were fabricated by the use of a fiber fusion splicer. In addition to the well-known scaling down of fiber dimensions, the innermost rings of air holes were found collapsed or significantly deformed, which results in almost doubling the diameter of the hollow core in the tapered region. The tapering of the PBF causes coupling of the fundamental core mode to a surface mode with approximately 10% higher effective mode index. An in-fiber core-surface mode interferometer was constructed by cascading two such tapers along the PBF. The interferometer was experimentally demonstrated for strain and temperature measurement, and the sensitivities of the interferometric peak wavelength to strain and temperature are measured to be -0.64 pm/microepsilon and 495.6 pm/( degrees C m), respectively.

Temperature sensitivity of a two-mode photonic crystal fiber interferometric sensor

The temperature sensitivity of a two-mode (TM) photonic crystal fiber (PCF) interferometric sensor was investigated theoretically and experimentally. In contrast to the conventional elliptical core TM fiber interferometric sensors, the temperature sensitivity of the PCF sensor has a nonmonotonic dependence on the operating wavelength, and was measured to be 0.083, 0.147, and 0.136 rad/degCmiddotm at 543, 975, and 1310 nm, respectively

Two-mode photonic crystal fibers

Index-guiding photonic crystal fibers with appropriate structural parameters support the fundamental and second order modes over a practically infinite wavelength range. The polarization principal axes and mode field patterns of the modes can be made stable by having different size air-holes along the orthogonal directions. The potential applications of such two-mode PCFs are discussed.

Two-mode operation in highly birefringent photonic Crystal fiber

A photonic crystal fiber with different air-hole diameters along the orthogonal axes is found to support only the first two modes over a very broad wavelength range (>650 nm). The second-order mode is approximately linearly polarized and has stable-intensity lope positions that do not change in response to environmental disturbances. Strain sensitivity as a function of operating wavelength of a two-mode interferometric sensor made from such a fiber is also investigated.

Directional Bend Sensing With a CO

In this paper, we present a directional bend sensor based on a long period grating (LPG) formed by introducing periodic grooves along one side of a photonic crystal fiber (PCF) with a focused CO2 laser beam. A bend sensitivity of 2.26 nm/m-1 within a range of - 5 ~ + 5 m-1 is experimentally demonstrated. Numerical simulation suggests that the directional response is the result of asymmetric cladding geometry resulted from collapse and/or deformation of air holes and asymmetric material-index modulation caused by one-sided illumination of the CO2 laser beam. The sensitivity could be further enhanced by increasing the area of the air-silica photonic crystal cladding and optimizing the size of individual air holes. The easy fabrication process and good linear response of the proposed sensor make it a suitable candidate for structural shape sensing in harsh environments.