i Lut Ho

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.

Evanescent-wave gas sensing using microstructure fiber

We report the experimental demonstration of evanescent-wave gas detection with a silica-air microstructure fiber. Detection sensitivity of ~6% of that of direction absorption spectroscopy per equal length has been achieved. 0]

Fiber-Optic Fabry–Pérot Acoustic Sensor With Multilayer Graphene Diaphragm

A fiber-optic Fabry-Pérot acoustic sensor with a ~100-nm-thick multilayer graphene diaphragm is reported. Acoustic testing demonstrates a pressure-induced deflection of 1100 nm/kPa and a noise equivalent acoustic signal level of ~ 60 μPa/Hz1/2 at the frequency of 10 kHz. The sensor exhibits a flat frequency response from 0.2 to 22 kHz and may be useful for highly sensitive acoustic sensing.

Performance analysis of a time-division-multiplexed fiber Bragg grating sensor array by use of a tunable laser source

The results of an investigation of the performance of a time-division-multiplexed (TDM) fiber Bragg grating (FBG) sensor array using a tunable laser source are reported. The system performance is found limited by the extinction ratio of the optical pulse modulator used for pulse amplitude modulation. Formulas that relate the crosstalk to the extinction ratio of the optical pulse modulator, the modulation parameters of the tunable laser, and the optical path differences among sensing channels are derived. Computer simulation shows that an array of 20 FBG sensors with 3 /spl mu//spl epsi/ resolution can be realized with a commercially available single Mach-Zehnder type optical pulse modulator of -35-dB extinction ratio.

Fast Response Microstructured Optical Fiber Methane Sensor With Multiple Side-Openings

We have demonstrated a fast response optical fiber methane sensor by use of a hollow-core photonic bandgap fiber with periodic microchannels fabricated by a femtosecond Ti:sapphire laser. A diffusion limited response time of ~3 s and a sensitivity of ~647 ppm have been achieved for a 7-cm sensing fiber with seven side-holes separated by 1 cm along the fiber. The side holes introduce very little loss and hence it would be practically possible to construct fiber gas sensors with long sensing length (up to a few tens of meters) to achieve highly sensitive distributed sensing without compromising the response time.

Ultra-sensitive all-fibre photothermal spectroscopy with large dynamic range

Photothermal interferometry is an ultra-sensitive spectroscopic means for trace chemical detection in gas- and liquid-phase materials. Previous photothermal interferometry systems used free-space optics and have limitations in efficiency of light–matter interaction, size and optical alignment, and integration into photonic circuits. Here we exploit photothermal-induced phase change in a gas-filled hollow-core photonic bandgap fibre, and demonstrate an all-fibre acetylene gas sensor with a noise equivalent concentration of 2 p.p.b. (2.3 × 10−9 cm−1 in absorption coefficient) and an unprecedented dynamic range of nearly six orders of magnitude. The realization of photothermal interferometry with low-cost near infrared semiconductor lasers and fibre-based technology allows a class of optical sensors with compact size, ultra sensitivity and selectivity, applicability to harsh environment, and capability for remote and multiplexed multi-point detection and distributed sensing.

Measurement of gas diffusion coefficient using photonic crystal fiber

We report the use of photonic crystal fiber (PCF) for the measurement of gas diffusion constant. PCF has uniform airhole columns along the fiber length that provide the basis for the study of gas diffusion based on the capillary method. The gas concentration within the airhole columns is monitored by measuring the attenuation of light through the PCF caused by the evanescent wave absorption of light by the gas sample.

Gas detection with micro and nano-engineered optical fibers

The prospects for realization of all-fiber gas sensors with hollow-core photonic bandgap fibers and tapered micro/nano optical fibers are investigated. Issues such as high background level and slow response time are discussed.

Compact In-Fiber Interferometer Formed by Long-Period Gratings in Photonic Crystal Fiber

We reported the design and implementation of an in-fiber Mach-Zehnder interferometer (MZI) based on a pair of long-period gratings (LPGs) written on a photonic crystal fiber (PCF). The LPG was fabricated by using a pulsed CO2 laser to carve grooves periodically along the PCF. The MZI relies on the interference between the fundamental core mode and a cladding mode of the PCF. The MZI was further demonstrated as a temperature sensor and a strain sensor. The temperature and strain sensitivities were measured to be 42.4 pm/degC and - 2.6 pm/muepsiv, respectively. We also fabricated an MZI on a single-mode fiber, which has a temperature sensitivity of 1215.56 pm/( degCmiddotm) and a strain sensitivity of + 0.445 pm/muepsiv.

Cantilever optical vibrometer using fiber Bragg grating

A cantilever optical vibrometer using a fiber Bragg grating (FBG) is developed. The sensor head is made by attaching an FBG to a triangular beam to provide position-independent strain for the purpose of acceleration measurement. The vibrometer demonstrates a noise-limited acceleration resolution of 2.8×10 –4 √Hz with repeatability error of less than 2.4%.