Xiaoqi Ni

A Hybrid Mach–Zehnder Interferometer for Refractive Index and Temperature Measurement

We fabricate and experimentally demonstrate a hybrid structured in-line Mach-Zehnder interferometer (MZI) for simultaneous measurement of refractive index (RI) and temperature. The MZI is composed of an embedded slender air cavity in a micro fiber cascaded to a piece of photonic crystal fiber (PCF). The interferometer is fabricated by fusion splicing and fused tapering. The interference spectra based on the air cavity and the PCF can be extracted through low-pass and band-pass filters, respectively. RI and temperature are interrogated through a coefficient matrix simultaneously by tracing two-dip wavelength shifts of the interference spectra. Novel structure, easy fabrication, simple system, and simultaneous measurement make it appropriate for dual-parameter sensing application.

High-resolution fiber Bragg grating based transverse load sensor using microwave photonics filtering technique.

In this paper, a new fiber Bragg grating (FBG) sensor exploiting microwave photonics filter technique for transverse load sensing is firstly proposed and experimentally demonstrated. A two-tap incoherent notch microwave photonics filter (MPF) based on a transverse loaded FBG, a polarization beam splitter (PBS), a tunable delay line (TDL) and a length of dispersion compensating fiber (DCF) is demonstrated. The frequency response of the filter with respect to the transverse load is studied. By detecting the resonance frequency shifts of the notch MPF, the transverse load can be determined. The theoretical and experimental results show that the proposed FBG sensor has a higher resolution than traditional methods based on optical spectrum analysis. The sensitivity of the sensor is measured to be as high as 2.5 MHz/N for a sensing fiber with a length of 18mm. Moreover, the sensitivity can be easily adjusted.

Demodulation of an optical fiber MEMS pressure sensor based on single bandpass microwave photonic filter

In this paper, a demodulation method for optic fiber micro-electromechanical systems (MEMS) extrinsic Fabry-Perot interferometer (EFPI) pressure sensor exploiting microwave photonics filter technique is firstly proposed and experimentally demonstrated. A single bandpass microwave photonic filter (MPF) which mainly consists of a spectrum-sliced light source, a pressurized optical fiber MEMS EFPI, a phase modulator (PM) and a length of dispersion compensating fiber (DCF) is demonstrated. The frequency response of the filter with respect to the pressure is studied. By detecting the resonance frequency shifts of the MPF, the pressure can be determined. The theoretical and experimental results show that the proposed EFPI pressure demodulation method has a higher resolution and higher speed than traditional methods based on optical spectrum analysis. The sensitivity of the sensor is measured to be as high as 86 MHz/MPa in the range of 0-4Mpa. Moreover, the sensitivity can be easily adjusted.

Optical Fiber Bragg Grating Pressure Sensor Based on Dual-Frequency Optoelectronic Oscillator

A dual-frequency optoelectronic oscillator (OEO) for high sensitivity pressure sensing is proposed and experimentally demonstrated. The oscillation frequency of the OEO is determined by a dual-passband microwave photonic filter in the OEO loop, which is implemented using phase shifted fiber Bragg gratings (PSFBGs) written in side-hole fiber to perform phase-modulation to intensity-modulation conversion. When the PSFBGs are experiencing a pressure, the wavelength difference between the two notches of the PSFBGs is slightly changed, which leads to a large frequency change of the two oscillating frequencies. Since the beat frequency between the two microwave signals is linearly proportional to the pressure, the high sensitivity pressure measurement can be realized through monitoring the beat frequency. The proposed approach is experimentally evaluated. High sensitivity pressure sensing with a sensitivity of 418.8 MHz/MPa is experimentally demonstrated. In addition, the sensing is insensitive to the variations of the environmental temperature.

Note: Simultaneous measurement of in-plane and out-of-plane displacement by using orthogonally polarized self-mixing grating interferometer

In this paper, we present an orthogonally polarized self-mixing grating interferometer (SMGI) for simultaneous measurement of in-plane and out-of-plane displacements. The measurement ranges in both directions are limited only by the length of grating. The orthogonally polarized lights emitted from a birefringent He-Ne laser are separated and enter the grating at ±1st-order Littrow angles. The diffraction beams re-enter the laser cavity and cause self-mixing interference. To differentiate the orthogonally polarized lights and obtain high resolution, phase modulation technique is introduced to extract phases from the orthogonally polarized SMGI signals. The measurement results show that the proposed system can reach a submicron accuracy in the experiment. This work provides a good way to achieve high precision two-dimensional displacement measurement with a robust system configuration.

An optical fiber MEMS pressure sensor using microwave photonics filtering technique

A fiber-optic micro-electromechanical systems (MEMS) extrinsic Fabry-Perot interferometer (EFPI) pressure sensor exploiting microwave photonics filtering technique is firstly proposed and experimentally demonstrated. A single-bandpass microwave photonic filter (MPF) which mainly consists of a spectrum-sliced light source, a pressurized EFPI, a phase modulator (PM) and a length of dispersion compensating fiber (DCF) is demonstrated. The frequency response of the filter with respect to the pressure is studied. By detecting the resonance frequency shifts of the MPF, the pressure can be determined. The theoretical and experimental results show that the proposed EFPI pressure sensor has a higher resolution and higher speed than traditional methods based on optical spectrum analysis. The sensitivity of the sensor is measured to be as high as 86 MHz/MPa in the range of 0–4MPa.