Qingxu Yu

Photoacoustic monitoring of trace gases by use of a diode-based difference frequency laser source

We present a compact mid-infrared laser spectrometer for trace-gas monitoring. Difference frequency generation in periodically poled LiNbO(3) is used as laser source, yielding a tuning range 3.2-3.7mum at a linewidth of 154 MHz. The relatively high average power of 3 to 5 mW favors detection with a small resonant photoacoustic gas cell. Measurements of methane yield a detection limit in the low parts in 10(6) by volume concentration range.

Simultaneous measurement of down-hole pressure and distributed temperature with a single fiber

A cost-effective sensing system combining the fiber Fabry?Perot interferometer-based pressure sensor with the distributed temperature sensor is presented. This sensing system can realize accurate pressure measurement of the oil reservoir and temperature profile measurement of the wellbore simultaneously. For the sensing wave bands of the pressure and temperature sensor are different, the wavelength-division multiplexing (WDM) technology was used to couple the two sensing signals into one single fiber. As the wavelength difference between Stokes peak and anti-Stokes peak is larger than the bandwidth of commercial WDM, a wide bandwidth WDM with gain flattening range up to 80?nm was custom-made. Experimental results showed that this sensing system can reach 0.1% accuracy for the pressure measurement with a 0?30 MPa working range and 1??C accuracy for the temperature measurement in a working range from room temperature to 300??C. Additionally, a resolution of below 1.4 kPa for the pressure measurement and 0.5??C for the temperature measurement had been realized. Field test results showed that this sensing system also has good long-term stability in high-temperature environment down hole.

Characteristics of a fiber-optical Fabry–Perot interferometric acoustic sensor based on an improved phase-generated carrier-demodulation mechanism

Abstract. The characteristics of a fiber-optic Fabry–Perot interferometric acoustic sensor are investigated. An improved phase-generator carrier-demodulation mechanism is proposed for obtaining a high harmonic suppression ratio and stability of the demodulation results. A gold-coated polyethylene terephthalate membrane is used as the sensing diaphragm. By optimizing the parameters and the demodulation algorithm, the signal-to-noise ratio (SNR) and distortion ratio of 50.3 dB and the total harmonic distortion of 0.1% at 114 dB sound pressure level (SPL) (@ 1 kHz) are achieved, respectively. The sensor shows good temperature stability; the variation of the response is within 0.6 dB as the temperature changes from −10°C to 50°C. A sensitivity of 40  mV/Pa at 1 kHz and a frequency response range of 100 Hz to 12.5 kHz are reached, respectively. The SNR of the system is 60 dB (Re. 94 dB SPL). The sensor may be applied to photoacoustic spectrometers as a high-performance acoustic sensor.

Loss analysis of physical contact fiber-optic connector induced by the endface geometry and the contact force

Abstract. Optical loss of fiber-optic connectors has a vital impact on fiber-optic-related systems. We analyze the contact loss caused by the endface geometry and the contact force. Based on analytical and simulated results, the analytical equations of the insertion loss (IL) and return loss (RL) as a function of the endface geometry and the contact force are derived. Then four fiber-optic connectors are experimentally tested. The experimental results are well consistent with the theoretical results, which show that there is an optimum relationship between the endface geometry and the contact force to ensure a low IL and a high RL.

Optical frequency domain reflectometry based single-mode fiber-optic distributed temperature sensor using synchronous polarization scrambling technique

In order to reduce the polarization-induced impact on the incoherent optical frequency domain reflectometry based single-mode fiber-optic distributed temperature sensor, a synchronous polarization scrambling technique with a low-speed electrically driven polarization controller (EPC) is presented. The polarization-induced error is derived by error analysis. By simulating the distribution of the states of polarization on the Poincare sphere, optimized EPC driven parameters are selected. The polarization scrambling process is synchronous with the frequency response measurement of the Raman backscattered light. Additionally, the scrambling period is set to be equal to the measurement time of each frequency response. Experimental results show that the polarization-induced error is ∼±3°C, and it is basically in accord with the result of a theoretical error analysis. By using the synchronous polarization scrambling technique, the polarization-induced fluctuation of the measured temperature distribution has been almost eliminated.

Fiber optic anemometer based on distributed Bragg reflector fiber laser

We present a novel fiber-optic anemometer based on a distributed Bragg reflector (DBR) fiber laser. The anemometer consists of a fiber laser pressure sensing element and a Venturi tube designed to convert the wind velocity into differential gas pressure that can be measured by the fiber-laser sensor. A DBR fiber laser that has two polarization modes is designed and fabricated as the major sensing element in this anemometer. The fiber-laser sensor is demodulated by the beat frequency of the two polarization modes, which has higher sensitivity and lower cost over wavelength-demodulated fiber-laser sensors. The experimental results show that the sensors response to wind velocity is quadratic in the measurement range of 8 ~ 40 m/s, the short term repeatability is better than 0.5%, and its sensitivity is impacted by the power of pump laser. Additionally, this anemometer has the potential capability to be used as a flow meter for both gas and liquid flows.

Research on fiber-optic cantilever-enhanced photoacoustic spectroscopy for trace gas detection

We demonstrate a new scheme of cantilever-enhanced photoacoustic spectroscopy, combining a sensitivity-improved fiber-optic cantilever acoustic sensor with a tunable high-power fiber laser, for trace gas detection. The Fabry-Perot interferometer based cantilever acoustic sensor has advantages such as high sensitivity, small size, easy to install and immune to electromagnetic. Tunable erbium-doped fiber ring laser with an erbium-doped fiber amplifier is used as the light source for acoustic excitation. In order to improve the sensitivity for photoacoustic signal detection, a first-order longitudinal resonant photoacoustic cell with the resonant frequency of 1624 Hz and a large size cantilever with the first resonant frequency of 1687 Hz are designed. The size of the cantilever is 2.1 mm×1 mm, and the thickness is 10 μm. With the wavelength modulation spectrum and second-harmonic detection methods, trace ammonia (NH3) has been measured. The gas detection limits (signal-to-noise ratio = 1) near the wavelength of 1522.5 nm is achieved to be 3 ppb.

Parylene-C diaphragm-based fiber-optic Fabry-Perot acoustic sensor for trace gas detection

We demonstrate a high-sensitivity fiber-optic Fabry-Perot acoustic sensor based on a thin Parylene-C diaphragm. The vacuum thermal evaporation deposition method is used to fabricate the Parylene-C nanofilm, which possesses strong adhesion and good compactness. Based on these characteristics, the Parylene-C diaphragm is fabricated with 9 mm in diameter and 500 nm in thickness. The noise limited equivalent acoustic signal level is 33.5 μPa/Hz1/2 at the frequency of 30 Hz. The pressure sensitivities of the acoustic sensor are more than 1000 mV/Pa at the frequency from 10 Hz to 30 Hz. The fundamental resonance frequency of the Parylene-C diaphragm is about 13 Hz. The acoustic sensor is applied in a multiple trace gas detection system based on photoacoustic spectroscopy. The detection limits of acetylene (C2H2), carbon monoxide (CO) and carbon dioxide (CO2) are achieved to be 0.25, 0.32 and 1.1 parts-per-million, respectively.

Au nanoparticles-based Localized S urface P lasmon R esonance R efractomete r

A localized surface plasmon resonance (LSPR) refractometer based on Au nanoparticles (NPs) was demonstrated in this paper. By immobilizing Au NPs onto the inner wall of micro-capillary, a LSPR-based refractometer was obtained. This sensor was highly sensitive to refractive index (RI) and offers a wide dynamic detection range, and it was sensitivitie-tunable caused by the extinction efficiency of the Au NPs in the evanescent field of micro-capillary inner surface. Then the micro-capillary can be used as both a sensing element and a flow cell, so that micro-measurement can be achieved. The response of LSPR to RI was verified experimentally and the results demonstrated that the absorbance of Au NPs increases between 1.328-1.451, and decreases between 1.451-1.470 with the increasing RI. The sensitivities were 109.6%/RIU in the RI of 1.328-1.394RIU, 395.7%/RIU in the RI of 1.394-1.397RIU and 1035.7%/RIU in the RI range of 1.451-1.470RIU. This refractometer was cost-effective, small-size and highly sensitive to be used for refractometer and can be extended for other physical or chemical parameter measurement for further investigations.

Glycoprotein Detection by Using the Surface Plasmon Resonance Technology

Specific detection of glycoprotein is performed by using the surface plasmon resonance technology. Ribonuclease B solutions with different concentration are measured in the range of 0.01-1 mg/mL and a good linearity is obtained.