To realize the highly sensitive detection of trace H2S in SF6, a multi-Mechanism Enhanced Photoacoustic Spectroscopy Analyzer (MEPSA) is proposed. A resonant Photoacoustic Cell (PAC) is used to enhance the photoacoustic signals. An ultrasensitive optical fiber acoustic sensor based on a silicon cantilever beam is used for mechanical resonance enhancement of photoacoustic signals. An Erbium-Doped Fiber Application amplifier (EDFA) was used to cascade a DFB to increase the excitation light power. The noise equivalent minimum detection limit of H2S under the SF6 background is 0.15 ppm.
In this paper, a new scattering enhanced photoacoustic (PA) spectroscopy (SEPAS) technique is introduced for trace gas detection. The porous materials are used to improve the absorption optical length of trace gas. The working mechanism of photoacoustic signal in porous materials is explained by simulation analysis. Through simulation analysis, the enhancement mechanism of the photoacoustic signal in the porous material is explained, and the experimental results objectively prove that SEPAS can achieve at least three times enhancement of the PA signal. The SEPAS technique proposed in this paper can increase the amplitude of the PA signal and improve the sensitivity of trace gas detection.
A low-frequency photoacoustic (PA) sensor based on Parylene-C diaphragm for micro-leakage gas detection is presented. The overall structure of the sensor head includes a cylindrical brass shell, a Parylene-C diaphragm, and a PA cavity regarded as a Fabry-Perot (F-P) cavity as well. The volume of the PA cavity is only 74 μL. A distributed feedback (DFB) laser is used as an acoustic excitation source. The PA pressure signal is obtained by measuring the deflection of the Parylene-C diaphragm using fiber-optic white-light interference (WLI) method. The PA sensor head is used for acetylene (C2H2) detection. A detection limit of 36 parts-per-billion (ppb) is achieved when the lock-in integration time is 1 s.
A single-mode fiber-optic Fabry–Perot interferometry (FPI) sensor based on optical cross-correlation demodulation is presented, with advantages of both low cost and small size. The sensor mainly consists of a light-emitting diode, a glass wafer, a linear charge-coupled device, a cylindrical lens, a fiber-optic coupler, a sensing head, and a circuit board. The cavity length of the FPI sensor is demodulated by the optical cross-correlation relationship between the glass wafer and the Fabry–Perot cavity. When the cavity length changes, the pixel number corresponding to the peak value of the cross-correlation image shifts accordingly. To optimize the demodulation parameters, a simulation model is established. The experimental results show that the resolution and variation range of cavity length measurement are achieved to be 0.6 nm and ±4 μm, respectively. The proposed fiber-optic sensor is promising for the temperature and pressure measurements in circumstances of intense electromagnetic interference and long distance.
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.
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.
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.
We report a high-resolution optical refractometer based on the long-period grating Michelson interferometer. The interferometer phase shift depends on the refractive index that surrounds the fiber probe. A cross-correlation signal-processing method is used to demodulate the interferometer phase shift. Experimental results show that a resolution of 3×10−6 refractive index unit (RIU) can be obtained using this cross-correlation signal processing method. In addition, a measurement sensitivity up to 3×103 deg/RIU is showed as the surrounding refractive index changing from 1.33 to 1.42. Such high-resolution and low-cost optical refractometers would find more applications in chemical or biochemical sensing fields.
We demonstrate a hybrid sensing scheme, combining the incoherent optical frequency domain reflectometry (IOFDR) based Raman distributed temperature sensor (DTS) with the extrinsic Fabry-Perot interferometer (EFPI) based pressure sensor, for down-hole monitoring. The pressure measurement is compensated by a fiber Bragg grating (FBG) based temperature sensor. By using a laser diode as the common light source, a hybrid Raman/EFPI/FBG sensing system has been developed with a single fiber. The stimulated emission light and the spontaneous emission light of the laser diode are used for DTS and EFPI-FBG interrogations respectively. There can be no overlap between the spectral range of the Raman backscattered light and the spectral range of the reflected light from the EFPI-FBG sensor. Distributed temperature and pressure can thus be measured by using wavelength-division multiplexing (WDM) technology. Experimental results show that, both the distributed temperature and the key-point pressure can be measured alternatively with little interference.
As an emerging biosensing technology, Surface Plasmon Resonance (SPR) technique, characterized by high sensitivity, label-free detection and real-time monitoring, has been extensively applied in biochemical analysis, environmental monitoring and refractive index measurement. In this paper, an angle modulated SPR spectrometer with high resolution is designed and manufactured. First, according to the modeling and simulation for the SPR spectrometer, several key parameters such as the light source, the thickness of golden film and Cr film are determined. Then, an angle modulated SPR spectrometer system based on 5-layers Kretchmann prism structure is developed for biochemical analysis. System performance is tested after the SPR spectrometer established. We test the power stability of the laser first, which is up to 1.504% (5min). Different concentrations of glycerol are measured to demarcate the system. Then, we measured the deionized water ten times continuously, and a resolution of 1.5×10-5 RIU is achieved. At last, different concentrations of glucose solution are measured, and the resonance angles are used to calculate the refractive index of the glucose solutions, which is more accurate than the result of Abbe refractometer. The relationship between concentration and refractive index is presented by liner fitting.
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.
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