A methane sensor based on dispersion spectroscopy is presented in this paper. A standard Mach-Zehnder modulator working in carrier suppression mode is adopted to generate a spectrum of a carrier and two sidebands. We aim at detecting the phase shift of the beatnote generated by the two sidebands in a methane concentration evaluation process. We put forward an analytical model to describe the dual-sideband heterodyne scheme and carry out experiments to demonstrate the model. Long-term tests show that the sensor has a minimum detection limit of 0.4 ppm·mHz-0.5 at an average time of 1 s. And in the condition of 1 atm and room temperature, a linear measurement range from 0.4 to 44955 ppm·m is achieved.
A new scheme for methane remote sensing is presented. Unlike a standard published remote sensor based on wavelength modulation spectroscopy (WMS), a reference cell is inserted into the measuring optical path. This scheme inherits the merits of WMS and can achieve high signal-to-noise ratio especially in a low concentration environment. Experimental results show that the presented remote sensor can detect ambient methane with a detection limit of 5 ppm m (parts per million · meter) at a distance of 10 m and 16 ppm m for 20 m. A methane leak test shows the sensor can detect a methane leak of 15 ml min−1 within a range up to 37 m.
To advance the practical application of optical coherence tomography (OCT) in the field of biomedical imaging, the imaging depth must be extended without sacrificing resolution while maintaining sufficient sensitivity. However, there is an inherent trade-off between lateral resolution and depth of field (DOF) in OCT. To address this shortcoming, this article proposes a multifocal Bessel beam spectral-domain optical coherence tomography (MBSDOCT) capable of increasing the DOF with unchanged lateral resolution and a high signal-to-noise ratio. The proposed technique is demonstrated by simulation and experiment. A three-focal MBSDOCT with an axicon lens theoretically achieved a DOF of ∼6 mm with a lateral resolution of ∼13 μm. In imaging experiments performed on the acinar cells of orange tissue, a measured DOF of ∼4 mm was demonstrated with a sensitivity penalty of ∼18.1 dB, relative to the Gaussian beam spectral-domain OCT, with a 9-mW light source.
A whole-fiber methane sensor under high absorbance based on phase sensitive chirped laser dispersion spectroscopy is presented in this paper. The laser source of the sensor is a tunable distributed feedback diode laser with a frequency of 1653.7 nm. A telecom-based electro-optical intensity Mach-Zehnder modulator working in carrier suppression mode is adapted to modulate the single frequency laser beam for generating a dual-sideband spectrum beside the carrier wave. Unlike previous proposed phase sensitive chirped laser dispersion spectroscopy scheme, the beatnote signal generated by the two sidebands is detected experimentally. The refractive index fluctuation around the 23 transition of methane is measured by detecting the phase variation of the dual-sideband beatnote signal through using the heterodyne interferometric method. A lock-in amplifier is employed in the phase demodulation process. By connecting the refractive index (the real part of the complex refraction index) and the absorption coefficient (the imaginary part of the complex refraction index) via Kramers-Kroning relation, the gas concentration information is retrieved from the optical dispersion measurement. Absorption-based wavelength modulation spectroscopy measures the gas concentration encoded in the optical intensity based on Beer-Lambert's law. However, the signal sensitivity of wavelength modulation spectroscopy decreases, and the signal even decreases while the gas concentration is raised in high absorbance condition, which leads to an uncertainty in concentration measurement. Experimental results demonstrate that wavelength modulation spectroscopy has better performance in low absorbance condition. The detection limit is about 38.1 ppmm. However, because the sensitivity decreases in high absorbance conditions, the upper detection limit of wavelength modulation spectroscopy is only 1500 ppmm. The dynamic range is defined through dividing the upper detection limit by the detection limit. Therefore, the wavelength modulation spectroscopy obtains a linear measurement dynamic range of 16 dB. Nevertheless, under the same experimental condition, the phase sensitive chirped laser dispersion spectroscopy has a much larger linear measurement range from 47.3 ppmm to 174825 ppmm with a dynamic range higher than 35 dB. Absorption-based gas measurement technique such as wavelength modulation spectroscopy can achieve a low detection limit by using long optical path at the expense of lower upper limit concentration. Phase sensitive chirped laser dispersion spectroscopy appears to be effective in high absorbance condition, which may be caused by high concentration or long optical path. Furthermore, by combining phase sensitive chirped laser dispersion spectroscopy and long optical path technique such as multi pass cell in sensor design, large linear measurement dynamic range and low detection limit can be obtained at the same time.
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