Gas phase Raman spectroscopy using hollow waveguides

The detection and characterization of molecular gases in a given sample is a relatively difficult challenge. Usually, this task is relegated to expensive and time consuming processes like mass spectrometry and gas chromatography. Furthermore, numerous industrial applications require such gas-phase analysis for pollution and process control. In particular, the operation of large natural gas-fired turbine generators requires real-time analysis of constituent hydrocarbon concentration in order to provide energy content information about the gaseous fuel, and subsequently, inputs for controlling fuel/air ratio and burner temperature. Herein, a novel technique for studying gaseous samples is presented which uses a new collection method for Raman spectroscopy. In our technique, gasses are introduced inside the light-guiding core of a hollow waveguide. Either lengths of hollow-core photonic-bandgap fiber or internally reflective capillary waveguides are used to both contain sample gases and collect Raman photons. The optical confinement characteristics of these types of hollow-waveguides allow a high power-density laser beam to propagate a long distance along with the low-volume gaseous sample. We have shown analytically that the Raman signal strength (power) collected using our gas cells can be hundreds of times larger than that which can be obtained in free-space. Along with this improvement in collected Raman power comes shorter minimum interrogation times and higher sensitivities to trace gasses. In general, the technique paves the way for the construction of a gas Raman spectrometer with low-cost components and high-accuracy

Fiber optic gas sensor

A gas sensor includes an in-fiber resonant wavelength device provided in a fiber core at a first location. The fiber propagates a sensing light and a power light. A layer of a material is attached to the fiber at the first location. The material is able to absorb the gas at a temperature dependent gas absorption rate. The power light is used to heat the material and increases the gas absorption rate, thereby increasing sensor performance, especially at low temperatures. Further, a method is described of flash heating the gas sensor to absorb more of the gas, allowing the sensor to cool, thereby locking in the gas content of the sensor material, and taking the difference between the starting and ending resonant wavelengths as an indication of the concentration of the gas in the ambient atmosphere

Pilot-scale testing of natural gas pipeline monitoring based on phase-OTDR and enhanced scatter optical fiber cable

Abstract In this paper, we present the results of lab and pilot-scale testing of a continuously enhanced backscattering, or Rayleigh enhanced fiber cable that can improve distributed acoustic sensing performance. In addition, the Rayleigh-enhanced fiber is embedded within a tight buffered cable configuration to withstand and be compatible for field applications. The sensing fiber cable exhibits a Rayleigh enhancement of 13 dB compared to standard silica single-mode fiber while maintaining low attenuation of ≤ 0.4 dB/km. We built a phase-sensitive optical time domain reflectometry system to interrogate the enhanced backscattering fiber cable both in lab and pilot-scale tests. In the laboratory experiment, we analyzed the vibration performance of the enhanced backscattering fiber cable and compared it with the standard single-mode telecom fiber. Afterward, we field validated for natural gas pipeline vibration monitoring using a 4-inch diameter steel pipeline operating at a fixed pressure level of 1000 psi, and a flow rate of 5, 10, 15, and 20 ft/s. The feasibility of gas pipeline monitoring with the proposed enhanced backscattering fiber cable shows a substantial increase in vibration sensing performance. The pilot-scale testing results demonstrated in this paper enable pipeline operators to perform accurate flow monitoring, leak detection, third-party intrusion detection, and continuous pipeline ground movement