Henry O. Edwards

Progress with optical gas sensors using correlation spectroscopy

The paper reviews recent progress on gas detection using real-time correlation spectroscopy. The general method relies on using a gas sample in a reference cell as a matched optical filter to detect, preferentially, similar absorption spectra in a measurement cell. All variations of the method have the advantage of excellent selectivity for gases with narrow line spectra, even when using broadband sources for illumination. They are also suitable for remote detection over optical-fibre leads. The recent progress is in two main areas. First, the earlier reported Stark modulation method has been extended to investigate a novel hygrometer. Secondly, we have developed a novel multi-line light source by combining a broadband optical source with a Michelson interferometer, where the latter contains a gas in one arm.

Gas sensors using correlation spectroscopy compatible with fibre-optic operation

This paper reviews methods for gas detection using real-time correlation spectroscopy. These methods involve using a gas sample in a reference cell as a matched optical filter to detect, preferentially, similar absorption spectra in a measurement cell. The methods all have the advantage of excellent selectivity, even using broadband sources, and are suitable for remote detection over optical-fibre leads. Our previously published methods using pressure and Stark modulation are reviewed. In addition, our recent theoretical treatment of the methods is presented. Finally, we describe new atmospheric-pressure measurements of gases using the method of phase-modulation spectroscopy.

Optimal design of optical fibers for electric current measurement.

The current sensitivity and bandwidth of the optical fiber current monitor are analyzed. The optimal sensitivity is proportional to the ratio of fiber attenuation to the Verdet constant at a specific fiber length. A selection of compound glasses has been investigated with a view to improving bandwidth and sensitivity over standard silica-fiber systems. A trial fiber of the most promising glass (Schott F7) has been fabricated and characterized.

Latest developments in gas sensing using correlation spectroscopy

This paper reviews latest developments in gas detection using real-time correlation spectroscopy. The general method relies on using a gas sample in a reference cell as a matched optical filter, to preferentially detect similar absorption spectra of the same gas in a measurement cell. All variations of the method have the advantage of excellent selectivity, provided the gases have narrow line spectra, even when using broadband sources for illumination. They are also suitable for remote detection over optical fiber leads. This paper describes work in three main areas. First, work on a Stark modulation method to investigate a novel hygrometer is reviewed. Second, a multiline light source, formed by combining a broadband optical source with a Michelson interferoemeter, which contains a gas in one arm, is described. Third, the most recent progress with a complimentary-beam correlation spectroscopy detection system using either alternately chopped or intensity modulated light sources, is described. The latter methods are very simple as they allow use of a fixed reference gas cell.

Tunable fiber laser source for methane detection at 1.68 um

A tunable fiber laser for spectroscopic gas detection is reported for the first time. The laser is based on a single-mode thulium doped fiber, which can operate at a wavelength around 1.684 micrometers , corresponding to a significant absorption line for methane. The fiber laser was pumped at 786 nm, a wavelength which is readily available with AlGaAs laser diodes and an optical threshold power of 43 mW was observed. An in-fiber photorefractive grating was used as the wavelength-selective output coupler for the laser. Simultaneous straining and heating of the grating induced a change in lasing wavelength, and a tuning range of up to 2 nm was demonstrated. This new tunable light source was configured within a methane detector and absorption spectra were recorded which demonstrate the presence of this gas. The large tuning range of the thulium fiber laser should allow the detection of many gas species with absorption bands in the wavelength region 1.65 micrometers to 2.05 micrometers .

Progress in fiber-remoted gas correlation spectrometry

The paper reports on the progress in gas sensing using real-time correlation spectroscopy, where a gas is used as a matched optical filter to recognize its own spectral absorption lines. The basic concept of correlation spectrometry involves the passage of light sequentially through two gas cells: a reference cell containing a known quantity of the gas to be detected. and a sampling cell where the presence of the gas is to be determined. An optical signal passing through both cells will suffer absorption due to the gas in each. If the absorption in the reference Cell is periodically modulated, then the total absorption depends on whether the gas absorption lines in the sampling cell correlate with those in the reference cell gas. Two methods of modulating the reference cell absorption are reported, pressure and Stark modulation. Results are presented for methane detection employing pressure modulation. The pressure fluctuations are generated within a compact resonant acoustic cell driven by a piezoelectric transducer. Also given are results for cross-sensitivity measurements with ethane as the contaminant gas. The Stark technique is applied to ammonia detection here, but can be used with many gases that exhibit a strong dipole moment.

Measurements of cross-sensitivity to contaminant gases using a highly selective, optical-fiber-remoted methane sensor based on correlation spectroscopy

An optical-fiber methane sensor that uses a correlation technique based on pressure modulation is reviewed. The sensitivity to methane was measured and the cross-sensitivity to a number of contaminant gases, such as ethane, has been determined. The performance of this system is compared with conventional gas-absorption measurement techniques in order to determine a figure of merit for the selectivity improvement.

Simple lead-insensitive heterodyne fiber optic vibrometer having a passive all-glass instrument head

A novel fiber-optic laser vibrometer has been developed in which a single highly birefringent fiber links the launch optics with a simple passive all-glass instrument head. The instrument utilizes heterodyne detection and can be made insensitive to undesired vibration effects acting on the film. The instrument may be greatly simplified and its performance enhanced if it is realized in all-fiber form.