David G. Moodie

Tunable Diode-Laser Spectroscopy With Wavelength Modulation: A Calibration-Free Approach to the Recovery of Absolute Gas Absorption Line Shapes

The principles and implementation of an alternative approach to tunable diode-laser spectroscopy with wavelength modulation are described. This new technique uses the inherent phase shift between diode-laser power modulation and frequency modulation to separate the residual amplitude modulation and the first derivative signals recovered at the fundamental modulation frequency. The technique, through analysis of the residual-amplitude-modulation signal, is absolute, yielding gas-absorption-line-shape functions, concentrations, and pressures without the need for calibration under certain defined operating conditions. It offers the simplicity of signal analysis of direct detection while providing all the advantages of phase-sensitive electronic detection. Measurements of the 1650.96-nm rotation/vibration-absorption-line-shape function for 1% and 10% methane in nitrogen at various pressures are compared to theoretical predictions derived from HITRAN data, and the excellent agreement confirms the validity of the new technique. Further measurements of concentration and pressure confirm the efficacy of the technique for determining concentration in industrial-process environments where the pressure may be unknown and changing. An analysis of signal strength demonstrates that sensitivity comparable to that of conventional approaches is achievable. The new approach is simpler and more robust in coping with unknown pressure variations and drift in instrumentation parameters (such as laser characteristics) than the conventional approach. As such, it is better suited to stand-alone instrumentation for online deployment in industrial processes and is particularly useful in high-temperature applications, where the background infrared is strong.

FIBRE OPTIC TECHNIQUES FOR REMOTE SPECTROSCOPIC METHANE DETECTION FROM CONCEPT TO SYSTEM REALISATION

Abstract Spectroscopic measurement of methane gas concentrations using absorption lines in the near infra red (the 1.67 μm region) has been demonstrated by several research teams. The detection technique has the advantage of access through optical fibres which are very transparent in this spectral region and the disadvantage of relative insensitivity compared with the fundamental absorption lines in the 3–4 μm area. This paper will describe a particular realisation of a fibre optic, highly multiplexed (up to 128 points) realisation of a methane gas detection system designed for safety monitoring applications and detection up to the lower explosive limit (5% by volume). This implementation is currently undergoing site trials and has combined advanced technological development especially in the signal processing domain with applications engineering, in particular with emphasis on the compatibility between the system performance, both technical and economic and potential applications. This approach which has involved close collaboration between the university research group and the specialist industrial provider of gas measurement systems promises to realise a cost effective and practical measurement system with state of the art technical performance.

Design of a fibre optic multi-point sensor for gas detection

Abstract We report the design of a multi-point fibre optic methane sensor using a DFB laser source with a branched fibre network and micro-optic cells. Measurements are performed through derivative spectroscopy, with line scanning and digital signal processing, to give sensitivities down to a few ppm metre. The form of the derivative signal obtained from the system is modelled theoretically and compared with the experimental signal. The main limitation in the signal to noise ratio of the system is due to interference effects (etalon fringes) from the cells and we show how these effects may be minimised.

Wavelength tunability of components based on the evanescent coupling from a side-polished fiber to a high-index-overlay waveguide

A number of in-line components utilizing the evanescent coupling between a side-polished fiber and a high-index slab overlay have recently been demonstrated. The wavelength response of the structure shows a series of resonances, the position of which must be precisely controlled for practical applications. We present experimental and theoretical results for the tuning of the resonance position through variation of the superstrate parameters.

Fiber-optic refractometer that utilizes multimode waveguide overlay devices

The evanescent field coupling resonances between a single-mode optical fiber and a multimode planar waveguide overlay are sensitive in position to the refractive index of the superstrate material in contact with the top surface of the overlay. By using lithium niobate and zinc selenide in the role of the overlay and Cargille refractive-index oil as the superstrate, this principle has been investigated for use in refractometry. The ability to resolve index changes of <1 × 10−5 has been clearly demonstrated for an open-loop mode of operation by using intensity modulation, and a method of closed-loop operation is proposed by using active materials such as lithium niobate in the role of the overlay to provide independent feedback control of the resonance position.

Tunable Diode Laser Spectroscopy With Wavelength Modulation: A Phasor Decomposition Method for Calibration-Free Measurements of Gas Concentration and Pressure

The principles and implementation of a phasor decomposition method for analyzing signals in tunable diode laser spectroscopy with wavelength modulation are described. This new technique enables recovery of the isolated and normalized residual amplitude modulation (RAM) signal from measured first harmonic signals at any chosen fundamental modulation frequency. Like the previously reported RAM technique, this new approach is absolute, yielding gas absorption line shape functions, concentrations and pressures without the need for calibration, under certain defined operating conditions. It represents an advancement of the RAM technique in that it obviates the need to operate at a specific high frequency to achieve phase quadrature between the RAM and derivative signals: the signals may be recovered at their maximum levels at any frequency. Measurements of the 1650.96 nm and the 1666.2 nm rotation/vibration absorption line shape functions for 1% and 10% methane in nitrogen at various pressures are compared to theoretical predictions derived from HITRAN data. The excellent agreement confirms the validity of the new technique. Further measurements of concentration and pressure confirm the efficacy of the technique for determining concentration in industrial process environments where the pressure may be unknown and changing. With the above features this new method is particularly suited to stand alone instrumentation for on-line deployment in industrial processes where the calibration factors in the conventional approach would present significant difficulties.

Photonics laboratory experiments for modern technology-based courses

The modern photonics and optical communications industries have placed ever increasing demands on the supply of skilled graduates who are competent in the design, installation and operation of photonics systems. In response to this demand, we have developed a range of photonics laboratory teaching experiments to support accompanying lecture courses by underpinning fundamental principles with hands-on experimental experience. These systems enable students and trainees to investigate experimentally the basic principles, characteristics, and design of optical waveguides, optical communications systems, optical amplifiers and fault location techniques for optical networks, with additional scope for open-ended investigation of real technical issues such as mode spectrum analysis in optical waveguide and optical pulse dispersion/bit rate limits in fiber communications systems. The educational and overall system design philosophies, hardware,and experiments are reported in this paper.

Gas detection based on optical correlation spectroscopy using a single light source

An alternative method of gas detection using optical correlation spectroscopy (OCS) has been investigated. A semiconductor optical amplifier (SOA) is employed to transmit broadband light through a reference and measurement cell containing the absorbing species, and the required 180° phase shift between the reference and measurement signal is achieved using an optical fibre delay line. The use of a single SOA light source reduces zero point drift errors that arise when two light sources are utilized and provides high compatibility with single mode optical fibre systems. A theoretical minimum detectable concentration of 0.15 ppm.m has been calculated and experimental results have been obtained for varying concentrations of acetylene (C2H2) gas at different pressures. The theoretical system response was determined using absorption data from the HITRAN database and compares well with the experimental results. Signal-to-noise ratio (SNR) analysis has been performed for our experimental system and an ideal system with minimal losses, an optimum filter and longer cell lengths.

Remote gas analysis using fibre optic links and near infrared absorption

This paper describes the potential application of optical fibre addressed systems in remote gas spectroscopy. The paper will first describe the basic principles of the spectroscopic measurements and the reasons why such measurements find applications complementing more established mid infrared and electrochemically-based systems. We shall describe some practical field trials and the results obtained there from. Finally we shall discuss the potential offered by new approaches in laser design and system architectures to enhance the range of addressable species and the sensitivity which these remote detection systems may achieve.

Large scale multiplexing a point sensor for methane gas detection

This paper has reported the design and installation of a highly multiplexed (45 point) methane gas detection system using single mode fibre optics linked to remote miniature open path absorption cells interrogated by a single DFB laser diode configured for frequency modulation spectroscopy. To our knowledge this is the first such system to be site tested. The system operates over a wide area with total link lengths from source to detector extending up to about 6 km and has proved to be rugged and stable through the operating conditions on an active methane gas producing landfill site. Initial results on a small scale trial system give optimism for future reliability and the system behaviour will continue to be closely observed. It also seems very probable that the data obtained from continuous positioning will give new insights into site dynamics. Within its present configuration the system could be readily extended to address 64 points though we believe that, for this application, a 128 point system is totally feasible and a 256 point system may be achievable.