Using integrating spheres as absorption cells: path-length distribution and application of Beer's law

We have modeled the path-length distribution in an integrating sphere used as a multipass optical cell for absorption measurements. The measured radiant flux as a function of analyte concentration is nonlinear as a result, deviating from that expected for a single path length. We have developed a full numerical model and introduce a new analytical relationship that describes this behavior for high reflectivity spheres. We have tested both models by measuring the optical absorption of methane at 1651nm in a 50mm diameter sphere, with good agreement with experimental data in the absorption range 0-0.01cm -1 . Our results compare well with previous work on the temporal response of integrating spheres

Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 1: single and dual pass cells

New designs for gas cells are presented that incorporate transmissive or reflective optical diffusers. These components offer simple alignment and can disrupt the formation of optical etalons. We analyse the performance-limiting effects in these cells of random laser speckle (both objective and subjective speckle), interferometric speckle and self-mixing interference, and show how designs can be optimised. A simple, single pass transmissive gas cell has been studied using wavelength modulation spectroscopy to measure methane at 1651 nm. We have demonstrated a short-term noise equivalent absorbance (NEA, 1 sigma) of 2x10(-5), but longer term drift of up to 3x10(-4) over 22 hours

Self-mixing interference effects in tunable diode laser absorption spectroscopy

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detecto

Use of diffuse reflections in tunable diode laser spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is an optical gas sensing technique in which the emission frequency of a laser diode is tuned over a gas absorption line of interest. A fraction of the radiation is absorbed by the sample gas and this can be determined from measurements of initial intensity and the intensity transmitted through the sample. The amount of light absorbed is related to the gas concentration. Additional modulation techniques combined with phase sensitive detection allow detection of very low gas concentrations (several parts per million). The advantages of using TDLAS for trace gas sensing include; fast response times, high sensitivity and high target gas selectivity. However, the sensitivity of many practical TDLAS systems is limited by the formation of unintentional Fabry-Perot interference fringes in the optical path between the source and detector. The spacing between the maxima of these fringes, in particular those generated in gas cells, can be in the same wavelength range as Doppler and pressure-broadened molecular line widths. This can lead to (1) interference fringe signals being mistaken for gas absorption lines leading to false concentration measurements or (2) distortion or complete obscuring of the shape and strength of the absorption line, such that the sensitivity of the instrument is ultimately limited by the fringes. The interference fringe signals are sensitive to thermal and mechanical instabilities and therefore can not be removed by simple subtraction techniques. Methods that have been proposed by previous workers to reduce the effects of interference fringes include careful alignment of optical components and/or mechanically jittering the offending components. In general the alignment of the optical components is critical. This often leads to complex and fragile designs with tight tolerances on optical component alignment, and can therefore be difficult and expensive to maintain in field instruments. This thesis presents an alternative approach based on the deliberate use of diffusely scattering surfaces in gas cells as a means of eliminating spurious signals due to Fabry-Perot etalons. However, their use introduced laser speckle that contributed an intensity uncertainty to gas detection measurements. A methodology for investigating the laser speckle related intensity uncertainty has been developed and confirmed. The intensity uncertainty has been quantified for the different gas cell geometries employing diffusely scattering surfaces including integrating spheres. Methods for reducing the speckle related intensity uncertainty were also investigated and are presented. It has been shown that under the right circumstances robust gas cell designs that do not suffer from Fabry-Perot etalon effects and are relatively easy to align can be realised. The performance was found to be comparable to a conventional cell design (e.g. 3ppm detection limit for a 10cm standard cell and 11ppm for a 10cm diffusive cell). The technique could potentially simplify instrument design, thereby aiding the transfer of technology to industry.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Use of diffuse reflections in tunable diode laser spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is an optical gas sensing technique in which the emission frequency of a laser diode is tuned over a gas absorption line of interest. A fraction of the radiation is absorbed by the sample gas and this can be determined from measurements of initial intensity and the intensity transmitted through the sample. The amount of light absorbed is related to the gas concentration. Additional modulation techniques combined with phase sensitive detection allow detection of very low gas concentrations (several parts per million). The advantages of using TDLAS for trace gas sensing include; fast response times, high sensitivity and high target gas selectivity. However, the sensitivity of many practical TDLAS systems is limited by the formation of unintentional Fabry-Perot interference fringes in the optical path between the source and detector. The spacing between the maxima of these fringes, in particular those generated in gas cells, can be in the same wavelength range as Doppler and pressure-broadened molecular line widths. This can lead to (1) interference fringe signals being mistaken for gas absorption lines leading to false concentration measurements or (2) distortion or complete obscuring of the shape and strength of the absorption line, such that the sensitivity of the instrument is ultimately limited by the fringes. The interference fringe signals are sensitive to thermal and mechanical instabilities and therefore can not be removed by simple subtraction techniques. Methods that have been proposed by previous workers to reduce the effects of interference fringes include careful alignment of optical components and/or mechanically jittering the offending components. In general the alignment of the optical components is critical. This often leads to complex and fragile designs with tight tolerances on optical component alignment, and can therefore be difficult and expensive to maintain in field instruments. This thesis presents an alternative approach based on the deliberate use of diffusely scattering surfaces in gas cells as a means of eliminating spurious signals due to Fabry-Perot etalons. However, their use introduced laser speckle that contributed an intensity uncertainty to gas detection measurements. A methodology for investigating the laser speckle related intensity uncertainty has been developed and confirmed. The intensity uncertainty has been quantified for the different gas cell geometries employing diffusely scattering surfaces including integrating spheres. Methods for reducing the speckle related intensity uncertainty were also investigated and are presented. It has been shown that under the right circumstances robust gas cell designs that do not suffer from Fabry-Perot etalon effects and are relatively easy to align can be realised. The performance was found to be comparable to a conventional cell design (e.g. 3ppm detection limit for a 10cm standard cell and 11ppm for a 10cm diffusive cell). The technique could potentially simplify instrument design, thereby aiding the transfer of technology to industry

Use of diffuse reflections in tunable diode laser absorption spectroscopy: implications of laser speckle for gas absorption measurements

Abstract We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detecto

A mechanically stable laser diode speckle interferometer for surface contouring and displacement measurement

Electronic speckle pattern interferometry (ESPI) is demonstrated using a simple configuration consisting of a wedged window and a beamsplitter. The window serves to produce a reference beam which is in-line with the scattered object beam. The system is almost common-path and therefore provides much better mechanical stability than conventional ESPI configurations, which have widely separated beam paths. The configuration has collinear observation and illumination directions and therefore has maximum sensitivity to out-of-plane displacement. Wavelength modulation through adjustment of the laser diode control current provides a convenient method of phase shifting without the need for external moving parts. Further, variation of the laser diode control temperature allows extended wavelength tuning to adjacent longitudinal modes, facilitating surface contouring measurements via the two-wavelength technique. The interferometer is demonstrated for surface displacement measurement with a 3.3 μm centre displacement measured over a 15 mm × 15 mm region of a flat plate. Contour measurements of a shaped object are made using an equivalent wavelength of 1.38 mm