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

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

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

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

All-electronic frequency stabilization of a DFB laser diode

A laser diode’s junction voltage is a sensitive measure of its temperature and can be used in a thermal control feedback loop. To compensate for the temperature dependence of the laser’s internal resistance, we have measured the dynamic resistance, ∂V/∂I, by modulating the injection current and measuring the demodulated voltage. The junction voltage was thus controlled while operating at fixed DC injection current. Over an external temperature range of 15°C to 35°C, this stabilised the centre frequency (wavelength) of a 1651 nm DFB laser diode with a residual mean frequency shift of 60 MHz (0.5pm), less than the uncertainty on the centre frequency of 80 MHz (0.7 pm). Under the same conditions, conventional thermistor control gave a systematic wavelength shift of −8.4 GHz (−76 pm), and control of the uncompensated forward voltage gave a shift of 9.9 GHz (90 pm)

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

Gas cells for tunable diode laser absorption spectroscopy employing optical diffusers. Part 2: Integrating spheres

We have studied the effects of random laser speckle and self-mixing interference on TDLS based gas measurements made using integrating spheres. Details of the theory and TDLS apparatus are given in Part 1 of this paper and applied here to integrating spheres. Experiments have been performed using two commercial integrating spheres with diameters of 50 mm and 100 mm for the detection of methane at 1651 nm. We have calculated the expected levels of laser speckle related uncertainty, considered to be the fundamental limiting noise, and imaged subjective laser speckle in a sphere using different sized apertures. For wavelength modulation spectroscopy, noise equivalent absorbances (NEAs) of around 5x10(-5) were demonstrated in both cases, corresponding to limits of detection of 1.2 ppm methane and 0.4 ppm methane respectively. Longer-term drift was found to be at an NEA of 4x10(-4). This lies within our broad range of expectations. For a direct spectral scan with no wavelength dither, a limit of detection of 75 ppm or fractional measured power uncertainty of 3x10(-3) corresponded well with our prediction for the objective speckle uncertainty