Method and apparatus for the spectroscopic detection of low concentrations of hydrogen sulfide gas

A spectroscopic apparatus comprising:

• a cavity defining a volume, the cavity having at least one gas inlet and one gas outlet for introducing gaseous samples into and extracting gaseous samples from said volume, and one optical inlet and one optical outlet for introducing light into said cavity and extracting an analyte absorption signal from said cavity;
• two mirrors having characteristic reflectivity disposed on opposite ends of said cavity and configured to form an optical path there between having a characteristic physical length;
• a light source disposed in optical alignment with the one optical inlet and configured to introduce the light into said cavity along said optical path, the light source wavelength tuning range selected to correspond with at least one absorption wavelength of a selected gaseous analyte;
• a driver electronics interfaced with the light source and configured to tune and modulate the wavelength of the light;
• wherein the mirrors and the light source are disposed such that the light introduced into the optical cavity along the optical path is reflected between the two mirrors such that a majority of the light undergoes multiple passes through the cavity prior to exiting the optical outlet, such that a cumulative optical path length traveled by the light while within the cavity is significantly greater than the physical length of the cavity;
• a detector disposed in optical alignment with the optical outlet and configured to detect the gaseous analyte's absorption signal at a selected absorption wavelength;
• wherein the absorption wavelength, the mirror reflectivity and the physical length of the cavity are selected to maximize the cumulative optical path length;
• and wherein reflectivity of the two mirrors and the cumulative optical path length are determined experimentally using a pulsed light source and monitoring a transient signal produced thereby, using the relationship:
• wherein (t) is an intensity at a time t, T=d/{c 1 ln(R)I} is a 1/e (first order) decay constant of a transient signal, d is the physical length of the optical path, c is the speed of light and R is the reflectivity of the mirrors” has been changed—A spectroscopic apparatus comprising:
• an optical cavity defining a volume, the cavity having at least one gas inlet and one gas outlet for introducing gaseous samples into and extracting gaseous samples from said volume, and one optical inlet and one optical outlet for introducing light into said optical cavity and extracting an analyte absorption signal from said optical cavity;
• two mirrors having characteristic reflectivity disposed on opposite ends of said optical cavity and configured to form an optical path there between having a characteristic physical length;
• a light source disposed in optical alignment with the one optical inlet and configured to introduce the light into said optical cavity along said optical path, the light source wavelength tuning range selected to correspond with at least one absorption wavelength of a selected gaseous analyte;
• a driver electronics interfaced with the light source and configured to tune and modulate the wavelength of the light;
• wherein the mirrors and the light source are disposed such that the light introduced into the optical cavity along the optical path is reflected between the two mirrors such that a majority of the light undergoes multiple passes through the cavity prior to exiting the optical outlet, such that a cumulative optical path length travelled by the light while within the cavity is significantly greater than the physical length of the optical cavity;
• a detector disposed in optical alignment with the optical outlet and configured to detect the gaseous analyte's absorption signal at a selected absorption wavelength;
• wherein the absorption wavelength, the mirror reflectivity and the physical length of the cavity are selected to maximize the cumulative optical path length;
• wherein reflectivity of the two mirrors and the cumulative optical path length are determined experimentally using a pulsed light source and monitoring a transient signal produced thereby, using the relationship:
• I(t)=I(t=0)exp(−t/τ)
• wherein I(t) is an intensity at a time t, τ=d/{c|ln(R)|} is a 1/e (first order) decay constant of a transient signal, d is the physical length of the optical path, c is the speed of light and R is the reflectivity of the mirrors;
• and
• wherein the selected gaseous analyte is H2S, and wherein the absorption wavelength is selected from one of 8000-9000 nm, 1900-2000 nm, and 1563-1626 nm regions.