Tunable diode laser gas analyzer

A method of determining laser transmittance through a sample containing an unknown quantity of a target gas, the laser transmittance being defined by absorbed laser transmissions through the sample and by corresponding unabsorbed laser transmissions, the method comprising:

• repetitively modulating the transmission wavenumber of a wavenumber-tunable laser about an absorption line of the target gas;
• disabling the laser by interrupting the laser drive current before each repetition of the transmission wavenumber modulating step;
• overdriving the laser after the disabling step and before each repetition of the transmission wavenumber modulating step;
• directing the laser transmissions through the sample;
• monitoring background intensity during the disabling step;
• measuring absorbed laser transmission intensities during the transmission wavenumber modulating step at discrete laser transmission wavenumbers corresponding to target gas absorption line wavenumbers;
• measuring lower and upper unabsorbed laser transmission intensities during the transmission wavenumber modulating step at lower and upper laser transmission wavenumbers which are below and above the target gas absorption line wavenumbers, respectively;
• determining the unabsorbed laser transmission intensities at wavenumbers between the lower and upper laser transmission wavenumbers from the lower and upper unabsorbed transmission intensities by interpolation.

A method of operating a wavenumber-tunable laser to determine concentration of a target gas within a sample, wherein the laser emits a laser beam at a variable wavenumber determined by the amplitude of the laser drive current, the method comprising the following steps:

• repetitively modulating the laser drive current amplitude to vary the laser wavenumber about an absorption line of the target gas;
• splitting the laser beam to direct a sample laser transmission through the sample and a reference laser transmission through a reference gas having a known concentration of the target gas;
• disabling the laser by interrupting the laser drive current before each repetition of the drive current modulating step;
• overdriving the laser with an increased drive current after the disabling step and before the drive current modulating step, the increased drive current being greater than the drive current at the beginning of the drive current modulating step;
• monitoring background laser intensity during the disabling step;
• and
• simultaneously measuring the transmission intensities of the sample and reference beam transmissions during the drive current modulating step at discrete laser wavenumbers corresponding to target gas absorption line wavenumbers.

A tunable laser trace gas analyzer for determining the concentration of a target gas in a sample, comprising:

• a wavenumber-tunable laser which emits a laser beam;
• a first sample cell for receiving the sample, the first sample cell forming an optical path through the sample along a first longitudinal axis;
• a reference cell for receiving a reference gas, the reference gas having a known concentration of the target gas, the reference cell forming an optical path through the reference gas along a second longitudinal axis, the reference cell being positioned relative to the first sample cell so that the first and second longitudinal axes intersect each other;
• beam splitter means positioned at the intersection of the first and second longitudinal axes and in the path of the laser beam emitted by the laser for directing a first portion of the laser beam through the sample in the first sample cell along the first longitudinal axis and for directing a second portion of the laser beam through the reference gas in the reference cell along the second longitudinal axis;
• a second sample cell positioned between the beam splitter means and the laser for receiving an additional volume of sample, the second sample cell being aligned along the first longitudinal axis to form an optical path also leading through the sample in the second sample cell;
• and
• first and second sensing means at remote ends of the first sample cell and the reference cell, respectively, for monitoring the intensities of the first and second portions of the laser beam after being transmitted through the first sample cell and the reference cell, respectively.

A tunable laser trace gas analyzer for determining the concentration of a target gas in a sample by comparison to a reference gas having a known concentration of the target gas, comprising:

• a wavenumber-tunable laser which emits a laser beam of varying wavenumber;
• a gas receiving assembly through which the laser beam is transmitted, the gas receiving assembly including first and second sample cells and a reference gas chamber;
• first and second radiation intensity sensors positioned at remote ends of the first sample cell and the reference gas chamber, respectively;
• a beam splitter within the gas receiving assembly adapted to direct a first laser beam portion to the first radiation intensity sensor and to direct a second laser beam portion to the second radiation intensity sensor;
• and
• the second sample cell being positioned between the beam splitter and the laser, the laser beam being directed through the second sample cell to the beam splitter.

A tunable laser trace gas analyzer for determining light absorption characteristics of a sample gas, comprising:

• a laser which emits a laser beam at a wavenumber which is variable in response to the amplitude of a drive current applied to the laser, the laser beam being transmitted through the sample gas;
• a first radiation sensor positioned relative to the laser to monitor the intensity of the laser beam after it is transmitted through the sample gas;
• laser control means for
• (a) supplying a drive current to the laser, the drive current having a variable amplitude;
• (b) repetitively modulating the drive current amplitude from a beginning ramp current value to an ending ramp current value to cyclicly vary the laser beam wavenumber over a wavenumber modulation band, the modulation band at least partially spanning an absorption line of a target gas in the sample gas;
• (c) disabling the laser by interrupting the laser drive current before each repetition of the drive current modulating;
• and
• (d) overdriving the laser with an increased drive current after the disabling step and before each repetition of the drive current modulating step to shorten the time required to restore the laser to its equilibrium excitation state, the increased drive current being greater than the beginning ramp current.