It is known that certain gases absorb electromagnetic radiation in selective manner as a function of the wavelength of said radiation when the substances concerned have a spectrum structure comprising lines that are relatively fine and separate, in contrast to a structure comprising bands.
This property is used to determine the absorption due to such substances, and thus to deduce the concentration thereof.
There is frequently a need to determine the concentration of a gas in a mixture with other gases without performing analyses that would require physical contact with the fluid. In any event, such analysis is not always easy when, for example, it is desired to detect a gas leak quickly.
Various physical methods are known for determining the concentration of a gas that possesses selective absorption for electromagnetic radiation over a range of wavelengths of the radiation.
Such physical methods enable measurement to be performed without making contact with the gas, unlike chemical measurements, and in addition they present good selectivity.
For example, a reliable method is known that is based on a Fourier transform interferometer and that consists in determining all of the absorption characteristics of the gas over a wide range of optical wavelengths. Nevertheless, that method is very complex.
It is also known chat the emission wavelength of a laser diode can be modulated (by modulating the injection current and/or the temperature of the laser) over a range of wavelengths that includes an absorption line of a gas whose concentration is to be determined, and to cause the modulated optical radiation emitted by the laser diode to pass through a cell that is filled with gas. An optical detector then serves to pick up optical radiation of varying amplitude that is characteristic of the gas and of its concentration.
Unfortunately, in general, the detected radiation is constituted by radiation due to modulation of the laser plus peaks superposed thereon due to differential absorption of the gas (with both kinds of radiation being attenuated by constant losses that are a function of wavelength and that are due to the various forms of opaqueness encountered by the modulated optical radiation coming from the laser diode), and it is therefore not possible to isolate the contribution in said radiation that is due solely to the gas.
A known method is mentioned in the article "Absorption measurements of water-vapor concentration, temperature, and line-shape parameters using a tunable InGaAsP diode laser", Applied Optics, Vol. 32, No. 30, pp. 6104-6116, Oct. 20, 1993, and which uses optical means to determine the concentration of water vapor while avoiding some of the problems mentioned above.
In that method, a laser diode is used and its emission wavelength is modulated by modulating the injection current and/or the temperature of the laser, with a range of wavelengths being chosen that covers the range of wavelengths over which water vapor presents differential absorption.
That laser diode emits modulated optical radiation which is subsequently split into two modulated optical radiations, one acting as reference radiation while the other passes through a cell filled with a gaseous mixture containing the water vapor whose concentration is to be determined, and thus acts as a measurement radiation.
At the output from the gas cell, an optical radiation detector picks up the measurement optical radiation of varying amplitude that is characteristic of water vapor and of its concentration, and transforms it into a measurement electrical signal. The reference radiation is also picked up by another optical radiation detector and it is transformed into a reference electrical signal.
The method described then consists in subtracting the measurement signal from the reference signal and in dividing the resulting electrical signal by the reference signal to deduce therefrom what optical radiation has been absorbed by the water vapor, and thus deduce the concentration thereof.
Nevertheless, that article specifies that in order to obtain a reliable absorption measurement, it is necessary for the amplitude of the two electrical signals from the two optical radiation detectors to be accurately balanced.
The slightest difference in amplification gain between them gives rise to distortion of the absorption measurement.
In addition, in the time interval between balancing the signals and obtaining the absorption measurement, any change in optical losses is confused with variation of said absorption to the detriment of measurement accuracy.
It is also specified in that article that various techniques have been tried for accurately balancing the electrical signals from the optical radiation detectors, but that none has given entire satisfaction. That problem is general to prior art absorption measurement methods and it can be asserted that methods known in the state of the art do not make it possible to determine absorption simply and reliably, and therefore do not make it possible to determine the concentration of a gas that presents selective absorption in a given range of wavelengths of electromagnetic radiation, since such methods cannot avoid being sensitive to inevitable parasitic losses.