Optics: measuring and testing – For light transmission or absorption – Of fluent material
Patent
1991-03-05
1992-12-22
Rosenberger, Richard A.
Optics: measuring and testing
For light transmission or absorption
Of fluent material
356433, 356435, 250343, G01N 2100, G01J 142
Patent
active
051737493
DESCRIPTION:
BRIEF SUMMARY
This invention concerns a method and an apparatus to spectroscopically measure the concentration of a gas in a sample whereby a light of a laser diode is intensity and frequency modulated by a modulation signal to lock the light to an absorption line of a gas within a reference cell of predetermined pressure and concentration. The modulated light is distributed through the gas cell and the reference cell and the intensity of the light passed therethrough is detected. A measurement signal and a reference signal is generated, respectively, representative thereof. The measurement signal is automatically compensated for nongasrelated transmission variations in the measurement path. The harmonics of the measurement signal and reference signal are detected after the nongasrelated transmission variations are compensated for and a concentration signal indicative of the concentration in the gas sample is generated.
BACKGROUND OF THE INVENTION
Tunable diode lasers have quickly become important for high resolution instruments for analysis of molecule spectra. Most of the applications for diode laser spectroscopy have so far been in basic research. Lead salt diode lasers emitting in the wavelength range of 3-30 .mu.m have been used to study absorption line parameters and for measurement of molecule constants. The possibility of indirectly determine different parameters such as pressure, temperature, or electrical field strength has also been successfully demonstrated. There has been a relatively small interest for the overtone bands around 1 .mu.m since these do not significantly contribute to any new information on the analyzed molecules. The main limitation for practical use of lead salt diode lasers in measurement systems has been the need for cooling of to temperatures below that of liquid nitrogen. AlGaAs and InGaAsP diode lasers emit in the 0.68-1.7 um wavelength range and can operate at temperatures up to 50 degrees centigrade A characteristic of these lasers compared to lead salt lasers is their high intensity modulation index which requires special signal analysis to obtain optimum sensitivity. A large number of molecules of interest in process control and environmental measurement applications have absorption bands in the 0.7-1.7 um range, although, these are relatively weak overtone bands. The superior performance of AlGaAs and InGaAsP lasers compared with lead salt lasers however, more than outweighs this difference and they have great potential to be used in instrumental applications.
A very interesting application is for measurement of oxygen concentration, which has many practical applications, e.g. in the medical area. Today, oxygen is usually measured using different electrochemical methods. For example, semiconductor sensors or the transformation of O.sub.2 to secondary products for later detection is frequently used. These methods have several limitations. They don't perform real-time measurements and are usually not suitable in explosive environments and moreover, they often influence the measurand since a small amount of the measured (oxygen) gas is consumed by the measurement instrument. These measurement instruments are also often sensitive to other types of gases or organic pollutions which results in a considerably limited lifetime.
From EP-Al-0 015 170 it is known that by using spectrometers with reference and measurements cells and by using optical fibers or glass prism and gas-tight boxes interference with the surrounding environment is avoided This is true concerning the elimination of unwanted contributions to the measurement result from other paths than the measurement path. However, the problem of eliminating the influence of nongasrelated transmission variations in the measurement path or optical probe is not addressed at all.
In DE-Al-3.633.93 the laser is not locked onto the frequency of the absorption line. Instead, the laser current is modulated and the absorption spectrum is studied. The absorption line is then sampled as the current is modulated to determine the gas absorption
REFERENCES:
patent: 3524066 (1970-08-01), Blakkan
patent: 4934816 (1990-06-01), Silver et al.
patent: 4937461 (1990-06-01), Traina
"Tunable Diode Laser Spectroscopy: An Invited Review," by Eng et al., Optical Engineering, vol. 19, No. 6, Nov./Dec. 1980.
"Measurements of Pressure-Broadening Coefficients of NO and O.sub.3 Using a Computerized Tunable Diode Laser Spectrometer," by Lundqvist et al., Applied Optics, vol. 21, No. 17, Sep. 1982.
"IR-Laser Spectroscopy for Measurement Applications in the Industrial Environment," by H. Ahlberg and S. Lundqvist, Institute of Electrical Measurements, Research Laboratory of Electro-Optics and Lasers, Chalmers University of Technology, Goteborg, Sweden, Dec. 1985.
"Measurements of Electric Field Strength in Gas Insulated High-Voltage Components Using Infrared Diode Laser Absorption Spectroscopy," by Svante Hojer et al., Applied Optics, vol. 25, No. 17, Sep. 1986.
"Trace Gas Detection Using 1.3-.mu.m InGaAsP Diode Laser Transmitter Modules," by Daniel T. Cassidy, Applied Optics, vol. 27, No. 3, Feb. 1988.
Ahlberg Henrik
Andersson Torbjorn
Lundqvist Stefan
Tell Robert
Altoptronic AB
Pham Hoa Q.
Rosenberger Richard A.
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