Application of the Methods of Laser Diode Spectroscopy for Practical Implementation of Contactless Measurement System of Concentration of Ethyl Alcohol Vapors in Human Breath
apability of fast tuning of semiconductor laser wavelength by 2–3 nm allows scanning the substance absorption lines. Small size of diode laser makes it possible to design quite compact analytical instrumentation on its basis.When radiation passes through absorbing gas the intensity of transmitted signal is determined on the basis of Bouguer-Lambert law:
where I0 is the initial intensity, α0(λ) is the substance absorption coefficient, С is the substance concentration, L is the length of measurement route.
Use of diode laser spectroscopy proves to be effective in case of detection of the substances which have narrow absorption lines, which are separately located in the spectrum. The classic example of TDLAS application is the task of methane detection in atmosphere. Absorption lines of this molecule located near the wavelength of 1.6 µm have high intensity, and the most important – the line width does not exceed 0.3 nm even at high concentration of methane. Therefore, it is possible to scan complete absorption line and even two lines retuning the radiation wavelength within the limit of 3 nm. At the wavelengths of about 1.6 µm the interfering impurities are practically absent in the atmosphere, and this fact allows applying the method of frequency modulation of laser radiation with the detection of the second harmonics, and this method also refers to traditional methods.
where υ0 is the laser frequency without modulation, hm is the modulation frequency, η is the modulation coefficient, ω = 2πf is the angular frequency of laser current modulation.
Assuming that the ratio is and , the expression of absorption law can be rewritten in the following form:
At the atmospheric pressure, the absorption line is equal to Lorentz line, and then applying the Fourier transformations to the dependence of current from detector it can be obtained that the coefficients for current capacity at the first and second harmonics of the frequency of current modulation f determine the gas concentration:
In Fig. 1 the layout of the method of laser radiation frequency modulation is shown graphically for methane detection .
The approach considered above refers to the traditional approaches for the detection of various gases in the atmosphere, and it has been successfully implemented in the devices for the substances with "narrow" absorption lines and upon the absence of absorption of "interfering" impurities, which can be contained in the atmosphere. It is much harder to detect substances when the width of substance absorption line is comparable with the width of laser spectral retuning and detection of weak absorption of target gas is performed against the background of the impurities, absorption of which is more intense by hundreds of times. Such situation occurs in the case of detection of complex organic substances, for example, vapors of ethyl alcohol under the conditions of strong radiation absorption by water vapors.
Some practical aspects of the development process of the system for contactless measurement of alcohol vapor concentration in human breath operating on the basis of measurement of the parameters of ethyl alcohol absorption spectrum within the near infrared band are considered below.
Selection of Spectral Range
for the Systems of Laser Diode Spectroscopy
When developing the devices operating on the basis of the principles of accurate measurement of absorption spectrums of various substances in the atmosphere using the methods of laser spectroscopy, the first question which must be answered by developer is the selection of the spectral range, within which the measurement will take place.
At first glance, in respect to organic substances the strongest absorption lines lie within the medium and far infrared region of optical spectrum, and within near infrared range the absorption on overtones and composite bands is weaker by many times in comparison with fundamental bands. By way of example, the transmission spectrums for methane and carbon dioxide molecules are given in Fig.2 .
If we rely upon the maximum absorption of substances, the detection of majority of organic compounds should be preferably performed within medium and far infrared ranges. On the other hand, the issue of the selection of spectral range is complex, and in any specific case it requires detailed consideration. Not only the absorption level of the substance of interest should be taken into account but also the presence of commercially available laser sources and radiation detectors, as minimum.
For example, let us consider the issue of selection of radiation detector. One of the criteria characterizing the optical radiation detector is the capacity reduced to the NEP (Noise Equivalent Power), and for the detector operating within near infrared region this value is lower by orders than for the detectors within medium infrared region and in case of correctly executed electronic route of designed device it will be expressed in the proportional increase of the ratio of received signal to noises. Noise characteristics of the detectors operating within far infrared range also can be reduced at the expense of introduction of the additional cooling systems to practically cryogenic temperatures. Undoubtedly, it generally deteriorates the operational characteristics and reliability and increases the system cost.
Selection of laser radiation source for the devices with "near action", or in other words for the devices which do not require great energy of laser radiation, is very rich now, and various laser semiconductor diodes, which overlap the range from hard ultraviolet to far infrared radiation, hold special place. In this area, the best results on the level of output power, suppression of phase noises and side modes of radiation are achieved within near infrared region in comparison with medium and far infrared range. The capability of operation of the lasers with fiber radiation yield within near infrared range up to 2 µm is also very important. Application of optical fiber allows getting rid of the excess optical elements, enhancing the device reliability and partially solving the problems connected with the interference of optical radiation. It can be noted that in actual practice the major limiting factor which restricts the sensitivity includes the interference effects in optical path, in particular.
In summary it can be concluded that the selection of device spectral range requires the accurate consideration in each specific case. Operation within the regions of strong absorption of substances will not always allow implementing the device, which meets the user requirements to the maximum.
By way of example, let us consider the contactless system of express test of alcohol intoxication.
Main technical specifications of the system:
Period of analysis of alcohol intoxication – not more than 1 sec.;
Period of readiness for the next test – not more than 1 sec.;
Analysis method – contactless;
Minimum set operation threshold: 0.2 per mille (90 µg/m 3);
Measurement range: 0.2–10 per mille;
Measurement error: 0.1 per mille.
The system appearance is shown in Fig. 3.
Peculiarities of Design
of the Measurement System
of Alcohol Concentration
in Human Breath
The system is intended for the introduction of total personnel control of alcohol intoxication at the entrance control and check points of enterprises, and therefore very stringent requirements are specified for the actuation time, resource and operation reliability. The system must be contactless and ensure the measurement of alcohol concentration in breath for 1 sec. and be ready for the following measurement in 1 sec., which will allow the continuous control without the formation of queues. The system must provide up to 20000 tests per day upon the continuous twenty-four hour operation during several years without replacement of sensor itself. In order to meet the specified technical requirements, the methods of diode laser spectroscopy refer to the most preferable and probably the only possible methods.
The system operates on the basis of the measurement of absorption of ethyl alcohol vapors in near infrared spectral region, in particular, near the wavelength of 1.4 µm. The tunable semiconductor laser diode with the distributed feedback scanning the set spectrum interval is applied.
The functional layout of the device is shown in Fig.4.
The system is executed in accordance with the traditional layout including the tunable semiconductor laser source scanning the set spectral region with the width of several nanometers and frequency of 2 kHz. The optical path is constructed according to the principle of open optical cell on the basis of multiple reflections from flat mirrors. The initial parameters of laser radiation are controlled by reference photodetector. The main complication of technical implementation is stipulated by the necessity of extraction of very weak differential absorption by alcohol vapors against the background of strong absorption of interfering impurities, in particular, by water vapors. Another peculiarity consists in the fact that during the exhalation the absolute humidity in test volume can vary by tens of times because the exhaled air is moistened almost up to 100% at 36 °C. In this article we are not going to consider the peculiarities of algorithm of useful signal extraction in details; we are going to consider more practical aspects connected with the substantiation of the selection of spectral range and structure of the system test volume in more details.
Initially, the absorption spectrum of alcohol vapors within various spectral ranges was considered. The absorption spectrum of ethyl alcohol is specified in Fig. 5.
As it is seen, there are three spectral ranges for the measurement of concentration of this complex molecule; the main absorption lines lie near 3.4 µm (2900 cm-1), overtone line – at 2.7 µm (3700 cm-1) and 1.4 µm (7190 cm-1) .
The range near 3.4 µm is not completely provided by detectors as well as radiation sources. The level of mean output power of laser diodes is lower by the order in comparison with the diodes operating at the wavelength of 1.4 µm and does not exceed 1 mW. Comparison of the level of side modes selection for the lasers of 3.4 and 1.4 µm is given in Fig. 6 . The level of 10 Dbm is worse for long-wavelength laser. Also, operation with optical fiber is impossible within this range. The cost of complete unit plays significant role. At the present time, the cost of laser diode for the wavelength of 3.4 µm exceeds the cost of diode for the wavelength of 1.4 µm by more than five times, and it is not acceptable for the commercial-type product.
Use of the wavelength of 2.7 µm is also associated with certain complications. The atmospheric transmission at 5 meters of optical path at the temperature of 20 °C and relative humidity of 60% is shown in Fig. 7. In this spectral region, there is very strong absorption by water vapors, which inevitably exist in the atmosphere, and if we will take into account the fact that the device is intended for the measurement of parameters of human breath having the humidity close to 100% at 36 °C, then the detection of low concentrations of alcohol vapors against the background of such strong absorption of interfering impurities will be complicated . The use of optical fiber for this range is also impossible.
As a result, the spectral region near 1.4 µm is the most acceptable for the solution of the task of contactless measurement of concentration of ethyl alcohol vapors; however, the relative absorption for this range is weaker by 50 times in comparison with 3.4 µm and by 10 times in comparison with 2.7 µm. The spectrum of atmospheric transmission within this range is specified in Fig. 8 . As it is seen, there are also regions of strong absorption by water vapors but there are the spectral windows, in which it is possible to measure the weak absorption by alcohol vapors.
During the design process, the particular attention was paid to the structure of the system optical path. Since the system must ensure measurements under continuous conditions upon the control of large flows of checked persons the construction of open cell must simultaneously provide high concentration of exhaled air, from one hand, and its fast removal from the cell, on the other hand. Various structures of optical cell were considered; numerical simulation of gas dynamic processes under different operation conditions was performed. The intensity and breath direction, structure of the deflector dispersing the air flow were varied. The layout of optical path and some simulation results are shown in Fig. 9.
During the process of experimental development of the system, the verification of numerical model was performed using the experimental method. The process of change of mean humidity in the test volume depending on the time, which was obtained on the system prototype, is shown in Fig. 10. Variation of mean humidity is good indicator of the process of filling of test volume during the exhalation. Taking the initial relative humidity of breath equal to 100% at the temperature of 36 °C in the model and in the experiment, selected structure of the measuring cell allows reaching the peak value of mean breath concentration up to 70% of the total volume of measuring cell. Herewith, emptying of the test volume by 95% of the previous exhale occurs in 1.3 sec., which makes it possible to perform the further measurement.
Further practical operation of commercial devices at the personnel check points of enterprises proved the functionality of selected structure of measuring cell. For various operation conditions, during summer and winter periods at high and low humidity the system provided continuous identification and breath control, even at the measurement rate once every 2 seconds.
The technical solutions represented in the article made it possible to develop, produce and successfully operate the system of detection of ethyl alcohol vapors in human breath using the methods of diode laser spectroscopy against the background of "interfering" atmospheric contaminants, absorption of which exceeds the absorption of target substance by two orders within the selected spectral range. Substantiated selection of the spectral range near 1.4 µm, which takes into account not only the level of radiation absorption by alcohol vapors but also the capability to work with commercially available and comparably inexpensive radiation sources and detectors, apply optical fiber, was performed.
Analysis of numerical simulation of the process of human breathing in the system test volume made it possible to develop the structure of the measuring cell, which allows reaching high peak values of mean concentration of exhaled air in measuring cell and emptying the volume for the next exhalation quickly. It allows applying the system for operation under the conditions of entrance control and check points of enterprises in case of considerable flows of people.
The article reviews some practical peculiarities of development of the systems for detection of low gas concentrations in the atmosphere using the methods of laser diode spectroscopy. Technical solutions for design of the systems of detection of complex organic substances are given in case of the presence of "interfering" contaminants by the example of the measurement system of ethyl alcohol vapor concentrations in human breath using the methods of laser diode spectroscopy.