Abstract:
The present invention provides a system and an apparatus for remotely detecting a gas molecule. The apparatus includes a diode laser for emitting radiation at a maximum absorption band of the gas molecule to be detected and a single mode fiber connected to the diode laser for narrowing spatial inhomogeneous of the radiation. The intensity of the laser diode depends on amount of the feeding current. Laser diode&#39;s temperature is stabilized by a thernistor and peltier element. The current feeding into the pump current adjusts and stabilizes the temperature of the diode laser. After going through an optical scheme, the radiation may be absorbed by the present gas molecule. The photodetector will detect whether absorption has occurred or not. This type of detection is utilized in detecting alcohol molecule in an enclosure such as a vehicle.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to a gas molecule detector. More specifically the invention relates to a laser device for detecting the presence of alcohol molecules from a distance. The device is controlled by a computer system.  
           [0003]    2. Description of the Related Art  
           [0004]    The colors of an object typically arise because materials selectively absorb light of certain frequency, while scattering or transmitting light of other frequencies. For example an object is red (wavelength range from 6300 and 6800 Å) if it absorbs all visible frequencies except those our eyes perceive to be “red.” Thus, we see the scattered wavelength range from 6300 and 6800 Å from that object.  
           [0005]    Similarly, gas molecules absorb at different frequencies. A predefined range of wavelength propagating through gas molecules are absorbed at the resonance frequencies of the atoms or molecules, so that one observes gaps in the wavelength distribution of the emerging wavelengths. Absorption lines of a gas molecule have its own intensity and spectral position. For example, absorption spectrum of simple molecule gas consists of narrow, isolated spectral lines. Alcohol (ethanol) like other complex organic molecule has spectrum that consists of many overlapping lines. Alcohol molecule has rather broad spectra about 100 times broader than isolated spectral line of a simple molecule. Selecting an alcohol spectrum for detecting the presence of its molecule can be complicate. The strongest and sharpest feature of ethanol absorption spectrum in near infrared range (1.387-1.414 μm) is Q-branch with maximum from 1.3924-1.3935 μm. On the other hand, the high intensity of absorption lines exists in mid infrared range (3-10 μm). In the present invention, the spectrum near 1.392 μm is preferred over mid infrared range because of the following principal:  
           [0006]    1. This spectral range is not hazardous for eyes if power of light sources is not more than 1 mW.  
           [0007]    2. Glass windows are transparent in this range.  
           [0008]    Traditionally, gas molecule detectors utilize infrared spectroscopy to detect the specified gas including alcohol. This type of detection requires complicated optical filters. The accuracy of detection depends on the sensitivity of these filters. Filter may detect more than one type of gas molecules where interference is highly probable.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides for a more sensitive detection of gas molecules without the use of optical filters by using laser technology, thereby eliminating interference from other gas molecules.  
           [0010]    An embodiment of the present invention utilized diode laser (“DL”) for alcohol detection because the radiation bandwidth is 10 −3  cm −1 . In laser diode, radiation is produced by the recombination of electrons and holes at a pn junction (semiconductor). A laser diode is small in size like other semiconductor devices. Its output can be modulated by varying the current.  
           [0011]    Diode lasers usually do not employ mirrors for feedback. This is because the refractive index is large enough to give considerable reflection at the semiconductor/air interface. Diode laser allows for fast scanning of the radiation frequency, so measurements are produced simultaneously in a predetermined wavelength range. It allows accounting of specific features of alcohol absorption band, that is important for selective measurements. Special improvements were made in the alcohol detector for subtracting of humidity variations, because water vapor absorption in used wavelength range is rather high. Special improvements were made in present invention for subtracting of various disturbances due to accidental sun illumination, vehicle window curvature and dirt on their surfaces, optomechanic vibrations. With the employment of diode laser, the improved alcohol detector is low cost and small and compact.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a block diagram of an alcohol detector of according to an embodiment of the present invention.  
         [0013]    [0013]FIG. 2 is a block diagram of an alcohol detector system according to an embodiment of the present invention.  
         [0014]    [0014]FIG. 3 is a block diagram of a computer system in accordance with an embodiment of the present invention.  
         [0015]    [0015]FIG. 4 is a pictorial representation of an alcohol detector controller in accordance with an embodiment of the present invention.  
         [0016]    [0016]FIG. 5 is a pictorial representation of interface module in accordance with an embodiment of the invention.  
         [0017]    [0017]FIG. 5( a ) is a DL current supply in accordance with an embodiment of the present invention.  
         [0018]    [0018]FIG. 5( b ) is a resistance-voltage transformer in accordance with an embodiment of the present invention.  
         [0019]    [0019]FIG. 5( c ) is a peltier supply in accordance with an embodiment of the present invention.  
         [0020]    [0020]FIG. 6 is a pictorial representation of a photodetector transformer/amplifier unit in accordance with an embodiment of the invention.  
         [0021]    [0021]FIG. 7 is a block diagram of software in accordance with an embodiment of the invention.  
         [0022]    [0022]FIG. 8 is a flowchart of signal processing in accordance with an embodiment of the invention.  
         [0023]    [0023]FIG. 8( a ) is a graph of DL current corresponding to point number in accordance with an embodiment of the present invention.  
         [0024]    [0024]FIG. 8( b ) is a graph of [DL current corresponding to point number] in accordance with an embodiment of the present invention.  
         [0025]    [0025]FIG. 9 is a flowchart for calculation of alcohol concentration in accordance with an embodiment of the present invention.  
         [0026]    [0026]FIG. 9( a ) is a graph of water and alcohol absorption factors corresponding to the wavelength in accordance with an embodiment of the present invention.  
         [0027]    [0027]FIG. 10 is a flowchart for DL temperature stabilization in accordance with an embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0028]    The description of the preferred embodiment of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.  
         [0029]    With reference now to the figures and in particular with reference to FIG. 1, a pictorial representation of alcohol detector  100  in accordance with an embodiment of the present invention is illustrated. Alcohol detector  100  involves diode laser (“DL”)  101  assembled with peltier element  120  and thermistor  121 , a temperature sensitive resistor. Diode laser  101  radiating power is proportional to transformed DL current  116 . Diode laser  101  wavelength depends on the temperature of the diode laser  101  from 1.3906 urn at 0° C. to 1.3933 urn at 40° C. um. Peltier element  120  and thermistor  121  assist in adjusting and stabilizing the temperature of diode laser  101  such that the emitted wavelength stays near alcohol absorption band at 1.392 um. Initially, thermistor  121  sets the temperature for diode laser  101  with the raw resistance voltage  112 . Peltier element  120  controls the temperature either by removing the heat by pumping heat away from the chamber adjacent to a device or adding heat to that chamber. In this case, the greater, the transformed pump current  113 , the more heat is removed from the chamber adjacent to diode laser  101 , thereby cooling it. In a preferred embodiment of the present invention, such assembly of diode laser, peltier element, and thermistor is commercially available from Sensors Unlimited, Inc. with part number, SU1393-DFB-TE. Moreover, peltier element  120  and thermistor  121  can be substituted by other temperature stabilizing components. Diode laser  101 , peltier element  120  and thermistor  121  are housed inside thermostatic enclosure  102 . Thermostatic enclosure  102  helps to keep the temperature of the assembly constant without the effect of the changing temperature of the outside environment. Outside of thermostatic enclosure  102 , the alcohol detector further includes optical components for analytical optical scheme and for reference optical scheme.  
         [0030]    In the analytical optical scheme, diode laser  102  radiation is channeled into a single mode fiber  103  of about two meters long. Single mode fiber  103  narrows or diminishes the concentration of radiation in which the inhomogenity of DL radiation is 0.3%. At various pumping current values, radiation is generated by different regions of the diode laser active area with different directional patterns and radiating power. As a result, the cumulative dependence of radiating power on pumping current varies for different angles of DL radiation pattern. In the used DL radiation pulse mode when radiation frequency is scanned by current within a pulse, the pulse shape of a photoreceiver signal varies depending on a part of DL radiation pattern falling on a photoreceiver platform. It turned out that the homogeneous laser radiation pattern can be most effectively obtained using a single mode optical fiber with 7 um diameter of central part. DL radiation transmitted through the fiber˜ 2   m  long results in the highly homogeneous radiation pattern at the fibre exit. The output of the single mode fiber  103  is diverged at a 10° angle obeying the Gaussian law.  
         [0031]    Then the radiation is passed through objective  104  to be adjusted by refraction in order to fully illuminates the cube reflector  105 . Between objective  104  and reflector  105 , the radiation may have pass through an enclosure, for example, a moving vehicle on the road with alcohol molecules within the enclosure. The absorption of the alcohol molecules occurs for the first time. In a preferred embodiment of the present invention, alcohol molecules may be detected here. The reflected radiation fully illuminates spherical mirror  106  having a 6.5 cm diameter, which is positioned behind objective  104 . The optical path between reflector  105  and spherical mirror  106  undergoes a second absorption of the alcohol molecules inside the enclosure. Because radiation passes through the enclosure twice, the absorption of the alcohol molecules amplifies. Spherical mirror  106  focuses the absorbed radiation on the sensing area of analytical photodetector  107 . Then, photodetector  107  generates raw analytical PD1 signal  114 .  
         [0032]    In the reference optical scheme, splitter  108  generates a reference radiation. Splitter  108  is positioned after objective  104 . The reference radiation passes through reference cell  109  having a predetermined concentration of water molecules. Next, the reference radiation is reflected by spherical mirror  110  to pass through the reference cell  109  again before reaching the sensing area of reference photodetector  111 . Reference photodetector  111  generates raw reference PD2 signal  115 . In a preferred embodiment, radiation is ready for detection when the reference radiation passes once through reference cell  109 . Photodetectors  107  and  111  of the Alcohol detector are Germanium photodiodes with sensing area near 2 mm 2  and Noise Equivalent Power (NEP) 10 −11  W/Hz 1/2 .  
         [0033]    Those of ordinary skill in the art will appreciate that the detector is capable of detecting alcohol or other gas molecules. This detailed description specifically describes alcohol molecules, which may be substituted by other detectable gas molecules having distinct absorption band. The diode laser may emit radiation according to the gas molecule absorption band. The reference cell&#39;s content may differ. The depicted example is not meant to imply molecule limitation with respect to the present invention.  
         [0034]    Referring now to FIG. 2, a block diagram of an alcohol detector system is shown in accordance with a preferred embodiment of the present invention. Alcohol detector system  200  includes computer system  201 , alcohol detector  202 , interface module  203 , photodetector transformer amplifier unit  204 , and software  205 . Software  205  initializes and synchronizes alcohol detector system  200 . It also provides for alcohol detector system signal processing and storing and analyzing data. Computer system  201  provides processing and control to alcohol detector system  200 . There are five signals communicating between computer system  201  and alcohol detector  202 . These signals must pass through either interface module  203  or photodetector transformer amplifier unit  204 . Computer system  201  receives three data inputs; amplified PD1 signal  210  and amplified PD2 signal  211  from photodetector transformer amplifier unit  204  and transformed resistance/voltage signal  212  from interface module  203 . These inputs are differential lines. Computer system  201  transmits two outputs, raw diode laser current  213  and raw pump current  214  to interface module  203 .  
         [0035]    On the other end of interface module  203  and photodetector transformer amplifier unit  204 , alcohol detector  202  transmits three data signals, raw PD1 signal  220  and raw PD2 signal  221  to photodetector transformer amplifier unit  204  and raw resistance voltage  222  from interface module  203 . Alcohol detector  202  receives two inputs, transformed tunable current  224  and transformed Pump current to interface module  223 .  
         [0036]    Referring now to FIG. 3, a block diagram of a computer system  201  is shown in accordance with a preferred embodiment of the present invention. Computer system  201  may employ a single microprocessor  301 , or in the alternative, multiple microprocessors on the system bus  302 . A storage device is connected to a memory bus  304 . An input/output (“I/O”) device may be integrated to the I/O bus  303  as depicted. A storage device includes memory devices such as hard disk drive  306 . I/O device includes an alcohol detector controller  305  for assisting in the control of an alcohol detector. Computer system  201  controls and communicates with the alcohol detector.  
         [0037]    Those of ordinary skill in the art will appreciate that the hardware depicted in FIG. 3 may comprise of multiple microprocessors, multiple storage devices, or multiple I/O devices. These devices may vary. For example, other peripheral devices, such as optical disk drives and the like, also may be used in addition to or in place of the hardware depicted. The depicted example is not meant to imply architectural limitations with respect to the present invention.  
         [0038]    Referring now to FIG. 4, a block diagram of an alcohol detector controller  305  is illustrated. Controller  305  involves three inputs and two analog outputs interfacing the alcohol detector  305  and a computer PCI bus. Controller  305  receives amplified analytical PD1 signal  401  amplified reference PD2 signal  402 . Controller  305  also receives transformed resistance/voltage signal  403 . Transformed resistance/voltage signal  403  is multiplexed with amplified analytical PD1 signal  401  and amplified reference PD2 signal  402 . A multiplexor  410  allows successive connecting of inputs to analog to digital converter (“ADC”)  411  with set update rate, which value can&#39;t exceed a predetermined sampling frequency, 1.25 MHz. Next, dither  412  may be used for smoothing of bits in ADC  411  output signals. A timer controlled by software serves as clock cycle for alcohol detector controller  305 . It may include a frequency divider that allows for frequency adjustments of output signal generation and data acquisition. A trigger is controlled by the timer. It serves as a synchronizational signal for the signal generation and data acquisition. If this triggering synchronization switches at a common frequency, it creates an operational frequency for the alcohol detector controller  305 .  
         [0039]    With regards to controller&#39;s outputs, data are stored in buffer memory  413 . A predetermined pulsed signal for DL current pulse is stored in buffer memory  413  for DL current. The data stored in the buffer memory  413  flows to the first digital-to-analog converter (“DAC1”). DAC 1  supplies continuous train of raw DL current  404 . Pump current for the peltier element must be calculated by the computer system. Then pump current data is transferred and stored in buffer memory  413  in which it flows to the second digital-to-analog converter (“DAC2”). DAC 2  supplies continuous train of raw pump current  405 . Controller  305  is installed in the computer PCI bus  406  and connected with Interface module and photodetector transformer/amplifier unit. Data exchange between controller  305  and computer through reads and writes of controller&#39;s  305  buffer memory  413 . In a preferred embodiment of the present invention, controller  305  is configured from a standard multifunctional NI-DAQ board of the PCI-MIO-16E-1 produced by National Instruments, Inc.  
         [0040]    Referring now to FIG. 5, a pictorial representation of interface module  203  in accordance with an embodiment of the present invention is illustrated. Interface module  203  involves three analog units: DL current supply  510 , resistance-voltage transformer  520 , and peltier current supply  530 . Interface module  203  provides interface for three signals between the alcohol detector  100  and alcohol detector controller  305 . In FIG. 5( a ), DL current supply  510  amplifies and transforms the pulse of raw DL current  517  into pulses of amplified DL current  518  feeding alcohol detector. It includes three operational amplifiers,  511 - 513 . Resistance R 1   514  and capacitance Cl  515  define frequency bandwidth. Resistance R 2   516  defines the current/voltage transformation factor. The output operational amplifier A 2    513  and resistor R 2    516  are chosen thermo stable for preventing drift of output parameters.  
         [0041]    Two other units of interface module  203 , resistance-voltage transformer  520  and Peltier current supply  530 , are intended for stabilizing and adjusting the diode laser temperature. The temperature of thermistor having good thermal contact with diode laser in alcohol detector  100  is measured in the Resistance/Voltage Transformer unit  520  as depicted in FIG. 5( b ). Resistance-voltage transformer unit  520  includes two operational amplifiers  521  and  522  and stable current supply  523 . Current supply  523  ensures that a current of 100 uA flows the thermistor R 1    524 . Resistance-voltage transformer unit  520  transforms raw resistance-voltage signal  526  into a voltage value for transformed resistance-voltage signal  525 . Transformed resistance-voltage signal  525  transmits to Controller  305  as one of the inputs, which is later transformed into degree value in the device software.  
         [0042]    In FIG. 5( c ), the raw pump current  531  from alcohol detector controller  100  is transmitted to Peltier current supply  530  of the Interface module  203 . Peltier current supply  530  constitutes a power amplifier for supplying differential voltage for transformed pump current  532 . The unit includes three operational amplifiers,  534 - 536 , resistance R 6    537  and capacitance C 2    538  restrict frequency bandwidth, resistances R 7    539  and R 8    540  restrict maximum output current for transformed pump current  532 . All units of the Interface module  203  are storage battery-powered; the batteries being very stable sources. Such independent power supply ensures stable operation and high values of a signal to noise ratio.  
         [0043]    Referring now to FIG. 6, a pictorial representation of a photodetector transformer/amplifier unit  204  in accordance with an embodiment of the present invention is illustrated. Photodetector transformer/amplifier unit  204  transforms raw analytical PD1 signal  601  and raw reference PD2 signal  602  respectively into differential amplified analytical PD1 signal  603  and amplified reference PD2 signal  604 . Amplified analytical PD1 signal  603  and amplified reference PD2 signal  604  are inputs of Alcohol Detector Controller  305 . Base scheme of these transformer-amplifiers is shown at FIG. 6. The first stage of the scheme is typical transimpedance amplifier A 9  where R 9  and C 3  are feedback resistance and capacitance respectively. Amplifier frequency bandwidth is defined by capacitance C 3 , transfer factor at low frequencies is defined by resistance R 9 . Second stage of the scheme is voltage amplifiers A 10  and A 11  for generating differential outputs. Photodetector transformer/amplifier unit  204  is also battery-powered for providing high signal to noise ratio.  
         [0044]    Referring now to FIG. 7, a block diagram of software  205  in accordance with an embodiment of the present invention is illustrated. Software  701  initializes and synchronizes alcohol detector system  200 . It also provides for alcohol detector system computer program instruction for signal processing  702 , diode laser temperature stabilization  703 , calculation of alcohol concentration  704  and other operations are produced in the base part of the program  705 .  
         [0045]    Referring now to FIG. 8, a flowchart of signal processing  702  according to an embodiment of the present invention is illustrated. The software provides instructions for signal processing for generating the pattern of pulses of DL current (step  801 ). The pulse pattern period must in proportionate to the digital to analog converter update rate. The pattern is then stored in the alcohol detection controller&#39;s buffer memory (step  802 ). The software further provides instructions for applying the pattern to the alcohol detection controller&#39;s digital to analog converter (step  803 ). In a preferred embodiment of the present invention, the pulse period is 3.6 ms with 0.85 duty factor. Therefore, the pulse is above the threshold current for 3.0 ms, and below the threshold current for 0.6 ms. The DAC&#39;s update rate is 500 kHz. Thus, in order to generate a pulse period, the signal processing  702  must generate 1800 points to be store in the buffer memory. Of the 1800 points, 1500 points is for generating current above the threshold level and 300 points for below. A graph of raw DL current with point number is depicted in FIG. 8( a ).  
         [0046]    Current pulse is high frequency square modulated with rather high modulation amplitude. If the controller update rate equals 500 kHz, the modulation period equals 12 us, so each period includes 6 points: three points at higher amplitude, three points at lower amplitude and so on. See FIG. 8( b ). As a result each pulse is divided in two branches: upper and lower. Each branch in the pulse is of trapezoid form with the same slope. So DL radiation wavelength is swept in each branch in different ranges. For ethanol detection the ranges of scanning diode laser radiation wavelength were chosen the following: 1.39262 urn-1.39284 urn for lower branch and 1.39262 urn-1.39274 urn for upper branch. The DAC in the alcohol detector controller, transform the pulsed pattern into a continuous raw DL current.  
         [0047]    Referring now to FIG. 9, a flowchart for calculation of alcohol concentration  704  according to an embodiment of the present invention is illustrated. The process for calculating alcohol concentration starts with the receipt of sampled data from the analytical photodetector signal at beginning of the current pulse (step  901 ).  
         [0048]    Three controller inputs (step  902 ): (1) photodetector signal from analytical channel (step  903 ), (2) photodetector signal from reference channel (step  904 ), (3) signal proportional to thermistor resistance (step  905 ), are used in present invention. They are applied to the controller ADC successively, so sampling frequency of each input is three times lower than the controller update rate and equals 166.6 kHz. Pulse duration in photodetector signals includes 500 points, duration between adjacent pulses includes 100 points, and pulse repetition period includes 600 points. Modulation period in the signals is two times more than duration between adjacent points; so even points form one branch (low), odd points form another branch (high).  
         [0049]    The first channel contains sampled analytical PD1 signals made up of a train of pulses having 3.6 ms period (step  903 ). The software separates the pulses for independent treatment of each pulse (step  906 ) according to a period or cycle of a pulse. In step  907 , the value of “zero signal” between two pulses is subtracted from each of the points respectively. “Zero signal” is PD signal when laser is switched off. This signal includes photodetector preamplifier output shift and value connected with illumination of photodetector by other light sources. The value of zero signal is averaged by 100 points between two adjacent pulses. Step  907  lessens interference of photodetector illuminated by another sources (i.e. light illumination reflected by pieces of glass or car windows). The result from subtracting zero signal is saved as background pulse (step  908 ). Next the process calculates the difference between the background pulse and the raw current (step  909 ). The independent pulse is further separated into two arrays: (a) an odd array for storing all the odd points; and (b) an even array for storing all the even points (step  910 ). Another procedure for lessening interference, in step  911 , is calculating the logarithm of the ratio of respective even point over odd point (e.g. Ln(even/odd)). Logarithm value is proportional to the difference of absorptions at the branches wavelength ranges and would lessen any low-frequency signal interference from mechanical vibration or interfering illumination.  
         [0050]    The predetermined Fourier Transform is stored in memory and accessible by the system. Unique features of absorption spectrum of alcohol and water in the range of wavelength scanning are used for their detection. FIG. 9( a ) shows the predetermined absorption lines of alcohol and water at wavelength range around 1.39268 um. The calculated concentration of water (content in the reference cell) and alcohol is distinguished from each other by mutual orthogonalization of the correlated factors of alcohol and water (steps  912  and  913 ) e.g. gas molecules to be detected and the content in the reference cell. In a preferred embodiment of the present invention, X denotes input processed signal (in this case, it is Ln(even/odd)), A denotes alcohol function (difference of alcohol absorption factors in the wavelength ranges corresponding to upper and lower parts of laser radiation), and W denotes water function. X, A, and W are one-dimensional arrays or vectors. The number of values in these arrays equals to (pulse point number)/2. For our parameters this number is equal to 300. Orthogonalization of X with respect to water is:  
           X   w   =X −( X*W ) *W/ ( W*W ),  
         [0051]    where (a*b) is scalar product of two vectors. Accordingly orthogonalization with respect to alcohol is:  
           X   a   =X −( X*A ) *A/ ( A*A ).  
         [0052]    The calculation of correlation factors may be produced after orthogonalization. Correlation factor of orthogonalized signal and alcohol function is: a=(X w *A), this value for water function is: w=(X a *W) in which a is proportional to ethanol concentration, and w is proportional to water concentration.  
         [0053]    Lastly, the concentration of water and alcohol is calculated by correlating the arrays with the predetermined absorption functions of alcohol and water (steps  914  and  915 ).  
         [0054]    Referring now to FIG. 10, a flowchart for DL temperature stabilization  703  according to an embodiment of the present invention is illustrated. Initially, the diode laser&#39;s temperature is set with the help of the thermistor (step  1001 ). First the process receives the transformed resistance/voltage signal (step  1002 ) from thermistor. With a predetermined load thermistor calibration function, the thermistor&#39;s actual temperature can be calculated (step  1003 ). Then with a set predetermined laser temperature and thermistor&#39;s actual temperature, the process calculates the temperature difference (step  1004 ). Next, the process calculates the PID (Proportion, Integral, Derivative) value (step  1005 ) in order to determine the pump current (step  1006 ). Initially, the diode laser and thermistor should have the same temperature until the diode laser generates more heat in which the temperature of the two components differs. As a result, thermistor&#39;s temperature is stabilized and not the diode laser. After the initial setting of the temperature, the process switches to line stabilization position (step  1010 ) for stabilizing DL temperature. The absorption line position within a recorded pulse is an unbiased criterion of DL true temperature. First, it receives the sampled data from amplified reference PD2 signal (step  1011 ). Each pulse is separated from the other (step  1012 ) for subtraction from zero signal (step  1013 ). The process repeats step  1013  one hundred times (100×) for one hundred pulse period before it takes the average value (step  1014 ). Next, with a preferred predetermined laser temperature and the calculated average value, the temperature difference is calculated (step  1015 ). Then the PID value must be calculated (step  1016 ) before the determination of pump current (step  1017 ). The difference between current absorption line position and predetermined one come to the input of PID (Proportion, Integral, Derivative) program module. Value from output of this module is applied to DAC  2  for feeding Peltier element. This value at n step of the program cycle (V n ) is calculated in conformity with formula:  
         
       V 
       n 
       =a*P 
       n 
       +b*I 
       n 
       +c*D 
       n  
     
         [0055]    where P n  is the difference (see above) at n step of the program cycle,  
           I   n     =       ∑   0   n                     P   1         ,                         
 
           Dn=P   n   −P   n−1 , a, b, c-factors.  
         [0056]    Because Pump current is not constant and must be determined, Pump current is tunable and directly stabilizes DL temperature. The determined Pump current is applied DAC 2  on the controller in which the Pump current is made continuous before channeling to the interface module. DL temperature variations directly affect the DL radiation wavelength variation. The stabilization of DL temperature ensures that DL will operate in the stable range near the maximum alcohol absorption band at 1.39268 um.  
         [0057]    Although preferred embodiments of the present invention have been described in the foregoing Detailed Description and illustrated in the accompanying drawings for alcohol detection, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of detecting other gas molecules which may require numerous rearrangements, modifications, and substitutions of steps without departing from the spirit of the invention. For example, each gas molecule having distinct absorption band would require a diode laser radiating at or near that band, the photodetector functions at the distinct absorption band, the predetermined DL current may differ in the sampled points and duration, the reference cell may differ in content. etc. Accordingly, the present invention is intended to encompass such rearrangements, modifications, and substitutions of steps as fall within the scope of the appended claims.