Abstract:
A laser spectrometer and method for measuring gas component concentration in a measurement gas, wherein light intensity from a wavelength-tunable laser diode is detected after irradiation of the measurement gas and a reference gas, and the concentration of the gas component is determined based on reduction of the light intensity by the absorption of light at the position of a selected absorption line of the gas component, and the position of the absorption line of the gas component is referenced based on a selected absorption line of the reference gas, and wherein there is a mixed operation consisting of actual measurements of fast concentration changes of the gas component to be measured and a short reference/standardization phase for wavelength referencing, line locking and standardization, where the duration of the actual measurement is measured such that measuring conditions remain constant and do not deviate from those during the reference/standardization phase.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. national stage of application No. PCT/EP2013/053247 filed 19 Feb. 2013. Priority is claimed on German Application No. 10 2012 202 893.5 filed 27 Feb. 2012, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to laser spectrometers and, more particularly, to a laser spectrometer and method for measuring the concentration of a gas component in a measurement gas. 
     2. Description of the Related Art 
     Laser spectrometers are particularly used for optical gas analysis in process measurement technology. Here, a laser diode generates light in the infrared range, which is guided through the process gas to be measured (the measurement gas) and is subsequently detected. The wavelength of the light is tuned to a specific absorption line of the gas component respectively to be measured, the laser diode periodically sampling the absorption line. To this end, the laser diode is driven periodically with a ramp-shaped or triangular (increasing and decreasing ramp) current signal. The concentration of the gas component of interest can be determined from the absorption detected at the position of the absorption line. 
     The intensity and wavelength of the light generated are nonlinear functions of the injection current and of the operating temperature of the laser diode. As a result, wavelength referencing is necessary in many cases. To this end, a reference gas is additionally introduced in a known concentration into the light path, and an absorption line of the reference gas is measured. The temperature of the laser diode can then be regulated via the position of the absorption line of the reference gas, such that the absorption line of the gas compared to be measured always lies at a particular position of the ramp of the current signal. In this case, the current ramp must be large enough for the laser diode sampling range resulting therefrom to cover both the absorption line of the gas component to be measured and that of the reference gas. 
     When shining through the measurement gas and reference gas, besides the wavelength-dependent absorption by infrared-active gas components, wavelength-independent absorption also takes place by optical components (e.g., windows) or aerosols (e.g., smoke particles). This makes normalization of the measurement necessary. To this end, the laser diode can be driven regularly with at least one burst current signal, the amplitude of which lies outside the value range of the ramp-shaped or triangular current signal, so that the light wavelengths generated with the burst current signal lie outside the wavelength ranges of the absorption lines of the gas components to be measured and other infrared-active gas components. This makes it possible to normalize the light intensity detected at the position of the absorption line to be measured, by division by the light intensity detected at the position of the burst current signal (EP 2 072 979 A1). 
     As explained above, in contemporary laser spectrometers a wavelength range that covers both the absorption lines of the gas components to be measured and the absorption lines for the wavelength referencing is sampled. In addition, a time window is required for the normalization of the measurement. Each sampling period therefore claims much more time than is necessary for the detection of a single absorption line. The time resolution of the measurement, in the case of rapidly varying gas concentrations, is thereby limited. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to increase the measurement speed in the laser-spectrometric determination of the concentration of a gas component in a measurement gas. 
     This and other objects and advantages are achieved by the method and the laser spectrometer in accordance with the invention by providing a method for measuring the concentration of a gas component in a measurement gas, by detecting the intensity of the light of a wavelength-tunable laser diode after shining through the measurement gas and a reference gas and determining the concentration of the gas component with the aid of the reduction in the light intensity due to the absorption of the light at the position of a selected absorption line of the gas component, the position of the absorption line of the gas component being referenced with the aid of a selected absorption line of the reference gas. In accordance with the method of the invention, the laser diode is driven periodically with a first increasing and/or decreasing current signal to sample the absorption line of the gas component wavelength-dependently in a sampling range which lies outside the absorption line of the reference gas and which is restricted to the immediate vicinity of the absorption line of the gas component. Next, the laser diode is driven regularly with a second increasing and/or decreasing current signal to sample the absorption line of the reference gas wavelength-dependently in a sampling range which either contains the two absorption lines of the gas component and of the reference gas or lies outside the absorption line of the gas component and is restricted to the immediate vicinity of the absorption line of the reference gas. The laser diode is then driven regularly with at least one burst current signal having an amplitude lying outside the value ranges of the first and second current signals to normalize the light intensity detected at the position of the absorption line with the intensity detected at the position of the at least one burst current signal. Finally the first current signal, the second current signal and the burst current signal are generated successively such that individual or a few, generated directly after one another, second current signals and burst current signals alternate with a multiplicity of first current signals generated directly after one another. 
     It is also an object of the invention to provide a laser spectrometer for measuring the concentration of a gas component in a measurement gas, where the laser spectrometer includes a wavelength-tunable laser diode, the light of which, after shining through the measurement gas and a reference gas, strikes a detector having a downstream evaluation device in which the concentration of the gas component is determined with the aid of the reduction in the light intensity due to the absorption of the light at the position of a selected absorption line of the gas component, the position of the absorption line of the gas component being referenced with the aid of an absorption line of the reference gas. 
     The spectrometer also includes a first signal generator for periodic driving of the laser diode with a first increasing and/or decreasing current signal to sample the absorption line of the gas component wavelength-dependently in a sampling range that lies outside the absorption line of the reference gas and which is restricted to the immediate vicinity of the absorption line of the gas component, a second signal generator for regular driving of the laser diode with a second increasing and/or decreasing current signal to sample the absorption line of the reference gas wavelength-dependently in a sampling range which either contains the two absorption lines of the gas component and of the reference gas or lies outside the absorption line of the gas component and is restricted to the immediate vicinity of the absorption line of the reference gas, at least one third signal generator for regular driving of the laser diode with at least one burst current signal having an amplitude lying outside the value ranges of the first and second current signals to normalize the light intensity detected at the position of the absorption line with the intensity detected at the position of the at least one burst current signal, and a time generator which controls the signal generators such that the first current signal, the second current signal and the burst current signal are generated successively, with individual or a few, generated directly after one another, second current signals and burst current signals alternating with a multiplicity of first current signals generated directly after one another. 
     With the method in accordance with the invention, or in the laser spectrometer in accordance with the invention, mixed operation occurs, consisting of the actual measurement (periodic microscan) of rapid concentration changes of the gas component to be measured and a short reference/normalization phase for the wavelength referencing, the line locking and the normalization. The duration of the continuous measurement must be dimensioned such that the measurement conditions remain constant and do not deviate from those during the reference/normalization phase. This applies above all to the transmission conditions, as well as the temperature and pressure. 
     Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be explained below with reference to the figures of the drawing with the aid of exemplary embodiments, in which: 
         FIG. 1  shows a schematic representation of an exemplary spectrometer in accordance with the invention having a laser diode; 
         FIGS. 2 to 6  show various examples of driving the laser diode; and 
         FIG. 7  is a flowchart of the method in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a laser spectrometer for measuring the concentration of at least one gas component of interest of a measurement gas  1 , which is contained in a measurement volume  2 , such as a measurement cuvette or a process gas line. The spectrometer contains a laser diode  3 , the light  4  of which strikes, through the measurement gas  1  and a downstream reference gas cuvette  6  filled with a reference gas  5 , a detector  7  with a downstream evaluation device  8  for delivering the measurement result  9 . The laser diode  3  is driven by a controllable current source  10  with an injection current i, the intensity I and the wavelength λ of the light  4  generated depending on the current i and the operating temperature of the laser diode  3 . The injection current i is generated in the form of different current signals. To this end, the current source  10  is driven via an adder  11  by different signal generators  12 ,  13 ,  14 ,  15 ,  16 , of which a first signal generator  12  generates a first ramp-shaped or triangular signal  17 , a second signal generator  13  generates a second ramp-shaped or triangular signal  18 , a third signal generator  14  generates a first burst signal  19 , a fourth signal generator  15  generates a second burst signal  20 , and a fifth signal generator  16  generates a sine signal  21 . A digital/analog converter  22  generates a bias signal  23 , with the aid of which the current source  10  generates a bias current for the laser diode  3 . The signal generators  12 ,  13 ,  14 ,  15 ,  16  are controlled by a time generator  24  in accordance with a table  25 , in which it is established which of the signal generators  12 ,  13 ,  14 ,  15 ,  16  generates the relevant signal  17 ,  18 ,  19 ,  20  or  21  when and how often directly in succession, i.e. with which number of periods. The generation of the ramp-shaped or triangular signals  17 ,  18  and the burst signals  19 ,  20  is carried out alternately, i.e., not simultaneously, while the sine signal  21  can only be generated together with the respective ramp-shaped or triangular signals  17 ,  18 . The table  25  is programmable and, as shown, may be implemented in the time generator  24  or, for example, in a superordinate control device  26  of the laser spectrometer. 
     The driving of the laser diode may be performed in different ways in the scope of the invention. For example, the adder  11  may be replaced with a switching device (multiplexer), controlled by the time generator  24 , which converts the signals  17 ,  18 ,  19 ,  20  into a signal sequence in accordance with the table  25 , and thereby drives the current source  10 . The signals  17 ,  18  may also have other increasing and/or decreasing signal profiles, such as a sine profile. 
       FIG. 2  shows a first example of the driving of the laser diode with the injection current i. In its time profile, the injection current i consists of different current signals  17 ′,  18 ′,  19 ′,  20 ′,  21 ′, which result from the driving of the current source  10  with the signals  17 ,  18 ,  19 ,  20 ,  21 . The wavelength λ of the light  4  generated follows the profile of the current i more or less linearly. The absorption line of the gas component to be measured lies at the position i abs , or λ abs , and that of the reference gas at the position i ref , or λ ref . 
     With the first ramp-shaped or triangular current signal  17 ′, the absorption line of the gas component is sampled in a sampling range that lies outside the absorption line of the reference gas  5  and is restricted to the immediate vicinity of the absorption line of the gas component. The sampling is performed over a prolonged time, such as one minute, with a multiplicity of sampling periods following one another directly. Owing to the relatively low amplitude of the current signal  17 ′, the period duration is correspondingly short, so that the measurement of the absorption line of the gas component can even follow rapid concentration changes of the gas component to be measured. 
     The sampling of the absorption line of the gas component is interrupted regularly, here for example at minute intervals, by a measurement of the absorption line of the reference gas  5 . To this end, the laser diode  3  is driven with the second ramp-shaped or triangular current signal  18 ′, the amplitude of which, in the example shown in  FIG. 2 , is large enough for the resulting sampling range to contain the two absorption lines of the gas component and the reference gas  5 . This second current signal  18 ′ is generated only for a short duration in the second range or less, for a single period or very few periods. 
     Before and/or after the second current signal  18 ′, the burst signals  19 ′ and  20 ′, respectively, used for the normalization of the measurement are generated. 
     In order to increase the measurement accuracy, the ramp-shaped or triangular current signals  17 ′ and  18 ′ may be modulated in a known way with the sine current signal  21 ′ with the frequency f. Owing to the nonlinearity of the absorption lines, the modulation of the injection current i with the frequency f results in a corresponding variation of the detected light intensity I with more less pronounced harmonic distortions. At the extreme position (absorption maximum) in the middle of the absorption line, the first harmonic with the frequency  2   f  dominates, while the proportion of the first harmonic in the intensity I decreases greatly in wavelength ranges outside the absorption maximum. The absorption occurring at the position of the absorption maximum can therefore be determined very accurately and free from interference in the evaluation device  8  by evaluating the  2   f  signal component. 
       FIGS. 3 to 6  show other exemplary embodiments of the driving of the laser diode  3 , in which the second current signal  18 ′ and/or the burst current signals  19 ′,  20 ′, or only one burst current signal, are generated in a different sequence. The second current signal  18 ′ may also be generated in the shape of a ramp ( FIGS. 4 and 6 ) instead of triangularly and/or with a small amplitude, restricting the sampling to the immediate vicinity of the absorption line of the reference gas  5  ( FIG. 3 ), in order to keep the interruption of the rapid periodic sampling of the absorption line of the gas component of interest as short as possible. A ramp-shaped signal form is naturally also possible for the first current signal  17 ′. 
       FIG. 7  is a flowchart of a method for measuring a concentration of a gas component in a measurement gas ( 1 ), by detecting an intensity (I) of light ( 4 ) of a wavelength-tunable laser diode ( 3 ) after shining the light through the measurement gas ( 1 ) and a reference gas ( 5 ), and by determining the concentration of the gas component aided by a reduction in the intensity (I) of the light due to absorption of the light ( 4 ) at a position (iabs, λabs) of a selected absorption line of the gas component, the position (iabs, λabs) of the absorption line of the gas component being referenced with aided by a selected absorption line of the reference gas ( 5 ). The method comprises driving the laser diode ( 3 ) periodically with at least one of (i) a first increasing current signal ( 17 ′) and (ii) a first decreasing current signal ( 17 ′) to sample the absorption line of the gas component wavelength-dependently in a sampling range which reside outside the absorption line of the reference gas ( 5 ) and which is restricted to an immediate vicinity of the absorption line of the gas component, as indicated in step  710 . 
     The laser diode ( 3 ) is then driven regularly with at least one of a second increasing current signal ( 18 ′) and (ii) a second decreasing current signal ( 18 ′) to sample an absorption line of the reference gas ( 5 ) wavelength-dependently in a sampling range which one of (i) contains two absorption lines of the gas component and the reference gas ( 5 ) and (ii) lies outside the absorption line of the gas component and which is restricted to the immediate vicinity of the absorption line of the reference gas ( 5 ), as indicated in step  720 . 
     Next, the laser diode ( 3 ) is driven regularly with at least one burst current signal ( 19 ′,  20 ′) having an amplitude lying outside the value ranges of the first and second current signals ( 17 ′,  18 ′) to normalize the light intensity (I) detected at the position (iabs, λabs) of the absorption line with the intensity (I) detected at the position of the at least one burst current signal ( 19 ′,  20 ′), as indicated in step  730 . 
     The first current signal ( 17 ′), the second current signal ( 18 ′) and the at least one burst current signal ( 19 ′,  20 ′) are generated successively such that individual or a few, generated directly after one another, second current signals ( 18 ′) and the at least one burst current signal ( 19 ′,  20 ′) alternate with a multiplicity of first current signals ( 17 ′) generated directly after one another as indicated in step  740 . 
     The method according to the invention is suitable for spectrometers in all bands (UV, VIS, IR). 
     While there have been shown, described, and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the methods described and the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.