Method and System for the Relative Referencing of a Target Gas in an Optical Measuring System for Laser Spectroscopy

Abstract

A method for operating an optical measuring system including a wavelength-tunable temperature-stabilized laser light source for measuring the concentration of a target gas component in a measured gas, wherein an instantaneous base current IDC_ZG,act corresponding to a wavelength λZG of a target gas absorption line is set so that a wavelength distance ΔλDC defined during calibration between a target gas absorption line for a target gas component and a reference gas absorption line for a reference gas component is maintained. During operation, a relative temperature difference in the laser light source, defined in advance during calibration, between the operating points selected at the time of calibration of the reference gas, with a base current IDC_RG,cal, and the target gas component, with a base current IDC_ZG,cal, is maintained by determining the required instantaneous base current IDC_ZG,act for the target gas component, as a function of an instantaneous base current IDC_RG,act for the reference gas. The system includes a measuring system for carrying out the method.

Description

TECHNICAL FIELD OF INVENTION

The invention relates to a method for operating an optical measuring system for measuring the concentration of a target gas component (ZG) in a measured gas, comprising a wavelength-tunable temperature-stabilized laser light source, wherein an instantaneous base current I_{DC_ZG,act} corresponding to a wavelength λZG of a target gas absorption line is set so that a wavelength distance Δλ_DC defined during the calibration between a target gas absorption line for a target gas component (ZG) and a reference gas absorption line for a reference gas component (RG) is maintained. The invention furthermore relates to a system for carrying out the method.

DISCUSSION OF RELATED ART

Optical measuring systems for measuring the concentration of a gas component in a measured gas, based on laser absorption spectroscopy (LAS), are known from the prior art in a wide variety of embodiments, as are a multitude of different methods for operating such an optical measuring system, such as wavelength modulation spectroscopy (WMS) and direct absorption spectroscopy (DAS). In LAS, exact setting of the laser wavelength to the wavelength of the absorption line for the target gas plays an important role in general. During calibration, the laser wavelength is usually defined by way of the operating point of the Peltier temperature (base laser temperature) and the base current I_DC of the laser light source. However, long-term changes in the laser and the influence of the outside temperature result in a different wavelength for the selected operating point, which deviates from the absorption wavelength of the target gas, even if the base current I_DC remains unchanged. Base current usually denotes a direct current (DC) having a maximum at the absorption line, with larger and smaller current values traversed around this, when the absorption line is being scanned. As a consequence, these changes also cause the sensor calibration to deviate from the specification limits, which then often necessitates a re-calibration of the optical measuring system.

So as to ensure that the wavelength of the target gas remains accurate (in particular in the absence of the target gas), usually one or more absorption lines for a reference gas are used, and the line locking principle is applied. The reference gas is either essentially permanently present in the process gas or is accommodated somewhere in the sensor itself, enclosed in a cuvette, for example. The cuvette can be implemented, for example, in the optical measuring path or in a special optical reference path (this requires a beam splitter and a further photodiode).

Often, it is not possible or useful to use the target gas itself as the reference gas, for example when this is not stable over a long time, when the line width associated therewith is too thin, which results from too short an absorption path and leads to a low absorption signal, or when this is too dangerous to produce or during operation.

As an alternative, a different gas (reference gas), which has one or more absorption lines in the vicinity of a target wavelength, can be used instead of the target gas. Reference is made to EP 2 307 876 B1 by way of example. This patent uses two CH₄ gas absorption lines to exactly reference the CO gas absorption line in the center. Often, however, the tuning range of the laser light source is not sufficiently wide to accurately reference a target gas absorption line in this way.

EP 2 307 876 B1 discloses a method for detecting at least one target gas by way of laser spectroscopy, using a laser light source having an emission wavelength that is monochromatic and tunable by varying the operating temperature or the operating current. So as to calibrate the wavelength scale of the laser light source relative to the varied operating temperature or the varied operating current, in the wavelength range of a band of the at least one target gas, a first laser light source tuning step is carried out across a first tuning width, wherein at least two absorption lines for a reference gas and at least one absorption line for the at least one target gas are present. Thereafter, a second laser light source tuning step is carried out across a second tuning width in the wavelength range of the band of the at least one target gas, wherein the second tuning width is narrower than the first tuning width, and wherein at least one of the at least one absorption line for the at least one target gas is present. The target gas and the reference gas are different gases. The first tuning step for calibrating the laser current or the laser temperature is carried out once with the absolute wavelength scale, and the second tuning step for detecting the at least one target gas is carried out consecutively several times. The second tuning width is compared to a calculated absorption spectrum, wherein non-iterative curve fitting using a linear regression algorithm is employed so as to calculate the concentration of the at least one target gas in one step. This procedure requires a reference gas that has at least two absorption lines in the vicinity of the target gas absorption line, which are within the tuning range of the laser. Moreover, this procedure is time-consuming since it requires that two reference lines be detected and evaluated.

For activating a wavelength-tunable laser diode in a spectrometer using no reference gas, it is known from DE 10 2013 202 289 A1 to predefine a power-time function, in accordance with which the laser diode is tuned periodically over a wavelength range, by ascertaining a current profile, which is used to activate the laser diode, from the power-time function and the measured values of the voltage present at the laser diode. The current profile that is used to directly activate the laser diode is generated by a control unit as a function of the control deviation between the power consumption (actual variable) of the laser diode and the predefined power-time function (target variable), wherein the voltage present at the laser diode and the current through the laser diode are continuously detected, for example measured, and the power consumption of the laser diode is ascertained continuously by multiplying the measured current and voltage values. No reference gas is used therein, and thus a different wavelength range is detected when the temperature of the Peltier element (Peltier temperature) on which a laser chip is disposed and/or the outside temperature change, even though the power-time function is predefined. This method is too imprecise for referencing a target gas. Moreover, the entire voltage across the laser is measured, which is too imprecise for determining the wavelength.

If only one reference gas line, which is not the target gas line, is available for the use of a reference gas, it is essential to ensure that the wavelength distance Δλ_DC between the target gas and the reference gas is kept constant during operation for different laser currents, and thus different laser temperatures. Moreover, the distance must also remain constant when the laser ages. In principle, it would be possible to simply scan a certain wave range. However, the detection limit is drastically limited due to present noise, resulting from optical interference phenomena, for example, and the presence of absorption lines for other gases in the measured spectrum. It is therefore important to adhere to the wavelength distance very precisely.

The wavelength distance between the target gas and the reference gas is typically set by a constant current distance ΔI. Due to the non-linear DC tuning behavior of the laser, a shift (drift) of the operating point, for example as a result of the influence of the outside temperature on the temperature stabilization or aging of the laser/electronics, results in a distance error for the wavelength distance. This results in the target gas component (absorption line for the target gas) being presumed to be in an incorrect location. In some instances, this results in considerable measurement errors.

Proceeding from this, it is the object of the claimed invention to propose a different, more precise, effective and easy to implement option for keeping the wavelength distance between the target and reference gas lines constant, despite changing laser properties, such as operating current, operating temperature or long-term drift.

SUMMARY OF THE INVENTION

This object is achieved according to the invention by a method for operating an optical measuring system for measuring the concentration of a target gas component in a measured gas having the features of described herein and by a system in the form of a measuring system described herein. Further advantageous embodiments can be derived from the respective dependent claims.

The core idea of the invention is, instead of using a fixed current difference value, to set the current difference in such a way that a previously defined relative temperature difference is maintained between the reference and target gas (peak) positions, which is to say between the respective absorption lines, for the calculation of the presumed wavelength position of the target gas. This temperature difference is calculated from the difference of the temperatures that are generated by the respective introduced electrical powers at an internal resistance of the laser light source at the peak positions of the target and reference gases at the time of calibration. This relative temperature difference is proportional to the wavelength distance of the absorption lines for the target and reference gases, and thus this procedure ensures that the target wavelength can always be exactly determined.

For example, this method advantageously allows applications in which reference gas and target gas absorption lines are significantly distanced from each other to be easily implemented. Monitoring for methane leaks shall be mentioned here by way of example, where, in addition to CH₄, C₂H₆ (ethane) also has to be measured to confirm that natural gas is involved. The reference gas used is methane (which, in this case, also represents the second target gas), which is accommodated in the optical path of the laser in the sensor for this purpose. Depending on the selected absorption lines, the distance between the reference gas (methane) and the target gas (ethane) is between 0.6 and 1.0 nm here. Since this distance is considerable, it is no longer possible to use a fixed current value. Additionally, there is an advantage in that the calibration is particularly simple, as compared to the conventional calibration methods known from the prior art.

The abbreviations RG and ZG used hereafter denote a reference gas, or a reference gas component in a gas, and a target gas, or a target gas component in a measured gas. The designations DC and AC used are common and thus known to a person skilled in the art as designations for DC voltage/direct current and AC voltage/alternating current. These refer to electrical currents, voltages and/or powers and indicate the respective type. Moreover, the abbreviation L is used for the laser light source or, in general, for the laser. The abbreviations RG, ZG, DC, AC and L are used for clarification in the claims and in the overall description, and in particular in the formulas below. The base current I_DC denotes, in summary, all the base currents corresponding to the respective operating states (calibration, instantaneous base current) and associated gases (RG, ZG).

According to the invention, in the method, during operation, a relative temperature difference in the laser light source, defined in advance during the calibration, between the operating points, selected at the time of calibration, of the reference gas (RG), with a base current I_{DC_RG,cal}, and the target gas component (ZG), with a base current I_{DC_ZG,cal}, is maintained by determining the required instantaneous base current I_{DC_ZG,act} for the target gas component, as a function of an instantaneous base current I_{DC_RG,act} for the reference gas.

The calibration of the measuring system is preferably carried out with the reference gas and the target gas. During the calibration, the base currents I_{DC_RG,cal} and I_{DC_ZG,cal}, assigned to the gas absorption lines for the reference gas RG and of the target gas component ZG are established, the associated electrical powers P_{DC_RG,cal} and P_{DC_ZG,cal} of the laser light source are determined, and a power difference ΔP_{DC, cal} is found therefrom, which is stored.

According to the invention, moreover, during operation of the measuring system, the electrical power P_{DC_RG,act} of the laser source assigned to the gas absorption line for the reference gas is then ascertained, the electrical power P_{DC_ZG,act} of the laser source assigned to the gas absorption line for the target gas ZG is determined as the sum of the electrical power P_{DC_RG,act} and the power difference ΔP_{DC, cal}, and the assigned base current I_{DC_ZG,act} is calculated from the electrical power P_{DC_ZG,act} of the laser source thus determined.

In other words, this means that, during the calibration, the DC laser currents of the peak positions of the reference gas and the target gas are established, the electrical DC power dropping across the internal resistance of the laser is ascertained for the two peak positions by detecting the internal resistance of the laser at the respective current position or, alternatively, an equivalent variable, for example using lock-in technology. At the time of calibration, the power difference ΔP_{DC, cal} of the DC powers, or at least an equivalent variable, is ascertained and stored in the sensor. The power difference is preferably determined according to formula F 10 listed below. As an alternative, it would also be possible to store the variables indicated in formula F 10 in the sensor.

During operation of the measuring system, the electrical DC power dropping across the internal resistance of the laser is determined for the instantaneous peak position of the reference gas. The internal resistance measurement can be implemented, in particular, by lock-in technology. Since the operating point may shift due to drift, the electrical power P_{DC_RG,act} is recalculated on a regular basis (and thus cannot be assumed to be a fixed value). Moreover, the instantaneous electrical power at the peak position for the target gas is determined by adding the instantaneous power for the reference gas peak and the power difference. From the instantaneous power of the peak for the target gas, the DC current for the peak position thereof is preferably ascertained according to formula F 11 listed below. Thereafter, the measuring scan for the target gas is carried out at the ascertained peak position for the target gas, and the gas concentration is ascertained therefrom. The base current defines the position at which the absorption characteristic curve has a maximum. Scanning with larger and smaller current values is carried out around this maximum to ascertain the concentration.

The proposed method is used for the initial calibration of the measuring system by the manufacturer and can also be utilized by the user for potentially necessary subsequent calibration.

In wavelength modulation spectroscopy, as is customary, a wavelength-tunable temperature-stabilized laser light source is used, which periodically varies a central base wavelength of the laser light of the laser light source by changing the base current via a relevant absorption line for the gas component at an operating point and, at the same time, modulates the same with a frequency (f) and a determinable amplitude by way of a modulation device. Using a light detector, the intensity of the laser light after it has passed through the measured gas is detected. An evaluation unit is used, which comprises means for the phase-sensitive demodulation of a measuring signal generated by the light detector at the frequency (f) and/or one of the harmonics thereof, wherein the laser light source is operated in a current-modulated manner with a base current I_DC and a modulation current I_AC and emits a laser beam of the wavelength having a wavelength modulation amplitude Δλ_AC, and the wavelength modulation amplitude Δλ_AC of the laser light is kept constant by way of variable setting of the current modulation amplitude ΔI_AC.

However, long-term changes in the laser may make it necessary to adapt the base laser temperature. However, this also necessitates a correction of the previously calculated power difference ΔP_DC,cal. If the laser temperature is being varied, formula F 15 should be preferred over formula F 11 for calculating the base current I_{DC_ZG,act}. For this purpose, the laser temperature is stored at the time of calibration.

According to a preferred method step, the respective internal resistances are required for calculating the electrical power at the respective base currents (for the target and reference gases). It should be noted that the exact internal resistance may iterate only after multiple measurements.

The internal resistance R_I of the laser light source is preferably determined from a voltage/current characteristic curve for the laser light source in which the voltage drop U_L across the laser light source is recorded as a function of the base current I_DC. The respective internal resistances are ascertained at the peak positions of the reference gas and the target gas. The voltage/current characteristic curve of the laser light source is usually recorded for the first time during the calibration of the optical measuring system. It is not necessary to measure the entire curve, but only a region around the operating point, so as to correctly determine the respective internal resistance R_I from the slope in this region. A multitude of measurements can be carried out in the region, and the result can be averaged. As an alternative, the slope can also be ascertained using lock-in technology, which has the advantage that noise is reduced. The resistance values are also ascertained during operation on a regular basis, and preferably these are re-determined during every concentration measurement.

During calibration and during regular operation of the optical measuring system, an evaluation unit comprising lock-in technology is preferably used for determining the concentration of a target gas component, so as to achieve noise reduction in the known manner, in particular in order to considerably lower the noise caused by the 1/f signal. A lock-in amplifier, which is also sometimes referred to as a phase-sensitive rectifier or carrier frequency amplifier, is an amplifier for measuring a weak electrical alternating signal, which is modulated with a reference signal having a known frequency and phase. The device represents an extremely narrow-band bandpass filter, thereby improving the signal-to-noise noise ratio. The advantage when using such a device is that DC voltages and AC voltages having different frequencies and noise are efficiently filtered.

According to the invention, the method described above is based on the formulas provided below. These are used during calibration and/or during operation. Values from calibration are denoted by “_cal” and values from operation are denoted by “_act.”

Heretofore, the instantaneous current position of the target gas (ZG) is determined in relative terms by the position of the reference gas (RG) using the following formula:

I _{DC_ZG,act} =I _{DC_RG,act} +ΔI  (F 1)

where ΔI is a fixed current value, which is established during calibration:

ΔI=I_{DC_ZG,cal}−I_{DC_RG,cal}  (F 2)

Due to the aforementioned non-linear DC tuning behavior of the laser, this procedure is not sufficiently exact for determining the position of the ZG. It is apparent from

$\begin{array}{cc}\frac{1}{\lambda }\frac{d\lambda }{\mathrm{dT}}=\frac{1}{n_{\mathrm{eff}}}\frac{{\mathrm{dn}}_{\mathrm{eff}}}{\mathrm{dT}}+\frac{1}{L_{\mathrm{eff}}}\frac{dL_{\mathrm{eff}}}{\mathrm{dT}}& \left(F3\right)\end{array}$

that a wavelength change can be described by a temperature change, where n_eff is the effective refractive index, and L_eff is the effective resonator length of the laser. Taking into consideration that the first term on the right side of the above formula is dominant over the second term, a wavelength change is given by:

$\begin{array}{cc}\mathrm{\Delta \lambda }=\lambda ·\frac{1}{n_{\mathrm{eff}}}\frac{{\mathrm{dn}}_{\mathrm{eff}}}{\mathrm{dT}}·\Delta T& \left(F4\right)\end{array}$

So as to calculate the presumed position of the target gas, instead of using a fixed current difference value (that was determined during calibration according to formula F 2), the current difference is set during ongoing operation in such a way that a defined temperature difference ΔT_act is maintained between the reference and target gas (peak) positions. This results in the following temperature at the instantaneous position of the target gas:

T _ZG,act =T _RG,act +ΔT _act  (F 5)

wherein the temperature at the position of the reference gas peak is given by:

T _RG,act =P _{DC_RG,act} ·R _th,act =I _{DC_RG,act} ² ·R _{I RG,act} ·R _th,act  (F 6)

wherein the electrical power is converted via the thermal resistance R_th,act into the temperature T_RG,act, and the power is produced by the laser current I_{DC_RG,act} in the internal resistance R_{I RG,act}.

The instantaneous temperature T_ZG,act at the position of the target gas peak is coupled to the sought laser current I_{DC_ZG,act} via the thermal resistance and electrical resistance:

T _ZG,act =P _{DC_ZG,act} ·R _th,act =I _{DC_ZG,act} ² ·R _{I ZG,act} ·R _th,act  (F 7)

If the laser temperature does not change during operation, then:

$\begin{array}{cc}\Delta T_{\mathrm{act}}=\Delta T_{\mathrm{cal}}=\Delta P_{\mathrm{DC},\mathrm{cal}}·R_{\mathrm{th},\mathrm{cal}}=\left(I_{\mathrm{DC}\_\mathrm{ZG},\mathrm{cal}}^2·R_{I\mathrm{ZG},\mathrm{cal}}-I_{\mathrm{DC}\_\mathrm{RG},\mathrm{cal}}^2·R_{I\mathrm{RG},\mathrm{cal}}\right)·R_{\mathrm{th},\mathrm{cal}}& \left(F8\right)\end{array}$

The aforementioned temperature difference ΔT_cal is calculated as the difference of the temperatures that are generated by the respective introduced electrical powers at the internal resistance at the peak positions of the target and reference gases at the time of calibration. Since the temperature difference is proportional to the wavelength difference between ZG and RG, this procedure ensures that the target wavelength is always exactly determined.

By inserting formulas (F 6), (F 7) and (F 8) in formula (F 5), the sought laser current at the position of the target gas is found:

$\begin{array}{cc}{I}_{\mathrm{DC}\_\mathrm{ZG},\mathrm{act}}=\sqrt{\frac{I_{\mathrm{DC}\_\mathrm{RG},\mathrm{act}}^2·R_{I\mathrm{RG},\mathrm{act}}+\Delta P_{\mathrm{DC},\mathrm{cal}}\frac{R_{\mathrm{th},\mathrm{cal}}}{R_{\mathrm{th},\mathrm{act}}}}{R_{I\mathrm{ZG},\mathrm{act}}}}& \left(F9\right)\end{array}$

wherein a power difference (“fixed power”) results using the values at the time of calibration:

ΔP_DC,cal=I_{DC_ZG,cal}²·R_{I ZG,cal}−I_{DC_RG,cal}²·R_{I RG,cal}  (F 10)

Assuming that the thermal resistance does not change during operation (R_th,act=R_th,cal), the following results for the peak position of the target gas:

$\begin{array}{cc}{I}_{\mathrm{DC}\_\mathrm{ZG},\mathrm{act}}=\sqrt{\frac{I_{\mathrm{DC},\mathrm{RG},\mathrm{act}}^2·R_{I\mathrm{RG},\mathrm{act}}+\Delta P_{\mathrm{DC},\mathrm{cal}}}{R_{I,\mathrm{ZG},\mathrm{act}}}}& \left(F11\right)\end{array}$

It should be noted that the exact internal resistance R_{I ZG,act} is not known initially and would have to be measured (at the location I_{DC_ZG,act}). After running through the formula several times, or after multiple measurements, however, the value iterates very quickly, and the internal resistance is correctly ascertained at the location I_{DC_ZG,act}.

If, additionally, the operating point of the base laser temperature T_L changes (for example, if the Peltier temperature is adapted during operation so as to compensate for excessively large wavelength shifts on the current scale due to the adaptation of the base laser temperature), from the constant relative temperature change condition:

$\begin{array}{cc}\frac{\mathrm{\Delta \lambda }}{\lambda }\cong \frac{\Delta T}{T}& \left(F12\right)\end{array}$

then the following relationship for the instantaneous temperature difference follows:

$\begin{array}{cc}\Delta T_{\mathrm{act}}=\Delta T_{\mathrm{cal}}\frac{T_{L,\mathrm{act}}}{T_{L,\mathrm{cal}}}=\Delta P_{\mathrm{DC},\mathrm{cal}}·R_{\mathrm{th},\mathrm{cal}}\frac{T_{L,\mathrm{act}}}{T_{L,\mathrm{cal}}}& \left(F13\right)\end{array}$

wherein the temperatures are to be used in Kelvin. The resulting current is given by:

$\begin{array}{cc}{I}_{\mathrm{DC}\_\mathrm{ZG},\mathrm{act}}=\sqrt{\frac{I_{\mathrm{DC}\_\mathrm{RG},\mathrm{act}}^2·R_{I\mathrm{RG},\mathrm{act}}+\Delta P_{\mathrm{DC},\mathrm{cal}}\frac{R_{\mathrm{th},\mathrm{cal}}}{R_{\mathrm{th},\mathrm{act}}}·\frac{T_{L,\mathrm{act}}}{T_{L,\mathrm{cal}}}}{R_{I\mathrm{ZG},\mathrm{act}}}}& \left(F14\right)\end{array}$

If a change in the thermal resistance can be neglected, it again follows that:

$\begin{array}{cc}{I}_{\mathrm{DC}\_\mathrm{ZG},\mathrm{act}}=\sqrt{\frac{I_{\mathrm{DC},\mathrm{RG},\mathrm{act}}^2·R_{I\mathrm{RG},\mathrm{act}}+\Delta P_{\mathrm{DC},\mathrm{cal}}·\frac{T_{L,\mathrm{act}}}{T_{L,\mathrm{cal}}}}{R_{I\mathrm{ZG},\mathrm{act}}}}& \left(F15\right)\end{array}$

It is commonly known that each laser light source circuit diagram can be replaced with an equivalent circuit, which comprises a laser emitter (active zone) and an internal resistance R_I connected in series thereto. As soon as a base current I_DC flows through the laser light source, a voltage U_L is present at the laser light source, which in part drops across the laser emitter as a partial voltage U_E and across the internal resistance R_I as a partial voltage U_RI, where U_E typically has a value of 0.9 to 1.1 V, depending on the laser type (having a wavelength close to that common in the telecom industry). In the case of WMS, additionally a modulation current I_AC is added to the base current; however, this does not affect this method since the temperature changes average out over time.

The instantaneous DC power at the position of the reference gas peak is calculated from the base current I_{DC_RG,act} flowing through the laser light source, and the voltage drop U_{RI_RG,act} occurring at the internal resistance R_{I_RG,act} as

P _{DC_RG} =I _{DC_RG,act} ·U _{RI_RG,act} =I _{DC_RG,act} ² ·R _{I RG,act}  (F 166)

The calibration of the optical measuring system takes place substantially according to the common method that is routine to the person skilled in the art by way of a known reference gas, which may also be a target gas, and the actual target gas itself. So as to establish the operating point of the laser light source, first the temperature of the temperature-stabilized laser light source is varied until the absorption signals for the reference and target gases are detected at a desired operating point. Thereafter, the power difference is ascertained and stored.

The system according to the invention is composed of a measuring system for carrying out the method, comprising a modulation device (4) for providing the base current I_DC for the laser light source, a receptacle for the measured gas, a light detector, and an evaluation unit (6) connected to the light detector and the modulation device, means for detecting the voltage present at the laser light source, means for detecting the laser base temperature, means for ascertaining the internal resistance of the laser light source, and means for controlling the base currents I_DC for the reference gas (RG) and the target gas (ZG). It is particularly preferred when the reference gas is disposed in the optical path of the laser in the sensor, so that the laser beam emitted by the laser light source first passes through the reference gas, and thereafter through the measured gas, to the light detector. This allows for extremely compact design of the entire measuring system and production of easy-to-handle and easy-to-transport gas detection devices.

The features and feature combinations mentioned above in the description, and the features and feature combinations mentioned hereafter in the description of the figures and/or shown only in the figures, can be used not only in the respective indicated combinations, but also in other combinations, or alone. It is not necessary for all the features recited in the claims to be implemented to carry out the invention. It is also possible to replace individual features of the independent or dependent claims with other disclosed features or feature combinations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described again in more detail hereafter based on the accompanying drawings. In the drawings, in schematic illustrations:

FIG. 1 shows an optical measuring system suitable for carrying out the method according to the invention;

FIG. 2 shows the equivalent circuit for the laser light source;

FIG. 3 shows a recorded voltage/current characteristic curve for determining the internal resistance of the laser light source;

FIG. 4 shows the wavelength λ_I as a function of the current in a schematic illustration of the target gas position and the reference gas position with a fixed current distance between the reference gas peak and the target gas peak, at a certain temperature, and the shift thereof due to temperature drift;

FIG. 5 shows a measuring curve for methane, serving as the reference gas and a further target gas, and ethane, serving as the target gas, at the target temperature (solid line) and after the drift (dotted line) caused by a change in the laser temperature;

FIG. 6 shows the distance error with respect to the target gas peak compared to the drift of the reference gas peak, measured using a fixed current difference and a fixed relative temperature difference based on an example; and

FIG. 7 shows a flow chart for fixing the wavelength distance to a reference wavelength.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically shows the basic design of an optical measuring system 1 for measuring the concentration of a target gas component ZG in a measured gas 2, based on wavelength modulation spectroscopy (WMS). The measuring system 1 comprises a wavelength-tunable temperature-stabilized laser light source 3, a modulation device 4, a light detector 5, and an electronic evaluation unit 6. The laser light source 3 emits a laser beam 7 of the wavelength λ_DC having a wavelength modulation amplitude Δλ_AC. The modulation device 4 periodically varies the wavelength of the laser light of the laser light source 3 by way of a reference absorption line and a target gas absorption line at an operating point and, at the same time, modulates the same in a triangular manner with a frequency (f) and a settable amplitude. This additionally comprises at least one DC and/or AC voltage source or a DC and AC current source 4a, and associated modulation means 4b for operating the laser light source 3. This can be used to variably set the respective base current I_DC and the modulation current I_AC. The modulation device 4 is connected directly to the laser light source 3. The light detector 5 detects the laser beam 7 originating from the laser light source 3 after this has passed through the measured gas 2, and generates a reception signal, which is dependent on the intensity of the laser light after it has passed through the measured gas 2, and is supplied to the evaluation unit 6. The evaluation unit 6 comprises means for the phase-sensitive demodulation of a measuring signal generated by the light detector 5 at the frequency (f) and/or one of the harmonics thereof. The evaluation unit 6 comprises two lock-in amplifiers 6a, 6b and a processing unit 6c. The processing unit 6c evaluates the demodulated reception signal of the light detector 5. Furthermore, an electrical connecting line 9 leads to the lock-in amplifier 6b by way of which the voltage present at the laser light source 3 is detected and the internal resistance is determined. As a function thereof, the control unit controls the base currents I_DC and modulation currents for the reference and target gases using the modulation device 4 so as to keep the wavelength distance between the reference gas and target gas constant. For this purpose, this comprises electrical control lines 8 and 10 to the modulation device 4. In the evaluation, primarily the above-described formulas F 11 or F 15 together with F 10 are used, among other things. Via the line 12, the Peltier temperature is transmitted to the control unit 6. In this exemplary embodiment, the reference gas RG is permanently present in the optical path during calibration. The target gas component ZG is then introduced into the measured gas chamber.

FIG. 2 shows the equivalent circuit for the laser light source 3. The laser light source 3 can thus be arithmetically replaced with a light emitter 3a, and an internal resistance R_I, 3b connected in series thereto. The laser light source 3 is operated in a current-modulated manner with a base current I_DC and a modulation current I_AC. Voltage U_L is present at the laser light source 3 and drops partially across the internal resistance 3b as a partial voltage U_RI and across the light emitter 3a as a partial voltage U_E.

FIG. 3 illustrates a current/voltage characteristic curve 10 recorded during the calibration of the optical measuring system 1 for determining the internal resistance R_{I RG} of the laser light source 3 at the location of the reference gas. The internal resistance R_{I RG} is determined from the relationship of the current/voltage characteristics of the laser light source 3 at the operating point 12 I_{DC_RG,cal} for RG. For this purpose, the current/voltage characteristic curve 10 (solid line) is provided with a linear approximation line 11 (dotted line) at the operating point 12 for determining the internal resistance R_{I RG} at the location of the reference gas. The slope of the approximation line 11 corresponds to the internal resistance R_I, 3b at the operating point 12 with R_{I RG}. An analogous procedure is applied at the location of the target gas for ascertaining R_{I ZG}.

In a schematic illustration, FIG. 4 shows, by way of example, the wavelength position of a target gas and that of a reference gas at a certain target temperature, wherein the two positions have a defined wavelength distance Δλ_DC with respect to one another, which via the DC tunability curve leads to a current distance ΔI. The absorption lines are shown with solid lines for the target temperature and with dotted lines for a drift induced by the reduced target temperature. The wavelength distance Δλ_DC is typically set by a constant current distance ΔI. Due to the non-linear DC tuning behavior of the laser, a shift (drift) of the operating point, for example as a result of the influence of the outside temperature of the sensor on the temperature stabilization or aging of the laser/electronics, results in a distance error ΔI_F for the position of the gas peak of the target gas GZ, resulting in measurement errors. The figure schematically shows the positions of the reference gas and the target gas at the time of calibration. Drift causes the position of the reference gas line to be shifted. Due to the non-linear relationship between the wavelength and the laser current, a difference, which is to say an incorrect wavelength having a wavelength distance error Δλ_F, that is, a distance error between the calculated and the actual target gas position, results for a fixed current distance ΔI. In the most favorable case, the error is small (typically <20 μA), and the sensor remains within the specification thereof in terms of the accuracy of the concentration. If the error grows larger, the error increases in a superlinear fashion. In the extreme case, the peak will no longer even be in the tuning range (for example, the target gas is no longer even measured in FIG. 4). The greater the distance between the reference gas and the target gas, the lower the allowed drift must be with this method; otherwise the distance error is >20 μA. The actual target gas position (associated base current) is at pos. 1, and the target gas position calculated via formulas F1 to F2 (associated base current) is at pos. 2. These two target gas positions deviate from one another by the distance error ΔI_F. It should be noted that the DC tunability curve itself is shown unimpaired by drift. In reality, the DC tunability curve itself can also change with long-term drift.

FIG. 5 shows, by way of example, a measuring curve, which is to say a spectrum for methane CH₄, having a lower peak, as the reference gas, and ethane C₂H₆, having a higher peak, as the target gas, at a certain target temperature (solid line G 1) and a temperature deviating from the target temperature (dotted line G 2). The deviation of line G 1 from line G 2 is caused by a change in the laser temperature and is intended to illustrate the influence of drift on the distance of the gas peaks. By way of example, the distance error ΔI_F is shown which results when, proceeding from a reference gas peak (here CH₄) having a fixed current distance, a target gas (ethane C₂H₆) is measured at a slightly different laser temperature. A shift in the reference gas peak by 0.96 mA results from the temperature reduction. It is apparent that the actual distance between the reference peak and the target gas peak has decreased from 1.669 mA to 1.568 mA. As was already mentioned above, this is caused by the non-linear DC tunability of the laser light source. A distance error ΔI_F of approximately 100 μA thus results. If the proposed method is applied using a fixed relative temperature change (formula (F 15)), the distance error ΔI_F is only 10 μA (FIG. 6). As a result, the target gas signal can be evaluated correctly.

FIG. 6 illustrates the dependence of the distance error ΔI_F on the drift values based on an example. This shows the distance error ΔI_F for the use of a fixed current difference according to formula (F 1) and a fixed relative temperature difference according to formula (F 15) for different drift values. The values based on formula F 1 are shown as dots P 1, and the values based on formula F 15 are shown as crosses P 2. It is apparent that, when using the proposed method, the distance error ΔI_F of the base current I_DC never exceeds 20 μA, which is sufficiently small to ensure that the measured concentration remains within the specifications.

FIG. 7 shows a flow chart for fixing the wavelength distance to a reference wavelength. In a first method step S1, which is to say during the calibration of the measuring system 1, firstly the DC base currents of the peak positions of the reference gas and the target gas are established and, secondly, the electrical DC power dropping across the internal resistance of the laser is ascertained for the two peak positions by determining the internal resistance of the laser at the respective current position. Thirdly, the power difference ΔP_DC,cal of the DC powers is ascertained and stored in the sensor.

In the subsequent second method step S2, which is to say during operation of the measuring system 1, initially the electrical DC power dropping across the internal resistance of the laser is ascertained for the instantaneous peak position of the reference gas by measuring the internal resistance using lock-in technology, then the instantaneous electrical DC power at the peak position for the target gas is calculated by adding the instantaneous DC power for the reference gas peak and the DC power difference between the reference gas and the target gas ascertained during calibration. Thereafter, the DC current for the peak position thereof is ascertained, wherein according to formula F 11 or F 15 the internal resistance of the target gas from the preceding measurement is used for this purpose. If F 15 is used, additionally the ratio of the instantaneous laser temperature to the calibration laser temperature is used. The laser temperature is stored at the time of calibration.

In the subsequent third method step S3, the actual measuring scan for the target gas is carried out based on the ascertained peak position for the target gas, and the gas concentration is ascertained therefrom.

The method steps S2 and S3 are preferably carried out multiple times in a loop, wherein the base current I_{DC_ZG} for the target gas position can be adapted between the runs, if the instantaneous base current I_{DC_ZG, act} deviates from the ideal base current I_{DC_ZG, cal}, which was ascertained during the calibration of the optical measuring system.

Claims

1. A method for operating an optical measuring system comprising a wavelength-tunable temperature-stabilized laser light source for measuring the concentration of a target gas component in a measured gas, an instantaneous base current IDC_ZG,act corresponding to a wavelength λZG of a target gas absorption line being set so that a wavelength distance ΔλDC defined during calibration between a target gas absorption line for a target gas component and a reference gas absorption line for a reference gas component is maintained, wherein during operation, maintaining a relative temperature difference in the laser light source, defined in advance during the calibration, between the operating points selected at the time of calibration of the reference gas, with a base current IDC_RG,cal, and the target gas component, with a base current IDC_ZG,cal, by way of determining the necessary instantaneous base current IDC_ZG,act for the target gas component, as a function of an instantaneous base current IDC_RG,act for the reference gas.

2. The method according to claim 1, wherein carrying out the calibration of the measuring system with a reference gas and a target gas, and during calibration; establishing the base currents IDC_RG,cal and IDC_ZG,cal assigned to the gas absorption lines for the reference gas and for the target gas component; anddetermining the associated electrical powers PDC_RG,cal and PDC_ZG,cal of the laser light source, finding a power difference ΔPDC,cal therefrom, and storing this and/or one or more equivalent variables in the measuring system,and, during operation of the measuring system:ascertaining the instantaneous electrical power PDC_RG,act of the laser source assigned to the gas absorption line for the reference gas; anddetermining the instantaneous electrical power PDC_ZG,act of the laser source assigned to the gas absorption line for the target gas as the sum from the instantaneous electrical power PDC_RG,act and the power difference ΔPDC, cal, and calculating the assigned instantaneous base current IDC_ZG,act from the instantaneous electrical power PDC_ZG,act of the laser source thus determined.

3. The method according to claim 2, wherein during operation, measuring the concentration of the target gas component using the base current IDC_ZG,cal calculated from the power PDC_RG,act and the power difference ΔPDC, cal, and ascertaining the gas concentration of the target gas component in the measured gas therefrom.

4. The method according to claim 2, wherein during operation of the measuring system, taking into consideration changes in a base laser temperature TL,cal of the laser light source by adapting the power difference ΔPDC, cal.

5. A method according to claim 1, wherein determining the internal resistance RI of the laser light source of the reference gas and the target gas from the respective slope of a voltage/current characteristic of the laser light source associated with the instantaneous base current IDC_RG,act or IDC_ZG,act.

6. A measuring system for carrying out the method according to claim 1, comprising: a modulation device for providing the base current IDC for the laser light source; a receptacle for the measured gas; a light detector, and an evaluation unit connected to the light detector and the modulation device; means for detecting the voltage present at the laser light source; means for detecting the laser base temperature; means for ascertaining the internal resistance of the laser light source, and means for controlling the base currents IDC for the reference gas and the target gas.