WO2012171786A1

WO2012171786A1 - Automatic identification of the wavelength of a laser

Info

Publication number: WO2012171786A1
Application number: PCT/EP2012/060016
Authority: WO
Inventors: Alexander Hartmann; Rainer Strzoda
Original assignee: Siemens Aktiengesellschaft
Priority date: 2011-06-16
Filing date: 2012-05-29
Publication date: 2012-12-20
Also published as: DE102011077634A1

Abstract

The invention relates to a spectrometer construction for wavelength modulation spectroscopy, in which a temperature ramp is applied in order to calibrate the wavelength of the laser, an absorption spectrum of the material is recorded as the temperature ramp is applied, at least two absorption lines are detected in the absorption spectrum, and the wavelength is calibrated by automatic comparison of the detected absorption lines with known absorption lines of the material.

Description

description

Automatic identification of the wavelength of a laser The invention relates to a spectrometer assembly for Wel ^¬ lenlängen-modulation spectroscopy with a tunable laser, a detector device for receiving the laser light after passage through a to be measured material, as well as egg ^¬ ner control device for laser and detector. The invention further relates to a method for wavelength calibration of the laser in a spectrometer setup for wavelength modulation spectroscopy.

For the measurement of gases by means of laser spectroscopy, it is necessary to change the wavelength of the laser to a known one

Einzu absorption line in the spectrum of the gas to be measured ask ^¬. Since in many spectra not a single Absorpti ^¬ onslinie occurs, but many different, under Umstän ^¬ also caused by different gases absorption lines, the appropriate absorption for measurement in the sensor system must be set first.

This setting, ie the calibration of the laser, was previously done manually. For this purpose, a spectrum was recorded and then compared by eye with theoretical known spectra. If a match of lines was found, the correct laser current or the correct laser temperature could be set. It is an object of the present invention to provide a spectrometer structure for laser spectroscopy, in which a calibration of the laser wavelength is made possible in an automated manner. It is another object of the present invention to provide a corresponding method for calibration.

The object is with respect to the spectrometer structure by a spectrometer structure having the features of claim 1 solved. As regards the method, there is a solution in a method having the features of claim 9.

The spectrometer setup according to the invention for wavelength modulation spectroscopy comprises a tunable laser and a device for adjusting the temperature of the laser. Furthermore, the spectrometer assembly comprises a Detek ^¬ gate means for receiving the laser light after passing through a material to be measured. Finally, the spectrometer design comprises a control device for controlling the laser and detector device and device for temperature adjustment.

The control means is arranged, at least the following steps auszufüh ^¬ ren for calibrating the wavelength of the laser:

Control of the temperature adjustment device such that a temperature ramp is passed through,

- Recording of the absorption spectrum of the material at

Traversing the temperature ramp,

Determination of at least two absorption lines in the absorption spectrum and

Calibration of the wavelength by comparison of the determined absorption lines with known absorption lines of the material.

The process for wavelength calibration of a laser in a spectrometer assembly for Wellenlän ^¬ genmodulationsspektroskopie comprising the steps of:

Passing through a temperature ramp for the laser,

- Recording of the absorption spectrum of the material at

Traversing the temperature ramp,

Determining at least two absorption lines in the absorption spectrum, and

Calibration of the wavelength of the laser by comparison of the determined absorption lines with known absorption lines of the material. Calibration is understood here to mean that, after the calibration has been carried out, it is known which wavelength the laser emits at room temperature or another temperature, or an analogous quantity.

The automatic identification of absorption lines and associated calibration offers several advantages. For egg ^¬ nen eliminates a corresponding manual step before a measurement. Furthermore, a possibly complex measurement of the wavelength of the laser by the manufacturer, for example by means of interferometer, is also eliminated. Advantageously, by the invention, the laser directly in the terminal, so in the spectrometer structure, on their wavelength and thus also ^¬ if tested for usability.

Another advantage is that a wider selection of lasers produced can be used. This is due to fluctuations in the laser intensity and also in the wavelength emitted at room temperature during the production of lasers. The fluctuations of the wavelength can be quite a few nanometers at a target wavelength of, for example, 760 nm. If one requires the laser manufacturer to supply only lasers which emit exactly at a predetermined wavelength, then this may only use a few percent of the lasers produced. By identifying the absorption lines in the spectrum and, for example, by preselecting several suitable absorption lines for a measurement, the exact wavelength emitted at room temperature is less important and significantly more lasers can be used. As a result, the price of a laser is significantly ^¬ Lich reduced.

Another advantage, which also occurs in the mass use ge to Ta, results from the automatic Linienidentifi ^¬ cation and is that the sensors must not be manually set to the correct absorption line, which in turn would require the use of expert personnel.

In a preferred embodiment and further development of the invention, after at least two absorption lines have been determined, a comparison of the temperature intervals found between the absorption lines is carried out with known intervals. The known intervals are present at ^¬ play as a database as the database HITRAN entnom- men. Here, it is considered preferable that the waves ^¬ change in length of the laser is proportional to the temperature change, and thus, the wavelength spacing between the Absorpti ^¬ onslinien to be taken for example from a database is proportional to the measured temperature gaps. By normalization, the distances can be compared with each other.

If the theoretical distances match the measured distances, then, according to an embodiment of the invention, the theoretical heights, ie intensities of the absorption lines, can then be calculated from the database and, in turn, compared with the measured strengths with the aid of normalization. Even though these line strengths coincide with those theoretically known in a certain error range, the wavelength of the laser is uniquely identified.

In a preferred embodiment of the invention, and a first stamp is first ^¬ raturwert and driven by drive therefrom the temperature ramp to a second temperature value at the temperature ramp. In other words, an increasing or decreasing temperature ramp will therefore pass through. A temperature minimum and as the second temperature value is a temperature ^¬ turmaximum is preferred as the first temperature value is used. In other words, a rising temperature ramp is preferably passed through.

According to a further preferred embodiment of the invention, at the beginning of the temperature ramp, a noise value for the Measurement spectrometer setup determined. This makes it possible in a ^¬ be Sonder preferred embodiment of the invention, and to consider only such in determining the absorption lines whose strength is greater than the noise value calculated. This increases the accuracy of the measurement and the safety of the calibration, as a recording of false absorption lines is avoided.

In order to achieve a further increase in accuracy and thus to avoid calibration errors, at least three absorption lines for the calibration are determined and taken into account. This allows two temperature intervals and three line weights to be used for the calibration. A preferred, but by no means limitative exporting ^¬ approximately example of the invention will now be further explained with reference to Figu ^¬ ren the drawing. The features are shown schematically. FIG. 1 shows a measuring setup for laser spectroscopy,

2 shows a temperature curve for the laser of Messauf ^¬ construction,

Figure 3 is a measured spectrum. FIG. 1 shows a measuring structure 10 of the exemplary embodiment. The measuring structure 10 includes a laser diode 11, a detector 12 and a control device 13. The laser diode 11, the detector 12 and the control device 13 are arranged in a common housing and connected to each other, wherein the housing is not shown in Figure 1. The detector 12 and the laser diode 11 are aligned with each other such that the light of the laser diode 11 after passing through a gas to be measured 14 at least partially incident on the detector 12. To set the temperature of the laser diode 11, a Peltier element 15 is connected to the laser diode 11. The Peltier element 15 is likewise controlled by the control ^device 13. The following describes how the exact Wellenlängenla ^¬ ge of the laser diode 11 is determined will be shown. The laser diode 11 of the embodiment emits in the range of 760 nm wave length ^¬, the exact emission wavelength is not known. In order to now the exact emission wavelength able to ermit ^¬ stuffs, the spectrometer assembly 10 controlled by the control device 13 passes through a calibration cycle.

For this purpose, a temperature curve shown in Figure 2 is traversed. Before the start of the calibration cycle, the

Laser diode 11 an initial temperature 21, for example, room temperature ^¬ . In order to be able to travel through the widest possible range with the temperature ramp, the temperature of the laser diode 11 is lowered in a first step to a minimum temperature 22. In this exemplary embodiment, the minimum temperature is determined by the performance of the pelletizing element 15. The power limit of the Peltier element 15 is determined by decreasing the negative slope of the temperature. Meanwhile, the maximum value of the system noise is picked up by the controller 13 and stored for later use.

Once the minimum temperature 22 has been reached, the positive temperature ramp begins. In this case, the temperature of the laser diode 11 is increased up to a maximum temperature 23. The Ma ^¬ ximaltemperatur 23 is in turn determined by the power limit of the Peltier element 15 in this embodiment. As before, this limit can be determined by a decreasing slope of the temperature ramp.

When passing through the temperature ^ramp from the minimum temperature 22 to the maximum temperature 23, the laser diode 11 and the detector 12 operate and the spectrum is recorded by the control device 13 and examined for absorption lines. In the exemplary embodiment to the second harmonized ^¬ specific spectrum is examined. FIG. 3 schematically shows a section of such a second harmonic spectrum. In the exemplary embodiment, the spectrum is first examined for local maxima 31, 33. For this purpose, examines the Steuerein ^¬ direction 13 continuously, the resulting values of Spekt ^¬ rums. If such a local maximum 31, 33 is found, then it is checked whether there are also local minima 32, 34 at respectively lower and higher temperature. These local minima 32, 34 to a local maximum 31, 33 are around cha ^¬ rakteristisch for absorption lines in the second harmonic form. Becomes a local maximum 31, 33 each have a minimum 32, 34 found in smaller and larger temperature, so it is an absorption line vomit ^¬ is chert.

Once the temperature ramp has passed through, the actual identification of the wavelength position begins with the determined absorption lines. In this case, it is advantageous on the basis of the scope of the theoretically known data for absorption lines if a restricted absorption line set is selected in advance, which is suitable for the present measurement task. This restricted selected data set is compared with the measured absorption lines. For this purpose, the comparison of the temperature intervals 35 between the local maxima 31, 33 and the theoretically known ones is started. If at least three absorption lines have been determined, at least two such temperature intervals 35 are available for the comparison. In the comparison it is determined whether the sequence of the temperature intervals 35 corresponds to one another of a theoretically known sequence in the selected data set.

Is now a range found in which the sequence of temperature intervals 35 corresponds to the theoretical temperature intervals so be vergli ^¬ chen next line heights. For this purpose, the theoretical heights are calculated from the theoretically known data of the database and then compared with the measured heights by means of a normalization. If the line heights also agree in a certain error frame, the given lines are identifiable. graces and thus the wavelength of the laser as a function of its temperature clearly known. Thus it can be driven 24 is a ge ^¬ desired wavelength of the laser, which is located at a certain absorption line from ^¬, by choosing a suitable temperature.

Claims

claims

1. spectrometer structure (10) for the wavelength modulation spectroscopy with

a tunable laser (11),

a device (15) for adjusting the temperature of the laser (11),

- a detector device (12) for receiving the laser light after passing through a material to be measured (14), - a control device (13),

characterized in that the control device (13) is made ^¬ designed for calibration of the wavelength of the laser (11) perform at least the following steps:

Controlling the device (15) for temperature adjustment so that a temperature ramp is passed through,

Recording the absorption spectrum of the material (14) when passing through the temperature ramp,

Determination of at least two absorption lines in the absorption spectrum,

A spectrometer assembly (10) according to claim 1, wherein the comparing step comprises first a comparison of temperature intervals (35) between detected absorption lines having known temperature intervals.

3. spectrometer structure (10) according to claim 2, ^wherein the comparison ^step after the comparison of Temperaturinterval ^¬ len (35) comprises a comparison of the strength of the absorption lines with known strengths.

4. spectrometer structure (10) according to one of the preceding arrival claims, in which the control of the means (15) for temperature adjustment is so audible that first a first temperature value (22) is driven and from there the Temperature ramp to a second temperature ^value (23) by ^¬ drive.

5. spectrometer structure (10) according to one of the preceding arrival proverbs, wherein the first temperature ^value (22) is a temperature ^¬ minimum (22) and the second temperature value (23) tem ^¬ ture maximum (23).

6. Spectrometer structure (10) according to one of the preceding arrival claims, in which the control device (13) is configured, at the beginning of the temperature ramp, a noise value for the

To determine the spectrometer structure (10) and to consider only those in the determination of the absorption lines, where the strength is greater than the noise value.

7. spectrometer assembly (10) according to one of the preceding claims ^¬, wherein the control means (13) is configured to determine at least three absorption lines for the calibration and use.

8. spectrometer assembly (10) according to one of the preceding claims ^¬, wherein the control means (13) is configured to use the second harmonic spectrum, and to iden ^¬ development of an absorption line first a local maximum

Is determined (31,33) is determined of the spectrum, then at a ermit ^¬ telten local maximum (31,33), whether associated local minima (32,34) to the local maximum (31,33) are provided around, and only in the presence of the local minima (32,34), the found local maximum (31,33) is stored as an absorption line.

9. A process for wavelength calibration of a laser (11) in a spectrometer assembly (10) for the Wellenlängenmodula ^¬ absorption spectroscopy comprising the steps of:

Passing through a temperature ramp for the laser (11),

Recording the absorption spectrum of a material (14) when passing through the temperature ramp, Determination of at least two absorption lines in the absorption spectrum,

- Calibration of the wavelength of the laser (11) by Ver ^¬ equal to the determined absorption lines with known absorption lines of the material.

10. The method according to claim 9, wherein in the comparison step first a comparison of temperature intervals (35) between determined absorption lines with known temperature intervals is performed, and then a comparison of the strength of the absorption lines with known starches.

11. The method according to claim 9 or 10, wherein first a first temperature value (22) for the laser (11) is driven and from there the temperature ramp is passed to a second temperature value (23).

12. The method according to any one of claims 9 to 11, wherein a temperature minimum (22) is used as the first temperature value (22) and a temperature maximum (23) is used as the second temperature value (23).

13. The method according to any one of claims 9 to 12, wherein at the beginning of the temperature ramp, a noise value for the spectrometer construction (10) is determined, and in determining the

Absorption lines are considered only those in which the strength is greater than the noise value.

14. The method according to any one of claims 9 to 13, wherein at least three absorption lines for the calibration are determined and used.

15. The method according to any one of claims 9 to 14, wherein the second harmonic spectrum is used and to determine an absorption line first a local maximum (31,33) of the spectrum is determined, then determined at a determined local maximum (31,33) whether there are associated local minima (32,34) around the local maximum (31,33), and only in the presence of local minima (32,34) is the found local maximum (31,33) as the absorption line vomit ^¬ chert.