DE102018117642B4 - System for wavelength-dependent spectroscopy and method for evaluating measurement signals in wavelength-modulated spectroscopy - Google Patents

Abstract

System (10) for wavelength-modulated spectroscopy, in particular for absorption and emission spectroscopy, comprising a signal evaluation device (12) with a signal input (58), a mixing device (56) and a local oscillator (42) for generating a time-dependent local oscillator signal which Mixing device (56) is coupled to the signal input (58) and the local oscillator (42) for generating a time-dependent demodulation signal by mixing a time-dependent input signal present at the signal input (58) and the time-dependent local oscillator signal, characterized in that the Local oscillator (42) is designed to generate the time-dependent local oscillator signal in the form of a time-dependent trigonometric function, which is formed by additive superposition of at least two time-dependent trigonometric functions, and that the local oscillator (42) is designed to specify the amplitude and the phase of each the M at least two time-dependent trigonometric functions.

Description

The present invention relates to a system for wavelength-modulated spectroscopy, in particular for absorption and emission spectroscopy, comprising a signal evaluation device with a signal input, a mixing device and a local oscillator for generating a time-dependent local oscillator signal, which mixing device is coupled to the signal output and the local oscillator for Generating a time-dependent demodulation signal by mixing a time-dependent input signal present at the signal input and the time-dependent local oscillator signal.

Furthermore, the present invention relates to a method for evaluating measurement signals, in particular in wavelength-modulated spectroscopy, more particularly in absorption and emission spectroscopy, in which method a time-dependent local oscillator signal is generated and mixed with a time-dependent input signal to generate a time-dependent demodulation -Signals.

Systems and methods of the type described above are used in particular in the frequency conversion of measurement or useful signals in order to convert the useful signals into a more suitable frequency range for further processing. In this case, the useful signal, also referred to below as the input signal or measurement signal, is mixed with a local oscillator signal in a mixing device, also referred to below as the mixer. The result of the mixing is referred to as an intermediate frequency or demodulation signal. The mixing can take place in particular with a demodulator or mixer.

If not just a single frequency is to be converted, but several frequencies at the same time, the local oscillator signal must also contain several frequency components. The frequency conversion then takes place again with a mixing device.

From the US 5,973,782A it is known to use a pulse width modulated signal, also referred to below as a PWM signal, as the local oscillator signal. A PWM signal is a square-wave signal whose mark-to-space ratio of a period is variable. According to Fourier, such a square-wave signal can be represented by superimposing an infinite number of sinusoidal oscillations with frequencies which correspond to an even multiple of the PWM basic frequency. These sine waves are called harmonics of the fundamental. The fundamental wave has the frequency of the actual square-wave signal. The amplitude and phase of each harmonic depends on the mark-to-space ratio of the PWM signal. If a PWM signal is used as the local oscillator signal, the mixer carries out a frequency conversion with the input signal for each frequency contained in the local oscillator signal.

Since the PWM signal always contains an infinite number of frequency components, an infinite number of frequencies are always mixed and converted when the local oscillator signal is mixed with the input signal.

In the US 5,047,639A a concentration detector is described. Wavelength modulated spectroscopy with detection of a plurality of harmonic frequency components is from US 6,356,350 B1 known.

It is therefore an object of the invention to improve the evaluation of input signals when using mixing devices.

In a system of the type described above, this object is achieved according to the invention in that the local oscillator is designed to generate the time-dependent local oscillator signal in the form of a time-dependent trigonometric function which is formed by additively superimposing at least two time-dependent trigonometric functions.

The solution proposed according to the invention makes it possible to design the local oscillator signal freely. In particular, the at least two time-dependent trigonometric functions can be specified as desired, for example as sine or cosine functions. As a result, a precisely defined local oscillator signal with very specific frequencies can be generated for the frequency conversion with the mixing device, for example the fundamental oscillation and only a certain number of harmonics or harmonics, and in particular also with any desired, i.e. in particular freely selectable or adapted to the respective task , amplitude and phase shifts. This has significant advantages over a PWM signal as a local oscillator signal, because with a PWM signal, the Amplitudes of the harmonics can only be set globally by a single specification, namely the mark-to-space ratio. The amplitude values of the harmonics are evenly scaled by the value of the square-wave amplitude. If individual harmonics are to be suppressed, additional filters must be used. The proposed development through optional specification of the local oscillator signal through the additive superimposition of the at least two time-dependent trigonometric functions makes it possible, in particular, to dispense with additional filters. The superposition of at least two time-dependent trigonometric functions is not to be understood here as a physically additive superposition, but rather as a mathematically theoretical addition as in a Fourier series. The local oscillator signal can be preset in a simple manner, so that the local oscillator is designed to preset the amplitude of each of the at least two time-dependent trigonometric functions. In particular, this makes it possible to specify not only the amplitude of the fundamental wave, but also the amplitudes of all the harmonics in a desired manner, namely, in particular, independently of one another. In particular, the amplitudes of all harmonics can be specified identically. Amplitudes of harmonics can be specified in a targeted manner without influencing the amplitudes of the other harmonics, which is always the case with a local oscillator signal in the form of a PWM signal. According to the invention, the local oscillator is designed to specify the phase of each of the at least two time-dependent trigonometric functions. This allows the phase angles between the fundamental wave and the harmonics of the local oscillator signal to be adjusted in any way. As described, this is not possible with a PWM signal. Only the phase angles and amplitudes of all harmonics can be relatively specified there, because the mark-to-space ratio of the PWM signal determines the amplitudes and phase angles of the individual frequency components.

It is advantageous if the local oscillator is designed to generate the at least two trigonometric functions in the form of sine or cosine functions. Sine and cosine functions can be generated in a simple manner, in particular with a function generator. In particular, their amplitudes, frequencies and phase angles relative to one another can be specified easily and precisely. Two trigonometric functions in the form of sine or cosine functions can be generated in particular by an oscillator or function generator with two outputs.

In order to simplify the processing of the input signal and also the time-dependent demodulation signal, it is advantageous if the local oscillator is designed to generate the time-dependent local oscillator signal corresponding to the additive superimposition of a maximum of 10 trigonometric functions. In particular, the additive superimposition can also be limited to a maximum of five trigonometric functions. In this way, time-dependent input signals can be converted into frequency-dependent signals with finite computing effort. This is particularly advantageous for wavelength-modulated spectroscopy, in particular absorption and emission spectroscopy.

The input signal can be easily mixed with the local oscillator signal if the local oscillator is designed to generate the at least two trigonometric functions with a demodulation frequency that corresponds to an integer multiple of a modulation frequency with which the input signal is modulated. This is a simple way of converting harmonics in the input signal into the desired frequency range.

It is advantageous if the local oscillator is designed to generate the at least two trigonometric functions with a demodulation frequency that corresponds to an integer multiple of a modulation frequency increased or decreased by a frequency value. In particular, this makes it possible to evaluate harmonics of the input signal separately using heterodyne measurement technology. Each spectral component of the input signal contains additional information about the course of an absorption curve in the frequency domain, for example. In this way, a frequency-dependent absorption spectrum or emission spectrum can be calculated from a time-dependent input signal. This is achieved in particular by selecting an unequal demodulation frequency for the modulation frequency, for example of an excitation light source, which differs from the modulation frequency by an integer multiple of the frequency value. The frequency value forms a sort of frequency shift between the modulation frequency and the demodulation frequency. Spectral components of the demodulation signal thus represent the harmonics of the input signal, for example from a radiation detector, in the high-frequency range and are separated from one another by the frequency offset, which corresponds to the specified frequency value.

A local oscillator signal can be specified in a simple manner if the local oscillator is designed in the form of a first function generator.

In order to be able to operate optical spectroscopy with the system, it is advantageous if it includes a spectroscopy device for measuring a time-dependent spectroscopy signal. The spectroscopy signal can, in particular, be an emission signal or an absorption signal of a solid, liquid or gaseous sample which is exposed to excitation light from an excitation light source, for example. In particular, a transmission and thus indirectly an absorption of the sample can be measured in this way.

The spectroscopy device preferably comprises an excitation light source for generating electromagnetic excitation radiation. In particular, absorption and emission spectra can be measured in this way. It is favorable if the excitation light source is designed in the form of an excitation laser. In this way, a sample can be excited in a very narrow band and its transmission or absorption can be measured.

The spectroscopy device can be designed in a simple and cost-effective manner if the excitation laser is designed in the form of a laser diode. In particular, the excitation radiation can be modulated in a simple manner with a corresponding laser diode driver, which modulates the wavelength or frequency of the laser radiation emitted by the laser diode as a function of the voltage applied to it. A tunable laser and an excitation light source that can be tuned as a function of the wavelength can thus be formed in a simple manner.

It is favorable if the spectroscopy device comprises a modulation device for generating a modulation frequency and if the modulation device is arranged and designed to interact with the excitation light source for modulating a wavelength of the excitation radiation with the modulation frequency. This configuration makes it possible, in particular, to measure an absorption or emission of a sample in a time-resolved manner, with the wavelength of the excitation radiation being changed continuously by the modulation frequency. In this way, spectroscopy signals can be generated over time and, for example, converted into the frequency space with the evaluation device in order to obtain frequency-dependent emission or absorption spectra.

The spectroscopy device can be designed in a simple manner if the modulation device is designed in the form of a second function generator. In particular, the second function generator can be synchronized with the first function generator via a common signal source.

According to a further preferred embodiment of the invention, it can be provided that the spectroscopy device comprises a sample cell, in particular in the form of an absorption or emission cell. In particular, the sample cell can be arranged and designed in such a way that the excitation radiation generated by the excitation light source can be applied to it. A sample cell can in particular be understood to mean any type of holder or container with which a sample, which can be gaseous, solid or liquid, can be positioned in a defined manner in order to expose it to the excitation radiation.

It is advantageous if the spectroscopy device comprises a detector and if the detector is arranged in such a way that radiation emitted or transmitted by a sample can be detected. In particular, the detector can be in the form of an optical detector. Furthermore, the sample can be arranged in particular in the sample cell. The detector can be designed in particular to detect radiation emitted and transmitted by the sample in a time-resolved manner. In particular, the detector is designed in such a way that it has an upper limit frequency, which corresponds to a time resolution of the detector, which is greater than the modulation frequency with which the wavelength of the excitation radiation is modulated.

It is advantageous if the detector has a signal output and is designed to generate a detector signal which can be output in particular at the signal output. The detector signal can then be further processed in a particularly simple manner, for example mixed with the local oscillator signal.

The system can be designed simply and inexpensively if the detector is designed in the form of a photodiode.

In order to improve the quality of the detector signal and in particular to improve the signal-to-noise ratio, it is advantageous if an amplification device is connected downstream of the detector or if the detector comprises an amplification device and if the amplification device is designed to amplify the detector signal.

In order to be able to evaluate the detector signal in a simple manner, it is favorable if the signal output of the detector is coupled to the signal input of the evaluation device. In particular, the detector signal can be transferred directly to the evaluation device.

According to a further preferred embodiment of the invention, it can be provided that the evaluation device comprises a signal processing device for processing the demodulation signal and a signal output device for outputting the demodulation signal. In particular, the evaluation device can be designed to convert the demodulation signal from the time domain to the frequency domain, for example by a Fourier transformation. The signal output device can thus, for example, output optical spectra as a function of wavelength or as a function of frequency. The signal processing device can in particular be designed in the form of a computer or include one. The signal output device can be in the form of a screen, a data memory or a printer, for example, or can include a screen and/or a data memory and/or a printer.

The object stated at the outset is also achieved according to the invention in a method of the type described at the outset in that the time-dependent local oscillator signal is generated by additive superimposition of at least two time-dependent trigonometric functions.

The at least two time-dependent trigonometric functions form a fundamental and at least one harmonic of the fundamental. The number of the desired harmonics can be specified in a simple manner by the number of time-dependent trigonometric functions. In this way, a frequency spectrum of the local oscillator signal can be configured in a practically random manner.

The local oscillator signal then does not contain any frequencies or harmonics that are not explicitly included in the additive time-dependent superimposition of the trigonometric functions. In this way, frequencies and consequently harmonics can be specifically eliminated if they are not desired for mixing with the time-dependent input signal. In this way, the evaluation of the input signal can be specifically simplified and thereby improved. In addition, filters can be dispensed with. If the local oscillator signal is formed by a PWM signal, harmonics can only be eliminated by using so-called notch filters in a targeted manner in order to filter out certain frequencies, i.e. to suppress them as completely as possible. However, these filters have the disadvantage that they undesirably affect all frequency components, in particular other harmonics that are not to be filtered out. It is favorable that the amplitudes of each of the at least two time-dependent trigonometric functions are specified. In particular, any desired amplitude ratios can be set, or all harmonics can be specified with the same amplitude. Furthermore, it is favorable that the phases of each of the at least two time-dependent trigonometric functions are specified. In particular, the phases can be specified in the form of phase shifts relative to a fundamental frequency that defines one of the at least two time-dependent trigonometric functions. In this way, the phase angles of individual harmonics can be adjusted relative to the fundamental and relative to one another.

Time-dependent trigonometric functions can be additively superimposed in a simple manner if the at least two time-dependent trigonometric functions are generated in the form of sine or cosine functions. In particular, cosine functions have the advantage that they are axisymmetric.

In order to simplify the evaluation of the input signal, it is advantageous if the time-dependent local oscillator signal is generated by additively superimposing a maximum of 10 trigonometric functions. In particular, it can be advantageous if a maximum of five trigonometric functions are additively superimposed. This then means in particular that a fundamental wave and four harmonics are superimposed to form the time-dependent local oscillator signal.

It is advantageous if the at least two trigonometric functions are generated with a demodulation frequency that corresponds to an integer multiple of a modulation frequency with which the input signal is modulated. In this way, the input signal can easily be converted into a desired frequency range.

In order to realize a frequency offset of the local oscillator signal to the input signal, which is modulated with the modulation frequency, which is necessary for heterodyne measurement technology, it is favorable if the at least two trigonometric functions are generated with a demodulation frequency that is an integer multiple of a frequency value increased or reduced modulation frequency corresponds. This frequency value then forms the frequency offset between the local oscillator signal and the modulated input signal.

The time-dependent local oscillator signal can be generated in a simple manner using a first function generator.

It is advantageous if a time-dependent spectroscopy signal is measured, which forms the input signal. In this way, the method can be used to evaluate spectroscopy signals. However, the input signal can also be any other measurement or useful signal.

In order in particular to be able to measure transmissions, absorptions or emissions of a sample, it is advantageous if electromagnetic excitation radiation is generated to excite a sample to be examined.

The wavelength of the electromagnetic excitation radiation is preferably modulated with a modulation frequency. In particular, this makes it possible to operate wavelength-modulated spectroscopy. A sample can be exposed to excitation radiation of different wavelengths in a time-dependent manner, so that a time-dependent spectroscopy signal that is dependent on the wavelength of the excitation radiation is generated, which can then be evaluated to determine the optical properties of the sample, for example its refractive index or the shape of the absorption curve . Since pressure and temperature in particular influence the refractive index and/or the shape of the absorption curve of the sample, the pressure and temperature in the sample cell can be determined indirectly from the measured optical properties. The time-dependent modulated excitation radiation can be used, for example, to generate a time-dependent spectroscopy signal of a sample. As described, this can then be converted from the time period into the frequency domain in order to obtain wavelength-dependent or frequency-dependent optical spectra of the sample.

Particularly narrow-band excitations of the sample are possible if the electromagnetic excitation radiation is generated in the form of laser radiation.

The modulation frequency can be easily generated with a second function generator. In particular, the second function generator can be synchronized with the first function generator via a common signal source.

According to a further preferred variant of the method according to the invention, it can be provided that the sample is exposed to the wavelength-modulated excitation radiation and that a transmission of the excitation radiation or an emission of the sample exposed to the excitation radiation is measured as a function of time. In particular, this procedure makes it possible to obtain wavelength-modulated spectra of an optically active sample, for example a gas, a liquid or a solid. If necessary, the time-dependent measured transmissions or emissions can be converted from the period to the frequency domain in order to obtain wavelength- or frequency-dependent transmission or emission spectra of the sample.

It is advantageous if the transmission or emission is measured as a detector signal with a detector as a function of time. In particular, such a measurement can be carried out using an optical detector. As described, if the modulation frequency with which the excitation radiation is modulated is known, the time-dependent detector signal can be used to evaluate the time-dependent spectra and to convert them from the time period into the frequency domain.

The time-dependent detector signal is advantageously generated with the detector from the time-dependent spectroscopy signal. In particular, the time-varying radiation can be measured with the detector and a current or voltage signal can be provided by the detector as a time-dependent detector signal.

Time-dependent optical spectra can be recorded in a simple manner if the detector is provided in the form of a photodiode.

In order to improve a signal-to-noise ratio of the detector signal, it is advantageous if the time-dependent detector signal is amplified.

In order to evaluate the spectroscopy signal, it is favorable if the time-dependent detector signal is used as the time-dependent input signal. The detector signal can then serve as a basis for an evaluation, in particular a conversion of the time-dependent detector signal into a wavelength- or frequency-dependent evaluation signal or demodulation signal.

In order in particular to obtain desired optical spectra for a sample as a function of wavelength or frequency, it is advantageous if the demodulation signal is processed and output in analog and/or digital form. With such a method, samples can be examined in the desired manner and characterized with regard to their optical properties.

1. 1. System (10) für die wellenlängenmodulierte Spektroskopie, insbesondere für die Absorptions- und Emissionsspektroskopie, umfassend eine Signalauswerteeinrichtung (12) mit einem Signaleingang (58), eine Mischeinrichtung (56) und einen Lokaloszillator (42) zum Erzeugen eines zeitabhängigen Lokaloszillator-Signals, welche Mischeinrichtung (56) mit dem Signaleingang (58) und dem Lokaloszillator (42) gekoppelt ist zum Erzeugen eines zeitabhängigen Demodulations-Signals durch Mischen eines am Signaleingang (58) anliegenden zeitabhängigen Eingangs-Signals und des zeitabhängigen Lokaloszillator-Signals, dadurch gekennzeichnet, dass der Lokaloszillator ausgebildet (42) ist zum Erzeugen des zeitabhängigen Lokaloszillator-Signals in Form einer zeitabhängigen trigonometrischen Funktion, welche durch additive Überlagerung von mindestens zwei zeitabhängigen trigonometrischen Funktionen gebildet ist.
2. 2. System nach Satz 1, dadurch gekennzeichnet, dass der Lokaloszillator (42) ausgebildet ist zum Erzeugen der mindestens zwei trigonometrischen Funktionen in Form von Sinus- oder Kosinusfunktionen.
3. 3. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass der Lokaloszillator (42) ausgebildet ist zum Erzeugen des zeitabhängigen Lokaloszillator-Signals entsprechend der additiven Überlagerung von maximal 10 trigonometrischen Funktionen, insbesondere von maximal fünf trigonometrischen Funktionen.
4. 4. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass der Lokaloszillator (42) ausgebildet ist zum Vorgeben der Amplitude jeder der mindestens zwei zeitabhängigen trigonometrischen Funktionen.
5. 5. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass der Lokaloszillator (42) ausgebildet ist zum Vorgeben der Phase jeder der mindestens zwei zeitabhängigen trigonometrischen Funktionen.
6. 6. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass der Lokaloszillator (42) ausgebildet ist zum Erzeugen der mindestens zwei trigonometrischen Funktionen mit einer Demodulationsfrequenz, die einem ganzzahligen Vielfachen einer Modulationsfrequenz entspricht, mit der das Eingangs-Signal moduliert ist.
7. 7. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass der Lokaloszillator (42) ausgebildet ist zum Erzeugen der mindestens zwei trigonometrischen Funktionen mit einer Demodulationsfrequenz, die einem ganzzahligen Vielfachen einer um einen Frequenzwert vergrößerten oder verkleinerten Modulationsfrequenz entspricht.
8. 8. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass der Lokaloszillator (42) in Form eines ersten Funktionsgenerators (40) ausgebildet ist.
9. 9. System nach einem der voranstehenden Sätze, gekennzeichnet durch eine Spektroskopieeinrichtung (14) zum Messen eines zeitabhängigen Spektroskopie-Signals.
10. 10. System nach Satz 9, dadurch gekennzeichnet, dass die Spektroskopieeinrichtung (14) eine Anregungslichtquelle (16), insbesondere in Form eines Anregungslasers (20), zum Erzeugen elektromagnetischer Anregungsstrahlung (18) umfasst.
11. 11. System nach Satz 10, dadurch gekennzeichnet, dass der Anregungslaser (20) in Form einer Laserdiode (22) ausgebildet ist.
12. 12. System nach einem der Sätze 9 bis 11, dadurch gekennzeichnet, dass die Spektroskopieeinrichtung (14) eine Modulationseinrichtung (28) umfasst zum Erzeugen einer Modulationsfrequenz und dass die Modulationseinrichtung (28) mit der Anregungslichtquelle (16) zusammenwirkend angeordnet und ausgebildet ist zum Modulieren einer Wellenlänge der Anregungsstrahlung (18) mit der Modulationsfrequenz.
13. 13. System nach Satz 12, dadurch gekennzeichnet, dass die Modulationseinrichtung (29) in Form eines zweiten Funktionsgenerators (28) ausgebildet ist.
14. 14. System nach einem der Sätze 9 bis 13, dadurch gekennzeichnet, dass die Spektroskopieeinrichtung (14) eine Probenzelle (32) umfasst, insbesondere in Form einer Absorptions- oder Emissionszelle, wobei insbesondere die Probenzelle (32) derart angeordnet und ausgebildet ist, dass sie mit der von der Anregungslichtquelle (16) erzeugten Anregungsstrahlung (18) beaufschlagbar ist.
15. 15. System nach einem der Sätze 9 bis 14, dadurch gekennzeichnet, dass die Spektroskopieeinrichtung (14) einen Detektor (34) umfasst, insbesondere in Form eines optischen Detektors, und dass der Detektor (34) derart angeordnet ist, dass von einer, insbesondere in der Probenzelle (32) angeordneten, Probe emittierte oder transmittierte Strahlung detektierbar ist.
16. 16. System nach Satz 15, dadurch gekennzeichnet, dass der Detektor (34) einen Signalausgang aufweist und ausgebildet ist zum Erzeugen eines Detektorsignals, welches insbesondere am Signalausgang ausgebbar ist.
17. 17. System nach Satz 15 oder 16, dadurch gekennzeichnet, dass der Detektor (34) in Form einer Fotodiode (36) ausgebildet ist.
18. 18. System nach einem der Sätze 15 bis 17, dadurch gekennzeichnet, dass dem Detektor (34) eine Verstärkungseinrichtung (38) nachgeschaltet ist oder dass der Detektor (34) eine Verstärkungseinrichtung (38) umfasst und dass die Verstärkungseinrichtung (38) ausgebildet ist zum Verstärken des Detektorsignals.
19. 19. System nach einem der Sätze 16 bis 18, dadurch gekennzeichnet, dass der Signalausgang des Detektors (34) mit dem Signaleingang (58) der Signalauswerteeinrichtung (12) gekoppelt ist.
20. 20. System nach einem der voranstehenden Sätze, dadurch gekennzeichnet, dass die Signalauswerteeinrichtung (12) eine Signalverarbeitungseinrichtung (48) zum Verarbeiten des Demodulations-Signals und eine Signalausgabeeinrichtung (52) zum Ausgeben des Demodulations-Signals umfasst.
21. 21. Verfahren zum Auswerten von Messsignalen, insbesondere bei der wellenlängenmodulierten Spektroskopie, weiter insbesondere bei der Absorptions- und Emissionsspektroskopie, bei welchem Verfahren ein zeitabhängiges Lokaloszillator-Signal erzeugt und mit einem zeitabhängigen Eingangs-Signal gemischt wird zum Erzeugen eines zeitabhängigen Demodulations-Signals, dadurch gekennzeichnet, dass das zeitabhängige Lokaloszillator-Signal in Form einer zeitabhängigen trigonometrischen Funktion erzeugt wird, welches durch additive Überlagerung von mindestens zwei zeitabhängigen trigonometrischen Funktionen gebildet wird.
22. 22. Verfahren nach Satz 21, dadurch gekennzeichnet, dass die mindestens zwei zeitabhängigen trigonometrischen Funktionen in Form von Sinus- oder Kosinusfunktionen erzeugt werden.
23. 23. Verfahren nach Satz 21 oder 22, dadurch gekennzeichnet, dass das zeitabhängige Lokaloszillator-Signal durch additive Überlagerung von maximal 10, insbesondere maximal fünf, trigonometrischen Funktionen erzeugt wird.
24. 24. Verfahren nach einem der Sätze 21 bis 23, dadurch gekennzeichnet, dass die Amplituden jeder der mindestens zwei zeitabhängigen trigonometrischen Funktionen vorgegeben werden.
25. 25. Verfahren nach einem der Sätze 21 bis 24, dadurch gekennzeichnet, dass die Phasen jeder der mindestens zwei zeitabhängigen trigonometrischen Funktionen vorgegeben werden.
26. 26. Verfahren nach einem der Sätze 21 bis 25, dadurch gekennzeichnet, dass die mindestens zwei trigonometrischen Funktionen mit einer Demodulationsfrequenz erzeugt werden, die einem ganzzahligen Vielfachen einer Modulationsfrequenz entspricht, mit der das Eingangs-Signal moduliert wird.
27. 27. Verfahren nach einem der Sätze 21 bis 26, dadurch gekennzeichnet, dass die mindestens zwei trigonometrischen Funktionen mit einer Demodulationsfrequenz erzeugt werden, die einem ganzzahligen Vielfachen einer um einen Frequenzwert vergrößerten oder verkleinerten Modulationsfrequenz entspricht.
28. 28. Verfahren nach Sätze 21 bis 27, dadurch gekennzeichnet, dass das zeitabhängige Lokaloszillator-Signal mit einem ersten Funktionsgenerator (40) erzeugt wird.
29. 29. Verfahren nach Sätze 21 bis 28, dadurch gekennzeichnet, dass ein zeitabhängiges Spektroskopie-Signal gemessen wird, welches das Eingangs-Signal bildet.
30. 30. Verfahren nach Satz 29, dadurch gekennzeichnet, dass eine elektromagnetische Anregungsstrahlung (18) erzeugt wird zum Anregen einer zu untersuchenden Probe.
31. 31. Verfahren nach Satz 30, dadurch gekennzeichnet, dass die elektromagnetische Anregungsstrahlung (18) in Form von Laserstrahlung erzeugt wird.
32. 32. Verfahren nach einem der Sätze 29 bis 31, dadurch gekennzeichnet, dass die Wellenlänge der elektromagnetischen Anregungsstrahlung (18) mit einer Modulationsfrequenz moduliert wird.
33. 33. Verfahren nach Satz 32, dadurch gekennzeichnet, dass die Modulationsfrequenz mit einem zweiten Funktionsgenerator (28) erzeugt wird.
34. 34. Verfahren nach Satz 32 oder 33, dadurch gekennzeichnet, dass die Probe mit der wellenlängenmodulierten Anregungsstrahlung (18) beaufschlagt wird und dass eine Transmission der Anregungsstrahlung (18) oder eine Emission der mit der Anregungsstrahlung (18) beaufschlagten Probe zeitabhängig gemessen wird.
35. 35. Verfahren nach Satz 34, dadurch gekennzeichnet, dass die Transmission oder Emission als Detektor-Signal mit einem Detektor (34) zeitabhängig gemessen wird, insbesondere mit einem optischen Detektor.
36. 36. Verfahren nach Satz 35, dadurch gekennzeichnet, dass mit dem Detektor (34) aus dem zeitabhängigen Spektroskopie-Signal das zeitabhängige Detektor-Signal erzeugt wird.
37. 37. Verfahren nach Satz 35 oder 36, dadurch gekennzeichnet, dass der Detektor (34) in Form einer Fotodiode (36) bereitgestellt wird.
38. 38. Verfahren nach einem der Sätze 35 bis 37, dadurch gekennzeichnet, dass das zeitabhängige Detektor-Signal verstärkt wird.
39. 39. Verfahren nach einem der Sätze 35 bis 38, dadurch gekennzeichnet, dass das zeitabhängige Detektor-Signal als zeitabhängiges Eingangs-Signal verwendet wird.
40. 40. Verfahren nach einem der Sätze 21 bis 39, dadurch gekennzeichnet, dass das Demodulations-Signal analog und/oder digital verarbeitet und ausgegeben wird.
41. 41. Verwendung eines Systems (10) nach einem der Sätze 1 bis 20 zur Durchführung eines Verfahrens nach einem der Sätze 21 bis 40.

The above description thus includes in particular the embodiments of systems for wavelength-dependent spectroscopy and methods for evaluating measurement signals in wavelength-modulated spectroscopy, defined below in the form of numbered sentences:

1. 1. System (10) for wavelength-modulated spectroscopy, in particular for absorption and emission spectroscopy, comprising a signal evaluation device (12) with a signal input (58), a mixing device (56) and a local oscillator (42) for generating a time-dependent local oscillator signal , which mixing device (56) is coupled to the signal input (58) and the local oscillator (42) for generating a time-dependent demodulation signal by mixing a time-dependent input signal present at the signal input (58) and the time-dependent local oscillator signal, characterized in that that the local oscillator is designed (42) to generate the time-dependent local oscillator signal in the form of a time-dependent trigonometric function, which is formed by additive superimposition of at least two time-dependent trigonometric functions.
2. 2. System according to sentence 1, characterized in that the local oscillator (42) is designed to generate the at least two trigonometric functions in the form of sine or cosine functions.
3. 3. System according to any one of the preceding sentences, characterized in that the local oscillator (42) is designed to generate the time-dependent local oscillator signal corresponding to the additive superimposition of a maximum of 10 trigonometric functions, in particular a maximum of five trigonometric functions.
4. 4. System according to any one of the preceding sentences, characterized in that the local oscillator (42) is designed to specify the amplitude of each of the at least two time-dependent trigonometric functions.
5. 5. System according to any one of the preceding sentences, characterized in that the local oscillator (42) is designed to specify the phase of each of the at least two time-dependent trigonometric functions.
6. 6. System according to one of the preceding sentences, characterized in that the local oscillator (42) is designed to generate the at least two trigonometric functions with a demodulation frequency which corresponds to an integer multiple of a modulation frequency with which the input signal is modulated.
7. 7. System according to one of the preceding sentences, characterized in that the local oscillator (42) is designed to generate the at least two trigonometric functions with a demodulation frequency which corresponds to an integer multiple of a modulation frequency increased or decreased by a frequency value.
8. 8. System according to any one of the preceding sentences, characterized in that the local oscillator (42) is designed in the form of a first function generator (40).
9. 9. System according to any one of the preceding sentences, characterized by a spectroscopy device (14) for measuring a time-dependent spectroscopy signal.
10. 10. System according to sentence 9, characterized in that the spectroscopy device (14) comprises an excitation light source (16), in particular in the form of an excitation laser (20), for generating electromagnetic excitation radiation (18).
11. 11. System according to sentence 10, characterized in that the excitation laser (20) is in the form of a laser diode (22).
12. 12. System according to one of sentences 9 to 11, characterized in that the spectroscopy device (14) comprises a modulation device (28) for generating a modulation frequency and that the modulation device (28) is arranged and designed to interact with the excitation light source (16) for modulation a wavelength of the excitation radiation (18) with the modulation frequency.
13. 13. System according to sentence 12, characterized in that the modulation device (29) is designed in the form of a second function generator (28).
14. 14. System according to one of sentences 9 to 13, characterized in that the spectroscopy device (14) comprises a sample cell (32), in particular in the form of an absorption or emission cell, with the sample cell (32) in particular being arranged and designed in such a way that it can be acted upon by the excitation radiation (18) generated by the excitation light source (16).
15. 15. System according to any one of sentences 9 to 14, characterized in that the spectroscopy device (14) comprises a detector (34), in particular in the form of an optical detector, and that the detector (34) is arranged such that one, in particular in the sample cell (32) arranged, sample emitted or transmitted radiation can be detected.
16. 16. System according to sentence 15, characterized in that the detector (34) has a signal output and is designed to generate a detector signal which can be output in particular at the signal output.
17. 17. System according to sentence 15 or 16, characterized in that the detector (34) is in the form of a photodiode (36).
18. 18. System according to one of sentences 15 to 17, characterized in that the detector (34) is followed by an amplification device (38) or that the detector (34) comprises an amplification device (38) and that the amplification device (38) is designed for Amplifying the detector signal.
19. 19. System according to one of sentences 16 to 18, characterized in that the signal output of the detector (34) is coupled to the signal input (58) of the signal evaluation device (12).
20. 20. System according to any one of the preceding sentences, characterized in that the signal evaluation device (12) comprises a signal processing device (48) for processing the demodulation signal and a signal output device (52) for outputting the demodulation signal.
21. 21. Method for evaluating measurement signals, in particular in wavelength-modulated spectroscopy, further in particular in absorption and emission spectroscopy, in which method a time-dependent local oscillator signal is generated and mixed with a time-dependent input signal to generate a time-dependent demodulation signal, thereby characterized in that the time-dependent local oscillator signal is generated in the form of a time-dependent trigonometric function, which is formed by additively superimposing at least two time-dependent trigonometric functions.
22. 22. Method according to sentence 21, characterized in that the at least two time-dependent trigonometric functions are generated in the form of sine or cosine functions.
23. 23. The method according to sentence 21 or 22, characterized in that the time-dependent local oscillator signal is generated by additive superimposition of a maximum of 10, in particular a maximum of five, trigonometric functions.
24. 24. Method according to one of sentences 21 to 23, characterized in that the amplitudes of each of the at least two time-dependent trigonometric functions are specified.
25. 25. Method according to one of sentences 21 to 24, characterized in that the phases of each of the at least two time-dependent trigonometric functions are specified.
26. 26. Method according to one of sentences 21 to 25, characterized in that the at least two trigonometric functions are generated with a demodulation frequency which corresponds to an integer multiple of a modulation frequency with which the input signal is modulated.
27. 27. The method according to any one of sentences 21 to 26, characterized in that the at least two trigonometric functions are generated with a demodulation frequency that is an integer corresponds to multiples of a modulation frequency increased or decreased by a frequency value.
28. 28. The method according to sentences 21 to 27, characterized in that the time-dependent local oscillator signal is generated with a first function generator (40).
29. 29. The method according to sentences 21 to 28, characterized in that a time-dependent spectroscopy signal is measured, which forms the input signal.
30. 30. The method according to sentence 29, characterized in that an electromagnetic excitation radiation (18) is generated to excite a sample to be examined.
31. 31. Method according to sentence 30, characterized in that the electromagnetic excitation radiation (18) is generated in the form of laser radiation.
32. 32. Method according to one of sentences 29 to 31, characterized in that the wavelength of the electromagnetic excitation radiation (18) is modulated with a modulation frequency.
33. 33. Method according to sentence 32, characterized in that the modulation frequency is generated with a second function generator (28).
34. 34. The method according to sentence 32 or 33, characterized in that the sample is exposed to the wavelength-modulated excitation radiation (18) and that a transmission of the excitation radiation (18) or an emission of the sample exposed to the excitation radiation (18) is measured as a function of time.
35. 35. The method according to sentence 34, characterized in that the transmission or emission is measured as a detector signal with a detector (34) as a function of time, in particular with an optical detector.
36. 36. Method according to sentence 35, characterized in that the time-dependent detector signal is generated from the time-dependent spectroscopy signal with the detector (34).
37. 37. The method according to sentence 35 or 36, characterized in that the detector (34) is provided in the form of a photodiode (36).
38. 38. Method according to one of sentences 35 to 37, characterized in that the time-dependent detector signal is amplified.
39. 39. Method according to one of sentences 35 to 38, characterized in that the time-dependent detector signal is used as a time-dependent input signal.
40. 40. Method according to one of sentences 21 to 39, characterized in that the demodulation signal is processed and output in analog and/or digital form.
41. 41. Use of a system (10) according to one of sentences 1 to 20 for carrying out a method according to one of sentences 21 to 40.

• 1: eine schematische Darstellung eines Systems für die wellenlängenmodulierte Spektroskopie;
• 2: eine schematische Darstellung der Erzeugung harmonischer Komponenten in der wellenlängenmodulierten Spektroskopie;
• 3: eine Darstellung von Frequenzspektren eines PWM-Signals für Puls-Pause-Verhältnisse zwischen 0,05 und 0,5 mit einer Schrittweite von 0,05;
• 4: eine beispielhafte Darstellung einer kosinusförmigen Modulation eines Anregungslasers mit zugehörigem Detektor-Ausgangssignal einer Probe;
• 5: eine mathematische Darstellung eines zeitabhängigen Lokaloszillator-Signal durch Überlagerung von Kosinuskurven;
• 6: mathematische Darstellung eines zeitabhängigen Lokaloszillator-Signals durch Überlagerung von Kosinuskurven mit ganzzahligem Vielfachen n der Modulationsfrequenz und Frequenzversatz für die Heterodyn-Messtechnik;
• 7: eine schematische Darstellung eines zeitabhängigen Lokaloszillator-Signals im Zeit- und Frequenzbereich mit der Formel in 8;
• 8: eine mathematische Darstellung eines zeitabhängigen Lokaloszillator-Signals durch Überlagerung von fünf Kosinus-Schwingungen;
• 9: eine beispielhafte Darstellung eines Demodulationssignals im Zeitraum (links) und im Frequenzraum (rechts);
• 10: eine beispielhafte Darstellung zweier Lokaloszillator-Signale mit einerseits drei und andererseits zehn additiv überlagerten Kosinus-Funktionen;
• 11: eine beispielhafte Darstellung zweier Lokaloszillator-Signale durch Überlagerung von jeweils fünf trigonometrischen Funktionen mit unterschiedlichen Phasenverschiebungen;
• 12: ein weiteres Beispiel einer Darstellung eines Eingangs-Signal und eines Lokaloszillatorsignal (links) sowie des durch Mischen erzeugten Demodulations-Signals (rechts); und
• 13: eine beispielhafte Darstellung des Vergleichs zwischen Messung und Simulation eines auf 1 normierten Absorptionsbands von isotopenreinem Kohlenmonoxid (CO).

The following description of preferred embodiments of the invention is used in conjunction with the drawings for more detailed explanation. Show it:

• 1 : a schematic representation of a system for wavelength-modulated spectroscopy;
• 2 : a schematic representation of the generation of harmonic components in the wavelength-modulated spectroscopy;
• 3 : a representation of frequency spectra of a PWM signal for pulse-pause ratios between 0.05 and 0.5 with an increment of 0.05;
• 4 : an exemplary representation of a cosine-shaped modulation of an excitation laser with the associated detector output signal of a sample;
• 5 : a mathematical representation of a time-dependent local oscillator signal by superimposing cosine curves;
• 6 : mathematical representation of a time-dependent local oscillator signal by superimposing cosine curves with integer multiples n of the modulation frequency and frequency offset for heterodyne measurement technology;
• 7 : a schematic representation of a time-dependent local oscillator signal in the time and frequency domain with the formula in 8th ;
• 8th : a mathematical representation of a time-dependent local oscillator signal by superimposing five cosine oscillations;
• 9 : an exemplary representation of a demodulation signal in the time domain (left) and in the frequency domain (right);
• 10 : an exemplary representation of two local oscillator signals with on the one hand three and on the other hand ten additively superimposed cosine functions;
• 11 : an exemplary representation of two local oscillator signals by superimposing five trigonometric functions each with different phase shifts;
• 12 : another example of a representation of an input signal and a local oscillator signal (left) as well as the demodulation signal generated by mixing (right); and
• 13 : an exemplary representation of the comparison between measurement and simulation of an absorption band normalized to 1 of isotopically pure carbon monoxide (CO).

In 1 an exemplary embodiment of a system for wavelength-modulated spectroscopy, denoted overall by the reference numeral 10, is shown schematically. It includes a signal evaluation device 12 and optionally a spectroscopy device 14.

The spectroscopy device 14 is designed to measure a time-dependent spectroscopy signal.

The spectroscopy device includes an excitation light source 16 for generating electromagnetic excitation radiation 18.

The excitation light source 16 can be embodied in particular in the form of an excitation laser 20 . The excitation laser 20 can be formed in particular by a laser diode 22 . For example, it can be a so-called surface laser (“Vertical Cavity Surface Emitting Laser”—VCSEL”), which is embodied in the form of a single-mode continuous-wave semiconductor laser. In this case, the laser light forming the excitation radiation 18 emerges perpendicularly to the surface of the doped planes of the semiconductor.

Optionally, the laser diode 22 can be tempered with a Peltier element stack, that is to say it can be kept at a constant temperature.

An output power of the excitation laser 20 is set via a diode current of the laser diode 22. The wavelength of the excitation radiation 18 emitted by the laser diode 22 is influenced by the diode current and the temperature of the laser diode 22 .

A driver device 24 controls and monitors the laser diode 22 and the optionally integrated Peltier element. The driver device 24, which can be embodied in particular in the form of a laser driver, can in particular provide a constant current for operating the laser diode 22.

The driver device 24 offers the option of connecting an external signal for modulation of the diode current and includes a modulation input 26 for this purpose.

in einen Diodenstrom um. Die Laserdiode 22 kann mit einer Modulationsfrequenz, welche in einem Bereich zwischen 0 kHz und 100 kHz liegt, betrieben werden.The driver device 24 converts an applied driver voltage, which is provided, for example, by a function generator 28 forming a modulation device 29, with a modulation coefficient $\mathrm{and}=10\frac{mA}{V}$

into a diode current. The laser diode 22 can be operated with a modulation frequency in a range between 0 kHz and 100 kHz.

The function generator 28 provides a periodic signal for modulating the diode current.

An optical waveguide 30 in the form of an optical fiber serves as the transmission medium for the laser light defined by the excitation laser 20 . To use an optical fiber 30 versus a free space transmission has the advantage that the water vapor contained in the air and other impurities cannot falsify a measurement signal.

In principle, both emission and absorption measurements can be carried out with the spectroscopy device 14 .

The excitation radiation 18 is guided to a sample cell 32 with the optical waveguide 30 . It can be a fiber-coupled absorption cell with a length of 80 cm, for example. This can be filled, for example, with isotopically pure carbon monoxide (CO) at a pressure of 1 bar. This sample cell can be used in particular as a reference cell for absorption measurements. For carbon monoxide, an absorption band that contains several different spectral lines lies in the wavelength interval from 1560 nm to 1595 nm.

The light transmitted through the sample cell 32 is recorded with a detector 34 in the form of an optical detector. The detector 34 can be designed in particular in the form of a photodiode 36 . In particular, this can be an indium gallium arsenide PIN photodiode for the wavelength range from 1000 nm to 1600 nm with an active area of 70.7 μm ² .

The terminals of the photodiode 36 are connected to an amplification device 38 .

in Spannungen umgewandelt werden.The amplifying device 38 can in particular amplify a detector signal, specifically by currents with a frequency of up to 200MHz with a transimpedance of Q=10 ² to ${10}^{\mathrm{8th}}\frac{V}{A}$

be converted into voltages.

In the manner described, a detector signal in the form of a current can be converted into a voltage signal.

With a further function generator 40, which forms a local oscillator 42, a signal can be generated heterodyne to the frequency of the function generator 28, the spectral components of which each have a frequency offset δ to the signal of the function generator 28.

In order to avoid a phase offset between the modulation signals of the function generators 28 and 40, a device can be used in particular in which both function generators 28 and 40 derive their system clock from a common signal source 43.

Optionally, a mixing device 44 can be provided for mixing the modulation signals of the two function generators 28 and 40. The mixing of the two modulation signals of the two function generators 28 and 40 results in a signal of frequency δ, which can be used in particular to trigger a measured value acquisition device 46. The measured value acquisition device 46 can be designed in particular in the form of an oscilloscope or also a simple analog/digital converter.

The measured values can be transmitted via a further suitable interface to a signal processing device 48 in the form of a computer 50 for further data analysis. In particular, a signal output device 52 in the form of a screen or a printer can also be coupled to the computer 50 in order to output the processed measurement signals.

For the heterodyne mixing of the detector signal (RF), optionally filtered by a filter 54 in the form of a notch filter, another signal is required which contains the frequency components Ω+δ, 2Ω+2δ,...,5Ω+5δ, where Ω is the modulation frequency of the Function generator 28 and δ represents the heterodyne frequency offset. This frequency mixture is referred to as a so-called one-shot signal or local oscillator signal (LO).

In principle, the number of frequency components can also be greater than the five frequency components shown. In particular, the number n=5 was selected here, since the signal amplitudes of harmonics of the detector signal decrease with increasing order and it is therefore no longer meaningful to evaluate harmonics, ie harmonics, from a certain number.

The detector signal (RF) optionally filtered with the filter 54 is mixed with the modulation signal of the function generator 40, optionally also with the one-shot signal, by means of the mixing device 56.

The mixing device 56 has a signal input 58 at which the detector signal is present. The mixing device 56 also includes a modulation signal input 60 . The mixing device, also referred to as a mixer, also has a mixer output 62 .

The mixing device 56 is designed to multiply the signals present at the signal input 58 and the modulation signal input 60 . These are output at the mixer output 62.

If two different, heterodyne signal frequencies are applied to the mixing device 56, for example the detector signal (RF) modulated with the modulation frequency of the function generator 28 and the described, optional one-shot signal (LO), sum and difference frequencies are generated at the mixer output 62. This process is called frequency translation and is useful for increasing or decreasing a signal's frequency.

The signal present at the mixer output 62 forms a so-called intermediate signal (IF) or demodulation signal of the detector signal (RF).

The heterodyne mixing of the detector signal with the one-shot signal removes the irrelevant high-frequency signal component at the modulation frequency. This improves the signal-to-noise ratio and the output signal contains the desired spectral components that are separated from one another by the frequency offset δ. In other words, the detector signal is down-converted into a lower frequency space in which the down-converted signal can be processed more accurately or better and in particular more cost-effectively with the downstream measured value acquisition device 46 .

A low-pass filter 64 is connected downstream of the mixer output 62 in order to attenuate signals with frequencies above the limit frequency of the low-pass filter 64 . All frequency components below the cut-off frequency pass through the low-pass filter 64 unhindered. The low-pass filter 64 serves in particular to suppress high-frequency, short-term signal fluctuations and to smooth the periodically recurring measurement signal and to emphasize long-term trends.

In 2 the process of wavelength-modulated spectroscopy is shown schematically.

The driver device 24 regulates the current of the laser diode 22. The laser diode 22 emits light of the frequency υ . In order to modulate the wavelength of the excitation laser 20, the driver device 24 is controlled with the modulation signal from the function generator 28 with a modulation deviation a, a detuning c and a modulation frequency f _mod =Ω. For example, f _mod =50 kHz. The modulation frequency is very much lower than the frequency of the excitation radiation 18.

The modulation of the laser frequency υ(t) is described by $$\upsilon \left(t\right)=\overline{\upsilon }+a•cOs\left({\omega }_{\text{model}}•t\right)+c\text{and}{\omega }_{\text{model}}=2\pi ·{ƒ}_{mO\mathrm{i.e}}$$

The modulated excitation radiation 18 is guided to the sample cell 32 via the optical waveguide 30 . There the excitation radiation 18 is partially absorbed by the sample in the sample cell 32 . This leads to a laser wavelength-dependent change in the amplitude of the absorption signal. One can assume, for example, that the incident intensity of the excitation radiation 18 is constant over the entire frequency range to be measured.

/ ist dabei die Länge der Probenzelle 32, die von der Anregungsstrahlung 18 durchstrahlt wird.The general course of the radiation attenuation or a transmission I υ(t)) is described by the Lambert-Beersche law with the form e ^-a(υ) . $$I\left(\upsilon ,l\right)=I_0\left(v\right)•e^{-a\left(\upsilon \right)•I}$$

/ is the length of the sample cell 32 through which the excitation radiation 18 passes.

The excitation radiation 18 u(t) with c=0 passes through the sample cell with the transmission I (υ) and is caught by the detector 34 .

The detector 34 converts the optical signal into an electrical signal. The electrical signal generated at the detector 34 by the optical signal contains a DC component and signal components at the fundamental wave of the modulation frequency and at its harmonic components, ie at the harmonics.

The amplification device 38 adapts the weak signal levels of the detector 34 for further evaluation with the signal evaluation device 12 .

The time-dependent term of I (υ(t)) is an even function. Therefore the detector signal can be represented by the following cosine Fourier series: $$I\left(\overline{\upsilon }+a•\text{cos}\left({\omega }_{mO\mathrm{i.e}}•t\right)\right)={\displaystyle \sum _{n-1}^{\infty }{H}_n}\left(\overline{\upsilon },a\right)•\text{cos}\left(n•{\omega }_{mO\mathrm{i.e}}•t+{\varphi }_n\right)$$

Mixing with the one-shot signal allows the individual harmonic Fourier components H _n (υ,a) to be selected and displayed.

The system 10 described thus makes it possible, in particular, to limit the number of harmonics and also, if desired, to adjust the phase offset as desired. In addition, amplitudes of additively superimposed cosine functions, i.e. the time-dependent trigonometric functions, can be specified as desired, namely with their amplitudes and with their phase relative to one another and in particular to the fundamental wave, which is generated with the function generators 28 and 40.

In contrast to this, when using a local oscillator signal in the form of a PWM signal, relationships between the amplitudes and phase offsets of the harmonics can be set globally only by the mark-to-space ratio. 3 shows an example of amplitude distributions with a local oscillator signal in the form of a PWM signal and additive superimposition.

Out of 3 it can be seen that the amplitude values and thus the existence of a certain harmonic in the PWM signal depends on the mark-to-space ratio, in 3 specified with "duty", is dependent. For example, at duty=0.5 only odd-numbered harmonics are included.

In 3 frequency spectra of a PWM signal are shown, plotted for a mark-to-space ratio between 0.05 and 0.5 with an increment of 0.05. The distribution of the harmonics for a mark-to-space ratio between 0.55 and 0.95 results in a distribution analogous to that shown.

In 4 the cosinusoidal modulation of the excitation laser 20 with a frequency of 1 kHz is shown as an example. The absorption signal of the sample in the sample cell 32 measured with the detector 34 is also shown in terms of time.

The two dips in the detector signal occur because the modulation as the cosine ramps up and down produces twice the wavelength absorbed by the sample. Therefore, the absorption, represented by the notch in the detector signal, i.e. the narrow-band drop in the intensity of the radiation transmitted through the sample measured with the detector, should always be axisymmetric, i.e. without phase shift, in relation to an extreme of the cosinusoidal modulation signal. This is through the in 4 shown measurement well confirmed.

As already explained, no PWM signal is used as the local oscillator signal, but a signal that is formed from the additive superimposition of cosine curves, this is also possible for sine curves. The superposition results according to the in 5 equation shown.

In 6 shown is the equation that applies to the superimposition of integer multiples of a frequency, such as is required for heterodyne measurement technology with a frequency offset of δ. Local oscillator signals according to the equations of 5 and 6 can easily be generated with the function generator 40.

In this way, the spectrum of the local oscillator signal can be configured freely, which means that the local oscillator signal contains exactly the frequency or frequencies that are required for the frequency conversion of the detector signal with the mixing device 44 . In this way, in particular, filters can also be saved, which must be implemented explicitly when using a PWM signal as a local oscillator signal.

If a frequency contained in the detector signal is not to be converted, the corresponding frequency is not inserted when the local oscillator signal is generated.

Furthermore, the amplitudes at each harmonic can be adjusted separately by the additive superimposition of the trigonometric functions. In particular, it is possible to specify all frequency components of the local oscillator signal with the same amplitude. Furthermore, the phases φ _n of each harmonic can also be set separately. Although this does not change the spectrum, it does change the shape of the curve of the local oscillator signal in the time domain.

Optimum phase adjustment of all harmonics contained in the local oscillator signal makes it possible, in particular, to achieve a homogeneous amplitude distribution of the time-dependent local oscillator signal. This has the advantage that the local oscillator signal does not contain any large peaks and a maximum amplitude of the harmonics can be achieved without overdriving the local oscillator input formed by the modulation signal input 60 .

If in the equation the 6 If δ≠0 is selected, a frequency offset of the local oscillator signal with respect to the modulation signal of the function generator 28, which is necessary for the heterodyne measurement technique, can be implemented.

drei unterschiedliche Amplituden nämlich 1, ½ und 2 sowie drei unterschiedliche Phasen, nämlich $\frac{2\pi }{3},$

, $\frac{-2\pi }{7}$

und $\frac{2\pi }{11}$

.In is an example of a local oscillator signal in the time and frequency domain with the in 8th given formula shown. There are: $${\omega }_{\text{model}}=2\pi •{ƒ}_{\text{model}}=1;\delta =0;n=5;-\pi \le t\le \pi ;$$

three different amplitudes viz. 1, ½ and 2 and three different phases viz $\frac{2\pi }{3},$

, $\frac{-2\mathrm{<figure-callout\; id="7"\; label="\pi "\; filenames="DE102018117642B4\_0016.png"\; state="\{\{state\}\}"><figure-callout\; id="2"\; label="\pi "\; filenames="DE102018117642B4\_0011.png,DE102018117642B4\_0014.png"\; state="\{\{state\}\}">\pi </figure-callout></figure-callout>}}{7}$

and $\frac{2\mathrm{<figure-callout\; id="11"\; label="\pi "\; filenames="DE102018117642B4\_0012.png,DE102018117642B4\_0015.png"\; state="\{\{state\}\}">\pi </figure-callout>}}{11}$

.

In the 8th The equation shown thus shows, by way of example, a local oscillator signal which is formed by additively superimposing five cosine oscillations.

In 7 on the left this local oscillator signal is plotted in the time domain, in 7 right in the frequency domain.

This in 7 The local oscillator signal shown is an example of the general formula according to 6 represent.

9 shows an example of the demodulation signal present at the mixer output 66 in the time and frequency domain. There are 6 harmonics in the local oscillator signal, which are also clearly visible in the demodulation signal.

In 10 shows examples of local oscillator signals that are created by superimposing three cosine functions or 10 cosine functions. A phase shift was omitted here. In addition, all cosine functions are superimposed with the same amplitude.

11 shows another example of two local oscillator functions. Both are formed by superimposing five cosine functions each. In the case of the function f(x), no phase shifts between the harmonics are provided, in the case of the function g(x), the third harmonic is phase-shifted by 180°. The second and fourth harmonics each form a sine function, which means that a shift of -90° is equivalent to a phase shift.

The advantage of randomly superimposing a finite number of trigonometric functions on the local oscillator signal is particularly evident 12 clear. In 12 On the left the detector signal (RF signal) and the local oscillator signal (LO signal) with δ= 10Hz are shown in the frequency range.

The demodulation signal present at the mixer output 62 after mixing the detector signal and the local oscillator signal is in the 12 also shown on the right in the frequency domain, but now shifted to lower frequencies, namely multiples of the frequency shift δ.

Finally shows 13 the excellent agreement of the absorption band of isotopically pure carbon monoxide CO measured with the system 10 and normalized to 1 in comparison with the theoretical value of the absorption band provided by the information system Hitran for the pressure of 1 bar set for the sample and the absorption length of 80 cm. The measured absorption band is at a wave number of about 6321.3 cm ^-1 , which corresponds to a wavelength of 1582 nm, and agrees very well with the theoretical value.

The systems and methods described above are particularly suitable for use in wavelength-modulated laser spectroscopy.

In principle, the signal evaluation device 12 can be used for any mixed applications.

Reference List

10

system

12

signal evaluation device

14

spectroscopy facility

16

excitation light source

18

excitation radiation

20

excitation laser

22

laser diode

24

driver setup

26

modulation input

28

function generator

29

modulation device

30

optical fiber

32

sample cell

34

detector

36

photodiode

38

reinforcement device

40

function generator

42

local oscillator

43

signal source

44

mixing device

46

Measured value acquisition device

48

signal processing device

50

computer

52

signal output device

54

filter

56

mixing device

58

signal input

60

modulation signal input

62

mixer output

64

low pass filter

Claims (20)

System (10) for wavelength-modulated spectroscopy, in particular for absorption and emission spectroscopy, comprising a signal evaluation device (12) with a signal input (58), a mixing device (56) and a local oscillator (42) for generating a time-dependent local oscillator signal which Mixing device (56) is coupled to the signal input (58) and the local oscillator (42) for generating a time-dependent demodulation signal by mixing a time-dependent input signal present at the signal input (58) and the time-dependent local oscillator signal, characterized in that the Local oscillator (42) is designed to generate the time-dependent local oscillator signal in the form of a time-dependent trigonometric function, which is formed by additive superposition of at least two time-dependent trigonometric functions, and that the local oscillator (42) is designed to specify the amplitude and the phase of each the at least two time-dependent trigonometric functions. system after claim 1 , characterized in that the local oscillator (42) is designed a) to generate the at least two trigonometric functions in the form of sine or cosine functions and/or b) to generate the time-dependent local oscillator signal corresponding to the additive superimposition of a maximum of 10 trigonometric Functions, in particular a maximum of five trigonometric functions. System according to one of the preceding claims, characterized in that the local oscillator (42) a) is designed to generate the at least two trigonometric functions with a demodulation frequency which corresponds to an integer multiple of a modulation frequency with which the input signal is modulated and/or b) is designed to generate the at least two trigonometric functions with a demodulation frequency that corresponds to an integer multiple of a modulation frequency increased or decreased by a frequency value, and/or c) is designed in the form of a first function generator (40). System according to one of the preceding claims, characterized by a spectroscopy device (14) for measuring a time-dependent spectroscopy signal. system after claim 4 , characterized in that the spectroscopy device (14) comprises a) an excitation light source (16), in particular in the form of an excitation laser (20), for generating electromagnetic excitation radiation (18), in particular the excitation laser (20) in the form of a laser diode (22) is formed, and / or b) comprises a modulation device (28) for generating a modulation frequency and that the modulation device (28) is arranged to interact with the excitation light source (16) and is designed to modulate a wavelength of the excitation radiation (18) with the modulation frequency, wherein in particular the modulation device (29) is designed in the form of a second function generator (28) and/or c) comprises a sample cell (32), in particular in the form of an absorption or emission cell, with the sample cell (32) in particular being arranged and formed in such a way that they are exposed to the excitation radiation (18) generated by the excitation light source (16). is storable. system after claim 4 or 5 , characterized in that the spectroscopy device (14) comprises a detector (34), in particular in the form of an optical detector, and in that the detector (34) is arranged in such a way that a sample emitted, in particular in the sample cell (32). or transmitted radiation can be detected. system after claim 6 , characterized in that the detector (34) has a signal output and is designed to generate a detector signal which can be output in particular at the signal output. system after claim 6 or 7 , characterized in that a) the detector (34) is designed in the form of a photodiode (36) and/or b) the detector (34) is followed by an amplification device (38) or the detector (34) comprises an amplification device (38) and the amplification device (38) is designed to amplify the detector signal. system after claim 7 or 8th , characterized in that the signal output of the detector (34) is coupled to the signal input (58) of the signal evaluation device (12). System according to one of the preceding claims, characterized in that the signal evaluation device (12) comprises a signal processing device (48) for processing the demodulation signal and a signal output device (52) for outputting the demodulation signal. Method for evaluating measurement signals, in particular in wavelength-modulated spectroscopy, further in particular in absorption and emission spectroscopy, in which method a time-dependent local oscillator signal is generated and mixed with a time-dependent input signal to generate a time-dependent demodulation signal, characterized in that that the time-dependent local oscillator signal is generated in the form of a time-dependent trigonometric function, which is formed by additive superposition of at least two time-dependent trigonometric functions, and that the amplitudes and the phases of each of the at least two time-dependent trigonometric functions are specified. procedure after claim 11 , characterized in that a) the at least two time-dependent trigonometric functions are generated in the form of sine or cosine functions and/or b) the time-dependent local oscillator signal is generated by additive superimposition of a maximum of 10, in particular a maximum of five, trigonometric functions. procedure after claim 11 or 12 , characterized in that a) the at least two trigonometric functions are generated with a demodulation frequency which corresponds to an integer multiple of a modulation frequency with which the input signal is modulated, and/or b) the at least two trigonometric functions are generated with a demodulation frequency , which corresponds to an integer multiple of a modulation frequency increased or decreased by a frequency value, and/or c) the time-dependent local oscillator signal is generated with a first function generator (40). procedure after Claims 11 until 13 , characterized in that a time-dependent spectroscopy signal is measured, which forms the input signal, with in particular an electromagnetic excitation radiation (18) being generated to excite a sample to be examined, with the further in particular the electromagnetic excitation radiation (18) in the form of laser radiation is produced. procedure after Claim 14 , characterized in that the wavelength of the electromagnetic excitation radiation (18) is modulated with a modulation frequency, in particular the modulation frequency being generated with a second function generator (28). procedure after claim 15 , characterized in that the sample is exposed to the wavelength-modulated excitation radiation (18) and that a transmission of the excitation radiation (18) or an emission of the excitation radiation (18) exposed sample is measured as a function of time. procedure after Claim 16 , characterized in that the transmission or emission is measured as a detector signal with a detector (34) as a function of time, in particular with an optical detector. procedure after Claim 17 , characterized in that a) the time-dependent detector signal is generated from the time-dependent spectroscopy signal with the detector (34) and/or b) the detector (34) is provided in the form of a photodiode (36) and/or c) the time-dependent detector signal is amplified and/or d) the time-dependent detector signal is used as a time-dependent input signal. Procedure according to one of Claims 11 until 18 , characterized in that the demodulation signal is processed and output in analog and/or digital form. Use of a system (10) according to one of Claims 1 until 10 for carrying out a method according to one of Claims 11 until 19 .

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