WO2020229531A1

WO2020229531A1 - System and method for photoacoustic spectroscopy

Info

Publication number: WO2020229531A1
Application number: PCT/EP2020/063314
Authority: WO
Inventors: Davide Iannuzzi
Original assignee: Stichting Vu
Priority date: 2019-05-13
Filing date: 2020-05-13
Publication date: 2020-11-19
Also published as: NL2023123B1

Abstract

System for photoacoustic spectroscopy, the system comprising: ° a light system comprising a light source, wherein the light system is configured to generate a light beam, to modulate the light beam at a modulation frequency, and to direct the light beam to a space; ° an optical waveguide, comprising a first measurement part; ° a vibration unit arranged in the space, comprising: ° a first fixation structure and a second fixation structure, wherein the first fixation structure and the second fixation structure are arranged remotely from each other in the space, and wherein the first measurement part of the optical waveguide is tightened between the first fixation structure and the second fixation structure, and wherein the first measurement part extends through the space and is in immediate contact with the space; ° a detection unit, configured to detect a first vibration of the first measurement part based on changes in optical propagation properties of the optical waveguide, the detection unit comprising: ° a sensor light source coupled to the optical waveguide, wherein the sensor light source is configured to generate a sensor light beam propagating through the optical waveguide; ° a sensor light detector coupled to the optical waveguide, wherein the sensor light detector is configured to measure the sensor light beam having propagated through the optical waveguide; ° a signal processing unit, configured to detect the first vibration of the first measurement part based on the generated sensor light beam and the measured sensor light beam.

Description

Title: System and method for photoacoustic spectroscopy

Field of the invention

The present invention relates to systems and methods for photoacoustic

spectroscopy. In particular, the present invention relates to the use of waveguides in photoacoustic spectroscopy.

Background of the invention

Photoacoustic spectroscopy, or PAS, is a method to measure traces of chemicals in a gas. A general introduction to the use of photoacoustic spectroscopy in gas or trace gas monitoring is provided in Harren et al.,‘Photoacoustic spectroscopy in trace gas monitoring’, Encyclopedia of Analytical Chemistry, 2000.

In photoacoustic spectroscopy, a gaseous molecule may be excited by absorbing electromagnetic radiation. The electromagnetic radiation may be light. During the following de-excitation of the molecule, the molecule may release energy in its surroundings due to collisions, causing an increase of a temperature in the surroundings of the, now de-excited, molecule. When the source of the electromagnetic radiation, such as a light source, is modulated at an acoustic frequency, the temperature in the surroundings of the excited, and then de-excited, molecule also changes with the acoustic frequency, causing pressure changes with the same frequency, which may be observed as an acoustic wave. By measuring the acoustic wave, for example by using a microphone, information associated with the excited molecule may be obtained, since the amplitude of the detected sound is proportional to the concentration of the probed molecules.

A system for photoacoustic spectroscopy is disclosed in J. Breguet, J.P. Pellaux, N. Gisin, Photoacoustic detection of trace gases with an optical microphone, Sensor Actuat. A - Phys. 48 (1995) 29-35. The system discloses an optical-fiber microphone constructed by winding and gluing a fiber coil on a thin plate of the gas chamber. The length of the fiber coil is modulated via the deformation of the cell wall due to the first longitudinal acoustic mode applied on the wall. Michelson and Sagnac interferometers were employed to detect the optical phase change, providing information on the gas in the gas chamber. Relying, at least partly, on photoacoustic cells or chambers which are designed and/or configured to resonate with an acoustic signal is common in the design of systems for photoacoustic spectroscopy. However, a drawback is that properties of the gas, the cell and the modulation frequency of the light source have to be aligned carefully with each other.

Summary of the invention

It would be desirable to provide systems and methods for photoacoustic spectroscopy of which the performance is less susceptible to design parameters.

In a first aspect of the invention a system for photoacoustic spectroscopy is provided, the system comprising:

• a light system comprising a light source, wherein the light system is configured to generate a light beam, to modulate the light beam at a modulation frequency, and to direct the light beam to a space;

• an optical waveguide, comprising a first measurement part;

• a vibration unit arranged in the space, comprising:

o a first fixation structure and a second fixation structure, wherein the first fixation structure and the second fixation structure are arranged remotely from each other in the space, and wherein the first measurement part of the optical waveguide is tightened between the first fixation structure and the second fixation structure, and wherein the first measurement part extends through the space and is in immediate contact with the space;

• a detection unit, configured to detect a first vibration of the first measurement part based on changes in optical propagation properties of the optical waveguide, the detection unit comprising:

o a sensor light source coupled to the optical waveguide, wherein the sensor light source is configured to generate a sensor light beam propagating through the optical waveguide;

o a sensor light detector coupled to the optical waveguide, wherein the

sensor light detector is configured to measure the sensor light beam having propagated through the optical waveguide;

o a signal processing unit, configured to detect the first vibration of the first measurement part based on the generated sensor light beam and the measured sensor light beam. The system for photoacoustic spectroscopy according to the first aspect of the invention may be suitable to detect a molecule or a type of molecule in a gas contained in the space. Detecting may include determining e.g. a presence, an amount, traces, a

concentration, a minimum concentration etc. of a molecule or a type of molecule in the gas.

To detect a molecule or a type of molecule in the gas, it may be beneficial to excite some molecules in the gas by a light or light beam generated by the light system. Light sources that may be suitable to generate the light beam include lasers and light-emitting diodes or LEDs. The light beam may have an optical wavelength that is substantially equal to a wavelength suitable to excite a molecule from a first state to an excited state. To cause de excitations of molecules at suitable instances, it may be beneficial to modulate the light beam. In particular, the amplitude and/or the wavelength of the light beam may be modulated by the light system. Modulating other aspect of the light beam may be possible and/or useful as well. An optical chopper may also be used to modulate the light beam. The light beam may be directed to the gas that may contain molecules that could be detected. To excite a molecule in the gas by the light, the light needs to be directed towards the gas. For this purpose, the light system may comprise one or more guiding optical waveguides, mirrors and/or other optical components that are suitable to guide and/or direct a light or light beam to the gas.

The (modulated) light beam may excite a molecule. An excitation of a molecule is generally followed by an de-excitation of the same molecule. By modulating the light beam, an excitation / de-excitation pattern of molecules may create a compression and

decompression of the gas, which may create a pressure wave propagating through the gas in the space.

The pressure wave may be a longitudinal wave propagating through the gas by means of an adiabatic compression and decompression of parts of the gas. The frequency of the pressure wave may be substantially equal to the modulation frequency of the light beam. The frequency may be outside of the human hearing range. An amplitude or intensity of the pressure wave may be correlated to the amplitude of the light beam. Also, an amplitude or intensity of the pressure wave may increase when the wavelength of the light beam becomes more equal to a wavelength suitable to excite the molecule.

The pressure wave may cause the first vibration of the first measurement part, tightened between the first fixation structure and the second fixation structure. The first measurement part is rectilinearly tightened (tensioned) between the first and second fixation structures, the tightening of the measurement part between the first fixation structure and the second fixation structure provides a tensioning force in the first measurement part of the optical fiber, the tensioning force extending along a length of the first measurement part of the optical fiber between the first fixation structure and second fixation structure. The tightening of the first measurement part between the first fixation structure and the second fixation structure is thus to be understood as a tensioning of the first measurement part between the first and second fixation structures to provide a tensioning force in the measurement part of the optical fiber, the tensioning force extending between the first and second fixation structures along the length of the first measurement part. Resonant and/or vibration properties of the first measurement part are accordingly affected by a length of the first measurement part between the first and second fixation structures as well as the tightening force, etc., similarly to a string of a musical instrument. The first measurement part extends through the space and is configured to be excited at the first vibration by a pressure wave generated by the light beam exciting molecules in the space. The pressure wave in the space, e.g. excited by the modulated light beam from the light source interacting with e.g. (gas) molecules in the space, may hence excite the first measurement part to vibrate.

The first vibration is associated with the first measurement moving through the space. The movement may be an oscillation around an equilibrium state. The equilibrium state may associated with the state of the optical waveguide when no pressure wave is impinging the waveguide. The vibration frequency or oscillation frequency of the first measurement part may be substantially equal to the frequency of the pressure wave. Alternatively or additionally, the vibration frequency or oscillation frequency of the first measurement part may be substantially equal to an entire multiple of the frequency of the pressure wave. The vibration frequency of the first measurement part may comprise a plurality of frequencies.

The first vibration of the first measurement part may causes deformations of the optical waveguide, which may result in changes of the optical propagation properties of the optical waveguide. In particular, the optical propagation properties of the first measurement part may change during the first vibration of the first measurement part.

The pressure wave may apply periodically a force to the first measurement part. The frequency of the pressure wave may be substantially equal to the modulation frequency. In case the frequency of the pressure wave is substantially equal to one of the natural frequencies of the first measurement part, the first measurement part may resonate with a first resonant frequency. Generally, the resonance of the first measurement part with the pressure wave may cause a larger amplitude of the first vibration of the first measurement part in comparison with first vibrations caused by pressure waves that do not coincide with a resonant frequency of the first measurement part.

Throughout the text, a first vibration, a second vibration etc. do not necessarily refer to harmonics of the resonant frequency. However, it some situations the first or the second vibration may have a frequency equal to the resonant frequency or an entire multiple thereof. A first resonant frequency, a second resonant frequency etc. may refer to any resonant frequency, wherein first, second, etc, not necessarily indicate the number of a harmonic. A fixation structure may comprise a clamp, glue or any other means to fixate part of an optical waveguide. Also, friction between a fixation part and part of the optical waveguide may cause a fixation of part of the first measurement part. The first fixation structure and the second fixation structure, including the first measurement part tightened between the two fixation structures, may be comprised in a vibration unit or form a vibration unit. The pressure wave may cause a first vibration in the first measurement part of the optical waveguide by directly impinging the first measurement part. Vibrations in an optical waveguide may be determined accurately with high sensitivity. The optical waveguide may be in immediate contact with the gas in the space. Various types of optical waveguides may be suitable to be used within the present invention. In particular, the optical waveguide may comprise an optical fiber, a strip waveguide, a rib waveguide, a segmented waveguide, a photonic crystal waveguide and/or a laser-inscribed waveguide.

To facilitate the first vibration of the first measurement part of the optical waveguide between the first fixation structure and the second fixation structure, the first fixation structure and the second fixation structure may have a predetermined position with respect to each other. Furthermore, the first measurement part of the optical waveguide between the first fixation structure and the second fixation structure is put under sufficient tense or strain or is sufficiently stretched, such that the first measurement part between the first fixation structure and the second fixation structure may vibrate upon receiving the pressure wave or pressure wave signal. The first measurement part may vibrate while the first fixation structure and the second fixation structure may not vibrate. In particular, the first vibration of the first measurement part may be substantially caused by the pressure wave.

Mechanical vibration properties of the optical waveguide between the first fixation structure and the second fixation structure, and/or of the first measurement part of the optical waveguide may depend on physical or mechanical properties of the respective part of the optical waveguide. These properties may include length, diameter, type of material, elasticity etc..

Vibrations in the optical waveguide may result in changes of optical propagation properties of the optical waveguide, which may be measured by the detection unit. For example, vibrations may cause changes in the strain of the optical waveguide, which result in changes in the optical path length. In case the strain in the optical waveguide is measured, an optical strain sensor or optical strain gauge may be used to obtain information on the strain and/or changes of the strain in the optical waveguide. Bragg gratings or Bragg reflectors may be comprised in or coupled to the optical waveguide in order to facilitate determining changes in the strain. Various systems, detections units or sensors to measure the changes in optical propagation properties of optical waveguides exist that may be suitable to be used within the present invention. Various detection units have in common that they comprise a sensor light source to generate a sensor light beam that may propagate through the optical waveguide and a sensor light beam detector to detect the sensor light beam after it has propagated through the optical waveguide. The sensor light beam may be reflected or partly reflected in the optical waveguide, possibly reversing a propagation direction. Changes in the optical propagation in the optical waveguide may result in changes in phase, amplitude and/or frequency of the sensor light, such that based on the generated sensor light and the detected sensor light, changes of the optical propagation properties in the optical waveguide may be determined. For example, an optical strain sensor may determine changes in the strain in optical waveguide, caused by vibrations. The sensor light bean may be modulated. For example, the wavelength and/or the amplitude of the sensor light beam may be modulated.

The system according to the first aspect of the invention enables the detection of molecules in a gas by employing the light source, the optical waveguide, the vibration unit and the detection unit. The vibration properties and/or resonant properties of the first measurement part may be designed and/or adjusted by determining a proper distance between the first fixation structure and the second fixation stricture, as well as by choosing an optical waveguide with suitable mechanical properties. Since no photoacoustic cell or gas chamber is required to resonate with a pressure wave, the alignment of properties of a photoacoustic cell with the properties of e.g. the modulation frequency of the light beam is not essential to the system. Consequently, the system according to the invention is less susceptible to design parameters or changes in the design parameters than systems previously known that rely on the resonant frequencies of the cell or the gas chamber. The system according to the invention is less susceptible to design parameters related to a photoacoustic cell that may be present. In particular, the system according to the invention may enable detecting molecules using photoacoustic spectroscopy without relying on the presence of a photoacoustic cell. Besides the technical benefits, the system according to the invention may also be cheaper to produce or to maintain, than systems that substantially rely on the vibration properties of the photoacoustic cell or gas chamber.

In an embodiment of the system, the modulation frequency is substantially equal to a first resonant frequency of the first measurement part and/or an entire multiple of the first resonant frequency of the first measurement part.

A pressure wave that may be generated in a gas has generally a frequency which is substantially equal to the modulation frequency of the light beam. The pressure wave causes a first vibration of the first measurement part by impinging on the first measurement part. To increase the amplitude of the first vibration of the first measurement part, it may be beneficial that the optical signal has a frequency which is substantially equal the first resonant frequency of the first measurement part and/or the entire multiple of the first resonant frequency of the first measurement part.

Consequently, it may also be beneficial that the modulation frequency is substantially equal to the first resonant frequency of the first measurement part and/or the entire multiple of the first resonant frequency of the first measurement part, in order to increase the amplitude of the first vibration of the first measurement part.

In case the modulation frequency is varying, it may be beneficial that the modulation frequency is substantially equal to the first resonant frequency of the first measurement part and/or the entire multiple of the first resonant frequency of the first measurement part.

In an embodiment of the system, the optical waveguide further comprises a second measurement part and the vibration unit further comprises:

• a third fixation structure and a fourth fixation structure, wherein the third fixation structure and the fourth fixation structure are arranged remotely from each other in the space; and wherein the second measurement part of the optical waveguide is tightened between the third fixation structure and the fourth fixation structure, and wherein the second measurement part extends through the space and is in immediate contact with the space, and

wherein the detection unit, further is configured to detect a second vibration of the second measurement part based on changes in optical propagation properties of the optical waveguide and wherein the signal processing unit, further is configured to detect the second vibration of the second measurement part based on the generated sensor light beam and the measured sensor light beam.

The detection unit is configured to detect or derive a first vibration of the first measurement part based on changes in optical propagation properties of the optical waveguide. In order to detect a first vibration it is beneficial that the changes in the optical propagation properties in the optical waveguide are strong, which may increase a signal strength. In order to increase the mechanical deformation of the optical waveguide due to vibrations, resulting in stronger changes in the optical propagation properties, it may be beneficial that the optical waveguide comprises a plurality of measurement parts that may vibrate due to a pressure wave propagating through the space and impinging the plurality of measurement parts. By arranging a second measurement part in the same space as the first measurement part both measurement parts may vibrate at the same time, causing larger changes in optical propagation properties of the optical waveguide. Similarly, arranging a third measurement, a fourth measurement part, etc. in the space may further increase the changes in optical propagation properties if the plurality of measurement parts vibrate simultaneously. The second and further measurement parts are likewise rectilinearly tightened (tensioned) between the respective fixation structures, the tightening of the measurement part between the respective fixation structures provides a tensioning force in the corresponding

measurement part of the optical fiber, the tensioning force extending along a length of the measurement part of the optical fiber between the respective fixation structures. The tightening of the first measurement part between the first fixation structure and the second fixation structure is thus to be understood as a tensioning of the first measurement part between the fixation structures to provide a tensioning force in the measurement part of the optical fiber, the tensioning force extending between the fixation structures along the length of the measurement part. Resonant and/or vibration properties of the measurement part are accordingly affected by a length of the measurement part between the fixation structures as well as the tightening force, etc., similarly to a string of a musical instrument. The second measurement part extends through the space and is configured to be excited at the second vibration by a pressure wave generated by the light beam exciting molecules in the space. The pressure wave in the space, e.g. excited by the modulated light beam from the light source interacting with e.g. (gas) molecules in the space, may hence excite the second measurement part to vibrate at the second vibration.

In an embodiment of the system, the first resonant frequency of the first measurement part is substantially equal to a second resonant frequency of the second measurement part.

To increase the signal associated with the vibrations, it may be beneficial that the light beam associated with the first wavelength is modulated at a frequency substantially equal to the first resonant frequency of the first measurement part or an entire multiple of the first resonant frequency of the first measurement part, and that the light beam associated with the second wavelength is modulated at a frequency substantially equal to the second resonant frequency of the second measurement part or an entire multiple of the second resonant frequency of the second measurement part.

In an embodiment of the system, the first resonant frequency of the first measurement part is substantially different from the second resonant frequency of the second measurement part.

To facilitate a differentiation between the first vibration of the first measurement part and the second vibration of the second measurement part, it may be beneficial that the first resonant frequency of the first measurement part is substantially different from the second resonant frequency of the second measurement part. This differentiation may be beneficial to detect different types of molecules or to further determine where a vibration occurred.

In an embodiment of the system, the light system further is configured to modulate the light beam at different frequencies comprising the modulation frequency and a second modulation frequency.

Various measurement parts, such as the first measurement part and the second measurement part may have distinct resonant frequencies. To facilitate the vibration of all a plurality of measurement parts that may not have the some resonant frequencies it may be beneficial that the light system further is configured to modulate the light beam at different frequencies comprising the modulation frequency and a second modulation frequency.

In an embodiment of the system, the second modulation frequency is substantially equal to a resonant frequency of the second measurement part and/or an entire multiple of the second resonant frequency of the second measurement part.

It may be beneficial that the modulation frequency is substantially equal to a first resonant frequency of the first measurement part and/or an entire multiple of the first resonant frequency of the first measurement part.

Similarly, it may be beneficial that the second modulation frequency is substantially equal to a resonant frequency of the second measurement part and/or an entire multiple of the second resonant frequency of the second measurement part.

In an embodiment of the system, the light system further is configured to generate a second light beam, to modulate the second light beam at a second beam modulation frequency, and to direct the second light beam to the space, wherein a first optical wavelength of the light beam is different from a second optical wavelength of the second light beam.

To detect molecules in a gas in the space, the optical wavelength of a light beam must be substantially equal to a wavelength that is suitable to excite the molecules. In case various molecules or types of molecules should be detected wherein each molecule or type of molecule requires a specific optical wavelengths of a light beam in order to excite the respective molecule or type of molecule, it may be beneficial to use a light system that is configured to emit, additionally to the light beam, a second light beam, wherein a first optical wavelength of the light beam is different from a second optical wavelength of the second light beam. The light beam and the second light beam may be comprised in a single beam. The light beam and the second light beam may be two separate beams. The light system may emit the light beam and the second light beam at the same time or at different time instances.

It may be possible to generate light beams with different optical wavelengths by using different light sources. For example, different lasers may be used. Additionally or alternatively, filters may be used to filter out specific optical wavelengths of a light beam, wherein changing a filter may result in a change of the optical wavelength of a light beam. An acoustic-optic or electro-optic modulator may also be used to shift an optical wavelength of a light beam. Other systems to change an optical wavelength exist as well.

In an embodiment of the system, the modulation frequency is different from the second beam modulation frequency.

To further improve the ability of the system to detect various molecules, it may be beneficial that a pressure wave associated with an excitation and de-excitation of a molecule or a type of molecule has a different frequency that another pressure wave associated with an excitation and de-excitation of another molecule or another type of molecule, since pressure waves with different frequencies may cause different vibrations in the optical waveguide. Therefore, it may be beneficial that the light beam associated with the first wavelength is modulated at a different frequency than the second light beam associated with the second wavelength.

In an embodiment of the system, the first optical wavelength is substantially equal to a first excitation wavelength associated with an excitation of a first molecule and wherein the second optical wavelength is substantially equal to a second excitation wavelength associated with an excitation of a second molecule.

To enable the detection of two different molecules, comprising the first molecule and the second molecule, it may be beneficial if the first optical wavelength is substantially equal to a first excitation wavelength associated with an excitation of a first molecule and wherein the second optical wavelength is substantially equal to a second excitation wavelength associated with an excitation of a second molecule. The first excitation wavelength is a wavelength of the light beam that may excite the first molecule. Similarly, the second excitation wavelength is a wavelength of the second light beam that may excite the second molecule. In an embodiment of the system, the first measurement part comprises a membrane.

In case the first measurement part comprises the membrane, the surface area of the first measurement part may be larger than a first measurement part not comprising a membrane. Having a larger surface area has the benefit that more energy of the pressure wave may be transferred to the first measurement part, which may result in a first vibration of the first measurement part having a larger amplitude.

The membrane may be flexible and may be made of various materials. The membrane may be added or attached to an existing first measurement part of the optical waveguide, or may become part of the optical waveguide during the production process of the optical waveguide or part of the optical waveguide. For example, a photonic integrated circuit, may be placed on or is incorporate into a membrane. In an embodiment, a light propagating part of the first measurement part may be suspended in the membrane, which may be referred to as a suspended waveguide.

A pressure wave impinging the first measurement part has generally the same frequency as the frequency of a pressure wave impinging the second measurement part, in particular if the frequencies of the pressure waves are associated with the modulation frequency of the light beam. In case the first resonant frequency of the first measurement part is substantially equal to a second resonant frequency of the second measurement part may cause similar vibrations in both measurement parts, which may further increase the changes of the optical propagation properties of the optical waveguide. This may in particular the case the modulation frequency is substantially equal to the first resonant frequency of the first measurement part and/or an entire multiple of the first resonant frequency of the first measurement part and the modulation frequency is substantially equal the second resonant frequency of the second measurement part and/or an entire multiple of the second resonant frequency of the second measurement part.

In an embodiment of the system:

• the light system further is configured to generate a further light beam, to modulate the further light beam at a further modulation frequency, and to direct the further light beam to a further space; • the optical waveguide further comprises a further first measurement part,

• the system further comprises a further vibration unit arranged in the further space remotely from the space, wherein the further vibration unit comprises:

o a further first fixation structure and a further second fixation structure,

wherein the further first fixation structure and the further second fixation structure are arranged remotely from each other in the further space, and wherein the further first measurement part of the optical waveguide is tightened between the further first fixation structure and the further second fixation structure, and wherein the further first measurement part extends through the further space and is in immediate contact with the further space;

• the detection unit further is configured to detect a further first vibration of the

further first measurement part based on changes in optical propagation properties of the optical waveguide and wherein the signal processing unit further is configured to detect the further first vibration of the further first measurement part based on the generated sensor light beam and the measured sensor light beam.

The system for photoacoustic spectroscopy may be suitable to detect molecules in spaces that are remote, disconnected or distant from each other. For example, the space and the further space are remote from each other. The optical waveguide extends in the space and in the further space. A further first measurement part is arranged in the further space. A light source may generate both the light beam and the further light beam. Also, the detection unit may detect a first vibration in the first measurement part and a further first vibration in the further first measurement part, reducing the need of multiple light sources and detection units, reducing e.g. costs and maintenance efforts. The detection unit may detect or derive a first vibration based on changes in optical propagation properties of the optical waveguide.

Multiplexing such as e.g. frequency multiplexing or wavelength multiplexing may enable that various signals associated with vibrations of measurement parts in the optical waveguide, such as the first measurement part or the further first measurement part, may be propagated through the optical waveguide to the detection unit. In case the detection unit comprises measuring changes of a strain in the optical waveguide or in at least one part of the optical waveguide, it may be beneficial to include Bragg gratings or Bragg reflectors in the optical waveguide to facilitate multiplexing, such as frequency and/or wavelength multiplexing. For example, wavelength multiplexing may be enabled by placing a first Bragg grating on one end of the vibration unit, and a second Bragg grating on another end of the vibration unit. The first and the second Bragg gratings form a pair. Similarly, a further first Bragg grating may be placed on one end of the further vibration unit, and a further second Bragg grating may be placed on another end of further the vibration unit. The further first and the further second Bragg gratings form a further pair. The pair and the further pair may be operated at different wavelengths form each other, such that a first vibration of the first measurement part can be distinguished from a further first vibration of the further first measurement part, even if both vibrations are identical.

In case of frequency multiplexing it may be sufficient to place a Bragg grating before the first vibration unit and another Bragg grating after the further vibration unit. A first vibration of the first measurement part can be distinguished from a vibration if both vibrations differ from each other.

In an embodiment of the system, a further first resonant frequency of the further first measurement part is substantially different from the first resonant frequency of the first measurement part.

In case the further first resonant frequency is different from the first resonant frequency, the first measurement part will generally vibrate differently than the further first measurement part, even if both measurement parts are impinged by pressure waves having equal frequencies. This further improves the possibility to determine whether a molecule is present in the space and/or is present in the further space.

In an embodiment of the system, the light system further is configured to vary the modulation frequency and to vary the further modulation frequency.

By varying the modulation frequency and the further modulation frequency may permit to determine whether a vibration in the optical waveguide is caused in the first space and/or in the further space, in particular if the first measurement part arranged in the space, has a different vibrational property than the further first measurement part arranged in the further space.

The modulation frequency and the further modulation frequency may be substantially equal and/or may be varied substantially equally. This may e.g. be obtained by a modulation of a source light beam emitted from the light source and a splitting of the source light into the light beam and the further light beam.

In an embodiment of the system, the further modulation frequency is different from the modulation frequency. If the further modulation frequency is different from the modulation frequency, generally also a pressure wave that may propagate through the space has generally a different frequency from the frequency of a further pressure wave that may propagate through the further space. Consequently, vibrations of measurement parts arranged in the space are generally different from vibrations of further measurement parts arranged in the further space. By associating certain vibrations in the optical waveguide with the space and by associating certain other vibrations in the optical waveguide with the further space, it may be possible to identify whether a pressure wave is propagating through the space and/or a further pressure wave is propagating through the further space. As a result, it may be possible to determine by the detection unit whether a molecule or type of molecule is present in the space and/or in the further space.

In an embodiment of the system, the modulation frequency is substantially equal to the first resonant frequency of the first measurement part and/or the entire multiple of the first resonant frequency of the first measurement part and wherein the further modulation frequency substantially equal to the further first resonant frequency of the further first measurement part and/or a further entire multiple of the further first resonant frequency of the further first measurement part.

Setting the modulation frequency equal to the first resonant frequency

and setting the further modulation frequency equal to the further first resonant frequency results in an increase of changes in the optical propagation properties of the optical waveguide.

In case the modulation frequency and the further modulation frequency are equal each other and vary equally over time, it may be beneficial that the modulation frequency and the further modulation frequency are substantially equal to the first resonant frequency of the first measurement part and/or the entire multiple of the first resonant frequency of the first measurement part. Furthermore, it may also be beneficial that the modulation frequency and the further modulation frequency are substantially equal to the further first resonant frequency of the further first measurement part and/or the entire multiple of the further first resonant frequency of the further first measurement part.

In an embodiment of the system, the system further comprises a container wherein the container comprises a pressure wave opening through which an inner space of the container is in direct contact with the surroundings of the container (e.g. the space), and wherein the first measurement part of the optical waveguide extends over the pressure wave opening.

The container may be used to contain a gas in the space. The optical waveguide may be connected or fixed to the container. A pressure wave may propagate or travel through the pressure wave opening. Since the first measurement part of the optical waveguide extends over or covers the pressure wave opening, the pressure wave may directly impinge the first measurement part, causing a vibration thereof. The container may further comprise a light beam opening, enabling the directing of the light beam to the gas in the space. Also, the container may comprise a gas inlet and a gas outlet. The pressure wave opening may function as the gas inlet and gas outlet. The container may further comprise one or more mirrors arranged in the inner space to reflect the light beam, possibly increasing the amount of molecules that may be excited.

In case the light system may be configured to generated a second light beam, also the second light beam may be directed through the light beam opening. In another embodiment, the container may comprise a second light beam opening through which the second light beam may be directed to the gas in the space.

In an embodiment of the system, the system further comprises a container wherein the first measurement part is arranged in the inner space of the container.

It may be beneficial to prevent gas in a container to leak to the surrounding

environment of the container or to prevent a gas in the surrounding to leak into the container. This may prevent the use of a pressure wave opening in the container. To enable that a gas in the space may propagate a pressure wave to the first measurement part of the optical waveguide, the first measurement part may be arranged inside the container.

In an embodiment of the system, part of the optical waveguide is wrapped around the container.

Wrapping the optical waveguide or part of the optical waveguide around the container may provide simple means to attach the optical waveguide to the container. By wrapping part of the optical waveguide around the container it may be easy to create multiple (e.g. at least two) measurement parts, by having multiple (e.g. at least two) parts of the optical waveguide extending over the pressure wave opening. The edges of the pressure wave opening may provide the first fixation structure, the second fixation structure, the third fixation structure etc. When two opposing edges of the pressure wave opening are substantially parallel to each other, the multiple measurement parts may have similar vibration properties, since the distance between two fixation structures in each pair of fixation structures is defined by the width of the pressure wave opening over which the measurement parts extend.

In an embodiment of the system, a container resonant frequency of the container is substantially different from the first resonant frequency of the first measurement part.

It may be beneficial that the container vibrates as little as possible, for example to avoid disturbances in surrounding instruments. On the other hand, in case molecules should be detected it is beneficial that the first vibration of the first measurement part is strong or has a large amplitude. To achieve a first vibration of the first measurement part, without vibration of the container it is beneficial that the container resonant frequency of the container is substantially different from the first resonant frequency of the first measurement part.

In an embodiment of the system, the relative position between the first fixation structure and the second fixation structure is adjustable.

By adjusting the relative position between the first fixation structure and the second fixation structure it is possible to adjust the vibration properties of the first measurement part.

It may be clear that various recited features that may be beneficial, may also hold only for some time periods. For example, the modulation frequency may be substantially equal to the first resonant frequency for some time periods. The time periods may be long enough to permit a detection of a molecule.

In a second aspect of the invention a method for photoacoustic spectroscopy is provided, the method comprising:

• generating a light beam, modulating the light beam at a modulation frequency, and directing the light beam to a space; wherein the light beam generates a pressure wave propagating through the space by exciting molecules in a gas in the space;

• causing a first vibration, by the pressure wave, of a first measurement part of an optical waveguide, wherein the first measurement part of the optical waveguide is tightened, i.e. tensioned, between a first fixation structure and a second fixation structure, and wherein the first measurement part extends through the space and is in immediate contact with the space;

• detecting the first vibration of the first measurement part based on changes in optical propagation properties in the optical waveguide, wherein detecting the first vibration of the first measurement part comprises: o generating the sensor light beam propagating through the optical

waveguide;

o measuring the sensor light beam;

o determining changes in optical propagation properties, in the optical

waveguide based on the generated sensor light beam and the detected sensor light beam.

The method according to the second aspect of the invention provides steps of functioning or operating of the system according to the first aspect of the invention. The method provides the same or similar advantages and benefits as the system.

These and other aspects of the invention will be more readily appreciated as the same becomes better understood by reference to the following detailed description and considered in connection with the accompanying drawings in which like reference symbols designate like parts.

Brief descriptions of the drawings

Figure 1 depicts a first embodiment of a photoacoustic system.

Figure 2 depicts an illustration on the operating of the first embodiment.

Figure 3 depicts an extension of the first embodiment with a second measurement part.

Figure 4 depicts an extension of the first embodiment with a membrane.

Figure 5 depicts another embodiment configured to detect a molecule in a space and/or in a further space.

Figure 6 depicts an embodiment comprising a container.

Figure 7 depicts another embodiment comprising another container.

Figure 8 depicts embodiment comprising a container without an optical signal opening. Figure 9 depicts an embodiment wherein a container is arranged in a space and a further container is arranged in a further space.

Figure 10 depicts a flow diagram of an embodiment of a method of photoacoustic spectroscopy. Detailed description of embodiments

Figure 1 discloses an embodiment of a system for photoacoustic spectroscopy according to the invention. The system is schematically depicted in Figure 1. For the purpose of illustrates a two dimensional representation of the system is provided. The representation may not be on scale.

The system is configured to detect molecules of methane or CH4 in a gas of a space 1. However, other molecules may be detected as well such as C02, H20, C2H4, C2H2, NH3, etc. if a suitable light source is used. A possible application of the embodiment is to detect gas leaks, wherein the system may be coupled to an alarm system.

The system comprises a light system 3, comprising a light source. In this case the light source is a laser. The laser emits light at a wavelength or first optical wavelength of 1654 nm. Other wavelengths are possible as well. However, in order to obtain a system of high efficiency, it is preferred that the wavelength of the emitted light coincides of with the wavelength associated with an absorption line of methane, that is a wavelength that is suitable to excite a methane molecule.

The light system 3 is configured to generate a light beam, to modulate the light beam at a modulation frequency, and to direct the light beam to the space 1. In this case, the light amplitude of the light beam is modulated at 10 kHz. Other frequencies are possible as. Also, other modulations are possible, such as a modulation of the wavelength of the light beam.

The light beam may be directed to the gas by pointing the light source towards the gas. Also, the light beam may be guided to the gas by means of a light guide 501.

The space 1 is schematically indicated by a dashed line. In other embodiments, the space 1 may be shaped differently. The space may be any space wherein the first

measurement part 5 of the system for photoacoustic spectroscopy may be installed. In Figure 1 , the light system 3 is inside of the space. However, in other embodiments of the system, the light system 3 may be outside or partly outside of the space 1.

The light system 3 may be configured to emit light beams with different optical wavelengths. For example, besides emitting a light beam having a wavelength of 1654 nm, another wavelength may be emitted that coincides with a wavelength associated with an absorption line of C02. In this way, both CH4 and C02 may be detected.

The system further comprises an optical waveguide 2, comprising a first measurement part 5. In this case the optical waveguide 2 is an optical fiber 2. The modulation frequency may be substantially equal to a first resonant frequency of the first measurement part 5 and/or an entire multiple of the first resonant frequency of the first measurement part 5. Since the light beam is modulated with a frequency of 10 kHz it is beneficial if the first resonant frequency of the first measurement part 5 and/or an entire multiple of the first resonant frequency of the first measurement part 5 is equal to 10 kHz. A vibration unit 5 is arranged in the space 1. The vibration unit 5 comprises a first fixation structure 7 and a second fixation structure 9, wherein the first fixation structure 7 and the second fixation structure 9 are arranged remotely from each other in the space 1 , and wherein the first measurement part 5 of the optical waveguide 2 is tightened between the first fixation structure 7 and the second fixation structure 9, and wherein the first measurement part 5 extends through the space 1 and is in immediate contact with the space 1. In this case the optical fiber 2 is clamped at the first fixation structure 7 and at the second fixation structure 9. By tightening the optical fiber 2 between the first fixation structure 7 and the second fixation structure 9, the first measurement part 5 is like a tightened string, similarly as strings on a guitar.

The system further comprises a detection unit 13, configured to detect a first vibration of the first measurement part 5 based on changes in optical propagation properties of the optical waveguide 2. The detection unit 13 comprises a sensor light source 15 and a sensor light detector 15. In Figure 1 , the sensor light source 15 and the sensor light detector 15 are within a single unit. In other embodiments, the sensor light source 15 and the sensor light detector 15 may be separated.

. Bragg gratings or Bragg reflectors 11 may be comprised in or coupled to the optical waveguide 2 in order to facilitate determining changes in the strain of the optical waveguide 2. In other embodiments, the Bragg reflectors 11 may be absent and changes in optical propagation properties may be detected by the detection unit 13 without using Bragg reflectors 11.

The sensor light source 15 is coupled to the optical waveguide 2, wherein the sensor light source 15 is configured to generate a sensor light beam propagating through the optical waveguide 2.

The sensor light detector 15 is coupled to the optical waveguide 2, wherein the sensor light detector 15 is configured to measure the sensor light beam having propagated through the optical waveguide 2.

The system further comprises a signal processing unit 17, configured to detect a first vibration of the first measurement part 5 based on the generated sensor light beam and the measured sensor light beam. The signal processing unit may be further configured to send a signal comprising information on the first vibration to other units.

Figure 2 illustrates the operating of the embodiment depicted in Figure 1. The light system 3 generates a light beam 201 and modulates the light beam 201 at a modulation frequency. The light beam is directed to the space 1. Methane molecules are built up of a number of atoms bonded to one another. These atoms are moving, but are also constrained by interatomic bonds. The leads to various vibration modes of the molecule 203 having respective frequencies of vibration.

The light beam 201 excites a molecule 203 of methane in a gas in the space 1 by increasing the amplitude of a vibration mode of the methane molecule 203.

The excitation of the molecule 203 is followed by an de-excitation of the same molecule 201. After an excitation, the extra energy of the molecule 203 is quickly transferred to other molecules in the vicinity by colliding with them. By modulating the light beam 201 , an excitation / de-excitation pattern of molecules may create a compression and decompression of the gas, creating a pressure wave 205 propagating through the gas in the space 1.

The pressure wave 205 causes a first vibration of the first measurement part 5 by directly impinging the first measurement part 5. The first vibration of the first measurement part 5 causes deformations of the first measurement part 5, resulting in changes of the optical propagation properties of the optical waveguide 2. The detection unit 3 may detect the first vibration of the first measurement part 5, based on the changes of the optical propagation properties of the optical waveguide 2.

Figure 3 depicts an embodiment of the system, similarly as the embodiment depicted in Figure 1. However, in Figure 3, the optical waveguide 2 further comprises a second measurement part 35 and wherein the vibration unit 19 further comprises a third fixation structure 37 and a fourth fixation structure 39, wherein the third fixation structure 37 and the fourth fixation structure 39 are arranged remotely from each other in the space 1 , and wherein the second measurement part 35 of the optical waveguide 2 is tightened between the third fixation structure 37 and the fourth fixation structure 39, and wherein the second

measurement part 35 extends through the space 1 and is in immediate contact with the space 1.

The detection unit 13, further is configured to detect a second vibration of the second measurement part 35 based on changes in optical propagation properties of the optical waveguide 2 and wherein the signal processing unit 17, further is configured to detect the second vibration of the second measurement part 35 based on the generated sensor light beam and the measured sensor light beam.

The first resonant frequency of the first measurement part 5 may be substantially equal to a second resonant frequency of the second measurement part 35. In this case, the first resonant frequency and the second resonant frequency may be designed to be 20 kHz.

The first fixation structure 7 may be the same fixation structure as the third fixation structure 37. The second fixation structure 9 may be a different structure as the third fixation structure 39. Other variations may be possible as well. The first resonant frequency of the first measurement part 5 may be substantially different to a second resonant frequency of the second measurement part 35.

The light system 3 may further be configured to modulate the light beam at different frequencies comprising the modulation frequency and a second modulation frequency.

The second modulation frequency is substantially equal to a resonant frequency of the second measurement part 35 and/or an entire multiple of the second resonant frequency of the second measurement part 35.

The light system 3 may further be configured to generate a second light beam, to modulate the second light beam at a second beam modulation frequency, and to direct the second light beam to the space 1 , wherein a first optical wavelength of the light beam is different from a second optical wavelength of the second light beam. In this case the second optical wavelength may be associated with an absorption line of C02, to enable detection of C02 besides methane.

The modulation frequency may be different from the second beam modulation frequency.

The first optical wavelength may be substantially equal to a first excitation wavelength associated with an excitation of a first molecule. The second optical wavelength may be substantially equal to a second excitation wavelength associated with an excitation of a second molecule. In the case the first molecule may be methane, whereas the second molecule may be C02.

Figure 4 depicts an embodiment of the system, similarly as the embodiment depicted in Figure 1. However, in Figure 4 the first measurement part 5 comprises additionally a membrane 401. The membrane 401 may be flexible and may be made of various materials.

Figure 5 depicts an embodiment of the system, similarly as the embodiment depicted in Figure 1. However, the light system 3 further is configured to generate a further light beam, to modulate the further light beam at a further modulation frequency, and to direct the further light beam to a further space 51. In this case, the further space may be at a different location where a leakage of methane should be detected. In particular, the embodiment according to Figure 5 permits to detect methane at multiple and different locations using a single light system 3 and a single detection unit 13.

The light system 3 comprises a light guide 501 and a light head 503, to direct the light beam to the space 1. The light guide 501 may be an optical waveguide. The light system 3 also comprises a further light guide 505 and a further light head 507, to direct the light beam to the further space 51.

The further first resonant frequency of the further first measurement part 55 may be substantially different from the first resonant frequency of the first measurement part 5.

The light system 3 may be configured to vary the modulation frequency and to vary the further modulation frequency. The further modulation frequency may be different from the modulation frequency.

The embodiment according to Figure 5 may be useful if one wants to determine if e.g. methane is present in the space and/or in the further space. At least some time instances the modulation frequency may coincide with a resonant frequency of the first measurement 5, and at least some time instances the further modulation frequency may coincide with a resonant frequency of the further first measurement part 55. If the first measurement part 5 and the further measurement part 55 vibrate differently, the difference is vibration may be used to determine to location of the vibration, i.e. the location where methane is present.

The further space 51 may be remotely apart from the space 1 , as is indicated by the dashed line 509.

The optical waveguide 2 further comprises a further first measurement part 55.

The system further comprises a further vibration unit 519 arranged in the further space 51 remotely from the space 1.

The modulation frequency may be substantially equal to the first resonant frequency of the first measurement part 5 and/or the entire multiple of the first resonant frequency of the first measurement part 5. The further modulation frequency may be substantially equal to the further first resonant frequency of the further first measurement part 55 and/or a further entire multiple of the further first resonant frequency of the further first measurement part 55.

The further vibration unit 519 comprises a further first fixation structure 57 and a further second fixation structure 59, wherein the further first fixation structure 57 and the further second fixation structure 59 are arranged remotely from each other in the further space 51 , and wherein the further first measurement part 55 of the optical waveguide 2 is tightened between the further first fixation structure 57 and the further second fixation structure 59, and wherein the further first measurement part 55 extends through the further space 51 and is in immediate contact with the further space 51.

The detection unit 13 further is configured to detect a further first vibration of the further first measurement part 55 based on changes in optical propagation properties of the optical waveguide 2. The signal processing unit 17 further is configured to detect the further first vibration of the further first measurement part 55 based on the generated sensor light beam and the measured sensor light beam. The detection unit 13 may be configured to detect a first vibration in the first measurement part 5 and/or further first vibration in the further first measurement part 55.

Figure 6 depicts an embodiment of the system comprising a container 603. The container may be made from various materials and have various shapes. In this case the container 603 is made by plastic, which may be cheaply fabricated. The container comprises a pressure wave opening 605 through which an inner space of the container is in direct contact with the surroundings of the container, and wherein the first measurement part 5 of the optical waveguide extends over the pressure wave opening 605.

The light system 3 comprises a light guide 501 to direct the light beam to the space 1 , via a light opening 601. The container 603 may be in entirely in the space 1. In further embodiments, the space may be completely or partly in the container. Generally, the molecule 203 to be detected, i.e. methane, is inside of the container.

The detection unit 13, comprises in Figure 6 a separate sensor light source 13a and a sensor light detector 13b. The sensor light source 13a and/or the sensor light detector may comprise processing units that may be communicatively connected to each other via a wired or wireless connected, schematically indicated by the dashed line 13c.

The container 603 may contain mirrors to reflect the light beam, which results in more excitations of molecules.

Figure 7 depicts another embodiment of the system comprising a different container 603. A difference with the embodiment depicted in Figure 6, is the presence of multiple signal openings 605. A single optical waveguide 2 is tightly wrapped around the container 603. An edge of a pressure wave opening is or functions as a first fixation structure 7. Another edge of the same pressure wave opening is or functions as a second fixation structure 9. Only the first fixation structure 7 and the second fixation structure 9 are explicitly indicated. More fixation structures are present, since the optical waveguide 2 extends over multiple pressure wave openings 605. The embodiment according to Figure 7 comprises a first measurement part 5, a second measurement part 35, and more measurement parts such as indicated e.g. by 705a, 705b, 705c. Other measurement parts are also present.

Figure 8 depicts an embodiment of the system wherein the first measurement part 5 is arranged in the inner space of the container 603. Openings through which the optical waveguide may enter or leave the inner space of the container 603 may function as a first fixation structure 7 and/or as a second fixation structure 9. The dashed lines in Figure 8 indicate parts that are not visible from the current view point of the container 603. In particular, the first measurement part 5 is not visible since it is inside of the container 603. The container may also comprise a gas inlet and/or a gas outlet through which the gas can enter and/or leave the inner space of the container 603.

Figure 9 depicts an embodiment which is similar to the embodiment depicted in Figure 5. However, in Figure 9 depicts a container 603 placed in the space 1 , a further container 9603 is placed in the further space 51. The vibration unit 19 is attached to the container 603 and the further vibration unit 519 is attached to the further container 9603. The containers 603 and 9603 are similar as the container according to Figure 6.

Figure 10 depicts a flow diagram of an embodiment of a method of photoacoustic spectroscopy. The steps may further be explained by Figure 2.

Step 1001 comprises generating a light beam 201 , modulating the light beam 201 at a modulation frequency, and directing the light beam to a space 1 ; wherein the light beam 201 generates a pressure wave 205 propagating through the space 1 by exciting molecules 203 in a gas in the space 1.

Step 1003 comprises causing a first vibration, by the pressure wave 205, of a first measurement part 5 of an optical waveguide 2, wherein the first measurement part 5 of the optical waveguide 2 is tightened between a first fixation structure 7 and a second fixation structure 9, and wherein the first measurement part 5 extends through the space 1 and is in immediate contact with the space 1.

Step 1005 comprises detecting the first vibration of the first measurement part 5 based on changes in optical propagation properties in the optical waveguide 1 , wherein detecting the first vibration of the first measurement part 5 comprises:

o generating the sensor light beam propagating through the optical

waveguide 2;

o measuring the sensor light beam;

o determining changes in optical propagation properties, in the optical

waveguide 2 based on the generated sensor light beam and the detected sensor light beam.

As explained in detail above, a system for photoacoustic spectroscopy is disclosed. The system comprises:

• an optical waveguide, comprising a first measurement part;

• a vibration unit arranged in the space, comprising: o a first fixation structure and a second fixation structure, wherein the first fixation structure and the second fixation structure are arranged remotely from each other in the space, and wherein the first measurement part of the optical waveguide is tightened between the first fixation structure and the second fixation structure, and wherein the first measurement part extends through the space and is in immediate contact with the space;

o a sensor light detector coupled to the optical waveguide, wherein the

o a signal processing unit, configured to detect the first vibration of the first measurement part based on the generated sensor light beam and the measured sensor light beam.

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which can be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. Further, the terms and phrases used herein are not intended to be limiting, but rather, to provide an understandable description of the invention.

The terms "a" or "an", as used herein, are defined as one or more than one. The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. The terms including and/or having, as used herein, are defined as comprising (i.e. , open language, not excluding other elements or steps). Any reference signs in the claims should not be construed as limiting the scope of the claims or the invention.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The term coupled, as used herein, is defined as connected, although not necessarily directly, and not necessarily mechanically.

Claims

1. System for photoacoustic spectroscopy, the system comprising:

• an optical waveguide, comprising a first measurement part;

• a vibration unit arranged in the space, comprising:

o a sensor light detector coupled to the optical waveguide, wherein the

2. The system according to claim 1 , wherein the first measurement part is tightened between the first fixation structure and the second fixation structure to provide a tensioning force in the first measurement part of the optical fiber, the tensioning force extending between the first fixation structure and second fixation structure along a length of the first

measurement part of the optical fiber.

3. The system according to claim 1 or 2, wherein the first measurement part extends through the space and is configured to vibrate at the first vibration by a pressure wave generated by the light beam exciting molecules in the space.

4. System according to any of the preceding claims, wherein the modulation frequency is substantially equal to a first resonant frequency of the first measurement part and/or an entire multiple of the first resonant frequency of the first measurement part.

5. System according to any of the preceding claims, wherein the optical waveguide further comprises a second measurement part and wherein the vibration unit further comprises:

wherein the detection unit further is configured to detect a second vibration of the second measurement part based on changes in optical propagation properties of the optical waveguide and wherein the signal processing unit further is configured to detect the second vibration of the second measurement part based on the generated sensor light beam and the measured sensor light beam.

6. System according to claim 5, wherein the first resonant frequency of the first measurement part is substantially equal to a second resonant frequency of the second measurement part.

7. System according to claim 5, wherein the first resonant frequency of the first measurement part is substantially different to a second resonant frequency of the second measurement part.

8. System according to any of the preceding claims, wherein the light system further is configured to modulate the light beam at different frequencies comprising the modulation frequency and a second modulation frequency.

9. System according to claim 8, wherein the second modulation frequency is

substantially equal to a resonant frequency of the second measurement part and/or an entire multiple of the second resonant frequency of the second measurement part.

10. System according to any of the preceding claims, wherein the light system further is configured to generate a second light beam, to modulate the second light beam at a second beam modulation frequency, and to direct the second light beam to the space, wherein a first optical wavelength of the light beam is different from a second optical wavelength of the second light beam.

11. System according to claim 10, wherein the modulation frequency is different from the second beam modulation frequency.

12. System according to any of the claims 10 - 11 , wherein the first optical wavelength is substantially equal to a first excitation wavelength associated with an excitation of a first molecule and wherein the second optical wavelength is substantially equal to a second excitation wavelength associated with an excitation of a second molecule.

13. System according to any of the preceding claims, wherein

• the light system further is configured to generate a further light beam, to modulate the further light beam at a further modulation frequency, and to direct the further light beam to a further space;

• the optical waveguide further comprises a further first measurement part,

o a further first fixation structure and a further second fixation structure, wherein the further first fixation structure and the further second fixation structure are arranged remotely from each other in the further space, and wherein the further first measurement part of the optical waveguide is tightened between the further first fixation structure and the further second fixation structure, and wherein the further first measurement part extends through the further space and is in immediate contact with the further space;

14. System according to claim 13, wherein a further first resonant frequency of the further first measurement part is substantially different from the first resonant frequency of the first measurement part.

15. System according to any of the claims 13 - 14, wherein the modulation frequency is substantially equal to the first resonant frequency of the first measurement part and/or the entire multiple of the first resonant frequency of the first measurement part and wherein the further modulation frequency is substantially equal to the further first resonant frequency of the further first measurement part and/or a further entire multiple of the further first resonant frequency of the further first measurement part.

16. System according to any of the claims 13 - 15, wherein the light system further is configured to vary the modulation frequency and to vary the further modulation frequency.

17. System according to any of the claims 13 - 16, wherein the further modulation frequency is different from the modulation frequency.

18. System according to any of the preceding claims, further comprising a container wherein the container comprises a pressure wave opening through which an inner space of the container is in direct contact with the surroundings of the container, and wherein the first measurement part of the optical waveguide extends over the pressure wave opening.

19. System according to any of the claims 1 - 18, further comprising a container, wherein the first measurement part is arranged in the inner space of the container.

20. System according to claim 18 or 19, wherein part of the optical waveguide is wrapped around the container.

21. System according to any of the claim 18 - 20, wherein a container resonant frequency of the container is substantially different from the first resonant frequency of the first measurement part.

22. System according to any of the preceding claims, wherein the first measurement part comprises a membrane.

23. Method for photoacoustic spectroscopy, the method comprising:

• causing a first vibration, by the pressure wave, of a first measurement part of an optical waveguide, wherein the first measurement part of the optical waveguide is tightened between a first fixation structure and a second fixation structure, and wherein the first measurement part extends through the space and is in immediate contact with the space;

• detecting the first vibration of the first measurement part based on changes in optical propagation properties in the optical waveguide, wherein deriving the first vibration of the first measurement part comprises:

o generating the sensor light beam propagating through the optical

waveguide;

o measuring the sensor light beam;

o determining changes in optical propagation properties, in the optical