Abstract:
A photoacoustic sensor, containing a resonance body, which at least partially delimits a space for receiving molecules to be detected, and a device for detecting an oscillation of the resonance body, including a device for optically detecting the location of at least one partial surface of the resonance body. A method for the photoacoustic detection of molecules in the gas phase and to a method for producing an optically integrated photoacoustic sensor.

Description:
BACKGROUND 
       [0001]    The invention relates to a photoacoustic sensor, comprising a resonance body which at least partly delimits a volume intended to hold molecules to be detected and an apparatus for identifying a vibration of the resonance body. 
         [0002]    A. A. Kosterev et al.: Quartz-enhanced photoacoustic spectroscopy, Optics Letters, Vol. 27, No. 21 (2002) 1902 has disclosed a device of the type mentioned at the outset. This known detection method discloses the use of a fork-shaped quartz crystal as a highly sensitive microphone, by means of which pressure variations in a gas phase can be detected. According to the known method, the pressure variations are generated by means of a laser diode, which selectively excites the molecules in the gas phase by means of spectrally narrow-band radiation. The sensitivity of the photoacoustic measurements can be increased due to the high Q-factor of the fork-shaped quartz crystal used for the detection. 
         [0003]    However, the method known from the prior art is disadvantageous in that the material selection for producing the fork-shaped element is restricted to piezoelectric materials. Hence, the signal processing requires the measurement of a signal voltage lying between a few picovolts and a few nanovolts. Measuring such small voltages is susceptible to electric disturbance signals. Furthermore, the known photoacoustic gas sensor cannot be used in explosive gas atmospheres because the piezoelectric sensor can cause an explosion by voltage sparkovers. Finally, the known measurement method cannot be used in hot gas atmospheres either because the piezoelectric materials used fail if the temperature is too high. 
         [0004]    Proceeding from this prior art, the invention is therefore based on the object of specifying a method for gas analysis, which can be applied universally, even at high temperatures and in potentially explosive regions. Furthermore, the method should have increased reliability with respect to electric disturbance signals. 
       SUMMARY 
       [0005]    According to the invention, it is proposed to excite the molecules situated in the gas atmosphere selectively by narrow-band laser radiation from a continuous wave laser or by appropriately shaped laser pulses with a duration of less than 200 fs. According to the invention, in order to detect the excitation that took place, a photoacoustic sensor is proposed, which comprises a resonance body which at least partly delimits a volume intended to hold molecules to be detected. This allows a pressure variation arising during the deexcitation of the optically excited molecules to be detected as a photoacoustic signal. 
         [0006]    The intensity of this photoacoustic signal is proportional to the concentration of the molecules to be detected. Different molecules can be excited by different wavelengths of the laser light used for the excitation or by differently shaped pulses of a short-pulse laser with pulse lengths of less than 200 fs. This affords the possibility of testing the composition of a gas mixture and/or the presence or absence of a molecule to be detected in the gas mixture. 
         [0007]    According to the present invention, the vibration of the resonance body is identified by means of an apparatus for optically capturing the location of at least one subarea of the resonance body. In another embodiment of the invention, the location of a subarea of the resonance body can be determined by means of an interference signal. This method is distinguished by a very high accuracy because the location can be used down to a fraction of the wavelength of the light used for the measurement. As a result of the long coherence length, it is advantageously possible to use the light of a laser for optically capturing the location of at least one subarea of the resonance body. 
         [0008]    In some embodiments of the invention, the resonance body can have at least two prongs, arranged approximately in parallel, which are respectively fixed to a connection element with a foot point and project freely at the end thereof opposite to the foot point. This results in the optical impression of a fork or a rake. In some embodiments, such a resonance body can simplify the spectroscopic detection of gaseous molecules if the at least two prongs, arranged approximately in parallel, at least partly delimit the measurement volume in which the molecules to be detected are situated. This enables a direct influence of the pressure variation arising when the molecules are excited on the at least two prongs arranged approximately in parallel. Furthermore, this geometry enables an efficient suppression of coupled-in air sound if the length of and/or the distance between the at least two prongs arranged approximately in parallel is selected to be smaller than the wavelength of the sound acting on the device. 
         [0009]    In some embodiments of the invention, the sensor comprises at least one beam displacer with at least one input and at least two outputs, designed to split an input laser beam into a plurality of output laser beams, the latter being provided for being reflected at different subareas of the resonance body. This results in a particularly low-disturbance and reliable evaluation of the photoacoustically induced vibration of the resonance body. As a result of the reflection at two subareas vibrating with different amplitudes, the two laser beams experience a phase shift that can be detected very easily as a measurable change in intensity after the two beams are brought together and subsequently separated according to polarization direction. 
         [0010]    In some embodiments of the invention, the laser light used to excite the molecules to be detected can be coupled-in as a free beam. To this end, some embodiments of the invention can make use of optical prongs such as slits, stops or lenses. In other embodiments of the invention, the light used to excite the molecules to be detected can be supplied by means of a waveguide. This enables a particularly space-saving design, which is not susceptible to faults, of the sensor proposed according to the invention. 
         [0011]    In some embodiments, the sensor proposed according to the invention can be integrated in a single substrate in monolithic form. The substrate can comprise an optically transparent material. In some embodiments of the invention, the substrate can comprise quartz glass or sapphire or magnesium oxide or langasite. In other embodiments of the invention, the substrate can comprise a polymer. In one embodiment of the invention, the substrate can comprise polymethyl methacrylate. 
         [0012]    In some embodiments of the invention, the substrate can be processed with the aid of a pulsed laser such that both waveguides and mechanically movable microstructures are produced in an optically transparent substrate. Here, the interaction of laser pulses, e.g. laser pulses with a duration of less than 250 fs, can induce a change in the refractive index. The change in the refractive index can be introduced into the material in a punctiform and three-dimensional fashion by focusing the laser beam used for the material processing. By overlapping the respective points it is possible to introduce a structure with a longitudinal extent, e.g. a waveguide, into the material. In some embodiments of the invention, the laser pulses used for processing the material can be modulated in terms of their amplitude and/or phase. 
         [0013]    In a further embodiment of the invention, provision can be made for areal regions in the substrate, which are exposed by the laser beam used to process the substrate, to be etched by means of an acid. By way of example, this can be brought about by means of HF. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In the following text, the invention is intended to be explained in more detail on the basis of exemplary embodiments and figures, without this restricting the general inventive concept. In detail: 
           [0015]      FIG. 1  shows the measurement principle, proposed according to the invention, for optical readout of the photoacoustic sensor according to one embodiment of the invention. 
           [0016]      FIG. 2  illustrates a first embodiment of a photoacoustic sensor, as proposed according to the invention. 
           [0017]      FIG. 3  illustrates a second embodiment of a photoacoustic sensor according to the present invention. 
           [0018]      FIG. 4  shows a third embodiment of the photoacoustic sensor according to the present invention. 
           [0019]      FIG. 5  shows an embodiment of a sensor system with a plurality of sensors. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0020]      FIG. 1  shows an embodiment of the measurement principle, proposed according to the invention, for optical readout of the photoacoustic sensor. The gas comprising the molecules to be detected surrounds a resonance body  11 . In some embodiments of the invention, the resonance body  11  can have at least two prongs, arranged approximately in parallel, which are respectively fixed to a connection element with a foot point and project freely at the end thereof opposite to the foot point. In this case, the volume  18  intended to hold the gas with the molecules to be detected is situated between the two prongs arranged approximately in parallel. 
         [0021]    The molecules to be detected are excited by means of a light beam  41 . The light beam  41  can be provided by means of a laser  42 . By way of example, the laser  42  can be a diode laser or a femtosecond laser. Accordingly, the light beam  41  comprises laser radiation with a narrow spectral band, which can selectively excite at least one electronic transition, a rotational transition or a vibrational transition in the molecules to be detected. To the extent that the laser  42  is a femtosecond laser, the light beam  41  comprises laser pulses with a duration of less than 200 fs, less than 100 fs, less than 50 fs or less than 20 fs. In this case, modulating the amplitude and/or the phase of the laser pulses can selectively excite at least one rotational and/or vibrational excitation in the molecules to be detected. 
         [0022]    The optical excitation of the molecules to be detected leads to the transfer of translation energy onto the molecules, which causes a pressure variation. The pressure variation in turn excites a vibration of the resonance body  11 . 
         [0023]    The vibration of the resonance body  11  is captured by means of an optical apparatus. The apparatus for capturing the vibration comprises a laser  10 , e.g. a He—Ne laser or a laser diode. The laser  10  provides a laser beam  12  that is provided to capture the vibration of the resonance body  11  by measuring the location of at least one areal region  116  of the resonance body  11 . To this end, an interferometer is available in the embodiment illustrated in  FIG. 1 . 
         [0024]    The interferometer comprises a beamsplitter  31 . The beamsplitter is an optical element that splits an incident light beam  12  into two light beams. In some embodiments of the invention, the beamsplitter can comprise a cube that is composed of two triangular glass prisms. A first part of the light is reflected at and a second part of the light is transmitted on the interface running diagonally through the cube. In other embodiments of the invention, the beamsplitter can comprise a semi-transparent mirror. The semi-transparent mirror can comprise a glass plate, which is provided with a vapor-deposited coating made of a metal or an alloy. The metal or the alloy can comprise aluminum. In some embodiments of the invention, the first part of the light and the second part of the light can each comprise approximately 50% of the irradiated light  12 . 
         [0025]    The light leaving the beamsplitter in a straight direction reaches a beam displacer  20 . The beam displacer  20  has at least one input  21  and at least two outputs  22 . The beam displacer  20  divides the entering light beam  12  into at least two emerging light beams  13 . In some embodiments of the invention, the beam displacer can comprise a Wollaston prism, a Rochon prism, a Glan-Thomson prism or a Nicol prism. The aforementioned prisms comprise a birefringent material. The refractive index of such a birefringent material depends on the polarization of the entering light  12 . As a result, light with a first polarization direction and light with a second polarization direction respectively take a different path through the material of the prism. As a result, a first light beam  13  with a first polarization and a second light beam  13  with a second polarization are available at the output of the beam displacer. 
         [0026]    One of the light beams  13  at the output of the beam displacer  20  can be used as a reference beam. The other light beam  13  at the output of the beam displacer  20  can be used as a detection beam. The two beams are reflected at different subareas of the resonance body  11 . As a result of the vibration of the resonance body  11 , the detection beam experiences a phase shift relative to the reference beam. Both reflected light beams are once again coupled into the beam displacer  20  through the outputs  22 . The light beams  13  reflected at the resonance body  11  are brought together in the beam displacer  20  and leave the beam displacer via the input  21  thereof in the direction of the beamsplitter  31 . 
         [0027]    In the beamsplitter  31 , the light reflected at the resonance body  11  is reflected and routed to a second beamsplitter  20   a.  In the beamsplitter  20   a,  the light reflected at the resonance body  11  is once again split into the reference beam and the detection beam as a result of the polarization dependence of the refractive index in the interior of the beamsplitter  20   a.    
         [0028]    A photodiode  35  enabling a spatially dependent detection of the incident light is available at the output of the beamsplitter  20   a.  In some embodiments of the invention, the photodiode  35  can be a quadrant photodiode. In other embodiments of the invention, the photodiode  35  can be formed by two photodiodes, of which one detects the detection beam and one detects the reference beam. 
         [0029]    The electric signal from the photodiode  35  is captured by means of electronics  171 . Here, the electronics  171  can comprise a voltage supply, by means of which it is possible to apply a bias voltage to the photodiode  35 . Furthermore, the electronics  171  can comprise a preamplifier and/or a discriminator, by means of which the signals from the photodiode  35  are processed further. Moreover, the electronics  171  can comprise an analog-to-digital converter in order to process further the analog output signals from the photodiode  35  by means of digital electronics. In some embodiments of the invention, the electronics  171  can comprise an arithmetic unit that forms the quotient of the difference between the two signals and the sum of the two signals as per the following formula: 
         [0000]    
       
         
           
             f 
             = 
             
               
                 
                   a 
                   + 
                   b 
                 
                 
                   a 
                   - 
                   b 
                 
               
               . 
             
           
         
       
     
         [0030]    In some embodiments of the invention, the electronics  171  can comprise an arithmetic unit that forms the difference between the two signals as per the following formula: 
         [0000]    
       
      
       f=a−b.  
      
     
         [0031]    In order to increase the detection sensitivity and to suppress background signals, the laser  42  can, in some embodiments of the invention, be modulated by the resonant frequency of the resonance body  11  or by a frequency that constitutes an integer multiple of the resonant frequency. This enables the measurement signal provided at the output of the electronics  171  to be evaluated by means of a lock-in amplifier  172 . The data from the lock-in amplifier  172  can be stored and/or visualized by means of a digital measurement system  173 . Here, the measurement system  173  may comprise a personal computer. 
         [0032]      FIG. 2  shows an exemplary embodiment of a photoacoustic sensor  1 , as proposed according to the invention. The photoacoustic sensor  1  comprises the resonance body  11  and parts of the interferometer  30  in an integrated design. According to the embodiment illustrated in  FIG. 2 , the resonance body  11  and the interferometer  30  are integrated in monolithic form in a single substrate  50 . 
         [0033]    Two recesses  18  and  51  have been introduced into the substrate  50 ; these free up two elongate prongs  114  and  113 . The recesses can be produced by means of micromechanical processing methods. The recesses  18  and  51  can be produced by means of etching, sawing, milling or another processing method, known per se, such that this results in the impression of a slit in the substrate  50 . The recesses can have a width of between approximately 10 μm and approximately 500 μm. The recesses  51  and  18  encompass the substrate  50  in its entire thickness. 
         [0034]    The prongs  114  and  113  are respectively connected to a connection element  111  at a foot point  112 . The end  115  opposite to the foot point  112  projects freely. As a result of this, the elongate prongs  114  and  113  can vibrate at a resonant frequency that depends on the geometric dimensions of the prongs  114  and  113 , and on Young&#39;s modulus of the material of the substrate  50 . In some embodiments, the prongs  113  and  114  can have a length of between approximately 100 μm and approximately 10 000 μm, and a width of between approximately 50 μm and approximately 1000 μm. 
         [0035]    Between the elongate prongs  113  and  114  there is a volume  18  provided for holding the molecules to be detected. A light beam  41  is available to excite the molecules, as described in conjunction with  FIG. 1 . Since the volume  18  is formed by a continuous recess which encompasses the substrate  50  in its entire thickness, the light beam  41  can be supplied as a free beam in the embodiment as per  FIG. 2 . To this end, the light beam  41  is irradiated in a plane that is approximately perpendicular to the plane of the measurement beam  13   a  and the reference beam  13   b.  This prevents the light beam  41  from being directly incident on the resonance body  11  or the elongate element  114  and the measurement signal from being falsified by the acting photon pressure. 
         [0036]    An interferometer  30  is available for optically capturing the vibration of the elongate element  114 . Like the resonance body  11 , the interferometer  30  is also integrated in the substrate  50  in monolithic form. The interferometer  30  comprises a beamsplitter  31 , as explained in conjunction with  FIG. 1 . Light from a laser is routed to the beamsplitter  31  by means of an optical waveguide. The optical waveguide starts at a plug-in connection  43   a,  which is provided for holding a glass fiber in a manner known per se. Further optical waveguides open at the outputs of the beamsplitter  31 ; these connect the beamsplitter  31  to respective beam displacers  20  and  20   a.    
         [0037]    The beam displacer  20  has one input  21  and two outputs  22   a  and  22   b.  The beam displacer  20  can comprise a birefringent material, as explained in conjunction with  FIG. 1 . A measurement beam  13   a  that is incident on a subarea  116   a  of the elongate element  114  is available at the output  22   a  of the beam displacer  20 . A reference beam  13   b  that is incident on a second subarea  116   b  of the elongate element  114  is available at the output  22   b  of the beam displacer  20 . Since the subarea  116   b  lies closer to the foot point  112  of the elongate element  114 , the vibration amplitude of the areal element  116   b  when the element  114  vibrates is less than the amplitude of the areal element  116   a.  As a result of this difference, the reflected light beams  13   a  and  13   b  experience a phase shift when the elongate element  114  vibrates and this can be measured by interferometry. 
         [0038]    The light reflected at the areas  116   a  and  116   b  is introduced into the beam displacer  20  via the outputs  22   a  and  22   b,  which now act as inputs of the beam displacer  20 , and is decoupled from the beam displacer  20  via the input  21 , which now acts as output. 
         [0039]    The light reflected at the areas  116   a  is subsequently reflected at the beamsplitter  31  and enters the second beam displacer  20   a.  The beam displacer  20   a  has two outputs  22   c  and  22   d.  Light from the reference beam  13   b  reaches the output  22   c  of the beam displacer  20   a  as a result of its polarization direction. Light from the measurement beam  13   a  reaches the output  22   d  of the beam displacer  20   a  as a result of its polarization direction, which differs from that of the beam  13   b.  The signals from the outputs  22   c  and  22   d  reach associated plug-in connectors  43   b  and  43   c  via appropriate optical waveguides, which are formed in the substrate  50 . The plug-in connectors  43   b  and  43   c  are also provided for holding a glass fiber in a manner known per se. The light can be routed to at least one photodiode and to evaluation electronics via this glass fiber, as explained in conjunction with  FIG. 1 . 
         [0040]    The optical elements of the interferometer  30 , such as the beamsplitter  31  and/or the beam displacer  20  and/or the beam displacer  20   a  and/or the optical waveguides that interconnect the aforementioned elements or connect these to the plug-in connectors  43   a,    43   b  and  43   c  can, in some embodiments of the invention, be introduced into the substrate  50  with the aid of laser material processing. 
         [0041]    In one embodiment of the invention, the laser material processing can comprise the irradiation of laser pulses that have a duration of less than 250 fs, less than 100 fs, less than 50 fs or less than 20 fs. It is possible to induce a change in the refractive index as a result of the interaction of the laser pulses with optically transparent materials such as e.g. quartz glass, magnesium oxide, langasite, sapphire, polymethyl methacrylate or other polymers. As a result, the substrate  50  has first regions with a first refractive index and second regions with a second refractive index. There is a jump in the refractive index, at which there can be total-internal reflection of light, at the transition regions between the irradiated and the non-irradiated regions. This allows the irradiated region to be used as an optical element, for example as a waveguide. By selecting a two-dimensional spatial coordinate and a position of the focus of the laser beam, it is possible to induce the change in refractive index in three dimensions in the substrate  50 . The surface roughness can be optimized by modulating the amplitude and/or phase of the laser pulses used for the material processing. 
         [0042]    Furthermore, exposed material can be attacked by an etching means which attacks unexposed material to a lesser extent. By way of example, the etching step can be carried out as a wet chemical or dry chemical etching step. This also makes it possible to produce the mechanical structures, for example the plug-in connectors  43  and/or the resonator  11 , by exposing and etching the substrate  50 . The recesses  51  and  18  can also be produced in this fashion. 
         [0043]      FIG. 3  shows a further embodiment of the present invention. The embodiment as per  FIG. 3  also comprises a resonance body  11 , which surrounds a volume  18  in which the molecules to be detected can be introduced. Provision is once again made for a light beam  41  to excite the molecules to be detected, as described in conjunction with  FIGS. 1 and 2 . 
         [0044]    An apparatus  17  is available for optically identifying the vibration of the resonance body  11 . The apparatus  17  comprises an interferometer  30 , as described in conjunction with  FIG. 2 . 
         [0045]    Deviating from  FIG. 2 , the photoacoustic sensor as per  FIG. 3  is not completely integrated in monolithic form. According to  FIG. 3 , the resonator  11  is made of a first material and the apparatus  17  for optically capturing the vibration of the resonator  11  is made of a substrate  50  made of a second material. This makes it possible to use respectively optimized materials, which achieve the respective objects to the best possible extent, for both the resonator  11  and also for the apparatus  17 . By way of example, a material can be selected for the apparatus  17  into which the optical components can be introduced in a particularly simple fashion by means of laser material processing. Alongside this, a material whose Young&#39;s modulus has been optimized to the extent that the resonant frequency of the resonator  11  is in a desired range for predeterminable geometric dimensions can be selected for the resonator  11 . Furthermore, the material of the resonator  11  can be selected such that the resonator  11  has good reflection properties, at least on those subareas  116  on which the laser beams  13  are reflected. 
         [0046]    The joint between the resonator  11  and the apparatus  17  can for example be brought about by adhesive bonding, being screwed together, a clamping point or welding, or by further contacting methods not explicitly mentioned. The recess  51  is obtained by shaping a step  52  in the substrate  50  when forming the joint between the apparatus  17  and the resonator  11 . 
         [0047]    In a development of the embodiment as per  FIG. 3 , the apparatus  17  can be present twice. This makes it possible to use the first apparatus  17  to monitor the vibration of the element  114 . The second apparatus  17 , which is attached on the resonator  11  symmetrically with respect to the volume  18 , can be used to capture the vibration of the element  113 . This makes it possible to distinguish between in-phase vibrations of the prongs  113  and  114  and opposite-phase vibrations of the prongs  113  and  114  in order thereby to identify a background signal that can arise as a result of surrounding sound acting on the resonator  11 . 
         [0048]      FIG. 4  shows a further embodiment of the present invention. The photoacoustic sensor as per  FIG. 4  is also integrated in monolithic form in a single substrate  50 . The photoacoustic sensor as per  FIG. 4  in turn comprises two elongate prongs  113  and  114 , which partly delimit a volume  18  and a recess  51 . The prongs  113  and  114  are respectively fixed to a connection element  111  with a foot point  112 . The end  115  opposite to the foot point  112  projects freely and thereby enables a bending vibration of the prongs  114  and  113 . Capturing the vibration of the element  114  is brought about by means of a Mach-Zehnder interferometer  30 . The Mach-Zehnder interferometer  30  comprises two optical waveguides  32  and  33 , which are routed in parallel in a subsection  34 . This results in evanescent coupling between the signals carried in the two waveguides  32  and  33 . 
         [0049]    During the operation of the photoacoustic sensor  1 , a coherent light signal, e.g. from a laser, is coupled in via the plug-in connection  43   a,  which is provided for holding an optical fiber. The light propagates through the waveguide  33  and reaches the subarea  116   a  of the element  114  as detection beam. 
         [0050]    As a result of the evanescent coupling between the waveguides  32  and  33  in the subsection  34 , part of the coupled-in light reaches the subarea  116   b  of the element  114  as reference beam. As a result of the vibration of the element  114 , the light signals reflected at the subareas  116   a  and  116   b  experience a phase shift. The reflected light signals once again run through the respective waveguides  32  and  33 . There is interference between the phase-shifted reflected signals in the region of the subsection  34 . The interference pattern can then be captured by means of a photodiode connected to the plug-in connector  43   b  by means of an optical fiber. The signal processing of the electric signal provided by the photodiode is then brought about as described in conjunction with  FIG. 1 . 
         [0051]    A light beam  41  is once again available for the photoacoustic excitation of the molecules to be detected that are situated in the volume  18 . The light beam  41  is once again coupled-in in a plane that runs perpendicular to the plane of the optical waveguides  32  and  33 . Contrary to the illustration in  FIGS. 2 and 3 , the light beam  41  in the embodiment as per  FIG. 4  is not routed into the volume  18  as a free beam, but rather it is routed there by means of a waveguide  40 . The waveguide  40  likewise ends in a plug-in connector  43   c,  which is provided for holding an optical fiber. The light beam  41  can be a narrow-band or broad-band beam, be continuous or pulsed, as already explained in conjunction with  FIGS. 1 and 2 . 
         [0052]    Independently of whether the light beam  41  is routed into the volume  18  by means of an optical fiber  40  or as a free beam, care should be taken that this light beam does not impinge on the prongs  114  and  113 . This can lead to a disturbance signal as a result of the photon pressure of the light beam  41 , which leads to a worsening detection limit or worsening sensitivity of the photoacoustic sensor. 
         [0053]    In order to produce the photoacoustic sensor  1  as per  FIG. 4 , the optical waveguides  32 ,  33  and  34  can be introduced into the substrate  50  by means of laser material processing, as described in conjunction with  FIG. 2 . In other embodiments of the invention, optical waveguides, e.g. glass fibers, can also be embedded into a substrate  50 . 
         [0054]      FIG. 5  shows a sensor system with a plurality of sensors  1   a,    1   b,    1   c,    1   d  and  1   e  as illustrated in  FIG. 4 . The plurality of sensors  1  are respectively connected to a multiplexer  174  by means of a fiber bundle  300 ,  301 ,  302 ,  303  and  304  of optical waveguides. The multiplexer  174  sequentially connects the respective sensors  1   a,    1   b,    1   c,    1   d  and  1   e  to a laser light source  10  and an apparatus  17  for evaluating the recorded output signals of the interferometer  30 . This enables a plurality of photoacoustic sensors  1 , which are respectively connected to the evaluation apparatus  17  in a cycle, to be kept available for a plurality of different molecules to be detected in one embodiment of the invention. This enables various different molecules to be detected in the gas phase to be examined with only short switching times of the multiplexer  174 . In another embodiment of the invention, a predeterminable molecule to be detected can be detected at different locations using only one light source and/or one evaluation apparatus. 
         [0055]    In order to ensure that individual photoacoustic sensors can be distinguished amongst the plurality of photoacoustic sensors, the photoacoustic sensors can have individual resonant frequencies in some embodiments of the invention. 
         [0056]    It is self-evident that the illustrated exemplary embodiments can be combined in order thus to obtain further, different embodiments of the invention. The description above should therefore not be construed as restrictive, but rather be considered explanatory. The claims below should be understood such that a mentioned feature is present in at least one embodiment of the invention. This does not preclude the presence of further features. To the extent that the claims define “first” and “second” features, this designation serves to distinguish between two identical features, without setting an order.