PHOTO-ACOUSTIC SENSOR HEAD AND PHOTO-ACOUSTIC MEASURING APPARATUS WITH IMPROVED INTERFERENCE SIGNAL SUPPRESSION

Abstract

The disclosure relates to a photoacoustic sensor head for detecting acoustic signals which are excited in a sample by absorption of pulsed measuring light, comprising a contact prism which is transparent for the measuring light and has a sample contact surface, a detection surface arranged opposite the sample contact surface and a light entrance surface arranged adjacent to the detection surface, as well as means for radiating the measuring light through the light entrance surface in the direction of the sample contact surface, wherein a detection device comprising at least one sound transducer is arranged in a manner covering the detection surface, characterized in that those portions of the measuring light which are reflected at the sample contact surface are directed to the detection surface or to the light entrance surface, wherein a material layer containing a material which absorbs the measuring light is arranged between the detection surface and the detection device.

Claims

1. A photo-acoustic sensor head for the detection of acoustic signals, which are triggered in a sample through absorption of a pulsed measuring light (40), comprising a contact prism transparent for the measuring light with a sample contact surface, a detection surface arranged opposite the sample contact surface and a light entry surface arranged adjacently to the detection surface as well as means for irradiating the measuring light through the light entry surface in direction of the sample contact surface, wherein a detection device comprising at least one sound converter is arranged so as to cover the detection surface wherein the portions of measuring light reflected at the sample contact surface are directed at the detection surface or the light entry surface, and wherein a material layer containing a material absorbing the measuring light is arranged between the detection surface and the detection device.

2. The photo-acoustic sensor head according to claim 1, wherein the material layer arranged between the detection surface) and the detection device is an adhesive for fastening the detection device to the contact prism.

3. The photo-acoustic sensor head according to claim 1, wherein the material absorbing the measuring light is a light absorber pigment.

4. The photo-acoustic sensor head according to claim 1, wherein the means for irradiating the measuring light comprise at least one optical fibre connected in a manner optically transparent manner for the measuring light to the light entry surface of the contact prism, wherein the measuring light exiting from the fibre end and thereupon fanning out illuminates the entire sample contact surface.

5. The photo-acoustic sensor head according to claim 4, wherein the means for irradiating the measuring light comprise a plurality of optical fibres adjacent to each other in a linear arrangement.

6. The photo-acoustic sensor head according to claim 1, wherein a backing material is arranged on the at least one sound converter of the detection device, the acoustic impedance of which is greater than that of the sound converter material.

7. A photo-acoustic measuring apparatus comprising a photo-acoustic sensor head according to claim 1, a light source for pulsed measuring light, a device for supplying measuring light to the sensor head, a device for measured data recording of the detection device and a device for lighting control, which causes the light source to emit measuring light pulses of a predetermined pulse duration at predetermined points of time, wherein the measuring apparatus comprises a device for measured data evaluation communicating with the device for lighting control and with the device for measured data recording, which causes the device for measured data recording to record the measured data of the detection device only during a plurality of non-overlapping time intervals, the temporal positions of which are predetermined as regards the points in time of emitting the measuring light pulses, and the interval lengths of which in total are smaller than the temporal distance between two successive measuring light pulses.

8. The photo-acoustic measuring apparatus according to claim 7, wherein the device for measured data evaluation causes the recorded measured data to be averaged across a plurality of measuring light pulses or performs this averaging.

9. The photo-acoustic measuring apparatus according to claim 7 of claims 7, wherein the plurality of time intervals after emitting a measuring light pulse comprises exactly two time intervals.

10. The photo-acoustic measuring apparatus according to claim 8, wherein the first time interval starts at the point in time of emitting the measuring light pulse and ends before an acoustic signal generated in the sample and entering into the contact prism through the sample contact surface reaches the detection surface.

11. A photo-acoustic sensor head for the detection of acoustic signals, which are triggered in a sample through absorption of a pulsed measuring light (40), comprising a contact prism transparent for the measuring light with a sample contact surface, a detection surface arranged opposite the sample contact surface and a light entry surface arranged adjacently to the detection surface as well as means for irradiating the measuring light through the light entry surface in direction of the sample contact surface, wherein a detection device comprising at least one sound converter is arranged so as to cover the detection surface, wherein the portions of measuring light reflected at the sample contact surface are directed at the detection surface or the light entry surface, wherein a material layer containing a material absorbing the measuring light is arranged between the detection surface and the detection device, wherein the material layer arranged between the detection surface and the detection device is an adhesive for fastening the detection device to the contact prism, wherein the material absorbing the measuring light is a light absorber pigment, wherein the means for irradiating the measuring light comprise at least one optical fibre connected in a manner optically transparent manner for the measuring light to the light entry surface of the contact prism, wherein the measuring light exiting from the fibre end and thereupon fanning out illuminates the entire sample contact surface, wherein the means for irradiating the measuring light comprise a plurality of optical fibres adjacent to each other in a linear arrangement, and wherein a backing material is arranged on the at least one sound converter of the detection device, the acoustic impedance of which is greater than that of the sound converter material.

12. The photo-acoustic measuring apparatus according to claim 8, wherein the plurality of time intervals after emitting a measuring light pulse comprises exactly two time intervals.

Description

[0029] Examples of how the photo-acoustic sensor head could be designed will now be explained in more detail, also with reference to the figures, in which

[0030] FIG. 1 shows possible designs of the photo-acoustic sensor head in lateral views a) and b) and in top views c) and d);

[0031] FIG. 2 shows a schematic plot of the measured pressure amplitudes of the interference signal (broken line) and the useful signal of the sample in the generated temporal separation.

[0032] FIG. 1 shows two possible designs of the inventive photo-acoustic sensor head. The part FIGS. 1a) and b) each depict a sample 10, on which a contact prism 20 is arranged such that the sample 10 and the contact prism 20 touch each other at the sample contact surface 30. The material of the contact prism 20 is transparent for the measuring light 40, which is irradiated via at least one optical fibre 60 through the light entry surface 50 into the contact prism 20 in direction of the sample contact surface 30.

[0033] Usually the measuring light 40 comprises wavelengths within the infrared spectrum, in particular the near (NIR) and medium infrared (MIR) range. For some purposes however the measuring light 40 may be visible light (VIS) or originate from another non-ionising spectral range.

[0034] Due to the transparency request the choice of material of the contact prism 20 orients itself on the wavelengths of the measuring light 40. As regards light in the medium infrared spectrum (MIR) for example, suitable materials are semi-conducting materials such as geranium, zinc selenide, silicone, indium phosphide, gallium arsenide or chalcogenide glasses, and as regards light in the near infrared spectrum (NIR) or visible spectrum (VIS) suitable materials are silicone dioxide (quartz, glasses), aluminium oxide (corundum, sapphire, ruby) or even some plastics (e.g. polyethylene).

[0035] The means for irradiating the measuring light 40 in the part FIG. 1a) are one or more optical fibres 60, which are arranged fixed to the light entry surface 50 of the contact prism 20. The fixing is not shown. The measuring light 40 exiting from the fibres 60 fans out in the contact prism 20 and illuminates the entire sample contact surface. A first portion of the measuring light 40 penetrates into the sample 10 and triggers useful signals, whereas a second portion of the measuring light 40 is reflected in direction of the detection surface 70. The detection surface 70 is covered by the detection device 80, which comprise at least one sound converter. As a rule the detection device 80 only comprises one single sound converter, which extends across the entire detection surface 70. The detection device 80 is for example glued onto the detection surface 70 of the contact prism 20 by means of the material layer 90, which also contains light absorber particles. The second portion of the measuring light 40 arriving in the material layer 90 is, as far as possible, fully absorbed, which triggers an interference signal. The detection device 80 records the interference signal, before a useful signal from the sample can arrive.

[0036] In the design of part FIG. 1b) a small portion of the measuring light 40 is also mirrored back onto the light entry surface 50 and thus into the exit end of the fibre 60. This case is typical if the measuring light 40 is irradiated vertically onto the sample contact surface 30.

[0037] The part FIGS. 1c) and d) each show a top view of the sensor heads from the part FIGS. 1a) and b), respectively, wherein the direction of view is onto the sample 10. The light entry surface 50 in FIG. 1c) is formed as a rectangle, so that even a plurality of optical fibres 60 can be arranged and fixed along the long axis of the rectangle. In FIG. 1d) the light entry surface 50 is in the centre of the detection surface 70 with the detection device 80. This configuration too, in which the light entry surface 50 is surrounded by the detection surface 70, is to be understood as adjacent arrangement of the light entry surface 50 to the detection surface 70.

[0038] A photo-acoustic measuring apparatus with a photo-acoustic sensor head according to the invention can be designed specifically for advantageous use of the sensor head, in that a measured data evaluation device is added, which takes appropriate account of the isolated interference signal occurring immediately upon irradiating the measuring light 40.

[0039] Apart from the sensor head the measuring apparatus comprises a light source for pulsed measuring light 40, a device for supplying the measuring light 40 to the sensor head, a device for measured data recording by the detection device 80 and a device for lighting control, which causes the light source to emit measuring light pulses of predetermined pulse duration at predetermined points in time. Moreover the measuring apparatus shall comprise a device for measured data evaluation communicating with the device for lighting control and the device for measured data recording. The device for measured data evaluation causes the device for measured data recording to record the measured data of the detection device 80 only during a plurality of non-overlapping time intervals, the temporal positions of which are predetermined as regards the points in time of emitting the measuring light pulses and the interval lengths of which in total are smaller than the temporal distance between two successive measuring light pulses.

[0040] In other words, the time span between emitting two successive measuring light pulses is divided into non-overlapping time intervals, of which some but not all, are being provided for measured data recording. The device for measured data evaluation specifies the time intervals with data recording after predetermination by the user. For example, the device for measured data evaluation comprises a stop watch which is reset on triggering a measuring light pulse, as well as a table with stop watch readings at which time intervals start and end, in which measured data are to be recorded. In one possible implementation the device for measured data evaluation instructs the device for measured data recording to activate or deactivate data recording when a tabulated stop watch reading is present. The device for data recording comprises at least one non-volatile electronic data memory, which digitally stores the voltage values received from the detection device 80 during the time intervals predetermined for data recording.

[0041] Preferably the device for measured data recording and the device for measured data evaluation form a constructional unit. They can be realised in a particularly simple manner by way of programming a conventional personal computer.

[0042] In order to remove random noise it is very advantageous to average the recorded measured data across a plurality of measuring light pulses, i.e. across a number of time intervals with respectively the same time reference for emitting a measuring light pulse. Preferably the device for measured data evaluation initiates this process in that it repeats its time requirements directed at the device for measured data across a sequence of measuring light pulses. The recorded measured data can be added up in the data memory of the device for data recording by way of the known boxcar averaging and then divided by the number of measuring light pulses, in order to determine a mean value.

[0043] FIG. 2 shows a schematic plot of the temporal course of the pressure amplitude (PA) for two successive measuring light pulses. Random noise has not been taken into account. The solid line curves represent the acoustic useful signal from the sample, which first has to propagate through the contact prism in order to reach the detection device. It therefore arrives distinctly after the interference signal (depicted as a broken line), which is generated immediately after the emission of a measuring light pulse. Both signals can be recorded in separate non-overlapping time intervals. The length of the second time interval (useful signal) can be predetermined by the user; in particular it can be very much larger than the length of the first time interval. Both time intervals together are shorter than the temporal distance between the measuring light pulses.

[0044] It appears to be sufficient in many cases and therefore also advantageous that the plurality of time intervals after emission of a measuring light pulse comprises exactly two time intervals. In this case it is preferably provided that the first interval begins at the point in time when the measuring light pulse is emitted, and ends before an acoustic signal generated and entering into the contact prism through the sample contact surface reaches the detection surface.