Equipment and method for temporally resolving recording of pulsed high-frequency electromagnetic radiation

Abstract

The present invention relates to an apparatus for the time-resolved capture of pulsed radiofrequency electromagnetic radiation (8), comprising a generator (2) configured to produce pulses of radiofrequency electromagnetic radiation when the apparatus is in operation, and a detector (3) configured and arranged to capture (10) the field strength of the pulses (8') reflected by the sample (4) as a function of time when the apparatus is in operation. The apparatus also includes a distance measuring system (12) and an evaluation device (9) connected to the detector and the distance measuring system. The distance measuring system is configured and arranged to capture (13) a change in the distance between the generator and the sample, and / or between the sample and the detector, as a function of time when the apparatus is in operation.The evaluation device is configured such that it calculates (14) a corrected function of the field intensity over time from the captured function of force over time and the captured function of the change in distance over time.

Description

[0001] The present invention relates to a device for time-resolved detection of pulsed electromagnetic high-frequency radiation comprising a generator, wherein the generator is configured such that the generator produces pulses of electromagnetic high-frequency radiation during operation of the device, and a detector, wherein the detector is configured and arranged such that the detector detects the field strength or the intensity of the pulses reflected by a sample as a function of time during operation of the device.

[0002] The present invention further relates to a method for time-resolved detection of pulsed electromagnetic high-frequency radiation comprising the steps of: generating pulses of electromagnetic high-frequency radiation with a generator, irradiating a sample with the pulses and detecting the field strength of the pulses reflected by the sample as a function of time with a detector.

[0003] Terahertz time-domain spectrometers have been used for some time as excitation-interrogation measurement techniques. A generated electromagnetic pulse in the terahertz frequency range is scanned in a detector using an optical pulse after passing through a sample or reflecting off a sample. This technique takes advantage of the fact that the optical pulse used for scanning is significantly shorter in duration than the pulse of the electromagnetic radiation in the terahertz frequency range. Using this measurement method, the electric or magnetic field of the electromagnetic terahertz pulses is measured with high temporal resolution. From the resulting time-domain field strength function, frequency domain data can be calculated, particularly using Fourier transforms, and information such as the layer thickness of a multilayer sample can also be obtained.

[0004] This sampling measurement method provides usable results as long as the time offset between the sampling optical pulse and the terahertz pulse is well-defined by the measuring apparatus and is not subject to disturbances. If the time offset between the sampling optical pulses and the terahertz pulses changes due to mechanical disturbances during sampling, this method yields a distorted function of the field strength of the terahertz pulse over time, the pulse spectrum is corrupted, and the measurement becomes unusable. However, mechanical vibrations are almost unavoidable, particularly in industrial environments and during robot-assisted measurements, which places high demands on the mechanical stability and, if necessary, mechanical decoupling of the measuring system.

[0005] One approach to reducing the influence of mechanical disturbances is to increase the sampling rate per pulse. The disturbance occurring within a measurement is relatively smaller the higher the sampling rate. However, the maximum possible sampling or measurement rate for a terahertz time-domain spectrometer is limited by the delay devices used. Furthermore, increasing the sampling rate does not provide a fundamental solution to the problem, but only mitigates it by transforming disturbances into a lower frequency range.

[0006] From US 2013 / 0204577 A1, an analytical model is known that simulates the propagation of radiation through a coated continuous web material, where the layer thickness and refractive index determine the velocity and direction of the transmitted radiation. The model predicts properties of the transmitted radiation based on properties of the incident radiation and initially assigned values ​​for the layer thicknesses. The layer thicknesses are determined in a procedure in which incident radiation with known properties is directed onto a coated web material, and then actual measurements of transmitted radiation are compared with the predicted properties. Radiation measurements are obtained using a terahertz time-domain spectroscopy device that generates a continuous reference, allowing the phase and amplitude of a sample pulse to be tracked and corrected.

[0007] Furthermore, WO 2014 / 063044 A2 discloses a system for interpreting terahertz radiation, comprising a terahertz transmitter configured to emit a pulse of terahertz radiation and a terahertz receiver configured to receive at least part of the pulse from the terahertz transmitter. The terahertz receiver is configured to output a signal based on the radiation received by the terahertz receiver.

[0008] JP 4403272 B2 describes the provision of a method and apparatus for measuring information about the amplitude intensity and phase of the terahertz pulse reflected by a sample, without being restricted by the shape of the sample, its physical properties, or the method for its mounting. The described spectrometer consists of: a spectroscopic system for irradiating the sample with the terahertz light pulse generated by the laser pulse and for detecting the light reflected by the sample; and a position measuring device for detecting the relative position between the sample and the spectroscopic system. A delay device of the spectroscopic system is corrected using the detected relative position information.This enables measurement, including information on the amplitude intensity and phase information of the terahertz light pulse reflected by the sample, without being limited by the shape of the sample, its physical properties, and the method of its mounting.

[0009] EP 2899498 A1 describes a method for characterizing a coated body by at least one coating parameter based on its adaptation to a physical model. The coated body comprises a substrate coated with a polymeric coating, such as a paint film, wherein the polymeric coating has at least one layer. The method is performed non-contact by a sensor system, which includes a transmitter system for emitting terahertz radiation, a detector system for detecting terahertz radiation, and a processing unit operationally connected to the transmitter system and the detector system.The method comprises: emitting a terahertz radiation signal by the transmitting system in the direction of the coated body, such that the terahertz radiation interacts with the polymer coating; detecting a response signal by the detector system, wherein the detected terahertz radiation signal has interacted with the polymer coating; determining model parameters of the physical model by optimizing the model parameters such that a predicted response signal of the physical model is matched to the detected response signal, wherein the model parameters represent optical properties of the polymer coating that describe the interaction of the terahertz radiation signal with the polymer coating; determining the at least one coating parameter from the determined model parameters, wherein the at least one coating parameter includes a thickness of the polymer coating.

[0010] Therefore, the object of the present invention is to provide a device and a method for time-resolved detection of pulsed electromagnetic high-frequency radiation, which reduces the influence of mechanical disturbances on the measurement.

[0011] At least one of the aforementioned problems is solved by a device according to claim 1 for time-resolved detection of pulsed electromagnetic high-frequency radiation, comprising a generator, wherein the generator is configured such that the generator produces pulses of electromagnetic high-frequency radiation during operation of the device, and a detector, wherein the detector is configured and arranged such that the detector detects the field strength of the pulses reflected by a sample as a function of time during operation of the device, further comprising a distance measuring system and an evaluation device connected to the detector and the distance measuring system, wherein the distance measuring system is configured and arranged such that the distance measuring system detects a change in the distance between the generator and the sample and / or between the sample and the detector as a function of time during operation of the device.and wherein the evaluation device is configured such that the evaluation device calculates a corrected function of the field strength over time from the recorded function of the field strength over time and the recorded function of the change in distance over time, wherein the corrected function of the field strength is calculated by taking the recorded field strength of one impulse at each time point in time , t at a time t'is transferred, which corresponds to the time at which the field strength would have been detected if the distance between the generator and the sample or between the sample and the detector had not changed during the duration of the pulse sampling, and wherein the detection of the change in the distance between the generator and the sample or between the sample and the detector as a function of time is carried out at a measurement rate of 100 kHz or more, preferably 150 kHz or more and particularly preferably 200 kHz or more.

[0012] Crucial to the present invention is that, independent of the generator and the detector for the pulses of the electromagnetic high-frequency radiation, i.e., in particular independent of the terahertz time-domain spectrometer, changes in the distance between the generator and the sample and / or between the sample and the detector are detected as a function of time.

[0013] In this way, the time base of the detected field strength of the high-frequency radiation pulses can be corrected so that it depends only on the time base specified by the device. For this purpose, the generator and detector for the high-frequency radiation, on the one hand, and the distance measurement system, on the other, must be separate and independent measurement systems.

[0014] In one embodiment of the invention, the distance measuring system is an interferometer or a radar system.

[0015] In one embodiment, an optical interferometer used as a distance measuring system according to the present invention has an accuracy in the range of 10 µm or better. In one embodiment, the distance measuring system has a sampling rate of 0.5 MHz or more.

[0016] In one embodiment of the invention, it is not necessary to determine the absolute distance between the generator and the sample and / or between the sample and the detector. Rather, the aim is to detect changes in this distance.

[0017] Therefore, in an embodiment of the invention in which the change in distance is determined using an interferometer or a radar system, no determination of the absolute distance is required.

[0018] In one embodiment of the invention, the frequency of the electromagnetic high-frequency radiation lies in a frequency range from 1 GHz to 30 THz, preferably from 100 GHz to 5 THz. This frequency range is referred to as the terahertz frequency range for the purposes of this application.

[0019] It is understood that the pulses of the electromagnetic high-frequency radiation are not monofrequent, but have a finite spectral bandwidth depending on the pulse duration.

[0020] While it is generally possible to detect the electric or magnetic field strength with time resolution using a detector for the pulses of electromagnetic high-frequency radiation, it will be advantageous for most embodiments of the invention to detect the field strength of the electric field.

[0021] In one embodiment of the invention, the device comprises a time-domain spectrometer, wherein the generator for the pulses of the electromagnetic high-frequency radiation and the detector for the pulses of the electromagnetic high-frequency radiation are components of this time-domain spectrometer. The time-domain spectrometer also comprises a short-pulse laser source configured to generate pulsed electromagnetic optical radiation during operation of the device. These short optical pulses then serve to drive the generator and to switch the detector.

[0022] Such generators and detectors for electromagnetic radiation in the terahertz frequency range, which are driven or switched by electromagnetic pulses, are in particular non-linear optical crystals, so-called photoconductive or photoconductive switches based on semiconductor devices and spintronic generators and detectors based on a multitude of metallic layers.

[0023] When using a photoconductive switch, possibly in combination with an antenna connected to it, a short electromagnetic pulse striking the photoconductive switch, with a corresponding electrical bias voltage, causes a brief current flow in the component and thus the emission of high-frequency electromagnetic radiation. The electromagnetic pulse on the detector side, on the other hand, serves to briefly switch the detector using the photoconductive switch, thereby making the electric field of the simultaneously incident high-frequency electromagnetic radiation measurable.

[0024] If a current is measured across the leads of the detector's photoconductive switch, the field of the electromagnetic terahertz radiation incident on the high-frequency component can be detected with time resolution. The electric field of the electromagnetic terahertz radiation incident on the detector drives charge carriers longitudinally across the switch. A current flow is only possible if the photoconductive switch is simultaneously closed, i.e., if the switch is being irradiated with the first electromagnetic radiation.

[0025] If an electromagnetic pulse used to switch or gate the photoconductive switch of the detector is short compared to the time course of the electric field of the pulse received by the detector in the terahertz frequency range, the electric field of the terahertz signal can be sampled or measured with time resolution.

[0026] For this purpose, a time offset is introduced between the terahertz pulses hitting the detector and the electromagnetic pulses used to switch the detector, and this offset is varied during the measurement.

[0027] It is understood that the terahertz time-domain spectrometer in an embodiment with a photoconductive switch as a detector has a suitable current or voltage amplifier which is connected on the one hand to the detector for detecting the currents via the switch and on the other hand to the evaluation device.

[0028] In one embodiment, the device includes a beam splitter configured and arranged such that, during operation of the device, it directs a first part of the optical pulses to the generator and a second part of the optical pulse to the detector. In one embodiment, such a beam splitter is a beam splitter, for example, a fiber fusion coupler. In another embodiment, such a beam splitter is implemented by a laser source that generates the optical pulses for the generator and detector in such a way that they are already provided in spatially separated beam paths.

[0029] Furthermore, in one embodiment, the device includes a delay device configured such that, during operation, the delay between the arrival of the radio frequency pulses and the optical pulses on the detector can be adjusted. The delay device is also connected to the evaluation unit, which is configured to control the delay device and thus the time delay between the radio frequency pulse and the optical pulse on the detector during operation.

[0030] In this embodiment, the delay device defines the time base for the detected field strength function. However, this time base requires no correction only if the actual delay between the electromagnetic high-frequency radiation and the optical radiation at the detector is not subject to any influences other than the temporal variation defined by the delay device. If, for example, the distance between the generator and the sample and / or between the sample and the detector changes due to mechanical vibrations, the time base defined by the delay device will be distorted.

[0031] The present invention now makes it possible to correct this time base by having the distance measuring system detect changes in distance between the generator and the sample and / or between the sample and the detector as a function of time. In the evaluation unit, a corrected function of the field strength as a function of time is then calculated from the detected function of the field strength as a function of time and the detected function of the change in distance as a function of time.

[0032] In one embodiment of the invention, the evaluation device is a suitably programmed computer or microprocessor with the necessary interfaces. In one embodiment, the interfaces serve to detect the field strength of the high-frequency radiation, to detect the change in the distance between the generator and the sample and / or between the sample and the detector as a function of time, and to calculate the corrected function of the field strength over time.

[0033] In one embodiment, the evaluation unit is connected via a control line to the delay line, for example, to the encoder of a linear actuator of the delay line. Furthermore, in another embodiment, the evaluation unit is connected to the detector for the high-frequency radiation. In yet another embodiment, the evaluation unit is connected to a detector of the distance measuring system in order to record and evaluate the change in the distance between the generator and the sample and / or between the sample and the detector as a function of time.

[0034] The evaluation unit is set up in such a way that it uses the recorded field strength of a pulse at each point in time to calculate the corrected function of the field strength over time. t at a time t'The time transferred corresponds to the point in time at which the field strength would have been recorded if the distance between the generator and the sample and / or between the sample and the detector had not changed during the sampling of the pulse.

[0035] At least one of the aforementioned problems is also solved by a method according to claim 4 for time-resolved detection of pulsed electromagnetic high-frequency radiation comprising the steps of: generating pulses of electromagnetic high-frequency radiation with a generator, irradiating a sample with the pulses, detecting the field strength of the pulses reflected by the sample as a function of time with a detector, detecting a change in the distance between the generator and the sample and / or between the sample and the detector as a function of time with a distance measuring system, and calculating a corrected function of the field strength over time from the detected function of the field strength over time and the function of the change in distance over time, wherein the corrected function of the field strength is calculated by determining the detected field strength of a pulse at each time point in time. t at a time t'is transferred, which corresponds to the time at which the field strength would have been detected if the distance between the generator and the sample or between the sample and the detector had not changed during the duration of the pulse sampling, and wherein the detection of the change in the distance between the generator and the sample or between the sample and the detector as a function of time is carried out at a measurement rate of 100 kHz or more, preferably 150 kHz or more and particularly preferably 200 kHz or more.

[0036] Insofar as aspects of the invention have been previously described with regard to the device for time-resolved detection of pulsed electromagnetic high-frequency radiation, these also apply to the corresponding method. Insofar as the method is carried out with a device for time-resolved detection of pulsed electromagnetic high-frequency radiation according to this invention, the device shall include the necessary features for this purpose. In particular, embodiments of the device are suitable for carrying out the method.

[0037] If at any time t or if no change in the distance between the generator and the sample and / or between the sample and the detector is detected by the distance measuring system around this time, the field strength remains constant at this point in time. t assigned, which is therefore solely determined by the time base specified by the delay device. However, if at that time tIf a change in distance is detected, the field strength is adjusted from the time specified by the delay device. t at a time t' transferred or shifted, which corresponds to the time offset between the optical pulse and the radio frequency pulse on the detector if no change in the distance between the generator and the sample and / or between the sample and the detector had occurred.

[0038] The method according to the invention is particularly suitable for determining layer thicknesses of a plurality of N Suitable for superimposed layers, such as paint layers. Therefore, in one embodiment of the invention, the sample has a plurality of N superimposed layers. S i in one layer thickness each d i on, whereby i even 1,2,3,..., N, where the layer thicknesses are derived from the corrected function of the field strength over time. d i all NLayers are determined.

[0039] To determine the layer thicknesses, the impulse response of the sample, i.e., the high-frequency radiation reflected by and interacting with the sample, is fitted with a model.

[0040] In one embodiment of the invention, this includes determining the layer thicknesses. d i the steps: a) Selecting a layer thickness d i , an absorption index k i and a refractive index n i for each layer S i , with i = 1,2,3,..., N, b) Calculating a time-dependent field strength E M (t) for the electromagnetic high-frequency radiation reflected by the sample using a model, wherein the model includes a corresponding number of N+1 The interfaces between a measurement environment and the sample, as well as between the individual layers, each have a time-dependent field strength. E j (t), with j = 0,1,2,3,...,N, taking into account the field strength E j (t) depending on the layer thicknesses d i , the absorption indices k i and the refractive indices n i to the time-dependent field strength E M (t) to be added, c) comparing the calculated time-dependent field strength E M (t) with the corrected function of field strength over time, where d) if there is a deviation Q between the calculated field strength E M (t) and the corrected field strength function is greater than a predetermined tolerance T , at least the layer thicknesses are varied and steps b) to d) are repeated until the deviation Q smaller than the tolerance T is, and e) providing the layer thicknesses d i as a result of the layer thickness determination.

[0041] In one embodiment, the absorption indices are also taken in step d). k i and the refractive indices n i varies to determine the layer thickness.

[0042] In one embodiment of the invention, the number of iteration steps is reduced by making assumptions about the dispersion, i.e., the frequency dependence of the absorption indices. k i and refractive indices n i within the frequency bandwidth of the electromagnetic high-frequency radiation used, and these assumptions are incorporated into the calculation in step b).

[0043] In one embodiment, the electromagnetic high-frequency radiation generated in the generator has a predetermined frequency bandwidth, and it is assumed that no dispersion occurs within the predetermined frequency bandwidth of the high-frequency radiation, i.e., the absorption indices are not affected over the frequency bandwidth of the electromagnetic high-frequency radiation used. k i and refractive indices n i assumed to be constant during the calculation in step b).

[0044] In an alternative embodiment, the electromagnetic high-frequency radiation generated in the generator has a predetermined frequency bandwidth and the frequency dependence of the absorption indices k i and the refractive indices n i In step b), a simple function describing the dependency, e.g. according to Drude-Lorentz, is assumed for the given frequency bandwidth during the calculation.

[0045] In another alternative embodiment, the electromagnetic high-frequency radiation generated in the generator has a predetermined frequency bandwidth and the frequency dependencies of the refractive indices n i and the absorption indices k i The specified frequency bandwidth is recorded separately for all layers in advance using calibration measurements, and the measured values ​​obtained in this way are used as the basis for the calculation in step b).

[0046] According to the invention, the change in the distance between the generator and the sample or between the sample and the detector as a function of time is detected at a measurement rate of 100 kHz or more, preferably 150 kHz or more and particularly preferably 200 kHz or more.

[0047] Further advantages, features and possible applications of the present invention will become clear from the following description of an embodiment and the accompanying figures. Figure 1 is a schematic representation of the device according to the invention for time-resolved detection of pulsed electromagnetic high-frequency radiation. Figure 2 is a schematic representation of the method according to the invention for time-resolved detection of electromagnetic high-frequency radiation with the device made of Figur 1 Figure 3 shows a layer thickness measurement on a sample with 3 layers without the spacing correction according to the invention. Figure 4 shows the measurement result of the layer thickness determination of the sample with 3 layers. Figur 3 however, with the distance correction according to the invention.

[0048] In the figures, identical elements are designated with identical reference symbols.

[0049] Figur 1 Figure 1 shows a terahertz time-domain spectrometer 11 as part of the device 1 according to the invention for time-resolved detection of pulsed electromagnetic high-frequency radiation in accordance with the present invention.

[0050] The time domain spectrometer 1 comprises a generator 2 for generating pulsed electromagnetic high-frequency radiation 8 and a detector 3 for detecting the electric field strength of the pulses reflected by a sample 4 as a function of time.

[0051] Sample 4 is a three-layer lacquer sample, and the terahertz time-domain spectrometer 11 is used to determine the thicknesses of all three layers of the lacquer sample 4. Both the generator 2 and the detector 3 are connected via optical fibers 5, 6 to a femtosecond laser as a short-pulse laser source as defined in the present application. The femtosecond laser is part of a Figur 1 The arrangement, designated with reference numeral 7 and shown only schematically, consists of short optical pulses generated by the femtosecond laser. These pulses are split into two beam paths using a fiber fusion coupler, also provided in the arrangement 7. One portion of the pulses is directed to generator 2 via fiber optic 5, and another portion is directed to detector 3 via fiber optic 6.

[0052] Furthermore, in arrangement 7, a delay line is provided as a delay device within the meaning of the present application, consisting of an adjustable optical path. This serves to delay the optical pulses reaching the generator 2 and those reaching the detector 3 relative to each other, in order to enable scanning and time-resolved detection of the electric field of the terahertz radiation 8' generated by the generator 2 and interacting with the sample in the detector 3.

[0053] Both generator 2 and detector 3 are photoconductive switches integrated into antennas for terahertz radiation. While the first switch / antenna combination 2 is used to generate the terahertz radiation 8, the second switch / antenna combination 3 is used for the time-resolved detection of the terahertz radiation 8' reflected by a sample 4.

[0054] When the photoconductive switch of generator 2 is briefly closed by means of the ultrashort optical pulses directed to the switch via optical fiber 5, the switch becomes momentarily electrically conductive. With a corresponding bias voltage, a current pulse flows through the switch, resulting in the emission of a high-frequency electromagnetic pulse. In the photoconductive switch, which forms part of detector 3, the electric field of an incident terahertz pulse then drives free charge carriers across the photoconductive switch when it is illuminated by an optical pulse emerging from optical fiber 6. A current can then be measured across the photoconductive switch of detector 3, which is proportional to the instantaneous electric field of the terahertz pulse.Since the optical pulse for switching detector 3 is temporally much shorter than the temporal extent of the oscillation of the electric field of the terahertz pulse, the terahertz pulse can be sampled with time resolution by delaying the optical pulse relative to the terahertz pulse on the photoconductive switch of detector 3.

[0055] For this purpose, the detector 3 is connected to an evaluation unit 9 via a measuring amplifier. This evaluation unit 9 also controls the delay line in the arrangement 7. The current position of the delay line then defines the time base for the acquisition of the measured function of the electric field over time.

[0056] In the right half of the Figur 1 The time dependence of the electric field of a terahertz pulse reflected by sample 4 is shown as an example. The figure labeled 10 shows the electric field strength plotted against time.

[0057] The signal obtained in this way, however, only reflects the actual course of the electric field over time if the distance between sample 4 and detector 3 does not change simultaneously. Otherwise, the time base is distorted by changes in this distance, as these changes in the time base are not reflected in signal 10. Signal 10 is then distorted.

[0058] According to the invention, the time base generated by the delay line in the arrangement 7 is corrected using the fluctuations in the distance between the sample and the detector 3. For this purpose, the device 1 according to the invention has, in addition to the terahertz time-domain spectrometer 11, a distance measurement system in the form of an optical interferometer 12. The interferometer 12 serves to detect changes in the distance between the sample 4 and the detector 3 at the same sampling rate at which the electric field is also detected using the terahertz time-domain spectrometer 11.

[0059] The change in distance of sample 4 from generator 2 and detector 3 is shown on the right side of the Figur 1 plotted as a function of time and labeled with reference numeral 13. For the schematic consideration of the Figur 1 It is assumed that the sample 4 performs a vibrational motion around a starting point, such that the distance between the sample 4 and the detector 3 changes in an essentially sinusoidal manner.

[0060] This function of recording the change in distance over time is also processed in evaluation unit 9 and, as is also the case in the right half of the Figur 1 As shown schematically, this is used to correct the time base of the recorded function 10 of the field strength over time. The result is a corrected function 14 of the field strength over time.

[0061] Based on the diagrams from Figur 2 It will now be explained in detail how the evaluation unit 9 calculates a corrected function 14 of the field strength over time from the recorded function 10 of the field strength over time and the recorded function 13 of the change in distance over time.

[0062] Figur 2 c) shows a representation of the path difference specified by the deceleration distance S between the terahertz pulse and the optical pulse on detector 3 with respect to time t '. The path difference introduced by the deceleration section corresponds to S a time delay τ , which electromagnetic radiation traveling through the delay path experiences compared to radiation in a reference path. This time delay τ is the time base specified by the delay line for the measurement.

[0063] Figur 2 c) It is assumed that the rate of change of the path difference with respect to time is constant. However, the path difference S between the terahertz pulse and the optical pulse on detector 3 is also subject to fluctuations due to changes in the distance. d between sample 4 and detector 3. Figur 2 a) shows the distance dplotted between sample 4 and detector 3 against time t The fluctuations in the distance are clearly visible. This change in the distance d over time t It follows that the actual path difference S compared to the elapsed time t unlike in Figur 2 c) The function shown is not linear, but has a profile, as exemplified in Figur 2 b) shown.

[0064] To now measure the electric field of the terahertz radiation 8 against time from Figur 2 d) To correct, for example, consider a first measurement point at time t 1 At this point t 1 is the path length difference between the terahertz pulse and the optical pulse on the detector 3 S 1 , which a delay τ 1 corresponds to this difference in path length. S 1 However, with an idealized time base determined solely by the delay distance, this corresponds to a time t' 1 . Accordingly, the measured value E 1 of the electric field E in the diagram Figur 2 d) from that point in time t 1 at that time t' 1 shifted. If this transformation is performed for all measurement points of the electric field E with respect to time. t from the raw data of Figur 2 d) by doing this, one obtains the corrected value and the distance fluctuations from Figur 2 a) purified function of the electric field E compared to time t ' out of Figur 2 e) .

[0065] The device according to the invention, in the embodiment discussed here, is used to determine the layer thicknesses of the three superimposed layers of sample 4. When sample 4 is irradiated with terahertz radiation pulses with a predetermined frequency bandwidth, the incident radiation is partially reflected at each interface, i.e., between the measurement environment and the sample, as well as between two adjacent layers. The time-dependent electric fields of these partial reflections superimpose to form the time-dependent electric field of the sample, which is detected with time resolution by the detector 3. Upon closer examination, the electric field comprises E P (t) The sample also exhibits multiple reflections, which arise from repeated reflections of the high-frequency radiation at the interfaces. The temporal sequence of the partial reflections and their phases depend on the material parameters of the layers.

[0066] To determine all three layer thicknesses of sample 4 with a plurality of N= 3 layers arranged one above the other S i , with i = 1, 2, 3 The following steps are carried out: Each of these layers has a refractive index n i , an absorption index k i and a layer thickness d i which influence the reflection and transmission properties of the layers for the electromagnetic high-frequency radiation used. In step a), for each layer S i a layer thickness d i , a refractive index n i and an absorption index k i The initial values ​​are selected. In a subsequent step b), a time-dependent electric field is applied. E m (t) The electromagnetic high-frequency radiation reflected by or transmitted through the sample is calculated using a model. The model comprises a number of N+1At the interfaces between the measurement environment and the sample, as well as between the individual layers, a time-dependent electric field is generated. E j (t), with j = 0, 1, 2, 3, where the electric fields E j (t) to the time-dependent electric field E M (t) of the model as a function of the layer thicknesses d i , the refractive indices n i and the absorption indices k i The refractive index is added. The model is based on the assumption that the refractive index n i and the absorption index k i each layer S i The electric field is constant over the frequency bandwidth of the high-frequency radiation used, i.e., independent of the frequency of the high-frequency radiation. Then, in step c), the calculated electric field is determined. E M (t) of the model with the detected electric field E P (t) the sample compared, whereby in step d) if a deviation Q between the calculated electric field E M (t) and the detected electric field E P (t) greater than a predetermined tolerance T is, the layer thicknesses d i , the refractive indices n i and the absorption indices k i Continue to vary and repeat steps b) to d) until the deviation is correct. Q smaller than the tolerance T is.

[0067] Is the deviation Q smaller than the tolerance T , in step e) the layer thicknesses d i provided as a result of the layer thickness determination.

[0068] Figur 3 Figure 4 shows measurement results of a corresponding determination of the three layer thicknesses of sample 4, with the correction in the evaluation unit 9 switched off. This means the layer thicknesses were determined based on the recorded function of the field strength over time. Plotted in the Figur 3 This is the result of the layer thickness measurement for the three layers of sample 4, designated as Layer 1 to Layer 3, against the serial number of the corresponding measurement. It is clearly evident that the individual measured values ​​exhibit a scatter of up to 2.5 µm around a mean thickness value.

[0069] In contrast, it shows Figur 4 The measurement results for determining the layer thicknesses of the three layers of the same sample 4 are shown. Again, the result of the layer thickness measurement for the three layers of sample 4, designated as Layer 1 to Layer 3, is plotted against the serial number of the corresponding measurement. In these measurements, however, the layer thickness determination was performed with correction applied. That is, the layer thicknesses were determined using the corrected function of the field strength over time. It is noteworthy not only that the scatter of the individual measured values ​​around a mean value is significantly reduced compared to the measurements without correction for each layer, but also that the absolute values ​​of the layer thicknesses have undergone a considerable correction. This demonstrates the significant influence of a distortion of the time base of the recorded function of the electric field with respect to time due to fluctuations in the distance of sample 4 from the detector 3.

[0070] For the purposes of the original disclosure, it is pointed out that all features as they can be deduced by a person skilled in the art from the present description, the drawings, and the claims, even if they are specifically described only in connection with certain other features, can be combined individually or in any combination with other features or groups of features disclosed herein, unless this has been expressly excluded or technical circumstances render such combinations impossible or pointless. A comprehensive, explicit description of all conceivable combinations of features is omitted here solely for the sake of brevity and readability.

[0071] While the invention has been illustrated and described in detail in the drawings and the preceding description, this illustration and description are merely exemplary and are not intended to limit the scope of protection as defined by the claims. The invention is not limited to the disclosed embodiments.

[0072] Variations of the disclosed embodiments are obvious to a person skilled in the art from the drawings, the description, and the accompanying claims. In the claims, the word "have" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude multiple features. The mere fact that certain features are claimed in different claims does not preclude their combination. Reference numerals in the claims are not intended to limit the scope of protection. Bezugszeichenliste

[0073] 1 Device for time-resolved detection of pulsed electromagnetic high-frequency radiation 2 Generator 3 Detector 4 Sample 5, 6 Optical fiber 7 Arrangement with short-pulse laser system, delay line, and beam splitter 8 Terahertz radiation generated by generator 2 8 Terahertz radiation interacting with sample 4 9 Evaluation unit 10 Detected electric field strength of the terahertz radiation as a function of time 11 Terahertz time-domain spectrometer 12 Optical interferometer 13 Distance as a function of time 14 Corrected electric field strength of the terahertz radiation as a function of time

Claims

1. Apparatus (1) for time-resolved capture of pulsed electromagnetic radio frequency radiation (8) comprising a generator (2), wherein the generator (2) is so adapted that in operation of the apparatus the generator (2) produces pulses of the electromagnetic radio frequency radiation (8), and a detector (3), wherein the detector (3) is so adapted and arranged that in operation of the apparatus the detector captures the field strength of the pulses reflected by a sample (4) as a function (10) of time, wherein the apparatus further has a distance measurement system (12) and an evaluation device (9) connected to the detector (3) and the distance measurement system (12), wherein the distance measurement system (12) is so adapted and arranged that in operation of the apparatus the distance measurement system (12) captures a change in a distance between the generator (2) and the sample (4) or between the sample (4) and the detector (3) as a function (13) of time, and wherein the evaluation device (9) is so adapted that the evaluation device (9) calculates a corrected function (14) of the field strength over time (10) from the captured function (10) of the field strength over time and the detected function (13) of the change in distance over time, wherein the corrected function (14) of the field strength is calculated by the captured field strength of a pulse being transferred at each time t to a time t' which corresponds to that time at which the field strength would have been captured if the distance between the generator and the sample (4) or between the sample (4) and the detector (3) would not have changed during the duration of the pulse, and wherein capture of the change in the distance between the generator and the sample (4) or between the sample (4) and the detector (3) as a function of time is effected with a measurement rate of 100 kHz or more, preferably 150 kHz or more and particularly preferably 200 kHz or more.

2. Apparatus (1) according to claim 1 characterised in that the distance measurement system (12) is an interferometer or a radar system.

3. Apparatus (1) according to one of the preceding claims characterised in that the apparatus (1) includes a time domain spectrometer (11) with a short pulse laser source which is so adapted that in operation of the apparatus it produces optical electromagnetic radiation in pulse form, the generator (2) for the pulses of the electromagnetic radio frequency radiation (8), the detector (3) for the pulses of the electromagnetic radio frequency radiation (8), a beam splitting device which is so adapted and arranged that in operation of the apparatus it passes a first part of the optical radiation on to the generator (2) and a second part of the optical radiation on to the detector (3), and a delay device which is so adapted that in operation of the apparatus a time delay between impingement of the pulses of the electromagnetic radio frequency radiation (8) and the pulses of the optical electromagnetic radiation on the detector (3) is adjustably variable with the delay device, wherein the delay device is connected to the evaluation device (9), and wherein the evaluation device (9) is so adapted that in operation of the apparatus it controls the delay device and the time delay.

4. A method for time-resolved capture of pulsed electromagnetic radio frequency radiation (8) comprising the steps: producing pulses of electromagnetic radio frequency radiation (8) with a generator (2), irradiating a sample (4) with the pulses of the electromagnetic radio frequency radiation (8), and capturing the field strength of the pulses reflected by the sample (4) as a function (10) of time with a detector (3), wherein the method further comprises the steps: capturing a change in a distance between the generator (2) and the sample (4) or between the sample (4) and the detector (3) as a function (13) of time with a distance measurement system (12), and calculating a corrected function (14) of the field strength over time from the captured function (10) of the field strength over time and the function (13) of the change in distance over time, wherein the corrected function (14) of the field strength is calculated by the captured field strength of a pulse being transferred at each time t to a time t' which corresponds to that time at which the field strength would have been captured if the distance between the generator and the sample (4) or between the sample (4) and the detector (3) would not have changed during the duration of the pulse, and wherein capture of the change in the distance between the generator and the sample (4) or between the sample (4) and the detector (3) as a function of time is effected with a measurement rate of 100 kHz or more, preferably 150 kHz or more and particularly preferably 200 kHz or more.

5. A method according to claim 4 characterised in that the sample (4) has a plurality of N mutually superposed layers Si each of a layer thickness di, wherein i = 1,2,3,..., N and wherein the layer thicknesses di of all N layers are determined from the corrected function of the field strength over time (10).

6. A method according to claim 5 characterised in that the operation of determining the layer thicknesses di includes the steps: a) selecting a layer thickness di, an absorption index ki and a refractive index ni for each layer Si, with i = 1, 2, 3, ..., N, b) calculating a time-dependent electrical field EM(t) for the electromagnetic radio frequency radiation reflected by the sample (4) by means of a model, wherein the model respectively takes account of a time-dependent electrical field Ej(t) with j = 0,1,2,3,..., N according to the number of N+1 interfaces between a measurement environment and the sample (4) and between the individual layers, wherein the electrical fields Ej(t) are added in dependence on the layer thicknesses di, the absorption indices ki and the refractive indices ni to the time-dependent electrical field EM(t), c) comparing the calculated time-dependent electrical field EM(t) to the corrected function of the electrical field over time, wherein d) when a deviation Q between the calculated electrical field EM(t) and the captured electrical field EP(t) is greater than a predetermined tolerance T the layer thicknesses di, the refractive indices ni and the absorption indices ki are varied for so long and steps b) to d) are repeated until the deviation Q is smaller than the tolerance T, and e) providing the layer thicknesses di as the result of the layer thickness determining operation.

7. A method according to claim 6 characterised in that the electromagnetic radio frequency radiation (8) has a predetermined frequency bandwidth and in step b) the absorption indices ki is assumed to be constant over the frequency bandwidth of the electromagnetic radio frequency radiation used and the refractive indices ni is assumed to be constant over the frequency bandwidth of the electromagnetic radio frequency radiation used.

8. A method according to claim 6 characterised in that the electromagnetic radio frequency radiation (8) has a predetermined frequency bandwidth and in step b) the absorption indices ki are assumed to be changing over the frequency bandwidth of the electromagnetic radio frequency radiation used and the refractive indices ni are assumed to be changing over the frequency bandwidth of the electromagnetic radio frequency radiation used, wherein the calculation in step b) is based on a function of the absorption indices ki and the refractive indices ni on the frequency.

9. A method according to claim 6 characterised in that the electromagnetic radio frequency radiation (8) has a predetermined frequency bandwidth and the frequency dependencies of the absorption indices ki and the refractive indices ni are predetermined in advance in calibration measurements over the frequency bandwidth for each of the layers and the predetermined frequency dependencies form the basis for the calculation in step b).

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